The analysis on the feasibility for using metal hydrides in the thermal protection system of cryogenic tanks in space was based on the heat capacity of ice as the phase change material (PCM). It was found that with ice the thermal protection system weight could be reduced by, at most, about 20 percent over an all LI-900 insulation. For this concept to be viable, a metal hydride with considerably more capacity than water would be required. None were found. Special metal hydrides were developed for hydrogen fuel storage applications and it may be possible to do so for the current application. Until this appears promising further effort on this feasibility study does not seem warranted

Pitts, William C.

English

NASA Technical Reports Server

PERFORMANCE ANALYSIS OF ADVANCED BPACECRAFT TPS 
Technical Progress Report 
for the period 
March 1, 1987 - October 31, 1987 
Cooperative Agreement NCC2-434 
Submitted to 
National Aeronautics and Space Administration 
Ames Research Center 
Moffett Field, California 94035 
Thermal Protection Materials Branch 
Dr. Howard E. Goldstein, Chief and Technical Monitor 
Thermosciences Division 
Dr. Jim Arnold, Chief 
Prepared by 
ELORET INSTITUTE 
1178 Maraschino Drive 
Sunnyvale, CA 94087 
Phone: 408 730-8422 and 415 493-4710 
Telex: (ESL) 62 44 2780 
K. Heinemann, President and Grant Administrator 
William C. Pitts, Principal Investigator 
9 December, 1987 
(hASA-CR-181532) P E R P C R W A N C E  ANALYSIS OF ~ a a - i 2 w +  
ALVANCED SPACECHAFT IPS Technical  Progress 
f i e ~ o r t ,  1 Mar. - 3 1  Cct. 1987 (Eloret 
Ccrp. )  1 4  F CSCL 01c U A C l a S  
G3/05 01  10990 
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https://ntrs.nasa.gov/search.jsp?R=19880003102 2020-03-20T09:18:23+00:00Z
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I 
O R I G I N S  PAGE IS 
OF POOR Q U A L m  INTRODUCTION 
The National Space Plane, NASP, and other future space vehicles rill 
have large liquid hydrogen, L82, fuel tmks. These rill require efficient 
insulation systems to minimire LE2 boiloff during 8 flight, including the 
critical launch phure. Many rysteur have been proposed that use combinations 
of: various insulation materials external to the tanks, Multi-Layer 
Insulation within the external insulation, and a foam liner inside the tank. 
A new concept, suggested by Howard Coldstein of NASA Ams, is to use a metal 
hydride in the m m e r  of 8 phase change uterirl, IWM, 
structure. This could be very desireable becru6e the cooling rschrrnim would 
be operating directly on the fuel tank. This paper rill give the re~ults of 
,a quick look at the feuribility of using the decolrporrition of metal hydrides 
ld a combined WY and structure d a i n  the insulation system of a liquid 
an part of the tank 
hydrogen fuel tank. 
For this application, hydrogen, Btored within the hydride would be 
released endothermicrlly BS the hydride reaches its critical temperature 
rmge, thus maintaining the tank temperature until all of the hydride has 
decomposed. Although the conapt of using wtal hydrides in fuel tank 
insulation system is apparently new, metal 
for storage of hydrogen for land based fuel csllr. The basic concept of 
storing hydrogen in a hydride is discussed in Reference 1 and shown in Fig. 
1. There the dissociation pressure is shorn u a function of the hydrogen 
ckposition within 8 typical hydride. The temperature remains relatively 
constant ILB the hydride dissociates from rutimtu hydrogen concentration to 
nearly 5er0 concentration. The temperature depend6 on the dissociation 
pressure. This reaction is reversible and generally without hysterisis. The 
numerous hydrides that exist provide 8 ride range of numbers that go with 
hydrides have been suggested 
ORIGINAL PAGE IS 
DE POOR QUALITY 
this typical curve form as well as a wide range of heats of formation. 
The procedure followed in this analysis w a s  to make parametric 
calculations to establish the range of numbers for Pig. 1 that are required 
for a PCM to be effective. Then a search is d e  for a metal hydride that 
satisfies the requirements. Practical problem such as the strength 
