R. A. CARPENTER Midwest Research Institute, Kansas City, M o .
Liquid Rocket Propellants: Fuels and Oxidizers of the Future A ROCKET demands the utmost in
design and operational efficiency from each of its parts. Unlike more conventional means of propulsion, such as propeller-driven airplanes or automobiles, which involve complicated heat transfer and mechanical linkages, the rocket moves simply and directly by expelling combustion products of high kinetic energy. The rocket engineer has largely succeeded in designing light-weight and efficient motors. Future increases in efficiency from the mechanical standpoint are likely to be incremental, although the reliability may be improved. The ultimate performance of any one rocket design depends on the energy content of the fuel-oxidant system. This is a field in which great improvements can still be made in the practical utilization of known materials. Solid propellants were used exclusively in the early rockets. Comparatively inefficient fuels, solid propellants were used because the primitive motors were merely metal tubes of suitable strength closed at
the forward end, and filled with chemicals which reacted with one another to produce gaseous products. Although many early enthusiasts recognized that liquid fuels and oxidizers offered higher performance, the problems of pumping and metering the materials to the reaction chamber were difficult to solve. In addition, liquid fuels and the necessary equipment were thought to be too expensive. As the size and payloads of rockets increased, and the technical barriers of pumping and metering were hurdled, it became apparent that, in a system such as a military rocket, exotic chemicals of higher cost are justified because of higher performance. A change in the -direction of liquid fuels, in fact, became mandatory. First used were such familiar available compounds as ethyl alcohol and hydrocarbons. With the number of possibly useful chemical elements definitely limited, researchers were able to plan a program to define, and then to develop manufacturing processes for, the combination of compounds believed to provide the ulti-
C^round was broken March 8 at Muskogee, Okla., for a $ 3 8 million facility to be operated by Callery Chemical Co. for the Navy. The plant will produce HiCal, a new high energy fuel, in tonnage quantities for further extensive Navy evaluation and testing. Characteristics of the fuel are under security wraps, but the Navy says that its components are hydrogen, boron, and carbon, and that it will not only give an increase in fuel efficiency but can be used efficiently at altitudes where ordinary fuels will not burn. Midwest Research Institute is one of several subcontractors who have been working on the Callery fuel. The author of this article has directed Midwest's subcontract work for the past five years.
mate in performance. T h u s began the search for the optimum chemical rocket propellant. Characteristics and Properties
T h e fundamental equation for rockets is
performance
'-V^x£*«Hi)^] (1) where vz = exhaust velocity, feet per second
ing Avogadro's law is that a given mass of a low molecular weight gas will contain more molecules than one of a higher molecular weight; consequently, use of a low molecular weight gas will result in a greater ex pansion in the nozzle of the engine and therefore a higher velocity in the exhaust gas. I n this partial periodic chart of the elements are the atoms that must be dealt with in the opti m u m propellent combination. H(1.008) Li(6.94) Na(23.0)
Be(9.01) Mg(24.3)
B(10.8) Al(27.0)
atmosphere becomes less important in limiting the speed. T h e r e is in sufficient oxygen to sustain combus tion in conventional engines. High energy fuels, therefore, are least val uable in " a i r breathing" mecha nisms. T h e y are most valuable when combined with oxidants carried with them in the same vehicle. O n page 45A, a plot is shown in which heat of combustion of various fuels in British thermal units per pound of fuel plus C(12.0) Si(28.1)
N(14.0) P(31.0)
0(16.0) S(32.1)
F(19.0) C1(3S.S)
Q
y = . ? ratio of specific heat at constant pressure to specific heat at con stant volume G = gas constant, ft.-lb./lb.-mole/ ° C. Τ = combustion chamber temperature,
°K.
