NASA Technical Memorandum 102339 

Evolutionary Growth for Space Station 

Freedom Electrical Power System 

Matthew F. Marshall 

Rockwell International 

Rocketdyne Division 

Canoga Park, California 

Kerry L. McLallin and Michael J. Zernic 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

24th Intersociety Energy Conversion Engineering Conference 

cosponsored by the IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE 

Washington, D.C., August 6-1 1, 1989 

~ 

(NA5A-TM-202339) EVOLUTIONARY GROWTH FOR N99-ZR5713 

SPACE STATXIIN FREFDQH ELECTRICAL PdHfR 

SYSTEY (NASA. Lewis Research Center) d p GSCt 225 uncl dS 

G3/LO 02326BO 
EVOLUTIONARY GROWTH FOR SPACE STATION FREEDOM 

ELECTRICAL POWER SYSTEM 

Matthew F. Marshall 

Rockwell International/Rocketdyne Division 

Canoga Park, CA 91303 

Kerry L. McLallin and Mike J. Zernic 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, OH 44135 

ABSTRACT 

Over an operational lifetime of at least 30 yr, Space 

Station Freedom will encounter increased space station 

user requirements and advancing technologies. The 

space station electrical power system is designed with the 

flexibility to accommodate these emerging technologies 

and expert systems and is being designed with the necessary software hooks and hardware scars to accommodate 

increased growth demand. The electrical power system is 

planned to grow from the initial 75 kW up to 300 kW. 

The Phase I station will utilize photovoltaic arrays to produce the electrical power; however, for growth to 

300-kW, solar dynamic power modules will be utilized. 

Pairs of 25-kW solar dynamic power modules will be 

added to the station to reach the power growth level. The 

addition of solar dynamic power in the growth phase 

places constraints in the initial space station systems 

such as guidance navigation and control, external thermal, truss structural stiffness, computational capabilities 

and storage which must be planned-in in order to facilitate the addition of the solar dynamic modules. 

INTRODUCTION 

The National Aeronautics and Space Administration 

(NASA) is currently in the hardware/software design, development, test, and evaluation phase of the Space Station Freedom (SSF) Program. Because the SSF is intended to be an orbital facility with a minimal useful life 

of 30 yr, its design must accommodate the addition of 

future hardware and software and changeout or upgrading of various components and elements, as well as inherent ease in maintenance. Establishing hooks and 

scars in the SSF initial design will not pin down a specific growth configuration but will permit viable growth 

options to be incorporated. Hooks refers to the ability of 

software to accept upgrades in capacity and capability, 

while scars refers to the hardware equivalent. 

Figure 1 shows the Phase I SSF. It contains one habitation module and three laboratory modules (United 

States, European, Japanese), has a crew of six to eight 

astronauts, delivers 75 kW of electric power to users, 

and has two to four active attached payload accommodations on the truss. The final growth station will have up 

to 300 kW of power available; a crew of 24; up to three 

habitation and six laboratory modules; three pressurized 

mini laboratories; provisions for 18 attached payload accommodations; facilities to maintain, service, and deploy 

orbital maneuvering vehicles, orbital transfer vehicles, 

and free flying platforms; and will provide support to the 

construction of large space structures. (11 

GROMTH SCENARIOS 

The path from the first phase of SSF to its growth 

configuration is a path of increasing user resource demands. There is a diverse spectrum of groups that would 

like to use SSF. First among these are the experiments 

users who are divided into several utilization scenarios: 

astrophysics and terrestrial observations, biological observations, and materials processing experiments. Astrophysics and terrestrial observation scenarios require an 

increase in the SSF truss work with provisions for attaching additional external payloads. Astrophysics growth 

scenarios require mounting on the top of SSF to facilitate 

D5145-1 

Figure 1. Space Station Freedom Phase I 

Configuration 

1 
astrophysics observations, while mounting to the bottom 

of SSF offers the widest field of view for terrestrial-looking experiments. Biological and materials processing 

growth scenarios require an increase in the pressurized 

the addition of laboratory modules. I laboratory volume. which can be accommodated though 

A second group of users desire to use SSF as a transportation node for spacecraft such as the Orbital Maneuvering Vehicle or Orbital Transfer Vehicle, which will be 

used in going from low Earth orbit to higher orbits in 

order to deploy and retrieve payloads. A third user 

group would like to use SSF to repair and refurbish satellites and payloads recovered with the vehicles from the 

transportation node users. The second and third utilization areas require additional truss structure as well as 

hanger space in order to dock the vehicles and satellites 

that will be serviced and repaired. 

