US3043099A - Monopropellant fuel - Google Patents

Description

This invention relates to fuels finding valuable use in both jet and rocket engines. In particular, it relates to fluid monopropell-ant fuels which are stable toward.

high rates of compression, have low freezing points and exhibit improved burning characteristics.

, A monopropellant fuel is a substance which contains sufiicient oxygen in its structure to sustain combustion Without additional oxygen. Such fuels find valuable use in situations where high thrust per unit weight is critical, as for example, in propelling jet aircraft, rockets, underwater missiles, andthe like. The monopropellants of this invention are particularly useful in devices of the type disclosed in assignees copending application Serial No. 183,111, filed September 5, 1950.

At the present time, liquid nitroalkanes are widely used as monopropellants. Unfortunately, these fuels are extremely sensitive to shock and detonate at compression rates as low as 20,000 psi/sec. It is also characteristic of these nitroalkanes that once detonation occurs, it readily propagates through the fuel. Hence, detonation of a nitroalkane in an engine is extremely hazardous, as the detonation is usually transmitted through the fuel lines to the main fuel storage supply, causing devastating explosions and extreme hazard. In addition to exhibiting undesirable explosive tendencies, the nitroalkanes have high freezing points which often render them impractical for arctic or high altitude use, where extreme temperatures are commonly encountered.

l have now found that fuels comprised of liquid nitroalkane and alkylene oxide exhibit monopropellant properties substantially equivalent to the pure nitroalkanes. They are stable to high compression rates and do not propagate detonation. Such fuels exhibit lower freezing points and viscosity than the corresponding nitroalkanes and can be used under both arctic and high altitude conditions.

According to the present invention, my improved monopropellant fuel is prepared by blending together alkylene oxide and nitroalkane. As the components are readily compatible, blending is usually achieved by simply pouring the ingredients together. Optimum results are obtained when the fuel contains from about 3% to about 50% by volume of alkylene oxide and from about 97% to about 5 by volume of nitroalkane.

The preferred liquid nitroalkanes are generally those having from 1 to 6 carbon atoms. It is also within the purview of this invention to use mixtures of liquid nitroalkanes having solid nitroalkanes such as the nitrated heptanes and octanes dissolved therein as monopropell-ant fuels. Due to their low molecular weight, nitromethane and nitroethane are ordinarily preferred. Among the other nitroalkanes useful as fuel components are: dinitromeflhane, tetranitromethane, dinitroethane, mononitropropane, dinitropropane, tri-nitropropane, mononitro butane, dinitrobutane, trinitrobutane, tetranitrobutane, dinitropentane, tetranitropentane, nitrated hexane, and mixtures thereof.

The alkylene oxide component is a compound or compounds having an oxygen atom bonded to two adjacent and mutually bonded carbon atoms. Ethylene .and propylene oxides are generally preferred due to their low molecular weight and greater commercial availability.

The fuels of this invention exhibit a high stability to 3,043,099 Patented July 10, 1962 7 2 shock. For example, fuel mixtures containing about ethylene oxide and about 95% nitromethane are insensitive to adiabatic compression rates as high as 150,000 p.s.i./sec. and are practicaly insensitive to gunfire. Furthermore, it has been found that alkylene oxide-nitroalkane fuels do not propagate detonation under such conditions, even though confined in a tube of /2" diameter. In addition, these fuels exhibit a high thermal stability; a fuel containing about nitrometh-ane and about 20% ethylene oxide being stable at temperatures as high as 765 F.

Large amounts of the alkylene oxide can be used for lower freezing points without impairing the monopropellant properties of the fuel. For example, a fuel containing 34% by volume ethylene oxide and 66% by volume nitromethane exhibits a freezing point of about 55 C., While at the same time being capable of delivering a specific impulse greater than lb.-sec. Even at low temperatures, these fuels exhibit low viscosities, as

illustrated bytwo representative fuels shown in Table I.

