Tankage systems for a methane fueled supersonic transport

N A S A TM X- 52378

N A S A T E C H N I C A L

t

M E M O R A N D U M

TANKAGE SYSTEMS FOR A METHANE FUELED

SU PERSON IC TRAN S PORT

by Joseph D. Eisenberg and Rene E. Chambellan

Lewis Research Center

Cleveland, Ohio

TECHNICAL PAPER proposed for presentation at Meeting on

Aircraft Design for 1980 Operations sponsored by the

’

American Institute of Aeronautics and Astronautics

Washington, D. C . , February 12- 14, 1968

\

\ NATIONAL AERONAUTICS A N D SPACE ADMINISTRATION * WASHINGTON, D.C. 1967

TANKAGE SYSTEMS FOR A METHANE FUELED

SUPERSONIC TRANSPORT

by Joseph D. Eisenberg and Rene E. Chambellan

Lewis Research Center

Cleveland, Ohio

TECHNICAL PAPER proposed f o r presentation at

Meeting on Aircraft Design for 1980 Operations

sponsored by the American Institute of Aeronautics and Astronautics

Washington, D. C. , February 12-14, 1968

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

TA-NKAGE SYSTEMS F O R A METHANE mTELED SUPERSONIC TRANSPORT by Joseph D. Eisenberg and Rene E. Chambellan Lewis Research Center National Aeronautics and Space Administration Cleveland, Ohio Abstract and be l o s t during f l i g h t .

Although l i q u i d methane f u e l promises an eco- The potential benefits afforded by t h e use of nomic improvement, its cryogenic nature r e s u l t s i n methane i n a Mach 3 SST were examined i n r e f e r - ence 2. The airplane configuration used i n t h a t on-board storage problems. A lightweight i n s u l a t i o n is required (1) t o l i m i t heat i n f l u x t o the l i q u i d study and i n t h e present one i s t h e SCAT 15F design methane, and ( 2 ) together with a defrosting system of t h e NASA Langley Research Center. It i s shown t o eliminate external i c e formation. The problem i n Figure 1 together with i t s pertinent data. I f remains t h a t , should t h i s f u e l be loaded i n a satu-. only p a r t of t h e void space J P is used as t h e f u e l , rated condition, as t h e a i r c r a f t climbs and pressure i n t h e wing is required for f u e l storage. If t h e lower-density methane is used, most of t h e available is reduced, much f u e l w i l l f l a s h off. Either using pressurized tanks or subcooling t h e f u e l w i l l solve volume i n t h e wing and fuselage i s used. Seventy With subcooled f u e l a pressurizing percent of the f u e l i s i n thewing, and t h i s requires t h i s problem.

gas is required. Low s o l u b i l i t y gases (e.g., use of some very shallow sections. O f t h e a i r c r a f t helium, neon) have low a v a i l a b i l i t y and may be used for t h e SST t h e SCAT 15F configurations considered only i f completely salvaged. Bladders or stand had t h e l a r g e s t volume available for f u e l storage.

sol- Other a i r c r a f t configurations might have even less pipes t o reduce contact a r e a may be used with uble gases (e.g., nitrogen) or condensible (e.g., volume so that the vehicle would have t o be methane g a s ) pressurizers. Analytical studies stretched i n some fashion with a consequent weight indicate t h a t both t h e pressurized tank and sub- and drag penalty.

cooled fuel approaches, separately or i n combina- estimated i n reference 2 t h a t t h e pas- t i o n , o f f e r potential solutions t o the tankage It w a s p r ob 1 em. senger capacity of a methane fueled a i r c r a f t could be increased by 31 percent and t h e d i r e c t operating Introduction c o s t reduced by 25 percent, compared t o a JP fueled a i r c r a f t . This included t h e benefit of both meth- The trunkline a i r c r a f t operator is constantly ane's higher heating value and i t s greater cooling capacity. This cooling capacity, it was assumed, seeking that airplane t h a t f l i e s f a s t e r and f a r t h e r is i n allowed more turbine blade cooling than is possible with greater econoniy than t h e airplane t h a t with a JP a i r c r a f t and t h i s permitted higher turbine current use. The American version of the supersonic transport now i n development is intended t o provide i n l e t gas temperature which resulted i n l i g h t e r engines.

a 200 percent increase i n speed, with no l o s s i n economy compared tij current a i r c r a f t . This vehicle These gains are a function of t h e f u e l systems w i l l u t i l i z e e s s e n t i a l l y the same gasoline/kerosene- f r a c t i o n which is the weight of t h e a i r c r a f t f u e l type f u e l s (frequently i d e n t i f i e d as JP) now used i n subsonic c r a f t . One way of i q r o v i n g the payload system per pound of f u e l c a r r i e d . Figure 2 displays t h e number of passengers as a function of t h e f u e l f r a c t i o n and t h e economy of follow-on versions of the supersonic transport is t o use a f u e l that is systems f r a c t i o n s and reveals t h a t substantial sys- superior t o JT type f u e l s i n heating value, heat tems weight increases over JP can be accepted with- out losing a l l benefit of methane; although, of sink capacity, cost and a v a i l a b i l i t y , and at the course, t h e l i g h t e r t h e system t h e greater t h e gain.

same t i m e is safer and more dense. Although meeting all of t h e s e requirements appears unlikely, the s t u d i e s reported i n references 1 and 2 have i n d i - It w a s evident from t h e study of references 1 and 2 t h a t t h e CHq offered several important advan- cated t h a t l i q u i d methane is a f u e l t h a t can meet tages t o t h e engine but posed some s i g n i f i c a n t prob- some of these c r i t e r i a .

lems i n terms of f u e l tankage. It is the purpose of t h i s paper t o discuss t h e tankage problems i n Table I compares t h e properties of JT and meth- g r e a t e r d e t a i l than has been previously done and t o ane f u e l s . The heating value of l i q u i d methane i s examine a number of possible solutions t o these 13 percent higher than that of JP and t h e heat sink capacity i s about four times as g r e a t . The range problem.

of flananability and t h e spontaneous i g n i t i o n temper- The Tankage Problem a t u r e suggest no increase i n i n - f l i g h t f i r e hazard.

