BOEING-STL 2000P0001

Hypersonic Airplane Space Tether
Orbital Launch System

NASA Institute for Advanced Concepts
Research Grant No. 07600-018
Phase I Final ReportBOEING-STL 2000P0001
07 January 2000

Hypersonic Airplane Space Tether
Orbital Launch System
NASA Institute for Advanced Concepts
University Space Research Association
Research Grant No. 07600-018
Phase 1 Final Report

Principal Investigator:

Thomas J. Bogar, Boeing/MDC Phantom Works
St. Louis, MO

Co-Investigators:

Michal E. Bangham, Boeing/MDC HuntsvilleRobert L. Forward, Tethers Unlimited, Inc.

Mark J. Lewis, University of Maryland
Contributors:

Robert P. Hoyt, Tethers Unlimited, Inc.
Donald Johnson, Boeing/MDC-Phantom Works, St. Louis
Joseph Stemler, Boeing/MDC-Phantom Works, St. Louis
Benjamin Donahue, Boeing/MDC, Huntsville
Laura Beth Fleming, Boeing/MDC, Huntsville
Brian J. Tillotson, Boeing S&CS, Seattle
Daniel Stuart Bowman, University of Maryland, College Park

Copies may be obtained from the NIAC Web Site: <http://www.niac.usra.edu/bogar>
or by writing Boeing/MDC-Phantom Works, P.O. Box 516, Mail Code S106-7250,
St. Louis, MO 63166

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TABLE OF CONTENTS
Page

TABLE OF CONTENTS..............................................................................................................ii
LIST OF APPENDICES..............................................................................................................iii
INTRODUCTION ...................................................................................................................... 1
TECHNICAL SUMMARY OF SELECTED BASELINE HASTOL DESIGN ...................... 2
HYPERSONIC AIRPLANE ........................................................................................................ 5
SPACE TETHER...................................................................................................................... 9

Rotovator™.......................................................................................................................... 10
LIFTether.............................................................................................................................. 15
CardioRotovator™ ............................................................................................................... 17
Tillotson Two-Tier Tether.................................................................................................... 19
HyperSkyhook...................................................................................................................... 19
HARGSTOL......................................................................................................................... 20
Rendezvous Simulations ...................................................................................................... 21
Space Tether Issues .............................................................................................................. 24
Relevance of Past, Present and Planned Tether Flight Experiments.................................... 28
TETHER GRAPPLE AND PAYLOAD ACCOMMODATION SYSTEM ........................... 32
Design Requirements ........................................................................................................... 33
Configuration Options.......................................................................................................... 35
Architecture Control and Payload Transfer.......................................................................... 46
Grapple and Payload Transfer Issues ................................................................................... 49
Further Study/Recommendations ......................................................................................... 51
CANDIDATE PHASE II TASKS .............................................................................................. 52
Mission Opportunities.......................................................................................................... 52
System Requirements Definition ......................................................................................... 52
Conceptual Design ............................................................................................................... 52
System Analysis ................................................................................................................... 53
Technology Development Planning ..................................................................................... 53
CONCLUSIONS.................................................................................................................... 54

REFERENCES.................................................................................................................... 55

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LIST OF APPENDICESPage

APPENDIX 1 - Hypersonic Airplane Space Tether Orbital Launch(HASTOL) System: Initial Study Results.................................................. A1-1
APPENDIX 2 - Hypersonic Airplane Space Tether Orbital Launch(HASTOL) System: Interim Study Results................................................ A2-1
APPENDIX 3 - Design and Simulation of Tether Facilities for theHypersonic Airplane Space Tether Orbital Launch(HASTOL) Architecture ............................................................................ A3-1
APPENDIX 4 - Hypersonic Airplane Space Tether Orbital Launch(HASTOL) System - NIAC Fellows Meeting ........................................... A4-1
APPENDIX 5 - Modular Space Tether Design and Construction .......................................A5-1

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INTRODUCTION
The Boeing Company - Phantom Works has led a team in a Phase I study which has shown
the technical feasibility of a completely new concept for moving payloads and passengers fromthe surface of the Earth into low Earth orbit at low cost and low acceleration levels without theuse of rockets as the main source of propulsion. The team includes the Boeing Company,
Tethers Unlimited, Inc. (TUI), and the University of Maryland (UMd). The concept, named theHypersonic Airplane Space Tether Orbital Launch (HASTOL) Architecture, is shown
schematically in Figure 1.

Figure 1. HASTOL System Architecture

The HASTOL architecture contains three major components: a hypersonic airplane, which
will transport the payload as high and as fast as possible using air-breathing propulsion; anorbiting space tether, the tip of which will be lowered down and slowed down by some means so
as to meet up with the hypersonic airplane; and a grappling system, part of which is at the tip of
the tether and part of which is on the payload, that will take control of the payload and, with the1

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lift supplied by the space tether, carry the payload on into orbit, then toss it to into its desired
final trajectory. Each component will be discussed in more detail in the remainder of this report.
The objective of the study is to examine the feasibility of such a system and optimize the
combined system of airplane, tether, and grapple in order to maximize the overall performance interms of payload mass and delivery rate, while minimizing the life cycle cost. As we shall show
later, our Phase I study effort has determined that there are a number of technically feasible ways
to implement the HASTOL architecture using existing design concepts and existing materials.
The Phase II study effort can now concentrate on choosing the best implementation andoptimizing it for minimum life cycle cost.

TECHNICAL SUMMARY OF SELECTED BASELINE HASTOL DESIGNIn the Phase I study effort we identified a number of different combinations of hypersonic
airplane and space tethers that can result in a Hypersonic Airplane Space Tether Orbital Launcharchitecture that can bridge the present gap in altitude and speed between atmospheric flight andspace flight. The best concept found to date is the simplest one, and is illustrated schematicallyin Figure 1. In this section we will give a brief summary of the baseline HASTOL concept thatemerged from the Phase I study. Following the summary will be more detailed discussions of theindividual systems in the HASTOL architecture, with even more detail available in the
appendices. Three of the five appendices consist of two papers1,2 already presented at
professional society meetings and the presentation given at NIAC Fellows Review Meeting in
Atlanta. The other two appendices will be submitted as papers3 for some future professionalsociety meetings.
For the hypersonic airplane portion of the baseline HASTOL system we use an existing
Boeing design4 for the DF-9, a dual-fuel airbreathing vehicle that has benefited from over a
million dollars in NASA/LaRC and Boeing funding during prior study efforts. The Boeing DF-9
hypersonic airplane is similar to the X-43 research vehicle in shape and uses engines similar to
those that will be tested in the X-43 in the Summer of 2000. The DF-9 has a 9 m (30 ft) long by
3 m (10 ft) diameter upward-opening central payload bay that can handle payloads up to 14 Mg(14 metric tons or 30,000 lb). It uses JP-fueled air-breathing turbo-ramjets up to Mach 4.5, and2

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slush-hydrogen and air/oxygen ram/scram engines above Mach 4.5. With a full fuel load attakeoff, the hypersonic airplane masses 270 Mg (590,000 lb) or a little less than 20 times the
14 Mg payload mass, and can deliver the payload to 100 km (330 kft) altitude at an apogee speedwith respect to the surface of the Earth of 3.6 km/s (12 kft/s) or approximately Mach 12. If weassume an eastward equatorial launch at the equator, the speed of the airplane with respect toinertial space is 4.1 km/s -- halfway to space.
The airplane is met as it approaches apogee by the lower end of an orbiting spinning spacetether facility consisting of a massive tether control station, a heavy 600-km-long tapered tether,and a homing grapple assembly at the tether tip. The tether facility center-of-mass (CM) is
90 km from the tether control station and 510 km from the grapple tip. The tether facility CM isin a slightly elliptical orbit with an apogee altitude of 700 km, and a perigee altitude of 610 km.
The facility is rotating around the CM such that the tip speed of the grapple end of the tether is
3.5 km/s. The orbital period is about 98 minutes while the rotation period is about 16 minutes.
At perigee, the orbital speed of the facility CM is 7.6 km/s eastward. The phase of the tether
rotation will be adjusted so that at perigee the tether is directly below the tether control station
with the grapple tip of the rotating tether moving at a speed of 3.5 km/s westward. This willproduce a net speed of the grapple tip with respect to inertial space of 4.1 km/s eastward,
matching the apogee speed of the hypersonic airplane. The homing grapple, letting out tether
from its onboard reel, meets with the airplane and connects to the payload in the open bay. The
tether rotation then lifts the payload into space at a mild 2.3 g’s acceleration.
Depending upon the needs of the payload, the tether adjusts its orbit eccentricity and energy,
and its rotation rate, by using electrodynamic tether propulsion and tether length pumping, which
require solar energy, but no propellant, then releases the payload into the desired final orbit ortransfer trajectory. After each payload pickup and toss, the tether facility restores its original
orbital and rotation state over a number of days until it is ready to pick up another payload.
Alternatively, incoming payloads can supply the energy and angular momentum given to
outgoing payloads. The total cycle time between payload pickups depends upon the final payload
trajectory desired and the power available from the solar panel array on the tether control station.

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The tether is predominantly made of high-tensile-strength (4.0 GPa) low specific density(0.97) highly-oriented-polyethylene AlliedSignal Spectra™ 2000 polymer used commercially tomake ship hawsers and fishing line. Braided into the Spectra™ lines will be aluminum wire foruse by the electrodynamic tether propulsion system. The outer 20 km of tether, near the grapple
tip, will warm from a nominal 40 C to about 80 C during its pass through the upper atmosphereat 100 km altitude. This portion of the tether will be made of Zylon™ PBO polymer with atensile strength of 5.8 GPa and a specific density of 1.56, which is almost as strong per unit
density as Spectra™ 2000, and maintains most of its strength up to 300 C. Assuming a safety
factor of 2.0 in the tether design, a payload mass of 14 Mg, and a tip speed requirement of
3.5 km/s, the required taper in the tether is 3-to-1, ranging from an equivalent cross-sectional
diameter of 3.2 cm near the facility CM to 1.2 cm at the grapple. The total mass of the tether is alittle more than 90 times the payload mass or about 1300 Mg. As stronger fibers become
available in the future, the tether mass will drop significantly. To insure a high probability(0.999+) of survival over many decades despite strikes from space debris, the tether will be
designed to use the TUI-patent-pending interconnected multiline open net Hoytether™ structure.
After the pickup of the payload at perigee, the apogee altitude of the tether facility CM will
drop. The amount of altitude drop depends upon the mass of the tether facility. If the tether
rotation rate is controlled so that the tether is pointing away from the nadir at the apogee point,then a considerable drop in altitude can be tolerated. To be conservative, we have assumed aworst case situation where after payload catch, control is lost over the tether rotation rate, so thatit can be pointing straight down at apogee. To keep the grapple tip (and payload) out of theatmosphere, the tether control station must mass 110 times the payload mass. Thus, the total
mass of the space tether facility will be a little more than 200 times the payload mass or about3000 Mg. One of the objectives of the follow-on Phase II study will be to find ways to lower thismass ratio without jeopardizing safety.
In summary, we have identified a baseline HASTOL architecture that works. In Phase II wewill optimize it.

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HYPERSONIC AIRPLANE
The technology for the hypersonic airplane portion of the HASTOL system is being
developed by Boeing and others elsewhere and is not part of the HASTOL effort. However,vehicle performance, flight trajectory requirements, and operational aspects peculiar to tether
rendezvous and payload transfer in support of development and optimization of the HASTOL
system are, and form a major portion of the hypersonic airplane portion of the HASTOL teameffort. The hypersonic vehicle portion of the HASTOL effort started with an existing design for
the DF-9, a multi-role hypersonic aerospaceplane shown in Figure 2. The DF-9 was developed
by Boeing for NASA Langley Research Center, to perform both long-range hypersonic cruise
missions and space launch missions. The vehicle is designed to operate from existing runways
and incorporates a low-speed propulsion system based on JP fueled, Air (core-enhanced) Turbo
Ramjets (AceTRs) for operations up to Mach 4.5 (46 kft/s or 1.4 km/s). Above Mach 4.5 aslush-hydrogen-fueled ram/scram system powers the vehicle.

Tank 1 (SH2)

PayloadTank 4 (JP-7)
LOX (Below)

MLG

Looking Aft
Highly Integrated
2-D Inlet
AceTRs(Internal)

Tank 2 (SH2)
Tank 3 (SH2)
Crew Station

Tank 5L(SH2)
Tank 6 (SH2)

Tank 7(SH2)
Linear Rocket

Payload
NLG

Ramjet/Scramjet
Highly Integrated
SERN

Air Core Enhanced
Turboramjets (AceTR)
Ramjet/Scramjet

GP94070004.ppt

Figure 2. Boeing-NASA/LaRC DF-9 Dual-Fuel Aerospaceplane

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While the design is optimized for long range cruise at Mach 10 (10 kft/s or 3.2 km/s) at
40 km altitude, the vehicle can also perform “pop-up”-type launches of satellites, as shown in
Figure 3, and incorporates a 3 m (10 ft) diameter, 9 m (30 ft) long upward-opening payload bay
for that purpose. The current design is capable of carrying a 14 Mg (14 metric tons or
30,000 lb.) payload to altitudes as high as 100 km (330 kft) at speeds as high as 3.6 km/s
(12 ft/s or Mach 12) with respect to the atmosphere of the rotating Earth. If we assume an
eastward equatorial launch at the equator, the speed of the airplane with respect to inertial space
would be approximately 4.1 km/s.

Launch AscentHigh Speed Climb
Mach 4.5 - 10
Pattern, Landing,
Post Flight
Operations

Alternate Upper
Stage to Tether
Rendezvous
Ballistic
Trajectory toTetherRendezvous

Subsonic
CruiseInitial Climb
Mach 0.8 - 4.5

Descend
Decel

Tanker Rendezvous
Inflight Refueling

Figure 3. Vehicle Pop-Up to Tether Rendezvous

During the future efforts of this continuing study, the existing DF-9 hypersonic airplane
design will be modified, as required, to perform HASTOL type missions and the modifications
incorporated into its performance simulation model. The results of the hypersonic vehicle trade
study will be incorporated in the overall HASTOL system assessment. Two principal variant
types are candidates for further consideration. In the first, the aircraft will rendezvous with thegrapple at altitude. This variant will be modified, as required, for each applicable HASTOL
concept and its rendezvous geometry. In order to optimize operations in these HASTOL modes
it will be useful to resize the existing auxiliary rocket engines and their propellant volumes forincreased altitude and velocity. An enhanced reaction control system with six-axis capability(including limited trajectory control) will also be required in lieu of the current 3-axis (attitudeonly) system.

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An alternate variant will have the hypersonic airplane carry the payload to an intermediate
altitude and speed. From there a small rocket upper stage would carry the payload to therendezvous with the grapple. This approach will require fewer modifications to the hypersonic
airplane design, but will have less payload capability at a particular size due to the mass requiredby the rocket upper stage. Both variants will then be used to evaluate the concept through several
trade studies and optimized with the tether and grapple studies.
The vehicle performance for the DF-9 hypersonic vehicle was modeled using the threedegree-of-freedom model (3DOF) in Holist5. Holist is a vehicle performance and sizing tool
developed with partial support from NASA/LaRC. The driving force behind Holist is the need tomodel the important discipline interdependencies that are critical to the performance ofhypersonic vehicles. Holist has been used in the Boeing hypersonics group to analyze the DF-9
Dual Fuel Mach 10 cruiser, Standoff Fast Reaction Missile (SFRW) concepts, and VTHL andHTHL Rocket Based Combined Cycle (RBCC) vehicle concepts for space access studies.
The 3DOF performance model in Holist was used to model the hypersonic airplane’s flightfrom takeoff through apogee and to the end of the turn back toward the launch site. The 3DOF
model is the most rigorous mission model available in Holist. Output from the model provides
the user with detailed information about the vehicle weight, flight condition, position,
aerodynamic state, and propulsion state. The 3DOF model is used when details of maneuvering
flight are required. The analysis runs in seconds or minutes depending on the number of phases
and the integration step size used. For the HASTOL study, various combinations of pull-up
Mach number, rocket shutdown altitude, and rocket thrust were used to calculate multiple apogee
altitude and velocity conditions, corresponding to various payload transfer points to the spinning
tether. Figure 4 shows the “achievable” apogees in the velocity/altitude space in solid blue dotsin the lower and right portions of the diagram, while the unachievable points are the open red
circles in the upper and left portions of the diagram.

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400,000
120

ft

❍

km

❍

❍

● Achievable Points❍ Unachievable Points

110

Altitude350,000
❍

100

●

❍

●

300,000
90

80

●

●

250,000
8,000

9,000

10,000

2,500

3,000

●

●

11,00012,000
3,500

ft/s
m/s

Velocity

Figure 4. Matrix of Analyzed Payload Transfer Points

Analysis of these unachievable apogees with the Holist 3DOF model showed that the vehicle
was unable to pull out during its re-entry mission phase without exceeding its structural limits.
A comparison of the normal load factor and dynamic pressure time histories for an achievable
and an unachievable apogee (the lower right and upper left cases shown in Figure 4) are shownin Figures 5 and 6, respectively. The combination of these two figures shows that if the airplaneis flown from one of its unachievable apogees at its maximum design load factor of 2.5 g’s, it isunable to pull-out of the dive after apogee before exceeding its maximum dynamic pressure limitof 2000 psf.

Figure 5. Normal Load Factor Along Descent Trajectory

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Figure 6. Dynamic Pressure Along Descent Trajectory

The situation is the same for all the apogees identified as unachievable in Figure 4. Changes
to the structural or aerodynamic design of the DF-9 would be necessary to expand the envelopeof achievable apogees. However, our Phase I study determined that changes to the design of theDF-9 hypersonic airplane were not necessary since the 100 km, 3.6 km/s apogee case wasdetermined to be sufficient for the HASTOL architecture. The existing design will do.

SPACE TETHER
The space tether component of the HASTOL architecture is required to reach down from its
orbital altitude hundreds of kilometers above the atmosphere, to place the grapple end of its
tether at an altitude in the upper atmosphere that the hypersonic airplane can reach in order that apayload transfer can take place. The end of the tether must also be slowed down, by one meansor another, from typical orbital speeds to a speed the hypersonic airplane can attain at that
altitude. The six different space tether facility concepts initially considered during the Phase I
contract effort were the: Rotovator™, LIFTether, CardioRotovator™, Tillotson Two-Tier Tether,HyperSkyhook and HARGSTOL. We found that nearly all of the concepts would work, and in
the Phase I effort we analyzed two of them in some detail, the Rotovator™ and the LIFTether.
The simplest concept is the Rotovator™, and it has been baselined for the Phase II effort.
However, all of the concepts will be revisited in the Phase II effort.

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Although in a typical space tether facility the mass of the tether will usually be less than themass of the tether control station, we do not want to ignore the tether mass entirely. So, for eachof the concepts, we estimated the mass of the tether alone, using the data we have for the tensile
strength and density of high strength materials that are presently available in commercial
quantities. If the mass of the tether alone started to exceed 200 times the mass of the payload,then that was an indication the particular scenario being considered was not engineeringly
feasible using presently available materials, although the application might become feasible in
the near future as better materials become available with higher tensile strengths at higher
operational temperatures.
As we shall see, presently available commercial materials will suffice to make the HASTOL
tethers needed. The primary message we want to leave with the Reader is:“We don't need magic materials like ‘Buckminster-Fuller-carbon-nanotubes’ to makethe space tether facility for a HASTOL system. Existing materials will do.”Rotovator™
The standard method of attaining a low tether tip velocity at low altitude is to use a longrapidly spinning orbiting tether, or Rotovator™, in a nearly circular orbit. The Rotovator™
concept was invented in 1967 by Artsutanov and reinvented by Moravec in 1977, who did thefirst thorough analysis6 of it. Since the Rotovator™ must reach down from orbital altitudes into
the upper atmosphere to match speeds with the hypersonic airplane, the length of the tether andthe orbital altitude are necessarily interrelated, with the orbital altitude of the tether center-ofmass (CM) being the length of the tether from the CM to the tip plus a nominal 100 km for the
thickness of the atmosphere. The longer the tether, the higher the orbital altitude and the slower
the velocity of the tether facility CM.
The mass of a rapidly spinning tether in free space is determined primarily by the tip speed of
the tether. Equation (1) in Appendix 1 shows specifically that the mass ratio of a spinning tether
grows exponentially with the square of the tether tip speed. The mass ratio of a long spinning
tether near the Earth will depend not only on the tip velocity of the tether, but also the gravity
gradient force which, in turn, depends upon the orbital altitude above the Earth and the length of
the tether.7-12 This will be true for most of the tether systems being considered for the HASTOL

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architecture. There is no simple analytical equation that takes both the tether rotation forces andgravity gradient forces into account, and the tether mass ratio needs to be numerically integrated
for each case.
We found, in general, that Rotovators™ that were very short had a lower orbital altitude andtherefore higher orbital velocity, so they needed a higher tip velocity to match speeds with ahypersonic airplane moving at a given hypersonic velocity. Thus, their mass ratio increased
exponentially as the square of the tip velocity. We also found that Rotovators™ that were very
long were orbiting more slowly, and thus needed less tip velocity, but because the gravity
gradient forces on the tether increased with tether length, the mass ratio increased because of theincreased gravity gradient force. We were able to find an intermediate-length Rotovator™
design that works. It is discussed in detail in Appendix 3 and summarized below. This design
has been selected as the baseline space tether for the Phase II study.
The baseline Rotovator™ space tether facility shown in Figure 7 is composed of a tether
control station (containing power supplies, tether reel, command and control, and ballast mass) ,a 600 km long tapered tether, and a grapple assembly at the end of the tether. The tether facility isplaced in a slightly elliptical orbit (e=0.0062) with a CM apogee altitude of 700 km, a perigee
altitude of 610 km, and a perigee velocity of a little over 7.6 km/s. The orbit was chosen to be
elliptical and payload capture was performed at perigee in order to reduce the amount of total
facility mass needed to keep the facility and tether above the atmosphere after the facility
captures a payload. The tether is set into rotation with a tip velocity of a little over 3.5 km/s.
The center of mass of the tether facility is located about 90 km from the tether control station, so
when the facility is at perigee altitude of 610 km, the tether control station is at an altitude of 700
km and the tether tip is at an altitude of 100 km, moving at a velocity of approximately 7.6 km/s 3.5 km/s = 4.1 km/s relative to the inertial reference frame, thus matching the speed of thehypersonic airplane. The atmospheric drag on the tether at 100 km altitude was calculated andfound to be negligible.

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Figure 7. Rendezvous of a Rotovator™ and a Hypersonic Airplane

Tether Diameter: The required effective diameter for the Rotovator™ tether was calculated
assuming it would be constructed of a material such as Spectra™ 2000, with a tensile strength of
4.0 GPa and a density of 970 kg/m3. Although in the final implementation the tether would
likely be a multiline Hoytether™ structure to provide tether survivability, in these simulations thetether was modeled as being a single-line structure, tapered to minimize the tether mass. The
required tether taper is illustrated in Figure 8 and shows that the required tether taper varies by afactor of roughly 3 from an equivalent tether diameter of about 3.6 cm (1.5 in.) at the tether CM
to about 1.2 cm (0.5 in.) at the tether tip. Along most of the tether, the safety factor was chosen
to be 2.0. At the tether tip, however, the safety factor is increased in order to provide extra safetymargin to handle transient loads due to payload capture. Because the portion of tether closer tothe tether control station has a much larger cross section, the transient loads created at the tether
tip due to payload capture become insignificant further up the tether, and thus the safety factor of
2 should be adequate. The total tether mass was calculated to be 1360 Mg, or approximately 90
times the payload mass. The station mass was calculated to be 1650 Mg, or approximately 110
times the payload mass. The total tether facility mass came to 3010 Mg, or just over 200 times
the payload mass. Recent additional simulations since those in Appendix 3 have already found
Rotovator™ designs with total facility mass ratios of 160 times the payload mass, and we expect
the required facility mass to drop even further as the Rotovator™ designs are optimized in the
Phase II effort.

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Tether Radius (cm)

2
1.6
1.2
0.8
0.4
0
0

100

200

300
Position (km)

400

500

600

Figure 8. Radius of a Stepwise Tapered Spectra™ 2000 Tether(15 Mg payload, 600 km long Rotovator™, tip velocity of 3.4 km/s)

Tether Heating: In addition to calculating the tether orbit and dynamics, our simulation
program also calculated the atmospheric drag and the tether temperature. The first scenario
studied was a rendezvous between the all-Spectra™ 2000 Rotovator™ and a hypersonic airplaneat an apogee altitude of 100 km and a velocity of 4 km/s. Figure 9 shows the altitude of thetether tip during the rendezvous period, while Figure 10 shows the temperature of the bottom
portion of the tether during the same time period. During the roughly 100 s the tether tip spends
within the upper atmosphere (altitude <130 km), the tether temperature increases only about
40°C from a nominal 40°C to a maximum of 80°C. This temperature rise might be problematicfor Spectra™ 2000, which loses strength rapidly with temperature. However, there exist several
commercially-available materials, such as PBO (sold by Tyobo of Japan under the name
Zylon™), that have strength-to-weight characteristics almost as good as Spectra™ 2000 and havesignificantly better temperature tolerance. PBO is also approximately 1.7 times as dense as
Spectra™, so a PBO tether would have a smaller diameter, and thus experience smaller drag andheating. Consequently, we conclude that the heat loading at 100 km is low enough that a tether
constructed of currently-available high-strength polymers (perhaps with some form of AOresistant coating) can accomplish the HASTOL mission.

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200
150
TetherTip
Altitude
(km)

100
50

0

675

700

725

750

775
Time (s)

800

825

850

825

850

Figure 9. Altitude of the Rotovator™ Tether Tip

100
80
Tether60
Tip
Temperature 40
(C)
20
0

675

700

725

750

775
Time (s)800

Figure 10. Temperature of the Rotovator™ Tether Tip

An important point to make about our Phase I study results so far, is that a simple orbiting
spinning Rotovator™ space tether facility built using existing space tether materials can be used
to pick up a payload from an existing design for a hypersonic airplane that is capable of taking a
payload to an altitude of 100 km (330 kft) altitude while moving at 3.6 km/s (12 kft/s or Mach12) with respect to the atmosphere, or 4.1 km/s with respect to inertial space.
Thus, the HASTOL system combination of a Boeing DF-9 hypersonic airplane and a Tethers
Unlimited, Inc. Rotovator™ space tether, is capable of taking payloads from the surface of the
Earth, putting them into space, and bringing payloads back. The other HASTOL concepts wewill discuss later may prove to be better after further analysis in Phase II, but this concept has
acceptable performance.

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LIFTether
The second concept1,13 for the space tether facility portion of a HASTOL system would use
two separate methods of operating the tether, one method for getting the grapple/payload at theend of the tether down into the atmosphere, and another method for getting the grapple back up
into orbit again. There are a number of variants for both the down and up options.
The simplest technique is illustrated in Figure 11. The tether length and tip speed of arotating tether are selected so that, without letting out any tether, the grapple hits the denser
portions of the upper atmosphere at about 80 km altitude, sufficiently ahead of the tether control
station to allow time for drag deceleration of the grapple down to a velocity that matches the
speed of the hypersonic airplane before the tether control station passes overhead. Since thedecrease in velocity of the grapple does not involve using the strength of the tether, the mass ofthe tether is not affected by the amount of velocity decrease needed. If the relative positions of
the grapple and tether control station are properly timed, the tether control station can be made to
be directly overhead the hypersonic airplane at the time of the payload transfer to (or from) thegrapple, while the distance between the two can be made to be equal to the total unreeled lengthof tether. As shown in Figure 11, with the tether control station directly overhead, the tether willsmoothly “lift” and accelerate a payload (or empty grapple) into orbit without requiring any
reeling in or out of the tether.

Figure 11. The LIFTether Concept

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Although the payload and the section of tether nearest the payload are moving at 3.5 km/s
relative to the facility’s center of mass, the bulk of the tether is rotating more slowly.
Consequently, once the tether pulls taught, the tether is again rotating at a lower tip velocity of
approximately 3.1 km/s. This allows us to size the bulk of the tether for a 3.1 km/s tip velocity
rather than 3.4 km/s, which reduces the mass of the tether considerably. The tether tip, however,must be designed with a higher safety factor to withstand the capture transients, and furthermore
the tether material must be chosen to survive the heating due to the aerodynamic drag at thelower pickup altitude of 80 km.
A detailed simulation was carried out of a LIFTether design composed of a 600 km long
Spectra™ 2000 tether picking up a payload from a hypersonic airplane that reaches apogee at 80km altitude with a velocity of 4.1 km/s (relative to the inertial reference frame). The tether taperand facility mass were identical to the Rotovator tether design, and the orbital velocity of thetether facility’s center of mass was approximately 7.5 km/s. The simulation was initiated withthe tether initially oriented parallel to its orbital velocity, rotating so that its tip velocity wasapproximately 3.0 km/s relative to its center of mass. As the tether dropped towards the local
vertical, its tip velocity increased to approximately 3.2 km/s due to gravity gradient forces. As
the grapple vehicle entered the upper atmosphere, it extended retractable aerobraking panels toincrease its cross-sectional area to 16 m2. The aerodynamic force on the grapple increased thevelocity of the tip an additional 0.3 km/s, giving it a total velocity of approximately 3.5 km/s
relative to the center of mass. Because the tether tip is rotating backwards relative to the centerof mass, this gave it a total velocity in the inertial frame of 4 km/s.
The temperature of the LIFTether tip as a function of time was calculated to increase from anominal 40°C to a maximum of 1000°C toward the end of its pass through the atmosphere and is
illustrated in Figure 15 of Appendix 3. Since Spectra™ 2000 melts at approximately 180°C,
Spectra™ clearly would not survive this maneuver. Even PBO/Zylon™, which can operate at
temperatures over 600°C, would not suffice. Consequently, for tether-airplane rendezvous atsuch low altitudes, the tether tip must be constructed of a high strength material with higher
temperature tolerance and higher heat capacity such as Titanium-coated Silicon Carbide
Textron™ fiber. Textron™ fiber maintains 65% of its strength at temperatures as high as
1200°C and so is suitable for this application even at these high temperatures.