A fourth user could be concerned with learning more 

about how persons function and live under long exposure 

to a weightless and limited habitable volume situation in 

preparation for future long duration missions to the 

moon or hlars. 

I The growth of SSF cannot occur without more available crew members. Regardless of how many missions 

or user experiments and payloads are on-orbit, the lack 

of crew resources will severely impact the scheduling 

and operations. One of the first things to be added is the 

quantity of crew available to operate experiments and 

payloads and to carry out science and technology missions, which implies the addition of more habitation 

modules. More laboratory modules are also needed to 

expand upon the facility's microgravity environment in 

order to conduct space research and development. In ad350 

300 

250 

zoo 

150 

100 

50 

dition to increasing the pressurized volume and crew 

availability, more external attached payloads will be 

manifested at various new locations on the SSF truss. 

Figure 2 shows an example of what a 275-kW growth 

station may look like (2j. Other growth configurations 

are possible based on using SSF as a transportation node 

for staging lunar, Mars, or other interplanetary missions. 

Therefore, growth of the Electric Power System (EPS) is 

inevitable and the EPS design and architecture must include an evolving capability so as to not preclude any 

future utilization and capability of SSF: 

GROWTH POWER DEMANDS 

Figure 3 shows growth power level requirements as a 

function of launch capacity and the types of uses of the 

station [2]. In order to allow aggressive research in life 

sciences or microgravity science and commercial production of materials, pharmaceutical products, and other experiment applications, SSF should accommodate growth 

to at least a 275-kW power capability. The capability to 

grow to this level is determined by the type of experir So 

r Small Pocket Lab 

Figure 2. A 275-kW Growth Station 

Space Stallon Freedom 

Ullllrallon Emphasls 

0 Mlcrogravlly Research 

and Malerlals Producllon 

0 Life Sclencer 

0 Mlcrogravlly Research 

A Observallonal Sclences 

I -. ..^*^. .,*..I---.- . .^ " 

0 50 100 150 200 250 300 350 400 

Annual UII Support (Ihousands of kg] 05145-3 

Figure 3. Space Station Growth Power Requirements 

2 
mentation being carried out on SSF and the level of 

launch support provided. 

The launch support dictates the riltc and viability of 

assembly, resupply, user payload quantity and interchangeability, and the addition or upgrading of SSF elements and systems. The lowest growth power level forecast of 150 kW occurs when observational science experiments represent the majority of the SSF utilization. 

This level is sufficient for an extensive observational science effort but is an unlikely scenario for some of the 

other planned uses. The highest power level anticipated 

is approximately 300 kW and is primarily driven by commercial research and development and production methods of materials and pharmaceuticals. However, this scenario requires extensive launch and return transportation 

support (probably a mixture of space shuttle and expendable launch vehicles [ELVs]) for resupply and returning processed materials back to Earth. This power 

level also implies a busy activity schedule and the associated crew and resources to complete the expected objectives [2]. 

ELECI'RIC POWER SYSTEM GROWTH 

Evolution and growth of the EPS addresses both the 

power generation (method and quantity) as well as the 

power distribution based upon the power demand. The 

power demand on the EPS comprises two parts: 

(1) power needed to run the inherent SSF facility and 

systems equipment (life support, lighting, propulsion, 

data processing, etc.) and (2) power for the equipment 

needed to operate experiments or payloads conducting 

research in various science and technology disciplines. 

