INTRODUCTION

This study of Reentry Vehicle (RV) systems and their associated

operations was conducted for the Department of

Transportation/Office of Commercial Space Transportation. The

purpose of the study was to investigate and present an overview

of reentry vehicle systems and to identify differences in mission

requirements and operations. This includes reentry vehicle

system background, system design considerations, description of

past/present/future reentry systems, and hazards associated with

reentry vehicles that attain orbit, reenter, and are recovered.

A general literature search that included the OCST data base,

NASA, Air Force, and other technical libraries and personal

contact with various government or private industry organizations

knowledgeable in reentry system vehicles was performed. A

reference page is provided at the end of this report. A history

of early manned reentry vehicle launches is shown in Appendix I. 

A listing of some of the agencies and companies found to be most

knowledgeable in the reentry vehicle area is provided in Appendix

II. The following sections provide more detailed information on

reentry system vehicles.

A. Background - The development of reentry vehicles began in the

late 1950's due to the need for Department of Defense and Central

Intelligence Agency photo reconnaissance of Soviet ICBM sites. 

NASA has also been involved in the use of reentry vehicles since

the early 1960's, including manned space programs Mercury, Gemini

and Apollo. The following sections describe the evolution of

reentry system development in the United States and foreign

countries:

1. Discoverer1 - The Discoverer program was of major

importance because it provided a vehicle for testing orbital

maneuvering capability and reentry techniques and it played

a large role in enabling the first United States manned

space flights to be conducted in Project Mercury. This

program also advanced technology required for the

development of the surveillance satellites used later by the

Department of Defense.
Between 1959 and 1962, there were 38 launches conducted at

Vandenberg Air Force Base by the US Air Force. The final

three missions all had similar ejection capsules and were

typical of many other missions. After 1962, this type of

work was classified. The purpose of these missions was to

provide military space research, develop reentry capsule

recovery techniques, and perform biological research.

The Discoverer was built by General Electric and launched

aboard a modified Thor Intermediate Range Ballistic Missile

(IRBM). It was boosted into orbit by a second stage Agena A

or B rocket. Once there, gas-jet thrusters oriented the

vehicle so that the reentry capsule could deorbit. After

the orbital portion of the mission was completed, ejection

of the capsule from the nosecone was accomplished and

retrorockets were fired to initiate reentry. After reentry

into the atmosphere, and while descending over the recovery

site, a parachute pulled the capsule away from its

heatshield, which fell into the Pacific Ocean. See Figures 1

and 22. The recovery capsule was then either retrieved by

ship after impact in the ocean or, in several cases, airsnatched while descending by C-119 transport aircraft.

The final three Discoverer missions all had similar ejection

capsules and were typical of many other missions. The

orbiting vehicle, which included the Agena second stage and

reentry capsule, was 7.62 meters long and 152.4 cm in

diameter. The bowl-shaped capsule was 84 cm in diameter,

68.6 cm in depth, and weighed approximately 227 kg.

The Discoverer program, up to the final unclassified mission

on February 27, 1962, resulted in 38 launches attempted, 26

spacecraft orbited, 23 attempted capsule recoveries, 8

successful air recoveries, and 4 successful sea recoveries. 

Two of the Discoverer missions did not contain reentry

capsules. On November 22, 1961, the Department of Defense

mandated that all military spacecraft launches would be

classified. The Tables of Earth Satellites, compiled by

Britain's Royal Aircraft Establishment, shows that 27

capsules were placed into orbit between 1963 and 1971,

designated only as the vehicles on which they were launched

(Atlas-Agena's B and D, Thor-Agena & Titan-Agena). Table 1

shows a recap of past Discoverer missions between 1959 and

19623.

Figure 2 is currently unavailable
2. Biosatellite1,4 - This late 1960's NASA program, directed

by Ames Research Center and built by General Electric, was

intended to study the prolonged effects of weightlessness

and radioactivity on living organisms. The agency planned

six Biosatellite flights: the first two with plants and

organisms, the second two with monkeys, and the third pair

with rodents. The program was suspended after the third

flight.