Table I Viscosity in Centistokes Fuel Temperatures -40 F. -20 F. 20 F Nitromethane so1id 1.2 approx 0.8 80% N itromethane. 1 89 66 20% Ethylene Oxide 60% Nitromethaue 84 7 54 40% Ethylene Oxide The new fuels of this invention are combusted in the conventional manner in jet engines. When used in such engines, the fuels are injected into the combustion chamher and ignited by means of spark plugs, or the like. The

gases produced by the resultant combustion are permitted to escape to the atmospherethrough a nozzle to create thrust for propulsion. Once combustion is initiated, it continues as long as fuel is provided.

The fuel, according to my invention, possesses the advantage that it is a self-combustible propellant and can be practically and efliciently employed in jet engines, without exposing the operator or apparatus to the danger of explosion and to the attending danger of propagation of the explosions to the main storage supply through the fuel lines. The novel fuels of my invention can be used in the same manner as nitroalkanes, and can be employed under operating conditions where the use of nitroalkanes alone is prohibited by their explosive characteristics. In addition, the monopropellant fuels of this invention can be used under extreme low temperature conditions.

I claim:

1. In the method of producing thrust for propulsion by combusting liquid nitroalkane in a combustion chamber, the improvement which comprises combusting said liquid nitroalkane in the presence of lower alk-ylene oxide.

2. The method of claim 1 wherein the alkylene oxide is present in an amount of from about 3% to about 5 0% by volume of the total fuel composition.

3. The method of claim 1 wherein the liquid nitroalkane is selected from the group consisting of nit-romethanes, nitroethanes, nitropropanes, nitrobutanes, nitropentanes, nitrohexanes, and mixtures thereof.

4. A fluid propellant composition consisting essentially of a mixture of a liquid nitroalkane and a lower alkylene oxide, said nitroalkane component being present in an being present in an amount from about 3 to about 50% by volume of the composition.

5. A monopropellant composition comprising a mixture of from about 97% toabout 50% by volume of liquid nitroalkane selected from the group consisting of nitromethanes, nitroethanes, nitropropanes, nitrobutanes, nitropentanes, nitrohexanes, and mixtures thereof, and alkylene oxide selected from the group consisting of ethylene oxide, propylene oxide, and mixtures thereof, in an amount of from about 3% to about 50% by volume of the total volume of the fuel composition.

6. A monopropellant composition comprising a mixture of nitromethane in an amount of from about 97% to about 50%, and ethylene oxide in an amount of from about 3% to about 50% by volume of the composition.

7. A monopropellant composition comprising nitromethane in an amount of about 95% by volume of the composition and ethylene oxide in an amount of about 5% by volume of the composition.

8. A monopropellant composition comprising nitromethane in an amount of about 66% by volume of the composition and ethylene oxide in an amount of about 34% by volume of the composition.

9. A monopropellant composition comprising nitromethane in an amount of about 80% by volume of the composition and ethylene oxide in an amount of about 20% by volume of the composition.

10. A monopropellant composition comprising nitromethane in an amount of about 60% by volume of the composition and ethylene oxide in an amount of about by volume of the composition.

References Cited in the file of this patent UNITED STATES PATENTS Hannum Oct. 25, 1955 OTHER REFERENCES Wilson: The Stability of Ethylene Oxide, JARS, vol. 23, November-December 1953, pages 3689.

Jet Propulsion Journal of the American Rocket Society, vol. 24, No. 2 March-April 1954), pages 111, 112, 121.

Journal of the American Rocket Society, vol. 23, November-December 1953, pp. 368-369.

Properties of Ethylene Oxide and Hydrazine Related to Their Use as Propellants, Thesis by William Clay Robison, California Institute of Technology, Pasadena, California, June 1953 (submitted in partial fulfillment of requirements for degree of Aeronautical Engineer), pp.

Claims (1)

1. IN THE METHOD OF PRODUCING THRUST FOR PROPULSION BY COMBUSTING LIQUID NITROALKANE IN A COMBUSTING CHAMBER, THE IMPROVEMENT WHICH COMPRISES COMBUSTING SAID LIQUID NITROALKANE IN THE PRESENCE OF LOWER ALKYLENE OXIDE.