The solar array blanket, defined as a substrate covered with interconnected and glassed solar cells, but excluding the necessary support structure, deployment, and orientation devices is considered. The interactions between the blanket and the structure that is used to package, deploy, support and, if necessary restow it, are addressed along with systems constraints such as spacecraft configuration, size, and payload requirements. The influence on blanket design is emphasized. The three main mission classes considered are low Earth orbital (LEO), intermediate, or LEO to GEO transfer, and geosynchronous (GEO). Although interplanetary missions could be considered to be a separate class, their requirements, primarily power per unit mass, are generally close enough to geosynchronous missions to allow this mission class to be included within the third type. Examination of the critical elements of each class coupled with considerations of the shuttle capabilities is used to define the type of blanket technology most likely required to support missions that will be flown starting in 1990

Scott-Monck, J. A.

English

NASA Technical Reports Server

BLANKET TECHNOLOGY WORKSHOP REPORT 
John A. Scott-Monck 
Jet Propulsion Laboratory 
Pasadena, California 
The s o l a r  a r ray  b lanket ,  de f ined as a subs t ra te  covered w i t h  i n t e r -  
connected and glassed s o l a r  c e l l  s, b u t  exc lud ing the  necessary support  
s t ruc tu re ,  deployment, and o r i e n t a t i o n  devices, cannot be considered as a 
complete ly  independent element o f  technology. P a r t i c u l a r l y  i n  l a r g e r   array 
designs, t h e  i n t e r a c t i o n s  between t h e  b lanket  and t h e  s t r u c t u r e  t h a t  i s  used 
t o  package, deploy, suppor t  and, i f  necessary restow it, can be s i g n i f i c a n t  
design d r i ve rs .  Systems c o n s t r a i n t s  such as spacecraf t  con f igura t ion ,  s i z e  
and payload requirements a l s o  may in f l uence  b lanket  design. 
extremely d i f f i c u l t  t o  assign a b lanket  s p e c i f i c  power goal  w i thou t  some 
knowledge o f  t h e  miss ion  c l a s s  t h a t  w i l l  u t i l i z e  it. 
intermediate,  o r  LEO t o  GEO t rans fe r ,  and geosynchronous (GEO). Although 
i n t e r p l a n e t a r y  miss ions cou ld  be considered t o  be a separate c lass,  t h e i r  
requirements, p r i m a r i l y  power p e r  u n i t  mass, are g e n e r a l l y  c l o s e  enough t o  
geosynchronous miss ions t o  a l l ow  t h i s  miss ion  c l a s s  t o  be inc luded w i t h i n  
the  t h i r d  type. Examination o f  t h e  c r i t i c a l  elements o f  each c l a s s  coupled 
w i t h  cons idera t ions  o f  t h e  s h u t t l e  c a p a b i l i t i e s  was used t o  d e f i n e  t h e  type  
o f  b lanket  technology most l i k e l y  requ i red  t o  support  ln issions t h a t  w i l l  be 
f l o w n  s t a r t i n g  i n  1990. 
There are t h r e e  user  groups c u r r e n t l y  opera t ing  i n  t h e  space environ- 
ment; NASA, t he  m i l i t a r y ,  and the  communications s a t e l l i t e  indus t ry .  NASA 
has i n t e r e s t s  i n  a l l  t h r e e  miss ion  classes, as does the  m i l i t a r y  t o  some 
degree, w h i l e  t h e  t h i r d  group i s  p r i m a r i l y  i n t e r e s t e d  i n  geosynchronous 
o r b i t s .  
l eve l s ,  r i s i n g  f rom the  present  1-3 kW t o  upwards o f  25 kW. 
Thus i t  i s  
The th ree  main miss ion  c lasses  considered were low Ear th  o r b i t a l  (LEO), 
A l l  t h r e e  user groups i n d i c a t e  t h e  need f o r  inc reas ing  a r ray  power 
LOW EARTH ORBITAL REQUIREMENTS 
Technology capable o f  s a t i s f y i n g  the immediate fo recas ted  needs f o r  t h e  
Ear th  o r b i t a l  a p p l i c a t i o n s  does e x i s t  i n  t h e  form o f  t h e  SEP t ype  array. 
Thus the  quest ion o f  s tandard iz ing  t h i s  technology was considered. I t  was 
the  general  consensus t h a t  t he  requirements p laced on t h e  a r ray  by miss ion  
