Steven R. Oleson and Melissa L. McGuire

Glenn Research Center, Cleveland, Ohio

COMPASS Final Report: Saturn Moons Orbiter 

Using Radioisotope Electric Propulsion (REP): 

Flagship Class Mission

NASA/TM—2011-216972

February 2011

CD–2007–19
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Steven R. Oleson and Melissa L. McGuire

Glenn Research Center, Cleveland, Ohio

COMPASS Final Report: Saturn Moons Orbiter 

Using Radioisotope Electric Propulsion (REP): 

Flagship Class Mission

NASA/TM—2011-216972

February 2011

CD–2007–19

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The COMPASS team wishes to thank the support of the Engineering Directorate, and specifi cally branch Chief Maria Babula and 

mission analysts of the Mission Design and Analysis Branch for their unending support and high caliber of engineer support. The

team would also like to thank the In Space Propulsion Offi ce for always providing challenging design study ideas.

Available from

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7115 Standard Drive

Hanover, MD 21076–1320

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

This report is a formal draft or working 

paper, intended to solicit comments and 

ideas from a technical peer group.

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2011-216972 iii

Contents

1.0 Executive Summary ............................................................................................................................. 1

2.0 Study Background and Assumptions ................................................................................................... 3

2.1 Introduction ................................................................................................................................. 3

2.2 Purpose ........................................................................................................................................ 3

2.3 Assumptions and Approach ........................................................................................................ 3

2.3.1 NASA Flagship Class Mission Definition .................................................................... 4

2.3.2 Design Starting Points ................................................................................................... 4

2.4 Growth, Contingency and Margin Policy ................................................................................... 4

2.5 Baseline System Design .............................................................................................................. 5

2.6 Mission Description .................................................................................................................... 6

2.6.1 Interplanetary Chemical Cruise Stage Trajectory Baseline .......................................... 6

2.6.2 Mission Analysis Event Timeline ............................................................................... 10

2.7 Launch Vehicle Details ............................................................................................................. 11

2.8 Science Instruments Overview.................................................................................................. 13

2.9 System Design Configuration Details ....................................................................................... 13

2.9.1 Cruise Deck Configuration ......................................................................................... 13

2.9.2 Aeroshell Configuration .............................................................................................. 13

2.9.3 REP S/C Configuration ............................................................................................... 13

2.10 Internal COMPASS Details ...................................................................................................... 13

2.10.1 GLIDE Study Share .................................................................................................... 14

2.10.2 GLIDE Study Container (Architecture) ...................................................................... 14

2.10.3 GLIDE Study Container(s) ......................................................................................... 14

2.11 Top Level Design (MEL and PEL) ........................................................................................... 14

2.11.1 Master Equipment List (MEL) .................................................................................... 14

2.11.2 Power Equipment List (PEL) ...................................................................................... 15

2.12 System Level Summary ............................................................................................................ 15

2.13 Design Concept Drawing and Description ................................................................................ 16

3.0 Subsystem Breakdown ....................................................................................................................... 17

3.1 Attitude Control System (ACS) ................................................................................................ 17

3.1.1 ACS Requirements ...................................................................................................... 17

3.1.2 ACS Assumptions ....................................................................................................... 17

3.1.3 ACS Design and MEL ................................................................................................ 17

3.1.4 ACS Trades ................................................................................................................. 20

3.1.5 ACS Analytical Methods ............................................................................................ 20

3.1.6 ACS Risk Inputs .......................................................................................................... 20

3.1.7 ACS Recommendations .............................................................................................. 20

3.2 Communications ....................................................................................................................... 20

3.2.1 Communications Requirements .................................................................................. 20

3.2.2 Communications Assumptions .................................................................................... 20

3.2.3 Communications Design and MEL ............................................................................. 20

3.2.4 Communications Trades.............................................................................................. 22

3.2.5 Communications Analytical Methods ......................................................................... 22

3.2.6 Communications Risk Inputs ...................................................................................... 23

3.2.7 Communications Recommendation ............................................................................ 23

3.3 Command and Data Handling (C&DH)—(Avionics)............................................................... 24

3.3.1 Avionics Requirements ............................................................................................... 24

3.3.2 Avionics Assumptions ................................................................................................ 24

3.3.3 Avionics Design and MEL .......................................................................................... 24

3.3.4 Avionics Trades .......................................................................................................... 25
NASA/TM—2011-216972 iv

3.3.5 Avionics Analytical Methods ...................................................................................... 25

3.3.6 Avionics Concerns, Comments, Recommendations ................................................... 25

3.4 Electrical Power System ........................................................................................................... 25

3.4.1 Power Requirements ................................................................................................... 25

3.4.2 Power Assumptions ..................................................................................................... 26

3.4.3 Power Design and MEL .............................................................................................. 26

3.4.4 Power Trades ............................................................................................................... 28

3.4.5 Power Analytical Methods .......................................................................................... 28

3.4.6 Power Risk Inputs ....................................................................................................... 28

3.4.7 Power Recommendation ............................................................................................. 28

3.5 Structures and Mechanisms ...................................................................................................... 29

3.5.1 Structures and Mechanisms Requirements ................................................................. 29

3.5.2 Structures and Mechanisms Assumptions ................................................................... 29

3.5.3 Structures and Mechanisms Design and MEL ............................................................ 29

3.5.4 Structures and Mechanisms Trades ............................................................................. 32

3.5.5 Structures and Mechanisms Analytical Methods ........................................................ 32

3.5.6 Structures and Mechanisms Risk Inputs ..................................................................... 32

3.5.7 Structures and Mechanisms Recommendation ........................................................... 32

3.6 Propulsion and Propellant Management ................................................................................... 32

3.6.1 Propulsion and Propellant Management Requirements .............................................. 32

3.6.2 Propulsion and Propellant Management Assumptions ................................................ 32

3.6.3 Propulsion and Propellant Management Design Trades ............................................. 33

3.6.4 Propulsion and Propellant Management Design and MEL ......................................... 35

3.6.5 Propulsion and Propellant Management Analytical Methods ..................................... 40

3.6.6 Propulsion and Propellant Management Risk Inputs .................................................. 41

3.6.7 Propulsion and Propellant Management Recommendation ........................................ 41

3.7 Thermal Control ........................................................................................................................ 41

3.7.1 Thermal Requirements ................................................................................................ 41

3.7.2 Thermal Assumptions ................................................................................................. 42

3.7.3 Thermal Design and MEL ........................................................................................... 42

3.7.4 Thermal Trades ........................................................................................................... 43

3.7.5 Thermal Analytical Methods ....................................................................................... 44

3.7.6 Thermal Risk Inputs .................................................................................................... 47

3.7.7 Thermal Recommendation .......................................................................................... 48

4.0 Cost..................................................................................................................................................... 48

4.1 Costing: Flagship Configuration ............................................................................................... 48

4.1.1 Costing Assumptions .................................................................................................. 48

4.1.2 Cost Estimates ............................................................................................................. 48

5.0 Trades ................................................................................................................................................. 50

6.0 Challenges, Lessons Learned, Areas for Future Study ....................................................................... 50

Appendix A.—Acronyms and Abbreviations ............................................................................................. 51

Appendix B.—Flagship Design Rendered Drawings ................................................................................. 53

Appendix C.—Study Participants ............................................................................................................... 55

References ................................................................................................................................................... 56
NASA/TM—2011-216972 1 

COMPASS Final Report: Saturn Moons Orbiter Using 

Radioisotope Electric Propulsion (REP): Flagship Class Mission

Steven R. Oleson and Melissa L. McGuire

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

1.0 Executive Summary

The COllaborative Modeling and Parametric Assessment of Space Systems (COMPASS) team was 

approached by the NASA Glenn Research Center (GRC) In-Space Project to perform a design session to 

develop Radioisotope Electric Propulsion (REP) Spacecraft Conceptual Designs (with cost, risk, and 

reliability) for missions of three different classes: New Frontier’s Class Centaur Orbiter (with Trojan 

flyby), Flagship, and Discovery. The designs will allow trading of current and future propulsion systems.

The results will directly support technology development decisions. The results of the Flagship mission 

design are reported in this document (see Figure 1.1). 

Figure 1.1.—Mission overview.
NASA/TM—2011-216972 2 

The mission chosen for this design is a science mission to the Saturn system to map Saturn and its 

moons Titan and Enceladus using REP to power Electric Propulsion (EP) thrusters and the S/C and 

science instruments. The requirement for the mission is to optimize the mass of a science orbiter delivered 

to the Saturn moon system. The REP S/C is launched in a Delta IV H. The S/C consists of three “stages”: 

a chemical cruise deck is used to perform one mid-course burn, an aeroshell is used to break into Titan 

orbit, and the Radioisotope-powered EP thrusters are used for operation between the moons and for 

atmospheric drag compensation about the moon Titan. After a chemical burn using the cruise stage, the 

aeroshell performs the aerocapture maneuver at Titan, and once at Titan, the REP powered EP stage 

provides propulsion for drag makeup at Titan and a 5-yr transfer from Titan to Enceladus. Figure 1.2 

shows the conceptual REP Vehicle with chemical propulsion cruise deck (stage) and aeroshell designed 

for this COMPASS session.

In Figure 1.2, the Cruise Deck (stage) is shown at the bottom, and the REP stage is shown inside the 

aeroshell. The major systems (heat shield, back shell) and stages (Cruise Deck) of the complete vehicle

are labeled.

Table 1.1 summarizes the Cruise Stage/Aerocapture/REP stage S/C and mission.

Figure 1.2.—Conceptual design REP science spacecraft (S/C) with Cruise Deck (REP stage inside 

aeroshell). 
NASA/TM—2011-216972 3 

TABLE 1.1.—MISSION AND S/C SUMMARY

Mission Titan Aerocapture in 8 yr, 1 yr Titan REP Drag makeup, 5 yr spiral down to 

Enceladus, 7 km/s, 

Total mass 

with growth 

System 30% system level growth (additional 105 kg carried at system level), American 

Institute for Aeronautics and Astronautics (AIAA) mass growth allowance (MGA) 

schedule used at subsystem level

3087 kg 

(wet)

Launch Launch: Delta IV Heavy, C3 48 km2/s2, 10 kg Expendable Launch Vehicle (ELV) 

adaptor, 49 kg available launch margin

3136 kg 

performance

Science Science: 70 kg, 250 W (based on Applied Physics Laboratory (APL) Titan flagship 

payload)

91 kg

Power Eight Advanced Stirling Radioisotope Generators (ASRG) with multilayer 

insulation (MLI), attached (loaded) in pairs, 1200 W end of life (EOL) (14-yr)

249 kg

Chemical Cruise Deck Cruise Deck provides deep space maneuver (bipropellant), thermal and 

communications during cruise to Saturn

954 kg 

(wet)

Aeroshell Aerocapture system sized from 2003 NASA Titan Aerocapture Studies, Aeroshell 

included in the REP Stage masses

315 kg

Mechanical Octagonal Al-Li bus with Propulsion and Science Decks, capable of carrying 6 g 

axial and 3.5 g lateral launch loads

100 kg

Electric Propulsion 3+1 Long Life Hall Thrusters, operated serially, 600 kg Xe propellant load

900 W into thruster, 1500 s, Specific Impulse (Isp), 200 kg Xe throughput each, 

30,000 hr

Single string power processing unit (PPU) (95%), thruster feed, thruster

187 kg 

(dry) 

Command and Data Handling 

(C&DH), Communications

RAD 750, 220 W, 10 kb/s Data rate, Ka Band, 2.1 m antenna 104 kg

Attitude Determination and

Control (AD&C) 

Two Star Cameras, Inertial Measuring Unit (IMU), four reaction wheels, Hydrazine 

Reaction Control System (RCS) propulsion system

30 kg

Thermal Hibernate S/C during interplanetary coast, Aerocapture shell consists of active 

radiators, post shell separation MLI and heaters

127 kg

2.0 Study Background and Assumptions

2.1 Introduction

The Executive Summary provides a framework for where this study fits into NASA science mission goals. 

The rest of the document addresses the issues of: mission category, propulsion type and engines, mission 

duration, scientific payload and its requirements, power requirements, communications requirements, launch 

vehicle and a specific mission targets (Titan and Enceladus). As with any study, there are many options among 

these mission elements. Many of them are traded for comparison in order to provide optimal scientific results, 

minimize cost and provide the highest probability of mission success (as defined by established goals). There 

is a preliminary discussion on past missions that serve as baseline designs.

2.2 Purpose

The goal of this study is to determine a preliminary S/C design for a Flagship class mission to study 

Saturn’s moons Titan and Enceladus focusing on the application of REP technology to enable and 

enhance a large scale, high power science mission.

2.3 Assumptions and Approach

The following section contains the description of the mission class that was the subject of this design 

study. The following details are the most current available at the time of this design session. 

This study will utilize the COMPASS S/C Conceptual Design team to provide complete Science 

Class Reference Mission Designs.
NASA/TM—2011-216972 4 

2.3.1 NASA Flagship Class Mission Definition

NASA Planetary Science Division (PSD) is developing plans for the next outer planet moon flagship 

mission. The moons under consideration are: Europa, Titan, Enceladus, and Ganymede. 

The Definition of a Flagship Class Mission is as follows (Ref. 1):

The largest missions, known as Flagship Missions, range in cost from $800 to $1400 million or 

$1400 to $2800 million. These missions will be crucial in allowing us to reach and explore difficult 

but high-priority targets. These critically important targets could help establish the limits of 

habitability, not just for our solar system, but also for planetary systems in general. In particular, 

they potentially provide an opportunity to identify pre-biotic organic molecules or even extant life 

beyond Earth, should it exist, in our own solar system. The targets of flagship missions include 

complex missions to the surface of Venus, the lower atmosphere and surface of Titan, the surface and 

subsurface of Europa, the deep atmosphere of Neptune and the surface of its moon Triton, and the 

surface of a comet nucleus in the form of cryogenically preserved samples.

2.3.2 Design Starting Points

The design began by comparing the performance of a launch on an Atlas 551/Star 48 and a Delta IV 

H class ELV on a direct injection trajectory to Saturn. A simple trade was performed to start the session, 

between the uses of the ASRG powered electric thrusters operating to Saturn versus the use of a chemical 

cruise stage (total ∆V to provide ~2.5 km/s). The chemical cruise stage was chosen as simpler since it 

avoids operating the EP thrusters while inside the aeroshell. The following is a list of goals of the S/C

mission design 

(1) Transfer optimal mass from Earth to Saturn

(2) Aerocapture at Titan with an aeroshell integrated around the REP stage

(3) Perform 1 yr mission of mapping Titan surface from an orbit of between 1200 and 1400 km using the 

ASRG power for science and S/C instruments, and for orbit maintenance (budgets ΔV ~ 100 m/s)

(4) Use REP stage to spiral out of Titan orbit. 

(5) Use the REP stage to spiral down to Enceladus and 

(6) Spend 1 yr at Enceladus doing science mapping.

2.4 Growth, Contingency and Margin Policy

Mass Growth: For dry mass elements in the system design, the COMPASS team uses the 

ANSI/AIAA R-020A-1999, “Recommended Practice for Mass Properties Control for Satellites, Missiles, 

and Launch Vehicles,” (Ref. 2). Table 2.1 shows the Percent Mass Growth separated into a matrix 

specified by level of design maturity and specific subsystem.