The Biosatellite consisted of a conical reentry vehicle,

which contained the recovery capsule, and a cylindrical

adaptor section. The entire configuration was approximately

206 cm long and the adaptor was 145 cm in diameter at its

base. See Figure 32. Depending on the experiment weight,

the capsule weight varied from 431 to 522 kg. The reentry

vehicle consisted of a bowl-shaped fibrous glass heat shield

101.6 cm in diameter. It contained the thrust cone

assembly, the recovery capsule, tracking equipment, and the

parachute assembly. The recovery capsule, which was mounted

inside the heat shield, was a blunt aluminum cone about 79

cm in diameter with 16,987 cu cm of payload space. See

Figure 42. Prior to reentry, position and attitude control

were provided by high-pressure nitrogen gas, three motionsensing gyros, and six cold-gas thruster jets. These

components and the retrorocket, which initiated reentry,

were located in the thrust cone assembly. The payload was

launched aboard a Thrust-Augmented Delta N (TAD) on its

first two missions and the Long Tank Thrust-Augmented Delta

(LTTA-Delta) for its final mission.

Bios 1 was launched from Cape Canaveral Air Force Station

(CCAFS) on December 14, 1966 on a TAD. A retrorocket

failure left the recovery capsule stranded in its 302 x 326

km orbit where it eventually decayed and reentered on

February 15, 1967, in the Australian area. Electronic and

visual searches of the Australian land area were made;

however, the capsule was not recovered. The most likely

impact point was estimated by the recovery personnel to be

on the eastern half of the continent or in the Tasman sea.



The second flight was more successful though not all mission

objectives were accomplished. The capsule, containing 13

plant and animal specimen experiments that would be

subjected to doses of radiation, was launched on September

7, 1967 aboard a TAD from CCAFS and was scheduled to orbit

for three days. The mission was cut short with a successful

recovery at 45 hours due to communication problems and the

threat of severe weather in the recovery area.

Bios 3, with a pig-tailed monkey on board, was launched on

June 28, 1969 on a LTTA-Delta from CCAFS. The monkey was

attached to 24 sensors to study the effects of

weightlessness on various bodily functions. He had been

trained to feed himself by pushing various buttons, but once

in orbit he became sluggish and refused to eat. NASA

aborted the scheduled 30-day mission after nine days. The

capsule containing the primate was successfully recovered.

3. DOD Satellite Film Canister Reentry Systems1 - Beginning

in the early 1960's the Air Force and the CIA began using

reentry capsules to retrieve reconnaissance film from

satellites. The project was conducted under the code-name

"Keyhole" and successive satellite generations were given

"KH" numbers that are still in use.

The early programs, Close Look and Area Survey, ran

concurrently from 1962 until 1984 with over 250 missions to

their credit. These programs were slowly phased out and

used as fillers with the advent of Big Bird in 1972. Big

Bird (KH9 Hexagon) was also known as LASP (Low Altitude

Surveillance Platform) and provided US intelligence with a

multitude of data. The final Big Bird was destroyed in the

Titan failure at Vandenberg AFB in 1986.

The LASP had a mass of over 11,000 kg in orbit and was 15.2

meters long and 3 meters in diameter. It deployed two large

solar arrays and a six meter antenna and carried four to six

pods used to return exposed film. The Titan 3D was specially

developed as the launcher. The earlier Area Survey and LASP

satellites had both radio transmitting (for area survey

pictures) and reentry pod capability (for close-look

capability). As technology has advanced, imaging systems

have developed to an extent that they can perform the closelook function, eliminating the need for film returns.

4. Current US and Foreign Reentry Vehicle

Concepts/Programs5 - US and foreign entities are increasing

research and development of new reentry systems. Stanford

University researchers have identified reentry capsule

services as one of the fastest growing international

aerospace markets in the 1990's. Estimates of the US market

alone yield an average yearly growth of nearly 50% during

the next five years, with 4-5 flights/year projected by
1995. Equal growth is anticipated in Europe and Japan. Most

of the current reentry systems being researched and

developed (domestic and foreign) are using technology from

the 1960's. Though heat shield materials and spacecraft

subsystems have been improved, the basic design remains the

same. Most of the current and proposed capsules are

ballistic in design to simplify mission operations.

a. Germany - Germany has vigorously pursued reentry capsule

studies of 10-15 separate concepts during 1986-1990. 

Presently, the EXPRESS (EXPeriment REturn Service from

Space) is under study. German experiments have flown on

Chinese and Soviet capsules and are planned for additional

flights over the next two years. Recent German studies have

tended to larger capsules (over 800 kg) to accommodate a

wide range of microgravity payloads. However, German

industry and space agency management appear close to

reaching a consensus that an initial capsule should employ

smaller Western launchers in early missions.

b. Italy - Italian interest in reentry systems dates back

to 1987-1988 and has focussed on small capsule options for

available Western ELV's. Technical efforts by Aeritalia and

support from the Italian Space Agency (ASI) are enabling

Italy to develop technology for reentry systems. ASI

support for reentry system technology has resulted in an

extensive definition and preliminary design study of the

Carina capsule.

c. France - France has not pursued reentry capsules as

aggressively as Germany and Italy. Preoccupation with the

major European programs (Hermes, Ariane 5), less emphasis on

microgravity science within the French space effort and a

solid domestic military reentry technology base have all

contributed to placing a lower priority on reentry capsules. 

