NASA Technical Memorandum 102347 

Toward an Electrical 

Power Utility for 

Space Exploration 

Robert W. Bercaw 

Lewis Research Center 

Cleveland, Ohio 

.'. 

Prepared for the 

European Space Power Conference 

sponsored by the European Space Agency 

Madrid, Spain, October 2-6, 1989 

i, 

2 

(BASA-TM-lQ2347) TOWARD AN ELECTRICAL PONEE 

UTILXTI FOR SPACE EXPLOEATIOW {Itas&. Lewis 

Research Center) 6 p CSCL 1033 

Unclas 

63/20 0225953 
TOWARD AN ELECTRICAL POWER UTILITY FOR SPACE EXPLORATION 

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Robert W. Bercaw 

Xational Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

ABSTRACT 

Plans for space exploration depend on today's technology programs addressing the novel requirements of 

space-based enterprise. The requirements for electrical power will be formidable: megawatts in 

magnitude, reliability for multi-year missions and 

the flexibility to adapt to needs unanticipated at 

design time. The reasons for considering the power 

management and distribution in the various systems 

from a total mission perspective--rather than simply 

extrapolating current spacecraft design practice-- 

are discussed. A utility approach to electric 

power, being developed at the Lewis Research Center, 

is described. It integrates requirements from a 

broad selection of current development programs with 

studies in which both space and terrestrial technologies are conceptually applied to exploration 

mission scenarios. 

Keywords: Space Exploration, Power, Electric Utility, Lunar Base, Mars Expedition 

1. INTRODUCTION 

It is the purpose of this paper to initiate a dialogue concerning the nature of the electrical power 

systems that will be needed to support space exploration of the sort envisioned by the "Bold New 

Initiatives:" extensive fleets of earth observing 

and planetary spacecraft, permanent lunar bases and 

manned missions to Mars. Studies of these missions 

performed by NASA and others now clearly show that 

power, with its extremely diverse range of applications, is the crucial resource. Most missions, 

especially the manned ones, will require orders-ofmagnitude more power than current missions and 

unprecedented system reliabilities. Moreover, 

requirements will arise in the course of the missions that can not be fully anticipated during 

system design. This paper will review the essential 

characteristics of such power systems, and in particular. define the capabilities that must be provided by the power management and distribution 

subsystem in order to achieve these characteristics. 

2. EXPLORATION REQUIREMENTS 

Following the recommendations of the National Commission on Space (Ref. 1) and the Ride Report (Ref. 

21, NASA has been conducting a variety of studies on 

planetary exploration missions and the technologies 

that will be required to support them. These are 

summarized in the Exploration Studies Technical 

Report (Ref. 3). The approach taken to identifying 

promising exploration options is shown in Figure 1. 

It proceeds through an iterative process of defining 

top-level strategies, performing a variety of case 

studies, and conducting broad trades on various 

technical options. It should be emphasized that 

this is an ongoing process that will be as strongly 

impacted by budgets and the political process as the 

technical results. 

ISTRATEGIES~ 

Hunun Erp.dtDn to PhobD. 

Human Erp.dtDna 0 LI.n 

LUMI anpoOt b Ea? Men Evolmn 

kumt4yudChalroul 

Figure 1 FY 1988 EXPLORATION STUDY APPROACH (Ref.3) 

Probably the most difficult challenge facing the 

development of future electrical power systems will 

be to not only meet the requirements of the exploration missions, but to do it in such a way that the 

development efforts are not rendered irrelevant by 

the all-but-inevitable alterations in the direction 

of the programs. To do so, requires that we take 

into account the entire envelope of requirements 

defined by the studies, rather than focusing on 

those of one specific system or mission. 

The studies make it clear that most of the missions 

must be accomplished through the integration of 

multiple diverse and complex activities rather than 

through the use of single integrated systems departlng from and returning to the surface of the Earth. 
This is illustrated in Figures 2 and 3 for two 

stages of a proposed human expedition to Phobos. 

3. UTILITY POWER 

It would be easy to conclude that the preceding 

requirements are so broad as to preclude a comprehensive approach to space electrical systems, and 

indeed current practice is to design end-to-end 

power systems whose resemblance to other systems 

tends to be purely coincidental (e.g. because they 

share a common design team). Terrestrial power 

utilities, however, successfully meet a similar--and an even broader--range of requirements. It is our 

belief that the current exploration studies are 

showing that the reasons that led to the creation of 

terrestrial electric utilities are rapidly emerging 

in space exploration and will lead to their analogues in space. The primary characteristics of 

terrestrial electric utilities, e.g., flexibility, 

user transparency, ease of maintenance and high 

system reliability, provide a system robustness that 

will be vital importance to space exploration. 

