A number of alternate fuels have been proposed for use with liquid oxygen. Here are some characteristics of these propellant combinations, in comparison with the standard fuels hydrogen and RP-1 (Rocket Propellant 1 = kerosene). I have only included fuels with an Isp greater than that of RP-1, and have ignored exotics like boranes. See the table notes at the end of the article for details of how the calculations were done. Tank Temp Mixture Fuel Bulk Vac Formula Ratio Dens. Dens. Tc Isp K O2/Fuel kg/m^3 kg/m^3 K 100:1 NON-HYDROCARBONS hydrogen, NBP 20 H2 6.0 70 358 3610 455.9 UDMH, RT 298 C2H8N2 1.6 786 972 3710 365.4 ALKANES methane, NBP 112 CH4 3.0 423 801 3589 368.3 ethane, NBP 184 C2H6 2.7 544 880 3671 364.6 propane, NBP 231 C3H8 2.7 582 905 3734 361.9 propane, 100K 100 C3H8 2.7 782 1014 3734 361.9 butane, NBP 273 C4H10 2.6 573 894 3734 360.9 RP-1, RT 298 C12H24 2.5 820 1026 3803 354.6 UNSATURATED HYDROCARBONS ethylene, NBP 169 C2H4 2.3 569 874 3888 366.9 propylene, NBP 225 C3H6 2.3 611 903 3842 364 1,2-butadiene, NBP 284 C4H6 2.1 645 914 3982 363.5 1,3-butadiene, NBP 269 C4H6 2.1 614 893 3917 359.4 methylacetylene, NBP 250 C3H4 1.9 671 919 4034 366.9 Hydrogen has an excellent Isp but a rotten bulk density. RP-1 gives an excellent bulk density, but a much lower Isp. There are a number of alternate propellants shown in the table which fall somewhere between the two, having a slightly higher Isp than RP-1 but a lower bulk density. UDMH is normally used mixed with straight hydrazine (N2H4) and is normally burned with N2O4 as an oxidizer. It burns perfectly well however with liquid oxygen, and is a superior fuel provided one can tolerate its toxicity and cost. The light alkanes are all readily available in industrial quantities, are good coolants. Going from RP-1 to methane gains 3.8 % in Isp, but costs about 22% in density. Other alkanes lie between the performance of methane and RP-1. Propane at room temperature is a non-starter for pump fed engines, as its vapor pressure is too high for light weight tanks. Propane is unusual in that it will not freeze solid if put in tanks in thermal contact with LOX tanks; it has been proposed therefore to use sub-cooled propane. Calculations done here show both propane at its normal boiling point, and at 100 K, about 10 K above LOX temperature. Sub-cooled propane (at LOX temperatures or slightly above) is a winner, with a bulk density nearly the same as that of RP-1, and a superior Isp. The light unsaturated compounds are also readily available in industrial quantities. These compounds may possibly give polymerization problems when used for engine cooling, but again, they may not (particular those which are very cold to start with may not warm up enough to cause problems). They don't seem to be superior enough to alkanes to make their use worth while, particularly considering that they generally have higher chamber temperatures than for alkanes with the same Isp. Table Notes: Isp calculations were performed on a partial equilibrium basis, assuming that recombination reactions stop at the nozzle throat (an approximation of real world behavior). Isp values are theoretical values for 100:1 expansion into vacuum, and assume a chamber pressure of 20 MPa (approximately 2900 psi). These conditions are representative of a high pressure, high expansion ratio engine that might be used for an SSTO. Fuel storage temperature is generally at Normal Boiling Point (NBP) which is the temperature at which a fuel will equilibrate at if tanks are vented to the atmosphere during filling. Exceptions are RP-1 and UDMH at Room Temperature (298 K), and one calculation in which propane is chilled to 100 K, about 10 K above its freezing temperature. Mixture ratios are expressed as the mass of oxygen divided by the mass of the fuel. The mixture ratios given are the optimum for maximal Isp, except for H2 which is calculated at a mixture ratio of 6:1, rather than the more optimal 4.4 which would give a higher Isp, but a much poorer bulk density. Fuel densities are the density in the fuel tanks. Bulk density is the overall density of the propellant combination, with the stated mixture ratio and oxygen at a density of 1140 kg/m^3 Tc is the chamber temperature - it is higher for unsaturated hydrocarbons than for alkanes, as the former have energy locked up in the structure of their molecules which adds to the energy available from oxidation. RP-1 is shown with a molecular formula of C12H24 ; this is an approximation to something like the average molecular formula (RP-1 has an H to C ratio of approximately 2, and an average molecular weight somewhere near that of hexadecane, C12H26)
Curator: Al Globus
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