As currently planned, SSF will take 21 shuttle flights 

and 3 yr to complete the assembly of the initial (Phase I) 

configuration. At the completion of Phase I, the EPS will 

be capable of delivering 75 kW of usable power. This 

power will be drawn upon to satisfy SSF housekeeping 

needs and operate equipment to accomplish scientific 

and technology objectives. The initial power generation 

system will consist of four 18.75-kW photovoltaic (F'V) 

power modules utilizing solar cells to transform incident 

solar energy into electrical power. Each PV module consists of two solar arrays, each approximately 100 ft long 

by 33 ft wide. The PV arrays were chosen for the Phase I 

station because of their lightweight deployable characteristics and their solar off-pointing power generating capability, which is especially important during the assembly 

buildup of SSF. However, the addition of PV modules 

for growth would impact the station guidance navigation 

and control (GNBrC) system and would require more frequent altitude reboost because of the increased drag area 

of the large solar array surface area. 

As more power generation capability is added, performance and efficiency become more important. Solar 

dynamic (SD) power offers performance, efficiency, and 

cost savings advantages over PV power that make it an 

excellent choice for the growth power system. 

SOLAIC DYNARZIC POWER GENEIWI'ION 

FOR GRO\Y1" 

The SD power generation system has been selected to 

provide the additional power required for SSF growth 

phases. This selection is based on the significant efficiency gains (approximately twice the power output at 

half the surface area of PV) and life cycle cost (LCC) 

savings projected for SD versus growth with a PV power 

generation system. Life cycle cost analyses have been 

performed to determine critical cost elements and to define the LCC for various power generation options. The 

LCC elements considered for these analyses consisted of 

the nonrecurring costs due to design, development, test, 

and evaluation as well as the recurring costs associated 

with production, growth, and operations. Each category 

of recurring cost included consideration of the hardware, 

launch and transportation, and deployment and installation costs. The LCC were evaluated for up to 30 yr of 

operation and for up to 300 kW of power generation capacity. Typical results from these analyses are presented 

in Figure 4. Life cycle costs for 30 yr (in fixed 1957 

dollars) are presented versus the final SSF power level 

for three cases. These cases illustrate the basic cost 

trends and the sensitivity to the split of PV and SD 

power. The data indicate that over the 30-yr life of SSF 

growing with SD will reduce LCC by $3.3 billion, compared to an all PV station, and that additional benefits 

could be realized by minimizing the level of PV power 

for the station. 

The LCC advantage of SD over PV is due primarily 

to the lower flight hardware costs required for growth, 

operations, and maintenance of the power generation 

system. Another significant advantage of SD is related to 

its high sun-to-user conversion efficiency. This translates into lower total surface area required to convert 

sunlight to electricity and hence lower on-orbit drag. The 

station can fly at lower altitudes while maintaining safe 

Leaend 

I p 

/ 41 I I I1 I J 

0 50 100 150 200 250 300 350 

Flnal Power Alter Growth (kW) D5145-4 

Figure 4. EPS Life Cycle Cost 

3 
orbital lifetimes, which will allow larger space transportation system (STS) payloads for station resupply. 

The Sb power generation is configured as individual 

modules, each producing a minimum of 25-kW net electric power. The SD configuration is shown in Figure 5. 

The module consists of a solar concentrator assembly 

with vernier pointing gimbals; a receiver/power conversion unit subsystem with thermal energy storage; a heat 

rejection assembly with a thermal control cold plate for 

the electrical equipment; WP-04-furnished beta gimbal, 

frequency changer, and controllers; and WP-02-furnished truss bays and cable trays. The SD module uses a 

recuperated Brayton cycle to convert solar energy into 

electricity via a turbine-driven alternator. At the minimum power design point, the SD module has a sun-touser efficiency of approximately 20%. A constant power 

level is delivered to the user continuously around the orbit, since the receiver incorporates thermal energy storage to support power generation during eclipse [3]. 

Some of the key SD module design requirements 

follow: 

(1) Provide a minimum of 25-kWe net power 

throughout any orbit at any time of the year 

within a prescribed orbit altitude envelope. 

(2) Provide 28.75 kWe (peaking power) for up to 

7.5 min during the sun and shade portions of 

the orbit (no more than 15 min total per orbit). 

(3) Provide automatic startup, shutdown and continuous operation capability. 

I (4) Employ common hardware, software, and standard interfaces to the maximum beneficial 

extent. 

I 

SD Electronlco 

(Electrlcal Equlpment) 

4983-303Rl 

145 

Figure 5. Solar Dynamic Module Configuration 

(5) Establish the moment of inertial such that dynamic instabilities do not occur. 