Flexibility - If exploration is indeed a process of 

discovery, it is also impossible to fully preplan 

the missions: changes in direction will be more the 

norm than the exception. This means that the electrical systems must be engineered with the flexi-. 

bility needed to adapt to both planned and unplanned 

redirections, emergencies, etc. The electrical 

system for lunar outpost, for example, must have the 

ability to evolve into one for a base that is planned after the outpost is operational. 

User transparency - Transparency refers to the 

ability of an electric utility to provide power on 

demand, to provide a simple interface (e.g. an 

outlet), and to isolate loads from transients and 

other disturbances caused by the other loads on the 

system. Transparency has not even been considered 

in most space systems both because of their paucity 

of power and the need to minimize their considerable 

costs: usually every watt was programed in advance, 

or elaborate procedures were established to avoid 

system upsets as loads are switched in and out. In 

the planning for Space Station Freedom, it was 

recognized that such procedures would become 

impossibly cumbersome and limit the complex and 

dynamic mix of activities which were envisioned. 

Consequently, 75 kilowatts of power were provided 

and a utility approach was taken to the power 

distribution system. Even so, attached loads have 

grown to several times the system capacity. In 

order to retain a large degree of user transparency 

NASA is developing autonomous systems for load 

scheduling and control of the power system. Since 

the exploration missions will be significantly more 

complex than Space Station Freedom, we believe that 

user transparency will be crucial to their success. 

On-orbit assembly - The terrestrial assembly of 

missions at Kennedy and other space centers requires 

extensive staff and facilities. This is true even 

for payload integration into the STS which was 

designed to be a flexible, general purpose vehicle. 

It is unreasonable to expect that the on-orbit 

assembly of a manned Mars probe as was shown in 

Figure 2 or the growth of a lunar base can be 

achieved without addressing all the issues that made 

these complexes necessary. One is the provision of 

electric power for a wide variety of operations. It 

is obvious that this becomes all but impossible 

unless the different systems have compatible power 

systems: for example, the power at a vehicle 

assembly center must be compatible with that of the 

vehicle stages in order to provide auxiliary power 

during their check-out. 

Figure 2 HUMAN EXPEDITION TO PHOBOS - EARTH 

ORBITAL OPERATIONS (Ref. 3) 

Lrutl UoImmm 

Figure 3 HUMAN EXPEDITION TO PHOBOS - MARS/PHOBOS 

ORBITAL OPERATIONS (Ref. 3) 

A corollary is that both space-based operations and 

a space-based infrastructure will become integral 

elements of space exploration. Operations will 

include on-orbit assembly, on-orbit servicing, 

planetary landing and assent, and rendezvous. The 

infrastructure will be comprised of space stations, 

vehicle accommodation (assembly) facilities, transportation depots, orbit transfer stages, and a 

variety of smaller systems such as orbital maneuvering vehicles, manned maneuvering units and communications relays. Furthermore, lunar and planetary 

bases will require infrastructures of similar 

complexity. 

The studies further show that a comprehensive 

approach to space electrical power management systems must be compatible with at least the following 

requirements: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

Scaleable from kilowatts to multiple megawatts 

System lifetimes and mean times between failure 

of up to 50 years 

A wide variety of sources ranging from photovoltaics to nuclear reactors 

A wide variety of storage systems including 

those on planetary rovers Compatibility with a broad range of natural and 

system induced environments, including planetary 

surfaces, LEO, deep space, reactor fields, etc. 

Capable of providing power for on-orbit servicing and of being serviced 

Compatible with on-orbit assembly; e.g. subsystems must be able to exchange or share power 

Capable of autonomous operation 

Minimum mass 

Maximum efficiency 

Assured system and personnel safety 

Minimum EM1 and other emissions 

2 
On-orbit Servicing - In its report to Congress, NASA 

identified a number of design criteria that must be 

followed in order to make on-orbit servicing 

practical. In addition to the limitations imposed 

on designs by replacement logistics, both EVA and 

robotic operations requirements dictate the use of 

orbital replacement units. Their design, in turn, 

mandates the careful definition of all interface 

standards including the electrical power busses. 

