Steven R. Oleson and Melissa L. McGuire

Glenn Research Center, Cleveland, Ohio

COMPASS Final Report: Enceladus Solar 

Electric Propulsion Stage

NASA/TM—2011-216973

February 2011

CD–2007–08
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Steven R. Oleson and Melissa L. McGuire

Glenn Research Center, Cleveland, Ohio

COMPASS Final Report: Enceladus Solar 

Electric Propulsion Stage

NASA/TM—2011-216973

February 2011

CD–2007–08

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.

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

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

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2011-216973 iii

Contents

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

2.0 Study Background and Assumptions....................................................................................................4

2.1 Introduction.................................................................................................................................4

2.2 Assumptions................................................................................................................................5

2.3 Growth, Contingency and Margin Policy ...................................................................................5

2.4 Mission Description ....................................................................................................................6

2.4.1 Mission Analysis Assumptions..................................................................................... 6

2.4.2 Mission Analysis Analytic Methods............................................................................. 7

2.4.3 Mission Trajectory Details............................................................................................ 9

2.4.4 Launch Window Sensitivity........................................................................................ 10

2.5 Mission Operations...................................................................................................................10

2.5.1 Duty Cycle Budget and Operations Phases................................................................. 10

2.5.2 S/C Trajectory and Pointing........................................................................................ 12

2.6 Solar Radiation Pressure ...........................................................................................................14

2.7 Slew Performance .....................................................................................................................14

2.8 Launch Vehicle Details: Atlas 551 ...........................................................................................14

2.9 ELV Performance Calculations.................................................................................................15

2.10 SEP Stage Launch Envelope.....................................................................................................17

2.11 System Design Trade Space......................................................................................................18

2.12 Internal COMPASS Details......................................................................................................18

2.12.1 GLIDE Study Share .................................................................................................... 18

2.12.2 GLIDE Architecture.................................................................................................... 19

2.12.3 GLIDE Study Container(s).......................................................................................... 19

3.0 Baseline Design..................................................................................................................................19

3.1 Baseline System Design............................................................................................................19

3.2 Top Level Design (MEL and PEL)...........................................................................................20

3.2.1 Master Equipment List (MEL).................................................................................... 20

3.2.2 Power Equipment List (PEL)...................................................................................... 20

3.3 System Level Summary ............................................................................................................20

3.4 Design Concept Drawing and Description................................................................................21

4.0 Subsystem Breakdown .......................................................................................................................22

4.1 Communications .......................................................................................................................22

4.1.1 Communications Requirements .................................................................................. 22

4.1.2 Communications Assumptions.................................................................................... 22

4.1.3 Communications Design and MEL............................................................................. 22

4.1.4 Communications Trades.............................................................................................. 23

4.1.5 Communications Analytical Methods......................................................................... 23

4.1.6 Communications Risk Inputs...................................................................................... 23

4.1.7 Communications Recommendation ............................................................................ 23

4.2 GN&C.......................................................................................................................................23

4.2.1 GN&C Requirements.................................................................................................. 23

4.2.2 GN&C Assumptions ................................................................................................... 24

4.2.3 GN&C Design and MEL............................................................................................. 24

4.2.4 GN&C Trades ............................................................................................................. 24

4.2.5 GN&C Analytical Methods......................................................................................... 24

4.2.6 GN&C Recommendation ............................................................................................ 24
NASA/TM—2011-216973 iv

4.3 Command and Data Handling (C&DH)....................................................................................25

4.3.1 C&DH Requirements.................................................................................................. 25

4.3.2 C&DH Assumptions ................................................................................................... 25

4.3.3 C&DH Design and MEL............................................................................................. 25

4.3.4 C&DH Trades ............................................................................................................. 26

4.3.5 C&DH Analytical Methods......................................................................................... 26

4.3.6 C&DH Risk Inputs...................................................................................................... 26

4.4 Electrical Power System ...........................................................................................................26

4.4.1 Power Requirements ................................................................................................... 26

4.4.2 Power Assumptions..................................................................................................... 26

4.4.3 Power Design and MEL.............................................................................................. 27

4.4.4 Power Trades............................................................................................................... 29

4.4.5 Power Comments, Concerns, and Recommendations................................................. 30

4.5 Structures and Mechanisms.......................................................................................................30

4.5.1 Structures and Mechanisms Requirements.................................................................. 30

4.5.2 Structures and Mechanisms Assumptions................................................................... 30

4.5.3 Structures and Mechanisms Design and MEL ............................................................ 30

4.5.4 Structures and Mechanisms Separation System.......................................................... 33

4.5.5 Structures and Mechanisms Analytical Methods........................................................ 34

4.5.6 Structures and Mechanisms Risk Inputs ..................................................................... 34

4.5.7 Structures and Mechanisms Recommendation............................................................ 34

4.6 Propulsion and Propellant Management ...................................................................................34

4.6.1 Propulsion and Propellant Management Requirements .............................................. 35

4.6.2 Propulsion and Propellant Management Assumptions................................................ 35

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

4.6.4 Propulsion and Propellant Management Trades.......................................................... 37

4.6.5 Propulsion and Propellant Management Analytical Methods..................................... 37

4.6.6 Propulsion and Propellant Management Risk Inputs.................................................. 40

4.6.7 Propulsion and Propellant Management Recommendation ........................................ 40

4.7 Thermal Control........................................................................................................................41

4.7.1 Thermal Requirements................................................................................................ 41

4.7.2 Thermal Assumptions ................................................................................................. 41

4.7.3 Thermal Design and MEL........................................................................................... 43

4.7.4 Thermal Trades ........................................................................................................... 44

4.7.5 Thermal Analytical Methods....................................................................................... 44

5.0 Software Cost Estimation...................................................................................................................45

5.1 Design Requirements................................................................................................................45

5.2 Assumptions..............................................................................................................................45

5.3 Summary ...................................................................................................................................45

6.0 Cost, Risk and Reliability...................................................................................................................46

6.1 Costing ......................................................................................................................................46

6.1.1 Costing Ground Rules and Assumptions..................................................................... 46

6.1.2 Cost Summary 1+1 SEP Stage .................................................................................... 46

6.2 Risk Analysis and Reduction ....................................................................................................47

6.2.1 Assumptions................................................................................................................ 47

6.2.2 Risk Summary............................................................................................................. 47

6.3 Reliability..................................................................................................................................47

7.0 Trade Space Iterations........................................................................................................................48
NASA/TM—2011-216973 v

7.1 Trade 1 : 1+1.............................................................................................................................49

7.2 Trade 2: 2+1..............................................................................................................................49

7.3 Trade 2: 3+1..............................................................................................................................50

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

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

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

Bibliography................................................................................................................................................56

NASA/TM—2011-216973 1

COMPASS Final Report: Enceladus Solar Electric Propulsion Stage

Steven R. Oleson and Melissa L. McGuire

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

1.0 Executive Summary

The results of the NASA Glenn Research Center (GRC) COllaborative Modeling and Parametric 

Assessment of Space Systems (COMPASS) internal Solar Electric Propulsion (SEP) stage design are 

documented in this report (Figure 1.1). The SEP Stage was designed to deliver a science probe to Saturn 

(the probe design was performed separately by the NASA Goddard Space Flight Center’s (GSFC) 

Integrated Mission Design Center (IMDC)). The SEP Stage delivers the 2444 kg probe on a Saturn 

trajectory with a hyperbolic arrival velocity of 5.4 km/s. The design carried 30 percent mass, 10 percent

power, and 6 percent propellant margins. The SEP Stage relies on the probe for substantial guidance, 

navigation and control (GN&C), command and data handling (C&DH), and Communications functions. 

The stage is configured to carry the probe and to minimize the packaging interference between the probe 

and the stage. The propulsion system consisted of a 1+1 (one active, one spare) configuration of gimbaled 

7 kW NASA Evolutionary Xenon Thruster (NEXT) ion propulsion thrusters with a throughput of 309 kg Xe 

propellant. Two 9350 W GaAs triple junction (at 1 Astronomical Unit (AU), includes 10 percent margin) 

ultra-flex solar arrays provided power to the stage, with Li-ion batteries for launch and contingency 

operations power. The base structure was an Al-Li hexagonal skin-stringer frame built to withstand launch 

loads. A passive thermal control system consisted of heat pipes to ‘north and south’ radiator panels, 

multilayer insulation (MLI) and heaters for the Xe tank. All systems except tanks and solar arrays were 

designed to be single fault tolerant.

Figure 1.1.—Mission overview.
NASA/TM—2011-216973 2

Figure 1.2.—Conceptual SEP Stage to deliver science payload package to Saturn.

Table 1.1 lists the major components and descriptions of each major subsystems that make up the 

SEP transfer stage.

TABLE 1.1.—MISSION AND S/C SUMMARY

Subsystem area Details Total mass 

with growth

Top level SEP 

Stage system

SEP Stage to provide probe mass of ~2500 kg to a Saturn transfer trajectory

Figures of merit (FOMs): Probe mass, stage cost, min probe capture velocity

1026 kg

Mission, 

Operations, 

GN&C

1+1 SEP Stage, 1- to 3-AU orbit range, <3 yr SEP Stage ops, 8 yr for Probe to reach 

Saturn, 3% additional Xe for nav/trajectory and other errors, 1000 km Earth flyby, SEP 

Stage thrust duty cycle 90%, ion thrusters can act as pointing control during thrusting for 

yaw and pitch, ion thrusters may be able to off-set solar perturbations (during thrusting) 

and/or dump momentum wheels

Attitude Control 

System (ACS)

Science Probe provides ACS for all coasting, commands ion thruster gimbal, solar array 

pointing, and probe provides roll control during all phases. control and actuators sized to 

accommodate stage mass properties

1.2 kg

Launch Atlas 551, C3 = 27 km2/s2, 20 day launch window, Adapter: 65 in, D1666, 10% launch 

margin taken from performance. Launch Loads: ~5g (See Section 2.9)

3864 kg

Science Science probe to be designed in IMDC subsequent session. Volume and mass to be 

determined. Probe allowable mass output from SEP Stage performance analysis

2444 kg

Power 18.5 kW power (with 200 W housekeeping), each array oversized by 1.5 kW to address 

degradation (cell, string, diode losses), two Ultraflex arrays design based on ST-8, CEV

Li ion batteries for pre-array deployment and contingency operations

321 kg

Propulsion One active 7 kW NEXT Ion engine with one cold spare, two Power Processing Units 

(PPU) (one is spare), 1% Xe unusable, <2 in. below adapter at launch, 66 cm thruster to 

thruster center separation, 99% of ion plume in 15° half angle cone, off-the-shelf (OTS) 

tanks

168 kg
NASA/TM—2011-216973 3

TABLE 1.1.—MISSION AND S/C SUMMARY

Subsystem area Details Total mass 

with growth

Structures and 

Mechanisms

Primary: Hexagonal, 63 in. diameter, Truss, Al-Li, Carry ~2500 kg probe and major 

stage components, Secondary: first order design based on component masses

Two thruster gimbals, two-axis range of motion: ±19°, ±17°, Single axis SA drives, solar 

array/boom deployment, louvers on radiators

98 kg

Communications Probe provides all communications support for Stage, Electric Propulsion (EP) thrust 

vector constrains communications link design

none

C&DH Use Digital Control Interface Units (DCIU) to run all probe functions; rely on probe for 

C&DH

12 kg

Thermal Passive heat-pipe system, body mounted radiators (north and south SADA faces) (main 

loads 500 W from PPU, 500 W from power distribution units (PDU), 100 layers MLI 

between thruster platform and S/C to prevent heat leak back, active tank heaters to 

prevent two-phase Xe, louvers on radiators

Radiation level, micrometeoroid environment

39 kg

The deployed SEP Stage developed in this study is shown in Figure 1.3. The two Ultraflex solar 

arrays are shown deployed and span 18.9 m at their widest point across. The arm placement of the solar 

arrays onto the stage is such that the 49° half angle thruster plume cone does not interfere with the arrays. 

A 3 m diameter dish antenna is placed on top of the, as yet, not designed science probe. Note that the 

science probe is shown in this design simply as a conical shape in order to allow for Expendable Launch 

Vehicle (ELV) packaging and stage layout during the design study.

Figure 1.3.—Conceptual SEP Stage deployed configuration.
NASA/TM—2011-216973 4

2.0 Study Background and Assumptions

2.1 Introduction 

Cassini performed several close flybys of Enceladus in 2005, revealing the moon’s surface and 

environment in greater detail. In particular, the probe discovered a water-rich plume venting from the 

moon’s South Polar Region (Figure 2.1). This discovery, along with the presence of escaping internal 

heat and very few (if any) impact craters in the South Polar Region, shows that Enceladus is geologically 

active today. 

NASA is considering several of Saturn’s moon for the next Flagship mission, and in light of 

Enceladus’ increased importance in terms of the interest of the scientific community, GSFCs IMDC was 

tasked with designing a science probe mission to Saturn’s moons of Titan and Enceladus. To help with 

this effort, the COMPASS team was tasked to design a SEP interplanetary stage to be used to send a “to 

be designed” IMDC scientific probe to the Saturn system. The NEXT ion propulsion system is well suited 

to future missions to the Saturn system. NEXT is used within the inner solar system, in combination with 

a Venus or Earth gravity assist, to establish a fast transfer to the Saturn system. The NEXT system 

elements are accommodated in a separable SEP module, or are integrated into the main S/C bus, 

depending on the mission architecture and performance requirements.