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Figure 12 shows the tether taper for a LIFTether designed to lift a 15-Mg payload into orbit.
The bulk of the tether would be made of a high-strength polymer such as Spectra™ 2000 or
Zylon™, but the bottom 20 km of tether would be constructed of Ti-coated SiC Textron™ fiber.
The total tether mass is 530 Mg, or approximately 35 times the payload mass. The Station massis 1650 Mg, or 110 times the payload mass. The entire tether facility mass is 2180 Mg, or 145
times the payload mass.

Tether Radius (cm)2

TEXTRON Segments

1.6
1.2
0.8
0.4
0
0

100

200

300
Position (km)

400

500

600

Figure 12. Tether Taper for a LIFTether with a 20 km Tip of Ti Coated Textron SiC

Since the LIFTether has a lower total mass than the Rotovator™ and can operate at a lower
altitude, which makes the requirements on the hypersonic airplane less severe, in our Phase II
effort we will continue to evaluate materials with high tensile strength at high temperatures so the
LIFTether concept can continue to be considered in a search for an optimal HASTOL
architecture. However, the higher tether temperatures, the uncertainty introduced byaerodynamic drag, and the drag and heating on the payload, have led us to choose the simpler
Rotovator™ as our baseline design for the Phase II study.
CardioRotovator™
The CardioRotovator™ concept13 consists of a tether control station in an elliptical orbit,
with a single long tapered tether. The tether rotation rate is chosen to be exactly twice the orbital
period. The phase of the rotation is chosen such that when the tether control station is at perigee,or closest to the Earth, the tether is pointing straight up, as is shown in Figure 13. Then, whentether control station is at apogee, or furthest from the Earth, the tether is pointing straight downat the Earth, reaching deep into the atmosphere for the payload pickup. As can be seen in

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Figure 13, at intermediate points, the tether is pointing away from the Earth and does not
penetrate below the tether control station altitude except near the touchdown point below theapogee point. The trajectory of the tip of the tether is approximately heart-shaped, which lead tothe name of “CardioRotovator™” for the system concept. This concept has the advantage that,
because the rendezvous between the tether tip occurs when the tether facility is at its apogee (andmoving at its slowest speed relative to the Earth), the rotation velocity of the tether would beapproximately 0.4-0.5 km/s slower than the tip velocity of an equivalent rotating tether in circularorbit. Due to the dependence of the mass ratio of the tether on the exponential of the square of
its tip velocity, this could significantly reduce the required tether mass.

Figure 13. The CardioRotovator™ Concept

However, analysis showed that this concept has several problems that likely render it
impractical. First, the payload pickup occurs when the tether is at apogee. Unless the tether
drops a return payload at the same time as it picks up the outbound payload, this will result in adrop in the perigee altitude of the tether facility. The mathematics of the orbital mechanics are
such that the tether facility would require a total mass on the order of 1000-2000 times thepayload mass in order to keep the tether facility from entering the atmosphere after a payload
capture. Second, this approach would require that the tether rotation be very carefully controlled
so that the tether is always above the facility at perigee. When the tether catches a payload,conservation of angular momentum will result in its angular velocity remaining constant, but its
orbital period will change due to its exchange of momentum with the payload. Consequently, the18

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tether facility would have to perform significant tether reeling maneuvers to maintain the propersynchronization between the tether rotation and its orbit. While this may be technically feasible,
any failure would result in the tether impacting the atmosphere, causing loss of the tether system.
For these reasons, we concluded that the CardioRotovator™ concept is less favorable than thesimpler Rotovator™ and LIFTether concepts.
Tillotson Two-Tier Tether
The Tillotson Two-Tier Tether (TTTT or
T4)14 illustrated in Figure 14, consists of a long,large, tapered “first stage” spinning tether, at theend of which is a smaller “second stage” spinning
tether. The T4 is essentially a two-stage
Rotovator™. The use of two tiers or two “stages”
in the design of a spinning tether decreases theoverall ratio of the tether launch system mass to
payload mass, in a manner similar to the benefitsof the lower mass ratio obtained when using a
two-stage rocket in a rocket launch system. The

Figure 14. Tillotson Two-Tier Tether

T4 approach to the design of the Rotovator™ for a
HASTOL system is much more complicated in design and dynamics than a simple one stage
Rotovator™. The plan is to baseline the one-stage Rotovator™ for the study, but to carry out
analyses of the T4 system in parallel. If the mass of the one-stage tether grows to where its mass
begins to cast doubt on the engineering or financial feasibility of the HASTOL concept, then wealways have the two-stage T4 concept available in order to drastically cut the tether mass needed.
HyperSkyhook
In 1995 Zubrin proposed15 the “Hypersonic Skyhook” as a solution to the mismatch between
the attainable atmospheric speeds of a hypersonic airplane and the orbital speeds of space tethers.
Since the orbital speed of the space tether decreases with increasing altitude of the tether systemcenter-of-mass, he proposed the use of very long non-spinning tethers or “skyhooks” reaching
down from very high altitudes (thousands of kilometers). His analysis showed that because a

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hanging tether must be tapered to support its lower end in the gravitational field of the Earth,achieving a HyperSkyhook tether tip rendezvous with a 5.0 km/s (16 kft/s or Mach 16) airplane
would require a HyperSkyhook tether mass of 25 times the payload mass. Trying to lower thetether tip speed to 4.0 km/s (13 kft/s or Mach 13) would require a HyperSkyhook tether mass
greater than 200 times the payload mass. In general, the non-spinning tether HyperSkyhook
concept does not look competitive with the spinning tether concepts. We will, however, revisit
this concept in our Phase II studies.
HARGSTOL
The final method of accomplishing the HASTOL concept is to compromise, and allow thepartial use of a rocket upper stage or a rocket-powered grapple to complete the payload transfer
between the hypersonic airplane and the grapple assembly at the end of the space tether. Thus,
instead of the HASTOL system, we will have the HARGSTOL or Hypersonic Airplane, Rocket
Grapple, Space Tether Orbit Launch system. This concept has a number of possible variations.
The normal method would be to have the rocket augmented grapple on the tip of the tether. The
tether system would slow the tip down as much as possible using one of the tether tip slowing
techniques, and the airplane would fly as fast and high as possible, and the rocket system on thegrapple would make up any speed difference. The grapple would need to be refueled periodically.
This could be done at each payload pickup, or there could be periodic pickups of propellant
tanks, with the empty tanks added to the tether control station ballast.
A variation on this concept would be to have the major part of the tether mass be a permanent
part of the space tether system, but the “tip” of the tether and the rocket grapple would be carriedby the hypersonic airplane. At some time interval before the rendezvous time, the grapple would
be separated from the airplane, pulling out the tether, which would be made of material capableof coping with the hypersonic heating and stress. The rocket grapple would then climb in
altitude and speed to meet up with the lower end of the space tether out in space away from theatmosphere, while the airplane stays in the atmosphere at an optimum cruise altitude. The
grapple grabs the end of the tether, the payload is pulled free from the airplane, and lifted into
space by the tether.

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The ultimate rocket grapple concept would have the rocket take the grapple from thehypersonic airplane all the way to the tether control station, pulling out tether from the payload.
Since for normal ∆V requirements the tether mass would be much larger than the payload mass,
it is obvious that a better technique would be to meet the downgoing tether from the tether
control station “halfway”. Finding the optimum ratio for the length of the airplane tether versus
the space tether would be part of the overall system optimization. This concept, with the rocket
grapple coming from the airplane without carrying the payload, would only be usable for taking
payloads into orbit.
The most important feature of all the possible HARGSTOL systems is that we are confident
we can make them work, no matter how poor the ultimate performance of the hypersonic airplane
and the space tether. All it requires is that the rocket grapple be loaded with enough propellant toclose the velocity gap. Since the mass ratio of the propellant to grapple-plus-payload is
exponential in the grapple ∆V, and the rocket ∆V is low because of the ∆V contributions of boththe airplane and tether, the propellant required should be low.
Rendezvous Simulations
In any rotating tether transport system, one of the most challenging tasks will be to enable therendezvous between the payload and the tether tip. For the tether to successfully capture thepayload, the payload and tether grapple vehicle must come together at nearly the same place inspace and time with nearly the same velocity. Because the payload is in free fall, and the tether isrotating, the payload and grapple vehicle will experience a relative acceleration equal toa = Vt2/L, where Vt is the velocity of the tether tip relative to the tether facility’s center of mass,and L is the distance from the tether tip to the center of mass. In the HASTOL tether designs
described above, Vt is approximately 3.5 km/s, and L is approximately 500 km, so thisacceleration is about 2.5 g’s. If neither grapple nor payload perform any maneuvering, the two
will coincide only instantaneously, providing a minimal rendezvous window.
Fortunately, it is possible to extend this rendezvous window to five seconds or more by using
tether deployment from the grapple vehicle. In this approach, the grapple vehicle will contain atether deployer and a tether brake. Prior to the rendezvous, the grapple vehicle will wind upsome of the tether into the deployer. As the tether nears the bottom of its swing, the payload will21

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use its guidance and thrusters to adjust its trajectory so that it will meet up with the grapple
vehicle. When the payload and grapple vehicle reach their closest approach to each other, thegrapple vehicle immediately releases the brake on the tether deployer and allows the tether todeploy at as low a tension as possible. This will put the grapple vehicle into an almost-free-falltrajectory which will match the free-fall trajectory of the payload, as illustrated in Figure 15. The
payload can then maneuver to close the gap and secure itself to the grapple vehicle. The length of
the rendezvous window will therefore be determined by the incremental length ∆L of tether
stored in the deployer, with the maximum window equal to ∆t = (2∆L / a ) .

Figure 15. Schematic of Tethered-Grapple Method for Increasing Docking Window

We have simulated this maneuver for a HASTOL Rotovator™ architecture in which theSpectra™ 2000 tether illustrated in Figure 8 picks a payload up from a 100 km, 4 km/s apogee.
In this simulation, the payload was launched into a trajectory that would meet up with the tether
tip. Once they came into close proximity, the grapple vehicle released the tether brake andallowed tether to pay out at very low tension for five seconds. At that point, the grapple vehicle
captured the payload and halted the tether deployment. Figure 16 shows the relative separation
between the payload and tether tip in the x and y directions. This plot shows that the tether
deployment maneuver extends the rendezvous window to about 5 seconds. The length of tether
deployed in this time was 486 m.

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100
Grapple Vehicle
Begins Deploying Tether

Separation (m)80Grapple Vehicle
Captures Payload
andApplies Tether Brake

60
40

²X
²Y

20
0
-20
250

255

Time (s)260265Figure 16. Relative Separation of Grapple Vehicle and Payload, with a Tether Deployment
Maneuver to Extend Rendezvous Window

Figure 17 shows the tether load level at the grapple vehicle. During the tether deployment,
the tension is essentially zero. When the grapple vehicle stops deploying tether, however, it
experiences a relatively strong transient tension spike up to about 70% of capacity, followed by alonger period transient that peaks at about 80%. These higher tension transients result from thefact that the deployment maneuver allows the payload and grapple to accelerate away from thetether facility for several seconds, and thus the tether must apply a larger force to them toaccelerate them into the tether rotation once the deployment is halted. This simulation result
indicates that the portion of the tether near the tether tip should be designed with a higher safety
factor to provide more margin for these tension transients.

Load Level at Tether Tip(Load/Capacity)

1

Grapple Vehicle
Begins Deploying
Tether0.8
0.6

Grapple Vehicle
Captures Payload
andApplies Tether Brake

0.4
0.2
0

250

260

270

280
Time (s)290300

Figure 17. Load Level on Bottom Segment of Tether, with a Tether Deployment
Maneuver to Extend Rendezvous Window

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Space Tether Issues
The Space Tether portion of the HASTOL system has a number of issues that must be dealt
with other than the method of operation, including surviving damage by meteorites and spacedebris, avoiding collisions with large spacecraft, fabricating the facility in space, and controlling
the tether orbit, spin and dynamics.
Tether Survivability: For a tether transport system to be economically advantageous, it
must be capable of handling frequent traffic for many years despite degradation due to impacts bymeteorites and space debris. Yet, the tether mass must be minimized to reduce the cost of
fabricating and launching the tethers. These two requirements present conflicting demands upon
the tether design that make conventional single-line tethers impractical for the HASTOL
application. For a single-line tether to achieve a high probability of survival for many years, it
must be very thick and massive. Fortunately, a low mass survivable tether design exists, called
the Hoytether™, which can balance the requirements of low weight and long life16.
As shown in Figure 18, the Hoytether is an open net structure where the primary load bearing
lines are interlinked by redundant secondary lines. The secondary lines are designed to be
initially slack, so that the structure will not collapse under load. If a primary line breaks,
however, the secondary lines become engaged and take up the load. Note in Figure 18, that four
secondary line segments replace each cut primary line segment, so that their cross-sectional area
need only be 0.25 of the primary line area to carry the same load. Typically, however, thesecondary lines are chosen to have a cross-sectional area of 0.4 to 0.5 of the primary line area, so
as to better cope with multiple primary and secondary line cuts in the same region of the tether.
This redundant linkage enables the Hoytether™ structure to redistribute loads around primary
segments that fail due to meteorite strikes or material failure. Consequently, the Hoytether
structure can be loaded at high stress levels, yet achieve a high margin of safety. This isdiscussed in detail in Appendix 2.

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Primary
Lines

First Level ofSecondary
Lines
Redistributes
Load toAdjacent
Nodes

SecondaryLines(initiallyunstressed)

Effects ofDamageLocalized
Second Levelof Secondary
Lines
Redistributes
Load Back to
Undamaged
Portion ofPrimary Line

SeveredPrimaryLine

0.2 to10’s ofmeters0.1- 1 meter

a.

b.

c.

Figure 18. The Hoytether™ Design and Its Response to a Cut Line

Collision Avoidance: There are many objects in space, ranging from micrometeorites tooperational spacecraft with 10-meter-long solar array panels. As shown in the previous section,
we can design interconnected multiple strand open net Hoytether™ structures that can reliably(>99.9%) survive in space for decades despite impacts by objects up to 30 cm (1 ft) or so in size.
Objects larger than 30 cm are all known and tracked by the U.S. Space Command. There areabout 6000 such objects in low and medium Earth orbit, of which an estimated 600 will be
operational spacecraft in the 2005 time frame. For an atmospheric tether application, we haveestimated that, if no traffic control measures are instituted, a 20 km long tether in an orbit grazing
the upper atmosphere has a 4% chance of striking one of the 6000 large objects during a one year
mission, and an 0.4% chance of striking one of the 600 operational spacecraft. Longer tethers
will have proportionately larger probabilities. It will therefore be incumbent on the HASTOL
operators to maintain contact with the U.S. Space Command and keep an accurate inventory of
the known large objects. They then need to control the tether facility CM orbital altitude andphase, the tether rotation rate and phase, and the tether libration and vibration amplitudes andphases, to insure that the tether facility components do not penetrate a volume of “protected
space” around these large orbiting objects. The requirements that need to be put upon theCommand and Control system to meet this need will be determined during the Phase II effort.

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Modular Space Tether Design and Construction: The space tether facility portion of the
HASTOL system will need to supply half or more of the energy and angular momentum needed
to put the payload into orbit. In order to keep the payload from dragging the space tether facility
down into the atmosphere after pickup, the facility must mass significantly more than the
payload. A minimum space tether facility mass would be 30 times the payload mass, while arobust system would be 50 to 200 times the payload mass. Because the full-up space tether
facility will necessarily be massive, it cannot easily be launched in one piece. We therefore will
design it in a modular fashion so that it can be launched in many separate modules. The types ofbasic modules have yet to be fixed, but probably would consist of a single large command andcontrol module, a grapple module, and a large number of power modules, winch modules, andtether modules. Further discussion of the modular design can be found in Appendix 5. The
details of the modular design of the space tether facility will be carried further in the Phase II
effort. The following summary of Appendix 5 should therefore be considered as merely one
example of what the modules would look like and how they would be interconnected into a spacetether facility.
The modules would be cylinders between one and two meters in diameter and height. A
small 1 m (3-ft) module would mass about 1 Mg and be 1/15th of the nominal payload mass of
15 Mg, while the largest 2-m (6-ft) module would mass about 6-8 Mg and be half the mass of thenominal payload mass. The modules will be designed so that only a few modules are needed toassemble a minimal but functional initial space tether facility consisting of the large commandand control module, a few power modules, a few winch modules, a number of tether modules,
and a grapple module
The command and control module would probably be put into place first by a heavy lift
rocket. Studies may show that it is more cost effective to put the other modules into orbit byrocket, but if the hypersonic airplane is available, we can start using the initial pieces of the
HASTOL system to begin to assemble itself. The power modules, winch modules, grapple
modules, and especially the tether modules, can be flown to Mach 12 (3.6 km/s) at 100 km
altitude, then boosted on into orbit by a rocket upper stage to a rendezvous with the commandand control module. There, they would be automatically assembled (with the aid of remotecontrol guidance), into a minimal, but functional, initial space tether facility. Assuming the26

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modules mass 1 Mg, the initial facility need only mass 50 Mg before it becomes capable ofhandling one module at a time at the full design tether tip speed of 3.4 km/s. Once the facility iscapable of rotating at the full design tip speed, this capability then eliminates the need for using
upper stage rockets to boost the modules from the hypersonic airplane up to the space tether
facility, and the initial HASTOL architecture has been “born”. This “infant” HASTOL now has
the ability to “grow” by “feeding itself” additional modules brought up by the hypersonic
airplane. (The analogy of “feeding itself” is an apt one, in that Oldson and Carroll9 have shownthat it is possible for a rotating tether system to “toss” a payload from its grapple at the end of thetether into a trajectory that ends up an orbit later with the payload coming to a gentle “dock” withthe tether control station - just like tossing a peanut into your mouth.)Once functional, that same initial HASTOL architecture can also make money by using its
hypersonic airplane to deliver 1 Mg communication satellites and deep space probes to the spacetether facility, which in turn delivers them to higher orbit or Earth escape. The HASTOL
architecture will thus be "in business" and “producing income” from almost its first day ofoperations. After doubling its size with 50 more “bites” of power, winch, and tether modules, itwill be able to handle 2 Mg payloads, “grow” itself even faster, and make even more money byhandling larger and larger payloads.
The use of modular design in the tether portion of the space tether facility eliminates any
concern about the manufacturability and packaging of a 600 km long tether. Since thecontinuous tether lengths required are only 20 km in length, they can each be fabricated in abouta month or so using standard braiding machine speeds. As a result, any desired tether delivery
schedule can be met by simply using more braiding machines in parallel.
Control of Tether Orbit, Spin and Dynamics: In order for the HASTOL system to achieve
low operational costs, the system must have means for maintaining and controlling the orbit, spinand dynamics of the tether system that do not consume large amounts of propellant. The mostimportant capability is for rapidly restoring the tether facility’s orbit after it has boosted apayload into orbit. In addition, the tether facility must have the capability to counteract
perturbations to its orbit and spin due to Earth oblateness, aerodynamic drag, and otherphenomena in order to maximize the frequency of opportunities for rendezvous with a launch

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vehicle. Fortunately, several tether techniques can provide the capabilities needed without
requiring propellant expenditure.
The primary technique to control the tether orbit and spin will be to use propellantlesselectrodynamic tether propulsion. Woven into the polymer tether nearest the tether control station
will be an aluminum wire conductor to be used by the High-strength Electrodynamic Force
Tether (HEFT) propulsion system10 built into the tether. Electrical power from the solar panel
array will be used to pump electric current through the conducting portions of the tether. The
current flowing along the length of the tether pushes against the Earth’s magnetic field. The
reaction force can be used to reboost the orbit of the tether facility after it boosts a payload. This
technique uses the mass of the Earth, coupled through its magnetic field, as a reaction mass, andthus requires no propellant. This technique can also be used to increase or decrease the rotation
rate of the tether system, prevent elliptical orbit precession, change the orbit ellipticity, energy,
angular momentum, and even inclination (slowly), as well as damp any librations or vibrations in
the tether.
The secondary technique for controlling the orbit and spin of the tether facility is to use tether
reeling maneuvers18-21. By using electrically powered winches to reel a tether in and out during
proper portions of its orbit or rotation, the tether system can “do work” against the Earth’s
gravitational potential, adding or subtracting energy from the tether’s orbit and/or rotation. The
spin rate of the tether can be increased by reeling the tether in slightly when it is near vertical(and the gravity gradient forces are high), and letting the tether back out when it is nearhorizontal (and the gravity gradient forces are low). Alternatively, the eccentricity or argumentof perigee of a tether’s orbit can be changed by reeling the tether in and out during its orbit.
Again, this method requires no propellant expenditure.
Lastly, once sufficient traffic to and from orbit has been established, the tether facility can beused to de-boost returning payloads and, in doing so, can regain the orbital momentum andenergy that it loses when it boosts a payload.
Relevance of Past, Present and Planned Tether Flight Experiments
HASTOL is still a paper concept, but there does exist a significant amount of experimental
flight data on tethers that are relevant to the success of a HASTOL program, and more is coming.

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There have been 17 tether flight experiments to date7, including one tether experiment still
functioning in space after 3.5 years. The two most publicized experiments have been theTethered Satellite System (TSS) experiments flown off the Shuttle Orbiter7,22, which involved a
1.6-m-diameter Italian satellite deployed upward from the Orbiter on the end of a 20-km long,
2.5-mm diameter conducting copper wire tether strengthened with a Kevlar™ core and insulatedwith a Teflon™ sheath and a Nomex™ jacket. The TSS-1 experiment in July 1992 was aborted
when the tether reel jammed. The TSS-1R reflight experiment in Feb 1996 was successful in
deploying the Italian satellite smoothly upward to almost the full length of the tether. The
motion of the 19.5-km length of conducting tether being dragged through the magnetic field of
the Earth produced the expected nearly 3500 V potential difference between the ends of thetether. Periodically, the plasma contactors on the Italian satellite and the Orbiter were activated,allowing electrons in the space plasma to flow into the conducting surface of the Italian satellite
at the positive end of the tether, down through the conductive wire in the tether, and out throughthe plasma contactors on the Orbiter back into the space plasma. The current through the tether
was limited by the control electronics to 0.5 A. This measurement demonstrated one of theobjectives of the experiment – that a 20-km long conducting tether could be used to convert
Orbiter kinetic energy into at least 1.75 kW of electrical power. The plasma contactors wereturned off, and the tether reeling was continued. With the current through the tether turned off,
and with no voltage drop along the tether, the voltage on the portion of the conductor still
remaining on the reel inside the Orbiter bay increased past 3500 V. Although the insulation had
been designed to withstand more than 10,000 V, for some reason the insulation failed, and asmall spark jumped through the insulation to the metal reel, which was at Orbiter “ground”
potential. With the plasma contactors off, there should have been little current flow. But thebare conducting surface of the Italian satellite turned out to be a excellent electron collector, 2-3
times better than predicted by the existing Parker-Murphy Theory. The Orbiter, with its everpresent "cloud" of gas emitted from its large surface area of materials with high outgassing
properties and its attitude control jets, also turned out to be an excellent negative electron emitterand positive ion collector. The resulting uncontrolled current flow through the spark jumped to
1.1 A, and the spark grew into a 3.85 kW arc that melted the copper and burned throughKevlar™, Teflon™ and Nomex™, causing the tether to part. The TSS-1R experiment was

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called a “failure”, but the very method of failure showed that a large area of bare conductor
alone, is sufficient to collect electrons from space, with no plasma contactor needed, and that aproperly designed electrodynamic tether would be 2-3 times more effective in space than theory
had previously predicted. In the HASTOL architecture, a major component of the space tether
portion of the system is the conducting wire braided into the Spectra™ strength portion of thetether to form the TUI-patent-pending High-Strength Electrodynamic Force Tether (HEFT).
Current from a solar power array pumped through the conducting tether will push against themagnetic field of the Earth, producing propulsive forces to restore the tether orbit and spin
between payload lifts – without the use of propellant. We know the HEFT concept will work in
the HASTOL architecture because of the TSS-1R data.
From 1993 to 1996, a number of smaller tether flight experiments7 were carried out by
NASA/MSFC and NASA/JSC using Small Expendable-tether Deployment System (SEDS)
tether systems built by Joseph Carroll of Tether Applications in San Diego (a consultant to both
TUI and Boeing). The Plasma Motor Generator (PMG) experiment in June 1993 deployed a
500-m long #18 American Wire Gauge (1.0 mm) Teflon™ insulated copper wire tether. The
electronic packages in the PMG experiment included plasma contactors at both ends of the tether
and a battery power supply to produce current flow in both directions along the tether, thus
demonstrating operation of the electrodynamic tether in both the power production andpropellantless propulsion mode. These experimental results are again of relevance to the HEFT
tether in the HASTOL Architecture.
Two other SEDS flight experiments7,23 each deployed a 20-km-long 0.75-mm-diameternonconducting polymer tether with a 26 kg payload at the end. The SEDS-1 experiment inMarch 1993 demonstrated that rapid tether deployment using springs to push off the payload, andsimple control laws to activate a brake to control the tether deployment rate would result in thetether being fully deployed with little shock and little residual dynamical motion, without therequirement for human intervention. This experiment is relevant to HASTOL since it showedthat the tether dynamics computer simulation models that had been developed by Carroll andothers would adequately predict actual tether dynamics performance in space. This tether was
deliberately cut at a designated time in its orbit to demonstrate the accurate deorbit of the payload
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left the tether hanging in space to determine its lifetime. It was cut in 5 days by a meteoroid ororbital debris (M/OD) impactor. This experiment showed the necessity for using open net
interconnected multistrand structures for space tethers, such as the Hoytether™ design to be usedon the HASTOL tethers, instead of the single compact braided line used in the SEDS-1 and -2
experiments.
The Naval Research Lab Tether Physics and Survivability (TiPS) experiment7,24 launched inJune 1996 on an NRO technology demonstration flight used another SEDS-type deployer with athicker braided polymer tether 4 km long. To increase the survival lifetime, the polymer strandsof the tether were spread out into a 2.5-mm-diameter open net by braiding the polymer strands
around a “fluffed-out” yarn core. This tether connects two spacecraft in an orbit at 1000 km
altitude, and they are still there as of this date. The open net structure has allowed the tether tosurvive cuts by space debris for over 3.5 years. The high strength polymer used in the TiPS
tether is Spectra™ 2000, the same material that will be used in the HASTOL tether. The factthat the polymer material has survived exposure to the ultraviolet and charged particle radiationand the vacuum of space for over 3.5 years is encouraging, in that it indicates that
Spectra™ 2000 is an acceptable candidate material for a first generation HASTOL system. The
stress level on the tether is low, however, so no estimate can yet be made of any strength
degradation of the Spectra™ 2000 by the space environment.
In 1997, NASA/MSFC funded an International Space Station (ISS) Electrodynamic Tether
Reboost study25. The study team included both Boeing and TUI. The 7-km-long electrodynamic
tether was designed to be made of aluminum wire braided with Spectra™ 2000 into a
10-mm-wide, 0.6-mm-thick tape massing about 100 kg. The 5 km portion near the ISS would be
insulated, while the outer 2 km would be bare to collect electrons from the space plasma. The
tether would be hung down below the ISS with a 200 kg ballast mass at the end. About 6 kW ofoff-peak power obtained from the 80 kW solar panel farm on the ISS would pump about 4 A of
current through the tether. The current in the tether would push against the magnetic field of the
Earth to provide a propellantless reboost thrust of about 0.5 N average, which is sufficient tocompletely overcome the estimated average air drag on the ISS. This would reduce, or evencompletely eliminate, the need for periodically hauling reboost propellant to the ISS with Russian
Progress rockets, potentially saving billions of dollars. Although this study has not turned into a

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planned experiment as yet, the effort put into the study by MSFC, Boeing, TUI and others on thedesign of the tether, power converter circuits, tether deployment mechanism, and tether dynamics
software, has already been relevant to the design of the High-Strength Electrodynamic Force
Tether (HEFT) in the TUI patent application, and will be relevant to the HEFT portion of the
HASTOL architecture.
The planned NASA/MSFC ProSEDS (Propulsive SEDS) tether experiment7,26 is scheduled
for launch in August 2000. This experiment will deploy a 5-km-long conductive tether in an
attempt to repeat the PMG and TSS-1R experiments in a more precise and controlled fashion,
with more detailed data collection. Both the deployment and the operation of this tether system
will produce operational flight data that will be relevant to the design of the electrodynamic
tether in the HASTOL architecture.

TETHER GRAPPLE AND PAYLOAD ACCOMMODATION SYSTEM
The actual payload transfer between the hypersonic vehicle and the tether systems isaccomplished by a combination of a tether grapple and payload accommodation system. This isbased on previous studies conducted for NASA. The system designer trades off the allocation of
capture functions between the payload accommodation system, which is an expendable device
used for each payload and the grapple assembly, which remains in orbit and is difficult to resupply.
The approach to this subtask was to begin by defining the basic design requirements andconstraints of the mission. Trade studies were then used to define what elements would perform
what functions. Several design concepts were used to test our ideas against the required
functions. This was repeated several times building on what was learned in each cycle. Since
this study is at the architecture level, trades were also conducted between other elements of thearchitecture such as the hypersonic aircraft, the tether control station, and the tether itself. The
products of the NASA-funded study of a LEO to GEO tether transportation study were used
extensively herein as were the lessons learned.