Px = pressure at exit, lb./sq. inch Po = pressure in chamber, lb./sq. inch M = average molecular weight of prod ucts g = acceleration due to gravity, feet/ sec.2 A more convenient expression of the performance of the engine is given in the equation vx = /,„ g, where I,p is the specific impulse (lb.-sec/lb.) and is used as a criterion of the fuel involved. It is an ex pression of the n u m b e r of seconds during which a unit mass of fuel can exert a unit force. Equation 1 shows that certain values of fuel properties have a de sirable effect on performance. T h e chemical energy contained in the fuel must be converted to heat energy and then to kinetic energy in as efficient a m a n n e r as possible. O b viously, to start with, the chemical energy should be highly concentrated. T h e heat energy released on com bustion in the fuel-oxidant system should be high, giving the m a x i m u m flame temperature consistent with the ability of the construction ma terials to withstand the temperature. T h e specific heat of the products should be low, which means that simple molecular structures should be present in the exhaust products. Thus, for example, helium would be a better product in the exhaust gases than carbon tetrachloride, and carbon monoxide would be better than carbon dioxide. Furthermore, the pressure differential between the chamber and the outside should be as high as possible. Additionally, the elements of low est atomic weight in the fuel-oxidant system would make the most efficient exhaust gas. Another way of stat
I n general, elements on the left side of the table are considered fuels, and those on the right are considered oxidants. T h e energy released on combination of two elements is roughly related to the distance be tween them in the periodic chart. For example, lithium and fluorine release more energy in forming lithium fluoride t h a n do carbon and oxygen in forming carbon monoxide. Therefore, the most desirable fueloxidant combination would consist of elements farthest to the left com bining with those farthest to the right. T h e plot on the next page is of the heat of combustion (with oxygen) in British thermal units per pound of various elements and some of their compounds vs. atomic number. T h e basis for these calculations is the con version of the elements to their stable oxides. T h e periodicity ex hibited in this plot is a reflection of the position of the elements in the periodic table. T h e dashed line represents the heat of combustion of gasoline and, for purposes of this dis cussion, separates the elements and compounds into two groups having a higher (or lower) heat of combustion than gasoline. If a better chemical fuel is to be synthesized, it must sur pass gasoline, which is readily and cheaply available from natural sources. I n the figure, note partic ularly the position of hydrogen and its vast superiority to the other ele ments, as far as fuel value is con cerned. Present hydrocarbon fuels are sufficient to provide high rocket speeds in relatively dense atmos pheres. Heat effects due to air friction severely limit any further in creases in velocity at lower altitudes. However, at higher altitudes, above 90,000 feet, external frictional heat caused by high velocity through the
the stoichiometric amount of oxygen required for the combustion is plot ted against atomic number. T h e calculations for hydrogen a r e : 2H2 + 0 2 —· 2H 2 0 Wt. of unit
2(2)
32
2(18)
Δ Η combustion for H 2 = B.t.u./lb.