requirements for the hydride are not within the scope of this preliminary 
analysis. It was assured that the weight of the fuel tank was the same 
whether or not it was partially corposed of a hydride. 
ANALYSIS MODEL 
This analysis was d e  using a simple one-dirensional del,Pig.2. 
The insulation material used outride the metal hydride PCY is LI-Ooo. A 
liquid hydrogen cmpatible foam is w e d  inside the PCY. The foam is lined 
on the inside with a thin aluminum shell. The total insulation thickness 
@I-Ooo plus fom) is varied in the analysis as well M their relative 
thicknesses. The PCy transition temperature m d  the dissocimtion heat 
crprcity of the PCH are also varied. 
A specid one-diremionl, trmsient heat-flow computer code 1118 
written for this maly~irr . The temperature dependant thermal conductivity of 
LI-OOO WWJ accounted for. For simplicty of the comparative analysis, the 
physical and thermal properties of the foaa were 8ssumed to be the saae as 
those of the LI-Ooo. The bad8 of comparison for the various combinations of 
these parmeters wae the integrated heat flow through the alu~inum liner. . 
A typical heat flux profile for a W P  flying a cruise trajectory ~ 8 8  
irposed on the top surface of the d e l .  
The dependence of liquid hydrogen boilof f weight on total insulation 
thickness is shorn in Fig. 4.  The lower pair of curves are for LI-800 
insulation with and without PCM included. For both cases, the optimum 
insulation thickness is between 0.7 and 0.8 inch. The use of the PCM reduces 
the minimum weight of insulation plus boiloff LE2 by about 20% for this 
case. Most of this weight reduction is in the mount of LE2 boiled off, and 
because the weight of the two ampnets are  comparable, this means that the 
boiloff weight is reduced by almst 40%. The upper pair of curves show the 
corresponding result for LI-2200 inrrulation. The total weight uaving is 
again about 20%, but becawe of the higher density of LI-2#K) (22 lb/ft3 
instead of 0 lb/ft3) the optimum insulation thickness is smaller and the 
amount of LE2 boiled off ir larger. 
The results shown in Fig. 4 were for cases with the PCY at a depth of 
1/4 of the total thickness of the insulation and foam, with the transition 
temperature at 125 F, and with the heat absorbing capacity of the 
decomposing hydride equivalent to a 0.4 inch sheet of ice. The performance 
of the PCM depends on all of these quantities. Some insight on how is shown 
in Fig. 5. The curves rhow the heat fluxes to the alminu~ liner of the tank 
ILB a function of time. The solid curve is for the case of no Pa. When the 
PCY is at a depth of 0.6 of the thicknesu, it is not very effective. For one 
thing, it takes longer for the temperature to r e d  the transitional level. 1 
1 
The major effect, though, is that the transition temperature is fixed at a 
level closer to the liner than it is for the 0.25 inch depth case. Thus the 
thermal gradient at the liner is larger. For the 0.5 level not all of the 
0 .4  inch equivalent ice heat absorbing capacity is utilised. At the 0.26 
level, it is used up near the end of ascent. For half this capacity, the 
__ 
~ . .  . . . . .  . .  _. .. . . ~ . .  . .  . * +  . -1 . 
I 
I 
m G I N A L  PAGE I$ 
‘OF POOP, PTJALTTY 
hydride is fully decompsed early in the ascent. When it is, the heat flux 
rises rapidly to the no PCY levels. The effect of transition temperature 
would appear the same on this figure as does the effect of PCY depth. 
The effect of PCM depth is shorn in more detail in Pig. 6. The 
minumum weights were determined from c u ~ e s  imilar to those of Fig. 4. As 
indicated by Fig. 6 the optimum depth is dependent on the available heat 
absorbing capacity. It is apparent that for all cases, the PQI should be 
placed relatively near the murface for maximum benefit. The higher the 
available heat capacity, the closer the POY should be placed to the surface. 
This trend is rlmst linear u1 shorn by Fig. 7 .  
A typical trend of minimum weight with the trrnsition temperature is 
shown in Fig. 8 .  It is apparent that a low transition temperature is 