The tankage problems, as previously ,noted, The p r i c e s of both JP and l i q u i d C q a r e sub- a r i s e basically because of the lower density and t h e j e c t t o debate, but they appear t o be about the same Although not c m - Several aspects of t h i s cryogenic nature of CH4.

on a c o s t per u n i t weight b a s i s , problem w i l l be discussed.

prehensively examined yet, t h e a v a i l a b i l i t y of meth- ane around t h e world is expected t o be as g o d as Although s u b s t a n t i a l increases i n systems that of Jp.

weight can be t o l e r a t e d , nevertheless any design techniques employed t o contain t h e f u e l must not i n - Not a l l of t h e methane properties are helpful.

cur too heavy a weight penalty or t h e potential Its density is only half t h a t of JP, requiring more b e n e f i t s can be l o s t . A penalty t o all systems is tank volume, and its one atmosphere boiling point i s t h a t of t h e pumps and plumbing t o g e t the f u e l from 201' R, more than 300' R b e l m ambient on the t h e tanks t o t h e engines. Assuming use of t h e sme ground, r e s u l t i n g i n a tendency for it t o b o i l away TM X-52378 techniques as those t o be used i n t h e JP SST and i f new.insulations a r e developed, weights w i l l be excluding insulation, from reference 2 , t h i s pen- even l e s s . Another method, a l s o mentioned i n r e f - a l t y is equal t o 2.09 percent of t h e f u e l weight. erence 1, i s t o reduce the insulation t o t h e point I n addition t o t h i s a r e t h e weights of: insula- where the boiloff r a t e a t c r u i s e is j u s t equal t o t i o n , b o i l o f f , pressurizer, tank, and unique sys- c r u i s e engine demand. Vapor pumps could then be tems associated with t h e various storage schemes. used t o pressurize and pump t h e vapor t o t h e en- These weights w i l l be evaluated except f o r those gines. The t o t a l weight penalty, vapor pumps p l u s which require detailed design f o r t h e i r d e f i n i t i o n . insulation, is greatly reduced. The ground hold Since some weights cannot be evaluated within the problem would remain, however. For t h i s discussion, scope of t h i s paper, t h e weights presented a r e used an insulation weight equal t o 2 . 0 percent of t h e only t o show t h e magnitude of t h e p e n a l t i e s associ- roughly 185,000 pounds of methane and an equal ated with each method, and no d e f i n i t i v e comparison amount of boiloff w i l l be assumed f o r all cases of of methods can be made. boiling f u e l . This 2 . 0 percent insulation f r a c t i o n w i l l a l s o be used i n a l l nonboiling cases, with but The unique problems i n designing methane tanks one exception which w i l l be noted.

can be described by reference t o t h e external en- vironment h i s t o r y f o r a t y p i c a l f l i g h t shown i n Pressure C h q e s f i g u r e 3 . One problem r e s u l t s from t h e difference The second category of problem is t h a t a s s o c i - a t e d with t h e reduction i n ambient pressure i n go- between t h e temperature of t h e air adjacent t o t h e s k i n of t h e airplane, which can be as high as the Consider ing from take-off t o c r u i s e a l t i t u d e .

stagnation temperature, and t h e f u e l temperature what occurs i n an a i r c r a f t i f the tankage concept which is 2 0 1 ' Rankine a t one atmosphere. Fig- is t h a t usually used f o r ground storage of a cryo- ure 3 ( a ) shows t h e a i r c r a f t Mach number as a func- genic, an insulated container vented t o t h e sur- i n t o t h e f l i g h t . It is seen that t h e t i o n of t i m e rounding atmosphere. The methane is loaded as a g r e a t majority of t h e f l i g h t is flown a t v e l o c i t i e s saturated l i q u i d i n t o t h e a i r c r a f t tanks, which It greater than Mach 1 with t h e cruise a t Mach 3. are, according t o current plans f o r t h e J P super- is these high v e l o c i t i e s that cause t h e high stag- i n t e g r a l with t h e wing and fuse- sonic transport, Tempera- nation temperatures shown i n f i g u r e 3(b). lage. Since a i r c r a f t wings and thus t h e majority t u r e differences a r e greatest a t t h e supersonic of t h e i n t e g r a l tanks can, i n general, hold a pres- sure d i f f e r e n t i a l of only 4 t o 6 p s i , as t h e air- c r u i s e condition where t h e temperature adjacent t o t h e s k i n can be nearly llOOo R. High thermal gra- c r a f t climbs t h e i n t e r n a l pressure must be reduced thus reducing t h e boiling temperature. The f u e l d i e n t s also e x i s t at take-off when the skin temper- a t u r e , even on t h e coldest winter day i n a polar w i l l then b o i l off a s u f f i c i e n t amount t o reduce its temperature t o t h a t of t h e new boiling point.

area, w i l l be about 200' R warmer than t h e f u e l .