52,000
52,000 X 4/36 = 5800 B.t.u./lb H 2 + oxygen
and for diborane: B2He + 30 2 — B 2 O s + 3H 2 0 Wt. of unit
28
3(32)
70
3(18)
Δ Η combustion for B 2 H 6 = 32,000 B.t.u./lb. 32,000 X 28/124 = 7200 B.t.u./lb. B2H6 + oxygen Where both fuel and oxidant must be carried along on the trip, com pounds of several elements have an advantage over hydrogen, because the relative weight of oxygen re quired for combustion to the stable oxides is less t h a n that for hydrogen. Hydrogen, lithium, beryllium, boron, magnesium, aluminum, and silicon all have possibilities in synthetic fuels. T h e optimum chemical fuel can not be determined solely on the basis of heat of combustion. T h e funda mental performance equation for rockets is limited in the operation of the best rocket motors we have today at approximately 3000° K. a n d 50 a t m . pressure. T h e heat of com bustion and the specific heat of the products determine the flame tem perature. T h e practical limit on combustion chamber temperature means that some diluent must be added to the stoichiometric propor tions of fuel and oxidant; for the theoretical flame temperature with out diluent is, in m a n y cases, far higher than 3000° K. However, as VOL. 49, NO. 4
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APRIl 1957
43 A
the temperature increases, liquids vaporize, m a n y molecules dissociate endothermically, and some oxides, such as beryllium oxide, sublime, so that the heats of vaporization, dissociation, and sublimation must be subtracted from the heat of combustion in calculating the flame temperaature. Even so, extra cooling gas is often provided. O n l y hydrogen, helium, oxygen, or nitrogen has sufficient heat capacity to convert high heats of combustion to propulsive energy without going to excessively high temperatures. No solids should be present in the exhaust gases, for they do not expand on flowing through a nozzle a n d thus can do no work. T h e molecular weight should be as low as possible. T h e pressure p a r a m e t e r is limited in two ways: T h e materials of construction used for the rocket c h a m b e r cannot stand m u c h more than the force of 50 a t m . or 750 pounds per square inch at high temperatures; and the fuel must be
pumped and metered into the c h a m ber against this pressure. A higher value would make this increasingly difficult. At high altitudes the lowpressure side of the nozzle m a y be at 0.1 a t m . a n d the pressure ratio thus m a y be 50 to 0.1. T o secure the full benefits from this high pressure drop, the size of the nozzle exit would have to be increased in impractical proportions T h e physical properties of the fuel are also important. T o minimize the cross-sectional area of the missile and the volume of the fuel and oxidant tanks, the density must be as high as possible. For instance, adding a high-density solid such as T N T to some working fluid such as hydrazine could m a k e a suitable fuel, except for the highly explosive properties of T N T . A liquid density as high as 1.5 would be desirable. T h e melting a n d boiling points should be far apart, and the liquid range should (ideally) be —60° to + 60° C. It is not possible to pres-
Atomic Number Heat of combustion of various compounds vs. atomic number of principal element 44 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
surize the fuel tanks to retain a highly volatile material in the liquid state, because of the resulting weight of the heavy walls of a pressurized vessel. T h e vapor pressure obviously should be as low as possible. T h e viscosity of the material also should be low, for p u m p i n g and metering purposes. Instability toward shock seriously limits the use of such materials as nitroglycerin and acetylene. T h e r m a l stability is also necessary because most of the engines operate with regenerative cooling of the rocket c h a m b e r walls. Fuel and oxidant are circulated through the walls, both to cool them and to recover heat lost in the walls. Therefore, the material should not decompose u p to 500° C. For these purposes, the fuel should have a high heat capacity and thermal conductivity. T h e fuel should be stable on storage, so that large amounts can be manufactured and accumulated at a rocket base. Although of less importance, ideally it should be nontoxic and noncorrosive. Factory filled a n d sealed, interchangeable fuel tanks will solve m a n y handling problems. T h e combustion characteristics of the fuel-oxidant system are as important as the physical properties of the fuel. T h e fuel and oxidant must be metered into the combustion chamber in precisely the amounts calculated to give the optimum specific impulse. If this is not done accurately, some fuel or some oxidant will beleft in the tanks when the other component is expended, resulting in increased dead weight of the vehicle and consequent shortening of range or velocity. Mixing must be complete to avoid hot spots in the combustion zone which might result in deterioration of the motor or surges in the power produced. T h e flame should be stable, and the rate of burning should not vary significantly with temperature and pressure. T h e ignition of the system should be spontaneous if possible. This is represented by the system aniline-nitric acid, which spontaneously burns when mixed. T h e ignition delay should be less t h a n 10 milliseconds. A longer delay might allow large amounts of the mixture to accumulate, which would detonate and destroy the motor when finally ignited. If the system is a monopropellant, such as nitromethane or hydrogen peroxide, it is usually
ROCKET
PROPELLANTS
Atomic Number Heat of combustion of fuel-oxygen systems vs. atomic number of principal element
necessary only to inject this material into the hot c h a m b e r and sponta neous combustion or decomposition will take place. Some bipropellants, however, require a sparkplug for ignition.