,desireable. However, this trend is limited by the fact that the PCY would 
decompose in the prelaunch environment if the transition tsrperature was too 
low. At the high end of the scale the PCY is ineffective because the 
transition temperature is not reached until well into the ascent unless the 
PCY is placed close to the murface. The 
PCM is placed close to the surface. Then, unless the absorbtion capacity is 
very large, the  hydride temperature l i m i t  w i l l  be exceeded when the  
decomposition i u  completed. 
The foregoing results were selected from a set of calculations in 
which the pertinent parameters were systematically varied. The raximum 
weight reduction due to the w e  of PCM was found to be about 2S%. For this 
case the PCM was 0.14 deep within the insulation, the transition temperature 
ww 126 F, m d  the heat absorbtion capacity was equivalent to 0.8 inch of 
ice. This is not a desireable design because the heat absorbtion requirement 
is so large m d  the trnk structure is dangerously close to the surfwe, but 
it does indicate the maximum benefit attainable for this type of heat 
protection system. 
The f i n a l  s t e p  of t h i s  study was t o  search the l i t e r a t u r e  for the  
best  metal hydride t o  match the  indicated requirements. These requirements 
are:  a large heat absorbtion capacity, a low t rans i t ion  temperature, a high 
rate of dissociat ion,  s t a b i l i t y  i n  the  prelaunch enviroruents, and su i tab le  
s t r u c t u r a l  charac te r i s t ics .  Unfortunately, most of the  work on t he  use of 
metal hydrides f o r  hydrogen storeage has been directed toward fue l  cell 
applications where a l o r  heat  absorbtion capacity and a high t r ans i t i on  
temperature is desired (Refs. 1 urd 2).  There are many b i q  and ternary 
hydrides with a wide rmge of charac te r i s t ics .  These charac te r i s t ics  can be 
varied s u b s t m t i r l l y  by t he  the  addition of impurities. 
The two mlrt l ike ly  candidates found were HgNi-H and PeTiH. They both 
,have lor  t r a m i t i o n  temperatures and high rates of decorposition. Both have 
a high storage capacity for hydrogen on a volume basis, but because of t he  
high dens i t i e s  of the  -tal component, t h e i r  capacity on a weight bas i s  is 
reduced t o  near t h a t  of water. The capacity of MgNi-H is h u t  10% higher 
than rater and t h a t  of PeTi-E is h u t  40% lower. There- i s  a l so  a question 
of t h e  s t a b i l i t y  of these hydrides i n  the  vacuum of space. 
I 
CONCLUSION 
The analysis  on the  f e a s i b i l i t y  for wing metal hydrides i n  the  thermal 
protection syster of cryogenic t d s  i n  space wae b e d  on t he  heat capacity 
of ice as t he  PCY. It was found that  w i t h  ice the  thermal protection system 
deight could be reduced by, at  wt, about 20 per cant over an al l  LI-OOO 
insulat ion.  For thilr concept to be viable ,  a metal hydride with considerably 
more capacity than rater would be required. None were found. Special metal 
hydrides were deroleped f o r  hydrogen fue l  storage applications and it r a y  be 
possible t o  do SO for the  current application. Until t h i s  appears promising 
further effort on t h i s  f e a s i b i l i t y  study does not seem rarrented. 
1 .  Hydrogen: Its Technology and Implications, Volure 11, Transition and Storage, 
Chemical Rubber Company Press ,  1977. 
. 
?igure 1. Pha8e change diagram’-for metal hydrider. 
, .  - .,* , 
"EAT LOAD 
1 1 
\ TANK 
Figure  2. O m  diwnmional rodel used for anrlysim. 
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# 07 
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# 01 
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B 
0 
(PCM DEPTH) 
(INSULATION THICKNESS) 
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EQUIVALEIT ICE 
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Figure 5. Dependence of heat flux to tank on model parameters. 
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Performance analysis of advanced spacecraft TPS

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