Document
NASA TECHNICAL N O T E h
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HIGH-ENERGY FUELS FOR
SUPERSONIC TRANSPORT RESERVES
by Joseph D. Eisenberg
Lewis Research Center
Cleveland, Ohio
N A T I O N A L A E R O N A U T I C S A N D SPACE A D M I N I S T R A T I O N W A S H I N G T O N , D. C . M A Y 1 9 6 7 TECH LIBRARY KAFB, N M O L 3 L O L O NASA TN D-3987
HIGH-ENERGY FUELS FOR SUPERSONIC TRANSPORT RESERVES
By Joseph D. Eisenberg Lewis Research Center Cleveland, Ohio NATIONAL AERONAUTICS AND SPACE ADMINISTRATION __ For sale by the Clearinghouse for Federal Scientific and Technical Information
Springfield, Virginia 22151 - CFSTI price $3.00
HIGH-ENERGY FUELS FOR SUPERSONIC TRANSPORT RESERVES
by Joseph D. Eisenberg
Lewis Research Center
SUMMARY
The reserve fuel requirements of the supersonic transports are expected to be about 10 percent of the g r o s s weight of the aircraft, an amount approximately equal to that of the payload consisting of both passengers and baggage. This analysis shows that the re duction in the reserve fuel resulting from the use of high-energy fuels leads to significant increases in payload and reductions in direct operating costs. Since high-energy fuel is used only for reserves (common fuels such as JP fuel o r kerosene, for example, continu ing to be the primary fuel), which are expended infrequently, such costly fuels as liquid hydrogen and ethyldecaborane may be considered. The basic aircraft used in this study is arrow-winged, JP-fueled, has a constant takeoff gross weight of 460 000 pounds, and flies a constant range of 3500 nautical miles. The high-energy fuels used for the re ethyldecaborane, and liquid methane. One case of liquid serves are liquid hydrogen, hydrogen reserves in a methane-fueled supersonic transport, and one case of ethyldeca borane reserves in a JP-fueled aircraft operating at extended range are also investigated.
The results a r e presented as a function of the fraction of main-fuel reserve energy The sensitivity of the results to fuel-system weight is replaced by high-energy fuels.
shown, and the direct operating costs are given for a spectrum of fuel prices.
For aircraft designed to use the high-energy reserves on the design range mission, the gains made with liquid-hydrogen reserves in the basic JP aircraft are the most inter esting. The calculated direct operating cost improvement is nearly 10 percent and, associated with it, is a gain in passengers of over 15 percent when compared with the all- JP-fueled aircraft. If ranges in excess of the aircraft design range a r e required, the decrease in number of passengers and the increase in direct operating cost that results may be reduced by the use of ethyldecaborane in the JP supersonic transport. However, this possible gain does not include any adverse effects on engine performance due to boric oxide deposits.
INTRO D UCTlO N
Current supersonic transport designs use JP o r kerosene fuel. There a r e , however, several fuels with much higher heating values than the JP fuels. A graphic example is liquid hydrogen, which has about three times the heating value. The higher the heating value and, therefore, the specific impulse, the lower the weight of fuel that is needed for a given propulsion energy requirement, and thus, a higher heating value fuel will gener ally yield a larger payload. Unfortunately, the higher heating value fuels usually have a higher cost, and thus, although the payload may increase, the cost p e r passenger may also increase. This increase in cost per passenger rules out the use of the high-energy fuels as the mission o r cruise fuel for commercial aircraft.
High-energy fuels can be used advantageously, however. When the weights of any of the supersonic transport designs a r e examined, the weight of the reserve fuel is ob served to be comparable to the weight of the passengers. Much of the reserve fuel is seldom used, and when it is not consumed, it is essentially dead weight which has, in a sense, merely displaced a sizeable number of passengers. Because part of the reserve fuel is infrequently used, a high energy, but high-cost fuel, may be utilized by replacing part of the reserves with it. By making the specification that all JP-fuel reserves a r e to be consumed prior to using the high-energy fuel, the weight savings is made, and the effects of the high cost of the high-energy fuel a r e minimized. This concept and its ap plication to supersonic transports was originally suggested by Roger W. Luidens of the Lewis Research Center. This principle is particularly applicable to supersonic trans ports since the ratio of reserve weight to payload weight is higher than f o r present sub sonic jet aircraft.
In addition to a high cost, some of the high-energy fuels have low densities that in crease the tankage weight, and low boiling points that increase the weights of insulation.
These weights decrease the potential gains. Only a mission study can determine the existence and size of the net gains.
This report analyzes the effect that the use of high-energy reserves has upon the payload capacity (the number of passengers) and the direct operating cost of a hypothet ical four-engine arrow-wing supersonic JP-fueled transport with a fixed takeoff gross weight of 460 000 pounds, flying a range of 3500 nautical miles with a cruise speed of Mach 3. Ethyldecaborane, methane, and liquid hydrogen a r e considered as reserve fuels. One case of liquid hydrogen in a methane supersonic transport and one case of ethyldecaborane in a JP supersonic transport operating at extended range a r e also inves tigated. A statistical analysis is used to determine the average amount of reserve fuel used. The effect of a change in reserve fuel on major fuel system components, aircraft aerodynamics, and aircraft structures is accounted for.
The aircraft performance and economics calculations in this study a r e based on the information presented in reference 1, which, in turn, was based on the large amount of data generated by the airplane industry during the current supersonic transport develop ment program. The basic configuration as specified in reference 1 is similar to the SCAT 15F vehicle proposed by the NASA Langley Research Center, who provided the nec essary aerodynamic data.
SYMBOLS
A a r e a of tank, f t C specif ic heat, Btu/ (lb) (OR) P aircraft fuel system fraction (total weight of aircraft fuel systems/total weight of FA aircraft (Ft, hwF, h FmWF, m)/(wF, h wF, m> basic high-energy fuel system fraction (fuel system weight f o r high-energy fuel/ Fh weight of high-energy fuel), see eq. (4) basic main fuel system fraction (weight of main fuel system/weight of main fuel) Fm
total high-energy fuel system fraction [Fh + (additional fuel system weight for
Ft, h high-energy fuel/weight of high-energy fuel)] I specific impulse, s e c k insulation conductivity, Btu/(ft) (hr)(OR) lift-to-drag ratio L/D P probability of fuel use greater than w distribution function of fuel use w P difference in temperature AT t thickness aircraft cruise velocity V weight of fuel wF weight of mission fuel W~~ weight of fuel required in excess of WMF AwF W A W F ~ W M F
-
W average w density, lb/ft 3 P
I
T time Subscripts : co arbitrary cutoff F fuel h high energy i insulation main m refers to point of high-energy substitution r reserve S structure available f o r heat sink (referring to temperature) S t total
METHOD OF ANALYSIS
In order to evaluate the effects on payload and direct operating costs resulting from the use of high-energy fuels f o r reserves, a representative supersonic transport and a representative mission are chosen. This transport and mission are the same as the basic aircraft and mission of reference 1 . Modifications to the basic aircraft are made when required by fuel and payload changes. The standard equations of motion and aero dynamics a r e employed in the calculations. Since, in this study, the only changes a r e in the fuels burned, only the assumptions affecting the fuel system a r e discussed in detail.