Today's Fuels
M a n y useful fuels are available today, but most of them present difficulties or limitations as chemical rocket propellants. Specifications for JP4, a kerosine hydrocarbon fraction, a r e :
Boiling range, ° F. Freezing point, ° F. Viscosity at —40° F., es. Heat of combustion (net), B.t.u./lb.
216-425 — 76 3.26 18,678
T h e actual specific impulse of this petroleum-derived material depends somewhat on motor design, ranges are from 200 to 250 seconds, and the cost is about $0.03 per pound. Moreover, it is not possible with this fuel to design missiles with sufficient exhaust velocity to be considered for interplanetary flight. T h e ac companying table shows other fuels a n d oxidizers now in use. Ammonia, cheap enough at a b o u t $0.04 per pound, provides a good working fluid for the rocket exhaust, but has a low density a n d boiling point. W h e n nitrogen in a com p o u n d changes to molecular nitrogen with a triple bond between the two atoms, a great release of energy re sults, even though no oxidation in the normally accepted sense of the word takes place; therefore, all nitrogencontaining compounds are poten
tially good fuels a n d explosives. Furthermore, the molecular nitrogen produced is a n excellent exhaust product, having a relatively low molecular weight and a satisfactory heat capacity.
Present Fuels and Oxidizers
Liquid oxygen, Os Liquid ozone, O» Hydrogen peroxide, H2Os White fuming nitric acid, WFNA Red fuming nitric acid, RFNA Mixed oxides of nitrogen, MON
Ammonia, HHi Aniline, C11H5NH1 Ethyl alcohol, C2H1OH Gasoline, C;H,« Hydrazine, N2H< tmej/m-dimethylhydrazine (CHt)tNNH» Hydrazine hydrate, Ν,Ηί.Η,Ο Methanol, CHsOH
Mixed acids, HiSO. + HNOa Nitrogen tetroxide, N2O,
Nitromethane, CHj~ NOa
VOL. 49, NO. 4
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APRIL 1957
45
A
Aniline has been used in m a n y instances as fuel. It combusts spon taneously with nitric acid, but has a relatively low specific impulse, is very toxic, a n d is comparatively ex pensive. Ethyl alcohol also costs about $0.04 a pound but has a low specific impulse. Hydrazine m a y cost a b o u t $0.25 a pound, which is acceptable. W h e n coupled with oxygen, it has a specific impulse lower than gasoline but pro vides hydrogen and nitrogen for a good working fluid a n d will probably be used to a great extent in the future as a diluent for other highenergy fuels. Methanol is in the $0.04-a-pound range, but has a low specific impulse. Nitromethane is a very useful monopropellant, but is somewhat expensive. T h e present liquid oxidizers con sist of oxygen and high density com pounds of nitrogen a n d oxygen. Liquid oxygen also costs about $0.04 a p o u n d and is very stable a n d rela tively safe. A great deal of informa tion has been built u p on handling this material, which is by far the best oxidizer available today. Further more, it is so good in this respect, t h a t there is little chance of its being displaced in the near future. Liquid ozone costs about $0.08 a p o u n d and m a y develop satisfactorily, but the hazards d u e to its instability must be overcome. T h e benefits to be de rived from its higher density and heat of decomposition are worth in vestigating. Hydrogen peroxide is a mono propellant, in that it decomposes into superheated steam a n d oxygen, releasing energy. T h e oxygen is then available to combine with a second fuel. Large amounts of hy drogen peroxide are handled today, despife its being unstable a n d d a n gerous. White fuming nitric acid is nitric acid containing not more t h a n 2 to 3 % of water. R e d fuming nitric acid contains dissolved NO2, nor mally about 1 4 % . Mixed oxides of nitrogen are usually mixtures of N O and N2O4. Mixed acid means mixed nitric and sulfuric acids. Ni trogen tetroxide is worth special mention, because it is becoming available on a large scale, a n d at about $0.06 a pound, it is a n ex tremely attractive oxidizer. All of these compounds of nitrogen and oxygen are very corrosive a n d difficult to handle. However, they sometimes produce a hyperbolic 46 A
system with a fuel, a n d they have the virtue of yielding molecular nitrogen as a combustion product, which is a good working fluid. W e can now consider some of the optimum propellent compositions a n d performance d a t a which are pos sible with present fuels a n d with some fuels of the future.