Figure 2.1.—Cassini S/C (probe example).
NASA/TM—2011-216973 5

2.2 Assumptions

The SEP Stage will be designed to deliver an, as yet not designed, science probe to Saturn. The 

assumptions for the probe to be delivered to Saturn are as follows

• Large antenna 

• Chemical propulsion 

• Radioisotope powered

The SEP Stage provides structural interface with the Atlas launch vehicle. In the stowed 

configuration, the SEP Stage solar arrays remain below the probe separation plane. For this study session 

the SEP Stage concept design assumes the following basis of design

• Estimate probe needs of mass and size based on past Saturn probes 

• Design SEP Stage based on past SEP S/C (Deep Space 1 (DS-1) and Dawn) 

• Utilize OTS products and components where possible to minimize costs 

• Single fault tolerant 

• New Frontiers Class (Flagship class in subsequent studies) 

• Mass growth (contingency) based on ANSI/AIAA R-020A-1999, additional growth carried at 

system level to total 30 percent for the stage 

2.3 Growth, Contingency and Margin Policy

Mass Growth: At the time of this study, the COMPASS team used the ANSI/AIAA R-020A-1999, 

Recommended Practice for Mass Properties Control for Satellites, Missiles, and Launch Vehicles for 

mass growth allowance calculations and allocations. Table 2.1 shows the Percent Mass Growth 

Allowance 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 at the vehicle 

level is calculated. The COMPASS team desired total growth to be 30 percent. Therefore 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 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.

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 margin on the bottoms up power 

requirements in modeling the power system. See Section 3.2.2 for the power system assumptions.
NASA/TM—2011-216973 6

TABLE 2.1—PERCENT MASS GROWTH ALLOWANCE

Code Design maturity

(basis for mass 

determination)

Percent mass growth allowance

Electrical/electronic 

components

StructureThermal controlPropulsionBatteriesWire harnessesMechanismsInstrumentation

0 to 5

kg

5 to 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

2.4 Mission Description

The baseline mission used an Atlas 551 to launch the SEP Stage with payload to a C3 of 27 km2/s2. 

The following baseline assumptions for the technology of the SEP Stage as related to trajectory modeling 

were used in the baseline mission modeling.

• 20 kW Class SEP Stage and Earth flyby to accelerate the SEP + probe 

– SEP Stage operates 1 AU ~ 3 AU then is disposed 

– Earth flyby safety considerations (probe plutonium powered) are to be taken into account 

during final mission modeling

– Two NEXT engines (1 +1), ~300 kg Xe propellant 

– Two solar arrays for beginning of life (BOL) of 17 kW (only 7 kW bus used—extra solar 

array used at >1 AU) 

• 20 day launch window chosen, worst case payload and Xe propellant load for window chosen as 

design basis 

2.4.1 Mission Analysis Assumptions

The following mission assumptions were used in the trajectory modeling of the low thrust SEP 

trajectory to Saturn.

Overall Assumptions for Mission Analysis

• The initial departure epoch is in the 2015 to 2016 time period. Epochs between 2015 and 2025 

also are to be achievable, but not necessarily with the same propellant loading. 

• Use a SEP system consisting of solar arrays, power distribution system and an EPS consisting of 

some number of thrusters, power processing units, propellant tanks and a distribution system for 

the propellant. 
NASA/TM—2011-216973 7

• Maximize the injected mass in the vicinity of Saturn while minimizing the hyperbolic excess 

velocity (Vhp) at Saturn arrival. 

• Use a launch vehicle from the United States’ existing fleet of ELVs. The ELV puts the S/C on an 

escape hyperbola. 

• The mission would use a Venus or Earth gravity assist(s) to increase the injected S/C mass. 

Power Assumptions for Mission Analysis

• The SEP system model includes a generic solar array whose power output is proportional to the 

inverse of the square of the distance from the S/C to the sun. This assumption provides a degree 

of conservatism since most solar arrays can provide slightly more power than inverse square 

because of low incidence and low temperature affects. In addition, array degradation is

incorporated into the power model. 

• Modeling power degradation with an inverse-square ratio without any specific array coefficients 

• Sweep of power level (P0)

– P0 = 10-, 15-, 16- and 17-kW cases investigated

• S/C power (SAP) = 1 + 2/R + 3/R2 (this permits modeling of increased S/C demand with distance 

(R), i.e., turning on heaters) where R = the distance to the Sun

• 2 percent array degradation per year 

Propulsion Assumptions for Mission Analysis

The nomenclature used for the propulsion system is N+1, where N represents the number of thruster 

strings required for the optimized mission, and the “+1” represents the spare string. 

• 1+1 Configuration (one thruster with redundant thruster available) 

• SEP system operations are assumed to occur with a 90 percent propulsive duty cycle during 

thrusting phases. The 10 percent downtime allows the S/C to perform navigation and 

communications functions on a regular basis.

The NEXT thruster can be operated across a wide throttle range. This capability has been modeled with 

curve fits of the extremes of the throttle table, representing either a high specific impulse (Isp) mode or high 

thrust mode. Based upon the results of a previous study, the high Isp model for NEXT is assumed to provide a 

higher final mass than the high thrust setting and was thus used for these analyses 

• NEXT coefficients for high Isp end of the performance envelope used as input to SEPTOP and 

MALTO for polynomial approximation of thrust and mass flow rate of the NEXT thruster

– ct = 3.646709187×10–2

, 3.774558883×10–3, 6.882669254×10–3, –6.815530390×10–4, 

2.334052793×10–5

– cm = 3.630027151×10–6

, –1.726624312×10–6, 6.466718696×10–7, –7.184580335×10–8, 

2.891651950×10–9

– Operating power ranges on the y axis (Figure 2.2)

 pmin = 1.252159689 

 pmax = 7.252

2.4.2 Mission Analysis Analytic Methods

The method was to find a low thrust trajectory to Saturn modeled using SEPTOP (2003 edition) 

exclusively. SEPTOP is a more precise version of VARITOP in that it offers the user a much more 

accurate simulation of a “real” EP thruster. It shares much with VARITOP including solutions techniques 

and the robustness of problems it can solve. It generally is used after a preliminary solution is in hand 

from VARITOP. The performance estimates SEPTOP generates are much more accurate. SEPTOP is a 

product of the NASA Jet Propulsion Laboratory (JPL) and is only available to employees of NASA 

centers. Non-NASA personnel should contact JPL directly for SEPTOP.
NASA/TM—2011-216973 8

Figure 2.2.—Available S/C power from the solar arrays versus distance from the Sun.

The optimization objective was to maximize payload and minimize arrival hyperbolic arrival velocity. 

Maximum injected mass is achieved by trajectory optimization. The Vhp is minimized by parametrically 

varying the transfer time from Earth departure to Saturn arrival. Although it may not be obvious, for a low 

thrust trajectory Vhp decreases with increased trip time until it attains a minimum value and then slowly 

increases. For this study, the computer program SEPTOP provided the optimization while the utility 

programs Vartable and Newpost (two VARITOP and SEPTOP utilities) used with Microsoft Excel 

permitted quick visualization of the optimization results. SEPTOP, as provided by Carl Sauer of JPL, 

obtains optimal trajectories using algorithms based upon classical optimal control theory. It has the 

capability of determining minimal propellant consumption while satisfying the transversality conditions 

associated the constraints on the mission. It provides the capability of selecting the optimal number of 

operating thrusters in multiple thruster configurations and of determining the correct throttle setting when 

presented with coefficients of the thruster and mass flow as a function of thrust system input power. 

A SEP rendezvous with Saturn is virtually impossible without a final thrusting period near Saturn. 

Because solar power drops below a minimum level necessary to power the thrusters around 3.5 AU, no 

EP thrusting is possible near Saturn. Hence, the SEP system essentially puts the S/C on a Saturn flyby 

trajectory. For the Saturn Flyby with Probes mission, this is sufficient, with SEP performing the majority 

of the mission velocity change required. For S/C capture into orbit around Titan or Saturn (for the 

Enceladus mission), a separate high thrust propulsion system or aero capture vehicle provides the 

necessary velocity change. 

In order to determine the optimal trajectory, the following gravity assist configurations were 

examined.

• Solar Electric Earth Gravity Assist (SEEGA)—chosen as optimal

• Solar Electric Venus Gravity Assist (SEVGA)—much less performance

• Solar Electric Jupiter Gravity Assist (SEJGA)—not very affective

• Solar Electric Earth Jupiter Gravity Assist (SEEJGA)—not available
NASA/TM—2011-216973 9

The next step in analysis would be a full verification with a detailed NEXT thruster model.

2.4.3 Mission Trajectory Details

It was determined that a single Earth gravity assist (EGA) was superior to either a Venus or a Jupiter 

fly-by. The less massive Venus imparts a smaller velocity increase even though the thruster system 

operates at a high efficiency level for a considerable duration in preparation for the fly-by and thereby 

also gains energy. Both Earth and Venus fly-bys are available across the 2015 to 2025 period. The JGA 

was not available throughout the entire 2015 to 2025 period because Jupiter moved away from an 

advantageous position for the Saturn mission. 

Figure 2.3 shows the trajectory from Earth to Saturn, calling out the areas of S/C thrusting and 

spacecraft (S/C) coasting along the arc. This particular solution does not have any coast phases during the 

inner solar system section of the trajectory. The Earth gravity assist occurs at approximately 670 days into 

the mission, after which the power, and resulting thrust tails off as the S/C travels towards Saturn.

The SEP Stage and probe are launched on an Atlas 551 to a C3 of 27 km2

/s2. The 20 kW Class SEP Stage goes 

through one Earth flyby to pick up acceleration. The SEP Stage thrusters then perform maneuvers and operate 

from 1 AU ~ 3 AU. At 3 AU, the SEP Stage is then disposed. Earth flyby safety considerations are taken into 

account in the choice of Earth flyby distance (note the Probe is plutonium powered). A 20-day launch window 

was chosen, worst-case payload and Xe propellant load for window chosen as design basis.

Figure 2.3.—EGA to Saturn trajectory.
NASA/TM—2011-216973 10

SEP Thruster and Power Details

• Two NEXT engines (1 +1), ~300 kg Xe propellant 

• Two solar arrays for BOL of 17 kW nominal (only maximum of 7 kW used—extra solar array 

area allows more thrust at >1 AU) 

– Note that the final design resulted in an 18.5 kW once the additional 30 percent power margin 

was built into the design.

2.4.4 Launch Window Sensitivity

Launch flexibility, including launch year capability and launch opportunities and windows of 

significant duration, is of primary interest to planetary science mission customers. Optimization of the 

nominal launch capability across launch dates spanning 10 yr revealed that the baseline configuration was 

capable of delivering roughly similar usable S/C masses annually with small variations in arrival Vhp. 

(Figure 2.4). Furthermore, the required SEP system propellant mass changed very little, while the mass 

delivered to the SEP starting orbit by the launch vehicle did vary. These variations arose from the annual 

variations in the relative Earth-Saturn geometry that required differing amounts of the Atlas 551 launch 

capability. Net mass penalties from years 2015 to 2018 can be mitigated by adjusting mission parameters, 

such as trip time and gravity assist altitude, for each launch year. Launch opportunity capability was 

addressed in these analyses, but is not presented here. Launch opportunity analysis using SEPTOP 

requires careful manipulation of constraints and optimization parameters. Results were achieved through 

several trial cases, with minimal impact to the primary mission capability, but further launch strategy 

development and analysis is warranted. Detailed launch window analyses have not yet been initiated. 

2.5 Mission Operations

2.5.1 Duty Cycle Budget and Operations Phases

The trajectory was run with a 90 percent duty cycle assumed. This duty cycle assumption was used to 

account for the following unmodeled activities accounting for the other 10 percent of thrusting time:

• 0.1 percent reserved for solar torque offset

• 1 percent used for timing events (e.g., checkout, flyby targeting)

• 1.9 percent saved for margin

• 7 percent remaining for communications uplink/navigation update

Figure 2.4.—Net delivered S/C mass for given launch date.
NASA/TM—2011-216973 11

The remaining 7 percent duty cycle translates into roughly 12 hr/week for comm. and nav. update and 

associated slews. Table 2.2 lists the Autonav commands used in the DS-1 mission. These commands and 

their time length were used to estimate the 7 percent remaining coasting period necessary in the SEP 

trajectory design.

TABLE 2.2.—MISSION PHASE AND OPERATING MODES

Command 

name

Description Argument Usage Time 

required

Nav_Do_OD Perform orbit determination None 1/wk 10 to 100 

min

Nav_Do_TCM Execute a TCM Duration 1/wk 1.5 to 24 hr

Nav_IPS_Off_Mes* Notify Nav of forced “engine off” None 1/wk 1 s

Nav_Man_Plan Perform maneuver planning None 1/wk 10 to 200 

min

Nav_Photo_Op Perform a nav picture taking processing session, edit and store data Duration 1/wk 1.5 to 8 hr

Nav_Reset* Stop all Navexec state machines None Seldom* 1 s

Nav_Set_IPS Start a mission burn None 1/wk 5 min

Nav_Start_Encntr Start an encounter sequence Seq. ID 4/encounter 1 min

Nav_Update_IPS Update the thrust vector during a mission burn None 2/day 1 min

Nav_Change_Mode Change an AutoNav operating mode Data vectors 2/mon 5 s

Nav_Data_Downlink Downlink a Nav file File ID 2/mon 20 s

Nav_Data_Update Update a Navigation file File ID 2/mon 20 s

Nav_IPS_Press Pressurize the main engine None 1/wk 1 to 30 min

Nav_ACM_Infoturn Optional desired pointing of the S/C after Nav event “turnspec” 1/wk 5 s

Nav_BBC_Deadband Optional desired deadband of the S/C after Nav event Deadband 1/wk 5 s

*Contingency or emergency back-up command

Table 2.3 lists the mission phases modeled in this analysis and the assumed operating modes of both 

the science probe and SEP Stage. This operational mode identification combined with the Power 

Equipment List (PEL) was used by the power system lead in determining the power load requirements.