Mission and Reserves
The basic mission is outlined in table I, and a flight profile in terms of Mach number and altitude is presented in figure 1. The Mach number and altitude coordinates are fixed in all cases until a speed of Mach 1 is reached. At higher Mach numbers the alti tude is increased when necessary in order to keep sonic boom overpressures on the ground no greater than 2 . 0 pounds p e r square foot. After completion of the climb phase, Mach 3 cruise is initiated and flown at the altitudes determined by maximizing the Breguet Range (ref. 2), that is, maximizing the product (L/D)(I). The total length of the flight is 3500 nautical miles, but in one case extended ranges a r e flown.
TABLE I. - MISSION REQUIREMENTS 3500 (4030 s mi) iange, n m i 3.0 Zruise Mach number Sonic boom overpressure limit, Ib/ft 2.0 Climb Cruise 1 . 5 Reserve requirements: Percent of mission fuel Cruise to alternate airport at cruise altitude and Mach number, n m i Mach 0.6 hold at 15 000-ft altitude, min ~~ Brequet cruise Sti I Fi I h
I
/
/
/ / 1 1.5 Mach number Figure 1 . - Typical mission profile. Takeoff gross weight, 460 OOO pounds; sonic boom overpressure limit, 2.0 pounds per square foot; takeoff gross weight per area of wing, 50 pounds per square foot \ \ \ -" 0 .04 .08 .12 Fraction of mission fuel, w Figure 2 . - Probability of reserve fuel use.
The amount of reserves, both main-fuel reserve and high-energy fuel reserve, ex pended p e r flight (when averaged over a large number of flights) must be known. These reserves are calculated by 'making use of a curve of probability of reserve fuel use 2). The abcissa is in t e r m s of the ratio AWF/WMF where WMF is the amount of (fig.
fuel required just to perform the planned mission (i. e. , predicted wind, predicted atmos pheric conditions, and no holding o r flying to alternate airfields), and AWF is the weight of fuel required in excess of WMF to perform the real mission. The ordinate P is the probability of requiring more than a given fraction of excess fuel. This curve is based on the probability curve presented in reference 3, which is based on experience with present-day long-range jet transports and adapted to supersonic transports. This figure shows that the frequency of use of the reserve drops off rapidly as AWF/WMF increases.
For example, 63.1 percent of all flights require more than the nominal fuel load to com plete the mission. An additional 10-percent fuel load, however, is adequate for all ex cept 2 percent of the flights. On the average flight, about 11 percent of the reserve energy is consumed.
The actual amount of fuel carried by a supersonic transport that is alloted for re serves is calculated from the nominal requirements listed in table I, which are specified in reference 4. Changes in the rules concerning reserves have been made by the Federal Aviation Agency from time to time, but these rules are typical for the super sonic transport.
Aircraft
The aircraft used a r e supersonic transport engine-airframe combinations consistent with reference 1. The aircraft configuration and characteristics, as presented in fig ure 3 and table II, respectively, are very similar to the SCAT 15F of the NASA Langley CD-8729 (a) Overall view.
A- A (b) Section detail of t a n k configuration. (Numbers i n parentheses a r e typical dimensions.)
Figure 3. - Layout of 203-passenger all JP-fuel supersonic transport. ( A l l dimensions a r e in ft.1 .
TABLE 1 1 . - AIRCRAFT CHARACTERISTICS
Wing planform area, f t 92 00 Aspect ratio 1.71 Fuselage outside diameter (max. ), in.
12 5 Fuselage length, f t 2 43 Seat pitch, in. 34 Number of seats abreast 5 Engine design airflow, lb/sec 47 0 Research Center. The SCAT 15F and the basic aircraft of this study are both four engine, advanced, fixed- sweep, arrow-wing, supersonic transport configurations. The aero dynamic performance is based on data supplied by the Langley Center. The resulting cruise lift-to-drag ratio for the study aircraft is about 9.2 (ref. 1 ) .
The aircraft has a constant takeoff gross weight of 460 000 pounds and a constant wing loading throughout this study. The wing a r e a and the wing weight a r e thus constant.
Each passenger has a weight of 200 pounds including baggage, and 116 pounds of furnish ings are required p e r passenger. The fuselage length varies with the number of passen gers, and with the fuel volume in some cases. The variation of fuselage weight with length is presented in figure 4. The length increase for passengers is determined by the 34-inch seat pitch and the 5 abreast seating as noted in table II. The increases in fuse lage length required to accommodate the high-energy reserve fuel a r e presented in fig ure 5 .
In connection with this fuselage increase, note that the basic JP-fueled supersonic ,/ /’
r
/
r
I 1 100 0 Fuselage length, ft Figure 4, - Variation of fuselage weight with length.
I
“r
i
: Frai
8 i n .6 . 8 1 on of J P f u e l reserve energy replaced by’l uid hydrogen %action of methane reserve energy replaced by liquid hydrogen (a) Basic case. (b) Fuselage extended for all liquid hydrogen. (c) Methane supersonic transport.
Figure 5. - Extension of fuselage for liquid-hydrogen reserves.
transport shown in figure 2 (p. 6) has a 27-foot-long section of constant cross section fuselage and a 5-foot section of tapered fuselage available f o r fuel storage. The empty fuselage space, like the empty space in the wing, results from the fact that the internal volume of the supersonic transport is determined by aerodynamics and associated engi neering considerations and not only by the volume of the fuel and payload that is to be con tained within it. Figure 5(a) presents the fuselage extension required by the JP-fueled supersonic transport with liquid-hydrogen reserves when 32 feet is available for fuel storage. Figure 5(b) differs from figure 5(a) in that the 32 feet is assumed unavailable f o r liquid hydrogen storage. Figure 5(c) is for a supersonic transport with methane mis sion fuel and liquid-hydrogen reserves.
The engine assumed for this study is an afterburning turbojet. The takeoff thrust-to weight ratio and the aircraft takeoff g r o s s weight are fixed, thus the size of the engines The engines oper is fixed also. The characteristics of the engine are listed in table III.
ate without afterburning until a speed of Mach 1 is reached at about 36 000 feet altitude, full afterburning throughout essentially the entire supersonic acceleration region, and
TABLE I I I . - ENGINE CHARACTERISTICS
TABLE IV. - WEIGHT BREAKDOWN FOR SUPERSONIC TRANSPORT WITH Number and type a4 Design compressor pressure ratio 1 0 ALL JP FUEL Design turbine inlet temperature, OF 2100 Maximum compressor bleed air for 6 . 6 Fuel weight, lb: turbine cooling, percent Takeoff and climb 60 304 Maximum afterburner temperature, OF Cruise 101 810 Design compressor efficiency 0.875 Letdown 5 22: Turbine efficiency .88 Reserves 34 454 Primary combustor efficiency .98 Total fuel weight Augmentor combustor efficiency 201 791 .93 Inlet pressure recovery at Mach 3.0 Component weight, lb: 0.850 Wing and verticle stabilizers Engine design airflow, lb/sec 82 879 Fuselage 24 842 aAfterburning turbojets.