Optimum Propellant Composition and Performance* Combustion temperature 3000° Κ.; expansion from 20 atm. to 1 atm. Free Hydrogen, Propellant NsHf-Fa NsH«-Hr-F 2 NHr-F, NHs-Hj-F, B-Hj-F,
Wt. % ,,
BÎHÎ~HΗOj
Β , Η . Hr-Oi HΗFJ
H5-O2
10
».
9 16 14 12 8 16
*βρ
279 323 280 323 363 363 363 361 3S4
* Carter, J. M., Aviation Age, p. 34 (April 1955).
W h e n excess fuel is added as a diluent to provide a n o p t i m u m work ing fluid, the specific impulse is in creased, although no further combus tion can take place. At the same time, the limit of specific impulse with the combinations represented is still below 400 l b . - s e c / l b . This figure of 400 is considered the ulti m a t e available from chemical fuels and normal combustion products. Fuels of the Future T h e g r a p h on fuel-oxygen shows that the ultimate chemical fuel p r o b ably will be a c o m p o u n d of hydrogen with one or more of the light metals. O n l y a limited n u m b e r of substances have a heat of combustion with oxy gen appreciably better t h a n that of gasoline. Some of t h e m are : 1. 2. 3. 4.
Hydrogen Lithium hydride Beryllium Beryllium hy dride 5. Methane 6. Acetylene 7. Sodium hydride
8. Magnesium 9. Aluminum hy drides 10. Aluminum borohydride 11. Silicon hy drides 12. Boron 13. Boron hy drides
O n l y compounds of hydrogen and boron meet the tests of desirable characteristics, properties, a n d avail ability. Lithium is almost as a b u n d a n t as
INDUSTRIAL AND ENGINEERING CHEMISTRY
zinc, but its ores are dilute, a n d there are few good ore deposits. I t is in d e m a n d in atomic energy as a fusion fuel and is also used in high-tempera ture greases. Its refinement to the metal involves electrolytic reduction ; any large expansion of lithium pro duction would require new electric power, with the consequent construc tion of generators and hydroelectric equipment. Lithium hydride, lith ium a l u m i n u m hydride, and lithium borohydride are solids, a n d there fore would not be particularly useful as fuels except in slurries. Beryllium hydride is also a solid, but the scarcity a n d difficult metal lurgy of ores, d e m a n d s in atomic en ergy, a n d toxicity combine to rule out beryllium compounds. Carbon forms hydrides (better known as the hydrocarbons) although the ones with the greatest heats of combus tion—that is, m e t h a n e and acetylene — a r e gases, a n d acetylene is rela tively unstable. Nitrogen forms two hydrides, a m m o n i a a n d hydrazine, discussed previously. Sodium forms a relatively stable solid hydride, as does magnesium. A l u m i n u m hy dride is a gas, but a l u m i n u m boro hydride has been mentioned fre quently as a rocket fuel a n d is, pe culiarly enough, a liquid. Silicon hydride is a low boiling material. From silicon on, t h r o u g h the periodic table, the molecular weights get too high to offer m u c h promise. T h u s for one reason or another the fore going substances are considered by combustion researchers to be unat tractive for liquid fuel development. Hydrogen has been mentioned fre quently as a fuel in its own right. Its cost is not unreasonable—about $0.50 per p o u n d — b u t its specific gravity is 0.14 as a liquid, a n d its boiling point is —426° F. I t is one of the best fuels from the standpoint of specific impulse. Hydrogen cannot be used as compressed gas, a n d its critical point is so low that it is very difficult to liquefy. However, pilot plant studies in liquefying hydrogen have been m a d e . Special p u m p s h a d to b e developed, a n d Freon, nitrogen, and helium used as refrig erants in order to lower the temper a t u r e of the hydrogen to the point where it could be liquefied by com pression. A n extremely high purity is required in liquid hydrogen be cause of the tendency for any con taminants to freeze and clog u p the lines, valves, and p u m p s . N o air can be admitted to the liquid hydro-