Reliability and lifetime - Sufficient experience has 

now been gained with space systems to make it clear 

that the system reliabilities required for the 

exploration missions cannot be achieved with any 

degree of certainty by merely improving component 

reliability; even if the components could be 

developed, validation procedures would be 

prohibitive. Therefore it must be achieved through 

intelligent redundancy management. The electric 

utilities have achieved extremely high reliabilities 

through the interconnection of numerous sources and 

loads on a redundant grid, and by using standardized 

interface elements that can be rapidly replaced or 

repaired. Essentially infinite system life is 

achieved by providing standards and methods so that 

individual elements can be replaced or upgraded 

without interruption of service. 

While the use of a transmission grid is not feasible 

in most exploration missions, with the notable 

exception of lunar and planetary bases, the 

principles are. Two factors are important. First, 

the degree of redundancy that must be designed into 

a system to assure mission success depends not only 

on the probability of failure, but also on the mean 

time to repair; i.e. a failure reduces the degree of 

system redundancy by one until it is repaired. 

Second, the transportation times associated with 

providing repairs or replacements from Earth will be 

enormous. In combination, these factors require 

systems to be significantly over designed unless 

spares and/or sources of backup power are locally 

available. For the complex missions envisioned, 

these options become practical only if the various 

systems are engineered for a high degree of spares 

commonality and with compatible electrical 

interfaces, respectively. 

4. ELECTRICAL SYSTEMS DEFINITION 

While the exploration missions will not be carried 

out for a number of years, it is important to 

develop a utility approach to their electrical 

systems now. It will both draw attention to 

significant technology issues, and hopefully reduce 

conflicts resulting from the need to resolve 

alternatives after opinions have become entrenched. 

The utility approach to space exploration power does 

not imply a monolithic approach to the engineering 

individual electrical systems, but it does require 

an understanding and awareness of the total mission 

requirements so that the individual systems are not 

sub-optimized at the expense of total mission goals 

and/or costs. In view of the diverse and immature 

state of the exploration missions. the evaluation of 

various approaches to electrical systems for them is 

a formidable task. Since the impact of any design 

decision depends on the details of the particular 

exploration mission that is ultimately chosen, it is 

necessary to develop an approach to making decisions 

that will yield valid results under all eventualities. 

We are approaching the problem of defining electrical systems for the exploration mission power 

systems through the iterative process shown in 

Figure 4. For a given mission. it consists of four 

interrelated activities: 1) the identification of 

the space domains and the drivers that they impose 

on the systems designs, 2) design studies of 

individual systems and the subsystems/components 

that comprise them, 3) mission analyses to identify 

requirements imposed by the systems interactions 

with each other, and 4) broad, mission wide, trade 

studies to evaluate the net results of the various 

design decisions. 

As appropriate, various system parameters, such as 

voltage or frequency, or interface elements, such as 

connectors, will be considered for standardization, 

and candidate standards will be proposed and evaluated. Clearly some degree of standardization is 

needed, but many options and combinations of them 

are possible. Because experience has taught us that 

the setting of such standards can be a contentious 

activity, we believe it is important to first 

establish a consensus on the need and requirements 

for them before embarking on an effort to define 

standards. 

In the first element of defining a utility-type 

electrical systems for an exploration mission, the 

space domains involved are analyzed to identify the 

system design drivers. These provide the general 

requirements to the mission analyses and the design 

studies. For example, transportation costs 

determine the mass sensitivity of the designs, while 

the degree of availability of resources or a local 

infrastructure determines the reliability/lifetime 

requirements. Environmental conditions, insolation 

and the length-of-night drive a variety of requirements. Most of these factors are already being 

determined in the ongoing mission definitions 

studies, but additional factors may be identified 

as the studies progress. 

Mission analysis, the second element. derives 

additional electrical system requirements from the 

mission scenario and requirements, including specifications of the power 1evel.the type of conversion 

and storage systems, and the type and number of 

attached loads. It also provides any requirements 

for growth, autonomous operation, transfer of power 

between systems during on-orbit assembly, or 

criteria for minimization of spares and other 

logistics. Again, much of this information is 

already being generated in the on-going mission 

studies, but additional work must be performed to 

extract the electrical system requirements. 

Using the requirements derived in the first two 

elements, the design studies establish the basic 

characteristics and figures of merit for the 

electrical systems for all the various vehicles, 

platforms, habitats needed for the mission under 

consideration. These studies are supported by conceptual design studies which provide the specificathe subsystems and components that comprise them. 

Finally, the above results are combined into broad 

trade studies that quantify the results of design 

approaches or parametric variations mission under 

consideration. As additional missions are defined 

and analyzed. we anticipate that the comparisons of 

the various results will both illuminate the true 

requirements for a space power utility and provide 

an early identification of the technology develop3 
ments that will be needed to carry out space 

exploration. 