Installed engines 44 641 Fuel system 4 217 Landink gear 20 056 Hydraulic and electrical system 6 831 Surface controls 4 007 Passengers and baggage 40 600 Furnishings, electronics, 30 136 passenger services, crew, crew baggage, emergency equipment, air conditioning, etc.
rakeoff, gross weight 460 000 TABLE V. - FUEL CHARACTERISTICS Characteristic JP fuel Ethyldecaboran Methane Liquid hydrogen Lower heating value, Btu/lb a18 60C 3 6 450 b21 000 c49 900 Density, lb/ft 5c 26 4.4 Typical cost, $/lb D. 0 184e 2.00 0.01846 0. 50 Ih/Im Relative specific impulse, 1. oc 1.23 1.12 2.70 Boiling or decomposition point at 81 a 660 20 1 37 1 atmosphere, O R Freezing point at 1 atmosphere, OR 37 5 445 164 Specific heat of liquid, c Btu/(lb)(OR) 0.47 0.7 0. 82 1.75 P' References 1 and 9 7, 8, and 9 ., 9, and 1 0 5, 6, 9, and 1 0 - aAt 536. 7 ' R.
bAt 201'b.
'At 36.7' R.
partial afterburning during Mach 3 cruise in o r d e r to satisfy the maximum Breguet range condition.
A weight breakdown f o r the basic JP-fueled aircraft flying the basic mission is pre sented in table IV. Note that the weight of reserves is 34 400 pounds, which is close to that of the total payload, passengers plus baggage. The fuel system weight is 2.09 per cent of the total weight of fuel.
F ue I C ha rac t e r i st ics
The fuels considered are JP, liquid hydrogen, ethyldecaborane, and liquid methane.
The JP fuel is the main fuel throughout the majority of this study. One case using meth ane as the main fuel is considered, however.
The characteristics of these fuels, which are taken from reference 1 and references 5 to 10 a r e listed in table V. All fuels considered have higher heating values in Btu per pound than does JP fuel as shown in table V.
Ethyldecaborane is a room-storable fuel that is slightly more dense than JP.
The combustion products of ethyldecaborane include boric oxide, a viscous solid that has been found to reduce drastically the performance of turbojet components.
However, such losses are ignored in the present study. Methane and liquid hydrogen a r e cryogenics, methane having about one-half the density of JP fuel and liquid hydrogen having about one-twelfth the density of JP fuel. The costs of the high-energy fuels a r e , in general, greater than the cost of JP fuel.
is a function not Although the actual change in engine performance f o r different fuels only of fuel but also of flight conditions, calculations based on real gases using the tables and methods of reference 9 indicate typical relative values of specific impulse I which will be applied to all engine conditions in this study. Implicitly assumed in these calcu lations a r e the assumptions that the engines operate with unchanged component efficien cies and net thrust when fuel is changed, and that dual fuel system components are pro vided when required to allow inflight changing of fuel. The resulting ratios of the high- energy fuel specific impulse to that of JP fuel are listed in table V.
From high specific impulses, all gains a r e derived; f o r the higher specific impulse means less weight of fuel required. A first approximation of the weight of high-energy reserves required is given by the relation Im
- w
F , m , r - W F , h , r 'h is the high-energy reserve weight required to replace a weight of main- where W F,h, r reserve fuel W * and Im and Ih are the main and high-energy specific impulses, F, m, r' respectively. In the actual performance calculations of this study, however, the changes in mission and reserve energy requirements resulting from the changes in the aircraft from case to case a r e taken into account.
Fuel System
Basic relation. - The fuel system related weights f o r the JP supersonic transport may be conveniently divided into two sections. The first is the engine portion, which con sists of such items as engine fuel pumps and engine fuel controls and is considered a part of the engine weight. Although changes in reserve fuel will force some modifications, the expected weight changes a r e small, and therefore, these engine changes a r e neglected in this study. The second portion is the aircraft fuel system, which consists of the follow ing items: Basic fuel system (all fuels including cryogenics): Pumps Valves Fuel lines and supports Miscellaneous plumbing Electrical components Heat exchangers Tank walls, baffles, sealant Additional fuel system (only for cyrogenic fuels): Insulation Pressure structure The basic fuel system is required for both the storable and cryogenic fuels. The addi tional fuel system is only required for cryogenic fuels.
The additional fuel system weight for high-energy fuels may consist of an insulation weight and a structural weight depending on the particular fuel. Using the definition for (the total fuel system fraction for the high-energy fuel) results in the following ex Ft, pression (see SYMBOLS):
' wS ' wi
FhWF,h Ft,h = wF, h where W is the weight of high-energy fuel and Ws and Wi a r e the weights of addi F , h tional structure and insulation, respectively.
The aircraft fuel system fraction FA then becomes, in terms of Fm and Ft,.,, Ft, hwF, h + FmWF, m (3) FA =--- W F , h + W F , m where W is the weight of main fuel. The quantities Fh, Ws, and Wi a r e computed F, m for each of the several high-energy fuels and f o r the varying amounts of these fuels car ried.
When these quantities a r e substituted into equations (2) and (3), FA becomes a varying aircraft fuel system fraction dependent on the characteristics of the high-energy fuel and on the amount of reserve energy replaced by the high-energy fuel.
Computation of weights. - To determine accurately Fh and thus F would neces t , h sitate a complete detailed fuel system design including pumps, piping, and other similar components. This complete design is beyond the scope of this report. Herein an approx imate method for determining these fractions is derived.
The high-energy fuel-system fraction Fh is determined from Fm by a scaling method based on the model shown in figure 6 (p. 14). In figure 6(a) a twelve tank JP fuel system is shown schematically. All tanks a r e assumed equal both in volume and in sur face area. The fuel is pumped to the four engines from one tank at a time. The reserve fuel is contained in two of the tanks. In figure 6(b), one tank of JP reserve is replaced by liquid hydrogen. An amount of liquid hydrogen having the same energy as a given amount of JP although lighter in weight is about four times as voluminous. Therefore, four tanks a r e required. These reserve tanks themselves and all their components a r e assumed to be the same in size, geometry, and weight as those used for JP fuel. In this case, as presented in figure 6(b), pumping of liquid hydrogen to the engines is now done from all four tanks simultaneously; thus, pumps, lines, etc., associated with each tank handle the same volume of fuel as in the original JP case.
