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journal or Proceedings. Released for general publication upon presentation. Full credit should be given to ASME, the Professional Division, and the author (5).

Potential of Hydrogen Fuel for Future Air

Transportation Systems

W. J. SMALL

Aero-Space Tech nologist, Configurations and Performance Section, HVD

D. E. FETTERMAN

Head, Advanced Aircraft Section, HVD

T. F. BONNER, JR.

Head, Aeronautical Engineering Section, SED

______ ~NAS~_L~o£LeyRe~s~e~a~rc~h~C~e~n~te~rl, ______________________ ___ Hampton, Va.

Recent studies have shown that hydrogen fuel can yield spectacular improvements in aircraft performance in addition to its more widely discussed environmental advantages. Its high heat of combustion permits major increases in engine per­.formance and aircraft range. its large heat sink capacity makes possible the development of long life engines and airframes for hypersonic aircraft through the use of active cooling. Environmentally, hydrogen is unusually clean, and unlike the fossil fuels, the supply is virtually inexhaustible (although energy must be provided for hydrogen production). This paper discusses the characteristics of subsonic, supersonic, and hypersonic transport aircraft using hydrogen fuel and compares their performance and environmental impact to that of similar aircraft using conventional fuel. The possibilities of developing hydrogen-fueled super­sonic and hypersonic vehicles with sonic boom levels acceptable for overland flight are also explored.

Contributed by the Intersociety Committee on Transportation for presentation at the Intersociety C..onference on Transportation, Denver, Colo., September 23-27, 1973.

i Manuscript received at ASME headquarters June 5, 1973.

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Potential of Hydrogen Fuel for Future Air

Transportation Systems

W.J. SMALL D. E. FETTERMAN

INTRODUCTION

One of the more serious problems facing us is the oncoming energy crisis •. Projections of petroleum production and U. S. demands (1)1 are shown in Fig. 1. The demand for oil in the United States has already outstripped domestic production and forced the importation of oil. Because of the depletion of domestic reserves, U. S. oil produ~tion is now declining. By 1985, the United States is expected to im­port over one-half of its petroleum needs (2). The availability of Alaskan North Slope oil will not provide a long-term solution, but will merely delay this decline in U. S. production until the mid 1980's (2).

Transportation now accounts for about one-half the .U. S. oil consumption. Today,

T. F. BONNER, JR.

hydrogen. In the sections that follow, results of our studies of subsonic, supersonic, and hypersonic hydrogen fueled aircraft are described. Each is foreseen to have a place in future markets, and for each, hydrogen provides sub­stantial performance benefits.

HYDROGEN ECONOMICS

Predictions of future fuel costs are very speculative; however, they are an important consideration in aircraft economic comparisons. Projections of future relative costs (per Btu) of JP and hydrogen are shown- in Fig. 2, taken from the data of reference (~). According to the author, these projections include an assumed 2.5 percent per year inflation rate. Recent experience shows that the cost of JP has risen much faster than this assumed 2.5 percent rate; for example, in 1971 and 1972, the average rate

;

I I i

U. S. aviation uses a minor share of total oil consQ~pti6n. However, air travel is projected to expand tenfold by 2000 (3) and would then consume almost one-half of U. S. oil production. Thus, aviation will become a substantial user of this dwindling natural resource.

of in~!ease was over 12 pe~nt (5). ~ Rea.!.~~_...,.......-_ projections of the pric~ of JP fuel derived from

One possible solution to the prOjected fuel shortage is development of alternate fuels. For aviation, a very attractive possibility is

1 Numbers in parentheses designate References at end of paper.

BARRELS OF CRUDE OIL

~

PER YEAR 20

10

WORLD OIL PRODUCTION BASED

U. S. PRODUCTION

crude oil indicate that it may increase fourfold by the year 2000 due to the increased costs of

FllEl PRICE f/BTU

4

3

/JUIz } FROM COAL

/JJP

/:'~ JLH2 (STEAM REFOR~ • /.: NATURAL GAS)

./' /' ./.

.......... . .-- JP DERIVED FROM CRUDE Oil

-.-------- -.~---- ----O~~~~~ __ ~~L_ ___ ~~~~~~

1900 1925 1950 1975 2rol 2025 o~---~------~------~ 1970 1980 1990 21m

Y~R YEAR

Fig. 1 Petroleum resources and consumption Fig. 2 Estimated fuel costs

2

ENERGY: 1 Ib H2 • 2.77 Ib KEROSENE

HEAT SINK: \ H2 (-360Dr - 1600oFI, I

1 Ib H2 • 38.05 Ib KEROSENE KEROSENE (lOOoF _ 4000FI

DENSITY:

1 1t3 H2 • 0.091 1t3 KEROSEN!

