LIQUID FUELS -- by Bruce Dunn (1997)


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)