Although only a liquid-hydrogen reserve substitution w a s used in this example, this model can be applied to any fuel used for reserve substitution. With this model, the fol lowing relation for the determination of Fh results: Fh = Fm- pF, m (4) pF, h where p- and p a r e the density of the main fuel and high-energy fuel, respec r , m F, h tively. This equation can be expected to give reasonable results when substantial quanti ties of the reserve fuel a r e replaced by high-energy fuels. This area is the one of inter est because all significant gains in aircraft performance and direct operating cost occur when large substitutions of high-energy fuels are made. In this study, the Fm for JP
I
I All-JP-fueled supersonic transport
7 Component I Fractitigofh;ystem
0.00174 Pumps .00262 Valves .00616 Fuel l i n e s and supports .00102 Miscellaneous p l u m b i n g .00092 Electrical components .00124 Heat exchangers Tank walls. baffles. a n d sealant .00720 0.02090 Total - M a i n f u e l (JP fuel), WF,
I t To engines &el p u m p
% % %
I Reserve = j
J P f u e l
I
1 - M i s s i o n JP fuel, WMF -7 Re- I (a) All-JP-fueled aircraft. Fuel flows to engines f r o m Reserve liquid-hydrogen -ls_erveJ JP reserve t a n k I
/mi[% % % p-J" %
WF, 1 - M a i n f u e l (JP fuel), m - l
To engines (b) JP-fueled a i r c r a f t w i t h about one-half reserve energy in liquid-hydrogen.
Fuel flows to engines f r o m liquid- hydrogen reserve tanks.
Figure 6. - Schematic of supersonic transport f u e l system 0.0209 (ref. 1). The fractional weights of the components making up this Fm are also presented in figure 6. F o r the one case in which methane is used f o r the main fuel, Fm is increased to 0.0396 (ref. 1). Both values for Fm are identical to those in refer ence 1. The values of Fh a r e computed based on these assumptions.
Note that the relation of equation (4) is.used only f o r reserve systems and that it is of Fm used for the aircraft with methane as the possibly a bit conservative. The value main fuel is not obtained by raising the Fm f o r JP by the scaling methods presented herein. The 0.0396 value is from reference 1, and it results when either a heavy insu lation o r a methane vapor pumping system is used. This 0.0396 value is scaled up for the liquid-hydrogen reserves in order to push the Fh f o r liquid hydrogen in the conserv ative direction.
' In using ethyldecaborane the value of FA was assumed equal to that for JP, ethyldecaborane fuel being j u s t slightly more dense than JP and also room storable.
Since liquid hydrogen and liquid methane are cryogenic and since the boiling away of these fuels during the flight would defeat the purpose of their use as reserves, a system of pressurization, subcooling, and insulation is used that eliminates boiloff. The meth ane and liquid-hydrogen reserves a r e stored in the fuselage. The fuselage is designed to hold a pressure of no less than the 5000-foot altitude pressure of 12.2 pounds per square inch absolute required for the passengers and crew, even at maximum altitude.
Therefore, both the methane and liquid-hydrogen reserves a r e also stored at a pressure of 12.2 pounds per square inch absolute at altitudes above 5000 feet and at ambient pres sure at lower altitudes.
A t the beginning of the flight, the liquid hydrogen is cooled to 27' R, just two degrees above its freezing point, the lowest temperature that seems practicable. Sufficient insu lation is then provided to allow a total heat leak throughout the flight so that the vapor pressure of the subcooled liquid will not exceed 12.2 pounds per square inch absolute.
(The insulation thickness ranges from 8 . 5 to 3.0 in. as the fraction of reserve energy replaced by liquid hydrogen varies from 0 . 1 to 1.0). For the storage of the liquid meth ane, an insulation thickness of 1 . 5 inches is used throughout. This amount of insulation is more than sufficient to contain the fuel for the cases considered. The methane is then cooled below its sea level atmospheric pressure boiling temperature in order that the total heat leak allowed will raise the vapor pressure to no more than 12.2 pounds per square inch absolute upon landing. At the end of the flight, the heated fuel must be either recooled aboard the aircraft or off -loaded for recooling, and fuel already subcooled must be.put aboard for the next flight.
An alternate method for handling cryogenic fuels that eliminates the problems of subcooling, such as pressurization and the off -loading, subcooling, and reloading of fuel is to load the fuel at its normal boiling condition into a tank capable of holding the 14. 7 pounds per square inch absolute pressure. The fuel tank pressure is then maintained at 14.7 pounds per square inch absolute throughout the flight, and some boiling occurs due to heat leaks into the tank. The insulation thickness is selected so that the direct oper ating cost is minimized. Although this method is not used as the basic one in this anal ysis, an example with liquid hydrogen using this method is presented.
The equation used to determine the insulation thickness for subcooled fuel is t. = &(ET AT) c P ATsWF The weight of insulation Wi is then I computed from the values of ti, pi, and A. The radiation-equilibrium
I
temperature f o r the fuselage exterior I, as a function of time along the flight is presented in figure 7 . (The fuselage exterior is also the fuselage tank ex 0 140 160 terior.) The characteristics of the Time along flight, m i n insulation used are Dresented in Figure 7. - Aircraft fuselage surface temperature as f u n c t i o n of time along f l i g h t table VI (p. 16).
TABLE VI. - FUEL TANK INSULATION DATA
Silica aerogel powder (ref. 13) Insulation Insulation covering High- temperatur e-resistant plastic f i l m (ref. 14) Insulation density, p , lb/ft3 Insulation thermal conductivity, 0.0114 k, Bh/(ft) (hr) (OR) Air pressure within insulation, 30 (0. 58 psia) mm Hg abs A second pressurization bulkhead is added to each end of the fuselage tank when liquid-hydrogen is used in order to protect further against liquid-hydrogen leakage. The weight of the bulkheads Ws is 169 pounds.
Figure 3(b) (p. 7) indicates the assumed fuel system f o r this study. This fuel sys tem is a multitank system very similar to the schematic model on which the fuel system scaling factors a r e based.
Derivation of Reserve-Use Equations
From table IV (p. l o ) , the amount of reserve that is carried aboard the aircraft due
to the requirements listed in table I (p. 5) is about 21 percent of the mission fuel. From 2 (p. 6), the probability of using more than this amount is 4x10-5. Although the figure amount of reserve carried aboard the supersonic transport is calculated from the reserve requirements given in table I, it is not surprising that the amount of reserve fuel de manded by the requirements has only a small chance of being exceeded on any one flight, since both the probability curve and the reserve requirements of table I are based on ex perience with flying aircraft. The need f o r reserve fuel above that aboard the aircraft would not necessarily mean the loss of the craft. It would probably only mean the neces sity of cutting short the mission and landing at an alternate location.
Using the information given in figure 2, a relation is developed for computing the average amount of main-fuel reserves and the average amounts of high-energy reserves consumed per flight. The stipulation is made that all main-fuel reserves a r e used prior to using any high-energy reserves.