SAFETY: COMPARABLE TO GASOLINE

Fig. 3 Properties of liquid hydrogen

domestic and imported oil. A similar trend will likely develop for natural gas, which would rapid­ly increase thS cost of hydrogen manufactured by steam reforming of natural gas. By the year 2000, then, the costs of liquid hydrogen and JP manu­factured from coal would be nearly equal and com­petitive with JP and hydrogen derived from tradi­tional sources.

50000

SPECIFIC 5000 IMPULSE,

sec Ib, THRUST

IIb/SEC'fuel 1 000

RAMJETS

-Stt_~_..:S~"CIRAMJETS , 5OOmm~""~~nru~~~~n7~~~ oeKETS

lOO~ __ ~ __ -L __ ~ ____ L-__ ~ __ -J

o 2 4 6 8 10 12 MACH NUMBER

Fig. 4 Typical propulsion system performance

four times the tankage volume as JP for equal energy (Fig. 3).

The cost of liquid hydrogen will be a Overall, it is the opinion of fuel experts strong function of the technology used to produce that liquid hydrogen should be considered about it and the quantities in which it is used. Liq- as hazardous as methane or gasoline. Hydrogen uid hydrogen costs based on the postulated eco- has much wider flammability limits and lower nomics of electrolysis of water using electrical ignition energy levels than petroleum fuel, but power generated by large nuclear breeder reactors is not explosive under unconfined conditions may also prove cost competitive. Sophisticated (9). Liquid hydrogen has several safetyadvan-manufacturing techniques, such as thermochemical tages. It vaporizes and rises when spilled, water splitting using the heat from large nuclear whereas gasoline or JP fuel evaporates slowly reactors, also hold the promise of producing low and its vapors tend to remain in the area of cost hydrogen (6). the spills. Also, hydrogen flames radiate little

__ .. ~._ -~-Ac-tua-l-l-y~,~e--C.()s.t-(...pe.r---Htu+-of-Uquid~~~~ruhea_t-.Rhen_c~Ql!lt!ared to gasoline flames. The im-

I .

hydrogen need not be as low as jet fuel to be pressive safety record of the space program in economically competitive. Because hydrogen- the handling of liquid "hydrogen is proof that, fueled aircraft require 20 to 40 percent less with proper procedures, hydrogen can be used with energy for a given mission, the two fuels may safety. become cost-competitive sometime in the 1980 l s when JP price is approximately eight-tenths that of hydrogen •

Unfortunately, there have been few stUdies on the overall economics of hydrogen aircraft. References (7) and (8) project that hydrogen aircraft will become profitable when liquid hydrogen costs approach 10 cents a pound (1972 dollars). Further studies will be needed to assess the impact of hydrogen fuel on aircraft development, manufacturing, and operating costs for all speed ranges.

LIQUID HYDROGEN CHARACTERISTICS

PERFORMANCE WITH HYDROGEN

Engine performance, structural weight frac­tion, aerodynam1c perfo~ance, and cruise veloc­ity are four essential items involved in aircraft cruise performance. For a given cruise velocity, liquid hydrogen has the effect of greatly favoring engine performance and somewhat degrading the structural weight fraction and aerodynamics. Large gains in specific impulse (pounds of thrust/pounds of fuel/sec) are achievable with hydrogen fuel, resulting from the increased energy content of hydrogen as compared to JP fuel. "Fig. 4 shows the approximate performance levels that can be

A fundamental attractive property of hy_ achieved by hydrogen and JP engines at their design

I I

dro en as an aircraft fuel is its hi h ener 'l'-:-~~~Lf=l:::::igh~=t-:::v--;e-:::l=o::-c=i=t~y::.'.-:;-;=:0a::f::-:t~h:-:e:-s-,e~v:-a-:cr:-:i:::O=u_ .. s--=e=ng~i:-::ne;-:;;;-"ty~p=e=s~'=-r~~~----! of combustion, two and three-quarters times the turbine power plants are the usual means 0 prop energy per pound as JP fuel. . However. because of sion for subsonic (M < 1) and supersonic speed.s its low density, liquid hydrogen requires almost (1 < M < 4), while ramjets and supersonic combus-

3

I

12000 [~ 10000

2

8000 ~ ~ ~ RANGE, n.mi. 6000 H2 -jp

4000 o--JP

2000

0 4 6 8 CRUISE MACH NUMBER r_11J1.I

Fig. 5 Potential vehicle performance, 750,OOO-lb gross weight, 300 passengers

JP FUEL IN WING

l""[o-----370.5,---.-rotl

cr!'"---:::c:::==-;;;?-::::::::::--=----,§;;J~

JPFUEl TAKE OFF WEIGHT' 1 500 000 Ib

EMPTY WEIGHT' 646 377 Ib

1I QUI D HYDROGEN FUEL TAKE OFF WEIGHT' 915 000 Ib

EMPTY WEIGHT· 521 963 Ib

Fig. 6 Subsonic cargo aircraft, payload = 265,000 Ib, range = 5070 n.m.