TABLE 2.3.—MISSION PHASE AND OPERATING MODES

Phase Max. phase duration 

(hr)

Probe mode SEP Stage

mode

Launch pad 480 Standby Standby 

Launch from Earth 1 Standby Standby 

Loiter before escape injection 1 Standby Standby 

Escape Injection 0.5 Standby Standby 

Separation from Centaur, cancel tip-off rates 0.5 ACS Standby 

Checkout 24 ACS Coast 

Phasing orbit before Earth flyby 16029 --------- ---------

Thrusting --------- Standby Thrust 

Coasting --------- ACS Coast 

Communication Attitude --------- ACS Comm. 

Earth flyby targeting phase 168 ACS Coast 

Earth flyby 24 ACS Coast 

Interplanetary cruise to ~2 AU 6168 --------- ---------

Thrusting --------- Standby/ACS Thrust 

Coasting --------- ACS Coast 

Communication attitude --------- ACS Comm. 

SEP Stage jettison and decommissioning 2 ACS Coast 

Probe-only mission 47710 Main Standby 
NASA/TM—2011-216973 12

2.5.2 S/C Trajectory and Pointing

The S/C attitude is near-Earth centered inertial (ECI) during the Earth flyby phasing orbit portion of 

the trajectory. The S/C attitude provides for tangential thrusting after the Earth flyby as shown in 

Figure 2.5. Arrows plotted along this phase indicate the direction of the thrust vector required to achieve 

the optimized trajectory.

The trajectory requires the thrust vector to point between +20° and –50° out-of-plane throughout the 

trajectory to achieve the 2.5° inclination change necessary to enter into orbit about Saturn. Saturn’s orbit 

is inclined 2.49° to the ecliptic plane. Figure 2.6 shows the Local Vertical Local Horizontal (LVLH) pitch 

and yaw angles for the S/C over the entire course of the powered trajectory.

The assumptions that went into the modeling of the thruster power profile throughout the trajectory 

are as follows:

• Up to 10 kWe of excess power at ~1 AU

• 2 percent per year degradation of arrays included in the modeling

• Between 350 and 750 We were available for non-propulsion S/C functions throughout the SEP 

portion of the trajectory

Figure 2.5.—Tangential thrusting during SEP operation.

Figure 2.6.—Attitude angle over mission time.
NASA/TM—2011-216973 13

Figure 2.7 shows the power profile and distance from the Sun as a function of days into the mission 

based on the above assumptions. Figure 2.8 shows gimbal angle over mission time.

Figure 2.7.—Power profile and distance to the Sun over mission time.

Figure 2.8.—Gimbal angle over mission time.
NASA/TM—2011-216973 14

2.6 Solar Radiation Pressure

Solar radiation pressure is a minor disturbance since the entire trajectory was outside of 1 AU, but 

was analyzed to ensure the EPS can provide sufficient control to the S/C. Solar torques can easily be 

counteracted by slightly angling the thruster off of the nominal profile. Alternatively, the solar radiation 

pressure torque could also be offset with momentum wheels on the probe that are despun with the ion 

thrusters. The total penalty is less than 0.1 percent equivalent efficiency decrease of the thruster.

2.7 Slew Performance

Slew performance was estimated for a 180° slew in the pitch axis for main antenna pointing to Earth

during coast periods.

Assumptions:

• ±7° deadbands

• 0.4 N thrusters in opposing pairs with 200 s ISP

• Simple Proportional Integral Derivative (PID) control

• Inertia matrix from solid model with assumed probe mass properties

• Moment arms attained from solid model

Given the above assumptions on deadbands, and the section of the trajectory during which antenna 

pointing is required, 133 slew maneuvers are required. Each maneuver involves a slew in pitch angle and 

then back to the original position. Assuming that the 180° pitch slew is worst-case for all slews, 

approximately 0.1 kg of propellant per slew maneuver is needed. Each slew maneuver takes 

approximately 600 s to complete. This yields a propellant mass total of 26.6 kg for 133 slew maneuvers to 

180° and 133 maneuvers back to 0° using the RCS thrusters.

M_slewpropellant = (0.1 kg/slew ∗133 slews) ∗ 2

2.8 Launch Vehicle Details: Atlas 551

The launch vehicle used in the baseline mission was the Atlas 551 ELV, see Figure 2.9 for an Atlas 

551 ELV chosen as the baseline launch vehicle for this study.

Figure 2.9.—Atlas 551 ELV.
NASA/TM—2011-216973 15

Figure 2.10.—Atlas V ELV.

Figure 2.10 shows the details of the Atlas V from the Lockheed Martin Atlas V fact sheet. These 

details were used by the configuration designer in packaging the stage into the payload fairing and by the 

structures engineer for load and launch variables used in structures calculations.

2.9 ELV Performance Calculations

The C3 range used for the Saturn mission and applied in the low thrust trajectory analysis was in the 

range of 18.6 to 67.3 km2

/s2. The mission analysis used a 10 percent ELV contingency factor without an 

adaptor mass fraction or explicit mass quotation. Figure 2.11 is the ELV performance for the Atlas 551 as 

modeled in the low thrust trajectory code used in this analysis.

The 5-m short fairing was chosen to maximize payload delivered to Saturn for the given trajectory C3.

Figure 2.12 is the performance of the 5 m fairing Atlas 551 for comparison to Figure 2.11. Note that SEP 

Trajectories require C3 of 20 to 30 km2

/s2.

A D1666/C22 adapter configuration was chosen to provide ample diameter for the SEP Stage. The 

Atlas V 551 performance to a C3 of 27 km2

/s2 is 3864 kg. The following calculations were used in 

determining the performance of the ELV to Saturn, and in backing out the remaining mass available for 

the science probe once the SEP Stage had been sized.

• Subtract out ELV margin of 10 percent = 386 kg

• ELV performance after margin (i.e., that which is available to the SEP Stage and Science probe) 

= 3478 kg

• ELV S/C adaptor = 73 kg taken out of the already subtracted ELV margin.

• ELV margin after adaptor = 314 kg (this reduces ELV margin to 8 percent)

• Subtract off the SEP Stage wet mass including the subsystem and system level growth 

(contingency) = 1034 kg (see Table 2.4 for details of the wet mass)

• Remaining mass available for the Probe = 2444 kg

• Chose lowest performance over 20 day launch window
NASA/TM—2011-216973 16

Figure 2.11.—Atlas 551 performance for C3-internal trajectory modeling.

Figure 2.12.—Atlas 551 5 m fairing performance versus C3 curve.

The performance available for the science probe is calculated at 2444 kg. This is the ELV 

performance minus the margin, minus the adaptor mass, minus the total wet mass of the SEP Stage. What 

remains is that which is allocated to the “to be designed” science probe.
NASA/TM—2011-216973 17

TABLE 2.4.—SEP STAGE MASS CALCULATION

Launch masses Value

Launch vehicle...............................................Atlas V (551)

ELV performance with margin ............................3864.8 kg

ELV margin (10%)................................................386.5 kg

ELV performance after margin............................3478.3 kg

Payload adaptor .......................................................72.6 kg

ELV margin less adaptor.......................................313.9 kg

New net ELV margin.................................................. 8.1%

SEP Stage wet mass................................................1034 kg

SEP performance to Saturn.....................................2444 kg

2.10 SEP Stage Launch Envelope

The solar arrays and SEP Stage configuration fits within the 5 m Atlas payload fairing envelope as 

shown in Figure 2.13.

The SEP Stage is designed to keep out of the “keep out zone” in the Atlas 5 m fairing adaptor as 

shown in Figure 2.14. Figure 2.15 shows the dimensions of the SEP Stage in launch configuration. The 

probe, as yet not designed, is not shown.

Figure 2.13.—Atlas 551 5 m fairing envelope.

Figure 2.14.—Atlas 551 5 m fairing payload keep out zone.
NASA/TM—2011-216973 18

Figure 2.15.—Dimensions of SEP and probe in Atlas V fairing.

2.11 System Design Trade Space

The trade space examined in these designed focused on the number of NEXT thrusters used in the 

EPS to perform the mission. Cases 2 and 3 were a larger, Flagship class, mission, and were launched on a 

Delta IV H rather than the Atlas 551. Details on these two trades can be found in Section 7.0 near the end 

of this report.

Case 1: EP thruster configuration 1+1 (Baseline)

• In this configuration, there is one active thruster and one spare to perform the delta V of the 

mission.

Case 2: EP thruster configuration 2+1

• In this configuration, there are two active thrusters and one spare to perform the delta V of the 

mission.

Case 3: EP thruster configuration 3+1

• In this configuration, there are three active thrusters and one spare to perform the delta V of the 

mission.

Case 3 will be documented in a future COMPASS report of the Saturn Flagship SEP Stage Summary, 

COMPASS document number CD–2007–09.

2.12 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.

2.12.1 GLIDE Study Share

GLobal Integrated Design Environment (GLIDE) 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.
NASA/TM—2011-216973 19

Share: https://glide.grc.nasa.gov/Saturn_SEP_Stage

2.12.2 GLIDE Architecture

The architecture set up to hold the following design studies is:

Architecture: Saturn_SEP_Stage

2.12.3 GLIDE Study Container(s)

Study name Summary GLIDE study container

Study 1 EP thruster configuration 1+1 SaturnSEP_1plus1

Study 2 EP thruster configuration 2+1 SaturnSEP_2plus1

Study 3 EP thruster configuration 3+1 SaturnSEP_3plus1

3.0 Baseline Design 

3.1 Baseline System Design

The deployed SEP module (Figure 3.1) and representative S/C are dominated by the large single axis, 

solar arrays to collect the power to operate a single NEXT thruster throughout the trajectory. Two 

thrusters are flown with one active and one a cold spare. Gimbals on the thruster provide thrust vector 

pointing and yaw and pitch control. Radiators are placed on the SEP module face just below the solar 

array drive assemblies so that they can view deep space during NEXT thruster operation. The science 

probe was designed as a cylinder probe of unknown design and notional antenna dish in the figure 

attached to the detailed SEP Stage and solar arrays. 

This SEP Stage is capable of delivering a 2444 kg probe on a Saturn trajectory with a hyperbolic 

arrival velocity of 5.4 km/s. The system assumed a growth of 30 percent mass, 10 percent power, and 6

percent on propellant margins. The SEP Stage relies on the probe for GN&C, C&DH, and 

Communications. The structure and packaging design of the SEP Stage is configured to carry the science 

probe and minimize packaging interference with Probe in shroud. All subsystems (except tanks and solar 

arrays) used a single fault tolerant design assumption approach.

Figure 3.1.—Conceptual SEP Stage deployed configuration.
NASA/TM—2011-216973 20

3.2 Top Level Design (MEL and PEL)

3.2.1 Master Equipment List (MEL)

Where the MEL (Table 3.1) captures the bottoms-up estimation of CBE and growth percentage line 

item by item from the subsystem designer. Table 3.2, in Section 3.2.2 wraps up those total masses, CBE 

and total mass after applied growth percentage. In order to meet the total of 30 percent at the system level, 

an allocation is necessary for system level growth. 

TABLE 3.1.—MASTER EQUIPMENT LIST (MEL)

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission 867.5 10 82.9 950.3

01 Saturn SEP S/C - - 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0 0.0 0 0.0 0.0

01.2 SEP Stage - - 867.5 10 82.9 950.3

01.2.1 Avionics and communications - - 10.2 20 2.0 12.2

01.2.2 GN&C - - 0.8 50 0.4 1.2

01.2.3 EP subsystem - - 267.6 16 44.2 320.8

01.2.4 Thermal control (non-propellant) - - 32.7 18 5.9 38.6

01.2.5 Structures and mechanisms - - 82.6 18 14.9 97.5

01.2.6 Propulsion - - 152.9 10 15.5 168.4

01.2.7 Propellant - - 311.7 0 0.0 311.7

3.2.2 Power Equipment List (PEL)

The power requirements for nominal loads of the SEP Stage are shown in Table 3.2. The power 

growth carried on the analysis was 15 percent. To provide for margin each array sized to 9350 W. The 

17000 W total solar arrays (plus margin taking the total to approximately 18.7 kW) account for 

degradation during the mission (cell, string, diode losses) as well as being oversized to provide sufficient 

propulsion power for the final thrusting leg up to ~5 AU. The power system’s batteries are sized based on 

the standby power collected from the subsystem requirements in the PEL.

