A SURVEY OF ROCKET PROPELLANTS
Another in the Series of Chemical Studies
PART 2
THE TERMS “exotic” or “zip” fuels are frequently mentioned in the literature and newspapers. The description “exotic” was applied in the early stages of rocket fuel development when new combinations, not in common usage at the time, were proposed. In 1952 the Navy set up Project Zip to develop superior fuels and some of the combinations to come from this research were dubbed “zip” propellants. Neither of these designations refer to any one specific system but either of them could apply to the boranes.
In theory, pure hydrogen with a yield of 52,000 BTU/lb is the most logical choice for a fuel; its density, however, is too low and even when liquidfied is ten times as bulky as gasoline. Beryllium, on combustion, gives up 29,000 BTU/lb but is rare, costly and highly toxic in chemical compounds. Boron’s 25,000 BTU/lb places it 40 percent above the less than 19,000 BTU/lb hydrocarbon rating and on combination with hydrogen produces an even higher heat content. Boron, therefore, seemed a likely candidate for investigation as a fuel possibility and the three basic compounds listed below were considered.
Diborane is the basic substance in the group from which the others are derived. An interesting sidelight on its combustion in air is that it yields boric oxide, the active reaction product in the extinguishing mechanism of trimethoxyboroxine, a recently developed agent used on fires involving certain metals. Like any other gas fire, diborane, burning with the green Hame characteristic of boron compounds, should not be extinguished unless its source can be shut off. Small laboratory leaks resulting in minor fires can be extinguished by carbon dioxide or dry chemical. Chlorinated hydrocarbons should never be used. The fire hazard of diborane and derivatives such as aluminum borohydride, Al(BH), is intensified by their spontaneous flammability in air.
The Callery Chemical Company advises that, “Diborane is now supplied as a compressed gas in 100-gram, 1-pound, 2-pound and 4-pound lots. The special returnable cylinders and crates are shipped by Railway Express. Since not more than five days are allowed in transit, diborane is shipped only on Mondays and Thursdays, so that the fifth day will not occur on a weekend.”
In addition to the use of boranes as a primary fuel, investigation is also in progress to test its use as an additive for increasing the efficiency of other systems. Pentaborane, for instance, added to aircraft fuel can increase range by 50 percent. One boron-based aircraft fuel related to the boranes, designated as HEF-2 by the Olin Mathieson Chemical Corporation, and used for oxygen breathing engines, while not spontaneously flammable can ignite on contact with hot surfaces. If it is involved in fire, foam is the most effective extinguishing agent.
Although the boranes are toxic, it is difficult to specify all the effects on the human body due to a lack of clinical experience. Diborane attacks the lungs in contrast to pentaborane and decaborane which affect the central nervous system. Detection of low concentrations of boranes is an exacting procedure because of the problems of chemically analyzing for them. Several monitoring devices are under development but there is no commercial borane detector on the market. A 3 percent aqueous solution of ammonia will neutralize spills and wash boranes from the skin, garments or floors.
While the exact composition of borane fuels is a secret, one surmise is that they are probably modifications of decaborane which changes to the liquid state upon the addition of hydrocarbon groups to its molecule. They are stable, can be transported safely and stored without deterioration. Because precise information on any one borane-based rocket fuel with regard to its chemical composition and physical characteristics is classified information, it is not possible to discuss specific fuels in this light and assign to each the proper extinguishing methods based upon stated properties.
Storage compatability
Where operations involve materials that are highly reactive and in addition are hypergolic on contact with certain other chemicals, the storage problem becomes a primary consideration and requires specification in the most minute detail.
—U. S. Marine Corps
Compatible Storage Groups
Group “A”
Nitric acid
Mixed acid (Nitric and Sulfuric)
Group “B”
Aluminum borohydride
Diborone
Other metal borohydrides
Pentaborone
Group “C”
Anhydrous ammonia
Hydrazine, anhydrous
Hydrazine, hydrate
Group “D”
Aniline
Monoethylaniline Furfuryl alcohol
Group “E”
Calcium permanganate
Solid sodium permanganate
Group “F”
Compressed hydrogen gas
Compressed inert gas
Group “G”
Compressed oxygen gas
Compressed inert gas
Group “H”
Ethyl alcohol
Methyl alcohol
Group “I”
Gasoline
Heptane
Kerosene
Octane
Pentane
Group “J”
Hydrogen peroxide
Group “L”
Liquid fluorine
Group “N”
Liquid hydrogen
Group “O”
Liquid oxygen
Group “R”
Nitromethane
Tetramethane
—U. S. Army
—U. S. Army
Storage computability is one phase of these requirements and listed at left is a portion of the table set up by the Army for fuels and oxidizers. Those chemicals in the same group are deemed to be compatible for storage because individual characteristics are such that their presence together does not increase the normal hazard of storage alone.