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Design Requirements
In order for successful rendezvous, docking, and transfer of the payload to occur, some basic
functions must be performed by one or more of the HASTOL system architecture elements,
which include the grapple assembly and payload accommodation system. The following basic
functions have been identified and the design process considered all of them:
1. Establishing and updating a known absolute location for rendezvous, capture, and transfer
2. Establishing and updating the relative position between the payload and the grapple
assembly
3. Recognizing the defined rendezvous point
4. Closing the gap to the rendezvous point
5. Payload/grapple docking
6. Payload separation from the hypersonic vehicle
7. Retention of the payload on the grapple during transfer
The results of earlier studies indicated that there is a high degree of cooperation required
between the payload accommodation system and the grapple assembly. In this case, thehypersonic aircraft must also become an element of the capture and must be integrated into thecapture event. In this approach the payload accommodation system provides the mechanisms tofacilitate the capture and provides a common interface to the grapple assembly. Earlier studies
indicated that any expendables such as propellants should be a part of the payload
accommodation system or the hypersonic aircraft to avoid re-supplying the grapple assembly.
This is a significant life cycle cost issue and will be treated in subsequent studies.
A general capture scenario was laid out in order to formulate grapple assembly design drivers
and requirements, and to establish a configuration trade space. The payload capture scenario isdefined by the following parameters:• Capture at an altitude of 80-100 km• Payload maximum weight of 14 Mg (15 English tons)• Capture when the hypersonic airplane is traveling at Mach 10-12

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BOEING-STL 2000P0001• Tether tip temperature of 45°-85°C• Very low dynamic pressure environment• Relative acceleration of mated payload/grapple assembly after capture of <2.5 g’s
Several grapple design requirements and drivers resulted from the definition of theseparameters. The atmosphere is not very dense at 80-100 km altitude. The grapple assembly will
therefore not need to be streamlined to any great extent, although it may have to withstandsignificant heating for its short duration in the atmosphere. The amount of heating will depend
upon the exact rendezvous altitude and speed and the effective cross-sections of the grapple
assembly and Payload accommodation system. Further analysis should show a clearer picture of
the effects of thermal cycling due to multiple atmospheric passages. It will also aid in futurematerial specifications.
The tethered grapple assembly motion at the point of capture must be in plane with the
payload. Control of either the grapple assembly or the payload (payload itself or the hypersonicaircraft) must be possible to ensure a successful docking. Structural loading of the grapple
assembly must be taken into account for rendezvous as well as for capture impact and transfer ofthe 14-Mg (14 metric tons or 30,000 lb.) payload. No damage must be allowed to occur to the
payload.
A conservative assumption is that there will only be one capture attempt possible per
mission. As a result, maximizing the capture opportunity window is a design objective. This
assumption has also resulted in a requirement for the capture to be as automated as possible; a
Go/No-Go decision initiated prior to the actual capture by ground control (or pilot, should there
be one) will be included in the design. It is assumed that abort modes will be defined prior toeach HASTOL mission for the specific client, though efforts are on-going to identify abort modes
that can be built into the system - for instance, establishing the bounds of an “attempt-to-transfer
window,” and a “payload out-of-bounds” window.
Other grapple design issues deal with the “no damage” to client payload policy andcommunications issues. The payload's safety during transfer must be ensured, which means
either the payload must not tumble during transfer, or it must be protected so limited tumbling

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can be accounted for at no consequence. The potential of communication loss at each phase of
the payload transfer scenario must also be considered.
Configuration Options
The general functions listed above were allocated among the hypersonic vehicle, payload, andgrapple assembly to define a system configuration trade space to drive out the implications eachconfiguration would have on the grapple assembly and payload accommodation system designand HASTOL system architecture as a whole. Seven configurations resulted from the functional
allocations between the hypersonic vehicle, payload accommodation system, and grapple
assembly, focusing on methods in which to remove the payload from the hypersonic vehicle:• Configuration 1 – Mechanical Arm on Hypersonic Vehicle• Configuration 2 – Mechanical Arm on Grapple Assembly• Configuration 3 – Atmospheric Vents Lift Payload out of Cargo Bay• Configuration 4 – Payload Powers Itself Out of Cargo Bay• Configuration 5 – Payload Ejected, Cradle/Clam Shell Mechanism captures
Free-falling Payload• Configuration 6 – Electromagnet on Grapple Assembly Removes Payload• Configuration 7 – Electromagnet on Payload Pulls it from Bay to Grapple Assembly
Configuration 3, with atmospheric vents being used to lift the payload out of the cargo bay,
was discarded based on the altitude range of 80 – 100 km of the HASTOL scenario for this study.
At those altitudes there is not sufficient atmosphere for atmospheric vents to be used for thispurpose (though the concept may become a viable candidate for other payload transfer concepts).
Each configuration definition has been placed in its own configuration table. A subsystemdefinition table follows each configuration table. The “causes” in the configuration table resultin the “effects” summarized in the subsystem table that follows. Every configuration assumes
that the hypersonic vehicle is the only HASTOL element with the required absolute positional
knowledge and that an electrodynamic tether is being used.

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Table 1. Configuration 1, Mechanical Arm on Hypersonic Vehicle
NOTE: Hypersonic Plane is only element with absolute positional knowledge
Relative
Positional
Knowledge

Finer
Maneuverability∆V
Knowledge and Control

X

Hypersonic Plane

Removes
P/L from
Plane

X

Mechanism(passive or
active)location

X

PayloadEnd Mass/Grapple
Assembly

X

X

Comments
Arm on plane does all the
maneuvering

X

Passive

X

Active; End Mass/Grapple
Assembly must carry onboard avionics for
positonal and velocity
knowledge relative toplane, including systems
for updating andprocessing thisinformation

TetherTether Central Facility(TCF)

Table 2. Configuration 1, Implied Subsystems
Required On-board
systems
Hypersonic Plane

Attitude, Vel. &∆ V Determ. Sys

Communications

X

X

Power
Power
Generation Storage

RCS
SystemTetherReel

Passive Docking
Ring

PayloadEnd Mass/Grapple
Assembly

Comments

X

X

LIDAR, batteries,
Active Docking Ring

X

Electrodynamic
tetherX

TetherTether Central Facility
(TCF)

X

The idea central to Configuration 1 (Figure 19) is amechanical arm on the hypersonic vehicle that performs
all of the necessary maneuvering to lift and orient the
payload so it can be captured by the grapple assembly.
The payload has a passive docking mechanism (adocking ring, perhaps) attached as a “payload adapter”

Track

Figure 19. Conceptual Mechanical
Arm on Hypersonic Vehicle

that will be discarded after the payload is transferred
into its destination orbit. The grapple assembly has the active docking mechanism, which will
require a power system (storage and distribution) that will store power generated by theelectrodynamic tether between missions. The grapple assembly does not have an on-board
reaction control system (RCS), but does have a passive guidance system element (beacon or

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optical cube) that interfaces with a guidance system on the hypersonic vehicle and arm. The
grapple assembly is moved entirely by the tether reel on the tether central facility (TCF). The
only communications system in this first configuration is the hypersonic vehicle’s
communication system.
Pros:• Reusable arm• Arm maintenance would be a ground operation between missions instead of an on-orbit
operation or an operation that involved tether central facility downtime• Lower cost from disposing of a passive docking ring as opposed to disposing of an active
docking mechanism with each mission• Hypersonic aircraft knows performance of both “sides” of system to transfer payload• Grapple assembly communicating some type of data to plane; boilerplate for futurecompletely automated transfers and abort procedures• Arm can grab payload and possibly orient it as well as lift it from cargo bay for transfer tograpple assembly• Hypersonic aircraft RCS use could extend capture windowCons:• Grapple assembly batteries required to power avionics as well as active capture
mechanism• Hypersonic aircraft RCS may not extend capture window sufficiently• Arm adds packaging/deployment complexity to aircraft design; additional weight andadditional power requirements• Questionable ability to perform “fine maneuvering” at scenario speeds• Possibility of tether tip motion precessing so that it is not only out of plane with thehypersonic vehicle/payload, but is also out of the reach of the mechanical arm. Further
mission analysis required to establish high probability bounds for the test case that would
be used in the arm design.

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Table 3. Configuration 2, Mechanical Arm on Grapple Assembly
NOTE: Hypersonic Plane is only element with absolute positional knowledge∆V
Knowledge

Relative Positional
Knowledge

Finer
Maneuverability
and Control

Docking
Mechanism
Removes P/L from (passive or active)Plane
location

Comments

Hypersonic Plane
PayloadEnd Mass/Grapple
Assembly

X

X

X

X

X

passive
docking

X

Robotic Arm
mechanism;
active docking

TetherTether Central Facility(TCF)

Table 4. Configuration 2, Implied Subsystems
Required On-board
systems
Hypersonic Plane

Attitude, Vel. &∆ V Determ. Sys

Communications

X

X

X

X

Power
Generation

Power
Storage

RCS
SystemTether
Reel

X

X

X

Comments
comm ground-link and with Grapple
Assembly

PayloadEnd Mass/Grapple
Assembly
Tethercomm gound-link and with HypersonicPlane, RCS refueling adds ops complexity,
tether reel adds weightX

Tether Central Facility(TCF)

Configuration 2 (Figure 20) places the mechanical arm
and all maneuvering responsibility on the grapple assembly.
This requires the grapple assembly to have on-board active
guidance and RCS systems, as it must seek the payload andmanipulate the arm to capture it. In addition to an on-orbit
power storage and distribution system, the grapple assembly
also makes use of it’s own tether deployment system. A

Figure 20. Conceptual
Mechanical Arm
on Grapple Assembly

portion of the tether extending from the TCF is still an electrodynamic tether which generates
the power which is stored in the grapple assembly’s power storage system, but the grapple
assembly deploys the power/arm/RCS/active guidance package along its own tether line. The
hypersonic vehicle now carries the passive guidance element of the capture system (beacon or
optical cube or other). The payload still has the disposable, passive, docking element. The
hypersonic vehicle may still need a means for orienting the payload so that a generic payload
adapter can be used for all payload clients, some of which may take up the entire cargo bay andmust be oriented “end up” in order to be captured.

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Pros:• Passive docking ring on payload has ease of manufacturing potential• Disposable passive docking ring on payload as opposed to more expensive, active
docking mechanism being thrown away with each mission• Fewer additional constraints imposed on hypersonic vehicle design• Additional tether reel on grapple assembly instead of only single Tether Central Facility(TCF) may result in quicker grapple responses to tether reeling• Capture window opportunity increased• Impact loads on main TCF tether (at capture) reducedCons:• RCS on grapple assembly must be replenished; on-orbit maintenance• Mechanism complexity is high
• Additional tether reel adds complexity to operations and system dynamics
Table 5. Configuration 4, Payload Powers Itself out of Cargo Bay
NOTE: Hypersonic Plane is only element with absolute positional knowledge
Relative
Positional
Knowledge∆V
Knowledge

Finer
Maneuverability
and Control

Docking
Mechanism
Removes P/L from (passive or active)Plane
location

Hypersonic PlaneX

PayloadEnd Mass/Grapple
Assembly

Comments
supplies guide beam to optical cube that’s on theGrapple Assembly, Payload follows beam forsuccessful capture

X

X

X

X

Atmospheric vents on P/L lift it out of cargo bay; active
docking using "radial clamp" idea Ben drew for NIAC
paper

X

passive docking mech with optical cube target;
passive docking ring

TetherTether Central Facility(TCF)

Table 6. Configuration 4, Implied Subsystems
Required On-board
systems

Position,
Vel. & ∆ V
Determ. Sys Communications

Hypersonic Plane

X

X

Payload

X

X

Power
Power
Generation Storage

X

RCS
SystemTetherReel

Comments

Disposable P/L adapter; adds capture
wt and adds recurring costX

End Mass/Grapple
Assembly
TetherTether Central Facility
(TCF)

X

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In Configuration 4 (Figure 21), the payload
powers itself out of the cargo bay either by using an
upperstage or a large RCS system built into a payload
adapter. Here is another instance where thehypersonic vehicle will probably need a means toorient the payload for release built into the cargo bay,

Figure 21. Concept for Payload Adapter
to Power Payload Out of Cargo Bay

or it must have a payload ejection system. The payload adapter also has an active docking
mechanism and power storage and distribution system to power avionics and RCS and docking
elements. The guidance system in this configuration is really a proposed design solution; the useof a guide beam from the hypersonic vehicle, which targets the grapple assembly. The payload
adapter’s guidance system commands the RCS to follow the beam path to the grapple assembly.
The grapple assembly has the passive docking element and any elements on the receiving end of
the guidance beam system that are required.
Pros:• Payload can maneuver itself and meet grapple assembly for capture• No fine path adjustments imposed on the hypersonic vehicle• Active docking mechanism on payload adapter reduces consumables on board the grapple
assembly, which increases ease of maintenance of the grapple.
• Loads on tether may not be as bad at capture due to cooperative payload• Guidance system with military heritage
Cons:• Payload adapter imposes load carrying capability on client payload• Payload adapter reduces size/weight of clients that can use the HASTOL system• Use of upper stage on payload also reduces size/weight of clients that can use HASTOL

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Table 7. Configuration 5, Payload Ejected, Cradle/Clam Shell
Mechanism captures Free-falling Payload
NOTE: Hypersonic Plane is only element with absolute positional knowledge
Relative
Positional
Knowledge∆V
Knowledge

Finer
Maneuverability
and Control

Removes P/Lfrom PlaneX

Hypersonic Plane

PayloadX

End Mass/Grapple
AssemblyX

Docking
Mechanism(passive or active)
location

X

Comments
Ejects P/LEjection from plane may requiredisposable encapulation for protection(dynamic pressure); optical cube on
P/LX

Rel.position to P/L known and updated;structural "net" or clamshell capture
mechanism

TetherTether Central Facility(TCF)

Table 8. Configuration 5, Implied Subsystems
Required On-board
systems

Relative
Positional,
Vel. & ∆ V

Communications

Power
Generation

Power
Storage

RCS
SystemTetherReel

Comments

Hypersonic Plane
PayloadEnd Mass/Grapple
Assembly

TetherX

optical cubeX

for on-board sensors and communication tolet operators know if/when P/L is acquired,and for net/clamshell open and closing, forpowering avionics for use of optical cube on
P/L to establish and update relative position
info.

X

X

X

Tether Central Facility
(TCF)

X

Configuration 5 (Figure 22) has the payload
being removed from the cargo bay in a simple
manner; it’s ejected from the bay and is left in freefall. The payload is enclosed in a protective capsule
which has a passive guidance element (beacon,other) on-board. The capsule would act like alaunch vehicle’s payload fairing, falling open andaway, discarded after the payload is released. The
grapple assembly has a simple capture mechanism;
either a cradle/launch tube arm or a bulldozer-likeFigure 22. Concept for Passive
Docking Mechanism on Tether Tip

clamshell that opens and closes to capture/release the payload.

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Pros:• Payload protected during transfer; clients happy• Simple grapple assembly concept without consumables (except long-term battery
replacement)• Simple mechanisms with higher reliabilities could be used
• Free-falling payload increases capture opportunity windowCons:• Grapple assembly does not have “smarts” to capture free-falling payload; may require toomuch precision from system for reliable, repeatable payload transfers• May not be able to ensure grapple/payload in plane with each other at capture instantunless RCS added to grapple assembly (adds consumables)• Capsule must be designed to withstand impact loads of capture without imparting them to
the P/L• Won’t allow for tether twisting unless RCS added with consumables or ingenious
cradle/launch tube design can be developed
Table 9. Configuration 6, Electromagnet on Grapple Assembly Removes Payload
Relative
Positional
Knowledge
Hypersonic Plane

X

Finer
Maneuverability∆V
Knowledge and Control

X

Docking
Mechanism
Removes P/L from (passive or active)
location
Plane

X

PayloadEnd Mass/Grapple
Assembly

Comments
must provide beam to optical cube on End
Mass to have relative positional knowledge

X

X

X

dumb ferrous lump to attract End Masselectromagnet, needs to be disposable

X

Provides optical cube for plane to know
relative position; Active electromagnet

TetherTether Central Facility
(TCF)

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Table 10. Configuration 6, Implied Subsystems
Required On-board
systems

Position,
Vel. & ∆ V
Determ. Sys Communications

Hypersonic Plane

X

X

X

X

Power
Power
Generation Storage

RCS
SystemTetherReel

X

Comments
Avionics and beam for rel. position;communicate whether or not P/L has left thecargo bay

PayloadEnd Mass/Grapple
Assembly

An optical cube for relative position
knowledge; sensors and comm to know
if/when capture successful

TetherTether Central Facility(TCF)

Configuration 6 (Figure 23) provides an option that attempts
to reduce the number of moving parts used in the capture
mechanism design. An electromagnet, within the grapple
assembly, lifts the payload out of the cargo bay and holds it
during transfer. Simply removing the current from theelectromagnet allows the payload to be smoothly released. Thisrequires the grapple assembly to have capacitors with quick

Payload w/ “Towel Bar”
Ferrous Adapter w/
Magnetic Isolators

Figure 23. Electromagnetic
Capture Concept

discharge capability. The payload needs a dumb, ferrous
adapter with magnetic isolation barriers to protect its avionics from the concentrated magnetic
field. A simplified analysis using the following equations was performed to check the feasibilityof this concept:
B=

F (72,134,000 )A 10 8( )F = load to be held, not lifted (lb.) = max. payload weight
B = flux density (Mx/in2)
A = area of holding surface (in2)
1 Wb = 108 Mx

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BLs
µN

I=

B = magnetic field of a solenoid (Tesla, T)
N = number of turns of wire
I = current (Amps)
Ls = length of solenoid (m)µ = permeability of a medium of magnetic field, assumed to be equal to µo (Tm/A)µ0 = 1.26E-06 (Tm/A)
1 T = 1 Wb/m2
The holding force, F, was assumed to be a constant (30,000 lb., the maximum payload
weight). The area was assumed to be circular, with the diameter varying from 60” to 120” in
10” increments. The magnetic field, B, was calculated over this varying area, and resulted infields of strengths ranging from 0.4 to 0.2 Telsa. Three values of B were used as constants tocalculate three sample sets of current values. Within a single sample set with constant B, thenumber of wire turns was held constant at values 500, 750, and 1000 to result in three data
series per constant B (Ls handled as a variable ranging from 6”-12”). Figure 24 follows as an
example of how these calculations were performed. Figure 25 shows the graphical results of the
0.2 Telsa magnetic field data set. Table 11 summarizes the calculation results for the three data
sets.

Equation 1

B=

Constant, F(lb)

Dia
(in)

F (72,134,000 )A 10 8Equation 2( )60 2827.4
70 3848.5
80 5026.5
90 6361.7
100 7854.0
110 9503.3
120 11309.7

BLsµNµ = permeability of a medium of magnetic field,
assumed to be equal to µo (Tm/A)µ0 = 1.26E-06 (Tm/A)

30,000A(sq.in.)I=

B(Mx/in 2 ) B (Wb/m2

1000
wire
turns

Ls
(in)

0.43
0.37
0.32
0.29
0.26
0.23
0.21

1000
1000
1000
1000
1000
1000

12
11
10
9
8
6

27,66523,713
20,74918,443
16,59915,09013,833Ls(m) I (A)
0.30
0.28
0.25
0.23
0.20
0.15

52
48
43
39
35
26

Figure 24. Example of Estimated Current Calculations for B Held Constant

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120

100

Current Req’d (A)

80

60

40

20

0
12

11

10

9

8

7

6

Solenoid Length (in.)

1000 wire turns
750 wire turns
500 wire turns

Figure 25. Current as a Function of Solenoid Length and Number of Wire Turns for B = 0.2 T
Table 11. Summary of Electromagnet Concept Feasibility Calculations
Number Solenoid Solenoid
Bof WireLength Diameter Current(in.)
Req’d (A)(Tesla) Turns (N)
(in.)
0.4
500
750
1000

12
12
12

60
60
60

208
139
104

500
750
1000

12
12
12

90
90
90

139
92
69

500
750
1000

12
12
12

120
120
120

104
69
52

0.3

0.2

Pros:
• Easy way to align payload and grapple assembly.; soft dock
• Simple• Makes use of electrodynamic tether; charge batteries or capacitors that will deliver a
steady current during capture/softdock, or could deliver high current quickly to pick up
payload from cargo bay just in time for docking mechanism to hard dock for transfer

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Cons:• Have to provide isolation barrier between any hypersonic vehicle avionics and Grapple
electromagnet• Must isolate payload avionics from grapple electromagnet• Increases “dumb” or wasted mass on payload (electromagnet barriers) and/or hypersonicaircraft
The data shows that sizing an electromagnetic system just to hold a maximum-weight
payload could require very high constant current values, but depending on physical limitations ofwire turns along a cylindrical length, some more feasible constant currents will suffice. The
amount of constant current required to size this system to perform work and actually pull thisweight out of the cargo bay depends on the distance over which one wishes this work to be
performed. It is expected at this early stage, that the current values required would be quite highfor any amount of work other than holding to be performed.
Another version of this configuration (Configuration 7) is to put the electromagnet on theP/L. The Pros and Cons and subsystem requirements are the same, but the electromagnet would
need to be on a payload adapter that would be disposed of (with each mission). The hypersonicvehicle avionics near the cargo bay would potentially need even more shielding, depending on
how long before the expected capture the magnet was “hot.”Architecture Control and Payload Transfer
The payload transfer must be approached as a system control problem due to the fact thatseveral elements must be controlled to successfully complete the payload transfer. The system
must be closely choreographed to maximize the payload transfer time and capture envelope.
While having to control multiple elements appears to add complexity to the system, it actuallyadds robustness in that there are several elements that can be adjusted to complete a payloadtransfer and the entire burden of adjusting the system is not allocated to the hypersonic aircraft.
The motions of all HASTOL elements are predictable with mathematical algorithms over therange of time required to complete a payload transfer. As the time scales increase beyond several
orbits the ability to precisely extrapolate position data becomes much more difficult. The

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accuracy of the predicted motion will improve as experience is gained from future flights anddemonstrations of tethers in orbit. Controlling the system motions will become easier in the
future due new instrumentation now being developed to provide high precision position andmotion data with inertial measurement units, differential GPS and optical tracking systems likethe LIDAR and LADAR systems now under development.
As described earlier, the payload transfer will occur when the hypersonic aircraft enters aballistic trajectory with an apogee near 90 km or higher. The hypersonic aircraft will use
onboard instrumentation and data relayed from the tether central station and tether grapple
assembly to accurately predict where in space and time the actual physical transfer will takeplace. The tether central station will make all required adjustments before the aircraft takes off
and will provide position and status of both the tether central station, the state and condition of
the tether, and the tether grapple assembly. The aircraft will launch and accelerate to rendezvous
with the tether grapple assembly. At the end of the air breathing cycle the aircraft will ignite theon board rockets to propel the vehicle to the rendezvous point. Prior to entering into the ballistic
rocket mode the aircraft will make the required adjustments based on differential GPS data,
LIDAR/LADAR, or automated optical tracking devices (optical reflectors mounted on thegrapple) to pin point the current grapple position and velocity data. The LIDAR/LADAR devices
are used to both track the grapple and to measure and map the atmospheric density at or near therendezvous point.
The earlier the corrections can be made to control the entire system the lower the thrust andfuel consumed. In this case, the system refers to the tether central station, tether grapple
assembly and the hypersonic aircraft. To set up the transfer, the tether central station and grapple
assembly will enter a fine control mode where the systems are configured to ensure very
predictable motion of the grapple during the approach to the rendezvous point. This is practicaldue to the mass/inertia of the grapple assembly, centrifugal forces from the motion of the tether
and the very low density of the atmosphere at these altitudes. The variation due to solar cycles
and fluctuations in the geomagnetic field are less radical at these altitudes so prediction of thedensity is easier. The grapple may be equipped with thrusters to adjust the velocity in cases
where density variations might cause a change in the predicted motion of the grapple. This
would be used only in the case of higher than normal densities since this imposes a re-supply

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requirement on the grapple. As long as the LIDAR systems can measure the atmospheric density
in the region of interest we will be able to predict the motion so corrections at the grapple should
not be required. As the rendezvous point becomes lower in the atmosphere this becomes a more
complex problem. It should be noted that much of the atmospheric absorption data anddevelopment of algorithms is being done for the Air Borne Laser where atmospheric absorption
is a critical problem to controlling the propagation of the laser to its intended target. The aircraft
will have the state data related to it through communications links and will compute the optimum
approach corridor. Once the approach corridor is determined, the aircraft will accelerate into thecorridor and will make any significant corrections at the beginning of the flight where there issignificant aerodynamics for control of the vehicle. On-board thrusters will be used during theballistic phase to make the minor corrections and maintain the attitude of the vehicle during theapproach phase.
In our point of departure design we will open the payload doors of the aircraft and will track
the grapple as it approaches from behind the aircraft. At or near the apogee of the aircraft the
payload will be raised out of the bay and captured by the grapple. This motion of the grapple andtether at this point will be very predictable due in part to the mass of the system and the lack ofany significant dynamic pressure from the atmosphere at these altitudes. The current concept is
to have strong electromagnets mounted on the payload accommodation package to pull the
payload into the capture envelope of the grapple to allow the actual mechanical capture. The
time frame that the payload and grapple are in near contact is short which allows quick verystrong pulsed electromagnets to complete the transfer. This approach works well for scenarios
where the physical capture envelope is on the order of meters. Several other payload transfer
techniques are described in the Phase 1 report which could also complete the transfer process forlarger capture envelopes. This one was selected as our point of departure because of the shorttime frame, small capture envelope expected, the predictability of the electromagnets and thesimplicity of the system.

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Grapple and Payload Transfer Issues
The rendezvous, docking, and transfer of the payload to and from the hypersonic vehicle tothe tethered grapple assembly will occur at around 330 kft (100 km) altitude in the presently
planned HASTOL scenario. The atmosphere is not very dense at that altitude. There will besignificant heating, but not much dynamic pressure despite the high velocities involved. The
grapple assembly will therefore not have to be “streamlined” to any great extent, although it will
have to withstand significant heating. The amount of heating will depend upon the exact
rendezvous altitude and speed, and the “height” of the upper atmosphere at the time of therendezvous.
A preliminary conceptual CAD drawing of a possible grapple assembly for the tether isshown in Figure 22. It features a circular attach ring at the bottom, which will mate with grapple
hooks on the payload. The attach ring is connected to the rest of the end mass via a six-degreeof-freedom multiple-shock-absorber-strut suspension cradle. In the suspension cradle, all of themembers are designed to compress as necessary, should the payload and grapple mechanism
contact at some non-zero speed or some slightly non-tangential angle. The struts in thesuspension cradle will also provide shock-absorber type damping of the resulting movement of
the attach ring relative to the heavier end mass cylindrical structure at the top, which contains atether winch, batteries, the RCS and its propellant, and command, control and guidance
electronics.
The ring and the suspension leg elements would be made of materials and designed towithstand heating from the hypersonic molecular flow at the rendezvous altitude. Thiseliminates any need for an aerodynamic cone or shroud, which would increase the aerodynamic
drag on the assembly compared to the mostly empty strut structure presented to thehydrodynamic molecular flow. The “ends up” cylindrical shape of the upper portion of thegrapple assembly is already aerodynamically stable. Adding a cone to it would not help
appreciably.
As a result of the relatively high rendezvous altitude of 100 km (330 kft), adding
aerodynamic surfaces on the grapple assembly will not be effective in maneuvering the grapple
toward a rendezvous with the hypersonic aircraft. The cylindrical portion of the grapple

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assembly will have a tether winch which will allow it to “leave” its normal position at the end of
the tether by letting out tether. The centrifugal acceleration from the rotation of the tether willcause the grapple assembly to “fly” in toward the payload in the hypersonic vehicle before thetether itself arrives overhead. The grapple assembly will have attitude control rockets for finecontrol, but to minimize the problem of refueling of the grapple assembly, it will be up to theattitude control system on the hypersonic aircraft to remove most of the position and velocity
errors during the rendezvous process.
The current concept is to fly the decelerating grapple assembly in so that it approaches thehypersonic airplane from behind. The attach ring would attach to the payload and pull the
payload up to the grapple. The hypersonic vehicle would then return to ground and the grapple
assembly, with payload, would continue its orbit to the correct location at which to release thepayload at the correct velocity to achieve the intended higher orbit.
After the grapple assembly exits the atmosphere, the time spent in space will be used to cool,
condition and recharge the batteries in the grapple assembly for the next aeropass. When thegrapple assembly will not be used to capture or deploy a payload for long periods of time, thetether will be shortened by either the grapple tether winch, or one of the other winches along thetether, to raise the minimum altitude of the tether tip and keep the grapple assembly above mostof the atmosphere.
The grapple assembly requires several internal functions to be successful for this kind of
mission, which make it similar to grapple assembly concepts developed earlier for exoatmospheric transfer of payloads. The grapple is attached to the end of a tether but some control,
independent of the tether, will still be required. A means for controlling motion in and out ofplane is necessary, as well as a mechanism to eliminate or control aerodynamic forces on thegrapple assembly during aeropass phases.
The hypersonic grapple would not use externally mounted solar panel arrays during theaeropass due to the high aerodynamic forces and heating rates during this phase. Two initial
options have been identified: a deployable/storable photovoltaic array or an electricallyconductive tether. Each would generate the required power, the latter while moving through the50

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earth’s magnetic field, and would store excess energy in batteries for use during the aeropass
phase.
In order to allow a reliable rendezvous, the grapple assembly must maintain location andattitude information and communicate with the hypersonic airplane. This can be done accurately
with a differential GPS similar to those systems being developed for landing commercial aircraft.
The approach velocities are too high to rely on human pilots on the ground so the system will
require autonomous rendezvous and capture (AR&C) capabilities. AR&C technologies, such asadvanced sensors for the final approach and rendezvous, are continuing to evolve, and are
maturing based on Russian, NASA, and more recently, DARPA investments.
Further Study/Recommendations
No one design solution can be offered at this early phase of development; however, several of
the configuration options have shown that some more investigation needs to be made intoquantifying the impacts of functional allocations within the HASTOL system architecture. Anyof these could be designed to make a working HASTOL system; the questions that must be
answered are, “Which one has the best reliability for the lowest operating cost,” and “Which one
is more easily adapted to take advantages of technology advances?”
Configurations 1, 4, 5, and 6 represent the diverse span of functional allocations within the
HASTOL architecture. Configuration 1, with the mechanical arm on the hypersonic vehicle, is amore traditional configuration that builds on a rich, past experience with Space Shuttle missions
and ISS design, testing, and cost data. A revolutionary, new means of payload delivery to spaceshould look at this traditional option (as a gauge, as well as a design solution) along with someother options that are more unconventional. Configuration 4 is the only option that requires thepayload to remove itself from the cargo bay. It introduces a different type of consumable to the
HASTOL system, the payload adapter, which may or may not be cost effective as well as
performance effective. Configuration 5, with the payload being ejected from the cargo bay, has asimple, grapple assembly operation concept; it does need more investigation into timing,
repeatability, and capture impact loading on the tether tip. Configuration 6, with theelectromagnetic grapple, should be investigated as a soft docking design solution. A lesspowerful electromagnet could be used to soft dock the grapple to the payload before a hard dock,51

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using any of the above hard dock suggestions, captured the payload. Though a mechanical armon the orbiting grapple assembly is viewed as unnecessarily complex, the tether-based solution toincreasing the capture window should be investigated for any type of grapple assembly that isused. The operational dynamics are a little more complex with this solution, but it has a greatchance of increasing the capture opportunity window, especially when teamed with other design
solutions aiming at that same end.