p lanners and spacecraf t  system designers w i  11 mandate con t inua l  modif i ca-  
t i ons ,  minor and major, which w i l l  tend t o  prevent  any h igh  degree o f  a r ray  
standard i za t  i on. 
The l a r g e r  power systems, planned f o r  t h e  fu tu re ,  must o f  necess i ty  be 
l e s s  expensive than sma l le r  a r rays  which can cos t  i n  excess o f  1000 d o l l a r s  
per  w a t t  EOL. Concern over  t h e  growing c o s t  o f  pho tovo l ta i c  ar rays was 
expressed. Al though t h e  a r r a y  t y p i c a l l y  makes up a very small f r a c t i o n  
(t5 percent)  o f  t h e  t o t a l  m iss ion  cost ,  t h e  steady increase i n  cos t  may make 
pxo tovo l ta i cs  l e s s  a t t r a c t i v e  f o r  f u t u r e  miss ions and thus make i t  e a s i e r  
f o r  a l t e r n a t i v e  power sources t o  compete. The SEP a r r a y  cos t  does no t  
appear t o  be a major  concern a t  t h i s  time, b u t  c o s t  o f  power w i l l  become 
very impor tant  as a r ray  requirements exceed 100 kW. Since LEO power cos ts  
401 
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are  determined by l i f e  cycle  considerations, i t  is  very important t ha t  the 
operating l i fe t ime of LEO arrays be maximized. Technology t o  guarantee a 
minimum l i fe t ime of 5 years  i s  c r i t i c a l ,  while 10 year  operation is  highly 
desirable.  
becomes more important as  the s i z e  of the array increases. To reduce th i s  
problem, higher e f f ic iency  c e l l s  which wil l  reduce array s i z e  f o r  a g iven  
power level,  wil l  be necessary, as  will  be in-orbit pa r t i a l  o r  f u l l  
restowage capabi 1 i ty .  
The shuttle launch capabi l i ty  f o r  LEO applications does not place 
s igni f icant  mass cons t ra in ts  on arrays. However, the s h u t t l e  launch cos t  i s  
determined by the amount of shu t t l e  bay volume taken up by the mission. 
T h u s  more at tent ion must be paid t o  properly packaging arrays and blankets 
ra ther  than t o  reducing their  mass. 
A key conern i n  t h i s  area i s  the question of o rb i t a l  "drag," which 
INTERMEDIATE OR TRANSFER ORBIT REQUIREMENTS 
Intermediate o r b i t s  (>500 km) which are used predominantly by mil i tary 
s a t e l l i t e s  place a c r i t i c a l  re uirement on radiation resistance.  Equivalent 
1 MeV fluences exceeding 1 ~ 1 0 ~ :  e/cm2 would be experienced by the blan- 
ket. 
thrust propulsion systems t o  boost payloads from LEO t o  GEO. 
s i gn i f i can t ly  reduced the amount of degradation presently observed i n  si l i -  
con s o l a r  c e l l s  i s  encouraged. The superior radiat ion resis tance predicted 
f o r  gallium arsenide argues f o r  i t s  consideration f o r  these orb i t s .  
a l te rna t ive  approach i s  t o  develop blankets capable of s u r v i v i n g  the tem- 
peratures necessary t o  anneal the  e l e c t r i c a l  degradation causea by the 
rad i a t  i on envi ronment . 
Radiation resis tance i s  a l so  required i n  order t o  consider u s i n g  low 
Effor ts  t o  
The 
GEOSYNCHRONOUS ORBIT REQUIREMENTS 
There was a unanimous agreement tha t  geosynchronous o r b i t  requirements 
are  the main dr iver  fo r  blanket technology. 
launch capabi l i ty  place a premium on lowering the array mass. 
always be pressure t o  reduce array mass i n  order t o  increase the s a t e l l i t e ' s  
" t r a f f i c "  (communication channels, e t c . )  potent ia l .  The IUS is  designed t o  
launch -2300 kg i n to  GEO.  
allowance f o r  the  so l a r  array wil l  be constrained t o  -150 ky. 
space based radar. T h i s  leads t o  the conclusion tha t  arrays possessing a 
beginning-of-life spec i f ic  power of -200 W/kg could be required. 
i ng the array spec i f ic  power t o  a blanket spec i f ic  power cannot be e a s i l y  
done s ince s t ruc tura l  and systems requirements must be considered. General- 
ly,  the f rac t ion  of the array mass resul t ing from the s t ruc ture  becomes 