The quantity AWF/WMF is represented by w in this derivation. From refer
ence 11, the average fraction of reserve fuel consumed w, if only one kind of fuel and
no fuel limitations are assumed, may be written where p is the distribution function f o r w the fraction of fuel being used. This is represented a s graphically in figure 8(a).
Figure 8(a) shows that for any w the prob - 0 (a) Graphic representation of equation (6). ability of using more than the fraction w is - m C 3 m
P=LW p d w = - l m p d w
= 5 c c 0 A k a 2 I 3 n 1 z: & 2 0 WO Thus, within this range Fraction of mission fuel, w (b) Two-fuel situation.
- - -
Figure 8. - Distribution of fuel used against @ - P the fraction of reserve fuel consumed.
dw and the average fraction for the reserve fuel consumed f o r the total range of P is from equations (6) and (8) Graphically, the integral represents the a r e a under the curve of P against w (fig. 2, P. 6 ) .
Figure 8(b) illustrates the case of cutting the range of w into two parts at wo to
represent the two-fuel situation. The magnitude of w for the main fuel only Wm is the
a r e a under the curve from 0 to wo. For this case, then, equation (9) becomes
w(-dP) - f w=w (w - wo)(-dP)
w=w Equation (10) reduces to
(w - wo)(-dP) of equation (lo), again with the assumption of no
The integral L=*
fuel limitation, represents the average weight fraction of fuel to the right of wo that would be used if it were still in main fuel. To determine the average weight fraction of high-energy fuel used v h it is only necessary to multiply this integral by Thus, In the actual situation, the reserve fuel is not infinite. There is sufficient reserve fuel, however, so that an arbitrary cutoff wco can be chosen in the area where fuel is still available. This cutoff results in negligible e r r o r s in the .computation of the frac tional amount of high-energy fuel consumed. This wco is used as the upper limit. The computations were actually done by summation. Using wco as the upper limit and the form in which the calculations were accomplished yield the following working equations: w=w w=w
-
A P wm = w A P +w0 cco
w =o w =w and w =w where A P is used for the absolute value of -dP.
To get the actual weights of reserve fuel consumed, im and th of equations
(13) and (14), respectively, a r e multiplied by WMF.
Computation of A i r c r a f t Passenger Load
The basic fixed-gross-weight aircraft, as described previously, is flown by a com puter program through the specific mission, which has also been discussed. Greater amounts of the high-energy fuel a r e substituted incrementally for main-fuel reserves with the necessary aircraft modifications being made in each case. The result is a series of aircraft fuel weights, system weights, and number of passengers. Thus, the payload, o r number of passengers, as a function of type and amount of reserve fuel is obtained.
The following main cases a r e studied: the JP-fueled aircraft with liquid-hydrogen reserves, with ethyldecaborane reserves, and with methane reserves. All three main cases utilize the 32 feet of fuselage length that can accept fuel. One case is investigated f o r the JP-fueled aircraft with liquid-hydrogen reserves with the assumption that no 1 8 fuselage volume is available f o r fuel storage and that fuselage extension is required for all liquid-hydrogen additions.
Also, the methane-fueled aircraft of reference 1 is examined with liquid hydrogen f o r the reserve requirement, and the standard JP aircraft using ethyldecaborane for re serve only for flights of more than 3500 nautical miles is investigated.
TABLE VU. - SUMMARY O F
Computation of Direct Operating Costs
FUEL PRICES Price, The economic value of changes in the aircraft is
I dollars/lb
best shown by the direct operating costs. Included in Main fuel the direct operating cost are the crew costs, fuel and oil costs, insurance costs, direct maintenance of flight 0.01846 JP equipment, and the depreciation of the aircraft. The Methane 0.01846 direct operating costs are computed by the formulas High-energy reserve fuel of reference 12, where the required inputs a r e taken from reference 1 as follows: 118 dollars per pound f o r airframe, 1 . 3 3 million dollars per engine, and a time between overhauls for the engine of 2000 hours.
Since the future costs of high-energy fuels a r e un certain, the high-energy fuels are studied over a spectrum of prices. The main fuels a r e kept constant in price, the JP-fuel price being assumed as 12 cents per gallon. A sum mary of prices used is shown in table VII.
RESULTS
The results a r e presented as the number of passengers and the direct operating cost for various fractions of reserve-fuel energy replaced by high-energy fuel. All but the last section of this report considers aircraft that are designed to use high-energy re serves on the design range mission. Liquid hydrogen, ethyldecaborane, and methane reserves a r e considered in turn. The last section considers the use of ethyldecaborane reserves in a previously designed JP-fueled aircraft flying at a range extended beyond the design value. The complete results are presented graphically in figures 9 to 23 (pp. 22 to 33). A summary of the most important results is presented in tables VI11 and M (pp. 20 and 21, respectively).
N
TABLE VIII. - SUMMARY OF RESULTS USING STANDARD ASSUMPTIONS~
Main Number of Fassenger Main fuel High-energy reserve change fuel ?assengem increase cost, fuel cost, optimized fraction cents/lb cents/lb cost high -energy fraction fuel --- ~ .................... _----_ 1.846 0.0209 0.000 JP 203 Liquid hydrogen Number of passengers .0481 1.000 2 37 0.1675 JP Liquid hydrogen Direct operating cost .0414 .760 231 ,1379 234 .1527 JP Liquid hydrogen Direct operating cost .0437 .845 .0481 1.000 237 .1675 JP Liquid hydrogen Direct operating cost ....................
Methane Methane .0396 . 000
Methane Liquid hydrogen Number of passengers .0668 1.000 Methane Liquid hydrogen Direct operating cost .0581 .700 50 Methane Liquid hydrogen Direct operating cost .0603 .750 Methane Liquid hydrogen Direct operating cost .0668 1.000 5
---
JP Ethyldecaborane Number of passengers .0209 1.000
JP Ethyldecaborane Direct operating cost .0209 . 520 2 00
JP Ethyldecaborane Direct operating cost .0209 . 525 100
JP Ethyldecaborane Direct operating cost .0209 .560 50 --- ....................
JP Methane .0264 1.000 I J P Methane Direct operating cost .0264 1.000 2 1.044 .0260 J P Methane Direct operating cost .0264 1.000 .If 1 1.041 .0288 aStandard assumption: 32-foot length of fuselage available for fuel.