LS

TAKE LO OFF

WEIGHT, Ib

.5

o

,-x

f-

f-

•

tin

CURRENT TECHNOLOGY

jp FUEL LH2 FUEL

PAYLOAD

jp FUEL

PAYLOAD

lH2

ONE ONE

SUPERCRlTlCAL AERO. ACTIVE FLIGHT CONTROLS COMPOSITE MATERIALS

FUTURE TECHNOLOGY • r-----~r------~,

JP FUEL LH2 FUEL

PAYLOAO

JP FUR PAYLOAD

LH2

ONE ONE

Fig., 7 Projected advanced cargo transports, Moruise = 0.85, range = 5070 n.m., payload = 250,000 lb

RANGE n.m.

7000

5000 ~ ~,~

",~

"""" "" jp FUEl~ """" LENGTH' 306 It "", EMPTY WEIGHT· 327.100 Ib

I ,~'

.....

~~--~~I~----~I~,~"~'--~I~--~~----~I-----71 6 ~--------·------------------------------------~~~--~.~5--__ ~.6~,~Mi.~7~T1 •. ~8----~.9~ __ ~1~.O~X~1~O ______ ~.

TAKE OFF WEIGHT, Ib tion ramjets (scramjets) are most efficient in the hyperSOnic speed regime (M > 4).

Because of the low density of liquid hydro­gen, the volume of a hydrogen aircraft will gen-erally be larger. than. that of a JP fueled ve-

Fig. 8 Comparative Mach 3 transport performance, 300 passengers, titanium structure

hic Ie. This increased size ana.t'he-added weightr-- Theresul ts shown in Fig • 5 for the Mach of the cryogenic fuel tanks and insulation will 3 to 8 speed range (8) were obtained by optimiz-result in structural weight fractions substan- ing wing loading, fuselage fineness ratio, wing tially above those of JP fueled aircraft (about thickness, engine size, and engine types. Super-50 percent greater for M = 3 aircraft). The sonic hydrogen aircraft achieve maximum range larger volume also contributes a drag penalty.. at about Mach 6 as opposed to JP aircraft which However, the performance of hydrogen engines more optimize at slightly under Mach 3 (10). The than compensates for these disadvantages and Mach 3 transport shown is a "current technology" results in the large range increase relative to titanium aircraft (with modifications incident JP fueled aircraft shown in Fig. 5. to hydrogen fuel) using hydrogen-burning versions

At subsonic speeds, the use of liquid hy- of an SST turbojet engine. The Mach 6 aircraft drogen nearly doubles aircraft range. A 750,000- of this study uses a shielded superalloy struc-Ib hydrogen-fueled subsonic aircraft of approxi- ture and turbOjet/ramjet engines. Refinements mately the weight of today1s jumbo JetJLc.o.ul ... d-------,in-aerodynamic,._structural, and propulsion con­fly nonstop near2y~al~~the world'with cepts in all of these speed ranges will, of 300 passengers. more prac e, alter the details of the3Le preliminar~~ _______ _ probably be designed for a shorter range with a results; however, the basic trends can be ex-larger payload. pected :to remain valid for both fuels.

4,

"

1 ,+ j

1 '/

I 'j

-.

c_

n' 'zwifihiit"tti" 1- r 7"" j(@t brecti' liter r'T;' "if'" ["It_

A P. psf

4

3

2

o

9 Mach 6 hypersonic transport

j

SUBSONIC TRANSPORT _ 4.4HR _--------,.......".,....",

Fig. 11 Implications of low sonic boom

SIDEliNE NOISE. dB

JP FUELED SST GROSS WT =750 IXXllb , I

120 HZ FUELED SST

110 GROSS WT = 510 IXXllb ~

100 WITH CURRENT FAR TAKE OFF REQUIREMENTS

90 WITH RElAXED TAKE OFF

80 REQUIREMENTS ~:p=rG'Q"'

: ~ ... ·-_Fig. 12

f ~\\\\\\\\'tNEARFIELD ~ Sideline noise at brake release. M a } SST

,

2 4

A P-..::o:::>. 1 "I

6 8

reduce the gross weight of the hydrogen aircI'att to approximately that of today's jumbo jets but with,greatly increased payload capability.

