From: Bruce Dunn <bruce_dunn@mindlink.bc.ca>
Newsgroups: sci.space.policy
Subject: Re: Kerosene vs Hydrogen fuelled SSTO rockets
Date: Sun, 09 Feb 1997 09:10:58 -0800

With respect to the subject of this thread, the choice of fuels for an
SSTO isn't just between kerosene and hydrogen.  A number of other fuels
are quite reasonable alternatives.  I reproduce below a cleaned-up
version of a post I recently made in sci.space.tech on the subject:


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               Fuel   Bulk         Vac.
                     Temp Formula   MR  Dens.  Dens.   Tc   Isp
                     K              O/F 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 (unsymmetrical dimethyl hydrazine) 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 to account for imperfect
chilling of propane by adjacent LOX tanks) 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 (MR) are expressed as the mass of oxidizer divided by the
mass of the fuel (O/F).  The MRs 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)


--
Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
http://www.genastro.com/  | 800-577-1117  | 604-876-7640
Reliable, low-cost transportation to low Earth orbit and beyond