CANDIDATE PHASE II TASKSIn the process of demonstrating the technical feasibility of the HASTOL concept during
Phase I, several areas were identified which would benefit from further investigation in Phase II.
These include not only refinement of the HASTOL concept from a technical standpoint, but also
development of the business aspects of the system. Areas identified as being worthy of further
study are described below.
Mission Opportunities
The spectrum of mission opportunities which can be satisfied with the HASTOL systemneeds to be established, along with the mission requirements which support those opportunities.
This may be accomplished by direct dialog with the potential user community, including NASA,the Department of Defense, and the commercial space industry.
System Requirements Definition
Based on the overall mission requirements generated above, a set of system requirements foreach of the major HASTOL systems should be developed. These requirements should address
such issues as payload characteristics, traffic rate, guidance and control requirements, g-force
limitations, acquisition and life-cycle costs, and system interface requirements. This will assurethat the system conceptual design will address the basic requirements, as defined by the potential
customers.
Conceptual Design
Taking the concept developed in Phase I as a point of departure, the HASTOL concept should
be refined through a more detailed design and analysis of each of the major systems. Studies

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should be to sufficient depth that it is clearly demonstrated that each system satisfies its statedrequirements. This can be accomplished through more detailed design of selected systems, tradestudies, modeling and simulation, and cost analyses.
System Analysis
There is a need to quantitatively assess the system concepts evolved through the follow-on
concept refinement study and, ideally, provide feedback to the trade studies. In addition, it will
be necessary to identify high risk areas (technical, cost, program) to indicate where technology
development areas need to be focused. Such areas may include the following:• Rendezvous and payload capture• Abort modes• Tether dynamics and structural integrity• Electrodynamic thrust control• Collision avoidance• Tether survivability• High-temperature tensile materials• Tether fabrication• Development cost uncertainties
Technology Development Planning
Defining a realistic technology development plan is key to gaining the confidence of acustomer that we are ready to move on to Phase III in the HASTOL system development. A
roadmap needs to be constructed which takes all major components of the system to a TRL of 7
or higher. This should address all of the concerns identified by potential customers. It should
include basic technology development, component design and materials selection, and technicaland cost trades. It should show early flight demonstration and qualification test plans, withground and flight testing.

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CONCLUSIONS
The fundamental conclusion of the Phase I HASTOL study effort is that the concept istechnically feasible. We have evaluated a number of alternate system configurations that willallow hypersonic air-breathing vehicle technologies to be combined with orbiting, spinning spacetether technologies to provide a method of moving payloads from the surface of the Earth into
Earth orbit. For more than one HASTOL architecture concept, we have developed a design
solution using existing, or near-term technologies. We expect that a number of the other
HASTOL architecture concepts will prove similarly technically feasible when subjected todetailed design studies. The systems are completely reusable and have the potential of drasticallyreducing the cost of Earth-to-orbit space access. In particular, we have:• Developed top-level system requirements• Conducted top-level trades to define a basic design approach• Selected a specific hypersonic aircraft concept• Defined an achievable aircraft apogee altitude / velocity envelope• Selected the Rotovator™ tether concept as our baseline tether system• Determined that the tether tip can withstand the aerodynamic and thermal loads as it dips
into the atmosphere using existing technology materials• Validated overlap of the hypersonic aircraft operating envelope with achievable conditions
for the tether tip for payload transfer• Identified a simplified grapple concept
No show-stoppers have been uncovered. Hence, all elements of the concept are in place forfurther development and refinement of the concept.

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REFERENCES
1. Robert L. Forward, Thomas J. Bogar, Michal E. Bangham, and Mark J. Lewis,“Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System: Initial Study
Results,” IAF-99-S.6.05, presented at the 50th International Astronautical Congress,
Amsterdam, The Netherlands, 4-8 October 1999.
2. Thomas J. Bogar, Michal E. Bangham, Robert L. Forward and Mark J. Lewis,“Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System: Interim Study
Results,” AIAA-99-4802, presented at the 9th International Space Planes and Hypersonic
Systems and Technologies Conference, Norfolk, VA, 1-5 November 1999.
3. Robert P. Hoyt, “Design and Simulation of Tether Facilities for the Hypersonic AirplaneSpace Tether Orbital Launch (HASTOL) Architecture,” submitted to the 36th
AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, AL, 17-19
July 2000.
4. L. F. Scuderi and J. L. Hunt, “Mach 10 Dual-Role Vehicle Concept,” Space Technology
and Applications International Forum, Albuquerque, NM, 1-4 February 1999.
5. J.N. Stemler, D.H. Petley, and J.G. Martin, “Concept and Future State of the Vehicle
Synthesis System ‘Holist’,” AIAA Paper 98-4742, 7th AIAA/USAF/NASA/ISSMD
Symposium on Multidisciplinary Analysis and Optimization, St. Louis, MO, September
1998.
6. Hans Moravec, “A Non-Synchronous Orbital Skyhook,” Journal of the Astronautical
Sciences, Vol. 25, No. 4, pp. 307-322, 1977.
7. M.L. Cosmo and E.C. Lorenzini, Tethers In Space Handbook - Third Edition, preparedfor NASA/MSFC by Smithsonian Astrophysical Observatory, Cambridge, MA, December
1997.
8. J. Carroll, “Preliminary Design for a 1 km/s Tether Transport Facility,” NASA OAST
Third Annual Advanced Propulsion Workshop, JPL, Pasadena, CA, 30-31 Jan 1992.

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9. J. Oldson and J.A. Carroll, “Potential Launch Cost Savings of a Tether Transport
Facility,” AIAA paper 95-2895, 31st AIAA/ASME/SAE/ASEE Joint Propulsion
Conference and Exhibit, July 1995.
10. Robert P. Hoyt and Chauncey Uphoff, “Cislunar Tether Transport System,” Paper
AIAA-99-2690, 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference,
Los Angeles, CA, 20-24 June 1999.
11. Robert L. Forward and Gerry D. Nordley, “Mars-Earth Rapid Interplanetary Tether
Transport (MERITT) System: I. Initial Feasibility Analysis,” Paper AIAA-99-2151, 35th
AIAA/ASME/SAE/ ASEE Joint Propulsion Conference and Exhibit, Los Angeles, CA,
20-24 June 1999.
12. M. E. Bangham, E. Lorenzini, and L. Vestal, “Tether Transportation System Study,”
NASA/TP-1998-206959, NASA Marshall Space Flight Center, Huntsville, AL, March
1998.
13. Robert L. Forward and Robert P. Hoyt, “Hypersonic Airplane Space Tether Orbital
Launch System,” U.S. Provisional Patent Application 60/146852, filed 3 August 1999.
14. Brian Tillotson, “Tether as Upper Stage for Launch to Orbit,” AIAA-89-1585-CP, 3rdInternational Conference on Tethers in Space, May 17-19, 1989.
15. Robert Zubrin, “The Hypersonic Skyhook,” Journal of the British Interplanetary Society,Vol. 48, No. 3, pp. 123-128, March 1995.
16. Robert P. Hoyt and Robert L. Forward, “Failsafe Multiline Hoytether Lifetimes,” Paper
AIAA-95-2890, 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, San Diego,
CA, 10-12 July 1995.
17. Robert L. Forward and Robert P. Hoyt, Failure Resistant Multiline Tether, U.S.
International Patent Application PCT/US97/05840, filed 22 April 1997.
18. M. Martinez-Sanchez and S.A. Gavit, “Orbital Modifications Using Forced Tether
Length Variations.” Journal of Guidance, Control and Dynamics, Vol. 10, pp. 233-241,
May-June 1987.

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19. G. Landis, “Reactionless Orbital Propulsion Using Tether Deployment,” Acta
Astronautica, Vol. 26, No. 5, pp. 307-312 (1992); paper IAF-90-254.
20. Robert P. Hoyt, “Maintenance of Rotating Tether Orbits Using Tether Reeling,”
Appendix F in Cislunar Tether Transport System, Tethers Unlimited, Inc. Final Report on
NIAC Phase I Contract 07600-011, 1999.
21. Robert P. Hoyt, “Cislunar System Dynamics Verification Through Simulation,”
Appendix C in Cislunar Tether Transport System, Tethers Unlimited, Inc. Final Report on
NIAC Phase I Contract 07600-011, 1999.
22. N.H. Stone, et al., “A Review of Scientific and Technological Results from the TSS-1R
Mission,” pp. 1-12 in J.K. Harrison, Compiler, NASA/MSFC, Tether Technology
Interchange Meeting, Huntsville, AL, 9-10 September 1997, NASA-CP-1998-206900,
January 1998.
23. Joseph A. Carroll, “SEDS Deployer Design and Flight Performance,” AIAA Paper 934764, AIAA Space Programs and Technologies Conference and Exhibit, Huntsville, Al,
21-23 September 1993.
24. Jim Barnds, et al., “TiPS: Results of a Tethered Satellite System,” pp. 13-30 in J.K.
Harrison, Compiler, NASA/MSFC, Tether Technology Interchange Meeting, Huntsville,
AL, 9-10 September 1997, NASA-CP-1998-206900, January 1998.
25. I.E. Vas, T.J. Kelley, and E. Scarl, “Application of an Electrodynamic Tether System to
Reboost the International Space Station,” pp. 305-334 in J.K. Harrison, Compiler,
NASA/MSFC, Tether Technology Interchange Meeting, Huntsville, AL, 9-10 September
1997, NASA-CP-1998-206900, January 1998.
26. Les Johnson and Judy Ballance, “Propulsive Small Expendable Deployer System(ProSEDS) Space Demonstration,” pp. 103-108, J.K. Harrison, Compiler, NASA/MSFC,
Tether Technology Interchange Meeting, Huntsville, AL, 9-10 September 1997, NASACP-1998-206900, January 1998.

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APPENDIX 1
HYPERSONIC AIRPLANE SPACE TETHERORBITAL LAUNCH (HASTOL) SYSTEM:
INITIAL STUDY RESULTS

IAF PAPER NO. 99-S.6.05

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IAF-99-S.6.05

HYPERSONIC AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) SYSTEM:
INITIAL STUDY RESULTS
Dr. Robert L. Forward, Vice President & Chief Scientist [Presenter]
Tethers Unlimited, Inc. 8114 Pebble Court, Clinton, WA 98236 USA
Dr. Thomas J. Bogar, Senior Principal Engineer, Hypersonic Design and Applications,Boeing/MDC Phantom Works, P.O. Box 516, St. Louis, MO 63166 USA
Michal E. Bangham, Senior Manager Advanced Engineering, Advanced Programs,
Boeing/MDC, P.O. Box 240002, Huntsville, AL 35824 USA
Dr. Mark J. Lewis, Associate Professor and Director of Center for Hypersonic Education andResearch, Aerospace Engineering Dept., University of Maryland, College Park, MD 20742 USA

ABSTRACTThe Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system is a novel
architecture for an Earth-to-orbit launch system consisting of: a completely reusable airbreathing
subsonic-to-hypersonic dual-fuel airplane which transports the payload from the ground to someintermediate point in the upper atmosphere; an orbiting space tether system which picks up the
payload from the intermediate point and takes it on into orbit; and a grapple system fortransferring the payload from the hypersonic airplane to the lower end of the space tether. The
system is revolutionary in that it minimizes, and perhaps even eliminates, the use of rockets for
Earth-to-orbit launch of satellite payloads and even passengers, while limiting the design
requirements for the reusable airbreathing hypersonic launch vehicle to less than 4650 m/s(Mach 15) and perhaps as low as 3100 m/s (Mach 10). There are four different options for thedesign of the space tether portion of the HASTOL system, all of which will work, although somedesign options promise better performance. The tethers can be built today using presently
available commercial fibers. As better materials with higher strength at higher temperatures
become available in the future, the performance and safety margin of the tethers can be improved
significantly. The space tethers required are long, typically 400 to 1600 kilometers in length,
while the total mass of the space tether portion of the HASTOL system is typically 20-50 times

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the payloads being handled. Most of that mass ratio requirement is driven by the fact that thetether system, including the Tether Central Station, must mass considerably more than the
payload it is handling, so that, upon pickup of the payload by the tether, the payload will not pull
the space tether system out of orbit.

INTRODUCTION
Boeing, Tethers Unlimited, Inc. (TUI), and the University of Maryland, have formed a team
to investigate the feasibility of a completely new concept for moving payloads and passengers
from the surface of the Earth into low Earth orbit at low cost, low risk, and low accelerationlevels. Our joint study effort has just come under contract funding, and this paper should beconsidered a preliminary report, rather than a finished piece of work. (You will be able to tellthat from the lack of equations and the poor quality of the graphics.)
HASTOL Architecture
The Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system contains threemajor components: a hypersonic airplane, which will transport the payload as high and as fast as
possible using air-breathing propulsion; an orbiting space tether, the lower tip of which will be
slowed down by one means or another, so as to meet up with the hypersonic airplane; and agrapple vehicle at the tip of the space tether that will take control of the payload, and with the lift
supplied by the space tether, carry the payload on into orbit. It would be desirable that the
HASTOL system operate in both directions, allowing for return of payloads from orbit to the
Earth’s surface. This is not a firm requirement, however, for a launch-only HASTOL system
would be useful in itself, since returning from orbit is much easier than launching into orbit. The
objective of our ongoing study is to optimize the combined system of airplane, tether, andgrapple in order to maximize the overall system performance in terms of payload mass anddelivery rate, while minimizing the life cycle cost.

Copyright  1999 by TUI, Boeing and University of Maryland.
Released to the IAF/IAA/AIAA to publish in all forms.

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BackgroundLet us first give some scale to the problem of launching a payload into space. In order to fly
an airbreathing vehicle directly into orbit requires an airplane capable of reaching horizontal
speeds of 7800 m/s (Mach 25) at 150 km altitude (orbital radius of 6528 km). Designs exist for
hypersonic airplanes capable of 3100 m/s (Mach 10), and concepts exist for faster planes of 3875
m/s (Mach 12.5) and higher, but the difficulty of making and operating the hypersonic airplanerises rapidly with increasing Mach number. There is another scale to the problem of putting
things into orbit. Since space is 100 km up, most people think that to get into space only
involves 100 to 200 km worth of travel. What they fail to realize is that every rocket launched
into orbit to date has had to travel thousands of kilometers down range to attain the necessary
7800 m/s orbital speed. Since the distance D that must be traveled at constant acceleration a toreach a final velocity V is D=V2/2a, to reach an orbital velocity of 7800 m/s at an acceleration of
one gee (a=9.8 m/s), requires covering a distance of 3100 km. At three gees acceleration, thedistance is 1035 km. Similar scaling laws apply to space tethers. If a rotating space tether is to
produce a change in velocity of a third of orbital speed, or 2600 m/s, then the tether length L for a
one gee acceleration at the tether tip needs to be of order L=V2/a=690 km. As will be illustrated
in the following section on HASTOL Space Tether Concepts, there are many designs for spacetether systems which can lower a payload grapple vehicle into the upper atmosphere at grapple
speeds with respect to the Earth’s atmosphere ranging from 4650 m/s (Mach 5) to 3100 m/s(Mach 10) and lower, but the difficulty of operating the space tether rises rapidly with decreasing
grapple speed. We are quite sure that the bridge between air and space can be crossed by using
the right combination of hypersonic airplane and orbiting space tether. Finding that optimum
combination is the objective of the ongoing study.

HASTOL SPACE TETHER CONCEPTS
There are many ways of designing the orbiting space tether component of the HASTOL
system. The five different space tether system concepts initially studied were the: HyperSkyhook,
Rotovator, CardioRotovator, CASTether/LIFTether, and HARGSTOL. In our initial analyses ofeach concept, we assumed that the tether system would have a Tether Central Station (TCS) that

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was many times more massive that the tether or the payloads being handled. This was assumed
so that the center-of-mass (CM) of the tether system was at the TCS. In reality, the TCS will be
only 5 to 20 times heavier than the payloads, and the CM of the tether system will not be exactly
at the TCS. These minor corrections can be taken into account later. Although the tether mass
will usually be less than the TCS mass, we do not want to ignore the tether mass entirely. So, foreach of the following concepts we have estimated the mass of the tether alone, using the data wehave for the tensile strength and density of high strength materials that are presently available incommercial quantities. If the mass of the tether starts to exceed 100 times the mass of the
payload, then that is an indication the particular scenario being considered is not engineeringly
feasible using presently available materials, although it might become feasible in the near future
as newer or better materials become available with higher tensile strengths at higher operationaltemperatures. As we shall see later, presently available commercial materials will suffice tomake the HASTOL concept work. Just a modest improvement over present-day materials in theratio of the tensile strength to the density by a factor of two will lower the tether masses to where
they are no longer a significant factor in the commercial feasibility of the concept. The primary
message we want to leave with the Reader is: “We don't need magic materials like
‘Buckminster-Fuller-carbon-nanotubes’ to make the HASTOL space tether. Present-day
materials will do.”HyperSkyhook
In 1995 Zubrin proposed1 the “Hypersonic Skyhook” as a solution to the mismatch between
the attainable atmospheric speeds of a hypersonic airplane and the orbital speeds of space tethers.
Since the orbital speed of the space tether decreases with increasing altitude of the tether systemcenter-of-mass, he proposed the use of very long non-spinning tethers or “skyhooks” reaching
down from very high altitudes. His analysis showed that because a hanging tether must be
tapered to support its lower end in the gravitational field of the Earth, achieving a HyperSkyhooktether tip rendezvous with a 5000 m/s (Mach 16) airplane would require a HyperSkyhook tether
mass of 25 times the payload mass. Trying to lower the tether tip speed to 4000 m/s (Mach 13)
would require a HyperSkyhook tether mass greater than 200 times the payload mass. Unless amajor breakthrough occurs in high strength tether materials, such as the commercial development

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of carbon nanotube fibers, it does not seem possible to push the non-rotating tether
HyperSkyhook concept down to speeds of 3100 m/s (Mach 10).
Rotovator™
The standard method of attaining a low tether tip velocity is to use a rapidly spinning tether,or Rotovator™. The Rotovator concept was invented in 1967 by Artsutanov and reinvented by
Moravec in 1977, who did the first thorough analysis2 of it. Since the Rotovator must reachdown from orbital altitudes into the upper atmosphere to match speeds with the hypersonic
airplane, the length of the tether and the orbital altitude are necessarily interrelated, with theorbital altitude of the tether center-of-mass (CM) being the length of the tether plus a nominal
100 km for the thickness of the atmosphere. The longer the tether, the higher the orbital altitude
and the slower the velocity of the tether system CM.
Rotovator™ Tether Mass: The mass of a rapidly spinning tether in free space is determinedprimarily by the tip speed of the tether, not the tether length or the tether tip acceleration. The
basic equation for the ratio of the mass MT of one arm of a spinning tether to the mass MP of the
payload plus grapple on the end of the tether arm, was derived by Moravec in 1978 in an
unpublished paper, based on a previously published paper2, and is: V  2   V 
 VT 
MTexp  T   erf  T = π VC MP  VC    VC 

(1)

Where the error function
erf (z ) =

2 
z3
z5

z
−
+
− ...π  1!×3 2!×5

(2)varies from erf(0)=0 to erf(>3)=1.0, while erf(1)=0.843, VT is the tether tip speed, andVC=(2U/Fd)1/2 is the maximum tip speed of an untapered tether, where U is the ultimate tensile
strength of the tether material, d is its density, and F>1 is an engineering safety factor derating
the “ultimate” tensile strength to a safer “practical” tensile strength. Equation (1) shows
specifically that the mass ratio of a spinning tether is a function of the ratio of the tether tip speed
to the characteristic velocity (VT/VC) only, and to first order does not depend on the tether tip
acceleration or the length of the tether. The exponential growth in the mass ratio with the squareof the velocity ratio seen in Eq. (1) means that attempting to achieve tip velocities significantly

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higher than the characteristic velocity of the material rapidly leads to unfeasible mass ratios.
Equation (1), however, is for spinning tethers in deep space, and does not include gravity
gradient forces, which can be significant for long tethers that are operating close to the Earth.
The mass ratio of a long tether near the Earth will depend not only on the tip velocity of thetether, but also the length of the tether and the gravitational acceleration on the tip of the tether.
This will be true for most of the tether systems being considered for the HASTOL architecture.
There is no simple analytical equation that takes these gravity gradient forces into account, andthe mass ratio needs to be numerically integrated for each case. Thus, we have used a numericalintegration program to generate a table of tether mass ratios for Rotovator tethers of various
lengths L, rotating at various tip speeds VT, with the center of mass of the tether at the orbital
radius RO=6378 km+100 km+L, and moving at a circular orbit velocity of VO=(GME/RO)1/2,,
where ME=5.98x1024 kg is the mass of the Earth, and the gravitational constant
G=6.67x10–11 kg–m/s3. The lower end of the orbiting, spinning tether then reaches down into theatmosphere to match speeds with a hypersonic airplane moving at a hypersonic velocity
VH=VO–VT–470 m/s, where 470 m/s is the velocity through inertial space of the atmosphere at
80 km altitude at the equator of the rotating Earth.
We found that rotating tethers that were very short had a lower orbital altitude and therefore
higher orbital velocity, so they needed a higher tip velocity to match speeds with a hypersonic
airplane moving at a given hypersonic velocity. Thus, their mass ratio increased exponentially as
the square of the velocity because of Eq. (1). We also found that tethers that were very long wereorbiting more slowly, and thus needed less tip velocity, but because the gravity gradient forces on
the tether increased with tether length, the mass ratio increased because of the increased gravity
force.
After a lengthy search through the parameter space, we found that there was a broad
minimum in the mass ratio that occurred when the tether length and the tether tip velocity weresuch that the centrifugal acceleration at the tether tip was approximately 16 m/s (1.6 gees). The
results are summarized in Table 1. For the calculation of the mass ratio, we used data available
for Spectra™ 2000, a polymer made by AlliedSignal with an ultimate tensile strength of 4.0 GPa,

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a specific density of 0.97, and a derated (safety factor of F=2) characteristic velocity of 2030 m/s.
This material, along with others, is discussed in more detail later in the paper.
In Table 1, the column labeled ‘2x’ is for a future material (Spectra™ X000?) that has twice
the tensile strength of presently available Spectra™ 2000, while the column ‘10x’ is a“placecard” for some far future material (derated carbon nanotubes?) that has ten times the ratioof tensile strength to density of the presently available Spectra™ 2000 fiber.
From looking at Table 1, we can see that the use of present-day Spectra™ in a HASTOL
system will enable the Rotovator system to work down to about 3875 m/s (Mach 12.5) without
the tether becoming too heavy. Column ‘2x’ indicates that it only takes a small improvement intether materials for the Rotovator concept to work down to 3100 m/s (Mach 10). Column ‘10x’
indicates that carbon nanotubes would be “overkill” as far as the Rotovator concept is concerned.
We don't need carbon nanotubes to make a HASTOL system, as we will need to retain someamount of mass in the tether in order to keep the tether system itself from being pulled out oforbit by the payload!
Table 1 - Minimum Mass Ratio Rotovator Tether Parameters for HASTOL Application
TetherOrbitalOrbitalTip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity

Velocity

Velocity

Mass Ratio

L

RO

VO

VT

VH= VO-VT-470 m/s

MT/MP

(km)

(km)

(m/s)

(m/s)

(m/s)

Mach

Spectra™

2x

10x

400

6878

7614

2494

4650

15.0

10.4

2.4

0.37

500

6978

7559

2749

4340

14.0

16.7

4.2

0.56

600

7078

7506

3006

4030

13.0

27.1

5.9

0.65

700

7178

7453

3263

3720

12.0

44.0

8.2

0.73

800

7278

7402

3522

3410

11.0

71.8

11.6

0.90

900

7378

7352

3782

3100

10.0

117.6

16.3

1.07

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CardioRotovator
The CardioRotovator concept consists of a Tether Central Station in an elliptical orbit, with asingle long tapered tether. The tether rotation rate is chosen to be exactly twice the orbital
period, with the phase of the rotation such that when the Tether Central Station is at perigee, orclosest to the Earth, the tether is pointing straight up, while, when Tether Central Station is at
apogee, or furthest from the Earth, the tether is pointing straight down at the Earth, reaching deep
into the atmosphere.
As can be seen in Figure 1, atintermediate points, the tether is pointing
away from the Earth and does not penetrate
below the Tether Central Station altitude
except near the touchdown point below theapogee point. The trajectory of the tip of
the tether is approximately heart-shaped,
which lead to the name of“CardioRotovator” for the system concept..
Unlike the circular orbit Rotovator system,
the tether length and tip velocity of a
CardioRotovator cannot be chosen
independently. Once a particular apogee

Fig. 1 - CardioRotovator Concept

radius RA is chosen, that determines thelength of the tether, since L=RA–(6378 km+100 km). Then, once a particular perigee radius RP ischosen, that, along with the apogee radius, fixes the orbital period P to be:(RA + RP )3 
P= π 

 2GM E 

(3)

The rotational period p of the spinning tether itself is then also determined, since the designof the CardioRotovator requires that p=P/2. This rotational period, together with the tether
length L, then determines the tether tip speed as VT=2pL/p=πL/P.

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In Table 2, we have tabulated some relevant examples of the CardioRotovator systemparameters, assuming that in all cases the perigee altitude of the Tether Central Station is
500 km, which is just outside the International Space Station nominal altitude of 400 km. With
these assumptions, the CardioRotovator tether tip acceleration levels were found to between 0.43
and 0.66 gees, acceleration levels easily accommodated by human passengers.
From comparing Table 1 for the Rotovator systems and Table 2 for the CardioRotovatorsystems, it is seen that the CardioRotovator gives somewhat better results than the Rotovator. In
general, however, the length of the CardioRotovator tether is much longer than the length of the
Rotovator tether, which leads to greater concern about collisions of the tether with other objects
in space. This concern is partially compensated by the fact that the CardioRotovator tether
spends most of its time at high altitudes where there is less traffic.
Table 2 - CardioRotovator Tether Parameters for HASTOL Application
TetherOrbitalOrbitalTip

Tip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity Velocity

Accel.

Velocity

Mass Ratio

L

RO

VO

VT

a

VH= VO-VT-470 m/s

MT/MP

(km)
1000
1200
1400
1600
1800

(km)
7478
7678
7878
8078
8278

(m/s)
7147
7004
6868
6737
6611

(m/s)
2076
2440
2789
3124
3445

(m/s2)
0.43
0.50
0.56
0.61
0.66

(m/s)
4601
4094
3608
3143
2695

Mach
14.8
13.2
11.6
10.1
8.7

Spectra
10.8
22.2
44.7
87.8
168.5

2x
3.1
5.2
8.4
13.4
21.0

10x
0.39
0.55
0.75
0.97
1.24

CASTether/LIFTether
The fourth concept for the space tether portion of a HASTOL system uses two separate
methods of operating the tether, one method for getting the grapple/payload at the end of thetether down into the atmosphere, and another method for getting the grapple back up into orbit
again. There are a number of variants for both the down and up options. We have yet to
calculate the tether masses needed for this option.
CASTether: The Cast Ahead Supersonic Tether or CASTether concept was originallyinvented decades ago by Guiseppe Colombo as a method of using a tether to “cast” a penetrator
sampling device onto an airless body like the Moon or Mercury from a spacecraft in low orbit
around the planetoid, then pulling the sample up out of the penetrator by the tether as theA1-10

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spacecraft orbited past overhead. The concept was analyzed by Chauncey Uphoff and Jerome
Wright, then of JPL4.
The CASTether technique can be implemented around Earth in many ways. The techniqueinvented by Colombo involved letting the grapple and payload, on the end of the tether, drift
ahead of the spacecraft in their common orbit, then having the spacecraft rapidly "reel in" thetether, decreasing the orbital speed of the grapple/payload, and causing it to fall into an ellipticalorbit that intercepted the atmosphere. An alternate technique would be to set the tether inrotation, then, at the proper instant in its rotation, release the brake on the tether winch and let
more tether reel out, “casting” the grapple forward and downward in the same manner as castinga fly with a fishing rod.
An even simpler technique, which is illustrated in Fig. 2, is to select a tether length and tip
speed of a rotating tether so that, without letting out any tether, the grapple hits the upper
atmosphere sufficiently ahead of the Tether Central Station to allow time for drag deceleration of
the grapple down to match the speed of the hypersonic airplane before the TCS passes overhead.
Since the decrease in velocity does not involve using the strength of the tether, the mass of thetether is not affected by the amount of velocity decrease needed.