Instead, concepts proposed have been driven by a desire to

fill secondary payload capacity on Ariane boosters. France

is currently working with Germany to develop a large Apollotype capsule (1,812 - 2,600 kg) for Ariane 4/5 and has

completed a concept definition study. The Microliner, a

Discoverer-type capsule flown to geosynchronous orbit, has

also been proposed as an Ariane 4 secondary payload.

d. Great Britain - The British have a strong reentry

technology base achieved during 1960-1980 through the

development and operation of strategic weapons systems. 

During 1988-1989, British Aerospace proposed the Multi-Role

Capsule (MRC) concept, which is a semi-ballistic

configuration that can be adapted for manned ascent or

rescue, or unmanned microgravity science missions. Work on

the MRC was initiated to compete for the NASA Space Station

crew rescue vehicle (ACRV) but it was not selected.

e. Japan - Japanese government and industry have targeted
reentry capsules as a strategic technology for development

during the early 1990's. The National Space and Development

Agency (NASDA) and the Ministry of International Trade and

Industry (MITI) agencies, with Nissan, Marubeni,Mitsubishi,

and other industrial and trading companies, are closely

monitoring US, German, and Italian capsule R&D programs. In

addition, NASDA is sponsoring the domestic development of a

small experimental capsule to collect reentry data in

support of their manned spacecraft project. This capsule is

scheduled to fly as a piggyback payload in 1993 on Japan's

two-stage heavy launch vehicle, the H-2.

f. Soviet Union - The Soviet Union's hard currency

requirements have pushed Glavkosmos, the Photon Design

Bureau and Soyuzkarta to rapidly commercialize and market

services in the West. An aggressive booster and services

pricing policy, reliable technology and a wide range of

services and systems have generated important successes,

most notably in Germany; Kayser-Threde and Intospace changed

suppliers to fly the Cosima-2 protein crystal experiment in

September, 1989, on the Resurs-F. A good ground

infrastructure capable of supporting Western scientists'

laboratory, communications, and data requirements

complements the on-orbit service during the pre-launch and

post-recovery phases.

The Soviet Union has implemented incremental improvements in

flight-proven systems flown for manned and military photoreconnaissance missions. The original Vostok spherical

module has been adapted for use as Photon, Resurs-F, and the

life sciences Biocosmos capsules. Glavkosmos recently

announced that its improved Nika reentry vehicle, outfitted

with solar panels for long duration flights, is slated for

commercial introduction in 1993-1994.

Outside the commercial realm, NASA and Soviet scientists

continue to cooperate on joint Biocosmos biosatellite

missions under the Joint US/USSR Biological Satellite

Program started in 1975. Since this began, American space

biologists have participated in seven missions covering

biological and radiation physics experiments. An additional

flight is expected in 1991 under current agreements.

g. China - The Chinese Academy of Space Technology (CAST)

undertook development of domestic reentry system technology

in the early 1970's in support of military photoreconnaissance and earth resources missions. The first

capsule flew in 1975 and has since flown over 10 missions

with land recoveries in central China.

The China Great Wall Industry Corporation has shown interest

in reentry system technology and has flown two microgravity

experiments for Matra Espace of France in August of 1987.

h. United States - Presently, in the United States, two

NASA-sponsored reentry capsule programs are expected during
the 1990's. First, the NASA Office of Commercial Programs,

CCDS (Centers for Commercial Development of Space), Center

for Aerospace Research, issued Requests for Proposals for

the Commercial Experiment Transporter (COMET) Flight Program

in the summer of 1990. The COMET program is designed to be

the first US space system to be launched, controlled, and

recovered by the commercial sector. This university-managed

program plans one launch a year for three to five years.

Payloads will range between 20 and 120 kg, with 15-30 days

mission duration.

In addition, the NASA Life Sciences Office LifeSat program

is currently under study with first launch attempt planned

for late 1994. LifeSat is a large sophisticated capsule

(1,200 - 1,400 kg) flown on a Delta-class booster. It is

intended for long duration (up to 60 days) radiation biology

experiments in support of NASA's Human Exploration

Initiative.