If t h e maximum pressure d i f f e r e n t i a l is 4 p s i , 9 . 0 percent of t h e weight of t h e f u e l would be The other environment problem is t h e reduction l o s t . I f t h e pmping system weight, t h e weight of i n external pressure as t h e a i r c r a f t climbs. Fig- insulation and t h e weight of boiloff from both ure 3 ( c ) presents the a i r c r a f t a l t i t u d e as a func- t i o n of t i m e i n t o the f l i g h t . The climb is seen t o heating and pressure a r e a l l added together and f i g u r e 2 is entered with t h e r e s u l t i n g 0.151 sys- be very rapid with cruise a l t i t u d e being above t e m fraction, it i s seen that t h e passenger gain 70,000 f t . The pressures resulting from t h e alti- tudes a r e shown i n f i g u r e 3 ( d ) . They start at is only 1 4 or about a 7 percent gain above t h e JP SST. Most of t h e potential gains have been l o s t .

14.7 p s i on t h e ground and drop t o about 0.5 p s i at c r u i s e a l t i t u d e .

Possible Tankage Systems Temperature During pre-take-off ground hold, a t an average It is necessary, then, t o design the tankage s y s t e m t h a t w i l l minimize the weight p e n a l t i e s . A e a r t h surface temperature, t h e f u e l is about 300° colder than t h e ambient temperature. This can numbel- of possible systems a r e categorized and pre- sented i n t a b l e 11. These systems a r e divided i n t o cause two problems. The wing surface may be cooled below t h e freezing temperature of water causing i c e two major categories describing t h e condition of it. This can be countered by insula- t h e methane when loaded aboard t h e a i r c r a f t as formation on e i t h e r a saturated l i q u i d or a subcooled l i q u i d .

t i o n and an e l e c t r i c deicing system.

The subcooled systems a r e f u r t h e r subdivided ac- cording t o t h e type of pressurizing gas or t h e The second problem, as previously noted, is t h e heat flow potential i n t o t h e tank. Insulation method of pressurization used.

is required here t o reduce t h e r a t e of heat flow Saturated Liquid Methane in order t o prevent excessive f u e l evaporation The f i r s t case, under Saturated Liquid Methane, loss. Since t h e problem of heat p o t e n t i a l is ac- that of venting the vapors overboard, is t h e system centuated a t cruise, protection must be adequate used above t o show t h e magnitude of t h e problem.

f o r t h e e n t i r e f l i g h t .

In figure 4, from reference 1, t h e use of in- It may be conceived t h a t t h i s evaporated f u e l could be reliquefied by r e f r i g e r a t i o n , but prelim- sulation t o c o n t r o l f u e l vaporization is shown.

b inary estimates of t h e system weights and t h e power These curves indicate that t h e r e i s a mlnimum t o t a l weight of i n s u l a t i o n plus evaporated f u e l . The demands associated with t h e required r a t e s i n d i c a t e t h a t these penalties would f a r exceed t h e penalty minimum penalty is 3500 pounds of boiloff from heating u t i l i z i n g 3500 pounds of insulation. I n incurred i n accepting t h e boiloff i t s e l f .

t h i s example, t h e physical c h a r a c t e r i s t i c s of Reference 2 states that It is a l s o possible that t h e boiloff could be s i l i c a aerogel were used.

pumped t o and burned i n the engines. However, insulation weight could be as high as 5900 pounds s i n c e t h e greater amount of evaporation is associ- i f p r a c t i c a l i n s t a l l a t i o n problem force t h e use a t e d with the reduction i n pressure due t o climb, of l e s s e f f e c t i v e insulations. On t h e other hand, t h e a i r c r a f t climb path m u s t be s o constrained t h a t e a t i r e volume; and t h i s requirement would be a r e - the boiloff r a t e does not exceed the engine f u e l s t r i c t i o n on the s t r u c t u r a l design.

requirement. The problems associated with pumping and pressurizing t h i s evaporated gas f o r engine use Combination Systems could be formidable; but a t t h e present time, they These methods of handling methane may a l s o be have not been f u l l y evaluated. used i n combination. In one method (proposed i n a NASA L e w i s patent disclosure), methane i s loaded a t One method t o prevent t h e evaporation associ- its normal boiling point i n some tanks and sub- ated with t h e decreased ambient pressure at a l t i - cooled i n others. A stand pipe is used i n the sub- tude is t o provide tanks that can hold one o r more cooled tanks t o reduce t h e area exposed t o t h e atmospheres of pressure. This can be done e i t h e r pressurizing gas and t h i s allows t h e use of warm by strengthening t h e a i r c r a f t structures t h a t con- methane gas. This method w i l l a l s o be discussed i n t a i n t h e f u e l or by using nonintegral, high- more d e t a i l .

pressure tanks. Nonintegral tanks have been de- vised and w i l l be discussed i n more d e t a i l l a t e r . Selected Tankage Systems Subcooled Liquid Methane A s examples of possible systems f o r t h e han- This problem of boiloff during climb can also d l i n g of l i q u i d methane, three a r e now presented i n be completely eliminated by loading the f'uel sub- more d e t a i l . These analyses a r e s t i l l f a r from be- cooled, corresponding t o a lower vapor pressure. ing s u f f i c i e n t f o r design purposes. The areas of Then during climb t h e i n t e r n a l tank pressure can be i n t e r e s t t h a t were beyond t h e scope of t h i s study d i f f e r from system t o system; and they w i l l be lowered t o t h e reduced vapor pressure without caus- ing b o i l o f f . This is basically t h e same s i t u a t i o n noted.

that e x i s t s when loading J P f u e l aboard an a i r c r a f t .