Fig. 2 - Cast Ahead Supersonic Tether (CASTether) Concept

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LIFTether: To complement the CASTether concept, we will use the Lift Into Freefall Tether
or LIFTether concept for increasing the velocity of the grapple from hypersonic speeds up toorbital speeds. The LIFTether concept was implicit in the original Colombo idea for obtaining
surface samples from small airless planetoids. It wasn’t until recently that it was recognized how
powerful the technique could be for use near a large gravitating body like Earth. If, during the
CASTether procedure, the relative positions of the grapple and Tether Central Station areproperly timed, the TCS can be made to be directly overhead the hypersonic airplane at the timeof the payload transfer to (or from) the grapple, while the distance between the two can be madeto be exactly equal to the total unreeled length of tether. As shown in Figure 3, with the TCS
directly overhead, the tether will smoothly “lift” and accelerate a payload (or empty grapple) into
orbit without requiring any reeling in or out of the tether. Although in Figure 3 the TCS is
moving at orbital speeds while the hypersonic airplane is moving some 4500 m/s slower, if along enough tether (L>600 km) is used, the “lift” of the tether on the payload is found to berelatively benign in terms of maximum acceleration on the payload. The maximum acceleration

Fig. 3 - Lift Into Freefall Tether (LIFTether) Concept

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on the payload occurs at the beginning of the lift, and is equal to a=∆V2/L, where ∆V is thevelocity difference between the TCS and the hypersonic airplane, and L is the length of thetether. For a tether length of L=600 km, and a velocity difference of ∆V=4500 m/s between theorbiting TCS and the hypersonic airplane, the total acceleration is only 4.3 gees, consisting of thelift acceleration of 3.4 gees, plus 0.85 gees from the Earth gravity as seen in the moving frame of
the 3000 m/s hypersonic aircraft flying a curved trajectory. Even this acceleration level would besmoothed out to a lower value by the elastic response of the tether. Moreover, the stress level on
a LIFTether is many times less than the stress level on a rotating tether with a tether tip speed
equal to the differential speed ∆V at the start of the “lift”, since the tether is not rotating at 4500
m/s. It is rotating at half that speed about its own center of mass (not the TCS CM), and the “endmass” at its rotating tip is only the outer portions of the tether material, not the payload.
HARGSTOL
The fifth method of accomplishing the HASTOL concept is to compromise, and allow thepartial use of rockets. Thus instead of the HASTOL system, we will have the HARGSTOL orHypersonic Airplane, Rocket Grapple, Space Tether Orbit Launch system. Instead of requiring
the hypersonic airplane and the space tether to close the entire velocity gap between the airplane
and the tether tip, we use rockets on the grapple at the end of the tether to contribute to the ∆Vrequirement. This concept has a number of possible variations. The normal method would be tohave the rocket augmented grapple on the tip of the tether. The tether system would slow the tip
down as much as possible using one of the tether tip slowing techniques, and the airplane would
fly as fast and high as possible, and the rocket system on the grapple would make up any speed
difference. The grapple would need to be refueled periodically. This could be done at eachpayload pickup, or there could be periodic pickups of propellant tanks, with the empty tanks
added to the Tether Central Station ballast.
A variation on this concept would be to have the major part of the tether mass be a permanent
part of the space tether system, but the “tip” of the tether and the rocket grapple would be carriedby the hypersonic airplane. Some time before the rendezvous time, the grapple would be
separated from the airplane, pulling out the tether, which would be made of material capable ofcoping with the hypersonic heating and stress. The rocket grapple would then climb in altitude

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and speed to meet up with the lower end of the space tether out in space away from theatmosphere, while the airplane stays in the atmosphere at an optimum cruise altitude. The
grapple grabs the end of the tether, the payload is pulled free from the airplane, and is pulled into
space by the tether.
The ultimate rocket grapple concept would have the rocket take the grapple from thehypersonic airplane all the way to the Central Station, pulling out tether from the payload. Since
for normal ∆V requirements the tether mass would be much larger than the payload mass, it isobvious that a better technique would be to meet the downgoing tether from the Tether Central
Station “halfway”. Finding the optimum ratio for the length of the airplane tether versus thespace tether would be part of the overall system optimization. This concept, with the rocket
grapple coming from the airplane without carrying the payload, would only be usable for taking
payloads into orbit. For two-way systems, it would be necessary to have the rocket grapple on
the end of the space tether, and have the rockets on the grapple capable of accelerating both thegrapple and the payload.
The most important feature of all the possible HARGSTOL systems is that we KNOW we
can make them work, no matter how poor the ultimate performance of the hypersonic airplane
and the space tether. All it requires is that the rocket grapple be loaded with enough propellant toclose the velocity gap. Since the mass ratio of the propellant to grapple-plus-payload is
exponential in the grapple ∆V, and the rocket ∆V is low because of the ∆V contributions of boththe airplane and tether, the propellant required should be low.

SPACE TETHER ISSUES
The space tether portion of the HASTOL system has a number of issues that must be dealt
with other than the method of operation, including surviving damage by meteorites and spacedebris, operating at hypersonic speeds in the upper atmosphere, avoiding collisions with otherspacecraft, and safe and reliable operation at low system mass.

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Tether Survivability
For a tether transport system to be economically advantageous, it must be capable of handlingfrequent traffic for many years despite degradation due to impacts by meteorites and space debris.
Yet, the tether mass must be minimized to reduce the cost of fabricating and launching thetethers. These two requirements present conflicting demands upon the tether design that makeconventional single-line tethers impractical for the HASTOL application. For a single-line tether
to achieve a high probability of survival for many years, it must be very thick and massive.
Fortunately, a low mass survivable tether design exists, called the Hoytether™, which canbalance the requirements of low weight and long life5. As shown in Fig. 4, the Hoytether is an
open net structure where the primary load bearing lines are interlinked by redundant secondarylines. The secondary lines are designed to be initially slack, so that the structure will not collapse
under load. If a primary line breaks, however, the secondary lines become engaged and take up
the load. Note in Fig. 4, that four secondary line segments replace each cut primary line segment,so that their cross-sectional area need only be 0.25 of the primary line area to carry the same load.
Typically, however, the secondary lines are chosen to have a cross-sectional area of 0.4 to 0.5 ofthe primary line area, so as to better cope with multiple primary and secondary line cuts in thePrimary
Lines

First Level ofSecondary
Lines
Redistributes
Load toAdjacent
Nodes

SecondaryLines(initiallyunstressed)

Effects ofDamageLocalized
Second Levelof Secondary
Lines
Redistributes
Load Back to
Undamaged
Portion ofPrimary Line

SeveredPrimaryLine

0.2 to10’s ofmeters0.1- 1 meter

a.

b.

c.

Fig. 4 - The Hoytether™ design and its response to a cut line

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same region of the tether. This redundant linkage enables the Hoytether™ structure to
redistribute loads around primary segments that fail due to meteorite strikes or material failure.
Consequently, the Hoytether structure can be loaded at high stress levels, yet achieve a highmargin of safety.
Tether System Collision Avoidance
There are many objects in space, ranging from micrometeorites to operational spacecraft with
10-meter-long solar array panels. We can design interconnected multiple strand open net
Hoytether™ structures that can reliably (>99.9%) survive in space for decades despite impacts by
objects up to 30 cm (1 foot) or so in size. Objects larger than one meter will impact all thestrands at one time, cutting the tether. These large objects could include operational spacecraft,
and they will also be damaged by the impact. Objects larger than 30 cm are all known andtracked by the U.S. Space Command. There are about 6000 such objects in low and medium
Earth orbit, of which an estimated 600 will be operational spacecraft in the 2005 time frame. For
an atmospheric tether application, we have estimated that, if no traffic control measures are
instituted, a 20 km long tether in an orbit grazing the upper atmosphere has a 4% chance of being
cut by one of the 6000 large objects during a one year mission, and an 0.4% chance of striking
one of the 600 operational spacecraft. Longer tethers will have proportionately larger
probabilities. It will therefore be incumbent on the HASTOL system operators to maintaincontact with the U.S. Space Command and keep an accurate inventory of the known large
objects. They then need to control the tether system CM orbital altitude and phase, the tether
rotation rate and phase, and the tether libration and vibration amplitudes and phases, to insurethat the tether system components do not penetrate a volume of “protected space” around theseorbiting objects.
Tether Safety Factor and System Reliability
When a tension member such as a tether is developed, it is normally designed to operate at aload level somewhat lower than the maximum it could support without breaking. This derating
provides margin of error in case of imperfections in the material or the construction. Typically, asingle line tether is designed to carry a maximum load that is 50% of its breaking limit. This
tether would thus have a "design safety factor" of F=1/0.50=2.0. For the Hoytether, we define

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the safety factor as the ratio of the maximum load capacity of both primary and secondary lines
to the design strees load SP of the primary lines alone.
F=[1+(NSAS)/(NPAP)]/SP(4)

where NP and NS are the number of primary lines and secondary lines, and AP and AS are theirrespective cross-sectional areas. For a typical tubular Hoytether™ there are twice as many
secondary lines as primary lines so NS=2NP and Eq. (4) reduces to F=[1+2AS/AP]/SP. For thecase where the secondary line area is half the primary line area or As=0.5Ap and the stress on the
primary lines is 67% of the ultimate tensile strength of the material or SP=0.67, then the
Hoytether™ safety factor would be: F=[1+2(0.5)]/0.67=3. This definition of the Hoytether™
safety factor provides the same measure of the strength-to-weight ratio of the Hoytether structure
as it does for a single-line tether. However, this definition of the safety factor does not accurately
represent the margin of safety for the Hoytether. Because the Hoytether has redundant links thatcan reroute stress around parts of the tether that have failed, it is possible to load the Hoytether ata large fraction of the capacity of the primary lines (i.e.- small “line safety factor”) and still havea large margin of safety against parting. To study the optimization of the Hoytether structure forhigh-load applications, we performed a series of simulations of variations of the structure using
our “SpaceNet” tether simulation program. The SpaceNet program uses a combination of finiteelement methods with a structural relaxation scheme to calculate the effects of damage tocomplex 3-D net structures such as the Hoytether. The results of our analyses indicate that thedesign of an optimal Hoytether depends upon how much of its mission duration will be spent
under high load. Consequently, there are two classes of Hoytether™ designs, one for tethers thatare always under high load, and one for tethers that are heavily loaded for brief periods only.
Continuous High Load Tether: If the tether will be under high load for most of its mission,then it should be designed with secondary lines slack at the expected load level. This will enable
the tether lines to remain spread apart at all times, minimizing the chances of a single impactor
cutting several lines. For this case, SpaceNet simulations showed that a near-optimal tether
design would be a cylindrical Hoytether with a large number of primary lines (~20) stressed at
75% of their maximum load and with initially-slack secondary lines that each have a crosssectional area 0.4 times that of a primary line. Simulations showed that splitting the tether up
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primary lines from overloading neighboring primary lines before the secondary lines could
become taut enough to take up the released stress and pass it around the cut primary line segmentto the uncut primary line segments above and below the cut segment. From Eq. (4), such a tether
will have a design safety factor of 2.4. However, the redundant nature of the structure will makethe Hoytether far more reliable than a single line tether with the same safety factor. Simulations
with the SpaceNet program have shown that this tether design can withstand multiple cuts on asingle level. In fact, even if all of the primary lines on one level are cut (one at a time), thesecondary lines will support the load.
Intermittent High Load Tether: The HASTOL Space Tether facility, however, would likely
be loaded at high levels for only a few hours at a time. Therefore, it is possible to reduce thetether weight by designing it to have slack secondary lines at the load level experienced during its
long “off-duty” periods, but to have the secondary lines bear a significant portion of the load
during a brief high-stress operation such as a payload catch-and-throw operation. During thehigh-stress period, the loading of the secondary lines will cause the structure to collapse to acylindrical tube. Once a payload is released and the stress is reduced, however, the tether lines
will spread back apart. If this high-load period is brief, it will only slightly increase the chancesof tether failure due to impact by a large object. Simulations indicate that a 20-primary line
Hoytether with the secondary line areas 1/4 of the primary lines can be safely loaded to 85% of
the primary line capacity during peak stress operations. The design safety factor of this tether
from Eq. (4) is F=1.75. In this paper we will use a slightly more conservative safety factor of
F=2.
Space Tether Materials
The space tether used in the HASTOL system will consist of a long strength member made ofa high strength, low density polymer, with a “hypersonic” tip made of a high strength at hightemperature, atomic-oxygen resistant material. Woven into the initial 10 km of the polymer
tether will be an aluminum wire conductor to be used by the Hoyt Electrodynamic Force Tether(HEFT) propulsion system3 built into the tether and used by the TCS to control the tether sustem
orbit and rotation parameters.

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High Strength Main Member The two candidate polymers for the high tensile strength main
portion of the tether, Spectra™ and Zylon™, are both stronger per pound than either steel or
Kevlar™ polymer fiber. Their “characteristic velocity”, defined as the maximum tip speed of an
untapered tether, is: VC=(2U/d)^1/2, where U is the ultimate tensile strength of the material and dis its density. Spectra 2000™ has a U=4.0 GPa, d=970kg/m^3 and VC=2872 m/s, while Zylon™
has a U=5.8 GPa, d=1560 kg/m^3 and VC=2727 m/s. Both of these materials are commerciallyavailable in tonnage quantities with reasonable prices and delivery times. These polymer
materials are sensitive to attack by atomic oxygen (AO), however, so the portions that get near
the atmosphere would probably be coated with a proprietary AO-resistant resin coating available
from Aeroplas.
Hypersonic Tip: The material for the hypersonic tip will not only have to withstand attack byatomic oxygen, but maintain a moderately high strength at high temperatures. We haveestimated that the tether temperature due to air drag heating will range from room temperature(300K or 27C) for a tether speed through the air of 3 km/s at 120 km altitude, up to as high as
2100K (1830C) for a relative speed of 5 km/s at 80 km altitude. The candidate materials andtheir ultimate tensile strength GPa (gigapascals) as a function of temperature are summarized in
the Table 3. (For reference, 1 GPa = 109 N/m2 = 145,000 psi). To allow a relative comparison of
the suitability of the various different tether materials for use in various rotating tether systems,
we also included in Table 3 the density d and the room temperature “characteristic velocity”
VC=(2U/d)1/2 of the material, which is the maximum attainable tip speed of an untapered rotating
tether made solely of that material. For reference, the melting points of some of the materials in
Table 3 are: Al–660C, Ti–1660C, Ni–1453C, W–3410C, Al2O3–2015C, SiC–2700C, andSiO2–1610C.

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Table 3 - Tether Material Tensile Strength (GPa) vs. Temperature
Material

VC

Density

20

300

600

800

1000

1200

(km/s)d (g/cc)

C

C

C

C

C

C

Spectra 2000

2.87

0.97

4.0

-

-

-

-

-

Quartz Glass (SiO2)

1.81

2.20

3.6

3.6

3.6

3.6

3.6

?

S-glass

1.94

2.50

4.7

?

?

?

?

?

Carbon

2.77

1.80

6.9

?

?

?

?

?

Carbon/Ni-coated

2.12

2.68

6.0

?

?

?

?

?

Tyranno (SiTiCO)

1.66

2.55

3.5

3.5

3.5

3.5

3.5

3.5

Textron β-SiC

2.19

2.93

7.0

6.6

6.0

5.6

5.2

4.5

0.72 β-SiC/Ti-coated

1.72

3.37

5.0

4.8

4.3

4.0

3.7

3.2

Altex (Al2O3/SiO2 )

1.21

3.30

2.4

2.4

2.4

2.4

2.4

1.5

Nextel (α-Al2O3)

1.30

3.88

3.3

?

?

?

?

?

0.65 Nextel/Al-coated

0.97

3.40

1.6

1.4

?

?

?

?

Tungsten Wire

0.55

19.35

2.9

2.9

2.9

2.9

2.9

2.9

HYPERSONIC AIRPLANE ISSUESThe technology for the hypersonic airplane itself is being developed by Boeing and others
elsewhere and is not part of the HASTOL effort. However, vehicle performance, flight trajectory
requirements, and operational aspects peculiar to tether rendezvous and payload transfer insupport of development and optimization of the HASTOL system are, and form a major portionof the hypersonic airplane portion of the HASTOL team effort. The hypersonic vehicle portionof the HASTOL effort will start with an existing design for a multi-role, hypersonic aircraft
developed by Boeing for NASA LaRC, which can perform both cruise and space launch missions(See Fig. 5). The vehicle is designed to operate from existing runways and incorporates a lowspeed propulsion system based on JP fueled, Air-core-enhanced Turbo Ramjets (AceTRs) for
operations up to Mach 4.5. Above Mach 4.5 a slush-hydrogen-fueled ram/scram system powers
the vehicle. While the design is optimized for long range cruise at Mach 10, the vehicle can also
perform “pop-up”-type launches of satellites, and incorporates a ten-foot diameter, thirty-footlong payload bay for that purpose.

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Tank 1 (SH2)

PayloadTank 4 (JP-7)
LOX (Below)

MLG

Looking Aft
Highly Integrated
2-D Inlet

Tank 2 (SH2)
Tank 3 (SH2)
Crew Station

AceTRs(Internal)

Tank 5L(SH2)
Tank 6 (SH2)

Tank 7(SH2)
Linear Rocket

Payload
NLG

Ramjet/Scramjet
Highly Integrated
SERN

Air Core Enhanced
Turboramjets (AceTR)
Ramjet/Scramjet

GP94070004.ppt

Fig. 5 - Mach 10 Dual-Fuel Airbreathing Aerospaceplane

An initial assessment of the HASTOL concept for this vehicle indicates that at the probable
80 km rendezvous altitude, the dynamic pressure and therefore the combustor pressure will be
too low for continuous cruise operation using current air-breathing hypersonic engine concepts.
However, the 80 km rendezvous altitude is readily attainable using a “pop-up” maneuver asshown in Fig. 6. This maneuver allows the safe staging of minimally streamlined, satellite/upperstage combinations at conditions where the velocity provided by the hypersonic aircraft could be
maximized and the performance of the upper stage optimized. The current design shown in
Fig. 5 is capable of carrying a 13,600 kg (30,000 lb.) combination upper stage and payload tospeeds of approximately 3,400 m/sec (11,120 ft/s or Mach 11) at altitudes as high as 85 km(280,000 ft).

Launch AscentHigh Speed Climb
Mach 4.5 - 10
Pattern, Landing,
Post Flight
Operations

Alternate Upper
Stage to Tether
Rendezvous
Ballistic
Trajectory toTetherRendezvous

Subsonic
CruiseInitial Climb
Mach 0.8 - 4.5

Descend
Decel

Tanker Rendezvous
Inflight RefuelingFig. 6 - Vehicle Pop-Up To Tether Rendezvous

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During the future efforts of this continuing study, the existing hypersonic airplane design will
be modified, as required, to perform HASTOL type missions and the modifications incorporated
into its performance simulation model. The results of the hypersonic vehicle trade study will beincorporated in the overall HASTOL system assessment. Two principal variant types will beassessed. In the first, the aircraft will rendezvous with the grapple. This variant will be
modified, as required, for each applicable HASTOL concept and its rendezvous geometry. In
order to operate in these HASTOL modes it will be necessary to resize the existing auxiliary
rocket engines and their propellant volumes for increased altitude and velocity. An enhanced
reaction control system with six-axis capability (including limited trajectory control) will also be
required in lieu of the current 3-axis (attitude only) system.
The second variant, the HARGSTOL concept, will have the hypersonic airplane carry the
payload to an intermediate condition. From there a small rocket upper stage will carry the
payload to the rendezvous with the grapple. This approach will require fewer systemmodifications, but will have less payload capability at a particular size due to the mass requiredby the rocket upper stage. Both variants will then be used to evaluate the concept through several
trade studies and optimized with the tether and grapple studies.

GRAPPLE AND PAYLOAD TRANSFER ISSUES
The rendezvous and capture/transfer of the payload from the hypersonic vehicle to thetethered grapple assembly will occur at around 80 km altitude, or near the perigee of the tether
trajectory. While the atmosphere is not very dense at that altitude, there will be significant
dynamic pressure due to the high velocities involved. The high dynamic pressure will allow theuse of aerodynamic control surfaces, which will minimize or eliminate refueling requirements forthe grapple vehicle. The current concept is to fly the decelerating grapple vehicle in so that it
approaches the hypersonic airplane from behind. A mechanism would be deployed from thegrapple vehicle that would attach to the payload and pull the payload up to the grapple. The
hypersonic vehicle would then return to ground and the grapple vehicle, with payload, would
continue its orbit to the correct location at which to release the payload at the correct velocity toachieve the intended higher orbit. After the grapple vehicle exits the atmosphere, the time spent

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in space will be used to cool and condition the grapple vehicle for the next aeropass. For periods
when the grapple will not be used to capture/deploy a payload, the tether will be shortened toraise the lower altitude of the grapple and keep the grapple vehicle above most of theatmosphere.
The grapple vehicle requires several internal functions to be successful for this kind of
mission, which make it similar to grapple assembly concepts developed earlier for exoatmospheric transfer of payloads6. The grapple is attached to the end of a tether but some
control, independent of the tether, will still be required. A means for controlling motion in andout of plane is necessary, as well as a mechanism to eliminate or control aerodynamic forces on
the grapple assembly during aeropass phases. Previous space grapple vehicles used storable
propellants and thrusters to maintain this control, but the high dynamic pressure should allow apure aerodynamic control concept that would eliminate propellant requirements. This eliminates
the refueling requirements and allows additional payload transfers for a selected configuration.
The hypersonic grapple would not use externally mounted solar panel arrays during theaeropass due to the high aerodynamic forces and heating rates during this phase. Two initial
options have been identified: a deployable/storable photovoltaic array or an electricallyconductive tether. Each would generate the required power, the latter while moving through theearth’s magnetic field, and would store excess energy in batteries for use during the aeropass
phase.
In order to allow a reliable rendezvous, the grapple vehicle must maintain location andattitude information and communicate with the hypersonic airplane. This can be done accurately
with a differential GPS similar to those systems being developed for landing commercial aircraft.
The approach velocities are too high to rely on human pilots on the ground so the system will
require autonomous rendezvous and capture (AR&C) capabilities. AR&C technologies, such asadvanced sensors for the final approach and rendezvous, are continuing to evolve, and are
maturing based on Russian and NASA investments.

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CONCLUSIONS
We have described a number of alternate system configurations that will allow hypersonicair-breathing airplane technologies to be combined with orbiting space tether technologies to
produce a method of moving payloads from the surface of the Earth into Earth orbit without theuse of rockets, and without subjecting the payloads and passengers to high accelerations or highrisk. The resultant Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system iscompletely reusable and has the potential to drastically cut the cost of Earth-to-orbit space
access.

ACKNOWLEDGEMENTS
This work was supported by a contract with the NASA Institute for Advanced Concepts(NIAC), Dr. Robert Cassanova, Director. The authors also acknowledge technical contributions
from their fellow co-workers, Dr. Robert P. Hoyt of Tethers Unlimited, Inc., Donald B. Johnsonof Boeing/MDC Phantom Works, and student Daniel Stuart Bowman of the University of
Maryland.

REFERENCES

1. Robert Zubrin, “The Hypersonic Skyhook,” Journal of the British Interplanetary Society,Vol. 48, No. 3, pp. 123-128, March 1995.
2. Hans Moravec, “A Non-Synchronous Orbital Skyhook,” Journal of the Astronautical
Sciences, Vol. 25, No. 4, pp. 307-322, 1977.
3. Robert P. Hoyt and Chauncey Uphoff, “Cislunar Tether Transport System,” Paper AIAA99-2690, 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Los Angeles, CA,
20-24 June 1999.

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4. Chauncey W. Uphoff, pages 69 and 72-75 in JPL Publication 80-26, Discussion Meetingon Gossamer Spacecraft (Ultralight Spacecraft): Final Report, edited by Roy G. Brereton(Jet Propulsion Laboratory, Pasadena, CA, 15 May 1980).
5. Robert P. Hoyt and Robert L. Forward, “Failsafe Multiline Hoytether Lifetimes,” Paper
AIAA-95-2890, 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, San Diego,
CA, 10-12 July 1995.
6. M. E. Bangham, E. Lorenzini, and L. Vestal, “Tether Transportation System Study,”
NASA/TP-1998-206959, NASA Marshall Space Flight Center, Huntsville, AL,
March 1998.

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APPENDIX 2
HYPERSONIC AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) SYSTEM:
INTERIM STUDY RESULTSAIAA PAPER NO. 99-4802

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AIAA-99-4802

HYPERSONIC AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) SYSTEM:
INTERIM STUDY RESULTS
Dr. Thomas J. Bogar [Presenter], Senior Principal Engineer, Hypersonic Design andApplications, Boeing/MDC Phantom Works, P.O. Box 516, St. Louis, MO 63166 USA
Michal E. Bangham, Senior Manager Advanced Engineering, Advanced Programs,
Boeing/MDC, P.O. Box 240002, Huntsville, AL 35824 USA
Dr. Robert L. Forward, Vice President & Chief Scientist
Tethers Unlimited, Inc. 8114 Pebble Court, Clinton, WA 98236 USA
Dr. Mark J. Lewis, Associate Professor and Director of Center for Hypersonic Education andResearch, Aerospace Engineering Dept., University of Maryland, College Park, MD 20742 USA

ABSTRACTThe Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system is a novel
architecture for an Earth-to-orbit launch system consisting of: a completely reusable airbreathing
subsonic-to-hypersonic dual-fuel airplane which transports the payload from the ground to someintermediate point in the upper atmosphere; an orbiting spinning space tether system which picks
up the payload from the intermediate point and takes it on into orbit; and a grapple assembly fortransferring the payload from the hypersonic airplane to the lower end of the space tether. The
system is revolutionary in that it minimizes, and perhaps even eliminates, the use of rockets for
Earth-to-orbit launch of satellite payloads and even passengers. For the hypersonic airplaneportion of the HASTOL system we use an existing Boeing design for the DF-9, a dual-fuelairbreathing launcher that has benefited from over a million dollars in NASA/LaRC and Boeing
funding during prior study efforts. The DF-9 has a 9 m (30 ft) long by 3 m (10 ft) diameter
upward-opening central payload bay that can handle payloads up to 14 Mg (14 metric tons or
30,000 lb). With a full fuel load at takeoff, the hypersonic airplane masses approximately 20
times the payload mass, and can deliver the payload to 100 km (330 kft) altitude at an apogee
speed of 3.6 km/s (12 kft/s) or approximately Mach 12. For the space tether portion of the

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HASTOL system, there are a number of design options, all of which will work, although some
options promise better performance. The tethers can be built today using presently available
commercial fibers. The tethers are long, typically 400 to 1600 km (1300 to 5300 kft) in length.
The total mass of the space tether plus the Tether Central Station typically will be 30-200 times
the payloads being handled. Most of that mass ratio requirement is driven by the fact that thetether system must mass considerably more than the payload it is handling, so that, upon pickupof the payload by the tether, the payload will not pull the space tether system down into theatmosphere. Thus, the advent in the future of better tether materials with higher strength athigher temperatures will not be used to lower the tether system mass significantly, but instead
will be used to increase the tether safety margins, lifetime, and system performance, by allowing
payload pickup at lower altitudes and lower speeds, thus decreasing the performancerequirements on the hypersonic airplane portion of the system.

INTRODUCTION
The Boeing Company, Tethers Unlimited, Inc. (TUI), and the University of Maryland, haveteamed to study the feasibility of a completely new concept for moving payloads and passengers
from the surface of the Earth into low Earth orbit at low cost and low acceleration levels without
the use of rockets as the main source of propulsion. Our joint study effort, funded by a $75,000
Phase I grant from the NASA Institute for Advanced Concepts, is halfway through its 6-monthterm. This paper builds upon work reported in a previous paper1, and should be considered aninterim report of the study results to date, rather than a finished piece of work.
HASTOL Architecture
The Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system contains threemajor components: a hypersonic airplane, which will transport the payload as high and as fast as
possible using air-breathing propulsion; an orbiting spinning space tether, the lower tip of which
will be lowered down and slowed down by one means or another, so as to meet up with thehypersonic airplane; and a grapple assembly at the tip of the space tether that will take control of

Copyright  1999 by TUI, Boeing and University of Maryland.

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the payload, and with the lift supplied by the space tether, carry the payload on into orbit. There,
after the space tether has used propellantless propulsion to change its orbit and rotation, thepayload will be tossed into its desired final trajectory. It would be desirable that the HASTOL
system function in both directions, allowing for return of payloads from orbit to the Earth’s
surface. This is not a firm requirement, however, for a launch-only HASTOL system would be
useful in itself, since returning from orbit is much easier than launching into orbit. The objectiveof our ongoing study is to optimize the combined system of airplane, tether, and grapple in orderto maximize the overall system performance in terms of payload mass and delivery rate, while
minimizing the life cycle cost.
BackgroundLet us first give some scale to the problem of launching a payload into space. In order to fly
an airbreathing vehicle directly into orbit requires an airplane capable of reaching horizontal
speeds of 7.8 km/s (26 kft/s or approximately Mach 25) at 150 km (490 kft) altitude or an orbital
radius of 6530 km (21,400 kft). Designs exist for hypersonic airplanes capable of level flight at
3.1 km/s (10 kft/s or approximately Mach 10), and concepts exist for faster planes of Mach 12.5
and higher, but the difficulty of making and operating the hypersonic airplane rises rapidly withincreasing Mach number.
There is another scale to the problem of putting things into orbit. Since space starts at about
100 km up, most people think that to get into space only involves 100 to 200 km worth of travel.
What they fail to realize is that every rocket launched into orbit to date has had to travel
thousands of kilometers down range to attain the necessary 7.8 km/s orbital speed. Since thedistance D that must be traveled at constant acceleration a to reach a final velocity V is D=V2/2a,to reach an orbital velocity of 7.8 km/s at an acceleration of one gee (a=9.8 m/s), requires
covering a distance of 3100 km. Similar scaling laws apply to space tethers. If a spinning spacetether is to produce a change in velocity of a third of orbital speed, or 2.6 km/s, then the tether
length L for a one gee acceleration at the tether tip needs to be of order L=V2/a=690 km. As will
be illustrated in the following section on HASTOL Space Tether Concepts, there are many
designs for space tether systems which can lower a payload grapple assembly into the upper

Released to the AIAA to publish in all forms.

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atmosphere at grapple speeds with respect to the Earth’s atmosphere ranging from 4.65 km/s
(15 kft/s or Mach 15) to 3.1 km/s (10 kft/s or Mach 10) and lower, but the difficulty of operating
the space tether rises rapidly with decreasing grapple speed. We are quite sure that the bridge
between air and space can be crossed by using the right combination of hypersonic airplane andorbiting space tether. Finding that optimum combination is the objective of our study.