TABLE 3.2.—POWER EQUIPMENT LIST (PEL)

Subsystem Nominal power 

(W)

Standby power 

(W)

C&DH 50 10

GN&C 10 0

Electrical Power 20 5

Thermal Control 0 3

Structures and Mechanisms 10 0

Propulsion 7080 0

Total SEP Stage 7170 18

3.3 System Level Summary

The Total Mass Growth for this study is set to 30 percent (subsystem growth 14.9 percent, system growth 

15.1 percent). Table 3.3 is the system integration table that captures the bottoms-up mass estimates for 

subsystems CBE (current best estimate) masses of their system level items. For clarification in this study, the 

term Contingency/Growth was mass that was “flown” to the destination and required propellant to perform the 

∆V along the mission. This is performance (or mass) delivered on the ELV that cannot be allocated to the 

science Probe. Subsystem leads applied growth (contingency) percent per subsystem item. Total dry mass 

growth (contingency) calculated for the system was 14.9 percent. The desired total growth (contingency) at the 
NASA/TM—2011-216973 21

system level was 30 percent as a ground rule of this analysis. Reaching the 30 percent total required an 

additional 83.9 kg shown in Table 3.3 as “System level growth (contingency)” in the last line of the table. The 

total SEP Stage mass (with subsystem level growth and system contingency reserves) was 1034.2 kg as shown 

for the line item “SEP Stage total mass with system growth”.

TABLE 3.3.—SYSTEM INTEGRATION SUMMARY

S/C system details

Master equipment list—Mass 

CBE 

(kg) 

Growth 

(kg) 

Total Mass 

(kg) 

Percent of 

dry mass 

Science Probe N/A N/A 2444 N/A 

SEP Stage ------- N/A 

Avionics and Communications 10.2 2.0 12.2 1.9

GN&C 0.8 0.4 1.2 0.2

Electrical Power 276.6 44.2 320.8 50.2 

Thermal Control 32.7 5.9 38.6 6.0

Structures 82.6 14.9 97.5 15.3 

Propulsion 152.9 15.5 168.4 26.4

Propellant 311.7 0.0 311.7 N/A 

SEP Stage total mass without system growth 867.5 82.9 950.3 

SEP Stage dry mass 555.8 82.9 638.7 

SEP Stage inert mass 558.9 N/A 641.8 

SEP Stage total mass with system growth 83.9 1034.2 

System level growth (contingency) tracking 

Dry mass growth (contingency) 

Desired total growth (contingency) 

Desired total growth (contingency) 

Mass system level growth (contingency)

14.9% 

30%

166.7 kg

83.9 kg

3.4 Design Concept Drawing and Description

Figure 3.2 shows the SEP Stage housing with the major components called out. The thrusters are on 

the bottom face and not shown in this drawing. The solar arrays are also not shown in this drawing but 

would be attached at the points labeled “Solar Array Drive”.

C&DH

• Digital Control Interface Unit (DCIU) controls power and 

• Propulsion system operation commands from probe

Power

• Two Ultra-Flex GaAs Triple Junction Solar Arrays 

• Total power at 1 AU 17 kW nominal (18.5 kW with contingency) but run at a maximum power of 

7 kW (8.5 kW with contingency), for single string redundancy at 1 AU

• Single axis array drives 

• Li-Ion batteries for launch and contingency ops 

Propulsion

• 1+1 gimbaled 7 kW NEXT ion propulsion system with 

• ~300 kg Xe propellant 

• Single thruster operation 

• Gimbal provides yaw/pitch control and ability to offset perturbations 

Structure

• Al-Li hexagonal, skin-stringer structure supports probe during launch 

• At end of SEP Stage operations, a LightBand separation system allows the probe to jettison the stage 

 

 

 

 

 . 7

 . 8

 m 4 .2 G ro

 e n cy ) 

 g e n c y

 t in g e n

 g en cy 

 t in g e n
NASA/TM—2011-216973 22

Figure 3.2.—Conceptual SEP Stage Main Components.

Thermal

• Passive heat pipes to ‘north and south’ radiator panels 

• MLI and heaters on Xe tank 

4.0 Subsystem Breakdown

4.1 Communications

4.1.1 Communications Requirements

Most of the communications functions will be performed by the systems on the science probe. 

Therefore, only minimal communications infrastructure is carried on the SEP Stage.

4.1.2 Communications Assumptions

Most of the Communications system was assumed to be part of the science probe. A 3 m dish antenna 

is shown in the S/C configuration based on assumptions of data link rates and data transfer requirements 

for the science probe.

4.1.3 Communications Design and MEL

For minimal communications on the stage, two omni-directional antennas were placed on back of 

propulsion stage. Antennas were included in the design to allow for communications with Earth while 

thrusters are running. Antennas are assumed to be on the rear or the propulsion unit and interface with the 

communications hardware in the payload. The SEP Stage was not responsible for probe communicants.

The signal is passed to the payload for processing.
NASA/TM—2011-216973 23

4.1.4 Communications Trades

No trades were run on the communications system components since only a minimal set of 

components were included.

TABLE 4.1.—COMMUNICATIONS SYSTEM MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - --- 867.5 10 82.9 950.3

01 Saturn SEP S/C - --- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - --- 867.5 10 82.9 950.3

01.2.1 Avionics and communications - --- 10.2 20 2.0 12.2

01.2.1.a C&DH - --- 9.5 20 1.9 11.4

01.2.1.b Communications - --- 0.7 20 0.1 0.8

01.2.1.b.a Transponder 0 0.0 0.0 0 0.0 0.0

01.2.1.b.b RF assembly 0 0.0 0.0 0 0.0 0.0

01.2.1.b.c Processing module 0 0.0 0.0 0 0.0 0.0

01.2.1.b.d Antenna 2 0.3 0.6 20 0.1 0.7

01.2.1.b.e Coaxial cable 2 0.0 0.1 20 0.0 0.1

01.2.1.b.f Installation—mounting and circuitry 0 0.0 0.0 0 0.0 0.0

01.2.1.b.g Miscellaneous no. 1 0 0.0 0.0 0 0.0 0.0

01.2.1.b.h Miscellaneous no. 2 0 0.0 0.0 0 0.0 0.0

01.2.1.b.i Miscellaneous no. 3 0 0.0 0.0 0 0.0 0.0

01.2.2 GN&C - --- 0.8 50 0.4 1.2

01.2.3 Electrical Power - --- 276.6 16 44.2 320.8

01.2.4 Thermal Control - --- 32.7 18 5.9 38.6

01.2.5 Structures - --- 82.6 18 14.9 97.5

01.2.6 Propulsion - --- 152.9 10 15.5 168.4

01.2.7 Propellant - --- 311.7 0 0.0 311.7

4.1.5 Communications Analytical Methods

Design was based upon hardware currently available from BAE, Maxwell and Aitech which greatly 

exceeds the capabilities of hardware used in DS-1. Changes to the model are manually made based upon 

feedback from other subsystem leads.

4.1.6 Communications Risk Inputs

Communications I/O not well defined.

4.1.7 Communications Recommendation

Further details are required from the science probe in order to better understand the requirements on

the communications system on the SEP Stage.

4.2 GN&C 

4.2.1 GN&C Requirements

The GN&C system must be able to make up for slew maneuvers during the coasting phase of the SEP 

Stage’s trajectory to the Saturn system. All other GN&C systems will reside on the science probe since 

they will be required after the SEP Stage has been jettisoned. 
NASA/TM—2011-216973 24

4.2.2 GN&C Assumptions

• The thruster configuration is for a 1+1 SEP Stage

• The SEP operates for a 1 AU to 3 AU orbit range

• The SEP Stage operates for < 3 yr

• It will take a total of 8 yr for the Probe to reach Saturn

• 3 percent additional Xe is carried for navigation/trajectory and other errors

• 1000 km Earth flyby

• SEP Stage thrust duty cycle 90 percent

4.2.3 GN&C Design and MEL

Ion thrusters can act as pointing control during thrusting for yaw and pitch. Ion thrusters may be able 

to off-set solar perturbations (during thrusting) and/or dump momentum wheels. Probe provides ACS for 

all roll control, coasting, and solar array pointing. The probe also commands the ion thruster gimbals. Sun 

sensors are the only additional ACS hardware component included in the SEP Stage and are used in 

conjunction with the solar arrays for pointing during maneuvers.

TABLE 4.2.—GUIDANCE, NAVIGATION AND CONTROL SYSTEM MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - --- 867.5 10 82.9 950.3

01 Saturn SEP S/C - --- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - --- 867.5 10 82.9 950.3

01.2.1 Avionics and Communications - --- 10.2 20 2.0 12.2

01.2.2 GN&C - --- 0.8 50 0.4 1.2

01.2.2.a IMU 0 0.0 0.0 0 0.0 0.0

01.2.2.b Star sensor 0 0.0 0.0 0 0.0 0.0

01.2.2.c Sun sensor 4 0.2 0.8 50 0.4 1.2

01.2.2.d ACS 0 0.0 0.0 0 0.0 0.0

01.2.2.e Flight control electronics 0 0.0 0.0 0 0.0 0.0

01.2.3 Electrical Power - --- 276.6 16 44.2 320.8

01.2.4 Thermal Control - --- 32.7 18 5.9 38.6

01.2.5 Structures - --- 82.6 18 14.9 97.5

01.2.6 Propulsion - --- 152.9 10 15.5 168.4

01.2.7 Propellant - --- 311.7 0 0.0 311.7

The slew maneuver RCS propellant is reported in the propulsion system MEL.

4.2.4 GN&C Trades

No trades performed.

4.2.5 GN&C Analytical Methods

GN&C components chosen from off the shelf catalogs. Calculations of the propellant mass required 

to produce the slew maneuvers is documented in Section 2.6.

4.2.6 GN&C Recommendation

Since the GN&C system has been assumed to reside on the science probe, this push back needs to be 

iterated with the science probe design team to make sure that the science probe is active during the SEP 

Stage portion of the trajectory.
NASA/TM—2011-216973 25

4.3 Command and Data Handling (C&DH)

The C&DH system is sometimes referred to as Avionics in documentation. Both terms are equally 

valid in describing this system.

4.3.1 C&DH Requirements

The SEP Stage C&DH systems were required to provide control interface for all SEP Stage 

propulsion, power and thermal systems. In this design, the Communications and Navigation functionality 

were handled by payload.

TABLE 4.3.—C&DH SYSTEM MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - --- 867.5 10 82.9 950.3

01 Saturn SEP S/C - --- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - --- 867.5 10 82.9 950.3

01.2.1 Avionics and communications - --- 10.2 20 2.0 12.2

01.2.1.a C&DH - --- 9.5 20 1.9 11.4

01.2.1.a.a DCIU 2 4.0 8.0 20 1.6 9.6

01.2.1.a.b Electrical integration assembly 0 0.0 0.0 0 0.0 0.0

01.2.1.a.c Data cabling (analog/digital) 1 0.8 0.8 20 0.2 0.9

01.2.1.a.d Power cabling to DCIU 1 0.8 0.8 20 0.2 0.9

01.2.1.a.e Remote interface unit 0 0.0 0.0 20 0.0 0.0

01.2.1.a.f Time generation unit 0 0.0 0.0 0 0.0 0.0

01.2.1.b Communications - --- 0.7 20 0.1 0.8

01.2.2 GN&C - --- 0.8 50 0.4 1.2

01.2.3 Electrical Power - --- 276.6 16 44.2 320.8

01.2.4 Thermal Control - --- 32.7 18 5.9 38.6

01.2.5 Structures - --- 82.6 18 14.9 97.5

01.2.6 Propulsion - --- 152.9 10 15.5 168.4

01.2.7 Propellant - --- 311.7 0 0.0 311.7

4.3.2 C&DH Assumptions

Most of the C&DH functions of the combined SEP/Science probe S/C will be performed by the 

science probe. Only the thruster control will be performed onboard the SEP Stage.

4.3.3 C&DH Design and MEL

The C&DH system design was based on NASA Solar Technology Application Readiness (NSTAR) 

DS-1 DCIU system, expanded to control power and thermal systems. A DCIU accepts and executes high 

level commands from the S/C computer and provides propulsion subsystem telemetry to the S/C data 

system.

The design has been expanded and includes two DCIUs to allow for control of the 1+1 thruster 

configuration. DCIUs were identical and cross-strapped to each thruster. The DCIUs use the 1553 

protocol to communicate to each other and the payload and the RS-422 to communicate with SEP Stage

systems. The avionics cabling is assumed to be minimal due to the small number of I/O channels and 

close proximity of the avionics hardware to their related systems. The block diagram of the C&DH 

system is shown in Figure 4.1.
NASA/TM—2011-216973 26

Figure 4.1.—C&DH block diagram.

4.3.4 C&DH Trades

Dedicated propulsion DCIU and remote interface unit versus a single DCIU to run the entire stage.

4.3.5 C&DH Analytical Methods

Design is based upon hardware currently available from BAE, Maxwell, and Aitech which greatly 

exceeds the capabilities of hardware used in DS-1. Changes to the model are manually made based upon 

feedback from other subsystems leads. The power estimates based on data from DS-1 avionics and 

currently available space rated hardware.

4.3.6 C&DH Risk Inputs

• C&DH I/O not well defined.

• C&DH component availability issues.

4.4 Electrical Power System

Figure 4.2 shows the conceptual SEP Stage with arrays deployed.

4.4.1 Power Requirements

Photovoltaic (PV) array power needs to be controllable based on S/C position and EP power 

demands. PV array must fit inside launch vehicle faring. PV array must not interfere with Saturn Probe 

(during launch as well as during cruse). PV array must be sized to allow for nominal degradation (.