US entrepreneurial companies have been active in

commercializing reentry technology that had been classified

or government-sponsored. COR Aerospace, Instrumentation

Technology Associates, Orbital Recovery Corporation, and

Space Industries have proposed both ballistic and lifting

reentry vehicle designs. Table 25 lists present reentry

system projects in the US and abroad.
B. Reentry Vehicle Design Considerations - Reentry vehicles

provide researchers in several diverse fields of interest with a

method of access to space for extended periods of time and

eventual intact recovery of the experiments on land, air or

water. Some vehicle capability factors that are considered when

designing these vehicles are: 1) being launched by a variety of

launch vehicles, 2) operating in low earth orbit as a free-flying

unmanned laboratory, and 3) an independent atmospheric reentry

with an air-snatch recovery or a soft landing at a preselected

site (land or water), providing the experimenter with rapid

access to the payload. Some specific design considerations are

as follows:

1. Shapes - The aerodynamic shape configuration (ballistic

or lifting) of a reentry vehicle determines the severity,

duration, and flight path of reentry experienced by the

vehicle. This, in turn, affects the vehicle systems

complexity and the heating loads on the payload. See Figure

56.

A lifting reentry vehicle has many operational advantages

over a non-lifting vehicle. Primarily, the reentry loads

can be minimized to almost any desired level, with

flexibility in landing site selection. The vehicle has the

ability to deviate its reentry trajectory to reach selected

landing sites "cross range" from the orbital track, and to

fine tune deorbit propulsion system errors. Spherical and

ballistic vehicles can only deorbit to selected sites which

are on the orbital ground track. Spherical shapes are used

on the Soviet Photon spacecraft, and the ballistic reentry

shape is used on the German EXPRESS (EXPeriment REturn

Service from Space) reentry vehicle.

A disadvantage of the lifting shape over the non-lifting

shape lies in the complexity and high cost associated with

guidance and control of the lifting vehicle. A failure of

the guidance or control system could render the vehicle

uncontrollable and cause it to diverge a great distance off

course.

The simple, blunt-body configuration similar to the NASA

Biosatellite, the Air Force Discoverer, and the Chinese

capsule shapes is the shape most often used. Once reentry

has been initiated, the body essentially falls uncontrolled

through the atmosphere with little excursion from the

nominal trajectory; however, there is the penalty of higher

g-loadings than a lifting shape. Various payload massvolume combinations for the blunt-body configuration are

shown in Figure 62.

2. Sizes - The size of a reentry vehicle has depended, for

the most part, on the capabilities of available launch

vehicles. For example, the largest size of a blunt-nose

configuration that is compatible with the Scout launch
vehicle is one with a 96.5-cm diameter base, while a Delta

II can accommodate a vehicle with a 254-cm diameter base. 

However, Ariane now offers a limited capability to configure

the launch vehicle to meet the boost requirements of the

reentry vehicle. In general, the government-funded vehicles

have been designed for the large (Delta II) class of

expendable launch vehicles while commercial design has been

targeted to a smaller class, such as the Scout, Pegasus, or

Amroc. The reentry vehicle user (government or commercial)

has the option of using a fully dedicated launch vehicle, or

riding "piggyback" as a secondary payload.

Other considerations that determine size are sufficient

resources for life science and other payloads, the adequacy

of available power, and costs. There are many other

tradeoffs and options associated with reentry vehicle

design. See Table 37 for candidate RV launch vehicles. See

Table 47 for the results of tradeoff studies by NASA Ames

Research Center for a set of selected vehicle sizes.

3. Subsystems Requirements - A payload module is normally

used in a reentry vehicle to separate the experiment from

the support systems. However, in some very limited cases,

where an experimental payload may not require any support

during the mission, the payload could be mounted inside the

payload envelope within the vehicle. In addition to a

payload, which contains life sciences, materials or other

experiments, a reentry vehicle usually requires several

support subsystems.


Many of these systems provide the same support that is used

for any other orbiting satellite, such as providing the

payload with electrical power, thermal control, command

signals and telemetry capability. However, for a reentry

vehicle, additional subsystems are normally used to effect

controlled deorbit, reentry, and an intact, soft landing or

air recovery of the vehicle. See Figure 78 for a block

diagram of typical reentry vehicle subsystems. Some typical

spacecraft subsystems are as follows; systems that apply

only to a reentry vehicle are shown in italics:

a. Attitude and Spin Control Subsystem - This system

normally is composed of sensors, control electronics and

several low thrust thruster assemblies that perform a

variety of functions, such as:

• to stabilize the reentry vehicle,

• to convert errors in orbit placement or to trim the

orbital period to adjust a projected orbital ground

track,

• to maintain reentry vehicle orbital altitude,

• to spin the reentry vehicle to induce artificial

gravity or to inertially fix the direction of the main

retro-rocket(s) thrust vector, and

• to trim the deorbit maneuver to null errors in the 

performance of the solid rocket burn.

b. Deorbit Propulsion Subsystem - This system provides the

required velocity decrement to deorbit the reentry vehicle

and place it on a trajectory that is aimed at the landing

site. A typical change in velocity requirement to do this

may be approximately 290 m/sec for low-altitude satellites

in near-circular orbit and for landing sites in the orbital

plane.

c. Structures - Structural design takes into consideration,

in addition to the loads imposed on any other satellite, the

loads imposed during deorbit, reentry, and landing.

d. Power Subsystem - The power source for the reentry

vehicle is a critical item, as with other satellites, and is

typically a tradeoff among batteries (and types of

batteries) vs fuel cells vs solar power systems.

e. Power Interface Units - These units control and

distribute primary power to the payload and the reentry

vehicle.

f. Tracking - A tracking aid, such as a transponder, is

normally required in the reentry vehicle as an aid in

recovery.
g. Communications Subsystems - Each payload normally has

its own dedicated data and control system that controls

payload functions and collects and stores its data between

reentry vehicle to Earth communication periods.

h. Reentry Vehicle Parachute Subsystem (or other

retardation system) - This system is designed to retard the

reentry vehicle's vertical velocity and provide a relatively

soft touchdown. For systems that have parachutes, two types

could be used for this application: a conventional type and

a lifting parafoil. The advantages of a conventional

parachute are reduced weight and less complexity. The

lifting parafoil has three advantages over the conventional

type: 1) being able to reduce the dispersions associated

with the deorbit and reentry trajectories by using its

maneuverability to glide to a predetermined point, 2) having

the capability of being manually controlled to minimize

landing area impact dispersions and, 3) by flairing, to

reduce the vehicle impact shock at touchdown.

i. Reentry Thermal Protection Subsystem - The function of

this system is to protect the reentry vehicle from

aerothermodynamic heating during atmospheric entry. 

Ablative material such as phenolic nylon, elastomeric

silicon material (ESM), and white oak have been used in the

past to protect against excessive heating. For protection

against the considerably lower heating rates that occur on

the conical skirt of the vehicle, two types of thermal

protection systems have been used: the ablative type or a

ceramic-based surface insulation type. Other methods have

been investigated, such as reusable heat shields.

j. Thermal Control Subsystem - The reentry vehicle poses

some unique design problems concerning the thermal control

and management aspects of the vehicle. If the modular

payload concept is used, the vehicle is intended to support

and accommodate a wide range of payloads through a generic

interface. This wide range of payload types with different

associated thermal requirements also implies that the

reentry vehicle and payload thermal control system must

accommodate a wide range of thermal loads. This includes

those generated by biological specimens. The payload module

may, depending upon the type of payload, require atmospheric

controls within the module. Some controls normally required

are:

• Control of temperature, humidity, and pressure

• Control of contaminate levels

• Provide for circulation of atmosphere

• Provide venting

C. Reentry Vehicle Operations8

1. Pre-Launch - Prior to launch, the payload is integrated

into the reentry vehicle. Installation of some experimental

specimens and support equipment into the reentry vehicle may

be performed before the reentry vehicle is mated to the

launch vehicle; however, final installation is normally

accomplished on the launch pad, with final payload and

reentry vehicle closeout being completed as close to launch

as practical. This is done to avoid contamination of the

payload and to allow monitoring of the experiments.

2. On-Orbit Considerations

a. Orbital Lifetime - An important advantage of the

reentry vehicle is its ability to maintain experiments

in orbit for long periods of time, depending on the

experiment and limited by the capability of the RV

support systems. Several factors affect the lifetime

of an RV and these include the power requirements,

orbital altitude and inclination, atmospheric density,

vehicle mass, coefficient of drag and exterior geometry

of the RV.

b. Orbit Selection - The RV can operate in a variety

of user-specified orbits or orbits specified by the

requirements of a shared launch. The microgravity

specifications of the experiment normally drives the

altitude of the orbit. Circular orbits within the

altitude range of 350 to 900 kilometers are normally

used. With a non-lifting RV, which has no lateral

maneuvering capability during atmospheric reentry, it

is necessary that the orbit inclination of the RV be
equal to or greater than the latitude of the preselected landing site in order to land at that site.