TABLE E. - LONG- FUSELAGE JP FUELED SUPERSONIC TRANSPORT LIQUID-HYDROGEN RESERVES
[No liquid-hydrogen storage space initially available in JP-fueled supersonic transport. ] Parameter that fuel Liquid- Number of Number of Fuselage Fuselage Passenger Direct oper- Direct oper- Direct oper reserve change hydrogen passengers passengers length, length, decrement, ating cost, ating cost, ating cost, ft optimized cost (standard ft passengers/ft cents/seat- savings savings frac assumptions) ,
----- ------
2 96 273 -- Number of passengers 221 2 37 0.696 214 231 280 260 .850 0.0270 0.0998 Direct operating cost 50 217 234 287 265 .773 1.034 .0354 .lo82 Direct operating cost 25 Direct operating cost 5 221 237 296 273 ’ .696 1.011 .0569 .1259
Improvements Using
Liquid Hydrogen
The initial case is that of liquid- hydrogen reserves in the basic JP-fueled supersonic transport. In this basic air craft (fig. 3, p. 7), a 32-foot length of is available for liquid-hydrogen fuselage storage. All liquid-hydrogen is stored in the fuselage tanks, and when required, the fuselage is extended for this purpose, as shown in figure 4 (p. 8). The values for F and FA used in these compu t, h tations are shown by the curves in fig \
.40 L ure 9. The discontinuity between the
basic JP aircraft system fraction and that of the hydrogen system fraction is due to the requirements for bulkheads,
I
insulation, etc., required for even mi nute quantities of liquid hydrogen.
Figures 10 and 11 present the num b e r of passengers and the direct oper ating cost, respectively, as functions of 4: the fraction of reserve energy in liquid . 2 4 .6 1.0 Fraction of reserve energy in liquid hydrogen hydrogen.
The curves based on the Figure 9. - Computed system fractions for JP-fueled su .sonic computed FA in figure 9 (shown as transport with liquid-hydrogen reserves.
solid lines) will be discussed for the most part.
The other curves of higher and lower FA a r e added in order to indicate the sensitivity of the number of passengers and the direct operating cost to a change in the aircraft fuel system fraction.
The curves in figure 10 show that the maximum increase in passengers occurs when the liquid hydrogen replaces all the JP reserves although the fuselage is extended 11 feet f o r fuel and 19 feet for passengers and the FA is more than doubled.
Figure 9 shows that the F with all liquid-hydrogen reserves is 0.406 and that the FA is 0.0481 for t , h this maximum passenger case. The increase in passengers is 34, o r a 16.75 percent increase. From the maximum points in the figure, it may be determined that a 0.01 change in system fraction FA (which is equivalent to a 0.17 change in F ) results in t, h a difference of four passengers. Thus, a sizable gain in passengers can be obtained even for quite high values of FA.
260 In figure 11, the direct operating costs
I I I ' J
A i r c r a f t f u e l system associated with the passenger gains are fraction, presented. The direct operating cost is in FA t cents per seat per statute mile. Curves a r e plotted not only for several values 2 4 of FA but also for several prices of liq
:-y
uid hydrogen. The current cost of liquid n p u t e d 7 hydrogen is about 50 cents per pound. The / 2% results using lower prices a r e included to / .05 show the sensitivity of direct operating VI L al P 221 cost to fuel price changes.
al v) VI The curve based on the computed FA x
-
i
L 0) shows that at the higher costs of liquid / = 21c 5 hydrogen, a minimum point on the curve .09 z
/
exists, but at low liquid-hydrogen cost, -Bas total replacement of reserves by liquid 20( air( hydrogen gives the best result. It may be computed from the minimum direct oper 1 9 ( ating costs that at 50 cents per pound for liquid hydrogen, the reduction in direct operating cost is about 10 percent, and at 18( /' / 5 cents per pound, the savings have risen slightly to 12.6 percent. These a r e both 17( significant gains, and their closeness Fraction of reserve energy ill .iquid hydrogen demonstrates that, in the case of liquid- Figure 10. - Number of passengers for JP-fueled supersonic transport w i t h liquid-hydrogen reserves.
hydrogen reserves in a JP aircraft, the high price of hydrogen does not make its use appear much less attractive. If minimum points of direct operating cost for vary ing FA a r e compared, the determination may be made that a 0.01 increase in FA ) would cause about a 20 percent reduction in direct operating cost savings.
(0. 17 in F t, h.
and thus FA noticeable savings in direct oper Thus, even with higher values of F t, h ating cost could still be obtained.
Although not presented in the curves, if the alternate method of carrying liquid hydrogen at the normal boiling condition is used, the savings in direct operating cost using $0.50 per pound liquid hydrogen is reduced to 6.0 percent and that using $0.05 per pound liquid hydrogen is reduced to 10.6 percent.
The aircraft performance for a given high- energy reserve replacement with different from those presented in the figures can be determined an Fm or F r , h 1.21
T,l I I I
Aircraft fuel svstem fracture;
'!
1. 2; FA \r \ \ ___ 1. 1I 0.0 \ \\ ~ _ _ ~ 1. 11 I ._ ___ Co-. of liquid- E \ v) hydrogen,
- I
\ dollarsllb z 1.u W v)
-
-
VI '., ~- ., c W V - 5- 1.ot 0 V
P
m operating c ._ c -costs m L
t
a , CL c 0 1.02 . 0 5 V \ a , L .
n
\
. o .98 .25 \ .25 . 0 5 . 9 4 .05 . M . 2 5
& Fral
.90
1 1
.86 I .6 . 8 1.0 o n of reserve energy in liquid hydrogen Figure 11. - Direct operating cost for JP'fueled supersonic transport w i t h liquid-hydrogen reserves.
8 1.0 Fraction of reserve energy in liquid hydrogen Figure 1 2 . - Number of passengers for JP-fueled supersonic transport w i t h liquid-hydrogen reserve; fuselage extended for all liquid hydrogen added.
__
l . 7 I
- --Cost of liquid- hydrogen, dollarsllb - _ _ 0. 50 -
I
__ __ \ \
/
__ / .25 __ 1’ L .- __ n - \.
1.02 .05 ~ 1.00 1.0 Fraction of reserve energy i n liquid hydrogen 13. - Direct operating costs for JP-fueled supersonic transport Figure w i t h l i q u i d hydrogen reserves; fuselage extended for a l l l i q u i d hydrogen.
by computing a new FA with the use of the weights in table IV (p. 10) and equations (1) and (3). This FA can then be used in conjunction with the sensitivity curves to find the desired aircraft performance.
Results of calculations made by
I
assuming that the 27-fOOt fuselage length of constant cross section and the 5-foot of tapered cross section a r e not available for liquid-hydrogen storage or passengers a r e presented in figures 12 and 13. Here the fuse - - .70 lage is extended for all liquid hydro gen added. All other assumptions
\
are the same as in the case just are pre discussed. These results
-
sented only for the computed FA.
