Projected energy demand for all NASA, DoD and civil missions for the time span 1981 to 1995 are illustrated. Typical energy cost range from about $300 to $2000 per kW-hr, with an average of about $800 per kW-hr for long-duration missions. At these levels, the cost of the required energy would be several billion dollars per year by about 1985 and might constrain the number and types of NASA programs to be carried out. NASA is extensively pursuing approaches for reducing nonrecurring costs. Two programs are presented for the development of an economical approach to space power systems. They are: (1) Economical Orbital Power (ECOP) with the objective to demonstrate the applicability of a commercial approach to the development of a low cost photovoltaic space power system; and (2) Space Power Experiment (SPEX) which has the objective to demonstrate the application of industrial hardware for space power systems

Teren, F.

English

NASA Technical Reports Server

AN ECONOMICAL APPROACH TO SPACE POWER SYSTEMS 
Fred Teren 
NASA L e w i s  Research Center  
A con t inu ing  concern of NASA i s  t h e  high cost of i t s  f u t u r e  
space programs and, i n  p a r t i c u l a r ,  t h e  cost of power and energy 
i n  suppor t  of space programs p r o j e c t e d  f o r  t h e  mid-1980's and 
beyond. F igure  1 i l l u s t r a t e s  p r o j e c t e d  energy demand f o r  
a l l  NASA, DOD and C i v i l  missions for  t h e  t i m e  span 1981 t o  
1995. I t  can be seen t h a t  t h e  p r o j e c t e d  energy demand i n c r e a s e s  
by about a f a c t o r  of  t e n  between 1 9 8 1  and 1 9 9 0 .  Typica l  energy 
costs have ranged from about $300 t o  about $2000  p e r  kW-hr, 
wi th  an average of about  $8OO/kW-hr f o r  long-durat ion missions.  
A t  t h e s e  l e v e l s ,  t h e  cost  of t h e  r equ i r ed  energy would be 
several b i l l i o n  d o l l a r s  pe r  year  by about 1985 and might 
c o n s t r a i n  t h e  numbers and t y p e s  of programs NASA w i l l  be a b l e  
t o  c a r r y  out .  Thus, it i s  important  t h a t  NASA f i n d  ways t o  
reduce t h e  cost  of space power systems. One way t o  accomplish 
t h i s  i s  t o  s t a r t  wi th  t h e  t r a d i t i o n a l  space -qua l i f i ed  systems 
approach and look f o r  ways t o  reduce costs through mod i f i ca t ions  
i n  procedures  and changes i n  components. Non-recurring c o s t s  
can be reduced by e l i m i n a t i o n  of custom and unique des igns  
f o r  each mission and by volume buys t o  t a k e  advantage of m a s s  
product ion techniques .  A l s o ,  unnecessary documents and 
s p e c i f i c a t i o n s  can be e l imina ted ,  and ways can be found t o  
s i m p l i f y  t h e  component p a r t s .  This  approach i s  be ing  pursued 
e x t e n s i v e l y  i n  t h e  NASA space power systems program. 
The advent  of  t h e  Space Transpor t a t ion  System ( i .e .  Space 
S h u t t l e )  w i l l  a f f e c t  t h e  c o s t  of space power systems i n  several 
ways. The m o s t  obvious e f f e c t  w i l l  be a s u b s t a n t i a l  r educ t ion  i n  
t h e  cost of t r a n s p o r t i n g  systems from E a r t h  t o  l o w  Ear th  o r b i t  
(LEO).  A less obvious bu t  p o t e n t i a l l y  very  important  e f f e c t  
i s  t h a t  a new approach t o  accomplishing mission r e l i a b i l i t y  
is  made p o s s i b l e .  H i s t o r i c a l l y ,  payload c o s t s  have t y p i c a l l y  
been d r i v e n  by requirements  t o  a s s u r e  t h a t  h igh ly  complex, 
advanced technology,  i r r e t r i e v a b l e ,  w e i g h t - c r i t i c a l ,  and 
s c h e d u l e - c r i t i c a l  v e h i c l e s  and s p a c e c r a f t  performed t o  f u l l  
requirements  on t h e i r  f i r s t  and on ly  f l i g h t .  
I n  o r d e r  t o  e s t a b l i s h  t h a t  t h e s e  systems have a h igh  
p r o b a b i l i t y  of performing f o r  t h e  mission d u r a t i o n ,  r e l i a b i l i t y  
requirements  w e r e  in t roduced  a t  t h e  o n s e t  of  t h e  program and 
i n t e g r a t e d  i n t o  n e a r l y  every a s p e c t  of t h e  development. They 
inc lude  such p r a c t i c e s  as conse rva t ive  d e s i g n s ,  redundancies ,  
use  of  high r e l i a b i l i t y  components, c l e a n  room f a c i l i t i e s ,  
2 65 
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established fabrication techniques, quality control, inspections 
comprehensive testing, etc. 
With the introduction of Shuttle it is possible to either 
service space systems in orbit, replace modules in space, or 
return faulty units to Earth for repair. Failure of a component 