The percent growth factors are applied to each subsystem, after which the total system growth of the 

vehicle level is calculated. The COMPASS team desired total growth to be 30 percent, and an additional 

growth is carried at the system level in order to achieve a total system growth of a 30 percent limit on the 

dry mass of the system. Note that for designs requiring propellant, growth in propellant is either book 

kept in the propellant itself or in the ∆V used to calculate the propellant necessary to fly a mission.

The COMPASS team uses the Discover Announcement of Opportunity (AO) definitions of 

Contingency and mass Margin.

From the Discovery AO: Definitions of Contingency and Mass Margin 

Contingency (or Reserve), when added to a resource, results in the maximum expected value for that 

resource. Percent contingency is the value of the contingency divided by the value of the resource less the 

contingency.
NASA/TM—2011-216972 5 

Margin is the difference between the maximum possible value of a resource (the physical limit or the 

agree-to limit) and the maximum expected value for a resource. Percent margin for a resource is the 

available margin divided by its maximum expected value.

Power Growth: The COMPASS team uses a 30 percent power growth assumption except in cases of 

electric propulsion since any reduction in power can be handled by increasing trip time. 

TABLE 2.1.—PERCENT MASS GROWTH ALLOWANCE

Code

Design Maturity

(Basis for Mass 

Determination)

Percent Mass Growth Allowance

Electrical/Electronic 

Components

Structure 

Thermal 

Control

Propulsion

Batteries

Wire Harnesses

Mechanisms

Instrumentation

0-5 

kg

5-15 

kg

>15 

kg

E Estimated

(preliminary sketches) 30 20 15 18 18 18 20 50 18 50

L 

Layout

(or major modification of 

existing hardware)

25 20 15 12 12 12 15 30 12 30

P 

Pre-Release Drawings

(or minor modification of 

existing hardware)

20 15 10 8 8 8 10 25 8 25

C Released Drawings

(calculated values) 10 5 5 4 4 4 5 5 4 5 

X 

Existing Hardware

(actual mass from another 

program) 

3 3 3 2 2 2 3 3 2 3 

A 

Actual Mass

(measured flight 

hardware)

0 0 0 0 0 0 0 0 0 0 

CFE Customer Furnished 

Equipment 0 0 0 0 0 0 0 0 0 0 

Following the conventions of CD–2007–16 (Ref. 3), the APL assumptions on science payload 

margins are applied to this science payload.

Science payload mass and power margins assumptions are:

• 30 percent margin should be added for mass estimates (mass estimates do not include any 

additional shielding for the REP mission)

• 30 percent margin should be added for power estimates (except for EP as noted above)

2.5 Baseline System Design

Due to the large gravity well of Saturn (almost 60 times that of Earth), spiraling down from a high 

orbit near the sphere of influence would require long periods of time (>5 yr) before approaching Titan. 

Alternatively, studies have shown that an aerocapture system can place a S/C directly into Titan orbit 

(Refs. 4, 5, and 6) with no trip time penalties using Titan’s atmosphere. Thus the baseline approach was to 

use an aerocapture system to place an REP S/C in low Titan orbit. The REP S/C is stored inside the 

aeroshell and goes into operation after aerocapture and the separation of the aeroshell. Options do exist 

where the REP can be used to reduce the Earth to Saturn transit time. 
NASA/TM—2011-216972 6 

Figure 2.1.—Baseline S/C design interior of aeroshell and cruise deck highlighted. 

Figure 2.1 shows the baseline design of the entire S/C. The subsystems on the cruise deck (stage) and 

the REP S/C are shown opaque. The aeroshell and the structure/radiator of the cruise deck are transparent.

The S/C is divided into three stages. The stack consists of a chemical cruise deck, an aeroshell and the 

REP stage for use in the Saturn Moon system. The chemical cruise deck contains the propulsion to 

provide a deep space chemical maneuver as well as necessary thermal, navigation, and communications 

systems for the REP S/C in the aeroshell during the Earth to Saturn transit.

2.6 Mission Description

The REP Flagship mission involved the use of three different propulsion systems: chemical, 

aerocapture, and electric propulsion, to deliver the REP S/C to the Saturn Moon system. The mission is 

broken up along the following mission stages

• The REP S/C is launched on a Delta IV H to a C3 of 48 km2/s2. 

• The cruise deck chemical stage provides a deep space maneuver with its bi-propellant system. 

• The chemical trajectory uses one Earth gravity assist (EGA) on the way to Saturn. 

• Upon arrival at Titan, an Aerocapture system enables the S/C to capture into a low Titan orbit 

using the local Titan atmosphere. 

• The REP powered Long Life Hall thruster EP Stage performs drag makeup at Titan, and then 

spirals the vehicle out of Titan orbit and then down to an orbit about the moon Enceladus (~5 yr 

trip time). 

2.6.1 Interplanetary Chemical Cruise Stage Trajectory Baseline 

After performing an initial trade between the uses of the main REP system versus a chemical cruise 

stage to perform the burn from the Earth to Saturn (Figure 2.2), a chemical cruise stage was chosen as the 

first propulsion system due to concerns operating the electric propulsion system while inside the 

aeroshell. 
NASA/TM—2011-216972 7 

Figure 2.2.—In-space trajectory from Earth to Saturn showing EGA. 

Based on previous aerocapture studies for Titan (Refs. 4, 5, and 6), the incoming velocity of the 

arrival trajectory at Saturn can be as high as 6.5 km/s for the current aeroshell design. Therefore, an 

Earth-Saturn trajectory was chosen such that the Vmp (hyperbolic excess velocity), reported below, was 

less than that target 6.5 km/s limit. 

• Launch vehicle Delta IV Heavy

• Launch date March 25, 2015 

• Maneuver October 9, 2016 

• Earth fly-by January 30, 2018 

• Arrival at Saturn date March 23, 2023 

• Transfer time 8 yr

• Launch mass 3087 kg (Delta IV-H)

• C3 48.36 km2/s2

• Post launch ∆V 0.699 km/s 

• Vmp Titan 5.99 km/s (less than the ~ 6.5 entry limit)

2.6.1.1 Aerocapture Maneuver at Titan

The aeroshell will be used to aerocapture into Titan’s orbit using Titan’s upper atmosphere, recently 

probed by Huygens. Aerocapturing will eliminate the need to start the REP system at the edge of Saturn’s 

gravity well, thereby significantly reducing mission time. The Aerocapture system is sized based on the 

2003 NASA Aerocapture studies (Refs. 4 and 5). The aerocapture maneuver and a subsequent periapsis 

burn by a dedicated bipropellant thruster system place the S/C in a 1200 km altitude circular orbit. The 

Aerocapture concept is illustrated in Figure 2.3. 

Titan Arrival Orbit Characteristics

• Arrival Vhp: 5.99 km/s 
NASA/TM—2011-216972 8 

Figure 2.3.—Concept of aerocapture maneuver at Titan. 

Titan bipropellant thruster system maneuver budget 

• Aerocapture maneuver: 30 m/s 

• Periapse raise: 200 m/s 

• Attitude control for rest of mission: 50 m/s 

Table 2.2 contains the parameters for modeling the aerocapture shell with regard to the Titan orbiter 

with resulting aerocapture data from the Titan Aerocapture mission (Ref. 7). Table 2.2 also contains the 

data from the Titan Aerocapture mission as well as similar Mars aerocapture data for comparison.

TABLE 2.2.—DESTINATION PARAMETERS FOR MARS AND TITAN ORBITS (REF. 7)

Destination/reference parameter Titan Mars

Entry velocity (km/s) a6.5 5.7

Nominal entry flight path angle (deg) –36 –14.2

Apoapsis/science orbit (km) 1700 1400

Atmosphere composition b95% N2, 5% CH4 95.3% CO2, 2.7% N2

Atmosphere scale height at aerocapture altitude (km) ~40 10.5

Atmospheric interface altitude (km) 1000 250

Aerocapture altitude (km) 200 to 400 40

L/D 0.25 0.25

M/CDA (kg/m2) c90 148

Theoretical corridor (deg) 3.5 ~1.4

Time from atmosphere entry to atmosphere exit (min) 42 10

Convective stagnation point heat rate (W/cm2) 46 (0.91 m nose rad) 30 (1.9 m nose rad)

Radiative aeroheating rate (W/cm2) 93 to 280 Negligible

Max g’s during aerocapture (Earth g’s) 3.5 2.5 to 3

Representative aerocaptures at Titan compared to Mars. 

a

Titan aerocapture entry velocity 6.5 to 10 km/sec, comparison given for 6.5 km/sec. b

Max CH4 atmosphere. c

For design trajectory and comparison; range: 56 to 84 kg/m2 dependent on aeroheating, TPS, vehicle diameter
NASA/TM—2011-216972 9 

2.6.1.2 Spiral Trajectory Operations at Saturn

The Radioisotope powered electric propulsion system is used to perform drag makeup for the orbit 

about Titan and to perform the spiral between the Saturn moons Titan and Enceladus. The REP S/C

spends 1 yr mapping Titan at a 1200 km apoapse orbit (note the data from the table in Figure 2.2 quotes a 

higher apoapse than the one applied in this mission, but the data is sufficient for the purpose of this 

analysis). The Altitude Control System (ACS) thrusters on the REP stage perform orbit maintenance (ΔV 

of 100 m/s). After the year of science mapping, the REP stage spirals out of Titan orbit and spirals down 

to Enceladus. The REP stage then spends 1 yr at Enceladus for science mapping. The total ΔV performed 

by the REP system is approximately 7 km/s at Enceladus. Stability of the REP spiral trajectory due to 

multibody affect needs to be assessed.

The aerocapture maneuver places the S/C in a 1200 km altitude circular orbit. The S/C will not spiral 

to Titan, though it may need the REP engine for some orbit maintenance at the low altitude. The spiral 

from Titan will take an estimated 1836 days. The transfer orbit was calculated using the Edelbaum 

equation (Refs. 8 and 9) and EP propellant flow. Using the Edelbaum methodology makes the delivered 

mass only dependent on Isp, and transfer time are dependent on power. For an acceptable trip, the specific 

mission parameters chosen were

• Transfer from Titan to Enceladus 

– Titan to Enceladus spiral time: 1836 days 

– Specific impulse: 1500 s 

– Thruster input power: 1000 W 

– ∆V: 7.054 km/s 

– Used propellant: 586 kg 

– Propellant with margin: 637 kg 

Figure 2.4 shows the delivered mass as a function of trip time and Isp (bottom axis) for the vehicle 

operating in the Saturn moon system. For the Titan to Enceladus spiral transfer, both trip time and mass 

fraction (propellant mass/delivered mass) are strongly dependant on specific impulse.

Spiral trades are only for an Edelbaum approximation (assumes 7.054 km/s ∆V regardless of specific

impulse). There may be significant gravity losses not taken into account in this analysis. Propellant 

reserve for trajectory margin is 5 percent. Propellant reserve for residuals is 3.6 percent. Table 2.3 lists the 

assumptions associated with the low thrust mission design. 

Figure 2.4.—Propellant mass and transfer time as a function of thruster operating Isp (s). 
NASA/TM—2011-216972 10

TABLE 2.3.—LOW THRUST MISSION DESIGN ASSUMPTIONS

Mass, Xe total....................................................................................... 636.57 kg

Mass, Xe useable ........................................................................586.1566414 kg

Mass, Xe navigation and trajectory margin ..................................................... 5%

Mass, Xe residuals........................................................................................ 3.6%

Specific impulse .........................................................................................1500 s 

Power, into the thruster.............................................................................1000 W

Time, transfer time ............................................................................... 2920 days

Time, spiral time................................................................................... 1836 days

∆V, deep space maneuver........................................................................0.7 km/s

∆V, chemical margin ...........................................................................0.035 km/s

∆V, spiral...............................................................................................7.05 km/s

Date, launch................................................................................. March 25, 2015

Date, arrival ................................................................................. March 23, 2023

Mass, launch mass.............................................................................. 3087.00 kg

Energy, C3......................................................................................... 48.36 km2

/s2

Target, flyby ................................................................................................. Titan

Target, Vhp............................................................................................5.99 km/s

Figure 2.5.—Power sensitivity (spiral phase).

The low thrust mission analysis performed for this mission was done using the low thrust 

optimization code VARITOP, developed at the NASA Jet Propulsion Laboratory (JPL). In order to 

simulate the 90 percent duty cycle of the electric thrusters, the thruster level was set to 90 percent. This 

interjected 10 percent coasting times into the trajectory.

The sensitivity analysis of spiral time to power into thrusters applied the following thruster 

performance curves shown in Figure 2.5. 

2.6.2 Mission Analysis Event Timeline

Main events during transfer

• Launch date: March 25, 2015 

• Deep space maneuver: October 9, 2016 

• Earth fly-by (EGA): January 30, 2018 

• Arrival date: March 23, 2023 
NASA/TM—2011-216972 11

Figure 2.6.—Mission event timeline.

Figure 2.6 is the straw man event timeline for the mission. All dates are relative to the launch date 

chosen in this mission analysis, and subject to change for different launch date configurations.

Table 2.4 captures the mission event timeline in terms of mass dropping as burns are completed 

throughout the mission, and as major portions of the S/C are jettisoned. The masses are then used as the 

initial masses for the next maneuver as the propellant budget is sized via the rocket equation.

TABLE 2.4.—MASS JETTISON HISTORY 

Mass jettisoning Mass 

(kg)

Launch wet mass 3038.0 

Cruise propellant (deep space maneuver) 623.94 

S/C at Saturn orbit entry 2414.0 

Cruise Deck dry Mass (bottoms-up growth) 311.2 

Cruise Deck dry Mass (with system growth) 340.0 

Residual prop 18.7 

S/C at Saturn post drop Cruise Deck 2055.4 

Aeroshell-heat shield (dropped) total 228.2 

Aeroshell-backshell (dropped) total 86.4 

Aeroshell total mass (bottoms-up growth) 314.6 

Aeroshell total mass (with system growth) 342.5 

Propellant with aeroshell 27.30 

S/C at Saturn post drop aeroshell 1685.6 

S/C at Saturn post periapse raise 1541.9 

2.7 Launch Vehicle Details

The baseline Launch Vehicle is the Delta IV H. An Atlas 551 with a Star 48 solid propellant stage 

was considered but lacked the performance required once the bottoms-up REP/Aerocapture/Cruise Deck 

S/C was designed. The Launch Vehicle performance for launch C3 is shown in Figure 2.7. This data was 

gathered through the NASA Kennedy Space Center (KSC) Launch Services website.
NASA/TM—2011-216972 12

Figure 2.7.—Delta IV H launch mass versus C3. 

Figure 2.8.—REP Flagship 

S/C in Delta V payload 

fairing.

Figure 2.8 shows the packaging of the REP S/C in the Delta IV H payload fairing. Note that the Delta 

IV payload shroud volume more than accommodates the 4 m diameter aeroshell thus allowing for options 

of a 5 m aeroshell to allow more room for the REP S/C and science payload. Options also may exist for 

adding Titan and Enceladus probes.
NASA/TM—2011-216972 13

2.8 Science Instruments Overview

Typically, the science payload mass delivered by the vehicle in a COMPASS design session is the 

figure of merit (FOM) of the analysis. In this design, the science payload capability was used as a FOM 

and was traded off as mass in order to fit the REP vehicle into the launch performance of the Delta IV H.

As a starting assumption, a 70 kg science package payload, with 30 percent growth on it was assumed.

Additional margin available in launch margin could be traded off for science payload, but would require 

additional propellant. This is left for future work.