·1

--CRUISE-MACI-I-NUM8ER~' __ --:-______ ---;;==~ SUPERSONICA~OrutF'TT~~-~~--~-----_~_

Fig. 10 SOnic boom overpressures, oruise oonditions

SUBSONIC AIRCRAFT

A comparison of "current technology" hy­

drogen and JP fueled SST's are shown in Fig. 8. The use of hydrogen fuel allows a one-third reduction in takeoff weight for an equivalent range. However. for an equivalent mission. the

The characteristics __ of_subsonic -t'current -------hydrogen aircraft's size is somewhat larger and technologytt JP and-hydrogen-fueled cargo ai~craft its empty weight nearly equal to its JP counter-are compared in Fig. 6 (ll). FoI' the speCified part. a result of the low density of liquid hy-mission. there is a reduction in takeoff weight drogen. Alternately. a hydrogen SST could be of 39 percent and ,an empty weight savings of over built for the same gross weight (,750.000 Ib) as 19 percent for the hydrogen aircraft. One obvious the JP fueled vehicle. but with almost one and difference between these two configurations is one-half times the range. Su~h an ~ircraft in the fuel tankage arrangement. where, in con­trast to the conventional practice of plaCing fuel in the wing, the liquid hydrogen is stored in large cryogenic fuselage tanks. ,Although these particular concepts were cargo aircraft, similar results could be expected for passenger configurations. The i~pact of advanced technology (composite materials, active f11ght controls, and supercritical aerodynamics) on these subsonic aircraft has also been assessed as illustrated in Fig. 7. Utilization of these concepts could

would have an obvious ,application to many long­range transpacific routes.

Of course. these supersonic transports would also benefit from the application of ad­vanced technology concepts (8. 10).

HYPERSONIC AIRCRAFT

Hypersonic aircraft, represent the ~ost dramatic departure from existing aircraft tech­nology that we have so far discussed (1. 8. 12-

5

16). Among the new technology requirements for attained that are low enough to be acceptable these vehicles are structures compatible with a for overland flight, vast transcontinental markets high-temperature environment, dual propulsion would open up for supersonic and hypersonic air-systems, and aerodynamic control_()v_~r_a_wide,----oraf~--.As .shown in Fig. 11, a hypersonic trans-speed range. As wilr1)edISCu-;~ed later, hy-' port coast-to-coast flight requires less than drogen is the only fuel that has been seriously 2 hr instead of over 4- hr by conventional jet. considered for cruise in this speed regime be- This includes time for overwater acceleration cause of engine cooling requirements. Fig. 9 and deceleration, since it may not be possible shows a typical hypersonic transport design from to tailor the aircraft for low sonic boom at all reference (12), which uses large nested ff pilloWi speeds. tanks ff and a blended wing body airplane design. NOISE Passengers sit above the tanks in a separate compartment, protected from the cold cryogenic tankage below and from the high temperature out­side environment by insulated cabin walls. Cruise altitudes would be over 100,000 ft, well above any severe weather systems. At cruise speeds of approximately 4000 miles an hour, travel times of only a few hours would separate most portions of the globe. -A 750,OOO-lb gross weight aircraft of this type could fly ~onstop over 6000 miles with a payload of 300 passengers. The applica­tion of advanced technology concepts and postu­_lated improved engine and aerodynamic performance could possibly reduoe aircraft gross weight to approximately 500,000 lb (8).

SONIC BOOM

Hydrogen subsonic aircraft should easily meet noise standards by using high bypass turbo­fan engines with acoustically treated nacelles of the same type successfully developed for 3P

fueled aircraft. For the Mach 3 configuration, studies have shown that dramatic engine noise reductions are possible with a hydrogen aircraft. These studies indicate that a hydrogen SST will optimize with about half the wing loading and substantially larger engines than conventional JP SST's, permitting takeoff with reduced power settings (13). The resulting redu?tion in sideline noise at brake release is shown in Fig. 12. The lOO-dB level corresponp.5 to a takeoff meeting current airline practice. That is, the engine power is not increased in the event of engine failure. The lowest noise level of 85

Commercial overland supersonic flight has dB corresponds to a condition where the major recently been prohibited in the United States portion of the runway is used for takeoff in because of sonic booms. Existing supersonic a very low (20 percent) throttle mode. Should aircraft designs were not compromised for low an engine failure occur, the remaining engines sonic boom requirements and produce the so-cal.re-dl--c::Co=u~laD-e~nc-rea-se-d-4n---pewer----to~make_.j.tILLQr the N-wave far field signatures illustrated in Fig. engine loss. This latter option is available 10. The intensity of such signatures depends to the hydrogen aircraft because of the large primarily on cruise altitude and secondarily on engine thrust available. Achievable takeoff noise aircraft weight. As Mach number increases, levels probably lie between these two extremes. cruise altitude also increases and the far field For hypersonic transports, the takeoff boom levels drop significantly (Fig. 10). Thus, turbojets are typically sized transonically and hypersonic aircraft tend to have the lowest are completely divorced from the cruise scramjet. sonic boom levels. However, if sonic boom re- This will allow a greater degree of freedom in quirements are brought into the design trades designing for both takeoff requirements and from the outset, the aircraft may be shaped to cruise efficiency with possible reduced takeoff produce the more complex "near-field" signature noise. (also illustrated in Fig. 10), and sonic boom levels on the ground may be substantially reduced for Mach 3 aircraft as well (17).