High Pressure Tanks With t h i s method, however, new a t t e n t i o n must be paid t o the s i t u a t i o n that e x i s t s a t take-off and The high pressure tanks of reference 3 i l l u s - low a l t i t u d e s . Here the vapor pressure is lower t r a t e one method f o r using methane loaded aboard than atmospheric and a gas is required t o f i l l any the a i r c r a f t as a saturated l i q u i d and avoiding the voids i n order t o prevent tank collapse and t o evaporation l o s s associated with the ambient pres- pressurize t h e emptying tanks. For JP fuel, t h e sure reduction during climb. I f t h e tank can hold pressurizing gas is normally air. Occasionally, one atmosphere, climb boiloff l o s s is eliminated.

nitrogen is considered. However, neither gas i s However, since heat leaks i n t o t h e tank a l s o cause boiloff or e l s e cause an increase i n i n t e r n a l pres- s u i t a b l e f o r subcooled methane since both oxygen and nitrogen a r e highly soluble i n it, about 1 0 sure, it may be desirable t o have tanks designed t o withstand more than one atmosphere pressure. I f percent by weight i n methane subcooled 25' R ( t a b l e I). The consequent l o s s i n a i r c r a f t per- two atmospheres of pressure rather than one can be is g r e a t . Relatively insoluble gases in- contained, this increase i n pressure is equivalent formance t o having 17' R of subcooling available t o combat clude Ha, He and Ne. Hydrogen i s questioned on grounds of s a f e t y due t o i t s inflammable nature. heating.

H e and N e a r e r e l a t i v e l y r a r e . I f , f o r example, a f l e e t of f i f t e e n hundred 460,000-pound supersonic As noted previously, most of the l i q u i d meth- ane is stored i n the wings. A t y p i c a l wing void transports f l y an average of t h r e e f l i g h t s per b y and use 24 pounds of helium per t r i p , then nearly available f o r f u e l storage is assumed f o r the tank computations. This is e s s e n t i a l l y a rectangular 40 million pounds of helium would be used per year, X 16" wide X 88" long or an amount about equal t o t h a t produced per year prismoid i n shape 24" deep where the length i s i n t h e spanwise direction.

at t h e present time. Thus, i f the scarce gases a r e t o be used, t h e pressurant cannot be allowed t o es- cape. A scheme f o r using He and retaining it w i l l Three types of tanks designed t o f i t i n t o t h i s space, shown schematically i n figure 5, have been be outlined.

studied. They a r e defined as "modified semimono- Another method f o r making use of soluble or coque tanks" composed of a framework of rings and s t r i n g e r s covered by a pressure-tight skin, con- condensable pressurizing gases is t o reduce or eliminate t h e area of gas i n contact with t h e vent i onal "membrane tanks " where the principal l i q u i d methane. The surface could be covered with loads i n the skin a r e t e n s i l e , and "filamentary r e - f l o a t i n g objects such as a l a r g e number of b a l l s or s t r a i n e d membrane tanks" where t h e outer skins of e i t h e r metal o r sealed nonmetallic f a b r i c a r e r e - cans. The tank could be f u l l and a stand pipe used f o r pressurization. Again only a small area is ex- s t r a i n e d by wires or threads attached t o t h e oppo- Or a bladder could s i t e skin. These filamentary restrained tanks a r e posed t o t h e pressurizing gas.

I f these methods be used eliminating a l l contact. c a l l e d unidirectional i f only one p a i r of opposite surfaces is so supported and b i d i r e c t i o n a l and tri- are used, d r y , C% f r e e air, or even warm gaseous d i r e c t i o n a l when two or three p a i r s of opposite methane, could be used f o r pressurization.

sides, respectively, a r e interconnected,'by these In a NASA-Lewis funded project, the use of filaments .

bladders t o separate soluble gases from Cryogenic f l u i d s is being investigated. With movable metal Titanium alloys such a s 6 Al-4V and 5 Al-2 bladders r o l l i n g s e a l s must be developed. P l a s t i c 1/2 Sn were considered f o r the design of metallic must s e a l well, be low i n porosity a f t e r tanks with an allowable t e n s i l e working s t r e s s of bladders '50,000 p s i . A minimum sheet metal thickness of numerous cycles, be easy t o replace, and they must r e t a i n t h e i r mechanical properties including 0.010 inch was assumed. The nonmetallic filament A l l tanks were assumed t o be made from Nomex, Dacron strength and f l e x i b i l i t y from 163' t o 1000° R.

or Nylon yard with t h e external surfaces of t h e of these demands have not yet been met i n any one m t e r i a l . &so, a tank must be clear of any mem- tank sealed with an elastomer which remains p l i a b l e over the range of service temperatures. These bers t h a t could prevent bladders from f i l l i n g the f a b r i c tanks would a l s o require a special protec- crease of 26 percent.

t i o n system t o prevent the temperature of the mate- rials from r i s i n g t o a point such t h a t s t r u c t u r a l In t a b l e I11 and figure 5, fuselage tanks and degradation could occur. All of these separate t h e i r c h a r a c t e r i s t i c s a r e presented. These have tank configurations, it should be noted, guarantee lower tank s t r u c t u r a l fractions than the wing tanks.

separation of the insulation from the f u e l since t h e insulation is placed on the tank exterior. Certain problems unique t o t h i s system and r e - quiring evaluation by examining a s p e c i f i c a i r c r a f t The various tank designs a r e canpared i n t a b l e i n some d e t a i l have not been taken i n t o account.