2.9 System Design Configuration Details

The REP S/C will be launched on a Delta IV H. The payload will be located on the inside of the 

aeroshell and not used until after the aeroshell separates at Titan. To first order, the S/C configuration is 

built around the following major components: 

2.9.1 Cruise Deck Configuration

The Cruise Deck is the name for the stage in this configuration that provides thermal protection and 

communications, and performs the deep space burn for the cruise to Saturn. It is separated just before 

aerocapture at Titan. The Cruise Deck includes: radiators to cool the REP S/C inside the aeroshell 

(including the ASRGs), a propulsion system to provide a single deep-space burn (ΔV of ~700 m/s), a 

medium gain antenna (MGA), all necessary structure and separation mechanisms, and miscellaneous 

avionics, cabling, thermal system.

2.9.2 Aeroshell Configuration 

The Aeroshell consists of a Heat Shield and a Back Shell. The Aeroshell is dropped after aerocapture 

into Titan’s atmosphere. Aerocapture system is sized on the 2003 NASA Aerocapture studies (Refs. 4, 5, 

and 6).

2.9.3 REP S/C Configuration 

The REP S/C is based on an octagonal frame with a propulsion/power deck and an upper avionics/ 

communications/science deck. The REP S/C is installed inverted in the aeroshell. The ~600 kg of Xe is 

carried in a single spherical Composite Overwrapped Pressure Vessel (COPV) tank with two hydrazine 

monoprop tanks on either side carrying the ACS/periapsis raise propellant. Four advanced Hall thrusters 

are mounted on the avionics deck since the shape of the aeroshell necessitated placing the 2.1 m fixed 

antenna below the propulsion deck. The eight ASRGs are spaced radially out from the propulsion deck to 

allow sufficient clearance with the aeroshell and maximize radiative cooling. The science instruments 

share the top of the avionics deck with the thrusters. Future assessments will need to consider the 

proximity of the instruments with the thrusters. The science instruments shown in the REP S/C are from a 

previous study (Ref. 3) and only representative of the final science payload. Further work is needed to 

integrate the actual science instrument set.

2.10 Internal COMPASS Details

COMPASS is a multidisciplinary collaborative engineering team whose primary purpose is to 

perform integrated vehicle systems analysis and provide trades and designs for both Exploration and 

Space Science Missions.
NASA/TM—2011-216972 14

2.10.1 GLIDE Study Share

GLIDE (GLobal Integrated Design Environment) is a data collaboration tool that enables secure 

transfer of data between a virtually unlimited number of sites from anywhere in the world. GLIDE is the 

primary tool used by the COMPASS design team to pass data real-time between subsystem leads.

The study share for this study is: https://glide.grc.nasa.gov/REP_Sept2007 

2.10.2 GLIDE Study Container (Architecture)

The convention is to keep the name of the study container and the study share the same. For the 

COMPASS team, the Microsoft Excel add-in GLIDE Options tool bar pick for the Study Container 

(Architecture) is:

Study Container (Architecture): REP_Sept2007 

2.10.3 GLIDE Study Container(s)

Reduced Science Case: REP_Flagship_Reducedscience 

Study Description 

Flagship class mission to the Saturn moon system. Baseline Design (six ASRG). A reduction in the 

science package for this iteration.

2.11 Top Level Design (MEL and PEL)

The Flagship mission to the Saturn moon system was divided into two distinct stages. To simplify the 

systems analysis and the dropping of mass during the mission phases.

2.11.1 Master Equipment List (MEL) 

2.11.1.1 Chemical Cruise Stage MEL

Table 2.5 lists the top level subsystems of the MEL of the Chemical Cruise Stage.

TABLE 2.5.—CRUISE DECK (STAGE) MEL

WBS 

no.

Description 

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - - 896.63 6.38 57.23 953.86 

03.1 Attitude Determination and Control - - 6.46 20.00 1.29 7.75 

03.2 Command and Data Handling - - 3.00 30.00 0.90 3.90 

03.3 Communications and Tracking - - 2.40 30.00 0.72 3.12 

03.4 Electrical Power Subsystem - - 3.00 50.00 1.50 4.50 

03.5 Thermal Control (Non-Propellant) - - 48.93 15.00 7.34 56.27 

03.6 Propulsion - - 74.38 30.00 22.31 96.69 

03.7 Propellant (Chemical) - - 642.63 0.00 0.00 642.63 

03.8 Structures and Mechanisms - - 115.83 20.00 23.17 139.00 

2.11.1.2 REP and Aeroshell MEL

Table 2.6 lists the top level subsystems of the MEL of the REP Stage and the Aeroshell.
NASA/TM—2011-216972 15

TABLE 2.6.—REP AND AEROSHELL MEL

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

01 REP S/C (Payload and Stage) - - 1784.15 10.90 194.52 1978.67 

01.1 Science Payload - - 70.00 30.00 21.00 91.00 

01.2 REP Bus - - 1714.15 10.12 173.52 1887.67 

01.2.1 Attitude Determination and Control - - 18.40 20.00 3.68 22.08 

01.2.2 Command and Data Handling - - 33.30 34.26 11.41 44.71 

01.2.3 Communications and Tracking - - 39.00 34.10 13.30 52.30 

01.2.4 Electrical Power Subsystem - - 223.24 11.51 25.70 248.94 

01.2.5 Thermal Control (Non-Propellant) - - 61.57 15.00 9.23 70.80 

01.2.6 Propulsion - - 145.64 28.22 41.10 186.74 

01.2.7 Propellant - - 847.51 0.00 0.00 847.51 

01.2.8 Structures and Mechanisms - - 345.49 20.00 69.10 414.59 

2.11.2 Power Equipment List (PEL) 

The power listing for nominal loads was collected and presented in Table 2.7. 

TABLE 2.7.—PEL

 

Propulsion

(W)

Avionics

(W)

Comm.

(W)

Thermal

(W)

GN&C

(W)

Power

(W)

Science

(W)

CBE 

total

(W)

30 % 

margin

Total

(W) 

Launch 0 22 0 33 27 63 0 145 48.3 193

Star 48 operation 0 22 420 33 27 63 0 565 174.3 739

S/C separation 16 22 420 33 27 63 0 581 174.3 755

S/C checkout 16 22 420 33 36 63 265 855 256.59 1112

REP thrusting 1016 22 0 33 29 63 0 1163 48.9 1212

REP coast 16 11 0 33 29 63 2 154 46.23 200

Communications 16 22 420 33 29 63 2 585 175.53 761

Aerocapture 27 22 420 33 29 63 0 594 174.9 769

Titan science 16 22 0 33 29 63 265 428 128.4 556

Spiral to Enceladus 1016 22 0 33 29 63 0 1163 48.9 1212

Enceladus science 16 22 420 33 29 63 265 848 254.4 1102

Thruster input power 950

Thermal waste heat (W) 906

ASRG waste heat (W) 2800

Avionics/comm. or REP (W) 250

2.12 System Level Summary

The S/C mass summary for the cruise deck and REP S/C is shown in Table 2.8. It shows the current 

best estimate (CBE) and growth masses for each subsystem as well as propellant masses for the 

propulsion system. The additional system level growth mass to put the total growth at 30 percent is also 

shown. 
NASA/TM—2011-216972 16

TABLE 2.8.—SYSTEM LEVEL SUMMARY WITH GROWTH CALCULATIONS

COMPASS study: Radioisotope Electric Propulsion (REP) Study date: November 29, 2007 

GLIDE container: REP_Sept2007: REP_Flagship_reducescience 

REP S/C MEL rack-up (mass) COMPASS 

REP design 

WBS 

no.

Main subsystems CBE mass 

(lkg)

Growth 

(kg)

Total mass 

(kg)

Aggregate 

growth 

(%)

01 REP S/C (payload and bus) 1784.2 194.5 1978.7 ----- 

01.1 Science Payload 70.0 21.0 91.0 30.0 

01.2 REP S/C 1714.2 173.5 1887.7 ----- 

01.2.1 AD&C 18.4 7.4 22.1 40.0 

01.2.2 C&DH 33.3 11.4 44.7 34.3 

01.2.3 Communications and tracking 39.0 13.3 52.3 34.1 

01.2.4 Electric power 223.2 25.7 248.9 11.5 

01.2.5 Thermal control 61.6 9.2 70.8 15.0 

01.2.6 Propulsion 145.6 41.1 186.7 28.2 

01.2.7 Propellant (Xe and chemical) 847.5 ----- -------- ----- 

01.2.8 Structures and mechanisms 345.5 69.1 414.6 20.0 

Estimated REP S/C dry mass 937 195 1131.2 20.8 

Estimated REP S/C wet mass 1784 195 1978.7 ----- 

03 Cruise Deck chemical stage 896.6 57.2 953.9 ----- 

03.1 AD&C 6.5 1.3 7.8 20.0 

03.2 C&DH 3.0 0.9 3.9 30.0 

03.3 Communications and tracking 2.4 0.7 3.1 30.0 

03.4 Electrical Power subsystem 3.0 1.5 4.5 50.0 

03.5 Thermal control (non-propellant) 48.9 7.3 56.3 15.0 

03.6 Propulsion 74.4 22.3 96.7 30.0 

03.7 Propellant (chemical) 642.6 0.0 642.6 0.0 

03.8 Structures and mechanisms 115.8 23.2 139.0 20.0 

Estimated cruise deck dry mass 254 57 311.2 22.5 

Estimated cruise deck wet mass 897 57 953.9 ----- 

Total estimated dry mass and wet mass Total growth 

Estimated Flagship S/C total dry mass 1191 252 1442.4 21.1 

Estimated Flagship S/C total wet mass 2681 252 2932.5 ---- 

System level growth calculations Total growth 

Desired system level growth 1191 357 1547.8 30.0 

Additional growth (carried at system level) ------- 105 -------- 8.9 

Total wet mass with growth 2681 357 3038.0 

Available launch performance to C3 (kg) 3087.0 

Launch margin available (kg) 49.0 

2.13 Design Concept Drawing and Description

Figure 2.9 shows a side view of the REP Titan/Enceladus Orbiter S/C. All dimensions are in metric 

units. 
NASA/TM—2011-216972 17

Figure 2.9.—REP stage inside aeroshell with cruise deck attached, with dimensions. 

3.0 Subsystem Breakdown 

3.1 Attitude Control System (ACS)

The starting design for the ACS on the REP vehicle is borrowed from New Horizons (NH) 

• ACS hydrazine 

– Off-the-shelf (OTS) blow-down similar to NH

– Single tank with ~20 kg hydrazine 

3.1.1 ACS Requirements

Autonomous operations, EP navigation approach. Provides full 6-degrees of freedom (DOF) control 

of the vehicle from separation from the Delta IV H through end of mission. 

3.1.2 ACS Assumptions

Much of the design is based on current hardware. 

• IMU has Deep Impact, MESSENGER, Cassini, and NEAR heritage

• Sun sensors and Star Trackers taken from IIE Team X design

3.1.3 ACS Design and MEL

Figure 3.1 is the conceptual block diagram of the attitude control system. While the propulsion system 

(ACS) is included in this block diagram, its mass (hardware and propellant) is being tracked in the 

propulsion system section of the MEL. Likewise, the instrumentation and computational hardware being 

used to compute the attitude control information is being kept in the C&DH (avionics) section of the MEL.
NASA/TM—2011-216972 18

Figure 3.1.—Block diagram of GN&C system.

Figure 3.2.—Northrop Grumman Scalable 

Inertial Reference Unit (NG SIRU) for 

space.

Figure 3.3.—Adcole Star Tracker.

3.1.3.1 ACS Model Summary

• One internally redundant IMU (NG SIRU) shown in Figure 3.2. 

• Two Star Trackers on the cruise deck and two on the aeroshell/REP S/C (Adcole Corporation) 

Figure 3.3. These star trackers were the ones used on the NH S/C (Ref. 10) 

• Eight Sun Sensors on the cruise deck and eight on the aeroshell/REP S/C (EDO Corp Barnes 

Engineering Division)

• Four Reaction Wheels (Valley Forge Bearcat 5 Nms reaction wheel,

http://www.vfct.com/aerospace/wheels/wheels2.html)

• GN&C software run on main C&DH computers 
NASA/TM—2011-216972 19

Table 3.1 lists the items in the ACS MEL for the COMPASS REP S/C design. Table 3.2 lists the 

items in the ACS MEL for the COMPASS REP/Aeroshell Stage design. All growth allowances follow the 

AIAA WGA schedule in Section 2.4. Figure 3.4 shows the avionics deck of the REP S/C. 

TABLE 3.1.—ACS MEL FOR CHEMICAL CRUISE DECK/STAGE

WBS 

no.

Description

General Chemical Stage 

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ---- 896.63 6.38 57.23 953.86 

03.1 AD&C - ---- 6.46 20.00 1.29 7.75 

03.1.1 GN&C - ---- 6.46 20.00 1.29 7.75 

03.1.1.a Sun Sensors 8 0.01 0.08 20.00 0.02 0.10 

03.1.1.b Reaction Wheels 0 0.00 0.00 0.00 0.00 0.00 

03.1.1.c Star Trackers 2 3.19 6.38 20.00 1.28 7.66 

TABLE 3.2.—ACS MEL FOR REP/AEROSHELL STAGE

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

01 REP S/C (Payload and Stage) - ---- 1784.15 10.90 194.52 1978.67 

01.2 REP Bus - ---- 1714.15 10.12 173.52 1887.67 

01.2.1 AD&C - ---- 18.40 20.00 3.68 22.08 

01.2.1.a GN&C - ---- 18.40 20.00 3.68 22.08 

01.2.1.a.b Sun Sensors 8 0.01 0.04 20.00 0.01 0.05 

01.2.1.a.b Reaction Wheels 4 1.27 5.08 20.00 1.02 6.10 

01.2.1.a.c Star Trackers 2 3.19 6.38 20.00 1.28 7.66 

01.2.1.a.d IMU 1 6.90 6.90 20.00 1.38 8.28 

Figure 3.4.—Bottom view of the REP stage highlighting avionics, communications, 

power, science, and ACS. 
NASA/TM—2011-216972 20

3.1.4 ACS Trades

Trades were conducted to use the REP S/C star trackers using openings in the aero backshell. While 

this is still possible, simplicity drove the addition of two star trackers to the cruise deck.

3.1.5 ACS Analytical Methods

OTS components were used in design. 

3.1.6 ACS Risk Inputs

None identified at this time. To be assessed.

3.1.7 ACS Recommendations

Analysis into the amount of ∆V necessary for station keeping and attitude control throughout the 

mission life needs to be performed to determine whether the 80 m/s assumption is sufficient. 

Additionally, further research is necessary to determine whether the start trackers and sun sensors are 

capable of operating at the distances of the Saturnian bodies at the EOL of the trajectory.

3.2 Communications

3.2.1 Communications Requirements

Provide uplink and downlink capability throughout the primary and/or extended mission. The 

communications system must meet science mission requirements of 8 hr/day of downlink pointed to Earth 

and a minimum 6.3 kbps downlink at 34-m disk (or about 147 Mbits/day of downlink including a 

minimum of 10 percent for housekeeping). 

3.2.2 Communications Assumptions

Communications requirements are assumed not changed from the REP Centaur orbiter mission in 

CD–2007–16 (Ref. 3), except more or less input power is available to communications system. Assume 

the Deep Space Network (DSN) will be capable of supporting Ka-band downlink via 34-m antenna by 

2024. The communications system design is based on the NH concept of two onboard-integrated 

electronics modules (IEM). Overall harness requirements are reduced if the NH IEM design is 

implemented. 