AIR POLLUTiON

Research into such possibilities is cur- The clean burning characteristics of hy-rently underway. The compromises required to drogen should contribute significantly to the achieve decreased boom levels may result in per- alleviation of atmospheric pollution by aircraft. formance losses. Hydrogen SST's. however •. may Hydrogen engines do not emit carbon dioxide, be able to trade some of their superior perform- carbon monoxide, unburned hydrocarbons or par-ance for lower boom levels and still provide ticulate matter, the only products of combustion

----~p~e~r~f~o)]r~maruflnmc~ealc~o~mmp~a~r~a~bbIlee-~GO~JJPP-S5S&TT-ddee554i~g~n~s~n~Q~t~c~o~n~-~-~b~e~i~n~g~water vapor and nitrous oxides (NOx )' figured for low boom. Should boom levels be NOx emissions are reduced since hydrogen is in-

6

.))sW-(;

BlOCK TIME. hour

rr we

14

O~--~----~----~----~ 2 4 6 8 lOx 103

RANGE. n. mi.

Fig. 13 Typical trip times -- transport aircraft

tr.oduced into the burners of turbine engipes as a gas, thus obtaining a uniform, 1ean-fue1-air distribution. The resulting reduction in tem­perature and residence time of air in the com­bustion zone could substantially reduce nitrous oxide emissions. Overall, hydrogen's impact on air pollution around airports and for flight within the troposphere should be less than any other known fuel. However, the effects of NOx and water vapor in the stratosphere, where super­sonic and hypersonic aircraft must cruise, is not yet firmly established and are the subject of current extensive government research programs (18) •

TYPICAL FUTURE HYDROGEN-FUELED AIRCRAFT

Fig. 14 World air traffic flows, billions of passenger miles in scheduled services, 1970

Fig. 15 Hydrogen system for B-57 airplane

hypersonic (Mach 5+) aircraft, with the subsonic -- ----- -~How wny--tne var:lous-ai;rc"I'at't-vTe-have-be-en-------a-i~c_ra.f'__t-f._l-y.ing----"-shor.t-ha~-pas.s-eng.er_r_QYte~~~~~~--I

discussing fit into the world airline picture and possibly long-range freight routes. Super-toward the end of this century? Obviously, economics and speed will play an important part in the selection of ~hese aircraft.

Speed has historically been the essential ingredient in the appeal of air transportation. Higher speeds not only increase the attractive­ness of air travel, but also increase aircraft productivity. The·effect of aircraft speed on trip time is shown in Fig. 13. A hypersonic transport (HST) will cut travel time by more than 50 percent compared to a Mach 3 SST at ranges greater than 5500 miles. A nonstop 6500-n.m. trip from Los Angeles to Sydney takes about 2.4- hr by HST versus 5 hr by SST, 14- hr by sub­sonic jet. At distances less than 3000 n.m., there is no great difference in time between a hypersonic or a supersonic transport, and the aircraft employed will be based on economic and environmental considerations, such as sonic boom

sonic and hypersonic aircraft will probably dominate the intercontinental overwater routes. Should supersonic and hypersonic aircraft with boom levels acceptable for overland flight prove feasible, then the resulting use of transcon­tinental routes would sharply increase the eco­nomic viability of these aircraft. The present substantial flow of traffic in short-, medium-, and long-range routes (Fig. 14-) (19) will by the year 2000 be approximately increased tenfold (3). Somewhat higher growth rates are projected for the long transpacific routes than for the shorter transatlantic routes (19).