ROCKET PROPELLANTS gen because it would solidify, a n d solid oxygen a n d liquid hydrogen will explode spontaneously. Storage vessels containing liquid hydrogen m a y become brittle a n d lose their impact resistance. Insulation of such vessels must be the best available, consisting in most cases of a n evacuated a n n u l a r space plus a radiation shield to prevent heat transfer by this means. H y d r o g e n exists in two forms, ortho a n d p a r a configurations, a n d unfortunately, appreciable heat is liberated when the ortho form changes to the p a r a form. Because this transformation occurs as the temperature of the hydrogen is lowered from a m b i e n t to liquefaction, the heat of vaporization plus the heat of transformation must be removed for each molecule condensed. As can be seen, liquid hydrogen is very difficult to h a n d l e a n d probably will never attain widespread use. O t h e r methods of using molecular hydrogen have been considered. O n e would be dissolving it in a hyd r o c a r b o n ; however, the solubility of the gas is so low a t 1 a t m . pressure that no appreciable increase in energy content of the system can be attained. Another would be absorbing the hydrogen on activated carbon, but only 0.0003 atom of hydrogen can be absorbed per carbon atom. Absorption on nickel results merely in formation of a nickel hydride, which has about one fourth the heat of combustion of gasoline. Polyatomic hydrogen with either three or four atoms has been postulated by m a n y people, b u t its existence has never been proved. Therefore, in order to get the desired a m o u n t of hydrogen into the fuel we t u r n to chemical combination. T h e hydrides of the elements shown in the figure o n p a g e 45 A have been studied in some detail. Of all the materials listed above, the boron hydrides seem most promising, from the standpoint of practical availability. Boron ores, although not in large proportion in the earth's crust, are very concentrated, and thus recovery is easy a n d inexpensive. Diborane, B2He, is a gas; however, it polymerizes into higher hydrides such as B 6 H 9 , a liquid, and B I O H H , a solid. Alkyl boranes increase the n u m b e r of molecular types for consideration. Synthetic chem-
istry must play a major role in the coming development of boron fuels by providing a large n u m b e r of new compounds of the a p p r o p r i a t e elements on which to base structureproperty correlations. T h e n it should be possible to "design" a material with most of the desirable handling properties, plus a n inherent high heat of combustion. T h e objective is to get the desired elements into a satisfactory physical state by using the rules a n d correlations that relate physical properties to molecular structures. F u t u r e oxidizers are also of great interest to rocketry, since we can raise the specific impulse by changing from one oxidizer to another. O n a weight basis, a rocket uses about five times as m u c h oxidizer as fuel. For instance, fluorine might be a good oxidizer. T h e m a x i m u m exothermic reaction with elements in their most stable states would be the combustion of hydrogen with fluorine to produce hydrofluoric acid. O n a molecular basis this is true, but because of the higher atomic weight of the fluorine, o n a weight basis the m a x i m u m exothermic reaction would still be with oxygen. However, fluorine is limited in availability, and is difficult to handle, so it is not considered promising as a long-range solution to the problem of increasing specific impulse. O z o n e has been studied for several years with the view of making possible liquid ozone in a stable state. This might be better t h a n liquid oxygen, but too litde is known about ozone a t the present time. Ozone releases heat energy in decomposing to oxygen, and this could give added impulse to the system. Mixtures of halogens such as chlorine trifluoride would have good physical properties, but, again, fluorine is in short supply. T h e same applies to fluorine monoxide. It can be stated wth a good deal of confidence that liquid oxygen will remain the best "work horse" oxidizer, although more fanciful compounds will be used for research rockets and highly specific purposes. Liquid oxygen boils at —183° C. a n d has a specific gravity of 1.14. I t m a y be distilled from liquid air a n d is comparatively cheap. It is noncorrosive a n d m a y be handled easily. Some precautions are necessary.