Figure 1 2 contains the changes in number of passengers, and fig ure 13 contains the direct operat / Figure 12 shows ing cost changes.
that the increase in passengers is now only 18. Thus, nearly half of the gain obtained by assuming avail
able fuselage liquid-hydrogen stor -
age space is lost. From figure 13 fueled a i r c r a f t it may be determined that, in the most probable price range of from . a3 0. 25 to 0. 50 dollars per pound, a . 2 Fraction of reserve energy in l i q u i d hydrogen the maximum direct operating Figure 14. - Computed system fractions for methane-fueled super cost savings is only about 3 per sonic transport w i t h liquid-hydrogen reserves.
cent. This case, however, with the extended fuselage is actually a limit, since there is empty volume in the wings of the JP air craft. Should there actually be no =I z ‘-Basic methane Fraction of reserve energy in liquid hydrogen Figure 15. - Number of passengers for methane-fueled super sonic transport w i t h liquid-hydrogen reserves.
empty fuselage the wings could be utilized. The results might be l e s s appealing than in the case in which considerable fuselage length is available because the wings w i l l require higher insulation weights due to their thin cross section and heavier tank weights to hold a pressure of 12.2 pounds per square inch. The gains would be greater than this bound a r y case, however, since the aerodynamic friction drag and fuselage structural weights would be less.
Figures 14, 15, and 16 refer to a supersonic transport with methane as the main fuel and liquid-hydrogen as the reserve fuel. The methane-fueled aircraft presented in reference 1 is used. Figure 14 shows that the F for all reserves in liquid hydrogen t , h is 0.398. Methane requires more volume than does JP fuel. Therefore, some methane is stored in the previously mentioned 32 feet of empty fuselage, and thus, some of this space is not available f o r liquid-hydrogen storage. From the number of passengers of figure 1 5 and the direct operating costs of figure 16, the fractional gains shown by the computed FA curves are seen to be about the same as those of the lengthened fuselage JP-fueled aircraft with liquid-hydrogen reserves. The smaller difference in specific impulse between methane and liquid hydrogen combined with the necessity of increasing fuselage length make liquid-hydrogen substitution appear far l e s s attractive f o r the methane-fueled aircraft than f o r the JP-fueled aircraft.
I /
l l
Cost of liquid h y d r q 1, d o l l a r i I -
- .- A 0 . x
E nputed f
m
-
G ' A .x
m a m
-
-....
'i .25
c m c
a -- U
4 2 -~
d :E
m 0 U
T-.-?* .02
.05
'I .M
1 .25
4 .05
I . 4 1
I -2
0 . 1 . 3 . 6 .7 1.0 Fraction of reserve energy in l i q u i d hydrogen Figure 16. - Direct operating cost for methane-fueled supersonic transport w i t h liquid-hydrogen reserves.
I m prove m e n t U si ng Et h y ldecabora ne
Figures 17 and 18 present the number of passengers and the direct operating cost, respectively, for a JP-fueled aircraft with ethyldecaborane reserves. A s in the case of liquid hydrogen, the maximum number of passengers occurs with total replacement of the JP reserves. The aircraft system I fraction FA using ethyldecaborane is a I, constant 0.0209 as in the case of JP, and I the curves computed by using this number
-
are discussed. The maximum passenger L m n increase from figure 17 is 14, or 6.9 per z cent. From figure 18, however, the max imum decrease in direct operating cost, which occurs with about one-half of the 0 1 reserve energy in ethyldecaborane, is Fraction of reserve energy in ethyldecaborane seen to be small.
Figure 17. - Number of passengers f o r JP-fueled supersonic transport w i t h ethyldecaborane reserves.
[ I Cost of'eth A i r c r a f t system e c a b r T I l b fraction, FA --- 0.03 . OM9 1.32 ------ . 01 I .- E 1.28 m z 1.24
-
c a l 0
.-
8 1.20 V D c .- 1,16 P 0 : . E 1. 12 ,ai g , t 1.08 / .
1.00 0 .1 1.0 . 3 Fraction of reserve energy in I yldecal pane Figure 18. - Direct operating cost f o r JP-fueled supersonic transport w i t h ethyldecaborane reserves.
This decrease is only a trifle over 3 percent even at the extremely optimistic as sumed price of 50 cents per pound, a price far lower than recent cost estimates that run as high as $20 per pound. Since additional adverse effects due to boric oxide depositions would further degrade this result, this fuel obviously offers no significant operational benefits.
Improveme nt U si ng Methane
The use of methane for reserve fuel in JP-fueled aircraft is presented in figures 19, 20, and 21. While figure 19 presents the computed FA and Ft, h, figure 20 presents the number of passengers, and figure 21 shows the direct operating cost.
From fig u r e s 19 and 20, the maximum passenger increase is determined to be about 3 percent, and it occurs with the total replacement of reserves by methane with Ft, h = 0.564 and FA = 0.0264. Since the price per pound of methane is close to that of JP fuels, the minimum direct operating costs occur when all reserves are methane; furthermore, the reduction in direct operating cost is just about that of the passenger increase, which is very modest.
. . . .
0 . 2 . 4 .6 .8 1.0 Fraction of reserve energy in methane Figure 20. - Number of passengers f o r JP-fueled supersonic transport w i t h methane reserves.
m L L . 2 . 4 .6 .8 1.0 Fraction of reserve energy in methane Figure 19. - Computed system weights for the JP-fueled super sonic transport with methane reserves.
0 . I . 2 . 3 . 4 . 5 .6 . 7 .8 . 9 1.0 Fraction of reserve energy in methane Figure 21. - Direct operating cost for JP-fueled supersonic transport w i t h methane reserves.
I f the aircraft is designed for methane reserves and if the price of methane is actu ally cheaper than JP, then a larger direct operating cost gain can be achieved by burning the methane as mission fuel and keeping part of the higher cost JP main fuel for re serves. If, for example, the price of methane is 0.01 dollars per pound, then the direct operating cost would drop an additional 0.035 cents per seat per statute mile by using the methane reserves in this manner, and the fractional reduction in direct operating cost would be 0.0614 rather than the 0.0288 gain obtained by using the methane only as re serve fuel.
Improvements Using Ethyldecaborane for Extended Range
The preceding sections have dealt with the effort to improve airplane performance at ,the design range. The case is now considered where the nominal design performance of ,the JP-fueled aircraft is presumably adequate, but it is desired to apply the airplane to longer range missions. The longer range is accomplished by reducing the passenger load and by adding an equal weight of fuel. However, the aircraft in this single instance is assumed capable of holding the extra fuel, and the seats a r e not removed. Thus, the air craft remains constant in empty weight, takeoff gross weight, and size.
The improvements obtained by using ethyldecaborane for extended range a r e pre sented in figures 22 and 23. In figure 22 the abscissa is the range beyond the design range in statute miles, and the ordinate is the number of passengers. The curves for all JP, JP reserve completely replaced by ethyldecaborane, and ethyldecaborane substitution for minimum direct operating cost a r e presented. At the design range of 3500 nautical
miles, o r 4030 statute miles (e. g. , New York to Berlin), the all JP-fueled supersonic
transport carries 203 passengers. For the flight from New York to Rome, 336 miles be yond the design range, the all JP aircraft can carry only 160 passengers. Using ethyl decaborane for reserves in an amount that minimizes the direct operating cost for an ethyldecaborane price of 2.00 dollars per pound yields a gain of 1 6 passengers, o r 22c I I I I I I I I All J P f u e l Ethyldecaborane substitution 2 C K e for m i n i m u m direct operating cost - _ _ Ethyldecaborane substitution for maximum passenger load 18C ' ,
(all reserves are ethvldeca- 1
B
16c VI L 0 ) cn c '\ x 14c VI m CL L .a ki 12c z 1 C K 8 C 6c 4c I I Extension of range beyond I30 sta 22. - Number of passengers for JP-fueled supersonic transport a t F i g u r e extended ranges.