4.4.2 Power Assumptions

PV array power level requirement for EP portion of S/C was determined by mission analysis. The 

mission analysis also provides S/C position and attitude information which drives array size and control 

requirements. Power Management and Distribution (PMAD) system designed for one fault tolerance. 

Power to EPS stage during launch and array deployment provided by launch vehicle.
NASA/TM—2011-216973 27

Figure 4.2.—Conceptual SEP Stage with arrays deployed.

4.4.3 Power Design and MEL

In order to provide the required 18.5 kW power (with 200 W housekeeping), each array was 

oversized by 1.5 kW to address degradation (cell, string, diode losses), two Ultraflex solar arrays design 

based on ST-8, CEV. Li Ion batteries for pre-array deployment and contingency operations. The power 

delivered by the solar array was dependant on distance to the sun throughout the mission. Figure 4.5

shows the distance to the sun of the SEP stage over the mission timeline and was used to size the solar 

arrays.

Figure 4.3 shows an example of an Ultraflex solar array. The schematic of the EPS system designed 

for this SEP Stage is shown in Figure 4.4.

TABLE 4.4.—POWER SYSTEM MEL

WBS no. Description Qty Unit 

mass

(kg)

CBE 

mass

(kg)

Growth

(%)

Growth

(kg)

Total 

mass

(kg)

SEP Saturn In-Space Propulsion Mission - ----- 867.5 10 82.9 950.3

01 Saturn SEP S/C - ----- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - ----- 867.5 10 82.9 950.3

01.2.1 Avionics and Communications - ----- 10.2 20 2.0 12.2

01.2.2 GN&C - ----- 0.8 50 0.4 1.2

01.2.3 Electrical power subsystem - ----- 276.6 16 44.2 320.8

01.2.3.a Solar arrays - ----- 198.8 15 29.3 228.1

01.2.3.a.a Solar array mass (cell and structure only) 2 93.7 187.4 15 28.1 215.5

01.2.3.a.b Solar array gimbals 2 5.7 11.4 10 1.1 12.5

01.2.3.b. Power management and distribution - ----- 62.8 16 10.0 72.8

01.2.3.b.a Power management/control electronics 1 61.2 61.2 15 9.2 70.4

01.2.3.b.b Power distribution/monitoring wiring harness 2 0.8 1.6 50 0.8 2.4

01.2.3.c Power cable and harness subsystem - ----- 7.0 50 3.5 10.6

01.2.3.c.a S/C bus harness 0 0.0 0.0 0 0.0 0.0

01.2.3.c.b PMAD harness 0 0.0 0.0 0 0.0 0.0

01.2.3.c.c EP harness 1 4.0 4.0 50 2.0 6.0

01.2.3.c.d Solar array to S/C harness 1 3.0 3.0 50 1.5 4.5

01.2.3.c.e Power cabling 0 0.0 0.0 0 0.0 0.0

01.2.3.d Battery system - ----- 8.0 19 1.5 9.5

01.2.3.d.a Battery assembly-primary 2 3.0 6.0 15 0.9 6.9

01.2.3.d.b Battery assembly-secondary 2 1.0 2.0 30 0.6 2.6

01.2.4 Thermal Control - ----- 32.7 18 5.9 38.6

01.2.5 Structures - ----- 82.6 18 14.9 97.5

01.2.6 Propulsion - ----- 152.9 10 15.5 168.4

01.2.7 Propellant - ----- 311.7 0 0.0 311.7
NASA/TM—2011-216973 28

Figure 4.3.—Solar array power versus S/C distance to the sun over the mission timeline.

Figure 4.4.—Ultraflex solar array.
NASA/TM—2011-216973 29

Figure 4.5.—Ultraflex solar array schematic.

Details of the above power system are called out in the bullets below.

Photovoltaic Array 

• Two AEC-able Ultraflex arrays at 8500 W (1 AU) each

• Provides 3000 W Contingency at 1 AU (17000 W at 1 AU assumed available for SEP trajectory)

• Each solar array populated with Spectrolab UTJ three-junction solar cells (93.7 kg each)

• PV cells arranged in series strings of 40 cells to provide Vmp of 80 V dc at 1 AU (125 V dc at 3 AU)

• Strings arranged in groups to provide 5 to 50 percent of array power at primary PDUs

• Moog SADA (single axis, 5.7 kg each)

PMAD

• Redundant primary PDUs, cross-tied (28.5 kg each)

• Power on buss controlled by string relays. Five percent strings represent 425 W at 1 AU and 

47 W at 3 AU

• Redundant 200 W 28 V dc-dc converters feed redundant secondary cross-tied PDUs (5.8 kg total)

• Provisions for 28 V dc feed/receive from launch vehicle and/or Probe 

4.4.4 Power Trades

The current baseline design uses probe for pad/contingency power. Solar array power is available to 

the science probe for transit operations. Option to use solar arrays in lieu of radioisotope power systems.

• Flat plate array versus Ultraflex

– Ultraflex chosen due to packaging and mass advantages

 Array power control options (peak power tracker versus sequential shunt vs. sequential 

switch)

– Sequential switch chosen for simplicity
NASA/TM—2011-216973 30

4.4.5 Power Comments, Concerns, and Recommendations

Given the cost, availability and safety issues associated with General Purpose Heat Sources (GPHS), 

only 28 kg available)) and the presence of a large solar array already aboard, does it make sense to 

examine PV power at Saturn?

4.5 Structures and Mechanisms

4.5.1 Structures and Mechanisms Requirements

The structural design of the SEP Stage must withstand applied loads from launch vehicle and 

thrusters and provide minimum deflections, sufficient stiffness, and vibration damping. The design will 

be such that the goal is to minimize weight while meeting the above physical requirements and the 

structure must fit within the confines of the launch vehicle chosen for this mission.

• Optimized for minimum weight

• Accommodate launch loads (Table 4.5)

– 5.5g maximum axial load

– 3.0g maximum lateral load

• Support satellite probe

– ~2500 kg (Calculated at 2444 kg)

• Support propulsion, navigation, and power systems

• Fit within confines of launch vehicle

• Representative stage/probe separation system

• Solar array and thruster gimbal mechanisms kept in power and propulsion sections, respectively

4.5.2 Structures and Mechanisms Assumptions

The following set of assumptions were applied to the structural design of the SEP Stage.

• Material: Al-Li 2090-T3

• Space frame architecture

– Riveted and fastened construction

• Satellite probe

– 2500 kg

– Center of gravity ~2 m above mounting surface of stage

Atlas 551 loads data was used for structural design. Table 4.5 shows the load limits for the Atlas 551 

on the structures.

The main structural will be made of Al and the structural elements will be made of tubular members.

Because the mounting area of the science probe to the main structure of the SEP Stage is unknown, 

assumptions will be made as to the method of connecting the two parts together.

4.5.3 Structures and Mechanisms Design and MEL

The SEP bus space frame architecture was a hex configuration (Figure 4.6 and Figure 4.7).

Aluminum sheets were inserted to the design for sheer stiffness and protection of components. Cross 

members were included to mount components. Rings were incorporated at the top and bottom to match 

respective separation system.
NASA/TM—2011-216973 31

TABLE 4.5—ATLAS 551 LOAD LIMITS

Load condition Direction Atlas V 40Z, 50Z Atlas V 4YZ, 5YZ

Steady state,

g

Dynamic,

g

Steady state,

g

Dynamic,

g

Launch Axial

Lateral

1.2

0.0

±0.5

±1.0

1.5

0.0

±1.5

±2.0

Flight winds Axial

Lateral

1.0 to 2.8

±0.4

±0.5

±1.6

1.0 to 2.8

±0.4

±0.5

±1.6

Strap-on 

separation

Axial

Lateral

----

----

----

----

3.3

0.0

±0.5

±0.5

BECO/BETO

(Max axial)

(Max axial)

Axial

Lateral

Axial

Lateral

5.5

0.0

3.0 to 0.0

0.0

±0.5

±0.5

±1.0

±1.5

5.5

0.0

3.0 to 0.0

0.0

±0.5

±0.5

±1.0

±1.5

SECO Axial

Lateral

----

----

----

----

----

----

----

----

MECO/CLE

(Max axial)

(Max axial)

Axial

Lateral

Axial

Lateral

4.5 to 0.0*

0.0

0.0

0.0

±1.0

±0.3

±2.0

±0.6

4.5 to 0.0*

0.0

0.0

0.0

±1.0

±0.3

±2.0

±0.6

Sign convention

Longitude axis: + (positive) = compression

– (negative) = tension

± may act in either direction

Lateral and longitudinal loading may act simultaneously during any flight event

Loading is applied to the S/C cg

“Y” in vehicle designator is number of SRBs and ranges from 1 to 3 (400 series) or 1 to 5 (500 series

“Z” in vehicle designator is number of Centaur engines and is 1 or 2

*Decaying to zero

The structural design provides support for

• Two ion thrusters and gimbals

– Two-axis range of motion: ±19°, ±17°

• Three high pressure Xe tanks

• Two low pressure units and one high pressure unit

• Two PPUs and radiators

– Louvers on the radiators

• Two SADAs

– Single axis solar array

– Solar array/boom deployment mechanisms

• Two DCIUs

• Separation unit components

– The LightBand separation system was used as the initial design point.

• Satellite probe

– Primary structure, 66.7 kg

– Secondary structure, total mass 15.9 kg
NASA/TM—2011-216973 32

Figure 4.6.—Main S/C bus wire model.

Figure 4.7.—Stage attached to payload fairing adaptor.
NASA/TM—2011-216973 33

TABLE 4.6.—STRUCTURES AND MECHANISMS SYSTEM MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - ----- 867.5 10 82.9 950.3

01 Saturn SEP S/C - ----- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - ----- 867.5 10 82.9 950.3

01.2.1 Avionics and Communications - ----- 10.2 20 2.0 12.2

01.2.2 GN&C - ----- 0.8 50 0.4 1.2

01.2.3 Electrical Power Subsystem - ----- 276.6 16 44.2 320.8

01.2.4 Thermal Control - ----- 32.7 18 5.9 38.6

01.2.5 Structures and Mechanisms - ----- 82.6 18 14.9 97.5

01.2.5.a Structures - ----- 82.6 18 14.9 97.5

01.2.5.a.a Primary structures - ----- 66.7 18 12 78.7

01.2.5.a.a.a Main bus structure 1 66.7 66.7 18 12 78.7

01.2.5.a.b Secondary structure (installations) - --- 15.9 18 2.9 18.8

01.2.5.a.b.a C&DH installation 2 1.2 2.4 18 0.4 2.8

01.2.5.a.b.b C&T installation 1 0.0 0.0 0 0.0 0.0

01.2.5.a.b.c GN&C installation 0 0.0 0.0 0 0.0 0.0

01.2.5.a.b.d Electric power installation 2 2.6 5.2 18 0.9 6.1

01.2.5.a.b.e Thermal control installation 0 0.0 0.0 0 0.0 0.0

01.2.5.a.b.f Electric propulsion installation 1 5.0 5.0 18 0.9 5.9

01.2.5.a.b.g Tanks support structure 1 3.3 3.3 18 0.6 3.9

01.2.5.b Mechanisms - ----- 0.0 0 0.0 0

01.2.6 Propulsion - ----- 152.9 10 15.5 168.4

01.2.7 Propellant - ----- 311.7 0 0.0 311.7

4.5.4 Structures and Mechanisms Separation System

An example of the LightBand considered as the separation mechanism between the SEP Stage and 

probe is shown in Figure 4.7 and Figure 4.8 illustrates the mechanisms of the Standard LightBand (SLB).

Flight Heritage

• December 2006: Three ANDE spheres successfully deployed in LEO on STS-116

• November 2006: Successful deployment of TacSat-2 in LEO 

• June 2006: Successful deployment of MITEX in geostationary orbit 

• April 2005: Successful deployment of XSS-11 into orbit 

LightBand Features and Benefits

• Lightweight: LightBand is typically 1/3rd the mass of an equivalent function V-band/Marman Clamp.

• Flight proven: LightBand has flown on multiple missions and successfully deployed satellites on each.

• Low shock: Non-explosive initiation combined with a low strain energy design produce an order-ofmagnitude less shock than pyrotechnics

• Low height: LightBand is half the height of V-band/Marman Clamp devices and 1/8th the cross 

sectional area allowing larger payload volume.

• Test verified: Every flight LightBand is tested for strength, vibration, thermal vacuum and separation 

reliability.

• Integrated solution: All of the subsystems of a separation system are completely included in the 

LightBand. 

• Low cost: LightBand typically costs 1/2 as much as an equivalent function V-band/Marman Clamp. 

LightBand reset cost is typically 1/100th that of V-band/Marman Clamp devices. Typical delivery is 8 

months ARO.
NASA/TM—2011-216973 34

Figure 4.8.—Standard LightBand (SLB).

1. The tensioned retaining line 

holds the hinged leaves into the 

upper ring. Tension measured by 

load cell and/or torque wrench and 

can be adjusted at any time.

2. The detensioner cuts 

the retaining line by 

radiantly heating it. The 

cut line retracts before it 

can burn or melt.

3. Redundant spring 

plungers at each leaf 

unlatch and rotate the 

leaves.

4. The separation springs push halves 

apart. The separation connectors 

disengage. No debris is generated. 

Separation switches change state. No 

tip off.

Figure 4.9.—How SLB works.

4.5.5 Structures and Mechanisms Analytical Methods

Preliminary structural analysis with given launch loads.

4.5.6 Structures and Mechanisms Risk Inputs

• Potential impact with foreign object or due to nearby operations.