I11 i n terms of t h e i r volumetric efficiency and t h e These a r e tank i n s t a l l a t i o n weights, plumbing con- r a t i o of tank weight t o t h e contained f u e l weight nection weights, tank r e l i a b i l i t y , inspection, r e - r a t i o ( t h e tank s t r u c t u r a l f r a c t i o n ) . Volumetric placement and t h e e f f e c t s of volume r e s t r i c t i o n s .

efficiency is the r a t i o of the net internal volume However, i f these penalties a r e not severe, t h e of t h e tank t o the net i n t e r n a l volume of the void system c e r t a i n l y offers a promising answer t o space available for f u e l storage. The character- l i q u i d methane tankage.

i s t i c s of t h e tanks were determined for i n t e r n a l of one and two standard atmospheres.

gage pressures No Loss Helium System A l l tanks a r e metallic except t h e two cases spe- In an e f f o r t t o avoid t h e weight penalties as- c i f i c a l l y noted. sociated with high-pressure tanks, a technique was devised t o use helium-pressurized subcooled methane Consider t h e wing tanks. The highest volu- with s p e c i a l provisions t o avoid any l o s s of helium metric efficiency, 99.5 percent i s obtained with t h e throughout the f l i g h t . Although there is a possi- the metallic t r i d i r e c t i o n a l filamentary restrained b i l i t y that helium may eventually be obtained inex- membrane tanks. The lowest volumetric efficiencies, pensively from very low y i e l d sources, t h e approach about 81.5 percent a r e realized with t h e conven- here is t h a t , a s previously noted, helium is a t i o n a l membrane tank, the modified semimonocoque scarce natural resource and must not be wasted.

tank, t h e s i n g l e lobe unidirectional filamentary Using t h e 460,000 pound supersonic transport, c a l - restrained tank, and the nonmetallic f a b r i c f i l a - culations of t h e weight of helium pressurizing gas mentary r e s t r a i n e d membrane tank. The volumetric required versus time i n t o t h e f l i g h t were made for e f f i c i e n c i e s f o r these four tank types a r e all two d i f f e r e n t cases and a r e presented i n figure 6.

about the same because the tank external configura- A constant helium gas temperature of 200' R, a tem- For all the configurations, perature j u s t s l i g h t l y higher than that of t h e sub- tions a r e very similar.

t h e volumetric efficiency is v i r t u a l l y independent cooled methane, and a 5 percent ullage space a r e of tank i n t e r n a l pressure. Tank weights increased assumed. I n i t i a l l y during ground hold, the pressur- k i t h tank pressure for all configurations except i z e r simply f i l l s the ullage spaces t o prevent tank the conventional membrane type tanks where t h e collapse and allow pumping of the f u e l . A s t h e s t r e s s e s were about 50 percent of the maximum al- plane begins its take-off and early climb, the lowable at two atmospheres i n t e r n a l pressure. This pressure remains close t o one atmosphere and helium vias due t o t h e minimum gage assumption. is added from a separate high-pressure storage con- t a i n e r i n t o the emptying f u e l tanks. As the air- In general, tank s t r u c t u r a l fractions run from c r a f t climbs higher, t h e ambient pressure f a l l s at roughly 3 percent a t one atmosphere t o 4 percent at a more rapid r a t e than can be achieved i n t e r n a l l y two atmospheres. Also, tanks with a higher volu- by allowing t h e He aboard t o expand i n t o the empty metric efficiency tend t o have a higher tank spaces r e s u l t i n g from fuel usage. Thus, i f ambient s t r u c t u r a l f r a c t i o n s and these factors may o f f s e t pressure were t o be maintained, helium would have one another i f void space is limited. The actual t o be released.

trade-off between volumetric efficiency and tank f r a c t i o n has not been investigated. This case is represented by t h e dashed curve of figure 6. Here t h e vapor pressure of t h e meth- The b i d i r e c t i o n a l filamentary restrained mem- ane i s assumed t o be negligible ( a s would be t h e brane tank w i l l be used as an example of this high case i f a methane s l u s h could be loaded). Early i n pressure tank system. This tank has a r e l a t i v e l y t h e f l i g h t 2 0 pounds of helium gas a r e required f o r high volumetric efficiency, 93 percent. I f it is pressurization. Later i n t h e f l i g h t , near t h e com- designed f o r 15 p s i i n t e r n a l pressure, it has a pletion of the climb, only 3 pounds of helium a r e tank s t r u c t u r a l f r a c t i o n of 2.92 percent. Adding required. Thus, i n t h i s case a 1 7 pound l o s s of t h i s t o t h e pump and plumbing systems fraction, t h e helium would occur.

2.00 percent insulation fraction and the 2.00 per- .

cent f r a c t i o n of heating boiloff, a systems frac- The a c t u a l method used i s represented by tk t i o n of 9.00 percent r e s u l t s . I f t h i s tank is de- s o l i d curve. Here a t y p i c a l vapor pressure of signed f o r 30 p s i , the temperatures can r i s e from 2.7 p s i is assumed, and no l i m i t is placed on t h e t h e loading temperature of 2 0 1 ' R up t o 2 1 8 ' R. pressure d i f f e r e n t i a l across t h e tank r e s u l t i n g

I

This is s u f f i c i e n t heat sink t o prevent all boiloff from t h e climb. However, the resulting,maximum from heat during ground hold and f l i g h t with an in- pressure d i f f e r e n t i a l across the tank d l i s only s u l a t i o n f r a c t i o n reduced t o 0.67 percent. Thus 5.6 p s i which is within the 4-6 p s i r a k e t h a t the both the pressure boiloff and the heat boiloff a r e basic J p type i n t e g r a l tanks can withstand. Thus, eliminated. The tank fraction, however, has r i s e n no s t r u c t u r a l weight increase is incurred. No t o 3.66 percent, but the systems fraction is re- helium is l o s t during the f l i g h t and t h e helium i n duced t o 6.42 percent. Thus, considering t h e t h e empty tanks can be recovered a f t e r landing.

weight penalty and providing that t h e minimum gages The r i g h t hand portion of both curves indicate cannot be reduced, tanks designed f o r two atmos- pheres pressure a r e superior t o those designed for w h a t occurs i f gas temperature remains constant j u s t one atmosphere. From figure 2, the 6.42 per- Here helium would have t o be added during descent.

cent t o t a l systems f r a c t i o n gives a passenger in- since t h e external pressure is constantly increas- ing. Since, however, t h i s helium would be recov- helium systems f r a c t i o n and possibly a 0.3 percent ered on landing, there is no l o s s problem.

boiloff f r a c t i o n associated with l e t down.