HYPERSONIC AIRPLANE
The technology for the hypersonic airplane portion of the HASTOL system is being
developed by Boeing and others elsewhere and is not part of the HASTOL effort. However,vehicle performance, flight trajectory requirements, and operational aspects peculiar to tether
rendezvous and payload transfer in support of development and optimization of the HASTOL
system are, and form a major portion of the hypersonic airplane portion of the HASTOL teameffort. The hypersonic vehicle portion of the HASTOL effort will start with an existing design2
for the DF-9 (See Fig. 1), a multi-role, hypersonic aircraft developed by Boeing for NASA LaRC.
The DF-9 can perform both cruise and space launch missions. The vehicle is designed to operatefrom existing runways and incorporates a low-speed propulsion system based on JP fueled, Aircore-enhanced Turbo Ramjets (AceTRs) for operations up to Mach 4.5. Above Mach 4.5 aslush-hydrogen-fueled ram/scram system powers the vehicle. While the design is optimized forlong range cruise at Mach 10, the vehicle can also perform “pop-up”-type launches of satellites,
and incorporates a 3 m (10-ft) diameter, 9 m (30 ft) long payload bay for that purpose.

Tank 1 (SH2)

PayloadTank 4 (JP-7)
LOX (Below)

MLG

Looking Aft
Highly Integrated
2-D Inlet
AceTRs(Internal)

Tank 2 (SH2)
Tank 3 (SH2)
Crew Station

Tank 5L(SH2)
Tank 6 (SH2)

Tank 7(SH2)
Linear Rocket

PayloadRamjet/Scramjet
Highly Integrated
SERN

NLG

Air Core Enhanced
Turboramjets (AceTR)
Ramjet/Scramjet

GP94070004.ppt

Fig. 1 - DF-9 Dual-Fuel Aerospaceplane

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The vehicle design does incorporate a linear rocket to provide thrust at altitudes where theairbreathing systems are ineffective. One important objective of the study will be to identify
HASTOL scenarios where the rocket is not needed.
Our initial assessment of the application of this particular vehicle design for accomplishing
the HASTOL concept indicates that at the probable 80 to 100 km rendezvous altitude, thedynamic pressure and therefore the combustor pressure will be too low for continuous cruise
operation using current air-breathing hypersonic engine concepts. However, a 100 km (330 kft)
rendezvous altitude was found to be readily attainable using a “pop-up” maneuver as shown in
Fig. 2. This maneuver would allow the safe staging of minimally streamlined payloads, or
payload/upper-stage combinations, at conditions where the velocity provided by the hypersonicaircraft could be maximized and the performance of the upper stage optimized. The current
design shown in Fig. 1 is capable of carrying a 30,000 lb. (14 Mg or 14 metric ton) combination
upper stage and payload to speeds of approximately 3.6 km/s (12 ft/s or Mach 12) at altitudes ashigh as 100 km (330 kft).
Apogee:Altitude 100 km
Speed 3600 m/s

Powered
Climb

Ballistic
Trajectory
to Tether
Rendezvous

Unpowered
Descent

Tanker Rendezvous
Inflight RefuelingFig. 2 - Vehicle Pop-Up To Tether Rendezvous

During the future efforts of this continuing study, the existing hypersonic airplane design will
be modified, as required, to perform HASTOL type missions and the modifications incorporated
into its performance simulation model. The results of the hypersonic vehicle trade study will beincorporated in the overall HASTOL system assessment. Two principal variant types will beassessed. In the first, the aircraft will rendezvous with the grapple assembly. This variant will be
modified, as required, for each applicable HASTOL concept and its rendezvous geometry. In

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order to operate in these HASTOL modes it will be necessary to resize the existing auxiliary
rocket engines and their propellant volumes for increased altitude and velocity. An enhanced
reaction control system with 6-axis capability (including limited trajectory control) will also be
required in lieu of the current 3-axis (attitude only) system.
A different variant that will also be studied, will have the hypersonic airplane carry the
payload to an intermediate condition. From there a small rocket upper stage will carry the
payload to the rendezvous with the grapple. This approach will require fewer systemmodifications, but will have less payload capability at a particular size due to the mass requiredby the rocket upper stage. Both variants will then be used to evaluate the concept through several
trade studies and optimized with the tether and grapple studies.

HASTOL SPACE TETHER CONCEPTS
There are many ways of designing the orbiting spinning space tether component of the
HASTOL system. The six different space tether system concepts initially studied were the:HyperSkyhook, Rotovator, CardioRotovator, CASTether/LIFTether, Tillotson Two-Tier Tether,and HARGSTOL. In our initial analyses of each concept, we assumed that the tether system
would have a Tether Central Station (TCS) that was many times more massive that the tether or
the payloads being handled. This was assumed so that the center-of-mass (CM) of the tether
system was at the TCS. In reality, the TCS will have a finite mass, and the CM of the tether
system will not be exactly at the TCS. These corrections will be taken into account in later, moredetailed, tether system design studies.
Although the tether mass will usually be less than the TCS mass, we do not want to ignore
the tether mass entirely. So, for each of the following concepts we have estimated the mass ofthe tether alone, using the data we have for the tensile strength and density of high strength
materials that are presently available in commercial quantities. If the mass of the tether starts toexceed 200 times the mass of the payload, then that is an indication the particular scenario being
considered is not engineeringly feasible using presently available materials, although theapplication might become feasible in the near future as better materials become available withhigher tensile strengths at higher operational temperatures.

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As we shall see later, presently available commercial materials will suffice to make the
HASTOL concept work. Just a modest improvement by a factor of two over present-day
materials in the ratio of the tensile strength to the density will lower the ratio of the tether mass tothe payload mass to the point to where they are no longer a significant factor in the commercial
feasibility of the concept. The primary message we want to leave with the Reader is: “We don't
need magic materials like ‘Buckminster-Fuller-carbon-nanotubes’ to make the space tether for a
HASTOL system. Present-day materials will do.”HyperSkyhook
In 1995 Zubrin proposed3 the “Hypersonic Skyhook” as a solution to the mismatch between
the attainable atmospheric speeds of a hypersonic airplane and the orbital speeds of space tethers.
Since the orbital speed of the space tether decreases with increasing altitude of the tether systemcenter-of-mass, he proposed the use of very long non-spinning tethers or “skyhooks” reaching
down from very high altitudes. His analysis showed that because a hanging tether must be
tapered to support its lower end in the gravitational field of the Earth, achieving a HyperSkyhooktether tip rendezvous with a 5.0 km/s (16 kft/s or Mach 16) airplane would require a
HyperSkyhook tether mass of 25 times the payload mass. Trying to lower the tether tip speed to
4.0 km/s (13 kft/s or Mach 13) would require a HyperSkyhook tether mass greater than 200 times
the payload mass. Unless a major breakthrough occurs in high strength tether materials, such as
the commercial development of carbon nanotube fibers, it does not seem possible to push thenon-spinning tether HyperSkyhook concept down to speeds of 3.1 km/s (10 kft/s or Mach 10).
Rotovator™
The standard method of attaining a low tether tip velocity is to use a rapidly spinning tether,or Rotovator™. The Rotovator concept was invented in 1967 by Artsutanov and reinvented by
Moravec in 1977, who did the first thorough analysis4 of it. Since the Rotovator must reachdown from orbital altitudes into the upper atmosphere to match speeds with the hypersonic
airplane, the length of the tether and the orbital altitude are necessarily interrelated, with theorbital altitude of the tether center-of-mass (CM) being the length of the tether plus a nominal
100 km for the thickness of the atmosphere. The longer the tether, the higher the orbital altitude
and the slower the velocity of the tether system CM.

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Rotovator™ Tether Mass: The mass of a rapidly spinning tether in free space isdetermined primarily by the tip speed of the tether, not the tether length or the tether tip
acceleration. The basic equation for the ratio of the mass MT of one arm of a spinning tether tothe mass MP of the payload plus grapple on the end of the tether arm, was derived by Moravec in
1978 in an unpublished paper, based on a previously published paper4, and is: V  2   V 
 VT 
MTexp  T   erf  T = π VC MP  VC    VC 

(1)

Where the error function
erf (z ) =

2 
z3
z5

z
−
+
− ...π  1!×3 2!×5

(2)varies from erf(0)=0 to erf(>3)=1.0, while erf(1)=0.843, VT is the tether tip speed, andVC=(2U/Fd)1/2 is the maximum tip speed of an untapered tether, where U is the ultimate tensile
strength of the tether material, d is its density, and F>1 is an engineering safety factor derating
the “ultimate” tensile strength to a safer “practical” tensile strength. Equation (1) shows
specifically that the mass ratio of a spinning tether is a function of the ratio of the tether tip speed
to the characteristic velocity (VT/VC) only, and to first order does not depend on the tether tip
acceleration or the length of the tether. The exponential growth in the mass ratio with the squareof the velocity ratio seen in Eq. (1) means that attempting to achieve tip velocities significantly
higher than the characteristic velocity of the material rapidly leads to unfeasible mass ratios.
Equation (1), however, is for spinning tethers in deep space, and does not include gravity
gradient forces, which can be significant for long tethers that are operating close to the Earth.
The mass ratio of a long tether near the Earth will depend not only on the tip velocity of thetether, but also the length of the tether and the gravitational acceleration on the tip of the tether.5-7
This will be true for most of the tether systems being considered for the HASTOL architecture.
There is no simple analytical equation that takes these gravity gradient forces into account, andthe mass ratio needs to be numerically integrated for each case. Thus, we have used a numericalintegration program to generate a table of tether mass ratios for Rotovator tethers of various
lengths L, spinning at various tip speeds VT, with the center of mass of the tether at the orbital
radius RO=RE+h+L, and moving at a circular orbit velocity of VO=(GME/RO)1/2,, where h=100 km

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is the nominal payload pickup altitude, RE=6378 km is the radius and ME=5.98x1024 kg is themass of the Earth, and the gravitational constant G=6.67x10–11 m3/kg-s2. The lower end of theorbiting, spinning tether then reaches down into the atmosphere to match speeds with ahypersonic airplane moving at a hypersonic velocity VH=VO–VT–470 m/s, where 470 m/s is thevelocity through inertial space of the atmosphere at 100 km altitude at the equator of the rotating
Earth.
We found that spinning tethers that were very short had a lower orbital altitude and therefore
higher orbital velocity, so they needed a higher tip velocity to match speeds with a hypersonic
airplane moving at a given hypersonic velocity. Thus, their mass ratio increased exponentially as
the square of the velocity because of Eq. (1). We also found that tethers that were very long wereorbiting more slowly, and thus needed less tip velocity, but because the gravity gradient forces on
the tether increased with tether length, the mass ratio increased because of the increased gravity
force. After a lengthy search through the parameter space, we found that there was a broad
minimum in the mass ratio that occurred when the tether length and the tether tip velocity weresuch that the centrifugal acceleration at the tether tip was approximately 16 m/s2 (52 ft/s2 or
1.6 gees).
Idealized Rotovator™ Results: The results of our first cut analysis are summarized inTable 1. It should be emphasized that in generating Table 1 we have made two highly idealistic
assumptions. First, we assumed that the Tether Central Station is much more massive than thetether. If this is not true, then the tether mass ratios given in Table 1 could rise by up to a factorof 2. The factor of 2 would be the case where there is no TCS at all, and the counterbalance tothe tether arm is an equally massive arm stretching out in the opposite direction from the CM.
Second, we assumed that we are dropping off a payload at the same time (or nearly the sametime) as we are picking up a payload. This assumption produces the ideal result that the load on
the tether does not change, the CM of the tether does not change, and the orbit of the CM of thetotal tether facility around the Earth does not change.
In prior studies of systems for picking up payloads from low Earth orbits and tossing them tothe Moon6 and Mars7, we showed that practical spinning tether systems could be designed
without using any of the above ideal assumptions, that were capable of carrying out those

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difficult payload pickup and toss tasks while massing less than 30 times the payloads being
thrown. As we move further into our HASTOL studies, we will go through the same procedureof replacing these idealistic first cut tether system designs with progressively more realistic
designs.
For the calculation of the tether to payload mass ratio, we used data available for Spectra™
2000, a polymer made by AlliedSignal with an ultimate tensile strength of 4.0 GPa (580,000 psi),a specific density of 0.97, and a derated (safety factor of F=2) characteristic velocity of 2030 m/s(6660 ft/s). This material, along with others, is discussed in more detail later in the paper.
In Table 1, the column labeled ‘2x’ is for a future material (Spectra™ X000?) that has twice
the tensile strength to density ratio of presently available Spectra™ 2000, while the column ‘10x’
is a “placecard” for some far future material (derated carbon nanotubes?) that has ten times theratio of tensile strength to density of the presently available Spectra™ 2000 fiber.
From looking at Table 1, we can see that the use of present-day Spectra™ in a HASTOL
system will enable the Rotovator system to work down to about 3.4 km/s (Mach 11) without thetether becoming too heavy. Column ‘2x’ indicates that it only takes a small improvement intether materials for the Rotovator concept to work down to 3100 m/s (Mach 10). Column ‘10x’
indicates that carbon nanotubes would be “overkill” as far as the Rotovator concept is concerned.
We don't need carbon nanotubes to make a HASTOL system, as we will need to retain someamount of mass in the tether in order to keep the tether system itself from being pulled out oforbit by the payload!
Table 1 - Minimum Mass Ratio Rotovator™ Tether Parameters for HASTOL Application
TetherOrbitalOrbitalTip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity

Velocity

Velocity

Mass Ratio

L

RO

VO

VT

VH= VO-VT-470 m/s

MT/MP

(km)

(km)

(m/s)

(m/s)

(m/s)

Mach

Spectra™

2x

10x

400

6878

7614

2494

4650

15.0

10.4

2.4

0.37

500

6978

7559

2749

4340

14.0

16.7

4.2

0.56

600

7078

7506

3006

4030

13.0

27.1

5.9

0.65

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Table 1 – (Continued) Minimum Mass Ratio Rotovator™
Tether Parameters for HASTOL Application
TetherOrbitalOrbitalTip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity

Velocity

Velocity

Mass Ratio

700

7178

7453

3263

3720

12.0

44.0

8.2

0.73

800

7278

7402

3522

3410

11.0

71.8

11.6

0.90

900

7378

7352

3782

3100

10.0

117.6

16.3

1.07

Realistic Rotovator™ Results: We recently have generated some new results where weassumed a more realistic design for the Rotovator™ Facility and a more realistic operational
scenario. In this analysis, we assumed a payload mass of 15 Mg (33,000 lb), grapple assemblymass of 0.5 Mg, tether length of 600 km, pickup altitude of 100 km (330 kft), and pickup
velocity of 4.1 km/s (13 kft/s), which requires the hypersonic aircraft to fly at 3.6 km/s (12 kft/s
or Mach 12) at the equator so that it can take advantage of the 470 m/s (1500 ft/s) rotation of the
Earth. This more realistic operational scenario assumed that there would be only a pickup of the
payload, without a compensating drop-off of a payload. This, in turn, required that the CM of the
Rotovator Facility be in an initially elliptical orbit with an eccentricity of 0.0062, so that after
pickup of the payload, the Rotovator Facility dropped into an orbit with a perigee such that the
tip of the spinning tether did not hit the atmosphere.
The analysis is still in the process of being optimized, but with the assumption that we use
Spectra™ 2000 material with a safety factor of 2, then the required mass of the tether alone wascalculated to be approximately 91 times the payload mass (about double that in Table 1), while
the mass of the Tether Central Facility needed to be 110 times the payload mass, for an overall
Rotovator Facility mass ratio of 201. If a stronger material becomes available, that has twice thestrength-to-density of Spectra™ 2000, then an optimized Rotovator Facility with a tether lengthof 600 km would operate in a slightly more elliptical orbit with an eccentricity of 0.0145. The
mass of the tether alone would now be only 11 times the payload mass, while to avoid the
payload from dragging the Rotovator Facility down into the atmosphere, the TCS mass would
actually have to increase to 120 times the payload mass! The total Rotovator Facility mass
would then be 131 times the payload mass. We expect that these total mass ratios will drop as
we optimize the system.

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Although these mass ratios are high, they are not impractical, considering that the Rotovator
Facility can be used to “build itself” by starting out small, then picking up tether and power
modules to build up the length, thickness, and taper of the tether, and picking up solar power
modules to build up the power supply needed by the propellantless electrodynamic tether
propulsion system6 that maintains the Rotovator Facility orbital altitude and spin speed. Carroll
has shown5 that tether facilities are capable of pickup up a payload with the end of a tether, then“tossing” the payload into an orbit where the payload later can rendezvous and dock with the CMof the facility (somewhat like tossing a peanut into your mouth)!
The important point to make about our study results so far, is that an orbiting spinning spacetether built using existing space tether materials, and using the simplest existing tether facility
design, can be used to pick up a payload from an existing design for a hypersonic airplane that is
capable of taking a payload to an altitude of 100 km (330 kft) altitude while moving at 3.6 km/s
(12 kft/s or Mach 12). Thus, the HASTOL system combination of a Spectra™ 2000 Rotovator™
and a DF-9 Aeropaceplane is capable of taking payloads from the surface of the Earth andputting them into space. The other HASTOL concepts we will discuss later may prove to bebetter, but this concept will suffice.
CardioRotovator
The CardioRotovator concept consists of aTether Central Station in an elliptical orbit, with asingle long tapered tether. The tether rotation rateis chosen to be exactly twice the orbital period.
The phase of the rotation is chosen such that when
the Tether Central Station is at perigee, or closest
to the Earth, the tether is pointing straight up, as isshown in Fig. 3. Then, when Tether Central
Station is at apogee, or furthest from the Earth,
the tether is pointing straight down at the Earth,reaching deep into the atmosphere for the payload
pickup.

Fig. 3 - CardioRotovator Concept

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As can be seen in Fig. 3, at intermediate points, the tether is pointing away from the Earthand does not penetrate below the Tether Central Station altitude except near the touchdown point
below the apogee point. The trajectory of the tip of the tether is approximately heart-shaped,
which lead to the name of “CardioRotovator” for the system concept. Unlike the circular orbit
Rotovator system, the tether length and tip velocity of a CardioRotovator cannot be chosen
independently. Once a particular apogee radius RA is chosen, that determines the length of thetether, since L=RA–RE+h. Then, once a particular perigee radius RP is chosen, that, along withthe apogee radius, fixes the orbital period P to be:(RA + RP )3 
P=π
 2GM E 

1/ 2

(3)

The rotational period p of the spinning tether itself is then also determined, since the designof the CardioRotovator requires that p=P/2. This rotational period, together with the tether
length L, then determines the tether tip speed as VT=2πL/p=πL/P.
In Table 2, we have tabulated some relevant examples of the CardioRotovator systemparameters, assuming that in all cases the perigee altitude of the Tether Central Station is
500 km, which is just outside the International Space Station nominal altitude of 400 km. With
these assumptions, the CardioRotovator tether tip acceleration levels were found to between 0.43
and 0.66 gees, acceleration levels easily accommodated by human passengers. Again, for
Table 2, we have assumed an idealistic situation where the TCS has an infinite mass and that apayload pickup is compensated by a payload drop-off.
By comparing Table 1 for the Rotovator systems with Table 2 for the CardioRotovatorsystems, it is seen that the CardioRotovator gives somewhat better results than the Rotovator. In
general, however, the length of the CardioRotovator tether is much longer than the length of the
Rotovator tether, which leads to greater concern about collisions of the tether with other objects
in space. This concern is partially compensated by the fact that the CardioRotovator tether
spends most of its time at high altitudes where there is less traffic.

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Table 2 - CardioRotovator Tether Parameters for HASTOL Application
TetherOrbitalOrbitalTip

Tip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity Velocity

Accel.

Velocity

Mass Ratio

L

RO

VO

VT

a

VH= VO-VT-470 m/s

MT/MP

(km)

(km)

(m/s)

(m/s)

(m/s2)

(m/s)

Mach

Spectra

2x

10x

1000

7478

7147

2076

0.43

4601

14.8

10.8

3.1

0.39

1200

7678

7004

2440

0.50

4094

13.2

22.2

5.2

0.55

1400

7878

6868

2789

0.56

3608

11.6

44.7

8.4

0.75

1600

8078

6737

3124

0.61

3143

10.1

87.8

13.4

0.97

1800

8278

6611

3445

0.66

2695

8.7

168.5

21.0

1.24

Two-Stage Rotovator
The Tillotson Two-Tier Tether (TTTT or
T4)8 consists of a long, large, tapered “firststage” spinning tether, at the end of which is asmaller “second stage” spinning tether as shownin Fig. 4. The T4 is essentially a two-stage
Rotovator. The use of two tiers or two “stages”
in the design of a spinning tether decreases theoverall ratio of the tether launch system mass to
payload mass, in a manner similar to thebenefits of the lower mass ratio obtained whenusing a two-stage rocket in a rocket launchsystem.
Since the ratio of the tether mass to theFig. 4 - Tillotson Two-Tier Tether (T4) Concept

payload mass of a spinning tether increases as
the exponential of the square of the tip velocity (see Eq. 1), large reductions in overall mass ratio
can be obtained by dividing up the total tip velocity required into nearly equal amounts. In atypical HASTOL scenario where a tether tip velocity of 3.6 km/s (12 kft/s) is needed to meet,

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say, a Mach 11 hypersonic aircraft moving at 3.4 km/s (11 kft/s), the tether mass required might
be 80 times the payload mass. If instead, the first stage tether rotates at a tip speed of 1.8 km/s
(6 kft/s), while the second stage tether also rotates at a tip speed of 1.8 km/s, the combined
velocities reach the 3.6 km/s needed for the pickup, but the combined mass of the two tethers
could be as little as 21 times the mass of the payload.
In the T4 concept, there will be a Tether Central Station (TCS) (assumed to have infinite
mass for this first cut analysis), around which will be spinning a one-arm first stage tether, with
an effective radius of rotation around the TCS of R1. At the end of the first stage tether will be a
stiff pivot bearing supported at both ends by the split end of the first stage tether, as shown in
Fig. 4. This pivot bearing will be the central support point for the spinning second-stage tether,which will have a radius of rotation R2. The total length, when both tethers are aligned along thenadir, will be L=R1+R2. Since we want the pickup to take place at an altitude h=100 km(330 kft), this determines the orbital radius of the TCS to be RO=RE+h+L, where RE is the radiusof the Earth. The orbital velocity is then just VO=(GME/RO)1/2, where G is the Newtonian
gravitational constant and ME is the mass of the Earth.
Dividing the total tip velocity VT evenly between the two stages gives each stage a tip
velocity of VT/2. To distribute the mass of the second stage tether evenly about its center ofrotation, the second stage tether must have two arms with equal mass on each side of the pivot
bearing. The second stage tether mass is therefore twice as great as a single-arm tether with tip
velocity VT/2. The first stage tether is a single-arm tether which must support the mass of the
payload plus the second stage tether. The total tether system mass is the sum of the first andsecond stage masses. The relative advantage of a T4 system compared to a single stage rotovatoris expressed by Equation 4, where M1(VT/VC) is the mass ratio for a single stage single arm tether
as shown in equation 1.
2

  V  V 
M2STAGE = 2  M1  T   + 3M1  T 
 2VC   2VC  (4)Equation 5 describes the total tether mass in more fundamental terms:

()

MT
VT
= 2π
MP
2VC

2

()

2
 V    V 2  VT VT T  exp  T  erf  VT exp  2erf
+ 3 π  2V  
 2VC 
2V

2V
2V

C
C
C
C




 

2

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In Table 3, we have tabulated some relevant examples of the T4 system parameters, assumingthat in all cases the total tip speed is evenly divided between the two stages. Because the second
stage tether has a shorter radius than a single-stage tether for equivalent total tip speed, theacceleration at the tip is higher than for a single stage system. For this analysis we assumed theratio of stage 1 tether radius to stage 2 radius is 5:1. With that assumption, the cases described inTable 3 have a maximum acceleration of 2.9 gravities, which is less than the 3-g maximum
acceleration experienced during a Space Shuttle launch. It is possible to change the ratio of stage
lengths and tip velocities to adjust system mass, acceleration, and dynamics.
The T4 approach to the design of the Rotovator for a HASTOL system is much more
complicated in design and dynamics than a simple one stage Rotovator. The plan is to baseline
the one-stage Rotovator for the study, but to carry out analyses of the T4 system in parallel. If
the mass of the one-stage tether grows to where its mass begins to cast doubt on the engineeringor financial feasibility of the HASTOL concept, then we always have the T4 two-stage concept
available in order to drastically cut the tether mass needed.
Table 3 - Tillotson Two-Tier Tether Parameters for HASTOL Application
1st Tier

2nd Tier

TetherTetherRadiusRadius

R1
(km)
333
417
500
583
667
750

R2
(km)
67
83
100
117
133
150

Total
TetherLengthTCS

TCS

Total Tip

HypersonicOrbitalOrbital

Velocity

AirplaneRadiusVelocity

NeededVelocity

RO
(km)
6878
6978
7078
7178
7278
7378

VO
(m/s)
7614
7559
7506
7453
7402
7352

VT
(m/s)
2494
2749
3006
3263
3522
3782

VH= VO-VT-470 m/s
(m/s)
Mach
4650
15.0
4340
14.0
4030
13.0
3720
12.0
3410
11.0
3100
10.0

Total Tether
to Payload
Mass Ratio

L
(km)
400
500
600
700
800
900

MT/MP
Spectra
2x
4.83
1.65
6.91
2.18
9.91
2.85
14.2
3.71
20.6
4.81
29.9
6.23

CASTether/LIFTether
Another concept for the space tether portion of the HASTOL system uses two separate
methods for operating the tether. The Cast Ahead Supersonic Tether or CASTether concept
involves “casting” the tether ahead of the Tether Central Station and using aerodynamic drag toslow the tip of the tether down to hypersonic speeds. The Lift Into Freefall Tether concept
involves arranging for the aerodynamically slowed tether to be vertical just as the TCS is passing

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overhead. The TCS will then initially “lift” the payload vertically upwards, then pull it along
behind into orbit. The two concepts are illustrated in our previous publication.1 We have yet tocarry out an analysis of this concept, so we will not discuss it further here.
HARGSTOL
The final method of accomplishing the HASTOL concept is to compromise, and allow thepartial use of a rocket upper stage or a rocket-powered grapple to complete the payload transfer
between the hypersonic airplane and the grapple assembly at the end of the space tether. Thus,
instead of the HASTOL system, we will have the HARGSTOL or Hypersonic Airplane, Rocket
Grapple, Space Tether Orbit Launch system. This concept has a number of possible variations.
The normal method would be to have the rocket augmented grapple on the tip of the tether. The
tether system would slow the tip down as much as possible using one of the tether tip slowing
techniques, and the airplane would fly as fast and high as possible, and the rocket system on thegrapple would make up any speed difference. The grapple would need to be refueled periodically.
This could be done at each payload pickup, or there could be periodic pickups of propellant
tanks, with the empty tanks added to the Tether Central Station ballast.
A variation on this concept would be to have the major part of the tether mass be a permanent
part of the space tether system, but the “tip” of the tether and the rocket grapple would be carriedby the hypersonic airplane. At some time interval before the rendezvous time, the grapple would
be separated from the airplane, pulling out the tether, which would be made of material capableof coping with the hypersonic heating and stress. The rocket grapple would then climb in
altitude and speed to meet up with the lower end of the space tether out in space away from theatmosphere, while the airplane stays in the atmosphere at an optimum cruise altitude. The
grapple grabs the end of the tether, the payload is pulled free from the airplane, and lifted into
space by the tether.
The ultimate rocket grapple concept would have the rocket take the grapple from thehypersonic airplane all the way to the Tether Central Station, pulling out tether from the payload.
Since for normal requirements the way to the Tether Central Station, pulling out tether from thepayload. Since for normal ∆V requirements the tether mass would be much larger than the
payload mass, it is obvious that a better technique would be to meet the downgoing tether from

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the Tether Central Station “halfway”. Finding the optimum ratio for the length of the airplane
tether versus the space tether would be part of the overall system optimization. This concept,
with the rocket grapple coming from the airplane without carrying the payload, would only beusable for taking payloads into orbit. For two-way systems, it would be necessary to have therocket grapple on the end of the space tether, and have the rockets on the grapple capable ofaccelerating both the grapple and the payload.
The most important feature of all the possible HARGSTOL systems is that we KNOW we
can make them work, no matter how poor the ultimate performance of the hypersonic airplane
and the space tether. All it requires is that the rocket grapple be loaded with enough propellant toclose the velocity gap. Since the mass ratio of the propellant to grapple-plus-payload is
exponential in the grapple ∆V, and the rocket ∆V is low because of the ∆V contributions of boththe airplane and tether, the propellant required should be low.

SPACE TETHER ISSUES6,9The space tether portion of the HASTOL system has a number of issues that must be dealt
with other than the method of operation, including surviving damage by meteorites and spacedebris, operating at hypersonic speeds in the upper atmosphere, avoiding collisions with otherspacecraft, and safe and reliable operation at low system mass.
Tether Survivability
For a tether transport system to be economically advantageous, it must be capable of handlingfrequent traffic for many years despite degradation due to impacts by meteorites and space debris.
Yet, the tether mass must be minimized to reduce the cost of fabricating and launching thetethers. These two requirements present conflicting demands upon the tether design that makeconventional single-line tethers impractical for the HASTOL application. For a single-line tether
to achieve a high probability of survival for many years, it must be very thick and massive.
Fortunately, a low mass survivable tether design exists, called the Hoytether™, which canbalance the requirements of low weight and long life9. As shown in Fig. 5, the Hoytether is an
open net structure where the primary load bearing lines are interlinked by redundant secondary

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lines. The secondary lines are designed to be initially slack, so that the structure will not collapse
under load. If a primary line breaks, however, the secondary lines become engaged and take up
the load. Note in Fig. 5, that four secondary line segments replace each cut primary line segment,so that their cross-sectional area need only be 0.25 of the primary line area to carry the same load.
Typically, however, the secondary lines are chosen to have a cross-sectional area of 0.4 to 0.5 ofthe primary line area, so as to better cope with multiple primary and secondary line cuts in thesame region of the tether. This redundant linkage enables the Hoytether™ structure to
redistribute loads around primary segments that fail due to meteorite strikes or material failure.
Consequently, the Hoytether structure can be loaded at high stress levels, yet achieve a highmargin of safety.

Primary
Lines

First Level ofSecondary
Lines
Redistributes
Load toAdjacent
Nodes

SecondaryLines(initiallyunstressed)

Effects ofDamageLocalized
Second Levelof Secondary
Lines
Redistributes
Load Back to
Undamaged
Portion ofPrimary Line

SeveredPrimaryLine

0.2 to10’s ofmeters0.1- 1 meter

a.

b.

c.