• Stage interface failures.

• Separation system does not work.

• Potential structural failure

• Solar array deployment failure (mechanisms)

4.5.7 Structures and Mechanisms Recommendation

The following items were suggestions for future cost savings in the structural design.

• Composite sandwich structures may be investigated for further mass reduction but would incur 

additional costs and may potentially be subject to degradation in space environment

Future detailed analysis is needed to determine

• Detailed mounting methods for components

• Detailed stress analysis

• odal analysis

4.6 Propulsion and Propellant Management

Figure 4.10 shows the propulsion face of the SEP Stage hexagonal core section. This shows the two 

EP thrusters and their mounting hardware. The SEP Stage in this design operated in a 1+1 configuration, 

with one operational thruster and one spare for redundancy.

1 2 3 4
NASA/TM—2011-216973 35

Figure 4.10.—NEXT ion thrusters as shown on 

bottom of SEP Stage.

4.6.1 Propulsion and Propellant Management Requirements

Because the NEXT Ion thruster was preselected as the thruster for SEP Stage, the primary information 

required to support the conceptual design of the SEP Stage for this mission consisted of the following:

• Propellant load for mission

• Components masses and sizes

• Power requirements

– Load; duty cycle

• Configuration requirements

– Position/proximity

4.6.2 Propulsion and Propellant Management Assumptions

Assumptions driven primarily by need to use mature technologies, including COTS where possible

(Table 4.7).

TABLE 4.7.—PROPULSION SYSTEM TRADE SPACE

Assumptions Action for Study Response Options/ Trades Issues

Subsystem 

sub-elements 

technologies 

preselected

The NEXT thruster, the thruster 

gimbal, the low and high pressure 

assemblies that comprise most of 

the Xe feed system, the PPU for the 

thruster are baselined for this study

Obtained all mass, sizing, and operational 

(power) information from in-house 

experts. This data was compiled into 

spreadsheet to roll up all of the subsystem 

characteristic

None for these 

sub-elements

No sensitivity

Propellant Load 

determined by 

mission analysis

Since the required propellant load 

mass was determined outside of the 

subsystem sheet, only had to 

estimate the size of the tanks 

required to store the Xe

While there is a simple model of sizing 

high pressure COPV tanks, it was 

calibrated with PSI tank data for high 

pressure COPVs. The resulting tanks were 

very close to the design configuration of an 

existing tank to ensure maturity

Xe storage 

density and 

pressure; No. of 

tanks

No sensitivity 

for propellant 

load; design 

choices limited 

by maturity 

requirement.

Changes in thruster 

operating points 

were accounted for 

in the mission 

analysis

Because the mission analysis was 

performed prior to the vehicle 

study, the thruster performance 

behavior was not required

The thruster’s performance information 

was limited to throughput which was 

driver in sizing Xe tanks. Power generation 

assumed to be sufficient to operate thruster

Thruster 

performance 

over mission 

profile

No sensitivity to 

thruster 

performance

4.6.3 Propulsion and Propellant Management Design and MEL

The propulsion subsystem is comprised of (EPS flow diagram is shown in Figure 4.11).

• 1+1 NEXT electrostatic ion thrusters
NASA/TM—2011-216973 36

Figure 4.11.—EPS flow diagram.

• Gimbals on each thruster for thrust vector control

• Three OTS carbon-overwrapped Ti high-pressure storage tanks for the Xe propellant (nominal)

• Xe distribution system to deliver the Xe from the tanks to the thrusters (LPA, HPA, lines)

• 1+1 PPU for delivering power to each ion thruster

One active 7 kW NEXT Ion engine with one cold spare, two PPUs (one is spare), 1 percent Xe 

unusable, <2 in. below adapter at launch, 66 cm thruster to thruster center separation, 99 percent of ion 

plume in 15° half angle cone, OTS tanks.

Table 4.8 lists the MEL for the Propulsion and Propellant management system as well as the 

propellant for this SEP 1+1 stage design.

TABLE 4.8.—PROPULSION AND PROPELLANT MANAGEMENT SYSTEM AND PROPELLANT MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - ----- 867.5 10 82.9 950.3

01 Saturn SEP S/C - ----- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - ----- 867.5 10 82.9 950.3

01.2.1 Avionics and Communications - ----- 10.2 20 2.0 12.2

01.2.2 GN&C - ----- 0.8 50 0.4 1.2

01.2.3 Electrical Power - ----- 276.6 16 44.2 320.8

01.2.4 Thermal Control - ----- 32.7 18 5.9 38.6

01.2.5 Structures and Mechanisms - ----- 82.6 18 14.9 97.5

01.2.6 Propulsion - ----- 152.9 10 15.5 168.4

01.2.6.a Primary EP system (thrusters, structure, thermal) - ----- 42.1 9 3.7 45.8

01.2.6.a.a Primary EP thrusters 2 12.7 25.4 4 1.0 26.4

01.2.6.a.b Electric propulsion system structure - ----- 12.0 18 2.2 14.2

01.2.6.a.b.a Miscellaneous 1 0.0 0.0 0 0.0 0.0

01.2.6.a.b.b EP thruster gimbal 2 6.0 12.0 18 2.2 14.2

01.2.6.a.c EPS thermal control system - ----- 4.7 12 0.6 5.2

01.2.6.a.c.a EPS MLI 1 0.6 0.6 12 0.1 0.7

01.2.6.a.c.b EPS heaters and sensors 1 4.1 4.1 12 0.5 4.5

01.2.6.b Propellant management - ----- 35.3 8 2.8 38.1

01.2.6.b.a Xe propellant tank(s) 3 10.3 30.8 8 2.5 33.2

01.2.6.b.b Feed system 1 4.5 4.5 8 0.4 4.9

01.2.6.c PPU - ----- 75.6 12 8.9 84.5

01.2.6.c.a PPU mass 2 34.4 68.8 8 5.5 74.3

01.2.6.c.b Cabling 1 6.8 6.8 50 3.4 10.2

01.2.6.c.c Miscellaneous 0 0.0 0.0 0 0.0 0.0

01.2.6.d RCS hardware - ----- 0.0 0 0.0 0.0

01.2.7 Propellant - ----- 311.7 0 0.0 311.7

01.2.7.a Primary EP propellant - ----- 311.7 0 0.0 311.7

01.2.7.a.a Primary EP propellant useable 1 308.6 308.6 0 0.0 308.6

01.2.7.a.b Primary EP propellant residuals (unused) 1 3.1 3.1 0 0.0 3.1

01.2.7.b RCS propellant (where needed) - ----- 0.0 0 0.0 0.0
NASA/TM—2011-216973 37

Figure 4.12.—Propellant Management System schematic.

A schematic of the propulsion and propellant management subsystem is shown in Figure 4.12. 

Derived from the NEXT thruster propulsion system S/C power, controller and DCIU shown for clarity 

(spare DCIU not shown).

4.6.4 Propulsion and Propellant Management Trades

• OTS versus custom tank design

• Ion engine location (gimbal interference, adapter clearance, contamination from separation 

system: none considered issues).

4.6.5 Propulsion and Propellant Management Analytical Methods

4.6.5.1 The NEXT Thruster Characteristics

• 0.54 to 6.9 kW thruster input power

• Ring-cusp electron bombardment discharge chamber

• 36 cm beam diameter, two-grid ion optics

• Beam current at 6.9 kW: 3.52 A

• Maximum specific impulse > 4170 s

• Maximum thrust > 236 mN

• Peak efficiency > 70 percent

• Xe throughput > 300 kg, 450 kg qualification level

– Analysis-based capability >450 kg

• Mass is 12.7 kg (13.5 kg with cable harnesses)
NASA/TM—2011-216973 38

Figure 4.13.—PM 1 Performance Acceptance Test.

Figure 4.13 shows the NEXT thruster in the Prototype Model Thruster (PM1) in Performance 

Acceptance Test.

4.6.5.2 NEXT Development Status

• PM1 delivered by Aerojet to GRC in January

– Flight-level design and fabrication processes

• Performance Acceptance Testing successfully completed at GRC

– Demonstrated throttling range from about 6.9 kW to 540 W, a 12.7:1 range

– At full power the specific impulse is about 4190 s, producing 236 mN thrust, with an overall 

thruster efficiency of 71 percent

– Consistent with performance demonstrated on multiple EM thrusters

• Comprehensive PM1 environmental test sequence completed at JPL

• System integration test planned for FY07

4.6.5.3 Gimbal Overview

Gimbal performance test and CAD illustration are shown in Figure 4.14.

• Breadboard gimbal 

– Designed and fabricated by Swales Aerospace 

– Flight-like design using JPL-approved materials with certifications 

• Stepper motors have space-rated option 

– Mass < 6 kg 

– Two-axis range of motion: ±19°, ±17° 

• Successful functional testing with PM1 engine 

• Gimbal passed two qual-level vibration tests and low-level shock tests with minor issues (fastener 

backout) 
NASA/TM—2011-216973 39

 

Figure 4.14.—Gimbal performance test and CAD illustration.

Figure 4.15.—NEXT thruster PPU schematic.

4.6.5.4 NEXT PPU

• EM PPU build is nearing completion at L3 ETI (schematic of NEXT thrust PPU shown in 

Figure 4.15)

• Modular beam supply and improved packaging provides performance and producability benefits 

over NSTAR approach 

• EM PPU is planned to be delivered to GRC for integration testing, in March 2007—part failure 

during functional testing and on-going recovery has delayed delivery 

• Thorough testing to follow 

– Single-string integration test 

 Functional testing on thruster load 

– Qual-level vibration testing and post-vibe functionals

– Qual-level thermal/vacuum test 

– EMI/EMC tests 

– Testing planned to be complete in FY2007 

• DCIU to be integrated in next development phase 
NASA/TM—2011-216973 40

Figure 4.16.—NEXT PMS.

4.6.5.5 NEXT Propellant Management System (PMS)

• All PMS assemblies are complete (Figure 4.16)

– Two high pressure assemblies (HPA), one flight-like 

– Three low pressure assemblies (LPA), one flight-like 

– Non-flight assemblies are identical except for use of lower cost equivalent parts 

• All assemblies have completed functional tests 

• Flight-like LPA and HPA successfully completed qual-level vibration testing and post-vibe 

functionals 

• Qual-level thermal/vacuum testing is pending 

• PMS will be delivered to GRC this year and tested in single-string integration test

4.6.6 Propulsion and Propellant Management Risk Inputs

• Electric thruster failure to start or operate

4.6.7 Propulsion and Propellant Management Recommendation

Preselection of NEXT thruster system due to technology maturity precludes significant changes in 

hardware. Changes have been driven by cost instead of mass. Remaining options limited to changes to 

propellant storage. Reduce number of Xe storage tanks (trade against structural/configuration impacts).

Increase Xe storage pressure to reduce storage tank size (trade against stronger tank requirements).
NASA/TM—2011-216973 41

4.7 Thermal Control

4.7.1 Thermal Requirements

To provide spreadsheet based models capable of estimating the mass and power requirements of the 

various thermal systems. The thermal system must account for heating and cooling issues of the solar 

arrays, thruster system, and stage electronics during the operating of the SEP Stage from launch at 1 AU 

from Earth through disposal roughly 3 yr into the trajectory to the Saturn system.

4.7.2 Thermal Assumptions

• Deep Space Operation, worst case at 1 AU 

Radiator Sizing

Table 4.9 lists the values of various constants and temperatures used as the radiator sizing 

assumptions. Note that the Radiator was modeled a thought it always sees deep space with a small (0.05) 

view factor to the solar array.

TABLE 4.9.—THERMAL SYSTEM 

RADIATOR SIZING ASSUMPTIONS

Variable Value

Radiator solar absorptivity.................................................0.14

Radiator emissivity ...........................................................0.84

Radiator Sun angle ............................................................. 90°

Radiator operating temperature .......................................320 k

Total radiator dissipation power ....................................850 W

Thermal Analysis Propellant Lines and Tanks

The dependence of insulation mass on the number of layers of insulation on the propellant tanks is 

shown in Figure 4.17. The modeling of the thermal insulation on the tanks and propellant feed lines was 

done using the assumptions shown in Table 4.10

Figure 4.17.—Insulation mass versus number of layers of insulation.
NASA/TM—2011-216973 42

TABLE 4.10.—THERMAL SYSTEM TANK INSULATION SIZING ASSUMPTIONS

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

Thermal Analysis—S/C Insulation

The mass of the S/C MLI on the engine bulkhead was modeled to determine the mass of the 

insulation and the heat loss. The model was based on a first principles analysis of the heat transfer from 

the S/C through the insulation to space. A near Earth thermal environment was used to size the insulation.

Assumptions used are shown in Table 4.11.

TABLE 4.11.—THERMAL SYSTEM TANK 

INSULATION SIZING ASSUMPTIONS

Variable Value

S/C MLI material................................................................................ Al

S/C MLI material density (ρisc)............................................2,770 kg/m3

MLI layer thickness (ti)........................................................... 0.025 mm

Number of insulation layers (ni) ....................................................... 100

MLI layer spacing (di) ................................................................ 1.0 mm

S/C radius (rsc).......................................................................... 0.825 m

Thermal Analysis—PMAD Cooling

Thermal control of the electronics and Active Thermal Control System (ATCS) is accomplished 

through a series of cold plates and heat pipes to transfer the excess heat to the radiators. The model for 

sizing these components was based on 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. Assumptions used are shown in Table 4.12.
NASA/TM—2011-216973 43

TABLE 4.12.—THERMAL SYSTEM PMAD 

COOLING SIZING ASSUMPTIONS

Variable Value

Cooling plate and lines material ..........................................................Al

Cooling plate and lines material density .............................. 2,770 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

4.7.3 Thermal Design and MEL

The thermal modeling provides power and mass estimates for the various aspects of the vehicle 

thermal control system based on a number of inputs related to the vehicle geometry, flight environment 

and component size.