If, further, it i s assumed t h a t the pressure r a t i o f o r This picture, however, was overly simplified compressing the gas is actually 15 with an associ- since it was based on t h e assumption of a constant a t e d loss of 0.4 percent i n s p e c i f i c impulse, then helium temperature. Actually, once a tank empties a 0.4 percent increase i n f u e l is required. A con- and no longer contains any low temperature methane, servatively high method of accounting f o r t h e e f - the temperature of t h e gas w i l l tend t o r i s e rapidly f e c t upon a i r c r a f t performance of t h i s increase i n and cause a correspondingly rapid r i s e i n pressure. f u e l weight is t o assume it equivalent t o 0.4 per- Even i n the nonempty f u e l tanks t h e helium tempera- cent increase i n systems fraction. Taking t h e to- t u r e w i l l r i s e s l i g h t l y since t h e external skin tal of these systems fractions, 5.39 percent, it is temperature r i s e s as t h e a i r c r a f t increases speed. seen from figure 2 t h a t a 28 percent gain i n pas- A method f o r constantly compressing, cooling and sengers r e s u l t s .

re-expanding the helium gas back i n t o the tanks is used t o maintain a constant low helium temperature. U n t i l the weight of t h e controls f o r t h e helium tank and the weights of the ducts, vapor The whole system i n a very schematic form is pumps and heat exchangers of t h e helium system are sham i n figure 7 . The helium gas is i n i t i a l l y re- determined, and u n t i l the r e l i a b i l i t y of the system leased from i t s high-pressure b o t t l e i n t o t h e f u e l is studied, no meaningful comparisons can be made tank ullage space. As it w8pms up it is then col- with t h e other example systems. However, it i s lected, compressed t o reduce heat exchanger size, seen t h a t t h i s method, too, offers a possible solu- and passed through t h e heat exchanger where some of t i o n t o the problem of methane tankage.

the f u e l headed f o r t h e engine is boiled thus cool- ing t h e helium. The helium is then expanded and Combined System reintroduced i n t o t h e f u e l tank i n its cooled s t a t e . Figure 8 presents a system that uses a combina- a turbine which The expansion takes place through t i o n of saturated l i q u i d methane loaded i n t o high- supplies most of t h e work f o r t h e compressor, thus pressure tanks, and subcooled l i q u i d methane with reducing t h e amount of work t h e engines must supply.

methane gas pressurization by means of a standpipe.

I f t h e compression r a t i o is as high as 15, t h e r e - A t takeoff the tanks, represented by A, containing duction i n s p e c i f i c impulse during cruise would be t h e f u e l f o r cruise and descent and t h e reserve fuel, only 0.4 percent. The r a t e of cooling required up about 7 0 percent of t h e t o t a l f u e l aboard t h e air- t o let-down is never more than 49 percent of the c r a f t , a r e completely f i l l e d . This f u e l is subcool- heat sink available from t h e heat of vaporization ed about 3 0 ° , but since there a r e no voids above t h e of t h e f u e l required by the engines, or 10 percent l i q u i d methane, t h e ambient pressure a g a i n s t t h e - of t h e t o t a l heat sink capacity of t h i s fuel.

walls is supported by the nearly incompressible f u e l i t s e l f . Thus, a pressurizing gas with a l l of i t s problems is not required within these tanks.

The s o l u t i o n of t h e heat problem could prove even more severe at let-down even though t h e ambient pressure is increasing, and, therefore, an increase In order t o control the i n t e r n a l pressure dur- i n i n t e r n a l tank pressure and thus an increase i n ing that portion of t h e f l i g h t i n which the ambient gas temperature can be tolerated. The d i f f i c u l t y pressure is greater than t h e methane vapor pressure, a standpipe is used with w a r m methane impinging a r i s e s from the f a c t that i n most descent modes t h e engines a r e cut back close t o i d l e . The heat flow- on the surface of the f l u i d i n t h e standpipe.

Ordinarily i n t h e f l a t f u e l tanks the sloshing of ing i n t o t h e helium, then, could not be removed by t h e f u e l required f o r f l i g h t , and the pressure of t h e f u e l prevents s t r a t i f i c a t i o n and t h e Warm meth- the gas would r i s e more rapidly than t h e ambient ane gas condenses out. However, i n t h e standpipe pressure causing an excessive pressure d i f f e r e n t i a l due t o its small area slosh would not be expected across t h e tank wall. However, i f a powered de- and s t r a t i f i c a t i o n t o occw t o any great extent, scent mode is used, such as using t h r u s t reversers, would, therefore, allow t h e use of t h e Warm gasi- t h i s problem would no longer e x i s t . Even i f the f i e d fuel.

low parer s e t t i n g mode is maintained, there is s t i l l a solution. Fuel could be s e n t i n t o the heat ex- Other tanks, B, contains all of t h e climb changers, boiled and dumped. The amount of f u e l f u e l (about 30 percent of the t o t a l ) at a tempera- required is a flmction of t h e speed with which t h e t u r e of 201O R with a resulting vapor pressure of airframe adjacent t o t h e f u e l cools down following 14.7 p s i . The tanks a r e strong enough t o hold at a reduction i n boundary layer temperature. If t h e l e a s t the one atmosphere pressure thus eliminating a i r c r a f t temperature drop is rapid, 0.3 percent of pressure boiloff from t h i s tank. The one atmos- t h e t o t a l f u e l weight would meet the heat sink re- from these tanks is used t o phere methane vapor quirements f o r t h e l e t dam. pressurize tanks A a t low f l i g h t a l t i t u d e s .