3.2.3 Communications Design and MEL

• REP orbiter communications subsystem consists of 

– A fixed 2.1-m diameter Ka-band high gain antenna (HGA) 

– Two IEMs as in the NH housing many S/C functions, including C&DH, instrument interface 

circuitry, telemetry interface, solid state recorder, and receiver and exciter sections of the 

communications subsystem 

– Two 200-W Traveling Wave Tube Amplifier (TWTA) to provide high power RF downlink 

output 

– RF switch assembly to interconnect antenna with two TWTAs and the rest of 

communications subsystem 

– Cabling 

• Link parameters for Ka-band downlink to 34-m ground stations 

– Ka-band downlink frequency: 32 GHz 

– Range to Earth ground station: 150.0 by 107 km 

– RF power: 200 W = 23.0 dBW 

– Antenna gain: 54.34 dBi (with 50 percent EFF at 32.0 GHz)

– Pointing loss: 1 dB 
NASA/TM—2011-216972 21

– Propagation and polarization loss: 2.0 dB 

– Equivalent isotropic radiated power (EIRP): 73.05 dBW 

• Cruise Deck

– Communications system on the cruise stage consists of cabling and a MGA to provide 

communications while the REP orbiter HGA is inside the aeroshell.

Table 3.3 and Table 3.4 list the items in the communications system MEL for the COMPASS Cruise 

Deck and the REP/Aerocapture Stage in the REP Flagship S/C design. All growth allowances follow the 

AIAA WGA schedule in Section 2.4. 

TABLE 3.3.—COMMUNICATIONS MEL CHEMICAL CRUISE DECK

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ---- 896.63 6.38 57.23 953.86 

03.3 Communications and Tracking - ---- 2.40 30.00 0.72 3.12 

03.3.2 MGA - ---- 2.40 30.00 0.72 3.12 

03.3.2.a Transponder 0 0.00 0.00 0.00 0.00 0.00 

03.3.2.b RF Assembly 0 0.00 0.00 0.00 0.00 0.00 

03.3.2.c Processing Module 0 0.00 0.00 0.00 0.00 0.00 

03.3.2.d Antenna 1 2.00 2.00 30.00 0.60 2.60 

03.3.2.e Cabling 2 0.20 0.40 30.00 0.12 0.52 

TABLE 3.4.—COMMUNICATIONS MEL REP/AEROCAPTURE STAGE

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

01 REP S/C (Payload and Stage) - ---- 1784.15 10.90 194.52 1978.67 

01.2.3 Communications and Tracking - ---- 39.00 34.10 13.30 52.30 

01.2.3.a X/Ka high gain antenna - ---- 27.00 31.48 8.50 35.50 

01.2.3.a.a Transmitter/receiver 2 4.00 8.00 30.00 2.40 10.40 

01.2.3.a.b Power amp 2 3.00 6.00 30.00 1.80 7.80 

01.2.3.a.c Switch unit 0 0.00 0.00 0.00 0.00 0.00 

01.2.3.a.d Antenna 1 9.00 9.00 30.00 2.70 11.70 

01.2.3.a.e Band pass filter 0 0.00 0.00 0.00 0.00 0.00 

01.2.3.a.f Band reject filter 0 0.00 0.00 0.00 0.00 0.00 

01.2.3.a.g Sensor 0 0.00 0.00 0.00 0.00 0.00 

01.2.3.a.h Cabling 2 2.00 4.00 40.00 1.60 5.60 

01.2.3.c Communications instrumentation - ---- 12.00 40.00 4.80 16.80 

01.2.3.c.a Coaxial Cable 2 6.00 12.00 40.00 4.80 16.80 
NASA/TM—2011-216972 22

3.2.4 Communications Trades

With the large power available from the idle EP system, more power than is usually enjoyed by a 

flagship S/C is available for science and communications. One use of the additional power is to increase 

the data rate and minimize the DSN contact time (and thus costs). 

3.2.5 Communications Analytical Methods 

The link budgets provide values of RF transmit powers at 40 and 200 W and antenna gains for Kaband. Note that X-band is not used in the final design; Ka-band to X-band link budgets are provided for 

comparison in Table 3.5. Link margins of 3 dB or better exist for all links. The Ka-band with 200 W RF 

output was baselined to maximize data rate/minimize DSN time. Table 3.5 lists the Communications Link 

budget analysis performed to size this system.

TABLE 3.5.—LINK BUDGET ANALYSIS

Item Units Ka-band downlink X-band downlink

Frequency GHz 32 32 7.75 7.75 7.75 7.75

Transmitter power W 40 200 40 40 200 200

Transmitter power dBW 16.0 23.0 16.0 16.0 23.0 23.0

Transmitter line loss dB 1.0 1.0 1.0 1.0 1.0 1.0

Transmit antenna beamwidth deg 0.31 0.31 1.29 1.29 1.29 1.29

Peak transmit antenna gain dBi 54.04 54.04 42.02 42.02 42.02 42.02

Transmit antenna diameter m 2.1 2.1 2.1 2.1 2.1 2.1

Transmit antenna pointing offset deg 0.1 0.1 0.37 0.37 0.37 0.37

Transmit antenna pointing loss dB 1.0 1.0 1.0 1.0 1.0 1.0

Transmit antenna gain (net) dBi 54.34 54.34 41.72 41.72 41.72 41.72

Equivalent isotropic radiated power dBW 66.06 73.05 53.74 53.74 60.73 60.73

Propagation path length km 150×107 150×107 150×107 150×107 150×107 150×107

Space loss dB 306.04 306.04 293.73 293.73 293.73 293.73

Propagation and polarization loss dB 2.0 2.0 2.0 2.0 2.0 2.0

Receive antenna diameter m 34.0 34.0 70.0 34.0 70.0 34.0

Peak receive antenna gain (net) dBi 78.5 78.5 72.5 66.2 72.49 66.2

Receive antenna beamwidth deg 0.02 0.02 0.04 0.08 0.04 0.08

Receive antenna pointing error deg 0.00 0.00 0.00 0.01 0.00 0.01

Receive antenna pointing loss dB 0.10 0.10 0.10 0.10 0.10 0.10

Receive antenna gain dBi 78.22 78.22 72.2 65.91 72.2 65.91

System noise temperature K 15.49 15.49 15.49 15.49 15.49 15.49

Data rate kbps 7 33 2 0.4 8 2 

Eb/No dB 5.10 5.10 5.10 5.10 5.21 5.10

Carrier-to-noise density ratio dB-Hz 43.29 50.28 37.25 31.0 44.24 37.96

Bit error rate -------- 10–5 10–5 10–5 10–5 10–5 10–5

Required Eb/No dB 2.0 2.0 2.0 2.0 2.0 2.0

Implementation loss dB 0.10 0.10 0.10 0.10 0.10 0.10

Link margin dB 3.0 3.0 3.0 3.0 3.11 3.0
NASA/TM—2011-216972 23

Figure 3.5.—HGA system. 

Figure 3.5 is a graphical representation of the communications system of a HGA courtesy of JPL.

3.2.6 Communications Risk Inputs

None assessed at this time.

3.2.7 Communications Recommendation

In the future, further analysis should be done considering the use of the communications system from 

the NH mission. Figure 3.6 shows a detailed block diagram of that communications system.
NASA/TM—2011-216972 24

Figure 3.6.—NH avionics layout. 

3.3 Command and Data Handling (C&DH)—(Avionics)

3.3.1 Avionics Requirements

The Design requirements, from the science payload and the REP Bus, for the C&DH system were as 

follows 

• Storage for TBD days of data (TBD, est. 8 to 16 GB) 

• Avionics for systems command, control, and health management 

• Payload control will be done by the C&DH system

• Single fault tolerant avionics

3.3.2 Avionics Assumptions

• All electronics are ≥65 Krad avionics

• Cabling is estimated as 50 percent of the avionics hardware mass

• Avionics spares are cold spares to minimize power consumption

• NH S/C was used as the starting point for the avionics hardware design

3.3.3 Avionics Design and MEL

All avionics components used in the design are based on commercially available components from 

BAE and SEAKR. There are two independent avionics boxes to provide for single fault tolerance. Each 

avionics box contains a GN&C/C&DH RAD6000 processor, 256 MB GN&C solid state memory card, 

SSR card, a Comm. interface card, and a payload interface card. The 1553 processor is used for 

communications between the GN&C processor and GN&C hardware, i.e., star trackers, IMUs, etc. The 

GN&C and C&DH computers communicate via the 1553 bus. 

Table 3.6 lists the components used in the COMPASS C&DH MEL design on the cruise deck and 

Table 3.7 lists the C&DH design in the REP/Aeroshell stage. These are the inputs from the subsystem 

lead. All growth allowances follow the AIAA WGA schedule in Section 2.4. 
NASA/TM—2011-216972 25

TABLE 3.6.—AVIONICS (C&DH) MEL CHEMICAL CRUISE DECK

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ---- 896.63 6.38 57.23 953.86 

03.2 C&DH - ---- 3.00 30.00 0.90 3.90 

03.2.1 C&DH - ---- 3.00 30.00 0.90 3.90 

03.2.1.a Flight computer 0 0.00 0.00 0.00 0.00 0.00 

03.2.1.b Command and telemetry computer 0 0.00 0.00 0.00 0.00 0.00 

03.2.1.c Data interface unit 1 2.00 2.00 30.00 0.60 2.60 

03.2.1.d Data bus operations amplifier 0 0.00 0.00 0.00 0.00 0.00 

03.2.1.e Operations recorder 0 0.00 0.00 0.00 0.00 0.00 

03.2.1.f Command and control harness (data) 1 1.00 1.00 30.00 0.30 1.30 

TABLE 3.7.—AVIONICS (C&DH) MEL REP/AEROSHELL STAGE

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

01 REP S/C (Payload and Stage) - ------ 1784.15 10.90 194.52 1978.67 

01.2.2 C&DH - ------ 33.30 34.26 11.41 44.71 

01.2.2.a C&DH - ------ 33.30 34.26 11.41 44.71 

01.2.2.a.a Flight computer 2 8.00 16.00 25.00 4.00 20.00 

01.2.2.a.b Command and telemetry computer 0 0.00 0.00 0.00 0.00 0.00 

01.2.2.a.c Data interface unit 2 2.00 4.00 30.00 1.20 5.20 

01.2.2.a.d Data bus operations amplifier 0 0.00 0.00 0.00 0.00 0.00 

01.2.2.a.e Operations recorder 2 1.10 2.20 30.00 0.66 2.86 

01.2.2.a.f Command and control harness (data) 1 11.10 11.10 50.00 5.55 16.65 

3.3.4 Avionics Trades

None performed to date, but comparison to NH hardware to be completed at a later date.

3.3.5 Avionics Analytical Methods 

OTS components were used in design. 

3.3.6 Avionics Concerns, Comments, Recommendations

• No ultra-stable oscillator (USO)/atomic clock included in avionics hardware. Should it be 

included in Communication system?

• Processing power of the RAD6000 is assumed to be adequate for GN&C, C&DH, and science 

payload 

• Storage requirements are driven by fly-by storage needs (which are still being estimated)

• Only one SSR would be active at a time and thus susceptible to SEUs 

• Total radiation dose is a concern with all deep space missions. This preliminary design has 

attempted to use only hardware which has already been proven in a deep space mission to assure 

the life of the electronics over the 12-yr mission. 

3.4 Electrical Power System

3.4.1 Power Requirements

Minimize power for non-propulsion during EP operation (minimize plutonium needed). 
NASA/TM—2011-216972 26

3.4.2 Power Assumptions

The baseline REP Stage design used eight ASRGs for the generation of power to be used by the 

science instruments, avionics and electric propulsion thrusters. The specific power assumptions of the 

ASRGs used are listed under Figure 3.7 in the following section. 

3.4.3 Power Design and MEL

Eight ASRGs (12 GPHS) are designed to provide 1120 W to power the REP S/C at beginning of life 

(BOL). The system is designed to provide 1040 W to the REP S/C at EOL (10-yr). There are negligible 

thermal interactions between the ASRGs. Figure 3.7 shows a typical ASRG with the main components 

called out in the graphic. The eight are connected together via a Shunt Regulator/Bus Protection (RBI) 

assembly. This RBI isolates the ASRGs from S/C bus and each other and follows load demands from S/C

bus. There is an approximately 6 percent loss through the RBI and monitoring circuitry (94 percent of 

power flows through to loads) with 53 W used for fault detection/monitoring. Included in this system is a

bus Capacitance of 3000 μf which provides some bus rigidity. Power cabling and harness systems design 

assumes a 1 percent line loss.

• Specific performance details on the ASRG unit are as follows:

– Power: 140 W at 28 ± 0.2 V BOL

130 W at 28 ± 0.2 V EOL (10 yr) 

EOM Deep Space (14 yr) 126 We 

– Mass: 20.2 to 21.5 kg with mounting isolator plate 

– Envelope: 30.5 cm W, 46 cm H, 76 cm L (12 in. W, 18 in. H, 30 in. L)

– Specific power: 6.7 We/kg 

Figure 3.7.—ASRG computer aided design (CAD) model. 
NASA/TM—2011-216972 27

ASRG Design Attributes

• Two Stirling converters 

– Co-axially aligned for dynamic balance 

– One GPHS module per converter 

• Integrated, single-fault tolerant controller 

• Autonomous operation and fault isolation from S/C

• S/C disturbance torque requirement < 35 N-m 

– Based on 1000 kg, 1-m cube S/C with 5-μrad pointing accuracy and a safety factor of 5 

Table 3.8 lists the items in the Power system MEL for the Cruise deck design. Table 3.9 lists the 

items in the Power system MEL for the COMPASS REP/Aeroshell S/C design. All growth allowances 

follow the AIAA WGA schedule in Section 2.4. 

TABLE 3.8.—POWER SYSTEM MEL CHEMICAL CRUISE DECK

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ---- 896.63 6.38 57.23 953.86 

03.4 Electrical power subsystem - ---- 3.00 50.00 1.50 4.50 

03.4.3 Power cable and harness subsystem - ---- 3.00 50.00 1.50 4.50 

03.4.3.a S/C bus harness 0 0.00 0.00 0.00 0.00 0.00 

03.4.3.b PMAD harness 0 0.00 0.00 0.00 0.00 0.00 

03.4.3.c Electric propulsion harness 0 0.00 0.00 0.00 0.00 0.00 

03.4.3.d RPS to S/C harness 0 0.00 0.00 0.00 0.00 0.00 

03.4.3.e Power cabling 1 3.00 3.00 50.00 1.50 4.50 

TABLE 3.9.—POWER SYSTEM MEL REP/AEROSHELL STAGE

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

01 REP S/C (Payload and Stage) - ------ 1784.15 10.90 194.52 1978.67 

01.2.4 Electrical Power Subsystem - ------ 223.24 11.51 25.70 248.94 

01.2.4.a RPS - ------ 155.76 10.00 15.58 171.34 

01.2.4.a.a RPS Main System 8 19.47 155.76 10.00 15.58 171.34 

01.2.4.a.b Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

01.2.4.b PMAD - ------ 32.48 15.00 4.87 37.35 

01.2.4.b.a Power management/control electronics 0 0.00 0.00 0.00 0.00 0.00 

01.2.4.b.b Power distribution/monitoring wiring harness 0 0.00 0.00 0.00 0.00 0.00 

01.2.4.b.c DC switchgear/shunt regulator 1 32.48 32.48 15.00 4.87 37.35 

01.2.4.b.d Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

01.2.4.c Power cable and harness subsystem - ------ 35.00 15.00 5.25 40.25 

01.2.4.c.a S/C bus harness 1 7.00 7.00 15.00 1.05 8.05 

01.2.4.c.b PMAD harness 1 7.00 7.00 15.00 1.05 8.05 

01.2.4.c.c Electric propulsion harness 1 7.00 7.00 15.00 1.05 8.05 

01.2.4.c.d RPS to S/C harness 1 7.00 7.00 15.00 1.05 8.05 

01.2.4.c.e Power cabling 1 7.00 7.00 15.00 1.05 8.05 
NASA/TM—2011-216972 28

3.4.4 Power Trades

A further trade will be made on the number of RTGs necessary to provide sufficient power to the 

thrusters and instruments to perform the mission.