The thermal control system consists of passive heat-pipes, body mounted radiators (north and south 

SADA faces) (main loads 500 Wth from PPU, 500 W from PDUs), 100 layers MLI between thruster 

platform and S/C to prevent heat leakback. Active tank heaters were included in the design to prevent 

two-phase Xe, louvers on radiators. The radiation level, micrometeoroid environment were other factors 

considered in the design. See Table 4.13 for the Thermal Protection System MEL and Figure 4.18 for a 

detailed illustration of the thermal system.

TABLE 4.13.—THERMAL PROTECTION SYSTEM MEL

WBS no. Description Qty Unit mass

(kg)

CBE mass

(kg)

Growth

(%)

Growth

(kg)

Total mass

(kg)

SEP Saturn In-Space Propulsion Mission - --- 867.5 10 82.9 950.3

01 Saturn SEP S/C - --- 867.5 10 82.9 950.3

01.1 Saturn Science Probe 0 0.0 0.0 0 0.0 0.0

01.2 SEP Stage - --- 867.5 10 82.9 950.3

01.2.1 Avionics and Communications - --- 10.2 20 2.0 12.2

01.2.2 GN&C - --- 0.8 50 0.4 1.2

01.2.3 Electrical Power - --- 276.6 16 44.2 320.8

01.2.4 Thermal control (non-propellant) - --- 32.7 18 5.9 38.6

01.2.4.a Active thermal control - --- 1.7 18 0.3 1.9

01.2.4.a.a Heaters (not tanks) 0 0.0 0.0 18 0.0 0.0

01.2.4.a.b Thermal control heaters circuit 2 0.2 0.4 18 0.1 0.5

01.2.4.a.c Data acquisition 1 1.0 1.0 18 0.2 1.2

01.2.4.a.d Thermocouples 25 0.0 0.3 18 0.0 0.3

01.2.4.b Passive thermal control - --- 24.9 18 4.5 29.3

01.2.4.b.a Heat sinks 2 3.5 6.9 18 1.2 8.2

01.2.4.b.b Heat pipes 1 0.7 0.7 18 0.1 0.9

01.2.4.b.c Radiators 1 9.3 9.3 18 1.7 11.0

01.2.4.b.d Engine bulkhead MLI 1 0.0 0.0 18 0.0 0.0

01.2.4.b.e Temperatures sensors 50 0.0 0.5 18 0.1 0.6

01.2.4.b.f Phase change devices 0 0.0 0.0 18 0.0 0.0

01.2.4.b.g Thermal coatings/paint 1 0.9 0.9 18 0.2 1.1

01.2.4.b.h Micrometeor shielding 2 0.6 0.9 18 0.2 1.1

01.2.4.b.i S/C MLI 1 5.5 5.5 18 1.0 6.5

01.2.4.c Semi-passive thermal control - --- 6.2 18 1.1 7.3

01.2.4.c.a Louvers 1 5.4 5.4 18 1.1 6.3

01.2.4.c.b Thermal switches 4 0.2 0.8 18 0.1 0.9

01.2.5 Structures and Mechanisms - --- 82.6 18 14.9 97.5

01.2.6 Propulsion - --- 152.9 10 15.5 168.4

01.2.7 Propellant - --- 311.7 0 0.0 311.7
NASA/TM—2011-216973 44

Figure 4.18.—Thermal system.

4.7.4 Thermal Trades

Traded location of radiators and insulation versus heaters.

4.7.5 Thermal Analytical Methods

Table 4.14 details the thermal protection data transfer. Inputs represent data, which the thermal 

system design required and output represents the data that will be provided to the SEP Stage design from 

the thermal system.

TABLE 4.14.—THERMAL PROTECTION DATA TRANSFER

Input Output

S/C dimensions (length, diameter) Heat pipe length and mass

Power management and electronics dimensions Cold plate size and mass

Waste heat load to be rejected

Distance from the Sun and S/C orientation

View factor of the solar arrays and their temperature

Radiator size and mass

S/C insulation mass and thickness

Thermal system components mass

Propellant tank dimensions and operating temperature Propellant tanks insulation mass and heater power level

Propellant line lengths and operating temperature Propellant line insulation mass and heater power level

Environmental Models

• Solar intensity based on S/C location
NASA/TM—2011-216973 45

Systems Modeled

• Micrometeor shielding on radiator

• Radiator panels

– The radiator panel area has been model along with a rough estimate of its mass. 

– The model was based on 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

– 1 AU thermal environment was used to size the radiator

• Thermal control of propellant lines and tanks

– 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 

– This modeling included propellant tank MLI and heaters and propellant line insulation and

heaters

– 1 AU thermal environment was used to calculate the heat loss

• S/C insulation

– The mass of the S/C MLI on the engine bulkhead was modeled to determine the mass of the 

insulation and heat loss 

• Avionics and PMAD cooling

5.0 Software Cost Estimation

5.1 Design Requirements

• Software to control the following systems

– Power 

 Solar array control 

 Power distribution unit 

 Solar array deployment and drives 

– Propulsion 

 PPU

 Low pressure assembly 

 High pressure assembly 

 Thruster gimbals 

– Thermal control 

5.2 Assumptions

• Estimates based upon “Space Mission Analysis and Design,” Chapter 16

• Payload will handle navigation and communications 

• Programming language will be C 

• Assembly instructions per SLOC of C = 7 

• Software on each DCIU is identical

5.3 Summary

Software on each DCIU is identical. Each DCIU controls one thruster independent of the other. 

DCIUs communicate with science payload using 1554 protocol. DCIUs communicate using 1554 

protocol. DCIUs communicate to propulsion subsystems using RS-422.
NASA/TM—2011-216973 46

SEP Stage software summary Software functionality not handled by SEP Stage

Functionality kwords of code Functionality kwords of code

Command processing 0.25 Command processing 1.0

Telemetry processing 0.25 Telemetry processing 1.0

Thruster control including gimbals 1.5 Altitude sensing (Sun sensor, Star 

Tracker...)

5.0

Fault detection monitoring 1.0 Autonomy 15.0

Fault correction 0.5 Kinematic integration 2.0

Power management 1.2 Altitude error determination 1.0

Thermal control 0.8 Precession control 3.3

Operating system 15.4 Ephemeris propagation 2.0

Total 20.9 kwords Orbit propagation 13.0

Complex ephemeris 3.5

Total 46.8 kwords

4k SLOC 7k SLOC

Figure 5.1.—SEP Stage software sizing.

6.0 Cost, Risk and Reliability

6.1 Costing

6.1.1 Costing Ground Rules and Assumptions

Estimates are for Phase C/D prime contractor costs without fee for the SEP Stage. Estimates 

developed at the subsystem and component levels using mostly mass-based parametric estimates.

Prototype development approach assumed. Quantitative risk analysis performed using Monte Carlo 

simulation based on mass and Cost Estimation Relationship (CER) uncertainties. Estimates reflect the 

mean (which is roughly the 64th percentile for the total) unless otherwise indicated.

6.1.2 Cost Summary 1+1 SEP Stage

Table 6.1 shows the cost summary for the SEP Stage.

TABLE 6.1.—COST SUMMARY 1+1 SEP STAGE

WBS

no.

DDT&E Flight hardware Ground spares Total 

(first flight)

01.2.1 Avionics and communications 4.3 3.6 2.0 9.9

01.2.2 GN&C 0.0 0.2 0.0 0.2

01.2.3 Electrical power subsystem 21.6 20.2 4.1 45.9

01.2.4 Thermal control (non-propellant) 5.2 0.6 0.4 6.2

01.2.5 Structures and mechanisms 9.5 3.1 0.0 12.5

01.2.6 Propulsion 15.3 9.8 5.3 30.4

Subsystem subtotal 56.0 37.4 11.8 105.1

Systems integration

IACO and STO 7.6 1.7 9.3

GSE 2.5 2.5

SE&I 3.1 4.4 7.5

PM 0.8 1.2 2.0

LOOS 3.7 3.7

Systems integration subtotal 17.6 7.3 24.9

Total prime contractor—Mean 73.6 44.7 11.8 130.1

Total prime contractor—10 percentile 42.8 31.5 7.3 81.6

Total prime contractor—90 percentile 115.9 60.2 17.1 193.2
NASA/TM—2011-216973 47

Figure 6.1.—Estimate distribution 1+1 SEP Stage.

6.2 Risk Analysis and Reduction

6.2.1 Assumptions

Each subsystem lead developed a risk for his/her subsystem. Risk attributes are based on the CEV risk 

values. A risk list is not based on trends of criticality. Some mitigation plans are offered as suggestions.

6.2.2 Risk Summary

These risks, with proper pro-active planning can be mitigated early to avoid becoming problems late 

in the development life cycle or during the extended mission.

Areas of concern for this mission include

• Stage interface problems

• Separation system issues

• Avionics components not available as well as undefined

• Foreign object debris 

• Untested solar array hardware

• Design of the probe S/C

• Possible Electric Thruster limitations

6.3 Reliability

Reliability analysis was not performed on this study. Future iterations of the SEP Stage design will 

include reliability.

1 2 3 4 5

Consequences

Likelihood

1

2

3

4

5

2

5, 6, 

7, 8

3, 4

1

1 2 3 4 5

Consequences

Likelihood

1

2

3

4

5

1 2 3 4 5

Consequences

Likelihood

1

2

3

4

5

1 2 3 4 5

Consequences

Likelihood

1

2

3

4

5

1 2 3 4 5

Consequences

Likelihood

1

2

3

4

5

2

5, 6, 

7, 8

3, 4

1

Med

High

Low

Criticality Criticality

2 x 4

2 x 4

2 x 4

2 x 4

3 x 4

3 x 4

4 x 4

5 x 4

L x C

Avionics I/O Not Well 

Defined

1 Avionics

Avionics Components 

Availability

2 Avionics

Separation System does 

not work

Launch 

Vehicle 6

Electric Thruster Failure to 

Start/Operate

4 Thruster

Navigation Sensor View 

Blockage

5 GNC

Solar Array Deployment 

Failure

7 Power

3 SE&I Stage Interface Failure

8 Structures Potential Structural Failure

No. Team Risk Title

2 x 4

2 x 4

2 x 4

2 x 4

3 x 4

3 x 4

4 x 4

5 x 4

L x C

Avionics I/O Not Well 

Defined

1 Avionics

Avionics Components 

Availability

2 Avionics

Separation System does 

not work

Launch 

Vehicle 6

Electric Thruster Failure to 

Start/Operate

4 Thruster

Navigation Sensor View 

Blockage

5 GNC

Solar Array Deployment 

Failure

7 Power

3 SE&I Stage Interface Failure

8 Structures Potential Structural Failure

No. Team Risk Title
NASA/TM—2011-216973 48

7.0 Trade Space Iterations

Trades were run on the number of active thrusters used to perform the DV of the trajectory. Cases 2 

and 3 were classified as Flagship class missions and will be documented in more detail in a future 

COMPASS final report on the Saturn Flagship SEP Stage (Table 7.1 to Table 7.4).

TABLE 7.1.—SEP STAGE COMPARISON

[SEP Stage comparison details (masses in kg)]

Flagship mission 

1+1 2+1 3+1 Delta Case 2

to Case 3

Total SEP Stage dry mass 638.7 1036.1 1160.6 124.5 

Total SEP Stage CBE dry mass 555.8 894.1 1003.0 108.9 

Bottoms-up growth (lbm) 82.9 142.0 157.6 15.6 

Bottoms-up growth (%) 0.15 0.16 0.16 0.00 

Required system level growth (30%) 0.15 0.14 0.14 0.00 

Additional system level growth 83.9 126.2 143.3 17.1 

Total SEP Stage with 30% system growth 722.5 1162.3 1303.9 141.6 

Total propellant 311.7 599.9 611.1 11.1 

Total wet mass (with 30% growth factor) 1034.2 1762.3 1915.0 152.7 

Probe mass 2444.0 3975.0 3946.7 –28.3 

TABLE 7.2.—SEP STAGE BOTTOMS-UP MEL COMPARISON

SEP Stage bottoms-up CBE mass 

(kg)

SEP Stage 1+1 2+1 3+1

C&DH 9.5 11.4 13.2 

Communications 0.7 0.7 0.7 

GN&C 0.8 0.8 0.8 

EP subsystem 276.6 495.8 500.2 

Thermal control (non-propellant) 32.7 42.9 56.8 

Structures and mechanisms 82.6 90.8 117.3 

Propulsion 152.9 251.7 313.9 

Propellant 311.7 599.9 611.1 

Total SEP Stage CBE wet mass 867.5 1494.0 1614.1 

Total SEP Stage CBE dry mass 555.8 894.1 1003.0 

TABLE 7.3.—SEP STAGE TOTAL MASS COMPARISON

SEP Stage bottoms-up total mass

(kg)

SEP Stage 1+1 2+1 3+1

C&DH 11.4 13.7 15.8 

Communications 0.8 0.8 0.8 

GN&C 1.2 1.2 1.2 

EP subsystem 320.8 586.0 592.6 

Thermal control (non-propellant) 38.6 50.6 67.1 

Structures and mechanisms 97.5 107.1 137.7 

Propulsion 168.4 276.7 345.4 

Propellant 311.7 599.9 611.1 

Total SEP Stage CBE wet mass 950.3 1636.0 1771.6 

Total SEP Stage CBE dry mass 638.7 1036.1 1160.6 
NASA/TM—2011-216973 49

TABLE 7.4.—FLAGSHIP MISSION (CASE 3) COMPARED TO BASELINE (CASE 1)

SEP Stage Comparison Details 

(masses in kg)

1+1 3+1 Flagship 

Total SEP Stage dry mass 638.7 1160.6 521.9 

Total SEP Stage CBE dry mass 555.8 1003.0 447.2 

Bottoms-up growth (lbm) 82.9 157.6 74.7 

Bottoms-up growth (%) 0.15 0.16 0.01 

Required system level growth (30%) 0.15 0.14 -0.01 

Additional system level growth 83.9 143.3 59.5 

Total SEP Stage with 30% system growth 722.5 1303.9 581.4 

Total propellant 311.7 611.1 299.4 

Total wet mass (with 30% growth factor) 1034.2 1915.0 880.8 

Probe mass 2444.0 3946.7 1502.7 

7.1 Trade 1 : 1+1

The baseline case documented in this report is the first trade using one active NEXT thruster and 

one spare.

