These oxidizers could be stored at room temperature.

The name “enriched storable oxidizers” (ESOs) has been coined for a family of optimized mixtures of between two and four oxidizer fluids. For most applications, the constituents of these mixtures would be nitrogen tetroxide (N2O4), nitrous oxide (N2O), and nitrogen dioxide (NO2); in some applications, the mixtures might include inhibited red fuming nitric acid [IRFN (which consists of red fuming nitric acid to which some hydrogen fluoride is added to reduce its corrosive effect]. The optimum proportions of these constituents would be different for different applications. ESOs were originally proposed for use in spacecraft and launch-rocket propulsion systems: ESOs could be especially useful in advanced spacecraft propulsion systems that could operate in multiple modes. ESOs might also be useful in special terrestrial applications that could include ramjet and scramjet aircraft engines.

Specific Impulse Versus Oxidizer/Fuel Ratio was calculated for methane burned with either (1) N2O4, (2) an ESO comprising a mixture of 65 mole percent of N2O with 35 mole percent of N2O4, (3) N2O, or (4) liquid O2. The combustion-chamber and exit pressures used in the calculations were 1,000 psi (≈6.89 MPa) and 0.05 psi (≈345 Pa), respectively. Stoichiometry favors higher oxidizer/fuel ratios for the ESO than for liquid O2; this is fortuitous because, as the corresponding plots show, the higher mixture ratio yields higher specific impulse.
ESOs would offer an attractive alternative to liquid oxygen and other previously known oxidizer fluids, including the individual constituents of these mixtures:

  • Unlike liquid oxygen and fluorine-based oxidizers, which must be stored under cryogenic conditions, ESOs could be stored at room temperature.
  • In comparison with most previously known oxidizer fluids other than oxygen, nitrous oxide, and nitrogen peroxide, ESOs would be less toxic.
  • In comparison with most previously known oxidizer fluids other than nitrous oxide, ESOs would be less corrosive, and would be more chemically stable in storage.

Calculations have shown that ESOs would offer high energy densities and that specific-impulse levels attainable by use of ESOs would approach those attainable by use of liquid oxygen with two hydrocarbon fuels — RP-1 (rocket propellant 1, which is similar to kerosene) and methane (see figure). ESOs would be hypergolic or nearly hypergolic with methane and RP-1 and with other fuels that include Jet-A (also similar to kerosene), hydrazine, and monomethyl hydrazine. A computational simulation has predicted that only benign exhaust products would result from burning methane or RP-1 with one of the ESOs (a mixture of 35 mole percent of N2O4 with 65 mole percent of N2O): These exhaust products would be primarily CO2, H2O, and N2, plus very small amounts of O2.

This work was done by R. L. Sackheim of Marshall Space Flight Center and J. R. Herdy, Jr., of Qualis Corp.

This invention is owned by NASA, and a patent application has been filed. For further information, contact Sammy Nabors, MSFC Commercialization Assistance Lead, at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to MFS-32407-1.

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