(6) Meet load demand changes, including peaking, 

turndown, and load shedding. 

(7) Remain operational indefinitely through orbital 

replacement unit (ORU) replacement. 

(8) Package two SD modules, along with the necessary truss, into one space shuttle launch. 

Although SD power will not be utilized until Phase LI, 

significant work must be done in Phase I to pave the way 

for the addition of SD modules for growth. The SD 

power system design effort in Phase 1 includes a task to 

define the software hooks and hardware scars required 

of SSF systems and elements to adequately support the 

addition of SD modules for growth phases of the program. The areas of consideration are briefly described in 

Table 1 [4]. As SD module requirements are defined in 

these areas, they are coordinated with the interfacing 

systems and elements and documented. If necessary, the 

program requirements documents would be updated to 

reflect the identified impacts. 

DISTRIBUTED ELECTRIC POWER 

SYSTEM GROCVTH 

The power management and distribution system 

(PMAD) is initially sized to be able to distribute more 

power than is available during Phase I. Major EPS components that are expected to last the lifetime of SSF are 

initially sized to handle the total maximum power capability of at least 300 kW. The hvo SSF alpha gimbals are 

each sized to allow 175 kW (350 kW total) of power to 

flow from the two outboard solar power modules to the 

inboard truss users. The cabling to the two inboard truss 

main bus switching units is also sized to 175 kW each. 

This initial sizing to the growth capability avoids the major complexity and power downtime of removing and replacing hardware or adding in greater capacity during 

growth. Space is provided in' the truss cable trays to allow the addition of cabling from the main bus switching 

units to growth power distribution and control units associated with the addition of pressurized modules or attached payload accommodations. Trades establish what 

level of growth is allowed for in the initial design. 

TECHNOLOGY TRAiiSPARENCI' 

The EPS has a built-in capability for addition or substitution of new technology with minimum impact to operations and interfaces. This feature is called technology 

transparency. The selection criteria for the appropriate 

degree of technology transparency in the design is a 

function of: 

(1) Technoloey Evolution Maturity. Structures and 

mechanisms have a high degree of maturity and 

4 
Table 1. Solar Dynamic Hooks and Scars 

Issues 

GNBC 

Power growth and utilization 

Structure 

Thermal environments 

Off-axis phenomena 

DMS 

Communication and tracking 

MSC 

~ ~~ 

Design Consideration 

SSF must be controllable with addition of SD modules. 

The power distribution system must accommodate additional power 

input by SD and accommodate SD power needs for startup, shutdown. and housekeeping. 

SSF truss and alpha and beta gimbals must accommodate mass, 

CG. and MOI of SD and be sufficiently stiff to allow accurate 

point and tracking. 

SSF must accommodate modification to thermal environment resulting from addition of SD modules. 

SSF must accommodate off-ads phenomena such as optical, radio 

frequency. and infrared concentrations and signatures. 

SSF must accommodate additional data input resulting from power 

growth. 

SSF must accommodate additional data input resulting from power 

The MSC must accommodate the masses and ranges of motion 

required to assembly the SD power module. 

growth. 

would therefore require less technology transparency than the relatively low maturity of automation and robotics or expert systems. 

(2) Predictabilitv of Uoerade and Technoloev Life. 

As an example, computers and software evolve 

rapidly and thus would warrant a high degree of 

technology transparency. 

(3) Develooment Time and Cost. Hardware evolves 

more slowly than software. 

(4) ADdicabilitv of Technoloev to Other Potential 

User Markets. The rate of development is in 

large part dependent on the needs. 

These selection criteria are applied to three categories of advancing technology (near-, mid-, and farterm) to determine the level of technology transparency 

in the baseline design. 

Near-term technologies are not currently mature but 

are incorporated in the completed Phase I configuration 

because their projected development conforms with the 

SSF schedule. Near-term technologies include controllers enhanced by expert systems for fault diagnostics, 

trend analysis, solar array orientation, and load distribution and switching (51. 