7.2 Trade 2: 2+1

Details about the second trade are shown in Table 7.5.

TABLE 7.5.—CASE 2 SYSTEM SUMMARY

S/C system details

Master equipment list—Mass 

CBE 

(kg) 

Growth 

(kg) 

Total Mass 

(kg) 

Percent of 

dry mass 

Science Probe N/A N/A 3975 N/A 

SEP Stage N/A 

Avionics and Communications 12.1 2.4 14.5 1.4

GN&C 0.8 0.4 1.2 0.1

Electrical Power 495.8 90.2 586.0 56.6

Thermal Control 42.9 7.7 50.6 4.9

Structures 90.8 16.3 107.1 10.3 

Propulsion 251.7 25.0 276.7 26.7

Propellant 599.9 0.0 599.9 N/A 

SEP Stage total mass without system growth 1494.0 142.0 1636.0

SEP Stage dry mass 894.1 142.0 1036.1

SEP Stage inert mass 900.0 N/A 1042.0

SEP Stage total mass with system growth 126.2 1762.3

System level growth (contingency) tracking 

Dry mass growth (contingency) 

Desired total growth (contingency) 

Desired total growth (contingency) 

Mass system level growth (contingency)

15.9% 

30%

268.2 kg

126.2 kg

Table 7.6 summarizes the launch vehicle performance hits that lead to the SEP performance to Saturn 

value. Note that the total SEP Stage wet mass with growth from Table 7.5 (1762) is subtracted from the 

ELV performance after margin value (5737 kg) in order to arrive at the SEP performance number of 

3875 kg. This number is the total mass available to the science probe.

 

 

 

 

 . 7

 . 8

 m 4 .2 G ro

 e n cy ) 

 g e n c y

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 g en cy 

 t in g e n
NASA/TM—2011-216973 50

TABLE 7.6.—CASE 2 MISSION ASSUMPTIONS AND SEP PERFORMANCE

Mission assumptions

Launch vehicle 

ELV performance after margin 

ELV performance with margin 

ELV margin (10%) 

Payload adapter 

ELV margin less adapter 

Net ELV margin 

ELV injected C3

Delta IV Heavy

5737.1 kg

6374.6 kg

637.5 kg

15.0 kg

622.5 kg

9.8% 

26 km2

/s2

SEP performance to Saturn 3975 kg

7.3 Trade 2: 3+1

Table 7.8 summarizes the launch vehicle performance hits that lead to the SEP performance to Saturn 

value. Note that the total SEP Stage wet mass with growth from Table 7.7 (1920) is subtracted from the 

ELV performance after margin value (5862 kg) in order to arrive at the SEP performance number of 

3942 kg. This number is the total mass available to the science probe.

TABLE 7.7.—CASE 3 SYSTEM SUMMARY

S/C system details

Master equipment list—Mass 

CBE 

(kg) 

Growth 

(kg) 

Total Mass 

(kg) 

Percent of 

dry mass 

Science Probe N/A N/A 3942 N/A 

SEP Stage N/A 

Avionics and communications 13.9 2.8 16.7 1.4

GN&C 0.8 0.4 1.2 0.1

Electrical power 500.2 92.4 592.6 50.9

Thermal control 60.5 10.9 71.4 6.1

Structures 117.3 20.3 137.7 11.8

Propulsion 313.9 31.5 345.4 29.6

Propellant 611.1 0.0 611.1 N/A 

SEP Stage total mass without system growth 1617.8 158.2 1776.0

SEP Stage dry mass 1006.7 158.2 1165.0

SEP Stage inert mass 1012.8 N/A 1171.0

SEP Stage total mass with system growth 143.8 1919.8

System level growth (contingency) tracking 

Dry mass growth (contingency) 

Desired total growth (contingency) 

Desired total growth (contingency) 

Mass system level growth (contingency)

15.7% 

30%

302 kg

143.8 kg

TABLE 7.8.—CASE 3 MISSION ASSUMPTIONS AND SEP PERFORMANCE

Mission Assumptions

Launch vehicle 

ELV performance after margin 

ELV performance with margin 

ELV margin (10%) 

Payload adapter 

ELV margin less adapter 

Net ELV margin 

ELV injected C3

Delta IV Heavy

5861.7 kg

6513.0 kg

651.3 kg

0.0 kg

651.3 kg

10.0% 

19 km2/s2

SEP performance to Saturn 3942 kg

 

 

 

 

 . 7

 . 8

 m 4 .2 G ro

 e n cy ) 

 g e n c y

 t in g e n

 g en cy 

 t in g e n
NASA/TM—2011-216973 51

Appendix A.—Acronyms and Abbreviations

AO Announcement of Opportunity

AU Astronautical Unit

BOL beginning of life

CAD computer aided design

C&DH Command and Data Handing

CBE current best estimate

CER cost estimating relationships

CEV Crew Exploration Vehicle

Comm Communications

COMPASS COllaborative Modeling and 

Parametric Assessment of Space 

Systems

COTS commercial off the shelf

DCIU Digital Control Interface Unit

DMR Design for Minimum Risk

DS-1 Deep Space 1

DSN Deep Space Network

DTE direct to Earth

ECI Earth centered inertial

EELV Evolved Expendable Launch 

Vehicle

ELV Expendable Launch Vehicle

EOL end of life

EP Electric Propulsion

EPS Electric Propulsion System

ESPA EELV Secondary Payload Adaptor

FEA finite element analysis 

FOM figure of merit

FY fiscal year

GSFC NASA Goddard Space Flight 

Center

GLIDE GLobal Integrated Design 

Environment

GN&C Guidance, Navigation and Control

GRC NASA Glenn Research Center

HPA High Pressure Assembly

HQ NASA Headquarters

IMDC I M Design Center

IP internet protocol

Isp specific impulse 

ISRU in situ resource utilization

JPL NASA Jet Propulsion Laboratory

KSC NASA Kennedy Space Center

LCT Lunar Communications Terminal

LEO low Earth orbit

LPA Low Pressure Assembly

LRS Lunar Relay Station

LSP Launch Service Program

LSTO Launch Service Task Order

LVLH Local Vertical Local Horizontal

MAC Media Access Control

MECO main engine cutoff

MEL Master Equipment List

MLI multilayer insulation

MPU Makeup Power Unit

NASA National Aeronautics and Space 

Administration

Nav navigation

NEXT NASA Evolutionary Xenon 

Thruster

NIST National Institute of Standards and 

Technology

NLS NASA Launch Services

NSTAR NASA Solar Technology 

Application Readiness

OMS Orbital Maneuvering System

OTS off the shelf

PDU power distribution unit

PEL Power Equipment List

PMAD Power Management and 

Distribution

PMS Propellant Management System

PN pseudo-noise

PPU Power Processing Unit

RCS Reaction Control System

S/C spacecraft

SADA Solar Array Drive Assembly

SEEGA solar electric Earth gravity assist

SEVGA solar electric Venus gravity assist

SEJGA solar electric Jupiter gravity assist
NASA/TM—2011-216973 52

SEEJGA solar electric Earth Jupiter gravity 

assist

SEP Solar Electric Propulsion

SEPTOP Solar Electric Propulsion 

Trajectory Optimization Program

SLB Standard LightBand

SN signal-to-noise

SPACE System Power Analysis for 

Capability Evaluation

SPU Solar Power Unit

TDRSS Tracking and Data Relay Satellite 

System

TWTA Traveling Wave Tube Amplifier

WBS work breakdown structure
NASA/TM—2011-216973 53

Appendix B.—Rendered Design Drawings

Figure B.1 is the SEP Stage with notional science probe attacked in the solar array deployed 

configuration. Figure B.2 shows the SEP Stage with the array deployed shown from the bottom of the 

thrust tube configuration with the NEXT thrusters shown.

Figure B.1.—SEP Stage with Science Probe in deployed array configuration.

Figure B.2.—SEP Stage with array deployed, looking at NEXT thruster face.

NASA/TM—2011-216973 55

Appendix C.—Study Participants

Solar Electric Propulsion (SEP) Stage to Saturn Design Session

Subsystem Name Center Email

Propulsion PI Scott Benson GRC Scott.W.Benson@nasa.gov

Propulsion PI Mike Patterson GRC Michael.J.Patterson@nasa.gov

Propulsion PI Luis Pinero GRC Luis.R.Pinero@nasa.gov

COMPASS Team

Lead Steve Oleson GRC Steven.R.Oleson@nasa.gov

System Integration, MEL and Final 

Report Documentation Melissa McGuire GRC Melissa.L.Mcguire@nasa.gov

Internal Editing and final Report 

documentation Leslie Balkanyi GRC Leslie.R.Balkanyi@nasa.gov

Mission John Riehl GRC John.P.Riehl@nasa.gov

Mission, Operations, GN&C Doug Fiehler GRC Douglas.I.Fiehler@nasa.gov

ELV, Integration and Test, 

Operations Jeff Woytach GRC Jeffery.M.Woytach@nasa.gov

Structures and Mechanisms Nelson Morales GRC Nelson.Morales-1@nasa.gov

Structures and Mechanisms John Gyekenyesi GRC John.Z.Gyekenyesi@nasa.gov

Thermal Tony Colozza GRC Anthony.J.Colozza@nasa.gov

Power Dave Wilt GRC David.M.Wilt@nasa.gov

Power PI Tom Kerslake GRC Thomas.W.Kerslake@nasa.gov

Command and Data Handling Jeff Juergens GRC Jeffrey.R.Juergens@nasa.gov

Communications O. Scott Sands GRC Obed.S.Scott@nasa.gov

Configuration Tom Packard GRC Thomas.W.Packard@nasa.gov

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—2011-216973 56

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Aerojet engines and propellant systems: http://www.aerojet.com/capabilities/spacecraft.php, accessed 

January 24, 2011.

ANSI/AIAA R–2001A–1999, “Recommended Practice for Mass Properties Control for Satellites, 

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4. TITLE AND SUBTITLE

COMPASS Final Report: Enceladus Solar Electric Propulsion Stage 

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6. AUTHOR(S)

Oleson, Steven, R.; McGuire, Melissa, L. 

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WBS 346620.01.03.01 

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John H. Glenn Research Center at Lewis Field 

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14. ABSTRACT

The results of the NASA Glenn Research Center (GRC) COllaborative Modeling and Parametric Assessment of Space Systems 

(COMPASS) internal Solar Electric Propulsion (SEP) stage design are documented in this report (Figure 1.1). The SEP Stage was designed 

to deliver a science probe to Saturn (the probe design was performed separately by the NASA Goddard Space Flight Center’s (GSFC) 

Integrated Mission Design Center (IMDC)). The SEP Stage delivers the 2444 kg probe on a Saturn trajectory with a hyperbolic arrival 

velocity of 5.4 km/s. The design carried 30 percent mass, 10 percent power, and 6 percent propellant margins. The SEP Stage relies on the 

probe for substantial guidance, navigation and control (GN&C), command and data handling (C&DH), and Communications functions. The 

stage is configured to carry the probe and to minimize the packaging interference between the probe and the stage. The propulsion system 

consisted of a 1+1 (one active, one spare) configuration of gimbaled 7 kW NASA Evolutionary Xenon Thruster (NEXT) ion propulsion 

thrusters with a throughput of 309 kg Xe propellant. Two 9350 W GaAs triple junction (at 1 Astronomical Unit (AU), includes 10 percent 

margin) ultra-flex solar arrays provided power to the stage, with Li-ion batteries for launch and contingency operations power. The base 

structure was an Al-Li hexagonal skin-stringer frame built to withstand launch loads. A passive thermal control system consisted of heat 

pipes to ‘north and south’ radiator panels, multilayer insulation (MLI) and heaters for the Xe tank. All systems except tanks and solar arrays 

were designed to be single fault tolerant. 

15. SUBJECT TERMS

Solar arrays; Propellants; Packaging; Launching; Deployment; Aerospace systems; Solar electric propulsion; Mission planning 

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