TECHNOLOGY STATUS

Propulsion

~----~l~i~m~,i~t&&i&ns on ~vepland flights·

The technology required for hydrogen-fueled turbojet engine development is essentially di­rectly transferable from JP technology. Experi­ments with hydrogen turbojets in the 1950 1 ;; ..

clearly showed the compatibility of hydrogen

I I

Indications are then that we could see a mix of subsonic, supersonic (Mach 1 to 4), and

7

F~~TSlNK: I ENGINE HEAT LOAD

2~_//

o 5 10 MACH NUMBER

15

Fig. 16 Available heat sink in hydrogen; fixed geometry scramjet

Fig. 17 Langley Hypersonic Research'Engine, structural assembly model

with standard aircraft engines. In 1957, a B-57 was successfully modified to burn hydrogen in one of its two J-65 engines (Fig. 15). Liquid hy­drogen was stored in the lett wing tip tank and pressurizing helium in the right tip tank. The liquid hydrogen was vaporized in a ram air heat exchanger' prior to being injected into the engine combustor. This test program, conducted by the Lewis Research Center, demonstrated both the practicality and the performance of hydrogen fueled turbojets (20). During this same time period, the Pratt and Whitney Aircraft Corpora­tion developed two engines to bUrn hydrogen, a modified J-57 turbojet and an Expander 'Cycle (21) turbojet for supersonic high altitude applica­tion. Instead of a standard turbine driven by combustion products, this engine used a turbine driven by hydrogen which had been vaporized in a heat exchanger in the engine exhaust duct. After

, t~aseGus hydrogen was injected into the combustor.

As 'shown in Fig. 4, ramjet or scramjet

8

VARIABLE GEOMETRY INLET

ADJUSTABLE DOOR

Fig. 18 Integrated hypersonic propulsion sys­tem installation, fixed geometry scramjet

engines become more efficient than turbojets at Mach numbers greater than about 3.5. At such speeds, rotating machinery is no'longer necessary to compress the air, since very high combustor pressures are available from the inlet alone. Also, the high total temperature of a hypersonic airstream precludes its use for engine cooling. The relative simplicity of ramjets or scramjets eases this problem by eliminating rotating ma­chinery and reducing the internal wetted surface areas. The basic engine duct can be cooled by internally Circulating the fuel through the

'internal surfaces of the engine before injection into the combustor. It is .in this application

.that the large heat sink capacity of hydrogen

•

; 'I

fue1~i-s---so-impor-ta.nt-f-Or-hyper-sWl1.c--fJ.ig,uht,,-,-. ______ _ The hydrogen fuel flow required for engine

cooling is compared to the fuel flow required for thrust in Fig. 16. As shown, fuel in excess of that required for thrust would have to be supplied to avoid overheating of the engine structure at Mach number 11. With hydrogen fuel, excess fuel heat sink capacity would be available at Mach numbers up to 10 or 11 that could be used for cooling other aircr,att com­ponents (see next section).

Several experimental hypersonic engines have been tested, the most advanced of which is the NASA-sponsored Hypersonic Research Engine. (ERE) (14, 15) (Fig. 17). Originally planned for flight testing on the X-15 research aircraft at Mach numbers from 3 to 8, the ERE is capable of operating in either a subsonic or a super­sonic combustion mode. When the X-15 program was terminated, an expanded ERE ground test program was initiated which is still in progress. The engine shown in Fig. 17 is the structural

!

assembly model ot the HRE (22). Hydrogen is ciroulated as a coolant in passages between the

f ! :-;

f' f

I ,. , ! t ~ f £ ~ . f

~

LOWER SURFACE, +2.0 9

1625

1600

Fig. 19 To1pical temperature distribution for radiation cooled hot structure. '", ... 8, a:t ti tude = 90,000 f't

UNIT WEIGHT.

Iblft2

15

10

5 ADVANCED HOT T STRUCTURE

ALUMI NUM COOLED STRUCTURE (SKIN-STRINGER)

·SKIN-STRINGER STRUCTURE (Hot) .l F . 0 4 I' MACH NUMBER

8

~,

t: . Fig. 20 Typical wing planform unit weights .,-- ---.-------

,-i: t i'

inner and outer walls prior to being injected in the combustor. The structural design concept of this flight weight, regeneratively cooled engine has been successfully demonstrated at Mach 7 in tests in the Langley 8-Foot, High Temperatures Structures Tunnel. Performance tests at Mach 5, 6, and 7 of the HRE in both the subsonic and supersonic combustion modes are' currently being conducted at the Lewis Research Center, Plumbrook Station.

In their basic form, neither ramjet nor the scramjet can produce low speed thrust, re­quiring that an auxiliary low speed acceleration system be provided. One approach to this re­quirement involves integrating turbojets with the hypersonic propulsion system as shown in Fig. 18. The turbojet acts as an acceleration device from takeoff through the supersonic speed range, while the scramjet provides supersonic and

+..-----hv~~nnie-~~airflow is provided

I.

I

to the turbojet engines by an adjustable inlet door during acceleration to about Mach 3. The

HEAT EXCHANGER (42 cu ft)

- COOLANT SUPPLY - COOLANT RETURN

Fig. 21 Aircraft cooling system

turbojet would then be shut down, the inlet

" /'

door closed, and acceleration continued on the scramjet engine alone. The scramjet shown in this figure is the Langley Fixed Geometry Scram­jet. which is designed to operate efficiently through a large Mach number range with minimal engine cooling requirements (14, 23, 24).