I The t o t a l weight of helium t o be carried a- A t t h e end of climb, the external pressure is board t h e a i r c r a f t is 120 pounds and t h e tank i n lower than the vapor pressure of the subcooled f u e l .

which it is c a r r i e d is estimated t o weigh The vapor pressure is then s u f f i c i e n t t 6 prevent 970 pounds, t h e t o t a l weight being about 0.6 per- tank collapse. During cruise t h e pumps expel t h e The helium is a t f u e l fran the subcooled tanks and send it i n t o t h e cent of t h e gross f u e l weight.

l i q u i d methane temperature. high-pressure tanks.

A t the end of cruise all f u e l remaining is Examinirg t h e system it is seen t h a t although There is s u f f i - t h e s t r u c t u r a l weight increase due t o pressurized stored i n the high-pressure tanks.

c i e n t storage volume i n t h e high-pressure tanks, tanks has been avoided, other weight penalties have been incurred. I n addition t o t h e i n i t i a l since t h e l e t dam f u e l plus reserve f u e l is l e s s than two-thirds the amount of climb fuel. I n order 2 . 0 9 percent systems f r a c t i o n and t h e 2 . 0 percent that the vapor pressure i n the tank w i l l be no l e s s i n s u l a t i o n fraction, there is the 0.6 percent than one atmosphere upon landing, a method f o r weights should be required. Actually these methods heating t h e f u e l stored i n the high-pressure tanks here represent merely the f i r s t probings i n t o t h e problem of l i q u i d methane tankage aboard a super- is included.

sonic transport. It is t o be expected t h a t improve- ments on these systems or t h e invention of b e t t e r This system, then, eliminates any need f o r an systems w i l l occur and actually reduce t h e weight i n e r t pressurizing gas while storing most of the penalties mentioned here.

f u e l i n a subcooled s t a t e i n i n t e g r a l tanks.

only 30 percent of t h e f u e l requires t h e penalty of a It may be concluded then, that although there high-pressure tank. Assuming that t h e high-pressure a r e no d e f i n i t i v e answers, the r e s u l t s of studies tanks a r e of t h e bidirectional filamentary r e s t r a i n - made so f a r indicate t h a t several approaches a r e ed membrane type designed f o r two atmospheres (dis- f e a s i b l e ; Much research remains t o be done, but it cussed previously), the tank s t r u c t u r a l fraction appears t h a t t h e tankage problem w i l l not prevent and t h e i n s u l a t i o n f r a c t i o n f o r t h e a i r c r a f t a r e t h e use of l i q u i d methane i n future a i r c r a f t .

1.10 percent and 1.60 percent, respectively. Add- ing these t o t h e 2 . 0 9 percent s y s t e m f r a c t i o n References r e s u l t s i n a t o t a l of 4.79 percent, j u s t s l i g h t l y lower than t h e helium pressurized system, allowing 1. Weber, R. J., Dugan, J. F., Jr., and Luidens, a 28 percent increase i n paylaod.

R. W., "Methane-Fueled Propulsion Systems,'' Paper No. 66-685, June 1966, A m , New York, This combined system has some of the problem N. Y.

areas of both the high-pressure tank system and t h e subcooled system; namely, the weights associated 2. Whitlow, J. B., Eisenberg, J. D., and Shovlin, with t h e i n s t a l l a t i o n of the high-pressure tanks M. D., "Potential of Liquid-Methane Fuel f o r and t h e i r r e l i a b i l i t y , and the controlling of in- Mach 3 Commercial Supersonic Transports," t e r n a l tank pressure and pressurizing methane gas TN D-3471, July 1966, NASA, Cleveland, Ohio.

i n t h e subcooled sections.

3. Chambellan, R . E., Lubomski, J. F., and As i n t h e other systems, a l l of these factors Bevevino, W. A., "Structural F e a s i b i l i t y Study have t o be taken i n t o account i n evaluating and of Preesurized Tanks f o r Liquid-Methane Fueled comparing it. However, here again, is a system Supersonic Aircraft," Proposed N A S A TN.

t h a t appears capable of making t h e use of l i q u i d methane advantageous.

Conclusions Property Fuel From studying t h e application of methane t o Methane J P supersonic transports, it appears that t h e use of =ethane is advantageous from t h e standpoints of Heat of combustion, Btu/lb 21,200 18,750 energy per pound, engine and combustor operation, and heat s i n k f o r cooling c r i t i c a l p a r t s of the Heat sink, Btu/lb 1,100 -250 high speed a i r c r a f t engines. Its p r i c e per pound is at l e a s t as l o w as t h a t of J p fuels. The Spontaneous ignition tem- 1,660 94.0 factors most l i k e l y t o determine whether it can be perature, OR used successfully a r e t h e problems of weight and systems complexity associated with the l i q u i d Lean flammable l i m i t , fuel- 0.028 0.035 methane tankage.