Mid-term technologies are currently under development but will not reach maturity until after the SSF Program Critical Design Review around 1992. These technologies can provide significant potential cost or operations bezefits and productivity enhancements to the SSF 

D658-0014 

EPS and are accommodated in the Phase I configuration 

through specific technology features. Examples of midterm technologies are autonomous intelligent controllers 

for fault detection and isolation, power management, 

and maintenance scheduling [SI. 

Far-term technology involves major technological innovations and unique new program needs that have yet 

to be identified. 

The WP-04 approach to technology transparency accommodates near-term technology in the Phase I design; 

mid- and far-term technologies are accommodated 

through software and hardware functional modularity. 

layering, isolation of functions from interfaces, physical 

and functional standardization of utilities, and interface 

standardization. This allows the EPS to take advantage 

of approaching technologies to improve the efficiencies 

and capabilities of the design. 

ACKNO\VLEDGMENTS 

The authors thank Tom Shanahan for the approach to 

technology transparency, Kelly Newton for examples of 

advanced technology, and Marty Garey for making this 

paper possible. 

REFERENCES 

[l] Space Station Program Office, 1988, “Space Station 

Program Definition and Requirements Section 3: Space 

Station Systems Requirements,” SSP 30000 Section 3, 

Revision G., Space Station Program Office, Reston, Virginia. 

5 
121 hlcDonnell Douglas Space Systems Company, Engineering Services undcr contract NAS1-18227 to NASALiriglry Rescnrch Center. 

[3] Rocketdyne, 1988, DR: SE-02. Power Svstems De- (SJ NASA, 1988, “Advancing Automation and Roboiics 

Scriotion Document, RURD 88-633, Rockwell Interna- Technology for the Space Station and for the U.S. Econtional, Rocketdyne Division, Canoga Park, California. omy - Progress Report 6.” NASA Technical Memorandum 100989, NASA-Johnson Space Center, Houston, 

(4J Rocketdyne, 1988, DR: 33-03, SuoDortine Enei- Texas. 

neerine Annlvsis and Data - Svstem Assessment RCDQ~I, 

RVRD 38-637, Rockwcll Inter notional, Rockcttiyne Ilivisicm, Cnuoga Park, Caliloiiiix 

D658-00 14/~l~ 

6 
I- N.iloonnl Aermulcs And 

NASA TM- 102339 

4. Title and Subtitle 

Eivlutionary Growth for Space Station Freedom 

Electrical Power System 

I 

1 7. Author@) 

N.iloonnl Aermulcs And Report Documentation Page 

SbI.3C.e I\JlIII1IBlfiltlOll I- 1 Report No. I 2. Government Accession No. 1 3. Recipient's Catalog No. 

I 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

Report Documentation Page 

SbI.3C.e I\JlIII1IBlfiltlOll 

11 Report No. I 2. Government Accession No. 1 3. Recipient's Catalog No. 

~, 

9 Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland. Ohio 44135-3191 

12 Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington. D.C. 20546-0001 

474-18 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

Matthew F. Marshall, Kerry L. McLallin, and Michael J. Zernic I 

7. Key Words (Suggested by Author(s)) 

Electric power system 

Evolution 

Growth 

Space Station 

E-5047 

10. Work Unit No. 

18. Distribution Statement 

Unclassified - Unlimited 

Subject Category 20 

9. Security Classif. (of this report) 

Unclassified 

16. Abstract 

20. Security Classif. (of this page) 21. No of pages 22. Price' 

Unclassified 

Over an operational lifetime of at least 30 yr, Space Station Freedom will encounter increased space station user 

requirements and advancing technologies. The space station electrical power system is designed with the 

flexibility to accommodate these emerging technologies and expert systems and is being designed with the 

necessary software hooks and hardware scars to accommodate increased growth demand. The electrical power 

system is planned to grow from the initial 75 kW up to 300 kW. The Phase I station will utilize photovoltaic 

arrays to produce the electical power; however, for growth to 300-kW, solar dynamic power modules will be 

utilized. Pairs of 25-kW solar dynamic power modules will be added to the station to reach the power growth 

level. The addition of solar dynamic power in the growth phase places constraints in the initial space station 

systems such as guidance navigation and control, external thermal, truss structural stiffness, computational 

capabilities and storage which must be planned-in in order to facilitate the addition of the solar dynamic modules. 

~ 

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