Fig. 5 - The Hoytether™ design and its response to a cut line

Tether System Collision Avoidance
There are many objects in space, ranging from micrometeorites to operational spacecraft with
10-meter-long solar array panels. We can design interconnected multiple strand open net
Hoytether™ structures that can reliably (>99.9%) survive in space for decades despite impacts by
objects up to 30 cm (1 ft) or so in size. Objects larger than one meter will impact all the strands
at one time, cutting the tether. These large objects could include operational spacecraft, and theyA2-20

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will also be damaged by the impact. Objects larger than 30 cm are all known and tracked by the
U.S. Space Command. There are about 6000 such objects in low and medium Earth orbit, ofwhich an estimated 600 will be operational spacecraft in the 2005 time frame. For anatmospheric tether application, we have estimated that, if no traffic control measures are
instituted, a 20 km long tether in an orbit grazing the upper atmosphere has a 4% chance of being
cut by one of the 6000 large objects during a one year mission, and an 0.4% chance of striking
one of the 600 operational spacecraft. Longer tethers will have proportionately larger
probabilities. It will therefore be incumbent on the HASTOL system operators to maintaincontact with the U.S. Space Command and keep an accurate inventory of the known large
objects. They then need to control the tether system CM orbital altitude and phase, the tether
rotation rate and phase, and the tether libration and vibration amplitudes and phases, to insurethat the tether system components do not penetrate a volume of "protected space" around theselarge orbiting objects.
Tether Safety Factor and System Reliability
When a tension member such as a tether is developed, it is normally designed to operate at aload level somewhat lower than the maximum it could support without breaking. This derating
provides margin of error in case of imperfections in the material or the construction. Typically, asingle line tether is designed to carry a maximum load that is 50% of its breaking limit. This
tether would thus have a "design safety factor" of F=1/0.50=2.0. For the Hoytether9, we define
the safety factor as the ratio of the maximum load capacity of both primary and secondary lines
to the design stress load SP of the primary lines alone.
F=[1+(NSAS)/(NPAP)]/SP(6)

where NP and NS are the number of primary lines and secondary lines, and AP and AS are theirrespective cross-sectional areas. For a typical tubular Hoytether™ there are twice as many
secondary lines as primary lines so NS=2NP and Eq. (6) reduces to F=[1+2AS/AP]/SP. For thecase where the secondary line area is half the primary line area or As=0.5Ap and the stress on the
primary lines is 67% of the ultimate tensile strength of the material or SP=0.67, then the
Hoytether™ safety factor would be: F=[1+2(0.5)]/0.67=3. This definition of the Hoytether™
safety factor provides the same measure of the strength-to-weight ratio of the Hoytether structure

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as it does for a single-line tether. However, this definition of the safety factor does not accurately
represent the margin of safety for the Hoytether. Because the Hoytether has redundant links thatcan reroute stress around parts of the tether that have failed, it is possible to load the Hoytether ata large fraction of the capacity of the primary lines (i.e.- small “line safety factor”) and still havea large margin of safety against parting.
To study the optimization of the Hoytether structure for high-load applications, we performeda series of simulations of variations of the structure using our “SpaceNet” tether simulation
program6. The SpaceNet program uses a combination of finite-element methods with a structural
relaxation scheme to calculate the effects of damage to complex 3-D net structures such as the
Hoytether. The results of our analyses indicate that the design of an optimal Hoytether depends
upon how much of its mission duration will be spent under high load. Consequently, there are
two classes of Hoytether™ designs, one for tethers that are always under high load, and one fortethers that are heavily loaded for brief periods only.
Continuous High Load Tether: If the tether will be under high load for most of its mission,then it should be designed with secondary lines slack at the expected load level. This will enable
the tether lines to remain spread apart at all times, minimizing the chances of a single impactor
cutting several lines. For this case, SpaceNet simulations showed that a near-optimal tether
design would be a cylindrical Hoytether with a large number of primary lines (~20) stressed at
75% of their maximum load and with initially-slack secondary lines that each have a crosssectional area 0.4 times that of a primary line. Simulations showed that splitting the tether up
into a large number of primary lines prevented the stress energy released by a cut of one of the
primary lines from overloading neighboring primary lines before the secondary lines could
become taut enough to take up the released stress and pass it around the cut primary line segmentto the uncut primary line segments above and below the cut segment.. From Eq. (6), such atether will have a design safety factor of 2.4. However, the redundant nature of the structure will
make the Hoytether far more reliable than a single line tether with the same safety factor.
Simulations with the SpaceNet program have shown that this tether design can withstandmultiple cuts on a single level. In fact, even if all of the primary lines on one level are cut (one ata time), the secondary lines will support the load.

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Intermittent High Load Tether: The HASTOL Space Tether facility, however, would
likely be loaded at high levels for only a few hours at a time. Therefore, it is possible to reduce
the tether weight by designing it to have slack secondary lines at the load level experienced
during its long “off-duty” periods, but to have the secondary lines bear a significant portion of theload during a brief high-stress operation such as a payload catch-and-throw operation. During
the high-stress period, the loading of the secondary lines will cause the structure to collapse to acylindrical tube. Once a payload is released and the stress is reduced, however, the tether lines
will spread back apart. If this high-load period is brief, it will only slightly increase the chancesof tether failure due to impact by a large object. Simulations indicate that a 20-primary line
Hoytether with the secondary line areas 1/4 of the primary lines can be safely loaded to 85% of
the primary line capacity during peak stress operations. The design safety factor of this tether
from Eq. (6) is F=1.75. In this paper we will use a more conservative safety factor of F=2.
Space Tether Materials
The space tether used in the HASTOL system will consist of a long strength member made ofa high strength, low density polymer, with a “hypersonic” tip made of a high strength at hightemperature, atomic-oxygen resistant material. Woven into the initial 10 km of the polymer
tether nearest the Tether Central Station will be an aluminum wire conductor to be used by the
Hoyt Electrodynamic Force Tether (HEFT) propulsion system6 built into the tether and used by
the TCS to control the tether system orbit and spin parameters.
High Strength Main Member: The two candidate polymers for the high tensile strength
main portion of the tether, Spectra™ and Zylon™, are both stronger per pound than either steel
or Kevlar™ polymer fiber. Their "characteristic velocity", defined as the maximum tip speed ofan untapered tether, is: VC=(2U/d)1/2, where U is the ultimate tensile strength of the material andd is its density. Spectra™ 2000 has a U=4.0 GPa, d=970 kg/m3 and VC=2872 m/s, while Zylon™
has a U=5.8 GPa, d=1560 kg/m3 and VC=2727 m/s. Both of these materials are commerciallyavailable in tonnage quantities with reasonable prices and delivery times. These polymer
materials are sensitive to attack by atomic oxygen (AO), however, so the portions that get near
the atmosphere would probably be coated with a proprietary AO-resistant resin coating available
from Aeroplas.

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Hypersonic Tip: The material for the hypersonic tip will not only have to withstand attackby atomic oxygen, but maintain a moderately high strength at high temperatures. We haveestimated that the tether temperature due to air drag heating will range from room temperature(300K or 27C) for a tether speed through the air of 3 km/s at 120 km altitude, up to as high as
2100K (1830C) for a relative speed of 5 km/s at 80 km altitude. The candidate materials andtheir ultimate tensile strength in GPa (gigapascals) as a function of temperature are summarized
in the Table 4. (For reference, 1 GPa = 109 N/m2 = 145,000 psi). To allow a relative comparisonof the suitability of the various different tether materials for use in various spinning tether
systems, we also included in Table 4 the density d and the room temperature “characteristic
velocity” VC=(2U/d)1/2 of the material, which is the maximum attainable tip speed of an
untapered spinning tether made solely of that material. For reference, the melting points of someof the materials in Table 3 are: Al–660C, Ti–1660C, Ni–1453C, W–3410C, Al2O3–2015C,
SiC–2700C, and SiO2–1610C.
Table 4 - Tether Material Tensile Strength (GPa) vs. Temperature
Material

VC

Density

20

300

600

800

1000

1200

(km/s)d (g/cc)

C

C

C

C

C

C

Spectra 2000

2.87

0.97

4.0

-

-

-

-

-

Zylon (PBO)

2.73

1.56

5.8

3.7

-

-

-

-

Quartz Glass (SiO2)

1.81

2.20

3.6

3.6

3.6

3.6

3.6

?

S-glass

1.94

2.50

4.7

?

?

?

?

?

Carbon

2.77

1.80

6.9

?

?

?

?

?

Carbon/Ni-coated

2.12

2.68

6.0

?

?

?

?

?

Tyranno (SiTiCO)

1.66

2.55

3.5

3.5

3.5

3.5

3.5

3.5

Textron β-SiC

2.19

2.93

7.0

6.6

6.0

5.6

5.2

4.5

0.72 β-SiC/Ti-coated

1.72

3.37

5.0

4.8

4.3

4.0

3.7

3.2

Altex (Al2O3/SiO2 )

1.21

3.30

2.4

2.4

2.4

2.4

2.4

1.5

Nextel (α-Al2O3)

1.30

3.88

3.3

?

?

?

?

?

0.65 Nextel/Al-coated

0.97

3.40

1.6

1.4

?

?

?

?

Tungsten Wire

0.55

19.35

2.9

2.9

2.9

2.9

2.9

2.9

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GRAPPLE AND PAYLOAD TRANSFER ISSUES8
In order for successful rendezvous, docking, and transfer of the payload to occur, some basic
functions must be performed by one or more of the HASTOL system architecture elements,
which include the grapple assembly. The following basic functions have been identified and thegrapple assembly design process must consider all of them:• Establishing a known absolute location for rendezvous, capture, and transfer and keeping
it updated• Establishing and updating relative position between the payload and the grapple assembly• Recognizing the defined rendezvous point• Closing the gap to the rendezvous point• Payload/grapple docking• Payload separation from the hypersonic vehicle• Retention of payload on grapple during transfer
Several grapple design requirements, drivers, and concerns have resulted from or areassociated with the identification of these functions. Rendezvous, docking, and payload transfer
will occur at around 100 km (330 kft) altitude in the presently planned HASTOL scenario. The
atmosphere is not very dense at that altitude. There will be significant heating, but not muchdynamic pressure despite the high velocities involved. The grapple assembly will therefore notneed to be streamlined to any great extent, although it will have to withstand significant heating
for its short duration in the atmosphere. The amount of heating will depend upon the exact
rendezvous altitude and speed, and the "height" of the upper atmosphere at the time of therendezvous. Later analysis will also show a clearer picture of the effects of thermal cycling due
to multiple atmospheric passages. It will also aid in future material specifications.
The tethered grapple assembly motion at the point of capture must be in-plane with the
payload. Control of either the grapple assembly or the payload (payload itself or the hypersonicvehicle) must be possible to insure a successful docking. Structural loading of the grapple

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assembly must be taken into account for rendezvous as well as for capture impact and transfer ofthe 14 Mg (14 metric tons or 30,000 lb) payload. No damage should occur to the payload.
A conservative assumption is that there will only be one capture attempt possible per
mission. As a result, maximizing the capture opportunity window is a grapple design objective.
This assumption has also resulted in a requirement for the capture to be as automated as possible;
a Go/No Go decision initiated prior to the actual capture by ground control or pilot, should there
be one, will be included in the grapple assembly design. It is assumed that abort modes will bedefined prior to each HASTOL mission for the specific client, though efforts are on-going to
identify abort modes that can be built into the system, for instance, establishing the bounds of an“attempt-to-transfer window,” and a “payload out-of-bounds” window.
Other grapple design issues deal with the “no damage” to client payload policy andcommunications issues. The payload's safety during transfer must be insured, which means
either the payload must not tumble during transfer, or it must be protected so limited tumbling
can be accounted for at no consequence. The potential of communication loss at each phase of
the payload transfer scenario must also be considered.
A preliminary conceptual CAD drawing of one
possible grapple assembly option for the tether is shownin Fig. 6. It features a circular “attach ring” at thebottom, which will mate with grapple hooks on the
payload. The attach ring is connected to the rest of theend mass via a six-degree-of-freedom, multiple-shockabsorber-strut suspension cradle. In the suspension
cradle, all of the members are designed to compress asnecessary, should the payload and grapple mechanism
contact at some non-zero speed or some slightly nontangential angle. The struts in the suspension cradle
will also provide shock-absorber type damping of theFig. 6 - Grapple AssemblyConcept Drawing

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heavier end mass cylindrical structure at the top, which contains a tether winch, batteries, thereaction control system and its propellant, and the command, control and guidance electronics.
The ring and the suspension leg elements would be made of materials selected to withstandheating from the hypersonic molecular flow at the rendezvous altitude. This eliminates any needfor an aerodynamic cone or shroud, which would increase the aerodynamic drag on the assemblycompared to the mostly empty strut structure presented to the hydrodynamic molecular flow.
The “ends up” cylinder of the upper portion of the grapple assembly is already aerodynamicallystable. Adding a forward-facing conical aerodynamic “shield” to it would not help appreciably.
The grapple assembly requires several internal functions to be successful for this kind of
mission, which make it similar to grapple assembly concepts developed earlier for exoatmospheric transfer of payloads10. As a result of the relatively high rendezvous altitude of
100 km (330 kft), adding aerodynamic “lift” surfaces on the grapple assembly will not beeffective in maneuvering the grapple toward a rendezvous with the hypersonic aircraft. The
cylindrical portion of the grapple assembly will have a tether winch will allow the grapple
assembly to “leave” its normal position at the end of the tether by letting out tether. The
centrifugal force from the rotation of the tether can be used to “cast” the grapple assembly intoward the hypersonic vehicle before the Tether Central Station arrives overhead. The grapple
assembly will have a reaction control system for fine control of its position, velocity, andorientation, but to minimize the problem of refueling of the grapple assembly, it will be up to thereaction control system on the hypersonic aircraft to remove most of the position and velocity
errors during the rendezvous process.
The current concept is to fly the decelerating grapple vehicle in so that it approaches thehypersonic airplane from above and behind (this holds true for all grapple assembly designs
considered). The attach ring would attach to the payload and pull the payload up to the grapple.
The hypersonic vehicle would then return to ground and the grapple assembly, with payload,
would be carried into space by the motion of the tether.
After the grapple assembly exits the atmosphere, the time spent in space will be used to cooland condition the grapple assembly, and recharge the batteries in preparation for the next
aeropass. When the grapple assembly is not going to be used to capture or deploy a payload for

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long periods of time, the tether will be shortened by either the grapple tether winch, or one of theother winches along the tether, to raise the minimum altitude of the tether tip and keep thegrapple assembly above most of the atmosphere.
The hypersonic grapple would not use externally mounted solar panel arrays during theaeropass due to the high aerodynamic forces and heating rates during this phase. Two options to
supply electrical power to the grapple assembly have been identified: a deployable/storable
photovoltaic array or an electrically conductive tether. Each would generate the required power,
the latter while moving through the earth’s magnetic field, and would store excess energy in thebatteries for use during the aeropass phase.
In order to allow for a reliable rendezvous, the grapple vehicle must maintain location andattitude information and communicate with the hypersonic airplane. This can be done accurately
with a differential GPS similar to those systems being developed for landing commercial aircraft.
The approach velocities are too high to rely on human pilots on the ground so the system will
require autonomous rendezvous and capture (AR&C) capabilities. AR&C technologies, such asadvanced sensors for the final approach and rendezvous, are continuing to evolve, and are
maturing based on Russian and NASA investments on docking technologies, and more recently,
DARPA investments in the ASTRO refueling vehicle concept.

CONCLUSIONS
We have described a number of alternate system configurations that will allow hypersonicair-breathing airplane technologies to be combined with orbiting spinning space tether
technologies to produce a method of moving payloads from the surface of the Earth into Earthorbit. The resultant Hypersonic Airplane Space Tether Orbital Launch (HASTOL) system iscompletely reusable and has the potential to drastically cut the cost of Earth-to-orbit space access.
The system is revolutionary in that it minimizes, and perhaps even eliminates, the use of rockets
for Earth-to-orbit launch of satellite payloads and even passengers.

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ACKNOWLEDGEMENTS
This work was supported by a contract with the NASA Institute for Advanced Concepts(NIAC), Dr. Robert Cassanova, Director. The authors also acknowledge technical contributions
from their fellow co-workers, Robert Hoyt of Tethers Unlimited, Inc.; Donald Johnson andJoseph Stemler of Boeing/MDC Phantom Works, St. Louis; Benjamin Donahue and BethFleming of Boeing/MDC, Huntsville; Brian Tillotson of Boeing S&CS, Seattle; and student
Daniel Stuart Bowman of the University of Maryland.

REFERENCES
1. Robert L. Forward, Thomas J. Bogar, Michal E. Bangham, and Mark J. Lewis,“Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System: Initial Study
Results,” IAF-99-S.6.05, presented at the 50th International Astronautical Congress,
Amsterdam, The Netherlands, 4-8 October 1999.
2. L. F. Scuderi and J. L. Hunt, “Mach 10 Dual-Role Vehicle Concept,” Space Technology
and Applications International Forum, Albuquerque, NM, 1-4 February 1999.
3. Robert Zubrin, “The Hypersonic Skyhook,” Journal of the British Interplanetary Society,Vol. 48, No. 3, pp. 123-128, March 1995.
4. Hans Moravec, “A Non-Synchronous Orbital Skyhook,” Journal of the Astronautical
Sciences, Vol. 25, No. 4, pp. 307-322, 1977.
5. J. Carroll, “Preliminary Design for a 1 km/s Tether Transport Facility,” NASA OAST
Third Annual Advanced Propulsion Workshop, JPL, Pasadena, CA, 30-31 Jan 1992.
6. Robert P. Hoyt and Chauncey Uphoff, “Cislunar Tether Transport System,” Paper AIAA99-2690, 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Los Angeles, CA,
20-24 June 1999.
7. Robert L. Forward and Gerry D. Nordley, “Mars-Earth Rapid Interplanetary Tether
Transport (MERITT) System: I. Initial Feasibility Analysis,” Paper AIAA-99-2151, 35th
AIAA/ASME/SAE/ ASEE Joint Propulsion Conference and Exhibit, Los Angeles, CA,
20-24 June 1999.

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8. Brian Tillotson, “Tether as Upper Stage for Launch to Orbit,” AIAA-89-1585-CP,
3rd International Conference on Tethers in Space, May 17-19, 1989.
9. Robert P. Hoyt and Robert L. Forward, “Failsafe Multiline Hoytether Lifetimes,” Paper
AIAA-95-2890, 31st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, San Diego,
CA, 10-12 July 1995.
10. M. E. Bangham, E. Lorenzini, and L. Vestal, “Tether Transportation System Study,”
NASA/TP-1998-206959, NASA Marshall Space Flight Center, Huntsville, AL,
March 1998.

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APPENDIX 3
DESIGN AND SIMULATION OF TETHER FACILITIES FOR THE
HYPERSONIC AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) ARCHITECTURE(UNPUBLISHED)

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DESIGN AND SIMULATION OF TETHER FACILITIES FOR THE
HYPERSONIC-AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) ARCHITECTURE
Rob Hoyt
Tethers Unlimited, Inc.

ABSTRACTIn this paper, we develop and evaluate designs for several tether concepts for the HASTOL
system, including a “Rotovator”, the “LIFTether” concept, and the “Cardiorotovator” concept.
Using numerical simulations of these tether systems, we examine the effects of hypersonicaerodynamic drag and heating on the tethers as they dip into the upper atmosphere, and study thetether load dynamics that result from capture of the payload. We also investigate the use oftether deployment to extend the rendezvous “window”. We find that a LIFTether that dips down
to 80 km altitude could, through proper use of aerodynamic drag and dynamical tether behavior,increase the ∆V capability of the tether relative to a simple Rotovator. However, of the threetether concepts, a Rotovator designed to pick payloads up from an altitude of 100 km is found tooffer the least system complexity and minimize the mass of the tether facilty. In addition, we
find that a simple tether deployment maneuver can extend the rendezvous window to facilitate
capture of the payload by the tether.
Introduction
The HASTOL architecture seeks to reduce by an order of magnitude the recurring cost oftransporting large payloads into Earth orbit by matching an air-breathing hypersonic airplanewith an orbiting tether platform to minimize the amount of fuel needed to deliver the payload
into orbit. In a HASTOL system, the hypersonic airplane would carry a payload up to an altitudeof 80-100 km at a speed of Mach 10-13. At its apogee, the airplane would meet up with the tipof a long rotating tether that swings down from a massive facility in Earth orbit. The airplane
would hand the payload off to a grapple vehicle at the tether tip, and the tether would pull the
payload up into orbit.

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The potential launch-cost savings of the HASTOL architecture would result from the large
reduction in ¨9WKDWPXVWEHSURYLGHGE\WKHODXQFKYHKLFOHDQGIURPWKHDELOLW\WRXVH
airbreathing engines for most of the ∆V it imparts to the payload. A conventional launch vehicle
would require a total ¨9RIRYHUNPVWRSODFHDSD\ORDGLQWR/(2,QWKH+$672/FRQFHSW
however, the launch vehicle only needs to provide a ∆V of about 3.5 km/s (Mach 12) to the
payload. If the hypersonic airplane takes off from near the equator, the Earth’s rotation will add
approximately 0.5 km/s to the velocity of the airplane. The remaining 3.5 km/s needed to deliver
the payload into orbit is provided by the rotating tether. Due to the exponential behavior of therocket equation, the reduction in launch vehicle ∆V from 7.5 to 3.5 km/s can result in a verylarge reduction in required propellant mass and launch vehicle size. This can result in a large
reduction in recurring launch costs.
In this work we examine the design of the tether component of the HASTOL architecture.
Several different concepts have been proposed for this tether, including a simple rotating tether innear-circular orbit, a “LIFTether”, which uses aerobraking to achieve rendezvous with the
payload, and a “CardioRotovator” in an elliptical orbit. We begin by describing the various
tether system designs. We then use a numerical simulation to model the operation of the first
two tethers in order to determine if the tethers can withstand the aerodynamic heating at thealtitudes of interest, and to determine if the tethers can sustain the dynamic loads that result frompayload capture. We will also use the simulation to determine if tether deployment maneuvers
can extend the window for rendezvous between the tether tip and the payload.
Tether Facility Designs
Each of the following tethers were designed to handle the 15 Mg payload that can belaunched by the Boeing DF-9 vehicle.
Rotovator
The first tether design studied was a simple rotating tether facility, also known as a
“Rotovator,” illustrated in Figure 1. The facility was composed of a central station (containing
power supplies, tether reel, command and control, and ballast mass), a 600 km long tapered
tether, and a grapple vehicle at the end of the tether. The tether facility was placed in a slightly
elliptical orbit with a perigee altitude of 611 km and a perigee velocity of 7.55 km/s. This was a

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Figure 1. Rendezvous of a rotating tether (“Rotovator”) and a hypersonic airplane.

slightly elliptical orbit, with an eccentricity of 0.0062. The orbit was chosen to be elliptical andpayload capture was performed at perigee in order to reduce the amount of total facility massneeded to keep the facility and tether above the atmosphere after it captures a payload. The tether
was set into rotation with a tip velocity of 3.4 km/s.
The tether sizing was calculated assuming it would be constructed of a material such as
Spectra 2000, with a tenacity of 4.0 GPa and a density of 970 kg/m3. Although in the final
implementation the tether would likely be a multiline structure to provide tether survivability, inthese simulations the tether was modeled as being a single-line structure, tapered to minimize thetether mass. The tether taper is illustrated in Figure 2. This tether was designed with a safety
factor (computed for nominal static loads) that varied along the length of the tether as shown in
Figure 3. Along most of the tether, the safety factor was chosen to be 2.0. At the tether tip,
however, the safety factor is increased in order to provide extra safety margin to handle transientloads due to payload capture. Because the portion of tether closer to the central station has amuch larger cross section, the transient loads due to payload capture are insignificant further up
the tether, and thus the safety factor of 2 should be adequate. The total tether mass is 1358 Mg,
or approximately 90 times the payload mass. The Station mass is 1650 Mg, or 110 times thepayload mass. The total Tether Facility mass is 3009 Mg, or just over 200 times the payload
mass. With these masses, the center of mass of the tether facility is located 89 km from theA3-4

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Tether Radius (cm)

2
1.6
1.2
0.8
0.4
0
100

0

200

300
Position (km)

400

500

600

Figure 2. Radius of a stepwise tapered Spectra 2000 tether designed to support
a 15 ton payload with a tip velocity of 3.42 km/s.

5

DesignSafety Factor

4
3
2
1
0
300
400
500
600
Position (km)Figure 3. Design safety factor profile for the Spectra 2000 tether shown in Figure 2(safety factor computed for nominal static loads). The safety factor is increased at thetether tip to provide protection against transient loads due to payload capture.
0

100

200

central station, so when the facility is at perigee, the station is at an altitude of 700 km and thetether tip is at an altitude of 100 km, moving at a velocity of approximately 4.1 km/s relative tothe inertial frame.
LIFTether
The LIFTether concept seeks to reduce the required tether mass by decreasing the nominal tip
velocity of the tether. In order to enable the tether to rendezvous with the payload, the velocity ofthe tether grapple vehicle and the section of tether nearest the tip is briefly increased just prior torendezvous through utilization of aerobraking, as illustrated in Figure 4.

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Figure 4. The LIFTether concept.

After the payload is captured, the tether pulls taught. Although the payload and the section oftether nearest the payload are moving at 3.5 km/s relative to the facility’s center of mass, the bulko the tether is rotating more slowly. Consequently, once the tether pulls taught, the tether isagain rotating at a lower tip velocity of approximately 3.1 km/s. This enables us to design thebulk of the tether to be sized for a 3.1 km/s tip velocity, which reduces the mass of the tether
considerably. The tether tip, however, must be designed with a higher safety factor to withstandthe capture transients, and furthermore it must be designed to survive the loads and heating due
to aerodynamic drag.
Figure 5 shows the tether taper for a design for a LIFTether. The bulk of the tether would bemade of a high-strength polymer such as Spectra 2000 or PBO, but the bottom 20 km of tether
would be constructed of Titanium-coated Silicon Carbide “TEXTRON” fiber, which has adensity of 3090 kg/m3 and a room-temperature tenacity of 4.7 GPa. The total tether mass is526.4 Mg, or approximately 35 times the payload mass. The Station mass is 1650 Mg, or 110
times the payload mass. The total Tether Facility mass is 2177 Mg, or 145 times the payload
mass.

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Tether Radius (cm)2

TEXTRON Segments

1.6
1.2
0.8
0.4
0
0

100

200

300
Position (km)

400

500

600

Figure 5. Stepwise tether taper for a LIFTether where the 20 km of tether nearest thegrapple vehicle is made of Textron SiC coated with Ti.

5

DesignSafety Factor

4
3
2
1
0
300
400
500
600
Position (km)Figure 6. Design safety factor profile for the Spectra 2000 tether shown in Figure 5(safety factor computed for nominal static loads). The safety factor is increased at thetether tip to provide protection against transient loads due to payload capture.
0

100

200

CardioRotovator
In the CardioRotovator concept proposedby Forward, a very long tether would be placed
into an elliptical orbit, rotating twice per orbit(relative to the inertial frame), as shown in
Figure 7. The rotation would be carefully
controlled so that the tether would be oriented
below the central facility when the tether is at
apogee, and oriented above the facility when
Figure 7. The Forward-CardioRotovator
Concept.

the tether is at perigee. This concept would

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have the advantage that, because the rendezvous between the tether tip would occur when thetether facility is at its apogee (and moving at its slowest speed relative to the Earth), the rotation
velocity of the tether could be approximately 0.4-0.5 km/s slower than the tip velocity of anequivalent rotating tether in circular orbit. Due to the exponent-of-the-square dependence of themass of a tapered tether on its tip velocity, this could significantly reduce the required tether
mass.
However, this concept has several problems that likely render it impractical. First, thepayload pickup occurs when the tether is at apogee. Unless the tether drops a return payload at
the same time as it picks up the outbound payload, this will result in a drop in the perigee
altitude of the tether facility. The mathematics of the orbital mechanics are such that the tether
facility would require a total mass on the order of 1000-2000 times the payload mass in order tokeep the tether facility from entering the atmosphere after a payload capture. Second, thisapproach would require that the tether rotation be very carefully controlled so that the tether isalways above the facility at perigee. When the tether catches a payload, conservation of angularmomentum will result in its angular velocity remaining constant, but its orbital period willchange due to its exchange of momentum with the payload. Consequently, the tether facility
would have to perform significant tether reeling maneuvers to maintain the propersynchronization between the tether rotation and its orbit. While this may be technically feasible,
any failure would result in the tether impacting the atmosphere, causing loss of the tether system.
For these reasons, we conclude that the CardioRotovator concept is less favorable than thesimpler Rotovator and LIFTether concepts, and it will not be analyzed further in this work.
Tether Facility Simulations
In order to examine and compare the feasibility of the Rotovator and LIFTether concepts forpicking payloads up from a hypersonic airplane, we have used the TetherSim program to model
these concepts during payload pickups from 100 and 80 km altitudes. The TetherSim program isa numerical simulation that includes models for orbital mechanics, tether dynamics, ionosphericdensity, geomagnetic fields, tether thermal behavior, and capture/release of payloads.1 In order1

Hoyt, R.P., “Cislunar System Dynamics Verification Through Simulation”, Appendix C in Cislunar Tether Transport
System, Tethers Unlimited, Inc. final report on NIAC Phase I contract NIAC-07600-011.

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to model the HASTOL concepts, the TetherSim code was extended to include models foratmospheric density, hypersonic aerodynamic drag on the tether, and aerodynamic heating of thetether. Figure 8 shows a screen capture of the TetherSim program simulating one of the
HASTOL concepts.

Figure 8. Screen capture of a TetherSim run.

Atmospheric Density Model
The model of the atmospheric density used was the NASA/GSFC MSISE 1990 model.2 Thismodel provides neutral densities and constituent densities across an altitude range of 0 to 1000
km. The model has the capability to calculate densities depending upon the date, time of day,and latitude and longitude data. In all of these simulations, we chose the date and time as midsummer, and early morning, local time.