or system in space need not imply mission failure. Thus, 
mission reliability can be achieved through a combination of 
component reliability, maintenance and servicing. This is 
what we refer to as the commercial approach to reliability. 
There are, of course, many reliable low cost commercial systems 
operating today which give dependable performance because 
they can be serviced. These systems, which were not designed 
for space application, include aircraft, automobiles, appliances, 
radios, television and large power generating equipment. With 
Shuttle making space servicing practicable, it is of interest 
to study the possible application of a low cost commercial 
approach to the design and fabrication of reliable space systems. 
Inherently related to the use of a commercial approach is 
the use of commercial components (i.e. components used in 
terrestrial and/or aircraft equipment). Relative to space- 
qualified components, commercial components typically are 
produced in larger numbers and at lower cost but may be less 
reliable. Thus, the use of commercial practices and components 
represents a tradeoff between cost, reliability, and servicing, 
in which low cost is emphasized and reliability is achieved 
through servicing and maintenance. 
The program objectives are therefore to demonstrate the 
applicability of a commercial approach and commercial components 
to the development of a low cost photovoltaic space power 
system. Once this has been achieved, the knowledge gained 
and .procedures established can be applied to other space 
systems as well. 
Before undertaking a substantial effort to identify and 
apply commercial practices to the development of Bpace power 
systems, the potential for substantial cost reduction should 
be established first. In order to do this, mission and system 
requirements must first be specified, so that a possible power 
system and its components may be identified. The power system's 
proposed mission is a circular 200-nautical-mile-altitude Earth 
orbit having inclination of 28 degrees. The system must 
supply 2 kilowatts of electrical power continuously to the 
load and is specified to consist of a solar array power 
source and batteries for energy storage. Excess energy from 
the array is stored during periods of sunlight and trans- 
mitted from storage to the load during solar occulation. 
A comparison of costs of space-qualified and commercially 
266 
available equipment is shown in table I for the principal 
categories of a space power system which is designed to meet 
the above specified requirements. A 5-kW solar array is 
required to deliver 2 kW average power to the load. The space- 
qualified solar array blanket is assumed to cost 1.5 million 
dollars, or $300/watt. Approximately one kW-hr of energy 
storage is required, and NiCd batteries are used for this 
purpose. Power processing includes a battery charger and bus 
regulator, and mechanical systems include a solar array drive 
and deployment mechanism. It is assumed that the commercial 
system will utilize industrial type components such as 
terrestrial solar arrays and avionics batteries and power 
converters. It can be seen that over half the cost of the 
space-qualified system is the cost of the solar array. On 
the other hand, a cost of $30/watt is typical for a terrestrial 
array. Substantial cost savings are also possible if commercial 
equipment is used instead of space-qualified equipment in the 
other categories. 
Overall, nearly an order of magnitude cost reduction is 
estimated to be possible - $390,000 for the commercial system 
vs. $2,650,000 for the space-qualified system. It is 
recognized, however, that the quoted costs for the commercial 
equipment are based on the use of the system in a terrestrial 
environment. Some modifications would surely be required 
if this equipment were to be used in the space environment, 
and these modifications would inevitably increase cost. 
However, these cost increases are expected to be small compared 
to the potential cost saving of over two million dollars. 
On the basis of these preliminary cost estimates, further pursuit 
of the commercial approach is justified. 
Our approach to the development of an economical approach 
to space power systems is to conduct two programs, which we 
call ECOP (Economical Orbital Power) and SPEX (Space Power 
Experiment). The objective of ECOP is to demonstrate the 
applicability of a commercial approach to the development of 
a low cost photovoltaic space power system. The objective 
of SPEX is to demonstrate the application of industrial 
hardware for space power systems. 
The ECOP program starts with studies and leads eventually 