5. CURRENT ACTIVITIES 

Information needed for the definition of a space 

power utility is now becoming available at a rapid 

and increasing rate: the NASA Office of Exploration 

is conducting a systematic investigation of the pros 

and cons of a wide variety of approaches to the 

basic strategies of space explorations; much environmental data has been gathered on the Moon and the 

planets; the electrical system for Space Station 

Freedom is being designed as a utility and is thus 

providing an invaluable base of engineering: and 

increasing attention is being paid to understanding 

operational problems such as on-orbit servicing. In 

the Power Technology Division at the Lewis Research 

Center, we are attempting to integrate this information in order to guide the definition of a utility 

approach to space power. 

We are also conducting our nearer term technology 

programs in a way that will shed further light on 

exploration issues. At the system level. we have 

been studying the requirements and design issues for a serviceable on-orbit technology test bed. 

Aprincipal objective is to be able to compatibly 

support orbital replacement units containing 

proprietary equipment from unrelated companies; the 

definition of suitable interface standards, and 

power and other bus structures is essential to success. A second example is a technology program 

to provide electromechanical actuators and an 

integrated power system for the Advanced Launch 

System Program. It extend the electric utility 

concept to the replacement of high maintenance 

hydraulic and pneumatic systems, and it also 

provides the requirements definition needed to 

extend the utility concept to launch vehicles. 

Lastly, in collaboration with the Space Station 

Freedom organization, we are developing autonomy 

technology by automating their test bed. We 

anticipate extending this activity to exploration 

missions when the current project is completed. 

At the component level, we have a variety of efforts 

directed at expanding the environmental envelope, 

e.g., temperature, radiation fields and plasmas, in 

which electrical systems can operate. We are also 

attempting to extend the power capabilities to the 

megawatt level while reducing the specific mass of 

typical components by an order of magnitude. 

6. CONCLUSIONS 

We have become convinced that the ambitious exploration goals can be achieved at reasonable costs 

only if a utility approach to the development of 

power is adopted. An effort has been initiated to 

define such an approach, and information is being 

collected through a combination of liaison with 

related projects/research, and enhancing our own 

ongoing technology research. Because a utility 

would impact qll those involved in exploration, we 

invite and encourage all others to join us in this 

quest. 

7. REFERENCES 

1. National Commission on Space 1986, Pioneering 

the Space Frontier, New York, Bantam Books 

2. Ride S 1987, Leadership and America's Future in 

Space, a report to the NASA Administrator 

3. Office of Exploration 1988, Exploration Studies 

Technical Report, FY 1988 Status, NASA TM 4075 

EXPLORATION 

ELECTRICAL UTluM 

DEflNlTlON 

STANDARDS . CANDIDATE 

IDENTIFICATION STANDARDS 

I 1 I 

L 

LOADS 

STORAGE 

ASSEMBLY 

TRANSPORT COSTS 

ENVIRONMENTAL 

EFFICIENCY t SUPPLY ROUTES t STABILITY AUTONOMY 

EVOLUTION & GROWTH 

INFRASTRUCTURE 

MASS 

RANCE EFFICIENCY 

COST 

RELIABILITY 

Figure 4 ELEMENTS OF UTILITY DEFINITION PROCESS 

4 
nv\sn 

National Aeronautics and 

1. Report No. 

NASA TM- 102347 

Report Documentation Page 

2. Government Accession No. 

4. Title and Subtitle 

Toward an Electrical Power Utility for Space Exploration 

7. Author@) 

Robert W. Bercaw 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

~ ~~ 

2. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

3. Recipient's Catalog No. 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-5055 

10. Work Unit No. 

506-4 1-4 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

5. Supplementary Notes 

Prepared for the European Space Power Conference sponsored by the European Space Agency, Madrid, Spain, 

October 2-6, 1989. 

6. Abstract 

Plans for space exploration depend on today's technology programs addressing the novel requirements of spacebased enterprise. The requirements for electrical power will be formidable: megawatts in magnitude, reliability 

for multi-year missions and the flexibility to adapt to needs unanticipated at design time. The reasons for 

considering the power management and distribution in the various systems from a total mission perspectiverather than simply extrapolating current spacecraft design practice-are discussed. A utility approach to electric 

power, being developed at the Lewis Research Center, is described. It integrates requirements from a broad 

selection of current development programs with studies in which both space and terrestrial technologies are 

conceptually applied to exploration mission scenarios. 

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

Space exploration; Power; Electric utility; 

Lunar base, Mars expedition 

18. Distribution Statement 

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Subject Category 20 

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