For the power system, two other options under consideration were as follows

Option 1 

• Direct drive the Hall thrusters 

• Use of dual alternator (providing 600 V and 28 V, 100 Hz AC) 

• 10/1 power ratio on dual alternators 

• The 600 V AC converts to 400 V DC

• Power to thrusters EOM 646 W 

• Power to payload EOM 76 W 

Option 2 

• DC/DC conversion to 28 V DC provided from ASRG as designed 

• DC/DC conversion to 400 V DC for hall thruster

• The current estimate of single 600 W DC/DC converter at 30 kg 

Table 3.10 lists the impact of trade in the number of SRGs and total power available, as well as 

excess power to be radiated.

• Eight SRGs provide (1120 W BOL, 1040 EOL) 750 W power into the thruster with excess 14 W 

EOL

• Loss of SRG would limit the power to ~650 W into thrusters

TABLE 3.10.—TRADE ANALYSIS OF VARYING NUMBER OF SRGS

Number SRG 4 5 6 7 8 

Power (EOL, 10 yr) 130 130 130 130 130

Total Power EOL (W) 520 650 780 910 1040

Into thruster (W) 250 400 500 650 750

PPU, Line Loss 25 40 50 65 75

Housekeeping (cruise only) 155 155 155 155 155

Housekeep margin (30%) 46.5 46.5 46.5 46.5 46.5

Excess 44 9 29 –7 14

3.4.5 Power Analytical Methods

The modeling of the ASRG units were based on current analysis being done at GRC in the area of 

ASRG development. 

3.4.6 Power Risk Inputs

Will there be power available in current configuration?

• If the angle between ASRGs were changed to 90°, better view 

• Vibration/thermal leak to science

3.4.7 Power Recommendation

Further analysis should be done on the alternate power options outlines in Section 3.4.4. 
NASA/TM—2011-216972 29

3.5 Structures and Mechanisms

3.5.1 Structures and Mechanisms Requirements

The REP S/C structure must contain necessary hardware for instrumentation, avionics, 

communications, propulsion and power. It must be able to withstand applied loads from launch vehicle 

and provide minimum deflections, sufficient stiffness, and vibration damping. This analysis assumed a 

maximum axial load of 6g. The goal of the design is to minimize weight of the components that make up 

the structure of the S/C bus, and must fit within the physical confines of the launch vehicle. 

3.5.2 Structures and Mechanisms Assumptions

The basic assumptions made in the design process of the S/C bus structure were

• Material: Al alloy 2090-T3

• Space frame with tubular members 

• Composite sandwich structure shelf assumed to be all Al using Al 2090-T3 face sheets and an Al

honeycomb core with the trade name, Alcore Higrid 

• Welded and threaded fastener assembly

3.5.3 Structures and Mechanisms Design and MEL

3.5.3.1 Chemical Cruise Stage

The cruise deck is used to perform the ∆V from Earth escape to Titan capture. A separate chemical 

cruise stage MEL (Table 3.11) was developed to track the items of the chemical cruise stage. This way, 

the stage and its mass were more easily dropped during the system modeling and mission modes to 

accurately calculate propellant loads at each mission phase.

TABLE 3.11.—STRUCTURES AND MECHANISMS MEL CHEMICAL CRUISE STAGE

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ------- 896.63 6.38 57.23 953.86 

03.8 Structures and mechanisms - ------- 115.83 20.00 23.17 139.00 

03.8.1 Structures - ------- 108.60 20.00 21.72 130.32 

03.8.1.a Primary structures - ------- 90.42 20.00 18.08 108.50 

03.8.1.a.a Main bus structure 1 10.77 10.77 20.00 2.15 12.92 

03.8.1.a.b Cruise deck structure 1 79.65 79.65 20.00 15.93 95.58 

03.8.1.b Secondary structures - ------- 18.18 20.00 3.64 21.82 

03.8.1.b.a Balance mass 0 0.00 0.00 0.00 0.00 0.00 

03.8.1.b.b Tank supports and bracketry 1 18.18 18.18 20.00 3.64 21.82 

03.8.2 Mechanisms - ------- 7.23 20.00 1.45 8.68 

03.8.2.e Adaptors and separation - ------- 7.23 20.00 1.45 8.68 

03.8.2.e.b Separation mechanism from LV 1 2.88 2.88 20.00 0.58 3.46 

03.8.2.e.d Separation mechanism REP probe 1 4.35 4.35 20.00 0.87 5.22 

3.5.3.2 Aeroshell

The aeroshell is modeled using the 2003 NASA Titan Aerocapture studies data (Refs. 4, 5, and 6). It 

can sustain aerocapture maneuvers up to 6.5 km/s. The aeroshell components are modeled in the main 

REP MEL (see Table 3.12).

Figure 3.8 is the REP stage of the vehicle with dimensions. The antenna is down, the ASRGs point 

out to left and right. The Aeroshell and Chemical Cruise Deck are not shown.
NASA/TM—2011-216972 30

Figure 3.8.—REP S/C with dimensions.

Figure 3.9.—ASRG loading access to REP S/C design.

The structural design of the Aeroshell and the shroud needs to allow for four access panels to the 

ASRGs while the REP S/C is being loaded on the pad. Figure 3.9 shows the REP S/C within the 

Aeroshell and within the ELV Fairing, noting where access points would need to be accommodated in the 

design. The four separate ports can be seen in the top down view on the right. 

Loading access through fairing

Loading access through aeroshell
NASA/TM—2011-216972 31

Figure 3.10.—REP S/C (after aeroshell separation). 

3.5.3.3 REP S/C 

Description of the Design

• Tubular space frame in octagonal configuration 

• Shelf of composite sandwich architecture with honeycomb core to mount hardware 

– Composite sandwich structure shelf assumed to be all Al using Al 2090-T3 face sheets and an 

Al honeycomb core with the trade name, Alcore Higrid. 

• Thin sheets for shear panels and to enclose structure

• ASRGs mounted to main bus through vibration isolators

Figure 3.10 gives the conceptual design of the base REP S/C inside the aeroshell.

All growth allowances follow the AIAA WGA schedule in Section 2.4. 

TABLE 3.12.—STRUCTURES AND MECHANISMS REP STAGE AND AEROSHELL MEL

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

01 REP S/C (Payload and Stage) - -------- 1784.15 10.90 194.52 1978.67

01.2.8 Structures and mechanisms - -------- 345.49 20.00 69.10 414.59

01.2.8.a Structures - -------- 330.63 20.00 66.13 396.76

01.2.8.a.a Primary structures - -------- 225.83 20.00 45.17 271.00

01.2.8.a.a.a Main bus structure 1 35.63 35.63 20.00 7.13 42.76

01.2.8.a.a.b Remove this cruise deck mass 0 0.00 0.00 20.00 0.00 0.00

01.2.8.a.a.c Aeroshell-heat shield (dropped) 1 190.20 190.20 20.00 38.04 228.24

01.2.8.a.b Secondary structures - -------- 104.80 20.00 20.96 125.76

01.2.8.a.b.a Aeroshell-backshell (dropped) 1 71.96 71.96 20.00 14.39 86.35

01.2.8.a.b.b Tank supports and bracketry 1 4.55 4.55 20.00 0.91 5.45

01.2.8.a.b.c SRG support structure 8 2.63 21.03 20.00 4.21 25.23

01.2.8.a.b.d SRG vibration isolation hardware 8 0.91 7.27 20.00 1.45 8.73

01.2.8.b Mechanisms - -------- 14.86 20.00 2.97 17.83

01.2.8.b.f Installations - -------- 14.86 20.00 2.97 17.83

01.2.8.b.f.a Science payload installation 1 1.76 1.76 20.00 0.35 2.12

01.2.8.b.f.b C&DH installation 1 1.33 1.33 20.00 0.27 1.60

01.2.8.b.f.c Communications and tracking installation 1 1.56 1.56 20.00 0.31 1.87

01.2.8.b.f.d GN&C installation 1 0.74 0.74 20.00 0.15 0.88

01.2.8.b.f.e Electrical Power Installation 0 0.00 0.00 0.00 0.00 0.00

01.2.8.b.f.f Thermal control installation 1 3.03 3.03 20.00 0.61 3.63

01.2.8.b.f.g Electric propulsion installation 1 6.44 6.44 20.00 1.29 7.73
NASA/TM—2011-216972 32

3.5.4 Structures and Mechanisms Trades

Next steps in the analysis are to perform a trade on the use of composite for the main bus 

compartment structure, sizing of space frame to accommodate requirements for antenna, SRGs, and 

instrumentation while fitting within confines of launch vehicle.

3.5.5 Structures and Mechanisms Analytical Methods

Preliminary structural analysis and modeling was performed using the given launch loads and 

dimensions of the desired S/C bus. The loads assumptions were 6g axial loading, 3.5g lateral loading (not 

concurrent with max axial loading), using Al 2090-T3 as the material and a 1.4 safety factor. An 

additional installation mass was held for each subsystem in the mechanisms section of the structures 

system. These installations were modeled using 4 percent of the CBE dry mass of each of the subsystems.

A 20 percent growth margin was applied to that installation mass.

3.5.6 Structures and Mechanisms Risk Inputs

Risk analysis to be performed.

3.5.7 Structures and Mechanisms Recommendation

Use a more detailed structural analysis for loads and vibrations using a modeling tool, i.e., finite 

element analysis (FEA). The use of graphite/polymer composites on the Shelf face sheets and outer sheets 

may provide for further weight savings but at increased cost.

3.6 Propulsion and Propellant Management

3.6.1 Propulsion and Propellant Management Requirements

This Flagship mission combined three different propulsion/capture technologies to perform various 

stages of the mission in order to deliver the maximum S/C mass or fit inside of the performance cap of the 

Delta IV H launch vehicle. These three modes were a chemical cruise stage, an aeroshell capture into 

Titan, and an REP powered EP spiral thrusting system. The two propulsion options: chemical cruise stage 

and REP moon spiral system will be discussed here.

The Chemical Cruise Stage was required to provide the necessary propulsion burns during the EarthSaturn Transit stage of the mission. These burns were required to provide necessary ∆V for orbit shaping 

and necessary ∆V for attitude control. Because these burns are completed during transit to Saturn, S/C

mass can be reduced substantially by dropping the no longer needed elements thereby reducing 

subsequent propulsion requirements later in the mission. 

The requirements for the REP system in spiraling from Titan to Enceladus and associated maneuvers 

are a result of a trade between trip time and payload mass. Trip time is inversely proportional to the Isp, 

while payload mass will follow the rocket equation. The requirements chosen for the purpose of this study 

are

• 1000 W 

• 1500 s Isp

• 587 kg throughput 

3.6.2 Propulsion and Propellant Management Assumptions

3.6.2.1 Chemical Cruise Stage

Both primary and attitude control thrusters are single-fault tolerant as they both have redundant 

engines in place for their respective burns. However, vehicle operations may be required to enable 

redundant use of attitude control thrusters. The fuel monomethyl hydrazine (MMH) and oxidizer 

(nitrogen tetroxide (NT)) are stored in two cylindrical tanks with a He pressurization system. All valving 
NASA/TM—2011-216972 33

is assumed to be dual-seat to insure required sealing. Propellant lines, tanks, and pressurization are zerofault tolerant (single string). Propellant tanks and lines are wrapped in tape heaters and insulation to 

insure that the propellant does not freeze, which could have very detrimental results. Further description 

of the propulsion system configuration is provided below.

3.6.2.2 REP Stage

The baseline system redundancy assumption was based on single string units. In other words, a 

propulsion system unit consists of a string of thruster, PPU, gimbal, and propellant management system. 

Spares or redundant units are assumed to consist of all of the above subsystems. It is important to note 

that the mission is already modeled with a 90 percent duty cycle. So, 10 percent of the time, the S/C is 

coasting along its trajectory.

The RCS propulsion subsystem follows the same design assumptions as the chemical propulsion 

system on the cruise stage

The baseline propulsion system design consists of the following items

• One active 1000 W Long Life Hall engine with two extra for life and one cold spare

• Four PPUs: no cross-strap

• Two-axis range of motion: TBD

• 1 percent Xe unusable

• Pounds per square inch spherical COPV Xe tank

• OTS hydrazine system with NH heritage

3.6.3 Propulsion and Propellant Management Design Trades

3.6.3.1 Chemical Cruise Stage

The only trade study made for the Cruise stage was to compare monoprop versus biprop propulsion 

option for the Auxiliary Propulsion System. 

• For the monoprop option: 

– Auxiliary thruster: Aerojet MR-104 thruster, 441 N, 239 s Isp

– Reaction thrusters: Aerojet MR-111 thruster, 4.4 N thrust, 229 s Isp

– Propellant storage configuration: Single fuel (hydrazine) COPV spherical tank with a 

blowdown pressurization system 

• For the biprop option: 

– Auxiliary thruster: Aerojet R-4D “HiPAT” thruster, 445 N thrust, 323 s Isp

– Reaction thrusters: Rocketdyne R-53 thruster, 8.9 N thrust, 295 s Isp

– Propellant storage configuration: Dual fuel (MMH) and Oxidizer (NTO) COPV cylindrical 

tanks with single dedicated He pressurization tank 

In this comparison, the Reaction Control Thrusters were assumed to operate on the same propellant as 

the Auxiliary thrusters. 

The Biprop option was selected because of an overall lower subsystem mass due to the higher engine 

specific impulse. Additionally, the four cylindrical tanks for the propellants were easier to fit into the S/C

volume than the larger volume of the single hydrazine tank. 

3.6.3.2 REP Stage

The trades to be considered in designing the two major propulsion systems on the S/C (main and 

RCS) are as follows

• Main REP propulsion system 
NASA/TM—2011-216972 34

– ~1000 W Xe Hall or Ion thruster 

 Two for life, one more for spare 

 Direct Drive System

– Up to 600 kg Xe stored in COPV tanks 

The possible main EP system options to be considered for this design are

• New Advanced Technology Small Hall Thruster (Figure 3.11) 

– Based on ongoing HiVHAC program at GRC 

– Optimized design to allow up to 2000 s Isp at powers below 1 kW 

– Allows long life needed for mission

• Derated HiVHAC

– Maximum Isp at 1 kW ~1570 s 

– Performance inadequate for range of REP missions

• Commercial-off-the-shelf (COTS) 

– SPT-70/BPT-600 

– 600 W, ~ 1500 s 

– Limited life/throughput (35 to 50 kg)

• Low power (20 cm) Ion 

The Advanced Hall thruster option was initially chosen both for its potential for Direct Drive 

operation (see power system discussion), and for its superior performance in terms of efficiency (or 

equivalently, thrust-to-power) at the low power levels characteristic of REP. The 20 cm ion thruster 

projected performance was inferior to that of the Hall below 1 kW and at 2000 s or less Isp. The 

commercial Hall thrusters increased system mass and complexity through the increased number of 

propulsion strings (13 or more) needed to meet lifetime and redundancy requirements. 