Structures Subsonic and supersonic (Mach 3) hydrogen

fuel transports could use conventional aluminum and titanium (and possibly composite) construc-tion techniques. The major technological re­quirement for this type of aircraft is the development of practical cryogenic tankage, insura'C:LOrf;--and-purgE!--sys-t'S'lTfs.---Hyp-er-s-O'nrc-ai-r--;----~

craft, however, operate in a high temperature environment. Typical surface temperatures are shown in Fig. 19 for a Mach 8 transport (14) under maneuver conditions. The structure of hypersonic aircraft must be able to cope with these severe temperatures and associated heating rates, reliably and without frequent or exten-sive refurbishment over the long service life typical of transport aircraft. Airline economics dictate that they also be easily maintained and be capable of rapid airport turnaround times. One hypersonic structural Cloncept is the hot structure in which high temperature materials are used in a load bearing structure. Insulation and thin exterior heat shields would be provided where necessary to limit the temperature rise ot the primary structure. These Ilhot struc1{ural ll

concepts demand major state-of-the-art advances ove~ present design and fabrication techniques. As illustrated in Fig. 20, hot structures for h¥pe~onic ~r~ consjderably heavjer 'than the aluminum structure of subsonic aircraft. The use of the "advanced hot structure" (a

9· ,

da inti' j i-an 'wes" "; raw·-

semi-monocoque spanwise-stiffened beaded skin) results in considerably reduced weights relative to conventional skin-stringer hot structure ar­rangements (25).

The long lifetime required for commercial transports makes the development of the "hot structure ll a formidable task. Active cooling systems (14, 26), however, hold the promise of reducing structural weight (Fig. 20) while simultaneously obtaining long life by utilizing for airframe cooling that part of the hydrogen heat sink not required for engine cooling (Fig. 16).

A schematic of an actively cooled hyper­sonic transport is shown in Fig. 21. Hydrogen is not circulated through the aircraft. Rather., a secondary coolant (water glycol in th:i.s case), circulated through tubes integrally formed into the aircraft skin, is used to transport the aerodynamic heat load to a heat exchanger. This heat load is, in turn, tran~erred to the hydrogen fuel on its way to the engines.

The total weight of a cooled structure, including the cooling system qeight (heat ex-

.changers, coolant, pumps, manifolds, etc.) is significantly lower than uncooled structures. Weight savings resulting from active cooling have been estimated to result in a 70 percent increase in payload (14). Advantages of this system include the use of lightweight aircraft materials, such as alurrdnum, titanium, and per­haps advanced composites, the use of existing

to become cheaper as its use increases. At some time within this oentury, hydrogen fuel is ex­pected to become relatively cheaper for air transportation than JP fuel, and contribute much less to noise and air pollution than any other known fuel. Givan the long lead times necessary· for· new aircraft development, the use of hydrogen should merit immediate increased attention by both government and industry to provide a sound technological base from which the aircraft industry can draw.

REFERENCES

1 Hubbert, M. King, "Man's Conquest of Energy, It's Ecological and Human Consequences," Environmental and Ecological Forum, 1970-1971, U.S. Atomic Energy Comm:i.ssion, 1972.

2 flU. S. Energy Outlook - An Initial Ap'­praisal, VoL II, II National Petroleum Council, washington, D. C., 1971.

3 Black. Richard E •• and Stern, John A •• IIExpanding Horizons for Long-Haul Air Tr~nsporta­tion,1I Paper No. 73-14, presented at AlAA 9th Annual Meeting and Technical Display, Washington, D.C., Jan. 8-10, 1973.-

4 Alexander, Arthur D., III, IIEc/Olnomic Study of Fut~e Aircraft Fuels (1970-2000)," NASA TM X-62,180, Sept. 1972.

5 Yaffee, Michael L., IIDOD, Airlines Face Energy Crises,1I Aviation Week and Space Tech­nology, Nov. 20, 1973, p. 54.

II

fabrication and subsystem technologies, and 6 Marchetti, C., IIHydrogen and Energy, II !

__ . __ the JJli.nimiza·Uon.....oLtlle.l'lllal._lLt.l'e.slLan<LwaJ:opage_\ ___ CllemicALRc.onolllY---Engineering ReJT.ieK,----"2'o~L5__________________ Ii

Although a major development program will be No.1 (No. 57), Jan. 1973. required to develop active cooling to a point 7 Peterson, R. H., and Waters, M. H., of flight readiness, all current indications are "Hypersonic Transports ~ Economics and Environ-that this is the most promising concept for long mental Effects," NASA TM X-62, 193, Oct. 1972 • I life structures required for oommercial trans- 8 Becker, John V., and Kirkham, Frank S., i ports. IIHypersonic Transports,lI NASA SP-292, Paper No. !

