Alternate Propellants for SSTO Launchers

Dr. Bruce Dunn

Adapted from a Presentation at:

Space Access 96

Phoenix Arizona

April 25 - 27, 1996

Introduction

The most commonly proposed propellant combination for an SSTO launcher is liquid oxygen and liquid hydrogen, at a mixture ratio of approximately 6.0. There have been a number of studies of alternate fuels for SSTO launchers, but they have been limited. To date, most studies have concentrated on methane, propane and RP-1 burned with liquid oxygen to the exclusion of other oxidizers and other fuels. These studies have often, but not always shown lower vehicle dry masses for hydrocarbon propellants (for the same payload size). The lowest dry masses of all are found in dual-fuel vehicles, using dense hydrocarbons early in the flight and hydrogen late in the ascent. These vehicles however suffer from mechanical and structural complexity over their single-fuel cousins, and are unlikely to represent the least expensive way to get a defined payload to orbit.

In the current study, a very simple model was set up to explore a wide range of propellant combinations for SSTO use. The model also is used to determine the advantages of pre-chilling propellants before loading, to increase their density. The model uses a constant volume SSTO (1000 cubic meters tank size) which can only load about 350 metric tons of liquid oxygen and liquid hydrogen. The same vehicle can hold up to approximately 1000 tons of alternate propellants when liquid oxygen is used as an oxidizer, and about 1300 metric tons of propellants when hydrogen peroxide is substituted for liquid oxygen. As propellant density is increased, there is some increase in structural mass (for example in thrust structures) and in engine mass. Much of the rest of the vehicle however has little or no increase in mass (avionics, thermal protection, landing gear, payload accommodation). In particular it should be noted that propellant tank structure is largely determined by internal pressure, and tank mass does not scale with the mass of tank contents. Also, while engine thrust must scale up in proportion to propellant load to allow takeoff, the engine turbopumps do not grow more powerful or heavier, as they must pump the same volume of propellant in the same time as for a hydrogen/oxygen vehicle. The model roughly models the increase in dry mass (see below) but by no means represents a proper analysis of the dry masses associated with different propellant combinations.

Methodology

• only propellants with environmentally friendly exhausts (no chlorine or fluorine compounds allowed)
• only propellants with sufficient stability to be seriously considered for use (no ozone, no acetylene)
• a total propellant volume of 1000 m^3
• payload delivered to a delta V of 9500 m/sec (this is a surrogate for a real-world delta V to LEO of perhaps 9200-9300 m/sec, plus maneuvering and landing propellant)
• engines have a chamber pressure of 20 MPa (2900 psi)
• engines have 100:1 expansion ratio (assumes altitude compensating nozzles to allow sea level operation)
• propellant density at different temperatures determined using equations of state from “Physical and Thermodynamical Properties of Pure Chemicals” except for UDMH and RP-1, taken from “Propellant Chemistry” by Stanley S. Sarner (Reinhold Publishing, 1966)
• Isp determined by an MS-DOS specific impulse program – program information and download
• SSTOs use an Isp of 90% of the theoretical shifting equilibrium Isp (this is a surrogate for the losses induced by less than 100% efficient engines, and for losses for atmospheric operations)

Dry Mass

• for LH2/LO2 propellant - 20 metric tons plus 3% of propellant weight for engines

• for all other propellants - 25 metric tons plus 2% of propellant weight for engines (vehicles other than LH2/LO2 have heavier thrust structures, accounting for the extra 5 tons allowed)
• 3% of propellant weight corresponds to engine specific thrust of approximately 500 N/kg, or thrust to weight of 50:1, similar to SSME
• 2% of propellant weight corresponds to engine specific thrust of approximately 800 N/kg, or thrust to weight of 80:1, similar to RD-170
• all vehicles burn the same volume of propellant in the same time period, therefore have pumps of the same power (this is reflected in the lower specific thrust of the LH2/LO2 engines)
• no change is made in non-engine dry mass when switching from LOX to H2O2 as an oxidizer (the room temperature storability of H2O2 is assumed to give enough mass savings to offset the more massive thrust structures required for the heavier propellant load)

Limitations of Modeling

• model vehicle parameters have been adjusted to match the approximate characteristics of hypothetical Vertical Takeoff Vertical Landing SSTOs - the model may not accurately reflect the trade-offs for other SSTO types
• payloads are for comparative purposes only - even competent designers cannot now agree on the structural mass and performance of hypothetical SSTO vehicles
• the model only roughly accounts for dry mass differences between equal volume SSTOs using hydrogen/oxygen and those using denser propellants
• comparisons between the payloads with hydrogen/oxygen and those with other propellants should be done cautiously - much better comparisons can be made when the propellant combinations under examination are roughly similar in density and Isp
• engine modeling ignores:
• theoretical vs. delivered Isp (other than use of an arbitrary 90% of theoretical Isp in calculations)
• more negative heat of formation of chilled propellants
• combustion temperature (which affects cooling and engine design)
• properties of fuels as coolants (including possibility of polymerization of unsaturated hydrocarbons)
• properties of fuels for gas generator use
• viscosity of propellants at reduced temperature (no check has been made to ensure that propellants aren’t syrup at 10 K above their melting points)