The possible EP thruster system options, once an EP thruster type has been chosen, to be considered 

are

• Hall

– Standard PPU 

– “Direct Drive” from Stirling Alternator

Because of limitations in the ASRG alternator design, the “Direct Drive” option was discarded and a 

standard PPU option was selected.

Figure 3.11.—Advanced Technology 

Small Hall Thruster. 
NASA/TM—2011-216972 35

3.6.4 Propulsion and Propellant Management Design and MEL

3.6.4.1 Chemical Cruise Stage

The cruise stage propulsion system has two functions. First, the auxiliary propulsion is used for orbit 

shaping which are performed with two Aerojet HiPAT (R-4D-derived thruster) bipropellant (MMH/NTO) 

thrusters operating at 441 N. The orbit shaping maneuvers are performed with a single thruster. The 

second thruster was operated in the event of a fault with the first engine.

Second, S/C attitude control requirements are performed with a set of twelve Rocketdyne R-53 8.9 N 

bipropellant thrusters. These thrusters were arranged in four pods of three thrusters that are arranged 

orthogonally in order to provide all attitude control thrusting. 

As mentioned above, the MMH fuel and NTO oxidizer are stored in two cylindrical carbon overwrapped (COPV) tanks each. It was decided to use two tanks for each liquid in order ease the volumetric 

conflicts with other elements in the S/C. Each tank is wrapped with tape heaters in order to maintain the 

required propellant temperatures. The propellant is delivered from the tanks to the thrusters through 

3/8-in. lines that are also wrapped with tape heaters to maintain propellant temperatures. While the 

propellant lines are zero-fault tolerant, each of the isolation and control valves are dual-seat to provide 

single fault tolerance.

The propellant tanks were pressurized with He gas that is stored at high pressure in a single metallic 

tank for fuel and oxidizer tanks, respectively. The He management subsystem is single-fault tolerant with 

dual-seat isolation valves and redundant pressure regulators. However, the He lines are zero-fault tolerant.

Figure 3.12 shows a schematic of the propulsion subsystem.

Table 3.13 lists the MEL for the propulsion system in the chemical cruise stage. All growth 

allowances follow the AIAA WGA schedule in Section 2.4. 

Figure 3.12.—RCS Configuration for Cruise Stage. 
NASA/TM—2011-216972 36

Table 3.14 lists the MEL for the propellant in the chemical cruise stage. Note that the propellant is 

modeled as an RCS system. All growth allowances follow the AIAA WGA schedule in Section 2.4. 

TABLE 3.13.—PROPULSION AND PROPELLANT MANAGEMENT SYSTEM MEL CHEMICAL CRUISE STAGE 

WBS 

no.

Description

Generic Chemical Stage

Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

03 Cruise Deck/Chemical Stage - ------- 896.63 ------- 57.23 953.86 

03.6 Propulsion - ------- 74.38 30.00 22.31 96.69 

03.6.1 Primary chemical system hardware - ------- 13.55 30.00 4.06 17.61 

03.6.1.a Main engine - ------ 13.55 30.00 4.06 17.61 

03.6.1.a.a Main engine 2 6.12 12.25 30.00 3.67 15.92 

03.6.1.a.b Main engine gimbal 2 0.65 1.30 30.00 0.39 1.69 

03.6.1.a.c Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

03.6.1.a.d Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

03.6.2 Propellant management (chemical) - ------- 50.47 30.00 15.14 65.61 

03.6.2.a Main engine propellant management - ------- 3.34 30.00 1.00 4.34 

03.6.2.a.a Fuel tanks 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.b Fuel lines 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.c Oxidizer tanks 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.d Oxidizer lines 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.e Pressurization system—tanks, panels, lines 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.f Feed system—regulators, valves, etc 1 3.34 3.34 30.00 1.00 4.34 

03.6.2.a.g Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.a.h Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.b RCS propellant management - ------- 47.13 30.00 14.14 61.27 

03.6.2.b.a Fuel tanks 1 30.54 30.54 30.00 9.16 39.71 

03.6.2.b.b Fuel lines 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.b.c Pressurization System—tanks, panels, lines 1 10.01 10.01 30.00 3.00 13.01 

03.6.2.b.d Feed System—regulators, valves, etc 1 6.58 6.58 30.00 1.97 8.55 

03.6.2.b.e Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

03.6.2.b.f Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

03.6.3 RCS hardware - ------- 10.36 30.00 3.11 13.47 

03.6.3.a RCS engines 4 2.59 10.36 30.00 3.11 13.47 

03.6.3.b RCS thruster subassembly 0 0.00 0.00 0.00 0.00 0.00 

TABLE 3.14.—PROPELLANT MEL FOR CHEMICAL CRUISE STAGE

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - --------- 896.63 6.38 57.23 953.86 

03.7 Propellant (chemical) - --------- 642.63 0.00 0.00 642.63 

03.7.1 RCS propellant - --------- 642.63 0.00 0.00 642.63 

03.7.1.a Fuel - --------- 241.38 0.00 0.00 241.38 

03.7.1.a.a Fuel usable 1 235.49 235.49 0.00 0.00 235.49 

03.7.1.a.b Fuel boiloff 0 0.00 0.00 0.00 0.00 0.00 

03.7.1.a.c Fuel residuals (unused) 1 5.89 5.89 0.00 0.00 5.89 

03.7.1.b Oxidizer - --------- 398.27 0.00 0.00 398.27 

03.7.1.b.a Oxidizer usable 1 388.56 388.56 0.00 0.00 388.56 

03.7.1.b.b Oxidizer boiloff 0 0.00 0.00 0.00 0.00 0.00 

03.7.1.b.c Oxidizer residuals (unused) 1 9.71 9.71 0.00 0.00 9.71 

03.7.1.c RCS pressurant 1 2.99 2.99 0.00 0.00 2.99 
NASA/TM—2011-216972 37

3.6.4.2 REP Powered Main EP System (Xe) for Spiral at Saturn System

The main EP system is comprised of:

• Four extended life, High Isp Hall Thrusters (one operating)

– One active 1000 W Long Life Hall engine 

 Two extra thrusters for life and one cold spare 

– 30,000 hr life, 300 to 600 V

• Dedicated PPU for each thruster; no cross-strap

• Dedicated gimbal for each thruster

– Two-axis range of motion: ±XX°, ±ZZ° 

• One spherical COPV high pressure (2800 psi) Xe tank 

• Propellant distribution system: Single string (zero-fault tolerant) Propellant Management System 

(PMS) to each thruster from balanced tank feed

• Thermal control system for propellant management subsystem

– Tape heaters and insulation used on Xe tank and feedlines 

• Total propellant

– 540 kg used

– 8.6 percent residual + margin unusable Xe

The Advanced Hall thruster assumed to be 50 percent heavier than SPT-70 thruster and have 

30,000-hr life capabilities.

Figure 3.13 is a schematic of the EP system and propellant management tankage, etc. The main 

electric propulsion subsystem is comprised of: four HiVHAC Hall Thrusters—three operating, one spare, 

Gimbals on each thruster for thrust vector control, one carbon-overwrapped (COPV) titanium-lined highpressure cylindrical storage tank for the Xe propellant (nominal), Xe distribution system based on newly 

developed pressure and flow control units and four PPU for delivering power to each ion thruster. 

Figure 3.13.—REP S/C main electrical propulsion subsystem configuration.
NASA/TM—2011-216972 38

The REP S/C chemical propulsion subsystem had two functions: 1) reaction/attitude control, 

2) orbit capture at destination. Hydrazine has been baselined as the propellant for this thruster system, 

which is comprised of 

• Twelve MR-103 thrusters positioned over the body of the S/C

– Aerojet hydrazine thrusters, TRL9; JPL “Minimum Impulse Thruster (MIT)” engine

– Thruster operates at 1 N, 227 s Isp

• Two MR-104A/C thrusters positioned side-by-side on the vehicle side opposite that of the main 

communication array 

– Aerojet hydrazine thruster, TRL 9

– Thruster operates at 441 N, 239 s Isp

– Single thruster is used, with second engine as spare

• Hydrazine propellant stored in two cylindrical COPV tanks 

– Similar to previous NH system

• Propellant distribution system similar to Cruise Stage RCS 

• Propellant tanks included blow down pressurization to drive thrusters 

• Thermal protection approach 

– Propellant tanks and feedlines are covered with tape heaters and insulation to maintain 

propellant as liquid

• Total propellant

– Useable 205 kg 

– 8 percent margin added 

Figure 3.14 shows the configuration of the chemical propulsion subsystem.

Figure 3.14.—REP S/C chemical propulsion subsystem. 
NASA/TM—2011-216972 39

Table 3.15 lists the propulsion system hardware MEL for the REP stage at Saturn. All growth 

allowances follow the AIAA WGA schedule in Section 2.4 and do not contain the additional 8.8 percent

carried at the system level.

TABLE 3.15.—PROPULSION SYSTEM MEL REP S/C INSIDE AEROSHELL

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

01 REP S/C (Payload and Stage) --- ------- 1784.15 10.90 194.52 1978.67 

01.2.6 Propulsion --- ------- 145.64 28.22 41.10 186.74 

01.2.6.a Primary EP system --- ------- 6.75 12.00 0.81 7.56 

01.2.6.a.a Primary EP thrusters 3 2.25 6.75 12.00 0.81 7.56 

01.2.6.a.b EPS power processing and control 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.c EPS structure --- ------- 0.00 0.00 0.00 0.00 

01.2.6.a.c.a EP thruster pod 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.c.b EP thruster boom 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.c.c Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.d EPS thermal control subsystem --- ------- 0.00 0.00 0.00 0.00 

01.2.6.a.d.a EPS MLI 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.d.b EPS heaters and sensors 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.a.d.c Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b Propellant management --- ------- 57.41 31.36 18.00 75.42 

01.2.6.b.a Xe propellant tank(s) 1 44.61 44.61 30.00 13.38 58.00 

01.2.6.b.b High pressure feed system 1 8.90 8.90 30.00 2.67 11.57 

01.2.6.b.c Low pressure feed system 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.d Residual Xe propellant (nondeterministic) 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.e Temperature sensors 1 3.90 3.90 50.00 1.95 5.85 

01.2.6.b.f Propulsion tank heaters 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.g Propulsion line heaters 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.h Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.i Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.j Miscellaneous no. 3 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.b.k Miscellaneous no. 4 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.c PPU --- ------- 12.00 12.00 1.44 13.44 

01.2.6.c.a PPU mass 3 4.00 12.00 12.00 1.44 13.44 

01.2.6.c.b Cabling 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.c.c Miscellaneous no. 1 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.c.d Miscellaneous no. 2 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.c.e Miscellaneous no. 3 0 0.00 0.00 0.00 0.00 0.00 

01.2.6.d RCS --- ------- 69.48 30.00 20.84 90.32 

01.2.6.d.a RCS tank subassembly 1 30.54 30.54 30.00 9.16 39.71 

01.2.6.d.b RCS propellant management subassembly 1 24.45 24.45 30.00 7.34 31.79 

01.2.6.d.c RCS thruster subassembly 12 1.21 14.48 30.00 4.34 18.82 

Table 3.16 lists the propellant used in this mission. Note, the margins and residuals are called out as 

separate line items in this mass listing, and no additional WGS is necessary on the propellants.

Figure 3.15 shows the power and propulsion deck of the REP S/C. The ASRGs are mounted to the bus 

structure via trusses, at a 45° angle between each other radially around the main bus.
NASA/TM—2011-216972 40

Figure 3.15—Flagship mission REP stage propulsion deck.

TABLE 3.16.—EP PROPELLANT MEL FOR REP STAGE

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

01 REP S/C (Payload and Stage) - -------- 1784.15 10.90 194.52 1978.67 

01.1 Science Payload - -------- 70.00 30.00 21.00 91.00 

01.2 REP Bus - -------- 1714.15 10.12 173.52 1887.67 

01.2.7 Propellant - -------- 847.51 0.00 0.00 847.51 

01.2.7.a Primary EP propellant - -------- 636.57 0.00 0.00 636.57 

01.2.7.a.a Primary EP propellant used 1 586.16 586.16 0.00 0.00 586.16 

01.2.7.a.b Primary EP propellant residuals (unused) 1 21.10 21.10 0.00 0.00 21.10 

01.2.7.a.c Primary EP propellant performance margin (unused) 1 29.31 29.31 0.00 0.00 29.31 

01.2.7.b RCS propellant - -------- 210.18 0.00 0.00 210.18 

01.2.7.b.a RCS used 1 205.05 205.05 0.00 0.00 205.05 

01.2.7.b.b RCS residuals 1 5.13 5.13 0.00 0.00 5.13 

01.2.7.c Pressurant 1 0.77 0.77 0.00 0.00 0.77 

3.6.5 Propulsion and Propellant Management Analytical Methods

Hall thruster and PPU performance and masses were based on published or in-house calculations by 

the GRC’s In Space Propulsion branch. Thruster performance over a range of specific impulse was 

examined as a series of custom designs, rather than a single thruster design capable of throttling over a 

range of Isp (Figure 3.16). Thruster mass was assumed to be 50 percent greater than a commercial thruster 

(SPT-70) operating at a similar power level. PPU performance and mass were based on a single module 

of a PPU unit under development and test at GRC. 
NASA/TM—2011-216972 41

Figure 3.16.—Flagship mission EP thruster performance assumption. 

3.6.6 Propulsion and Propellant Management Risk Inputs

There were three risk issues that were identified for detailed risk analysis. These issues are

• Failure of the electric thruster to start/operate.

• Impingement of RCS thruster plumes on sensitive S/C surfaces

• Freezing of chemical propellant in lines or tanks 

3.6.7 Propulsion and Propellant Management Recommendation

Future trades to on the main propulsion system are as follows 

• Derated HiVHAC (mass, trip time penalties)

• Low power ion thruster (8 cm) 

Further trades on the secondary propulsion system to reduce mass are

• Utilize primary propulsion for some maneuvering, and modeling the trade between attitude 

control using Wheels versus propulsion. 

3.7 Thermal Control

3.7.1 Thermal Requirements

The thermal requirements are dominated by the need to place the ASRG system inside of the 

aeroshell during transit and aerocapture itself. A substantial amount of waste heat (125 Wth) needs to be 

removed from the ASRGs while inside of the aeroshell. Once the Aeroshell is jettisoned, the ASRGs will 

see deep space and Titan or Enceladus’ surface and no longer have thermal issues.

The thermal requirements after the aerocapture were to provide a means of cooling the S/C during 

operation as well as provide heat to vital components and systems to maintain a minimum temperature 

throughout the mission. 

The maximum heat load to be rejected by the thermal system was 125 W from the electronics, and the 

desired operating temperature for the electronics and propellant was 300 K. The ASRGs have dedicated 

built in thermal control systems and therefore were not part of the S/C thermal system. 
NASA/TM—2011-216972 42

3.7.2 Thermal Assumptions

The thermal subsystem approach was based on that used in the COMPASS SEP Enceladus design 

(CD–2007–08 (Ref. 11)).

The assumptions utilized in the analysis and sizing of the thermal system were based on the 

operational environment. It was assumed that the worst case operational conditions would be in near 

Earth space. The following assumptions were utilized to size the thermal system. 