25, Nov. 1971, pp. 429-445. CONCLUSIONS 9 Chelton, D. B., IISafety in the Use of

Liquid hydrogen has great potential as an aviation fuel, due to its large energy content and cooling capacity, its minimal environmental impact and potentially unlimited supply. Its use as a fuel greatly improves the performance of subsonic and supersonic aircraft. Hydrogen fuel makes possible hypersonio transports which can carry travelers to any portion of the globe within a few hours.

The cost of conventional aircraft trans­portation is projected to rise due to rising petroleum fuel prices and the indirect, but none the less real cost of reducing noise and air pollution levels. Hydrogen fuel is expected

10

Liquid Hydrogen,1I Technology and Uses of Liquid Hydrogen, Chapter 10, Scott, R. B., ed., Pergamon Press, 1964, pp. 359-378.

10 Nichols, Mark R., Keith, Arvid L., Jr., and Foss, Willard E., Jr., "The Second Genera­tion Supersonic Transport, II NASA SP-292, Paper No. 24, Nov. 1972.

11 Advanced Transport Technology Engineer­ing Staff, IIA Fuel Conservation Study For Trans­port Aircraft Utilizing Advanced Technology and Hydrogen Fuel, II LTV Aerospace Corp., NASA CR 112204, Nov. 1972.

12 Jarlett, F. E., "The Hydrogen Fueled Hypersonio Transport,"'paper presented at ASME Annual AViation and Spaoe Conference, Beverly

,

I ;

'"

I K.' t f. t

,

Hills, Calif., June 16, 1968, pp. ~6~-~70. 13 Nagel, A. L., and Becker, John V., "Key

Technology for Airbreathing Hypersonic Aircraft," Presented at the AIAA 9th Meeting and Technical Display, Washington, D.O., Jan. 1973.

l~ Becker, John V., IINew Approaches to Hypersonic Aircraft,1I Seventh Congress of the International Council of the Aeronautical Sciences, Rome, Italy, Sept. 14-18, 1970.

15 Henry, J. R., and MoLellan, C. H., IIAirbreathing La·unch Vehicle for Earth-Orbit Shuttle ~ New Technology and Development Ap­proach, II Journal of Aircraft, Vol. 8, No.5, May 1971, pp. 381-387.

16 Witcofski, Robert D., "Hydrogen Fueled Hypersonic Transports, If Presented at the American Chemical Society Symposium on Non-Fossil Chemical Fuels, Boston, Mass., April 9-l~, 1972.

17 Ferri, Antonio, IIAirplane Configurations for Low Sonic Boom,1I Third' Conference on Sonic Boom, . NASA SP-255, Oct. 29-30, 1970, pp. 255-275."

18 Goldberg, Arnol; "ClimatiC Impact Assessment f'or High-Flying A;1.rcraf't Fleets. II Astronautics and Aeronautics, Deo. 1972.

19 "Studies of the Impact of Advanced Technologies Applied to Supersonic Transport Aircraf't, Task II, Market Analysis, Oral Review," Lockheed Airoraft Corp., NASA Contraot NASl-1l940,

Mar. 7, 1973. 20 Lewis Laboratory Staff, '''Hydrogen for

Turbojet and Ramjet Powered Flight, II NACA RM E57D23, April 26, 1957.

21 Mulready, R. C., "Liquid Hydrogen Engines, II Technology and Uses of' Liquid Hydrogen, Chapter 5, Scott, R. G., ed., Pergamon Press, 196~, pp. 1~9-l80.

22 Airesearch Staff, '''Hypersonic Research Engine Project, Phase II," Structures and Cooling Development, Airesearoh Mfg. Co., NASA OR 112055-112057; April 1972.

23 Henry, John R., and Anderson, G. Y., "Design Considerations for the Airframe-Integrated Scramjet," Presented at the First International Sympoiium on Airbreathing Engines, Marseille, France, June 19-23, 1972.

2~ Henry, John R., and Beach, H. Lee, "Hy_ personic Airbreathing Propulsion Systems," NASA SP-292, Paper No.8, Nov. 1971, pp. 157-172.

25 Plank, P. P., et ale. "Hypersonic Cruise Vehicle Wing Struoture Evaluation," Lockheed Missile and Space Co., NASA OR-1568, May 1970.

26 Helenbrook, R. G.. and Anthony', F. M., "Design ot a Conveotive Cooling System tor a Mach 6, Hy'personio Transport Airframe," Bell Aerospace·Co., NASA OR 1918, Dec. 1971.

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