- 1 I I I I I I I All JP f u e l Ethyldecaborane s u b s t i t u t i o n for m i n i m u m direct operating costs --- Ethyldecaborane s u b s t i t u t i o n for cost of maximum pass ger load (all
3.80 c reserves a r e el
-
3.40 YI c m a l 2 3.00
=
I
m - .- m > m 2. 60 \ VI c c m V z- 2. M 0 U
k'
m c ._
' ' 1
5 1.80 a , n c % .L 1.40 a 1.00 0 MO 400 600 800 lo00 1Mo Extension of range beyond 4030 statute miles (3500 n mi), s m i Figure 23. - Direct operating cost for JP-fueled supersonic transport at extended ranges.
10 percent. A 21 percent passenger increase is possible if all the reserves a r e in ethyl decaborane. The percent gains a r e larger at the longer range extensions, f o r example, from New York to Athens.
The ordinate of figure 23 is in cents per available seat per statute mile. Here the
direct operating cost on the New York to Rome trip is 1.26 cents per available seat -
statute mile with all JP fuel and only 1.16 cents per available seat per statute mile with ethyldecaborane priced at 2.00 dollars per pound. Although this is a savings of nearly 8 percent, it is still much higher than the 1.072 cents p e r seat mile for the all-JP-fueled if extended range flights a r e required, ethyl aircraft flying the design range. Thus, decaborane reserves could reduce the resulting increase in direct operating cost while increasing, with only minor aircraft modifications, the number of passengers carried.
CONCLUDING REMARKS
This analysis, based on the statistical frequency with which reserves a r e used, has shown (as summarized in tables VIII and IX, pp. 20 and 21, respectively) that high- energy fuels, even though costly, can improve supersonic transport economics. The magnitude of these gains is a function of the reserve that the aircraft is forced to c a r r y by regulation, however, and a major reduction in The Federal Aviation Agency require ments, although unlikely, would substantially reduce these gains. The outstanding case is that of liquid hydrogen used in the JP-fueled aircraft. Both the increase in number of passengers and the reduction in direct operating cost appear very attractive. Note, how ever, that to achieve these improvements in direct operating cost with liquid-hydrogen reserves, the boiloff must be essentially eliminated. This elimination of boiloff appears feasible by subcooling the liquid hydrogen and by using insulation. A question arises as to the safety of liquid hydrogen. Liquid hydrogen, however, has already been chosen f o r use in manned space flight, and it is the projected fuel f o r hypersonic aircraft. There fore, it is quite probable that the dangers associated with the use of liquid hydrogen can be minimized by technical advances.
Using ethyldecaborane as a reserve fuel yields only a small reduction in direct oper ating cost. Since the expected reduction in engine performance caused by the boric oxide deposits is ignored even in obtaining these modest improvements, the use of this fuel does not appear at all attractive.
An aircraft using high-energy fuels only for reserves has one advantage over an Since the mission fuel is still the aircraft using high-energy fuels also as mission fuel.
more conventional JP fuel, should it ever be required, the aircraft could be operated usefully between airports where high-energy fuels are not obtainable. The number of passengers or the range would be reduced in such a case, but use of the aircraft would still be possible.
The idea of high-priced, high-energy fuel being substituted for low-priced, low- energy fuel for the reserves is not necessarily limited to supersonic transport applica tions. In any vehicle in which the reserves a r e sizable, seldom used, and recoverable, this substitution would very probably result in gains in performance and direct operating cost. Indeed, although the low level of current and expected direct operating costs do not make it seem appealing, even the subsonic aircraft could benefit by using this high energy- reserve replacement concept.
Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio, October 26, 1966, 126-15-02-02-22.
I
REFERENCES
1. Whitlow, John B., Jr. ; Eisenberg, Joseph D. ; and Shovlin, Michael D. : Potential of Liquid-Methane Fuel for Mach 3 Commercial Supersonic Transports. NASA TN D-3471, 1966.
2. Dommasch, Daniel 0. ; Sherby, Sydney S. ; and Connolly, Thomas F. : Airplane Aerodynamics. Second Ed., Pitman Publishing Corp., 1957.
3. Stickle, Joseph W. : Consideration of Fuel Requirements f o r Supersonic Transport Operation. Conference on Aircraft Operating Problems. NASA SP-83, 1965, pp. 193-202.
4. Anon. : Request for Proposals f o r the Development of a Commercial Supersonic Transport. Federal Aviation Agency, Aug. 15, 1963.
5. Anon. : Liquid Hydrogen Technology. Rep. No. AE 62-0774, General Dynamics/ Astronautics, Sept. 1962.
6. Vance, R. W. ; and Duke, W. M., eds. : Applied Cryogenic Engineering. John Wiley and Sons, Inc., 1962.
7. Anon. : Recent Hef-3 Property Information. Descriptive Data 2709, Beech Aircraft Corp., Mar. 17, 1959.
8. Pusanski, Barbara: Self-Ignition Temperatures and Flash Points of Some High- Energy Fuels. NACA RM E56C16a, 1957.
9. Hall, Eldon W. ; and Weber, Richard J. : Tables and Charts for Thermodynamic Calculations Involving Air and Fuels Containing Boron, Carbon, Hydrogen, and Oxygen. NACA RM E56B27, 1956.
10. Perry, John H . , ed. : Chemical Engineers' Handbook. 4th ed., McGraw-Hill Book Co., Inc., 1963.
11. Hald, Anders: Statistical Theory, with Engineering Applications. John Wiley and Sons, Inc., 1952.
12. Anon. : Standard Method of Estimating Comparative Direct Operating Costs of Transport Airplanes. Air Transport Association of America, June 1960.
13. Anon. : Santocel A- Monsanto's Silica Aerogel for Efficient Thermal Insulation.
Tech. Bull. 1-180, Monsanto Chemical Co., Oct. 1959.
14. Anon. : Dupont H Film - Summary of Properties. Bulletin H-1, E. I. DuPont
de Nemours and Co., 1965.
NASA-Langley, 1967 - 2 E-3540 “The aeronautical and space activities of the United States shall be
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edge of phenomena in the atmosphere and space. The Adminhtration shall provide for the widest practicable and appropriate dissemination of information concerning its activities and the results thereof.” -NATIONAL AERONAUnCS AND SPACE ACT OF
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