For instance, in loading, tanks and lines must be cooled by either undercooling the oxygen or allowing some evaporation to take place. T h e r e is some frostbite danger to personnel d u e to spillage or touching extremely cold metal and there must be some precaution against trapping the liquid in a sealed system. T h e specific impulse derivable from liquid oxygen is a m o n g the highest on a weight basis, but because of the higher density of hydrogen peroxide and nitric acid these materials are somewhat better on a volume consideration. Other Hypothetical Propellant Possibilities Because the actual performance of any rocket-driven vehicle depends only on the velocity of the exhaust gases, it is worth while to consider other means of imparting high velocity to suitable working fluids (gases). Some other chemical reactions are possible which do not involve materials in the stable state. A good example is the recombination of atomic hydrogen. Irving Langmuir invented the atomic hydrogen torch, which uses an electric discharge to split u p the hydrogen molecule into two hydrogen atoms, which then recombine, giving off approximately 90,000 B.t.u. per pound, or enough for a specific impulse of 700 seconds. However, the problem here is that the half life of the hydrogen atom at room temperature is about 1 second, and hydrogen atoms cannot be practically isolated, liquefied, a n d stabilized. Atomic hydrogen could be m a d e in the rocket from nuclear electricity, or some method of stabilizing the atoms might be found, so that it could be stored. However, all surfaces a n d any other chemical compounds present catalyze the recombination. T h e big problem in the use of any such high energy system would be to control the heat release. O t h e r free radicals (dissociated or excited fragments of molecules), which can recombine to give u p energy or decompose further to the elements, are N H , C H 2 , C H 3 , a n d H 0 2 . After the energy of recombination is recovered, the resulting molecules— e.g., ethane from C H 3 — m a y still be combusted chemically to provide additional heat. VOL. 49, NO. 4
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APRIL 1957
47 A
Another example of an unstable fuel is the metastable white form of white phosphorus. The approximate energy content of some of these materials is :
Energy of Recombination of Free Radicals AH , B . t . u . / L b .
Radical
9,000 18,000 90,000 5,300 2,500
NH CH H CH, CÎH&
Another source of energy for the working fluid is referred to often as the thermonuclear rocket. In this case, fission or fusion is used to heat hydrogen, which then propels the vehicle. Fission of uranium-235 releases 36 χ 109 B.t.u. per pound or approximately 1,000,000 times that obtained from burning hydrogen and oxygen. T h e accompanying table shows the efficiency of several work ing fluids in a thermonuclear pro pulsion system operating at 3000° K. and a pressure drop of 300 to 1.5 pounds. Again, the position of hy drogen relative to all of the other possible materials is noted. This superiority would probably make hydrogen the most desirable working fluid, despite problems in transporta tion and storage.
W o r k i n g Fluids f o r Thermonuclear Propulsion"
Pile
Compound (MW)
Heat Content, B.t.u./Lb.
Jgp
Hydrogen, H»(2.076) Helium, He(4.003) Nitrogen, N , ( 2 8 . 0 1 6 ) Oxygen. O i ( 3 2 . 0 ) Argon, A ( 3 9 . 9 4 ) Ammonia, N H a ( 1 7 . 0 ) Hydrazine, N 2 H