a i r r a t i o Due t o its cryogenic nature, methane is sub- Rich flammable l i m i t , f u e l - 0.095 0.270 j e c t t o boiloff from heat leaks and from reductions air r a t i o i n pressure. Insulation is required t o control the heating r a t e and high-pressure tanks or subcooling Density, l b / f t 3 26 50 can be used t o eliminate boiloff. I f subcooling is used a pressurizing gas is required. Only the Boiling point (1 atrn), % 201 810- scarce gasses such as helium a r e r e l a t i v e l y insol- uble in subcooled methane. I f they are used systems Freezing point (1 atm), OR 163 375 a r e required t o conserve them. I f a soluble or condensible gas is used the contact between the Heat of vaporization, Btu/lb 219 120 pressurizing gas and t h e subcooled methane must be minimized or eliminated. The boiloff or t h e tankage Liquid s p e c i f i c heat, Btu/lb-'R 0.82 0.47 weights cannot be great or t h e potential gains of Liqui methane w i l l be l o s t . Further, the systems cannot Gas s o l u b i l i t y percent by weight.

be s o complex that r e l i a b i l i t y cannot be achieved.

Methane subcooled 25' R: The several a l t e r n a t i v e approaches t h a t have Nitrogen -10 0.02 been examined here involve various degrees of N o v a l i d weight complexity and various weights.

Helium -0.003 0.00005 comparison can be made between one system and another, since some of the factors affecting weight Table I. Fuel Properties require complete designs f o r t h e i r evaluation.

However, these preliminary calculations do indicate a possible increase i n passengers of up t o 28 per- cent. a s u b s t a n t i a l improvement could still Thus be expected even i f some increases i n system Saturated Liquid Methane Subcooled Liquid Methane * l . Vent vapors overboard 1. Nonsoluble, noncondensable pressurizers 2. Reliquefy vapors a. Hydrogen 3. Fwnp vapors t o t h e engine b. Neon 4. Pressurized wing *c. Helium *5. Pressurized tanks 2. Soluble o r condensable pressurizers a. Floating b a l l s *b. Standpipe c. Bladder "Denotes methods employed i n selected example systems Table 11. Summary of Liquid Methane Tankage Systems Wing Tanks Tank Pressure = 15 p s i Tank Pressure = 30 p s i Volumetric Volumetric ~ a n k m e {see f i g . 5) Efficiency Tank Weight Efficiency Tank Weight

d Fuel Weight % Fuel Weight

81.7 0.0318 81.7 0.0417 Modified semimonocoque T r i p l e lobe conventional membrane 81.1 0.0279 81.1 0.0279 Single lobe unidirectional filamentary 81.8 0.0241 81.1 0.0256 r e s t r a i n e d membrane Double lobe unidirectioaal filamentary 91.1 0.0332 91.1 0.0341 r e s t r a i n e d membrane Bidirectional filamentary restrained membrane 93.0 0.0292 93.0 0.0366 T r i d i r e c t i o n a l filamentary r e s t r a i n e d 99.6 0.0322 99.5 0.0446 membrane Nonmetallic f a b r i c filamentary restrained membrane 81.8 0.0282 87.8 0.0556 Fuselage Tanks 99.8 0.0205 99.7 0.0372 Modified semimnocoque Single lobe unidirectional filamentary

79.7 0. o n 0 79.7 0.0127

r e s t r a i n e d membrane Bidirectional filamentary restrained membrane 93.6 0.0162 93.6 0: 0250 Nonmetallic f a b r i c filamentary r e s t r a i n e d membrane 79.7 0.0258 79.7 0.0509 Table 111. Comparison of Various 'Pressurized Tank Configurations All tanks a r e of titanium unless otherwise noted I(- ~

Take off gross weight, Ib

460, OOO

*

Range, nautical miles 3500

Engine

Afterburninc

turbojet

Engine turbine inlet

temperature, O R

JP 2660

Methane 3260

Methane-fueled

aircraft

L c T ._

CD-9475

Figure 1. - Aircraft.

-

,-Methane SST of

VI L W (3, W VI

-

VI m CI \c

-

L W n E

-

z

-

,r Reference JP

200 - a’ a i r c r a f t

0 .02 .04 .06 .08 .10 .12 .14 .16 .18 .20

Fuel systems fraction

Figure 2. - Effect of fuel system weight on airplane payload.

W - L L c W F n ::!!!---1, W E a E W c

0 L!!J 1

(a) Mach number. (b) Temperature.

I-

;t: u c

E 20

a

! - t a

-30 0 30 60 90 120 150 -30 0 30 60 90 120 150

Time i n t o flight, m i n

(c) Altitude.

(d) Pressure.

Figure 3. - Airplane environment.

,-Weight of insulation

-P

Insulation weiqht7. -0 0'

I I I I I I

&2 3 4 5 ,:I03

Insulation weight, Ib

Figure 4. - Fuel insulation and boiloff due to heating.

WING TANKS

Triple lobe conven-

w

Filamentary Restrained

FUSELAGE TANKS

Filamentary re-

Figure 5 . - Pressurized wing tank and fuselage tank configurations.

L/

Maximum tank

Liquid methane pressure

press u re, I d i ffe renti a ,, I

psi psi

I

w

- . =

Climb Cruise Descent

0 10 20 30 ' 120 130 140 150

Time into flight, min

Figure 6. - Weight of 200" R helium pressurizing gas required

in fuel tanks as a function of time into the flight.

Fuel flow -+ Heat exchanger7 / r V a P o r f l o w 7 To engine, *Fuel flow LEmptying tanks of CH4 Figure 7. - No-loss h e l i u m pressurized system.

r Cont r o l valve

201" R methane at

4 e a t d u r i n g c r u i s e

exchanger

Figure 8. - Combined saturated-subcooled l i q u i d methane tankage system.

NASA-CLEVELAND, OHIO E - 4 2 5 9