Although the nominal mission lifetime may be of a

specified duration, certain payloads may require

shorter durations and emergency deorbit due to an

unexpected experimental condition. Another

consideration is that recovery at a single site could

be delayed by one or more days because of local weather

conditions, and a missed deorbit opportunity could

cause a considerable delay while waiting for the RV

orbit to reposition itself coincident with the landing

site. Finally, if parachute recovery is used, it is

less difficult to recover during daylight, especially

during the cool morning hours when the surface winds

are generally lower.

One approach to accomplishing these requirements is to

choose a proper altitude, inclination and nodal

placement to ensure that the ground track of the

satellite permits deorbit and landing at a designated

recovery site twice a day during the mission. These

orbits are termed "integer orbits" in that the orbits

have a repeating ground track each day. Some

correction of the orbital parameters after launch may

be necessary to adjust for insertion errors and to

synchronize the integer orbit ground tracks with the

landing site. Other corrections may be required to

adjust for drag effects, however, in most cases these

will be negligible. It should be noted that the above

orbit selection scenario is probably only possible for

a dedicated launch of an RV. Shared launches may not

permit the independent selection of all orbital

characteristics.
If the first payload deposited in orbit is an RV, all

orbital maneuvers are limited by the capabilities of

the RV Attitude Control System (ACS). These maneuvers

may be necessary to make the projected ground track

coincident with the landing site. However, if the RV

is the last payload deposited in orbit, it is possible

that the launch vehicle upper stage may have sufficient

remaining performance (if it is restartable) to alter

the RV orbital parameters.

3. Deorbit - At the completion of the orbital flight

phase, the vehicle may be commanded by ground control,

or by on-board sequence programmer commands, to

position itself for the deorbit retrothrust maneuver.

In an example of a deorbit maneuver, the Attitude

Control System takes readings on the local horizon in

two orthogonal directions and uses gyroscope

measurements to determine the flight direction. The

Attitude Control System thrusters position and

stabilize the longitudinal axis (and therefore the

thrust vector) in the correct attitude with respect to

the orbital velocity vector. Thrusters then spin the

vehicle about the longitudinal axis to stabilize

inertially the thrust vector to within +/- 10 (a

commonly used 3-sigma tolerance) relative to the

required deorbit attitude. Thereafter, at the

appropriate time, the main retrorocket(s) fire to

decelerate the vehicle and provide a sufficient

velocity decrement to deorbit the RV and place it on a

trajectory that brings it to a descent over the landing

site.

4. Reentry - Following the deorbit maneuver, a set of

reverse spin thrusters are used to despin the RV to a

relatively low rate of spin in anticipation of entering 

the upper atmosphere. The ballistic RV, for example,

retains the attitude of the deorbit burn maneuver and

encounters the atmosphere at an angle of attack of

approximately 90 - 110 degrees, depending primarily on

orbital altitude. At a low rate of spin, the RV

reorients itself into a nose-forward attitude. A

lifting RV must be reoriented after the deorbit burn to

a nose forward attitude and must be attitude-controlled

throughout the reentry phase because of the location of

the center of gravity. A summary of the time history

of altitude, velocity, and acceleration for a typical

ballistic atmospheric entry is shown in Figure 89. As

an aid to recovery, a radar beacon is normally used on

the RV to give the landing site personnel knowledge of

the vehicle track from reentry to landing.

Debris is a potential safety and liability concern

during reentry. This results from the jettisoning of
capsule parts. The jettison technique is useful in

reducing the reentry and recovery weights, and

simplifies the ablative shield and parachute systems

design due to the reduced weights. However, it

increases the amount of debris and the associated

hazards due to unplanned impacts. Technological

developments in the areas of ablatives and parachute

systems design have resulted in the capability of RV's

to reenter as a single unit. This reduces orbital and

reentry debris while reducing the subsystem assembly

replacement requirements.

5. Terminal Descent - The RV descends over the landing

site through the altitude range of the generally

prevalent high winds (i.e., the jet stream) to

approximately 6000 meters, for a typical system, where

deceleration is initiated. Deceleration for RV's has

been accomplished primarily by parachute. At the 

appropriate altitude, a pilot chute is deployed, which

pulls out a drogue chute, thereby slowing and

stabilizing the vehicle. The drogue chute, in turn,

pulls the main parachute from the RV.

The parachute may be of conventional form or may be a

ram-air-filled lifting parafoil. If a conventional

parachute is used, the RV will descend vertically to

the surface (or to the air-snatch site) somewhere in

the area defined by the nominal landing dispersions. If

a parafoil is used and is deployed at an altitude of

approximately 6000 meters, typical performance

characteristics provide the RV with a maneuvering

circle of about 16 km in radius.