to the design, fabrication and testirfg of a 2-kW space power 
system. The studies will define and compare commercial and 
space-qualified approaches to the design, fabrication and 
testing of a photovoltaic space power system and estimate 
the cost which would result for each approach. The specific 
power system type to be considered is a photovoltaic system, 
with rechargeable batteries for energy storage. Batteries, 
rather than fuel cells, are specified.for energy storage because 
of the availabiiity of both space-qualified and industrial 
267 
t ypes .  The system i s  t o  o p e r a t e  i n  a l o w  Ea r th  o r b i t  and 
i s  t o  provide 2 kW of average power t o  t h e  load .  A c o n t r a c t  
has r e c e n t l y  been i n i t i a t e d  w i t h  Solarex  t o  s tudy  t h e  
commercial approach t o  space power system development. A 
second c o n t r a c t  i s  a n t i c i p a t e d ,  t o  concep tua l ly  des ign  and 
estimate t h e  cost of a space power system us ing  t h e  t r a d i t i o n a l  
space-qual i f ied  approach. The So la rex  c o n t r a c t  w i l l  examine 
t h e  commercial approach i n  d e t a i l ,  i nc lud ing  t h e  approach t o  
des ign ,  f a b r i c a t i o n ,  documentation and R&QA. Considered i n  
t h e  s tudy  w i l l  be costs,  manpower, methods, p r a c t i c e s  and 
procedures  involved f o r  a complete c y c l e  of a new product  
f r o m  conceptua l  des ign  t o  a f i n i s h e d  f a b r i c a t e d  product .  These 
s t u d i e s  w i l l  est imate t h e  cost  of space power s y s t e m s  
developed through t h e  use  of commercial and space-qual i f ied  
approaches.  This  w i l l  e s t a b l i s h  wi th  m o r e  assurance  t h e  
p o t e n t i a l  for  cost  r eguc t ion  of space power systems through 
t h e  use  of a commercial approach. 
The nex t  s t e p  i n  t h e  ECOP program i s  t o  des ign  and 
f a b r i c a t e ,  under c o n t r a c t ,  a 2-kW pho tovo l t a i c  space  power 
system, us ing  a commercial approach as de f ined  i n  t h e  e a r l i e r  
s tudy.  By conduct ing t h i s  program as a con t r ac t ed  e f f o r t ,  
t h e  cost of commercial power systems w i l l  be f i r m l y  e s t a b l i s h e d .  
The choice  of a 2-kW power l e v e l  a l lows  p o s s i b l e  use of t h e  
system by f r e e - f l y e r  experiments  and a l lows  a system t o  be 
developed a t  low cost which i s  s t i l l  l a r g e  enough t o  supply 
informat ion  about  t h e  cost  of  f u t u r e  m u l t i k i l o w a t t  s y s t e m s .  
P o t e n t i a l  u s e r s  w i l l  be  contac ted  du r ing  t h e  des ign  phase,  
and an  a p p r o p r i a t e  tes t  program f o r  t h e  power system w i l l  be 
evolved through c o n s i d e r a t i o n  of u s e r  needs.  
Concurrent wi th  t h e  above program, L e w i s  i s  conducting 
an e f f o r t  t o  des ign ,  b u i l d  and f l i g h t  t e s t  a smal l  ( l ess  than  
100 w a t t )  p h o t o v o l t a i c  space power system. This program, c a l l e d  
SPEX (Space Power Experiment) ,  w i l l  demonstrate t h e  a p p l i c a t i o n  
of i n d u s t r i a l  hardware f o r  space power systems. L e w i s  
eng inee r s  w i l l  d e f i n e  t h e  system, select and purchase com- 
merc i a l ly  a v a i l a b l e  components, i n t e g r a t e  t h e  system and d e f i n e  
and conduct a l i m i t e d  tes t  program. 
The SPEX experiment i s  a low c o s t ,  s o l a r  a r r a y  - b a t t e r y  
power system. The power system c o n s i s t s  of t e r r e s t r i a l  s o l a r  
a r r a y s  and an a v i o n i c s  b a t t e r y  and dc  t o  dc power conve r t e r .  
The b a t t e r y  charge  scheme i s  based on t h e  c a p a b i l i t y  of t h e  
b a t t e r y  t o  accep t  a l o w  r a t e  overcharge f o r  an i n d e f i n i t e  
t i m e  per iod  so no b a t t e r y  charge  i s  r equ i r ed .  
A l l  costs w i l l  be  accounted f o r  i n  the  SPEX program and 
compared wi th  p r e d i c t i o n s  made by t h e  ECOP s t u d i e s .  The SPEX 
power system is  scheduled t o  be f l i g h t  tested on t h e  long- 
du ra t ion  experiment f a c i l i t y  (LDEF) . 
268 
TABLE I 
Cost of space power systems using space-qualified and 
commercial components. System operates in LEO, delivers 2 kW 
average. 
Category Space Qualified 
Solar Array $ 1 . 5 M  
Batteries 180K 
Power Processing 360K 
Mechanical Systems 225K 
Systems Integration and 400K 
Qualification Testing 
Comer c i a 1 
$ 150K 
5K 
10K 
75K 
150K 
$ 2.65M $ 390K 
269 



An economical approach to space power systems

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