• The view factors for the radiator to the Earth, lunar surface and SRG radiators were assumed to 

be 0.1, 0.25, and 0.1 respectively. 

• The maximum angle of the radiator to the Sun was 15°. 

• The radiator temperature was 320 K. 

During cruise and prior to aerocapture a radiator system mounted on the REP stage deck will be used 

to reject the heat from all the internal systems including the ASRGs. After aerocapture and jettison of the 

SEP stage a separate thermal control system will be used to reject heat from the S/C components. This 

second radiator system is housed inside the aeroshell and is not operational until after the aerocapture. 

3.7.3 Thermal Design and MEL

3.7.3.1 Cruise and Aerocapture Phase

During the cruise phase the ASRGs and avionics are inside of the aeroshell. In order to cool them 

sufficiently a pumped loop radiator system is used to connect the ASRGs to a radiator on the cruise deck 

which is to be jettisoned before aerocapture. A separate loop cools the lower temperature avionics on the 

REP S/C using the cruise deck radiator.

Since the cruise deck must be jettisoned for the aeroshell to work properly, no external cooling will 

be available during the 10 to 40 min trip through Titan’s upper atmosphere. This transit will also incur 

additional heat radiated from the ionized atmospheric nitrogen. It is assumed that the aero backshell will 

protect the REP S/C sufficiently. The heat of the operating ASRGs and avionics is assumed to be cooled 

sufficiently during this short time by boiling off the radiator coolant still in the lines.

Table 3.17 lists the items in the Thermal system MEL for the COMPASS REP S/C design. All 

growth allowances follow the AIAA WGA schedule in Section 2.4 and do not contain the additional 

8.8 percent carried at the system level.

TABLE 3.17.—THERMAL MEL FOR CHEMICAL CRUISE STAGE

WBS 

no.

Description

Generic Chemical Stage

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

03 Cruise Deck/Chemical Stage - ------ 896.63 6.38 57.23 953.86 

03.5 Thermal control (non-propellant) - ------ 48.93 15.00 7.34 56.27 

03.5.3 Semi-passive thermal control (cruise deck) - ------ 48.93 15.00 7.34 56.27 

03.5.3.a Louvers 0 0.00 0.00 15.00 0.00 0.00 

03.5.3.b Thermal switches 0 0.00 0.00 15.00 0.00 0.00 

03.5.3.c RTG radiator 1 37.70 37.70 15.00 5.66 43.36 

03.5.3.d RTG coolant loop 1 11.23 11.23 15.00 1.68 12.91 

03.5.3.e RTG cold plates 0 3.46 0.00 15.00 0.00 0.00 

Table 3.18 lists the items in the Thermal system MEL for the COMPASS REP S/C design. All 

growth allowances follow the AIAA WGA schedule in Section 2.4 and do not contain the additional 

8.8 percent carried at the system level.
NASA/TM—2011-216972 43

TABLE 3.18.—THERMAL MEL FOR REP/AEROCAPTURE 

WBS 

no.

Description

REP Flagship Mission Aeroshell 

(November 7, 2007)

Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

01 REP S/C (Payload and Stage) --- ------ 1784.15 10.90 194.52 1978.67 

01.2.5 Thermal control (nonpropellant) --- ------ 61.57 15.00 9.23 70.80 

01.2.5.a Active thermal control --- ------ 16.90 15.00 2.54 19.44 

01.2.5.a.a Heaters 15 1.00 15.00 15.00 2.25 17.25 

01.2.5.a.b Thermal control/heaters circuit 2 0.20 0.40 15.00 0.06 0.46 

01.2.5.a.c Data acquisition 1 1.00 1.00 15.00 0.15 1.15 

01.2.5.a.d Thermocouples 50 0.01 0.50 15.00 0.08 0.58 

01.2.5.a.e Miscellaneous no. 1 0 0.00 0.00 15.00 0.00 0.00 

01.2.5.a.f Miscellaneous no. 2 0 0.00 0.00 15.00 0.00 0.00 

01.2.5.b Passive thermal control --- ------ 41.17 15.00 6.18 47.34 

01.2.5.b.a Heat sinks 4 3.46 13.85 15.00 2.08 15.93 

01.2.5.b.b Heat pipes 1 1.37 1.37 15.00 0.21 1.58 

01.2.5.b.c Radiators 1 4.69 4.69 15.00 0.70 5.39 

01.2.5.b.d MLI 1 14.51 14.51 15.00 2.18 16.69 

01.2.5.b.e Temperature sensors 25 0.01 0.25 15.00 0.04 0.29 

01.2.5.b.f Phase change devices 0 0.00 0.00 15.00 0.00 0.00 

01.2.5.b.g Thermal coatings/paint 1 3.60 3.60 15.00 0.54 4.14 

01.2.5.b.h Micrometeor shielding 0 0.00 0.00 15.00 0.00 0.00 

01.2.5.b.i S/C RTG MLI 1 0.00 0.00 15.00 0.00 0.00 

01.2.5.b.j S/C engine MLI 1 2.90 2.90 15.00 0.43 3.33 

01.2.5.c Semi-passive thermal control --- ------ 3.50 15.00 0.52 4.02 

01.2.5.c.a Louvers 1 2.70 2.70 15.00 0.40 3.10 

01.2.5.c.b Thermal switches 4 0.20 0.80 15.00 0.12 0.92 

3.7.4 Thermal Trades

Prior to aerocapture, the thermal system utilizes a pump loop cooling system connected to a radiator 

on the REP stage deck. This cooling system removes excess heat from all of the ASRGs and the S/C 

components. 

After the aerocapture, the thermal system is used to remove excess heat from the electronics and other 

components of the system as well as provide heating to thermally sensitive components during periods of 

inactivity. 

Excess heat is collected from a series of Al cold plates located throughout the interior of the S/C. 

These cold plates have heat pipes integrated into them. The heat pipes transfer heat from the cold plates to 

the radiator, which radiates the excess heat to space. The portions of the heat pipes that extend from the 

S/C body and are integrated to the radiator are protected with a micro meteor shield. The radiator has 

exterior louvers on it to provide some control over its heat transfer capability. 

The radiator was sized with approximately 50 percent margin in its heat rejection area. This added 

margin insures against unforeseen heat loads, degradation of the radiator and increased view factor 

toward the sun or other thermally hot body not accounted for in the analysis. 

To provide internal heating for the electronics and propulsion systems a series of electric heaters are 

utilized. These heaters are controlled by an electronics controller, which reads a series of thermocouples 

through a data acquisition system. 

MLI is also utilized on the S/C, and propellant system to regulate and maintain the desired 

temperatures. 
NASA/TM—2011-216972 44

3.7.5 Thermal Analytical Methods

Objective

To provide spreadsheet based models capable of estimating the mass and power requirements of the 

various thermal systems. The models were produced based on a first principles analysis. They were 

structured to be easily adapted to various missions. 

The analysis performed to size the thermal system is based on first principle heat transfer from the 

S/C to the surroundings. This analysis takes into account the design and layout of the thermal system and 

the thermal environment to which heat is being rejected or the vehicle is being insulated from. For more 

detailed information on the thermal analysis, a summary white paper entitled “Spacecraft Preliminary 

Thermal System Sizing for Trade Study Analysis” was produced. This paper is presently under 

publication as a NASA Contractor Report. 

The thermal for the Aerobraking portion of the mission was based on the Mars Science Laboratory 

(MSL). Radiator pump loop cooling was used on the REP S/C while inside the aeroshell.

3.7.5.1 Thermal Environmental Models

Calculations of solar intensity are based on S/C location. A standard solar intensity model was used to 

determine the solar flux on the S/C throughout the mission profile. This model is detailed in the report 

titled “Spacecraft Preliminary Thermal System Sizing for Trade Study Analysis.”

3.7.5.2 Thermal Systems Modeled

Figure 3.17 shows a conceptual design of the Cruise Deck (stage) thermal system. This system 

consists of a pump loop coolant system and radiator panel. The coolant system rejects heat from the 

ASRGs and the S/C internal components during the cruse phase of the mission prior to aerocapture. 

After the aerocapture maneuver is completed a heat pipe based thermal control system is utilized to 

maintain the S/C components at their desired temperatures. The main components from this system are 

illustrated in Figure 3.18. 

Figure 3.17.—Cruise Deck thermal protection system.
NASA/TM—2011-216972 45

Figure 3.18—Illustration of the main components of the S/C thermal control system. 

3.7.5.2.1 Micrometeor Shielding on Radiator

No specific micrometeor shielding was used on the S/C. The aeroshell structure was deemed 

sufficient to protect the vehicle during the near earth and cruse phases. The radiator coolant lines that 

were outside of the aeroshell were the only components that had dedicated micrometeor shielding. 

3.7.5.2.2 Radiator Panel Modeling

• The radiator panel area has been modeled along with a rough estimate of its mass. 

• The model was based on a first principles analysis of the area needed to reject the identified heat 

load to space. From the area a series of scaling equations were used to determine the mass of the 

radiator. 

• Worst-case thermal environment was used to size the radiator.

TABLE 3.19.—RADIATOR THERMAL ASSUMPTIONS

Variable Value

Radiator solar absorptivity ................................................................ 0.14

Radiator emissivity............................................................................ 0.84

Radiator Sun angle ............................................................................. 90°

Radiator operating temperature ...................................................... 320 K

S/C radiation dissipation power..................................................... 250 W

SRG radiator thermal dissipation ................................................ 2800 W

3.7.5.2.3 Thermal Control of Propellant Lines and Tanks

• Power requirements and mass have been modeled. This modeling included propellant tank MLI 

and heaters and propellant line insulation and heaters. 

• Worst-case thermal environment was used to calculate the heat loss.

• The model was based on a first principles analysis of the radiative heat transfer from the tanks 

and propellant lines to space. The heat loss through the insulation set the power requirement for 

the tank and line heaters. See Figure 3.19 for details of the insulation mass versus number of 

insulation layers and the subsequent heat loss for that number of layers. Figure 3.19 is used to 

size the mass of the MLI used in the thermal protection system. 
NASA/TM—2011-216972 46

Figure 3.19.—Thermal system assumptions. 

Table 3.20 lists the environmental and engineering assumptions used in the tank insulation 

calculations. 

TABLE 3.20.—THERMAL ASSUMPTIONS FOR TANK INSULATION CALCULATIONS

Variable Value

Tank surface emissivity (εt) ........................................................................................................... 0.1

MLI emissivity (εi) ...................................................................................................................... 0.07

MLI material .................................................................................................................................. Al

MLI material density (ρi) ................................................................................................ 2,770 kg/m3

Internal tank temperature (Ti) ................................................................................................... 300 K

MLI layer thickness (ti) ...................................................................................................... 0.025 mm

Number of insulation layers (ni) ..................................................................................................... 10

MLI layer spacing (di) ............................................................................................................ 1.0 mm

Tank immersion heater mass and power level............................................. 1.02 kg at up to 1,000 W

S/C inner wall surface emissivity................................................................................................ 0.98

S/C outer wall surface emissivity................................................................................................ 0.93

Line foam insulation conductivity...............................................................................0.0027 W/m K

Line foam insulation emissivity .................................................................................................. 0.07

Propellant line heater specific mass and power......................................0.143 kg/m at up to 39 W/m

Line foam insulation density ................................................................................................56 kg/m3

3.7.5.2.4 S/C Insulation 

The mass of the S/C MLI on the engine bulkhead MLI was modeled to determine the mass of the 

insulation and heat loss. The model was based on a first principles analysis of the heat transfer from the 

S/C through the insulation to space. The near earth thermal environment was used to size the insulation. 

Table 3.21 lists the thermal environment assumptions used in sizing the S/C insulation. 
NASA/TM—2011-216972 47

TABLE 3.21.—THERMAL ENVIRONMENT 

ASSUMPTIONS FOR S/C INSULATION

Variable Value

S/C MLI material ................................................................................. Al

S/C MLI material density (ρisc) ...............................................2770 kg/m3 

MLI layer thickness (ti) ............................................................ 0.025 mm

Number of insulation layer(ni).............................................................. 25

MLI layer spacing (di) .................................................................. 1.0 mm

S/C radius (rsc) ............................................................................. 1.145 m

3.7.5.2.5 Avionics and Power Management and Distribution (PMAD) Cooling

Mass estimates for the Active Thermal Cooling System (ATCS) have been completed. The 

components of the system included cold plates and heat pipes. The model was based on a first principles 

analysis of the area needed to reject the identified heat load to space. From the sizing a series of scaling 

equations were used to determine the mass of the various system components (Table 3.22). 

TABLE 3.22.—THERMAL ENVIRONMENT ASSUMPTIONS

FOR AVIONICS AND PMAD COOLING

Variable Value

Cooling plate and lines material ................................................................. Al

Cooling plate and lines material density....................................... 2770 kg/m3

Number of cooling plates ............................................................................ 2 

Cooling plate lengths............................................................................. 0.5 m

Cooling plate widths.............................................................................. 0.5 m

Cooling plate thickness.......................................................................... 5 mm

Heat pipe specific mass.................................................................. 0.15 kg/m

3.7.5.3 Thermal Assumptions

• Deep Space operation

• Radiator always sees deep space with a small (0.05) view factor to the Sun

• The ASRGs produced 2800 W waste heat for rejection by the radiators. The 

communications/avionics/EP system produced a maximum of 250 W of waste heat for rejection 

by the radiators. 

3.7.6 Thermal Risk Inputs

The risks associated with the thermal system are based mainly on the failure of a component of 

multiple components of the system. The majority of the system operation is passive and therefore has a 

fairly high reliability. Some of the major failure mechanisms are listed below. 

• Pumped coolant loop failure. This can occur due to a leak in the coolant line or a pump failure. 

This type of failure would cause a loss of the mission. Heat pipe failure. This can be due to 

cracking due to thermal stresses, micrometeor impact or design defect. This likelihood of this 

type of failure is low. The impact of this failure would be a loss of all or a portion of the S/Cs 

capability. 

• Heater system failure. This would most likely be due to wire breakage or a controller failure. The 

likelihood of this type of failure is low. The impact of this failure would be a loss of certain 

components or propulsion capability once the vehicle is exposed to an extended period of cold. 
NASA/TM—2011-216972 48

• Radiator louver failure. The thermal controller on the system can fail due to an electronics failure 

or power failure. Subsequently this will cause a failure of the radiator louvers reducing the 

effectiveness of the radiator and limiting control of the thermal system. The louvers can also 

experience mechanical failure causing them to be held in a fixed position or limiting their range 

of motion. 

3.7.7 Thermal Recommendation

To improve the reliability of the system and compensated for the identified failure risks the following 

system design changes can be made.

• A redundant coolant loop and coolant pumps can be utilized to minimize the risk of a coolant 

system failure during the cruse portion of the mission. 

• Redundant heat pips can be utilized for each cold plate. The heat pipes can be individually run to 

the radiator to provide independent cooling paths. The radiator can be separated into two 

independent units providing additional redundancy. 

• Redundant heating system controllers can be utilized. The heaters can be wired individually so 

that a single heater failure does not bring down any additional heaters. Additional insulation can 

be added to the S/C to insure that the interior components do not drop below their desired 

minimum temperature based on a know shadow period of operation. 

• The radiator louvers can be designed to fail opened to a specified angle. This will enable the 

radiator to continue to operate, although not optimally, for the remainder of the mission. 