Quantity-distance tables are also charted for fuels and oxidizers. They specify minimum allowable distance between storage of fuels and oxidizers, minimum distance of each substance from inhabited buildings, public railways, public highways and the minimum distance between storage facilities in order that flame or explosion in one will not propagate to another.
Fire hazard
The fire hazard of propellants varies with the components involved. A consideration of physical and chemical characteristics will give some understanding of extinguishment problems. Among the important related properties are density, solubility or miscibility in water, vapor pressure, flammability limits, toxicity and combustion temperatures of the propellants.
Many of the fuels are alcohols or hydrocarbons and fires can be handled in accordance with standard procedures for this type of operation. In some cases, how’ever, these basic fuels contain additives which may be highly toxic and indicate the use of a self-contained breathing apparatus.
In general, the rocket fuels of today, unless otherwise indicated, are miscible with water which continues to be our primary extinguishment agent in the forefront as the most potent weapon for fighting propellant fires. Application in the form of a coarse fog or spray at not less than 100 pounds per square inch pressure, will fulfill the requirements for extinguishment and dilution. One Army test involved a propellant system containing an aniline-furfuryl alcohol as the fuel and red fuming nitric acid as the oxidizer. Each component was in a small earthen pool separated by a wall which was blasted away to initiate the combustion. Application of water at the rate of 1,500 gallons per minute extinguished the fire in 45 seconds.
In the laboratory, small quantities can be handled efficiently with carbon dioxide or dry powder extinguishers.
Where more stringent requirements are not specified, storage and handling should be in conformance with I.C.C. regulations.
The future in fuels
While the results of present research and development may appear to approach the ultimate, still more seemingly fantastic possibilities are being explored. On the boards are a number of nuclear fission designs, one of which envisions a small atomic pile to heat hydrogen several thousand degrees and blast it through the exhaust nozzle.
Power through the utilization of free radicals may be the supreme achievement in space technology. Free radicals are unstable fragments of chemical compounds which have been observed in flame and high speed reactions, existing only for millionths of a second. Upon recombination, these free radicals release the enormous heat necessary to approach a maximum degree of efficiency. Normally gases such as oxygen and hydrogen exist in the molecular form of two atoms combined (O2,H2). In the upper atmosphere there is a large source of these gases in the uncombined single atom or free radical state (O, H). Two directions of research are in progress to harness this vast reservoir of power. An attempt is being made to isolate and freeze free radicals in the laboratory and another announced method makes use of the atmospheric free radicals. Nitric oxide has been used as a catalyst in the 60 to 70 mile atmosphere region, the level of the single-atom oxygen layer. A solid catalyst to line the rocket cents so that the recombination process can take place within the device and create thrust producing heat has been reported by the Air Force’s Cambridge (Mass.) Research Center as a promising possibility.
BIBLIOGRAPHY
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Schneider, “Rocketry: A New Means of Transportation”; (Background material for Rocket Exhibition—Queens College, Feb. 1958)
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Schechter, “How Toxic are High Energy Fuels?”; (National Safety News, Vol. 77, No. 4, April 1958)
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Malcolm, “New Fuel Extinguishment Research”; (Proceedings U.S. Naval Civil Engineering Research and Evaluation Laboratory Symposium on Fire Extinguishment Research and Engineering, November 1954)
New York Times, “Moon Shot Date in Dispute”, Vol. CVII, No. 36, 663, Page 1 Col. 7, June 11, 1958)
New York Times, “U.S. Likely to Make SolidFuel Missiles Key Defense by ’65,” Vol. CVII, No. 36,667, Page 1 col. 3, June 15, 1958.
New York Times, ” ‘Super Fuel’ to Drive Rocket Indefinitely in Space,” Vol. CVI, No. 36,324, Section 4, Page 47 col. 5, July 7, 1957)
New York Times, “New Satellite Is Regarded as Test Sphere and Is Expected to Yield Little Data,” Vol. CVII, No. 36,578, Page 15 col. 1, March 18, 1958)
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“Diborane”; (Callerv Chemical Company, Technical Bulletin C-020, March 15, 1958)