smaller as the array s i z e  increases. T h u s ,  the overall  spec i f ic  power of 
the array would l i ke ly  be greater  f o r  a large array ( > l o  kW) than a smaller 
array (-1 kW) even though the same spec i f ic  power blanket was used f o r  both. 
A set of technology requirements necessary t o  support a 1990 launch was 
established. From previous experience, i t  was determined t h a t  these goals 
would have t o  be accomplished by 1985. 
blanket ( B O L )  including bus  mass, could be achieved by 1985 without the need 
f o r  a major "breakthrough" i n  blanket technology. T h i s  wil l  require tha t  by 
S h u t t l e  cos t s  and the  IUS 
There w i l l  
Except f o r  a few unique s i tua t ions ,  the mass 
Power leve ls  of 25 kW and up wil l  be required f o r  space platforms and 
Translat- 
I t  was determined tha t  a 300 W/kg 
402 
1985 s i l i c o n  s o l a r  c e l l s  50-60 pm t h i c k  w i t h  an AM0 e f f i c i e n c y  o f  14 percent  
are i n  product ion.  
down very t h i n  c e l l s  must have been reduced t o  p rac t i ce .  The present methoa 
of cover ing s o l a r  c e l l s  w i l l  have t o  b e  replaced. 
suggested were t h i n  (25-50 pm) i n t e g r a l  covers o r  organic  .encapsulants. The 
approach t o  in te rconnect ing  must be mod i f i ed  t o  accomodate welding o f  
extremely t h i n  in terconnects .  To reduce t h e  mass o f  t h e  bus system it w i l l  
be necessary t o  operate b lankets  near 200 vo l ts .  
concerning a r ray  space charg ing e f f e c t s  and plasma i n t e r a c t i o n s .  
agreed t h a t  t h i s  was a p o t e n t i a l  major  problem, and f u r t h e r ,  t h a t  very 
l i t t l e  i s  a c t u a l l y  understood i n  t h i s  area. 
t h a t  space experiments aimed a t  p rov id ing  i n fo rma t ion  on the  e f f e c t s  o f  
plasma leakage on b lanket  performance be i n s t i t u t e d  once the  s h u t t l e  i s  
operat ional .  I n  add i t ion ,  as advanced b lankets  are developed, they  should 
be f l i g h t  t e s t e d  as q u i c k l y  as poss ib le ,  p r e f e r a b l y  on an annual basis.  
New and innova t i ve  processes f o r  assembling and l a y i n g  
Poss ib le  a l t e r n a t i v e s  
This  l e d  t o  a d iscuss ion  
It was 
It was s t r o n g l y  recommended 
1990 REQUIREMENTS 
It was conceded t h a t  b lanket  s p e c i f i c  power must probably reach 
400 W/kg EOL ( 1 ~ 1 0 ~ ~  e/cm2) t o  assure t h a t  a r rays  can s a t i s f y  t h e  
requirements f o r  advanced geosynchronous missions. 
revo lu t i ona ry  r a t h e r  than  evo lu t i ona ry  developments would be necessary. 
example, t he  s o l a r  c e l l s  employed i n  t h i s  advanced b lanket  would have t o  be 
15 t o  16 percent  e f f i c i e n t  (EOL) a t  opera t ing  temperature. The c e l l ' s  mass 
would have t o  be equ iva len t  t o  no more than 50 um o f  s i l i c o n .  Obviously, 
advanced encapsulants o r  covers would be requi red.  Although low mass blan- 
k e t s  imp ly  minimum back sur face  r a d i a t i o n  sh ie ld ing ,  t he  t rade-o f fs  seem t o  
i n d i c a t e  t h a t  reduced s h i e l d i n g  does r e s u l t  i n  improved EOL s p e c i f i c  power 
i n  the  cases s tud ied  t o  date. Fu r the r  reduct ions i n  harness mass w i l l  be 
requi red,  t hus  making even h igher  a r r a y  opera t ing  voltages, i n  t h e  order  o f  
1000 V, necessary. Al though no t  discussed i n  any great  d e t a i  1, concepts 
such as concent ra t ion  and more e f f i c i e n t  o p t i c a l  coupl ing,  which would 
reduce the  b lanket  opera t ing  temperature, were deemed appropr ia te areas f o r  
development. 
k e t s  (>400 W/kg EOL) would be ava i lab le ,  bu t  i t  was agreed t h a t  work aime-d 
a t  developing t h i s  technology should begin immediately. 
The members f e l t  t h a t  
For 
It was n o t  poss ib le  t o  accura te ly  f o r e c a s t  when advanced blan- 
403 


Blanket technology workshop report

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