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Hypersonic Drag and Heating Model
The drag and heat load on the tether was calculated using the MSISE atmospheric density
model and a hypersonic model developed by Stuart Bowman and Mark Lewis at U. Maryland.3
In calculating the drag and heating on the tether segments, the simulation assumed that theatmosphere is rotating with the Earth; this results in a relative velocity between the tether and theatmosphere that is approximately 0.5 km/s lower than would be calculated if one assumed thatthe atmosphere was motionless in the inertial frame.
Rotovator Simulation
The first scenario studied was a rendezvous between the all-Spectra 2000 Rotovator
illustrated in Figure 2 and a hypersonic airplane at an apogee altitude of 100 km and a velocity of
4 km/s (Mach 13). Figure 9 shows the altitude of the tether tip during the rendezvous period.
Figure 10 shows the velocity of the tether tip relative to the inertial frame and relative to thetether facility’s center of mass. Figure 11 shows the temperature of the bottom portion of thetether. During the several-hundred seconds the tether tip spends within the upper atmosphere,
the tether temperature increases only about 40°C. This temperature rise might be problematic forSpectra 2000, which loses strength rapidly with temperature. However, there exist several
commercially-available materials, such as PBO (sold by Tyobo of Japan under the nameZYLON), that have strength-to-weight characteristics almost as good as Spectra 2000 and havesignificantly better temperature tolerance. PBO is also approximately 1.7 times as dense as
Spectra, so a PBO tether would have a smaller diameter, and thus experience smaller drag andheating. Consequently, we conclude that the heat loading at 100 km is low enough that a tether
could be constructed of currently-available high-strength polymers (with some form of AOresistant coating) that could achieve this mission. Figure 12 shows the perigee altitude of thetether facility’s center of mass before and after the payload pick-up. Because the payload ismoving 3.5 km/s slower than the facility, the facility must transfer some of its orbital momentum
and energy to the payload, and thus the facility’s perigee drops by about 60 km. This perigee
drop could be reduced by using a larger ballast mass on the central station.

2

http://nssdc.gsfc.nasa.gov/space/model/atmos/msise.html. The GSFC web site provides FORTRAN code for this model. In
this work, the code was translated into C++ for compatibility with the TetherSim code.

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200
150
TetherTip
Altitude
(km)

100
50

0

675

700

725

750

775
Time (s)

800

825

850

825

850

Figure 9. Altitude of the tether tip.

5000

Tether Tip Velocity,
Inertial Frame

Velocity (m/s)

4000
3000

Tether Tip Velocity
w.r.t. COM

2000
1000
0

675

700

725

750

775
Time (s)800

Figure 10. Velocity of the tether tip relative to an inertial frame andrelative to the facility’s center of mass.

3

Bowman, S. and Lewis, M.

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100
80
Tether60
Tip
Temperature 40
(C)
20
0

675

700

725

750

775
Time (s)

800

825

850

Figure 11. Temperature of the tether tip.

600

COM Perigee
Altitude
(km)

480
360
240
120
0

0

100

200

300
Time (s)400

500Figure 12. Perigee altitude of the tether facility.

LIFTether Simulations
Case A: All Polymer Tether
The next simulation was of of a LIFTether design composed of Spectra 2000 picking up a
payload from a hypersonic airplane that reaches apogee at 80 km altitude with a velocity of 4.1
km/s (relative to an inertial frame). The tether taper and facility mass were identical to the
Rotovator tether design, and the orbital velocity of the tether facility’s center of mass wasapproximately 7.5 km/s. The simulation was initiated with the tether initially oriented parallel toits orbital velocity, rotating so that its tip velocity was approximately 3.0 km/s relative to its
center of mass. Figure 13 shows the altitude of the tether tip during the rendezvous.
As the tether dropped towards the local vertical, its tip velocity increased to approximately
3.2 km/s due to gravity gradient forces. As the grapple vehicle entered the upper atmosphere, it

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extended retractable aerobraking panels to increase its cross-sectional area to 16 square meters.
Figure 14 shows the velocity of the tether tip in the inertial frame and its velocity relative to thetether system’s center of mass during the period when the grapple vehicle is below 130 km
altitude. Examination of the trace of tether tip velocity with respect to the center of mass reveals
that the aerobraking succeeded in increasing the tip velocity an additional 0.3 km/s, giving it atotal velocity of approximately 3.5 km/s relative to the center of mass. Because the tether tip isrotating backwards relative to the center of mass, this gave it a total velocity in the inertial frameof 4 km/s.
200
160
TetherTip
Altitude
(km)

120
80
40
0

200

250

300

350

Time (s)Figure 13. Altitude of the tether tip. High-strength polymer LIFTether reaching down to 80 km.

5000

Tether Tip Velocity,
Inertial Frame

Velocity (m/s)

4000
3000

Tether Tip Velocity
w.r.t. COM

2000
1000
0

200

250

300

350

Time (s)Figure 14. Velocity of the tether tip relative to an inertial frame and relative to the facility’s centerof mass. High-strength polymer LIFTether reaching down to 80 km.

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Figure 15 shows the temperature of the bottom section of tether during the rendezvous. At
this low altitude, aerodynamic heating increases the tether’s temperature by almost 1000 K.
Since Spectra 2000 melts at approximately 180°C, Spectra clearly would not survive thismaneuver. Even PBO/Zylon, which can operate at temperatures over 600°C, would not suffice.
Consequently, for tether-airplane rendezvous at such low altitudes, the tether tip must be
constructed of a high strength material with higher temperature tolerance and higher heat
capacity.
1000

Tether Tip
Temperature (C)

800
600
400
200
0

200

250

300

350

Time (s)Figure 15. Temperature of the tether tip. High-strength polymer LIFTether
reaching down to 80 km.

Case B: High-Temperature Composite Tether Tip
The rendezvous at 80 km was simulated again, this time using the LIFTether design with thebottom 20 km of tether constructed of high-temperature tolerant Titanium-coated TEXTRONSCS-6 Silicon Carbide fiber. Figure 16 shows the altitude of the tether tip during therendezvous, and Figure 17 shows the velocity of the tether tip relative to the inertial frame andrelative to the tether facility’s center of mass. As in Case A, the trace of the tether tip velocity
relative to the center of mass shows that the aerobraking increases the tether tip velocity.
However, the ∆V achieved by the aerobraking is smaller than in Case A. This is because
although the radius of the bottom portion of the LIFTether shown in Figure 5 is roughly the sameas the bottom portion of the all-Spectra Rotovator shown in Figure 2, the TEXTRON material isover three times as dense as Spectra 2000, so the bottom 20 km of the LIFTether is 4 times as
massive as the bottom 20 km of the all-Spectra tether. Consequently, the aerobraking force isless effective at decelerating the heavier TEXTRON tether segments.

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150

TetherTip
Altitude
(km)

100

50

0

250

290

330

370
Time (s)410

450

Figure 16. Altitude of the tether tip. CAST/LIFTether with tip made of Ti-coated
β-Textron reaching down to 80 km.

5000

Tether Tip Velocity,
Inertial Frame

Velocity (m/s)

4000
3000

Tether Tip Velocity
w.r.t. COM

2000
1000
0

250

290

330

370
Time (s)410

450

Figure 17. Velocity of the tether tip relative to an inertial frame and relative to the facility’s centerof mass. LIFTether with tip made of Ti-coated β-Textron reaching down to 80 km.

Figure 18 shows the increase in temperature of the TEXTRON tip of the LIFTether. Using
this high-temperature composite, the temperature increase during the period the tether is in theupper atmosphere is only about 40°C. The large change relative to the all-Spectra tether is due tothe larger mass and higher heat capacity of the TEXTRON material. This temperature rise iswell within the capabilities of the TEXTRON material.
The load level on the section of tether nearest to the grapple vehicle is shown in Figure 19.
Immediately after payload capture, the tether load level increases to approximately 0.4 (safety
factor of 2.5), rises over half a minute to 0.5 (safety factor of 2.0), then slowly drops back down
to 0.4. Thus even despite the tether dynamics resulting from sudden loading of the tether as thepayload is captured, the tether remains above a safety factor of 2.0.

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100

Tether Tip
Temperature (C)

80
60
40
20
0

250

290

330

370
Time (s)410

450

Figure 18. Temperature of the tether tip. LIFTether with tip made of Ti-coated
β-Textron reaching down to 80 km.

Load Level at Tether Tip(Load/Capacity)

1
0.8
0.6
0.4
0.2
0

250

290

330

370
Time (s)410

450

Figure 19. Load level at the tether tip before and after tether capture. LIFTether with tip made of
Ti-coated β-Textron reaching down to 80 km.

Shows the perigee altitude of the tether facility’s center of mass before and after the payload
capture. Because the payload is moving approximately 3.5 km/s slower than the tether facility,
when the tether captures the payload, the tether facility transfers some of its orbital momentum
and energy to the payload. As a result, the tether facility’s perigee drops from 616 km to 506 km.
This large change in the orbit results from the fact that the LIFTether system modeled had a total
mass of only 145 times the payload mass. To keep the tether from falling too deep into theatmosphere, this facility would either have to ensure that the tether is oriented above the facility
when it is at perigee or reel in about 100 km of tether within half an orbit. As both of theserequirements would likely be difficult to achieve, a better solution would be to increase the total
mass of the tether facility so that its orbit is not so strongly perturbed by the payload capture.
Thus, although the LIFTether concept can reduce the required tether mass, the total system mass
would likely have to be equal to the mass of a Rotovator system due to purely orbital mechanics
considerations.

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750
600
TetherTip
Altitude
(km)

450
300
150
0

250

290

330

370
Time (s)410

450

Figure 20. Perigee altitude of the tether facility’s center of mass. LIFTether with tip made of
Ti-coated β-Textron reaching down to 80 km.

Rendezvous Window
In any rotating tether transport system, one of the most challenging tasks will be to enable therendezvous between the payload and the tether tip. For the tether to successfully capture thepayload, the payload and tether grapple vehicle must meet with the same position and the same
velocity. Because the payload is in free fall, and the tether is rotating, the payload and grapple2

vehicle will experience a relative acceleration equal to = Vtip /L, where Vtip is the velocity of thetether tip relative to the tether facility’s venter of mass, and L is the distance from the tether tip tothe center of mass. In the HASTOL tether designs described above, Vtip is approximately 3.5
km/s, and L is approximately 500 km, so this acceleration is about 2.5 gees. If neither grapple
nor payload perform any maneuvering, the two will coincide only instantaneously, which is arather small rendezvous window.
Fortunately, it may be possible to extend this rendezvous window to a period of several
seconds or more by using tether deployment from the grapple vehicle. In this approach, thegrapple vehicle will contain a tether deployer and a tether brake. Prior to the rendezvous, thegrapple vehicle will wind up some of the tether into the deployer. As the tether nears the bottomof its swing, the payload will use its guidance and thrusters to adjust its trajectory so that it willmeet up with the grapple vehicle. When the payload and grapple vehicle reach their closest
approach to each other, the grapple vehicle immediately releases the brake on the tether deployer
and allows the tether to deploy at as low a tension as possible. This will put the grapple vehicle
into an almost-free fall trajectory which will match the trajectory of the payload, as illustrated in

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Figure 21. The payload can then maneuver to close the gap and secure itself to the grapple
vehicle. The length of the rendezvous window will therefore be determined by the amount oftether stored in the deployer, with the maximum window equal to∆t = √(2l/a).

Figure 21. Schematic of tethered-grapple method for increasing docking window.

Using TetherSim, we have investigated this maneuver for a HASTOL system in which theSpectra-2000 tether illustrated in Figure 2 picks a payload up from a 100 km, 4 km/s apogee. In
this simulation, the payload was launched into a trajectory that would meet up with the tether tip.
Once they came into close proximity, the grapple vehicle released the tether brake and allowed
tether to pay out at very low tension for five seconds. At that point, the grapple vehicle captured
the payload and halted the tether deployment. Figure 22 shows the relative separation between
the payload and tether tip in the x and y directions. This plot shows that the tether deployment
maneuver extends the rendezvous window to several seconds. The length of tether deployed inthis time was 486 meters. Figure 23 shows the tether load level at the grapple vehicle. During
the tether deployment, the tension is essentially zero. When the grapple vehicle stops deploying
tether, however, it experiences a relatively strong transient tension spike up to about 70% of

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100
Grapple Vehicle
Begins Deploying Tether

Separation (m)80Grapple Vehicle
Captures Payload
andApplies Tether Brake

60
40

²X
²Y

20
0
-20
250

255

Time (s)260265Figure 22. Relative separation of grapple vehicle and payload, with a tether deployment
maneuver to extend rendezvous window.

Load Level at Tether Tip(Load/Capacity)

1

Grapple Vehicle
Begins Deploying
Tether0.8
0.6

Grapple Vehicle
Captures Payload
andApplies Tether Brake

0.4
0.2
0

250

260

270

280
Time (s)290300

Figure 23. Load level on bottom segment of tether, with a tether deployment
maneuver to extend rendezvous window.

capacity, followed by a longer period transient that peaks at about 80%. These higher tension
transients result from the fact that the deployment maneuver allows the payload and grapple toaccelerate away from the tether facility for several seconds, and thus the tether must apply alarger force to them to accelerate them into the tether rotation once the deployment is halted.
This result indicates that the portion of the tether near the tether tip should be designed with aneven higher safety factor to provide more margin for these tension transients.

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CONCLUSIONS
We have developed analytical designs of three tether facility concepts for the HASTOL
system. The CardioRotovator concept was eliminated due to the high facility masses required tokeep it in orbit and the complexity of maintaining a proper synchronization between its rotationand orbit. A Rotovator tether facility designed to pick payloads up from a 100 km, 4 km/s
apogee would require a total mass of approximately 200 times the payload mass, with the tether
massing about 90 times the payload. Simulations of the Rotovator and a LIFTether designed topick payloads up from a 80 km, 4.1 km/s apogee indicate that utilization of aerodynamic dragmight enable a LIFTether design to reduce the amount of tether mass required relative to the
Rotovator. However, the primary mass driver for the system is the amount of total facility massneeded to keep the station and tether from deorbiting after catching a payload. Thus, although a
LIFTether could minimize the tether mass, it does not significantly reduce the total system mass.
Moreover, since the density of the upper atmosphere varies significantly with solar conditions
and other phenomena, accurately predicting and controlling a LIFTether would likely prove to berather difficult. Consequently, we conclude that the most viable tether concept for the HASTOL
system is a rotating tether designed to pick payloads up from as high an altitude as the hypersonic
airplane can reach. In addition, we investigated the use of tether deployment to increase thewindow of opportunity for rendezvous between the payload and tether tip, and found that therendezvous window can be extended to a period of several seconds or more, depending on thelength of tether that can be deployed. This maneuver will, however, result in larger tension
transients, and thus will require higher safety factors for the portions of tether nearest to thegrapple vehicle.

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APPENDIX 4
HYPERSONIC AIRPLANE SPACE TETHER ORBITAL LAUNCH(HASTOL) SYSTEM - NIAC FELLOWS MEETING

9 NOVEMBER 1999

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Discussion Topics

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CardioRotovator Concept

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Tillotson Two-Tier Tether (T4) Concept

Rotovator Tether Mass Ratios
TetherOrbitalOrbitalTip

Hypersonic Airplane

Tether to Payload

LengthRadiusVelocity

Velocity

Velocity

Mass Ratio

L

RO

VO

VT

VH= VO-VT-470 m/s

MT/MP

(km)

(km)

(m/s)

(m/s)

(m/s)

Mach

Spectra
™

2x

10x

400

6878

7614

2494

4650

15.0

10.4

2.4

0.37

500

6978

7559

2749

4340

14.0

16.7

4.2

0.56

600

7078

7506

3006

4030

13.0

27.1

5.9

0.65

700

7178

7453

3263

3720

12.0

44.0

8.2

0.73

800

7278

7402

3522

3410

11.0

71.8

11.6

0.90

900

7378

7352

3782

3100

10.0

117.6

16.3

1.07

CardioRotovator Tether Mass Ratios
TetherOrbitalOrbitalTip

Tip

LengthRadius Velocity Velocity Accel.

Hypersonic Airplane

Tether to Payload

Velocity

Mass Ratio

VH= VO-VT-470 m/s

MT/MP

L

RO

VO

VT

a

(km)

(km)

(m/s)

(m/s)

(m/s )

2

(m/s)

Mach

Spectra

2x

10x

1000
1200
1400
1600
1800

7478
7678
7878
8078
8278

7147
7004
6868
6737
6611

2076
2440
2789
3124
3445

0.43
0.50
0.56
0.61
0.66

4601
4094
3608
3143
2695

14.8
13.2
11.6
10.1
8.7

10.8
22.2
44.7
87.8
168.5

3.1
5.2
8.4
13.4
21.0

0.39
0.55
0.75
0.97
1.24

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High-strength Electrodynamic Force Tether (HEFT) Facility

Dual-Fuel DF-9 Dual Role Vehicle
Tank 1 (SH2)

PayloadTank 4 (JP-7)
LOX (Below)

MLG

Looking Aft
Highly Integrated
2-D Inlet
AceTRs(Internal)

Tank 2 (SH2)
Tank 3 (SH2)
Crew Station

Tank 5L(SH2)
Tank 6 (SH2)

Tank 7(SH2)
Linear Rocket

PayloadRamjet/Scramjet
Highly Integrated
SERN

NLG

Air Core Enhanced
Turboramjets (AceTR)
Ramjet/Scramjet

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Dual-Fuel DF-9 Dual Role Vehicle

Dual-Fuel DF-9 Performance

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Matrix of Payload Transfer Points Analyzed
400,000
120

ft

❍

❍

km

● Achievable Points

110❍ Unachievable Points❍

Altitude350,000
❍

100

●

❍

●

300,000
90

80

●

●

250,000
8,000
2,500

9,000

10,0003,000●●

11,00012,000
3,500

ft/s
m/s

Velocity

Normal Load Factor Along Descent Trajectory

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Dynamic Pressure Along Descent Trajectory

Hoytether Failsafe Tether Design

Primary
Lines

First Level ofSecondary
Lines
Redistributes
Load toAdjacent
Nodes

SecondaryLines(initiallyunstressed)

Effects ofDamageLocalized
Second Levelof Secondary
Lines
Redistributes
Load Back to
Undamaged
Portion ofPrimary Line

SeveredPrimaryLine

0.2 to10’s ofmeters0.1- 1 meter

a.

b.

c.

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High Temperature Tether Materials
Tensile Strength (Gpa) vs. Temperature
VC

Density

20

300

600

800

1000

1200

(km/s)d (g/cc)

C

C

C

C

C

C

Spectra 2000

2.87

0.97

4.0

-

-

-

-

-

Quartz Glass (SiO2)

1.81

2.20

3.6

3.6

3.6

3.6

3.6

?

S-glass

1.94

2.50

4.7

?

?

?

?

?

Carbon

2.77

1.80

6.9

?

?

?

?

?

Carbon/Ni-coated

2.12

2.68

6.0

?

?

?

?

?

Tyranno (SiTiCO)

1.66

2.55

3.5

3.5

3.5

3.5

3.5

3.5

SiC

2.19

2.93

7.0

6.6

6.0

5.6

5.2

4.5

SiC/Ti-coated

1.72

3.37

5.0

4.8

4.3

4.0

3.7

3.2

Altex (Al2O3/SiO2 )

1.21

3.30

2.4

2.4

2.4

2.4

2.4

1.5

Nextel ( $O2O3)

1.30

3.88

3.3

?

?

?

?

?

0.65 Nextel/Al-coated

0.97

3.40

1.6

1.4

?

?

?

?

Tungsten Wire

0.55

19.35

2.9

2.9

2.9

2.9

2.9

2.9

Material

Textron
0.72

HASTOL Grapple Assembly

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Grapple to Payload Attachment Option

Conclusions

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APPENDIX 5
MODULAR SPACE TETHER DESIGN AND CONSTRUCTION

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MODULAR SPACE TETHER DESIGN AND CONSTRUCTION
Robert L. Forward
Email: forward@tethers.com
Phone & Fax: +1-360-579-1340
Tethers Unlimited, Inc.
8114 Pebble Court, Clinton, WA 98236<http://www.tethers.com>
The space tether facility portion of the HASTOL system will need to supply half or more of
the energy and angular momentum needed to put the payload into orbit. In order to keep the
payload from dragging the space tether facility down into the atmosphere after pickup, the facility
must mass significantly more than the payload. A minimum facility mass would be 30 times thepayload mass, while a robust system would mass 50 to 200 times the payload mass. Because theultimate full-up space tether facility will necessarily be massive because we want it to handle
payloads of 15 Mg or more and the laws of physics requires that it mass 30-200 times more thanthe payloads, it cannot easily be launched in one piece. We therefore will design it in a modular
fashion so that it can be launched in many separate modules. The basic modules have yet to be
fixed, but probably would consist of a single large command and control module, one or two
grapple modules, depending upon whether the tether is single- or double-ended, and a large
number of power modules, winch modules, and tether modules. The details of the modular
design of the space tether facility, the design of the individual modules, and the method ofassembling the separate modules to construct the first functional minimal space tether facility,
and then “grow” it into the ultimate full-up space tether facility, will be carried further in the
Phase II effort. The following description should therefore be considered as merely one“example” of what the modules would look like and how they would be interconnected into aspace tether facility.
The modules would be cylinders between one and two meters in diameter and height. A
small 1-m (3-ft) module would mass about 1 Mg and be 1/15th of the nominal payload mass of
15 Mg, while the largest 2-m (6-ft) module would mass about 6-8 Mg and be half the mass of thenominal payload mass. The modules will be designed so that only a few modules are needed toassemble a minimal but functional initial space tether facility consisting of the large command

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and control module, a few power modules, a few winch modules, a number of tether modules,
and a grapple module. These would form an initial space tether facility which would not only
service customers and start bringing in a revenue stream, but would be used to haul up additional
modules and add them to itself to increase its length, width, taper, power, and lift capability.
The command and control module would probably be put into place first by a heavy lift
rocket. Studies may show that it is more cost effective to put the other modules into orbit byrocket, but if the hypersonic airplane is available, we can start using the initial pieces of the
HASTOL system to begin to assemble itself. The power modules, winch modules, grapple
modules, and especially the tether modules, can be flown to Mach 12 (3.7 km/s) at 100 km
altitude, then boosted on into orbit by a rocket upper stage to a rendezvous with the commandand control module. There, they would be automatically assembled (with the aid of remotecontrol guidance), into a minimal, but functional, initial space tether facility. Assuming themodules mass 1 Mg, the initial facility need only mass 50 Mg before it becomes capable ofhandling one module at a time at the full design tether tip speed of 3.4 km/s. This now
eliminates the need for using upper stage rockets to boost the modules from the hypersonic
airplane up to the space tether facility, and the initial HASTOL architecture has been “born”.
This “infant” HASTOL now has the ability to “grow” by “feeding itself” space tether facility
additional modules brought up by the hypersonic airplane. (The analogy of “feeding itself” is an
apt one, in that Oldson and Carroll1 have shown that it is possible for a rotating tether system to“toss” a payload from its grapple into a trajectory that ends up an orbit later with the payload
coming to a gentle “dock” with the tether control station, just like tossing a peanut into yourmouth.)
Once functional, that same initial HASTOL architecture can also make money by using its
hypersonic airplane to deliver 1 Mg communication satellites and deep space probes to the spacetether facility, which in turn delivers them to higher orbit or Earth escape. The HASTOL
architecture will thus be “in business” and “producing income” from almost its first day ofoperations. After doubling its size with 50 more “bites” of power, winch, and tether modules, itwill be able to handle 2 Mg payloads, “grow” itself even faster, and make even more money byhandling bigger and bigger payloads.

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The tether control station would consist of the command and control module, power modules,
and winch modules. The power modules would consist of unfolding solar power panels,
standard power converters to supply power to the command and control module, and high power
converters that supply electrical current to the winches that drive the tether length pumping
propellantless propulsion system and the conducting portions of the tether that drive theelectrodynamic tether propellantless propulsion system. A key aspect of the power module
design would be to insure that the stacked power modules produced a cooperative array of solar
power panels that could follow the Sun with minimal shading of one panel by another as thetether orbits and spins.
The tether portion would consist of a large number of identical tether modules. Each tether
module would consist of a standardized deploy-only canister module about a meter in diameterand height with unfolding 50 cm spreader struts to spread out the tether, and unfolding 2 mtriangular separator trusses with interconnection fittings to interconnect the module canisters into
a “frame” many meters across. These separation frames would be spaced every 20 km along thetether, keeping the tether lines spread out from each other. Inside each canister would be would a20-km-long, 24 primary line Hoytube™ type Hoytether™, which, after deployment, would bespread out by the spreader struts into a 1-m-diameter by 20 km long sparse net tube. Six or more
Hoytether canisters would be interconnected by their built-in separator trusses into an open
hexagonal array many meters across. In advanced versions, the arrangement of Hoytethers
between the “frames” formed by the separator trusses each 20 km along the tether, will itself
follow the failsafe Hoytether design philosophy, with some of the Hoytethers being straight“primary” lines that carry the load, while other Hoytethers are slack diagonal “secondary” backup
lines.
Being modular in design, the tether can be beefed up in width and taper as needed by simply
adding tether modules between the periodically-placed frames. The final full space tether
structure would have the spacing between the outer Hoytethers be 20 m or more, in order toinsure that the structure would survive even a strike by a full-sized spacecraft. Except by strikes
by full-sized spacecraft, such a tether structure would never need repair during its 100-year
operational lifetime, so there would be no need for “repair robots”.

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In exploring the optimal means to assemble such a modular structure, we will investigate
methods of coupling the tethers to the frames using the same grapple mechanism design as isused at the tip, with a semi-intelligent grapple at each end of the deploy-only canister containing
the Hoytether, so additional tether modules could self-attach to the frames (with some remotecontrol help from the ground), after having been hauled into rough position by a “crane” winch
module running between frames.
Near the tip, the design would change from deploy-only tether modules to specialized winch
modules, which would have, at one or both ends, a built-in winch box with 20 km of thick-line
two primary Hoytapes or three primary Hoytubes that it can wind in and out rapidly. These rapid
winching 2-3 primary line Hoytether designs would have failsafe lifetimes measured in years ascompared to centuries for the 24 primary line Hoytethers, so as the winch winds the tether in, it
would check the lines in the Hoytether, and if it notices weakness or breaks, it would signal
ground control, and a new winch and grapple, with a new 20 km of Hoytether would come up,
and replace the winch and grapple with the damaged line. There would be perhaps up to 12
winch layers, each capable of reeling in 20 km of tether at some speed of x meters per second.
The whole system can then reel in at a rate of 12x m/s. The last grapple module, which would
connect to and pick up the payload, would be a single large custom winch and grapple
mechanism, with either a very thick single cable or perhaps a two-primary one-secondary thickline half-Hoytape 20 km long. The line material would be chosen for strength at hightemperature and the ability to withstand weakening by atomic oxygen. The grapple winch would
also check the line(s) on its tether after each use and the whole grapple module would be
replaced if any damage is found.
All the winches would have sufficient battery capacity to carry out a wind-in of the tether
against the centrifugal load, but the winch drives should be motor-generators so that upon lettingout the tether, most of the energy gained from lowering the load mass in the centrifugal force
field of the spinning tether is converted into electricity and put back into the batteries. The
makeup energy would come from an electrodynamic tether generator build into the non-windup
portion of the tether.

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The use of modular design in the tether portion of the space tether facility eliminates any
concern about the manufacturability and packaging of a 600 km long tether. Since thecontinuous tether lengths required are only 20 km in length, they can each be fabricated in abouta month or so using standard braiding machine speeds. As a result, any delivery time greaterthan a month can be met by simply using more braiding machines in parallel. A braided
Hoytether has a unique structure, with typically 24 “primary” lines running along the length of
the tether and 48 “secondary” lines spiraling around the tether, 24 clockwise and 24
counterclockwise. Each line is braided from 8 strands. The secondary lines are interconnected tothe primary lines where they cross by interbraiding the primary line strands along with thesecondary line strands for an interval, before taking the secondary lines out to cross over to theadjacent primary line. This interbraiding produces a slipless interconnection without using knots
or producing high curvature in the strands that would cause stress buildup. This produces a line
interconnection where the tensile strength of the strands in the connected region is close to thatelsewhere on the strand. Standard braiding machines, which are essentially “Maypole” braiding
machines, cannot produce the Hoytether structure, but a simple modification will allow an arrayof standard braiding machines to fabricate the structure. The Hoytether fabrication array would
consist of a braiding machine for each primary and secondary line in the Hoytether, arranged in acircle, 24 “primary line” machines and 48 “secondary line” machines in pairs between each“primary line” machine. Each machine would have 8 bobbins, one for each strand. The standard
braiding machines would be modified so that they can pass bobbins back and forth between
adjacent braiding machines. The resulting array would have “primary line” machines thatinterbraided the same 8 bobbins most of the time, but periodically would trade bobbins with thefour “secondary line” machines adjacent to it to form the braided interconnections. The“secondary line” machines would braid 8 bobbins for a while, until the interconnection region
was reached, and after the interconnection was made, would end up with 8 different bobbinsfrom the “secondary line” machine on the other side of the “primary line” machine, resulting inspiraling clockwise and counterclockwise secondary lines.
Although such an array of simple braiding machines has never been assembled, ourfabrication vendor, Flemings Textiles Ltd of Kilmarnock, Scotland, thinks it would be astraightforward process to assemble one, given the funding to design the machine modifications

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and build the prototype. Flemings Textiles is a major textile fabricator that specializes in
producing custom industrial fabrics rather than the lower-margin clothing and household goods
fabrics. They, in turn, are owned by Scott & Fyfe, Ltd. of Tayport, Scotland, a much larger
company which has the financial resources and production space to handle very large production
jobs. One of their recent contracts was to produce Velcro™-backed custom carpeting for one of
the Boeing airplanes. Since we see no particular need to be concerned about the feasibility oftether production, these foreign companies will not be involved in the Phase II effort.
1. J. Oldson and J.A. Carroll, “Potential Launch Cost Savings of a Tether Transport
Facility”, AIAA paper 95-2895, 31st AIAA/ASME/SAE/ASEE Joint Propulsion
Conference and Exhibit, July 1995.

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