Results

• Propellant and payload details are given in tables 1 to 3, while propellant properties are given in table 4. Other values (such as for example the dry weights, loaded weights and propellant fractions of vehicles) can be calculated from the characteristics of the model.

• In the current model, most propellant combinations beat hydrogen/oxygen. This is a direct result of assuming a constant-size rather than constant-mass vehicle for all propellants, regardless of density. In practice, a hydrogen/oxygen vehicle would be built much larger than one with dense propellants, in order to deliver an equivalent payload.

• With propellants at their normal boiling point (or at 25 C for those storable without excessive pressure), there are 4 propellant combinations that give a 10% or better performance relative to the classic combination of RP-1 and LO2. The high performers (UDMH, methylacetylene, propargyl alcohol, and cyclopropane) are marked in bold.

• With pre-chilled propellants at 10 K above their melting points, the increase in payload attributable to the higher density ranges from approximately 20 to 30%, with occasional propellants showing greater increases. With prechilling, propylene and 1,2 butadiene also become interesting.

• Hydrogen peroxide as an oxidizer gives much poorer performance than liquid oxygen. It is notable however that some combinations of fuel with peroxide equal or exceed the performance of LH2/LO2, and no fewer than 8 separate fuels are at least 10% better performing than RP-1 when coupled with peroxide. Particularly notable is the combination of peroxide and propargyl alcohol, in which both propellants are room temperature storable liquids and the delivered payload is 40% greater than that using RP-1.

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For use with LO2

The Good

LO2/H2

• the classic high specific impulse combination - well understood with engines in production
• not an impressive performer in this model, but performs well in systems which are not volume limited
• about a 32% increase in payload for prechilling

LO2/RP-1

• the classic medium specific impulse combination - well understood with engines in production
• other denser hydrocarbon blends may give slightly improved performance (see JP-10 under description of cyclopentadiene)
• about a 23% increase in payload for prechilling

LO2/propane

• if stored at room temperature under pressure or even at its NBP, is not dense enough to compete with RP-1
• if prechilled, is marginally better than RP-1 but can’t compete with propylene or methylacetylene

LO2/UDMH

• about 16% more payload than RP-1
• gains an additional 23% payload when prechilled
• a well understood, if somewhat toxic and expensive fuel
• combination has been burned in Soviet engines
• even better performance would be obtained with 50/50 UDMH/N2H4 (not modeled here)

LO2/methylacetylene

• about 21% more payload than RP-1
• gains an additional 25% payload when prechilled
• non-toxic, with same shipping restrictions as propane
• commercial MAPP gas is likely to have a performance intermediate between methylacetylene and propylene

LO2/propargyl alcohol

• about 13% more payload than RP-1
• gains an additional 21% payload when prechilled
• higher density but less favorable formula and enthalpy of formation than methylacetylene
• a stable liquid at room temperature
• toxic and suspect carcinogen

LO2/1,2-butadiene

• gives about 9% more payload than RP-1
• gains an additional 37% payload when prechilled
• not to be confused with 1,3-butadiene, which is the common “butadiene” of polymer chemistry
• liquid at room temperature under slight pressure (about 1.5 atmospheres)

LO2/propylene

• no better than RP-1 at its normal boiling point, but gains 39% more payload when prechilled
• when prechilled, outperforms prechilled RP-1 by 14%

The Bad and the Ugly

• O2/methane - poorer than RP-1 and more difficult to handle
• O2/ethylene - no improvement on RP-1 and more difficult to handle
• O2/ethane - no improvement on RP-1 and more difficult to handle
• O2/cyclopropane - high performing but extremely expensive, and no better than methylacetylene
• O2/1,3-butadiene - no substantial improvement on RP-1
• O2/butane - poorer than RP-1
• O2/1,3-cyclopentadiene - only slightly better than RP-1, and difficult to handle (dimerizes spontaneously)
• O2/furfural alcohol - very dense but poorer than RP-1
• O2/o-xylene - poorer than RP-1