4.0 Cost

4.1 Costing: Flagship Configuration 

4.1.1 Costing Assumptions

S/C costs reflect mean estimate (approximately 60 percent confidence level). Quantitative risk 

analysis on S/C cost based on potential mass growth and CER uncertainty. The ASRG is assumed to be 

flight ready by its own development project. The S/C fee assumed at 10 percent and is not applied to 

science instruments (assumed to be furnished equipment). NASA project office/technical oversight are 

based on 5 percent of all other costs. Phase A based on 5 percent of S/C costs. Mission ops cost assume

9-yr cruise to Titan, 1 yr of science at Titan, 5-yr cruise to Enceladus, and 1 yr of science at Enceladus. 

Reserves of 30 percent have not been applied to Launch Services or Mission Ops costs. Costs for 

aerocapture technology development and demonstration are not included. 

4.1.2 Cost Estimates

Table 4.1 lists the life cycle cost estimates for this REP Flagship mission. Table 4.2 lists the S/C 

Estimate Details.

TABLE 4.1.—LIFE CYCLE COST—FY08 ($M)

NASA Project Office/Technical Oversight ...................................................... 71

Phase A ............................................................................................................ 39

S/C with Science Instruments......................................................................... 715

S/C Prime Contractor Fee ................................................................................ 65

Launch Services............................................................................................. 250

Mission Operations ........................................................................................ 350

Contingency (30%) ........................................................................................ 267

Life Cycle Cost ........................................................................................... 1,757
NASA/TM—2011-216972 49

TABLE 4.2.—S/C ESTIMATE DETAILS—FY08 $M

WBS element Element name DDT&E 

total

Flight 

hardware

S/C total

REP S/C (Payload and Stage)

0.1.1 Science Payload 34.4 28.1 62.4

01.2.1 Attitude Determination and Control 10.0 11.1 21.1

01.2.1.a.a Sun Sensors 1.7 4.0 5.8

01.2.1.a.c Star Trackers 1.3 2.5 3.8

01.2.1.a.d IMU 7.0 4.5 11.6

01.2.2 C&DH 30.8 5.1 35.9

01.2.2.a.a Flight Computer 3.7 4.0 7.6

01.2.2.a.c Data Interface Unit 0.4 0.5 0.9

01.2.2.a.e Operations Recorder 0.1 0.1 0.3

01.2.2.a.f Command and Control Harness (data) 5.5 0.5 6.0

Flight Software/Firmware 21.1 21.1

01.2.3 Communications and Tracking 14.1 6.8 21.0

01.2.3.a.a Transmitter/Receiver 4.7 1.8 6.5

01.2.3.a.b Power Amp 2.1 1.8 3.9

01.2.3.a.d Antenna 3.5 2.3 5.8

01.2.3.a.h Cabling 1.0 0.3 1.3

01.2.3.c.a Coaxial Cable 2.8 0.6 3.5

01.2.4 Electrical Power Subsystem 17.6 189.8 207.5

01.2.4.a.a RPS Main System 11.4 185.1 196.5

01.2.4.b Power Management and Distribution 4.3 3.3 7.7

01.2.4.c Power Cable and Harness Subsystem (C and HS) 1.9 1.4 3.3

01.2.5 Thermal Control (Non-Propellant) 6.8 2.6 9.4

01.2.5.a Active Thermal Control 0.7 2.1 2.7

01.2.5.b Passive and Semi-Passive Thermal Control 6.1 0.5 6.6

01.2.6 Propulsion 25.1 14.7 39.8

01.2.6.a.a Primary EP Thrusters 1.1 1.2 2.3

01.2.6.b.a Xe propellant tank(s) 4.0 0.9 4.9

01.2.6.b Balance of propellant Mgt System 4.4 2.5 6.9

01.2.6.c.a PPU Mass 3.1 2.9 6.0

01.2.6.d.a RCS Tank Subassembly 2.7 0.6 3.3

01.2.6.d.b RCS Propellant Management Subassembly 8.0 4.5 12.4

01.2.6.d.c RCS Thruster Subassembly 1.9 2.0 4.0

01.2.8 Structures and Mechanisms 31.5 13.4 44.9

Subtotal 170.3 271.6 441.9

Systems integration 89.0 53.0 142.0

Integration, assembly and check out 7.8 10.0 17.8

System test operations 8.4 ------- 8.4

Ground support equipment 15.8 ------- 15.8

System engineering and integration 28.0 28.6 56.6

Project management 14.4 14.4 28.8

Launch operations and orbital support 14.5 ------- 14.5

Total prime cost 259.3 324.6 583.9

Cruise Deck/Chemical Stage

03.1 Attitude Determination and Control 2.9 5.0 7.9

03.1.1.a Sun Sensors 1.6 3.3 5.0

03.1.1.c Star Trackers 1.2 1.7 2.9
NASA/TM—2011-216972 50

TABLE 4.2.—S/C ESTIMATE DETAILS—FY08 $M

WBS element Element name DDT&E 

total

Flight 

hardware

S/C total

03.2 C&DH 0.9 0.3 1.1

03.2.1.c Data Interface Unit 0.4 0.2 0.6

03.2.1.f Command and Control Harness (data) 0.4 0.1 0.6

03.3 Communications and Tracking 1.1 0.5 1.6

03.3.2 MGA ------- ------- -------

03.3.2.d Antenna 0.9 0.4 1.4

03.3.2.e Cabling 0.2 0.1 0.3

03.4 Electrical Power Subsystem 0.4 0.5 0.9

03.4.3.e Power Cabling 0.4 0.5 0.9

03.5 Thermal Control (Non-Propellant) 8.8 3.3 12.1

03.5.3.c RTG Radiator 6.6 1.5 8.1

03.5.3.d RTG Coolant Loop 2.2 1.8 4.0

03.6 Propulsion 12.3 3.2 15.5

03.6.2.b.a Fuel Tanks 2.8 0.7 3.5

03.6.2.b.c. d RCS Propellant Management Subassembly 5.0 1.7 6.7

03.6.3.a RCS Engines 4.4 0.8 5.3

03.8 Structures and Mechanisms 15.7 4.6 20.4

03.8.1 Structures 9.5 4.3 13.8

03.8.2.e.b Separation mechanism from LV 2.7 0.1 2.8

03.8.2.e.d Separation mechanism REP Probe 3.6 0.2 3.7

Subtotal 42.1 17.4 59.5

Systems integration 26.3 4.6 30.9

Integration, Assembly and Check Out 1.9 0.6 2.5

System Test Operations 2.4 ------- 2.4

Ground Support Equipment 4.3 ------- 4.3

System Engineering and Integration 8.5 3.1 11.6

Project Management 5.6 0.9 6.5

Launch Operations and Orbital Support 3.5 ------- 3.5

Total prime cost 68.4 22.0 90.4

Vehicle Integration 19.7 20.8 40.5

S/C Total 347.4 367.4 714.8

5.0 Trades

Several trades were made including use of REP instead of chemical during cruise from Earth to 

Saturn, cruise deck equipment (propulsion, navigation, communications), and options for entry probes. 

6.0 Challenges, Lessons Learned, Areas for Future Study

A more aggressive mission using an SEP stage based on previous studies may allow for increased 

delivered mass and potentially science landers at the Saturn moons (Ref. 12).
NASA/TM—2011-216972 51

Appendix A.—Acronyms and Abbreviations

ACS Attitude Control System

AD&C Attitude, Determination and 

Control 

AIAA American Institute for 

Aeronautics and Astronautics 

Al aluminum 

ANSI American National Standards 

Institute 

AO Announcement of Opportunity 

APL Applied Physics Laboratory 

ASRG Advanced Stirling Radioisotope 

Generators

ATCS Active Thermal Cooling System 

BAE British Aerospace 

BOL beginning of life

C&DH Command and Data Handling 

CAD computer aided design

CBE current best estimate

CEV Crew Exploration Vehicle

Comm Communications 

COMPASS COllaborative Modeling and 

Parametric Assessment of Space 

Systems

COPV Composite Overwrapped Pressure 

Vessel

COTS commercial-off-the-shelf 

DSN Deep Space Network

EGA Earth Gravity Assist

EIRP equivalent isotropic radiated power 

ELV Expendable Launch Vehicle

EOL end of life

EP Electric Propulsion

FEA finite element analysis 

FOM figure of merit

GLIDE GLobal Integrated Design 

Environment 

GN&C Guidance, Navigation and 

Control 

GRC NASA Glenn Research Center

He helium 

HGA high gain antenna 

HQ NASA Headquarters

IEM integrated electronics module

IMU Inertial Measuring Unit

Isp Specific Impulse

JPL NASA Jet Propulsion Laboratory

KSC NASA Kennedy Space Center

Li lithium

LSP Launch Service Program

LSTO Launch Service Task Order

MEL Master Equipment List

MESSENGER MErcury Surface, Space 

ENvironment, GEochemistry, and 

Ranging 

MGA Mass Growth Allowance

MIT Minimum Impulse Thruster 

MLI multilayer insulation

MMH/NTO monomethyl hydrazine and 

nitrogen tetroxide bipropellant 

system

MSL Mars Science Laboratory 

NASA National Aeronautics and Space 

Administration

Nav navigation

NEAR Near Earth Asteroid Rendezvous

NH New Horizons

OTS off-the-shelf

PEL Power Equipment List 

PMAD Power Management and 

Distribution

PMS Propellant Management System

PPU power processing unit 

PSD Planetary Science Division

RBI Regulator/Bus Protection

RCS Reaction Control System

REP Radioisotope Electric Propulsion

RF radio frequency 

RTG Radioisotope Thermal Generator

S/C spacecraft

SEAKR SEAKR Engineering, Inc. 

SEP Solar Electric Propulsion 

TBD to be discussed
NASA/TM—2011-216972 52

TWTA Traveling Wave Tube Amplifier

USO ultra-stable oscillator

WGA Weight Growth Allowance 

Xe xenon 
NASA/TM—2011-216972 53

Appendix B.—Flagship Design Rendered Drawings

Figure B.1 through Figure B.4 show the REP stage rendered views. Figure B.1 highlights the science 

instruments. Figure B.2 shows the bottom face of the EP thrusters. Figure B.3 showing face on view of 

the science instruments side. Figure B.4 also shows the bottom face of the EP thrusters from a slightly 

different view.

Figure B.1.—REP stage rendered view, showing science instruments.

Figure B.2.—REP stage rendered view, showing EP thrusters on bottom face. 
NASA/TM—2011-216972 54

Figure B.3.—REP stage rendered view, face on showing science instruments side. 

Figure B.4.—REP stage rendered view, showing EP thrusters on bottom face.
NASA/TM—2011-216972 55

Appendix C.—Study Participants

Radioisotope Electric Propulsion (REP) Design Session

Subsystem Name Center Email

In-Space Program Len Dudzinski GRC Leonard.A.Dudzinski@nasa.gov

In-Space Program Scott Benson GRC Scott.W.Benson@nasa.gov

Lead Steve Oleson GRC Steven.R.Oleson@nasa.gov

System Integration, MEL and 

Final Report Documentation Melissa McGuire GRC Melissa.L.Mcguire@nasa.gov

Documentation Les Balkanyi GRC Leslie.R.Balkanyi@nasa.gov

Launch Vehicle Integration TBD GRC TBD

Ground Systems GRC with APL support TBD

Mission John Dankanich GRC John.W.Dankanich@nasa.gov

Operations, GN&C Doug Fiehler GRC Douglas.I.Fiehler@nasa.gov

Structures and Mechanisms John Gyekenyesi GRC John.Z.Gyekenyesi@nasa.gov

Propulsion Tim Sarver-Verhey GRC Timothy.R.Verhey@nasa.gov

Propulsion Jim Gilland GRC James.H.Gilland@nasa.gov

Thermal Tony Colozza GRC Anthony.J.Colozza@nasa.gov

Power Paul Schmitz GRC Paul.C.Schmitz@nasa.gov

Command and Data Handling Jeff Juergens GRC Jeffrey.R.Juergens@nasa.gov

Communications O. Scott Sands GRC Obed.S.Scott@nasa.gov

Communications Bin Nyugen GRC Binh.V.Nguyen@nasa.gov

Configuration Tom Packard GRC Thomas.W.Packard@nasa.gov

Communications, Avionics and 

Software T.C. Nguyen GRC Thanh.C.Nguyen@nasa.gov

Cost Tom Parkey GRC Thomas.J.Parkey@nasa.gov

Risk/Reliability Anita Tenteris GRC Anita.D.Tenteris@nasa.gov

Risk/Reliability Bill Strack GRC bstrack@wowway.com
NASA/TM—2011-216972 56

References

1. NASA Solar System Exploration Office (http://solarsystem.nasa.gov/missions/future4.cfm).

2. ANSI/AIAA R-020A-1999, Recommended Practice for Mass Properties Control for Satellites, 

Missiles, and Launch Vehicles.

3. CD–2007–16, “COMPASS Final Report: Radioisotope Electric Propulsion (REP) Centaur Orbiter 

New Frontiers Mission,” Sep. to Nov. 2007. 

4. C.G. Justusa, Aleta Duvalla, Vernon W. Kellerb, Thomas R. Spilkerc and Mary Kae Lockwood, 

“Connecting atmospheric science and atmospheric models for aerocapture at Titan and the outer 

planets,” Available online 12 February 2005. 

[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6T-4FG4VCY1&_user=141925&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000011798&_version

=1&_urlVersion=0&_userid=141925&md5=e18ccf2b7ce6cd5887132d2d5f155378]. 

5. M. Lockwood, “Titan Aerocapture Systems Analysis,” AIAA–2003–4799, 39th AIAA/ASME/SAE/ 

ASEE Joint Propulsion Conference and Exhibit, Huntsville, AL, July 20–23, 2003. 

6. J. Olejniczak, CA; D. Prabhu and M. Wright, N. Takashima, B. Hollis and K. Sutton, “An Analysis of 

the Radiative Heating Environment for Aerocapture at Titan,” AIAA–2003–4953, 39th AIAA/ 

ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Huntsville, AL, July 20-23, 2003. 

7. Lockwood, Mary Kae (2003). “Titan Aerocapture Systems Analysis,” AIAA–2003–4799, 39th 

AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Huntsville, AL, July 2003. 

8. Edelbaum, T.N., 1964, “Optimum Low-Thrust Rendezvous and Station Keeping,” AIAA Journal, 

vol. 2, no. 7, pp. 1196–1201. 

9. Edelbaum, T.N., 1965, “Optimum Power-Limited Orbit Transfer in Strong Gravity Fields,” AIAA 

Journal, vol. 3, no. 5, pp. 921–925. 

10. http://www.adcole.com/en/ 

11. CD–2007–08, “COMPASS Final Report: Enceladus Solar Electric Propulsion Stage,” Mar. 2007.

12. Kerslake, Thomas W.; Haraburda, Francis M.; Riehl, John P., “Solar Power System Options for the 

Radiation and Technology Demonstration Spacecraft,” AIAA–2000–2807; E–12362; NAS 

1.15:210243; NASA/TM—2000-210243, July 2000. 
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The COllaborative Modeling and Parametric Assessment of Space Systems (COMPASS) team was approached by the NASA Glenn 

Research Center (GRC) In-Space Project to perform a design session to develop Radioisotope Electric Propulsion (REP) Spacecraft

Conceptual Designs (with cost, risk, and reliability) for missions of three different classes: New Frontier’s Class Centaur Orbiter (with 

Trojan flyby), Flagship, and Discovery. The designs will allow trading of current and future propulsion systems. The results will directly 

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15. SUBJECT TERMS

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