6. Retrieval Methods - Three retrieval methods have

been used to recover RV's. Of the three RV system

retrieval methods, air-snatch, water and land, water

recovery has been the most widely used by the United

States. However, each of the methods has its own

distinct advantages and disadvantages. Following is a

brief discussion on each of these methods:

a. Air-snatch - This method was used only to support

Air Force programs and used modified military aircraft

to intercept the descending reentry vehicle parachute

lines over the ocean. This reduced impact loads and

contamination risks, and removed retrieval operation

activities from populated areas.

Air-snatch places costly demands on the retrieval

operation, however, as it requires skilled pilots and

modified aircraft. Contingency recovery plans, either

water or land, must also be incorporated into the

design to allow for poor weather in the retrieval zone

or missed air-snatch attempts.

b. Water recovery - This has been the most widely used

method for the retrieval of unmanned reentry vehicles. 

It is advantageous over the air-snatch method because

it reduces the retrieval crew training requirement. 

However, this method may require a large recovery crew

to insure rapid access to the RV following splashdown. 

The RV system design is also complicated by the need

for flotation systems and recovery aids such as dye

markers and beacons. Additional factors include the

risk of loss of the RV if the flotation systems fail,

as well as the increased refurbishment demands caused

by exposure of the RV to seawater.

c. Land recovery - This is being considered as a

retrieval method for some reentry vehicle systems.

State-of-the-art parachutes and/or parafoils should

provide the capability to land an RV without damage to

the payload. Various mechanisms such as crushable

material in the nosecone may have to be included in the

vehicle design to absorb the impact loads and diminish

their effects on the payload module.
It is desirable by some experimenters that retrieval

personnel be able to access the RV quickly to ensure

minimum thermal damage to the payload as a result of

being exposed to the local environment of the landing

site. One major concern with land-based recovery is

that it presents additional safety considerations

because of population centers.

D. Reentry Vehicle Hazards - The problems associated with

launching reentry vehicles differ from other launches into space

in that a planned deorbit, reentry, and recovery of the payload

is an integral part of the mission. This poses potential hazards

should a failure occur during the deorbit, reentry, or recovery

phases of the mission. The primary risks are from impact of the

payload, impact of other RV debris, or the possible dispersal of

hazardous materials. Failures of the payload attitude control

system, deorbit propulsion system, and/or reentry deceleration

systems influence the location of the impact area of the RV

hardware. Most inland landing sites have populated areas either

near or in the surrounding areas that could be exposed to impact

hazards resulting from failures or errors during deorbit and/or

reentry. If a system failure occurred which left the RV in

orbit, it will eventually reenter, and potentially survive to

impact somewhere on the surface of the Earth. If a normal

deorbit is achieved and a subsequent failure occurs, the impact

dispersion area may become larger but should remain centered

around the planned impact point.

As an example, for a nominal 479 km altitude orbit, the change in

velocity required for deorbit is 290 m/sec. A retrorocket has

been estimated to deliver the total impulse required for this

velocity change to within +/- 0.5% (3-sigma). For a given

retrovelocity increment, a non-lifting RV will follow a deorbit

trajectory resulting in along-track dispersion due to errors in

thrust impulse. Shown in Figure 98 is an analysis of dispersion

for various orbital conditions with a thrust impulse error of -

0.5%. For a nominal 479 km altitude, the total dispersion

footprint due to the combined 3-sigma errors of +/- 10 in thrust

direction and +/- 0.5% in the thrust magnitude is shown in Figure

10(a)8. The impact dispersion shown is approximately 30 km

uprange, 29 km downrange, and a crossrange dispersion of 6 km due

to an out-of-plane thrust direction error of +/- 10. The

Attitude Control System (ACS) thrusters may be used to supplement

the main retro-thrust in order to fine tune the velocity

increment, thereby reducing the dispersions due to thrust

magnitude errors. This is possible by using on-board, highprecision accelerometers and a microprocessor to compute the

actual velocity decrement achieved. This shows how much plus or

minus velocity increment must be supplied by the ACS thrusters. 

Knowing the velocity error, the ACS thrusters are activated to

null this error in the total velocity. The resultant dispersion

ellipse, using this method, is about 6 km in radius as shown in

Figure 10(b)8. This example is for deorbit from 479 km altitude;
for higher orbits, the dispersions will be proportionately larger

for the same amount of retropropulsion.