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For Use with H2O2

The Good

H2O2/RP-1

• peroxide and kerosene type fuels are a classic storable combination
• there is however no experience with high pressure turbopump fed engines
• other denser hydrocarbon blends may give slightly improved performance (see JP-10 under description of cyclopentadiene)

H2O2/UDMH

• about 16% more payload than RP-1
• a well understood, if somewhat toxic and expensive fuel
• is hypergolic with peroxide
• even better performance would be obtained with 50/50 UDMH/N2H4 (not modeled here)

H2O2/methylacetylene

• about 16 % more payload than RP-1
• non-toxic, with same shipping restrictions as propane
• commercial MAPP gas is likely to have a performance intermediate between methylacetylene and propylene
• possibly hypergolic with peroxide

H2O2/propargyl alcohol

• about 40 % more payload than RP-1
• compound is structurally “methyl acetylene alcohol”
• higher density but less favorable formula and enthalpy of formation than methylacetylene
• a stable liquid at room temperature
• possibly hypergolic with peroxide
• toxic and possibly carcinogenic

H2O2/propylene

• about 15% more payload than RP-1
• inexpensive and easily handled (properties are similar to propane)
• possibly hypergolic with peroxide

H2O2/1,2-butadiene

• gives about 9% more payload than RP-1
• not to be confused with 1,3-butadiene, which is the common “butadiene” of polymer chemistry
• availability and price uncertain
• liquid at room temperature under slight pressure (about 1.5 atmospheres)
• possibly hypergolic with peroxide

The Bad and the Ugly

• H2O2/hydrogen - almost no payload
• H2O2/methane - poorer than RP-1 and more difficult to handle
• H2O2/ethylene - no improvement on the chemically similar propylene and more difficult to handle
• H2O2/ethane - poorer than RP-1 and more difficult to handle
• H2O2/cyclopropane - high performing but extremely expensive, and no better than propargyl alcohol
• H2O2/propane - poorer than RP-1 and more difficult to handle
• H2O2/1,3-butadiene - only marginal improvement on RP-1, and poorer than alternatives
• H2O2/butane - much poorer than RP-1
• H2O2/1,3-cyclopentadiene - poorer than some alternatives and difficult to handle (dimerizes spontaneously)
• H2O2/furfural alcohol - very dense but poorer than RP-1
• H2O2/o-xylene - no improvement on RP-1

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Data Tables

Table 1: Performance of Liquid Oxygen at Normal Boiling Point (90K) plus Fuels at either 298 K or their NBP

		Oxidizer	Fuel	Fuel	Overall	Vacuum	90% of	Payload	% of
	MR^*	Density	Temp	Density	Density	Isp	Vacuum	metric tons	LOX/RP-1
		kg/m^3	K	kg/m^3	kg/m^3	100 to 1	Isp	to LEO
H2	6	1140	20	70	358	469.2	422.3	9.35	56%
methane	3.5	1140	112	423	828	386.4	347.8	12.78	77%
ethane	3.2	1140	184	544	904	384.3	345.9	15.29	92%
propane	3.1	1140	231	582	924	382.2	344.0	15.21	91%
butane	3	1140	273	573	914	374.1	336.7	14.32	86%
RP-1	2.7	1140	298	820	1031	375.9	338.3	16.64	100%
o-xylene	2.6	1140	298	875	1052	372	334.8	15.51	93%
furfural alcohol	1.5	1140	298	1126	1134	356.9	321.2	10.62	64%
ethylene	2.6	1140	169	569	891	388.4	349.6	16.56	100%
propylene	2.7	1140	225	611	924	385.7	347.1	16.8	101%
1,2-butadiene	2.6	1140	284	645	940	387.1	348.4	18.21	109%
1,3-butadiene	2.5	1140	269	614	916	382.9	344.6	15.16	91%
1,3-cyclopentadiene	2.4	1140	298	796	1011	378.8	340.9	17.27	104%
cyclopropane	2.6	1140	240	698	969	388.8	349.9	20.36	122%
UDMH	1.8	1140	298	786	982	385.4	346.9	19.32	116%
methylacetylene	2.3	1140	250	671	941	391.1	352.0	20.17	121%
propargyl alcohol	1.6	1140	298	944	1056	378.1	340.3	18.8	113%

*MR is optimal, except 6 used for H2 to increase bulk density

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Table 2: Performance of Liquid Oxygen at Melting Point+10 K (64K) plus Fuels at MP+10K

		Oxidizer	Fuel	Fuel	Overall	Vacuum	90% of	Payload	% of	% increase
	MR	Density	Temp	Density	Density	Isp	Vacuum	metric tons	LOX/RP-1	for chilling
		kg/m^3	K	kg/m^3	kg/m^3	100 to 1	Isp	to LEO
H2	6	1262	14	77