It should be understood that this example uses a ballistic, nonlifting RV and that the impact dispersions determined in the

analysis are due only to errors in thrust direction and

magnitude. Other factors may cause significantly larger

dispersion areas, such as errors in orbital parameters, timing

sequences, and variations in atmospheric density. In addition,

for a lifting RV, attitude control system errors could cause very

large dispersions by causing the vehicle to "pitch" out of the

planned trajectory. Also, this example does not include

dispersion areas for other reentry vehicle component parts that

are jettisoned during reentry and terminal descent.


APPENDIX I - Manned Reentry Programs

1. Project Mercury1 - NASA's first manned space program

launched its initial research and development flight on

September 9, 1959 aboard an unmanned Atlas D. The program

consisted of two more unmanned flights, two with chimpanzees

aboard and six manned flights. Though the capsules used for

the primates and the astronauts were not reusable, they did

employ much of the technology learned from the Discoverer

missions.

The Mercury capsule, which could accommodate one astronaut,

was conical with a cylindrical neck at the top. It was 3

meters tall and 180 cm in diameter at the base. The body of

the capsule was covered with titanium skin and the ablative

fiberglass heat shield and retro-rocket package were

attached to the base. The typical Mercury capsule weighed

approximately 1300 kg while in orbit. The final five

Mercury missions were flown on a modified Atlas D, which

provided 360,000 pounds of thrust. The launch vehicle

weighed 118,000 kg and its height, including the 5-meter

escape tower, was 28 meters. Table A.13 illustrates key

facts about the Mercury program.
2. Project Gemini1 - NASA's second set of missions leading to

a moon landing used a two-man spacecraft and had astronauts

practicing docking maneuvers, extravehicular activities and

guided reentry. The Gemini capsule was an outgrowth of the

Mercury capsule's conical design. The reentry module had

rendezvous and recovery, reentry control, and cabin sections (50

percent more than Mercury) and was 230 cm in diameter at the

base. It was made primarily of titanium, with external skin of

beryllium and nickel alloy. The adaptor module, which was

jettisoned before deorbit, contained retrograde and equipment

sections.

Modified Titan II ICBM rockets were used for all the Gemini

missions. The hypergolic liquid propellant vehicle with a

430,000 pound-thrust first stage and the 100,000 pound-thrust

second stage had a total height of 33 meters. Table A.23 gives

highlights of Project Gemini.
3. Project Apollo1 - NASA's premier project of the 1960's

culminated in July 1969 when Neil Armstrong took his one small

step on the lunar surface.

The Apollo reeentry vehicle, which housed a three-man crew, was

3.66 meters high and had a base diameter of 4 meters. The

interior structure was primarily aluminum and the sides were

stainless steel eith an ablative coating. The base was covered

by an ablative heat shield. The total weight of the module was

about 5400 kg. Onboard systems included communications, guidance

and navigation, environmental control, attitude control,

batteries and drogue and main parachutes for earth landing.

The saturn V rocket, which launched all but the first manned

Apollo mission, was 111 meters high and weighed 2.7 million kg. 

Its three stages had a combined throust of 8.7 million pounds.

Table A.33 gives a summary of the manned Apollo missions.
APPENDIX II

The following is a limited list of primary agencies and

organizations, foreign and domestic, that have done research

in developing reentry vehicle systems. It should be noted

that NASA Ames Research Center is the lead governmental

agency in the US that is doing this research.

UNITED STATES

NASA Ames Research Center

GE Aerospace

Stanford University

University of Tennessee

Science Applications International Corporation (SAIC)

COR Aerospace

Instrumentation Technology Associates

Orbital Recovery Corporation

Space Industries, Inc.

GERMANY

Dornier

MBB/ERNO

FRANCE

MAN Technologie

Matra Espace

ITALY

Aeritialia

GREAT BRITAIN

British Aerospace

JAPAN

National Space and Development Agency (NASDA)

Ministry of International Trade and Industry (MITI)

CHINA

Great Wall Industry Corporation

USSR

Glavcosmos
REFERENCES

1. Janes Spaceflight Directory, 1988-89

2. GE Aerospace, "Satellite Recovery Vehicle", April, 1989

3. TRW Space Log 1957 - 1987

4. NASA, "Biosatellite Project", Dec., 1969

5. SPACE, "Concerning Re-entry Capsules", May-June, 1990

6. NASA Technical Memorandum L58E07a

7. Stanford University, "Overview of Policy, Market and

Technology Options for Commercial Reusable Re-entry Satellites",

June, 1989

8. NASA Technical Memorandum 101043, Oct 1988

9. NASA, "RRS - A Multipurpose Reusable Reentry Satellite"