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.

---

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

---

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

---

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

---

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	395	469.2	422.3	12.38	61%	32%
methane	3.5	1262	101	438	890	386.4	347.8	15.61	77%	22%
ethane	3.2	1262	100	638	1024	384.3	345.9	20.64	101%	35%
propane	3.1	1262	95	718	1065	382.2	344.0	21.34	105%	40%
butane	3	1262	145	725	1065	374.1	336.7	20.81	102%	54%
RP-1	2.7	1262	234	867	1124	375.9	338.3	20.4	100%	23%
o-xylene	2.6	1262	235	907	1138	372	334.8	18.82	92%	21%
furfural alcohol	1.5	1262	269	1151	1215	356.9	321.2	13.16	65%	24%
ethylene	2.6	1262	114	640	994	388.4	349.6	21.36	105%	29%
propylene	2.7	1262	98	753	1067	385.7	347.1	23.27	114%	39%
1,2-butadiene	2.6	1262	147	801	1088	387.1	348.4	25.01	123%	37%
1,3-butadiene	2.5	1262	174	750	1056	382.9	344.6	21.3	104%	41%
1,3-cyclopentadiene	2.4	1262	198	896	1127	378.8	340.9	22.12	108%	28%
cyclopropane	2.6	1262	156	794	1084	388.8	349.9	25.75	126%	26%
UDMH	1.8	1262	206	860	1081	385.4	346.9	23.78	117%	23%
methylacetylene	2.3	1262	180	751	1046	391.1	352.0	25.21	124%	25%
propargyl alcohol	1.6	1262	231	1013	1153	378.1	340.3	22.83	112%	21%

---

Table 3: Performance of H2O2 at 298 K plus Fuels at NBP or 298 K

		Oxidizer	Fuel	Fuel	Overall	Vacuum	90% of	Payload	% of
	MR	Density	Temp	Density	Density	Isp	Vacuum	metric tons	H2O2/RP-1
		kg/m^3	K	kg/m^3	kg/m^3	100 to 1	Isp	to LEO
H2	15	1440	20	70	648	384.1	345.7	3.81	45%
methane	8.4	1440	112	423	1147	345.8	311.2	5.32	63%
ethane	7.8	1440	184	544	1213	346.1	311.5	7.24	85%
propane	7.7	1440	231	582	1231	345.6	311.0	7.42	88%
butane	7.5	1440	298	573	1222	345.4	310.9	7.13	84%
RP-1	7.2	1440	298	800	1312	343.7	309.3	8.48	100%
o-xylene	6.7	1440	298	875	1329	343.2	308.9	8.65	102%
furfural alcohol	3.8	1440	298	1126	1361	337.3	303.6	6.03	71%
ethylene	7	1440	169	569	1209	351.1	316.0	9.81	116%
propylene	7.2	1440	225	611	1236	349.5	314.6	9.73	115%
1,2-butadiene	6.7	1440	298	645	1241	351.4	316.3	10.92	129%
1,3-butadiene	6.8	1440	298	614	1228	348.9	314.0	9.15	108%
1,3-cyclopentadiene	6.6	1440	298	796	1301	347.6	312.8	10.41	123%
cyclopropane	7.1	1440	240	698	1273	351.5	316.4	11.91	140%
UDMH	4.5	1440	298	786	1251	349	314.1	9.85	116%
methylacetylene	6.4	1440	250	671	1247	349.2	314.3	9.86	116%
propargyl alcohol	4.1	1440	298	944	1306	349.8	314.8	11.83	140%

---

Table 4: Propellant Properties: First density is at NBP, or for those propellants which are liquid at near-ambient conditions, 298 K (bold). RP-1 has no defined NBP or MP: values shown are density 820 at 298 K, and 867 at 233 K Second density is at Melting Point plus 10 K (except liquid hydrogen chilled density taken as 77 at melting point (slush point) of 14 K)

		Hf	Hf	NBP	Density, NBP	MP	Density, MP +10
		kcal/mole	kJ/mole	K	kg/m^3	K	kg/m^3
liquid oxygen	O2	-3.08	-12.89	90	1140	54	1262
hydrogen peroxide	H2O2	-32.53	-136.11	423	1440	272	1460
liquid hydrogen	H2	-2.15	-9.00	20	70	14	77 @14 K
methane	CH4	-21.4	-89.54	112	423	91	438
ethane	C2H6	-23.7	-99.16	184	544	90	638
propane	C3H8	-23.6	-98.74	231	582	86	718
butane	C4H10	-33.9	-141.7	273	573	135	725
RP-1	...CH2...	-5.7	-23.85	NA	820	NA	867
o-xylene	C8H10	-5.8	18.99	418	875	225	907
furfural alcohol	C5H6O2	-52.2	-218.40	443	1126	259	1151
ethylene	C2H4	8.1	33.89	169	569	104	640
propylene	C3H6	4.7	19.66	225	611	88	753
1,2-butadiene	C4H6	33.6	140.70	284	645	137	801
1,3-butadiene	C4H6	21.2	88.7	269	614	164	750
1,3-cyclopentadiene	C5H6	31.7	132.63	314	796	188	896
cyclopropane	C3H6	13	54.39	240	698	146	794
UDMH	C2H8N2	11.9	49.79	336	786	196	860
methylacetylene	C3H4	39.8	166.52	250	671	170	751
propargyl alcohol	C3H4O	10.1	42.2	387	944	221	1013

---

Fuel Characteristics

Methane: CH4 / Ethane: C2H6 / Propane: C3H8 / Butane: C4H10

• from natural gas, inexpensive and widely available
• fuel grades have added sulfur compounds, which may cause cooling passage deposits

RP-1: ..CH2..

• a large variety of grades of kerosene-like materials are available, which differ in their chemical composition, volatility, density, and coking characteristics
• SSTO market is big enough to support production of special grades designed for specific engines

o-xylene: C8H10

• the best of the three xylene isomers, could most probably be produced in pure form if there were a market

furfural alcohol: C5H6O2

• very dense - formerly used as a hypergolic fuel with nitric acid
• produced from furfural, which comes from the action of heat and acid on certain woody materials
• in 1980, annual production >100,000 tons per year and price about $1.25/kg

ethylene: C2H4

propylene C3H6

1,3-Butadiene: C4H6

• major industrial chemicals used for making polyethylene, polypropylene, and synthetic rubber
• costs on the order of $1 per kg or less
• might cause polymerization problems if used for regenerative cooling
• polymerization might be reduced by prechilling

1,2 butadiene

• 1,2-butadiene is not a major commercial chemical, but is an impurity in the production of 1,3-butadiene
• better density and heat of formation than 1,3-butadiene
• methods exist for isolating it from product streams, but there is currently no market for the compound

1,3-cyclopentadiene

• a major industrial chemical - 1990 production about 150,000 tons per year
• must be kept chilled to consider use - at room temperature it spontaneously dimerizes at 3.5% per hour
• dimer is a solid at room temperature, and energetically less favorable
• the tetrahydro derivative is stable, and has been used as a high density fuel for cruise missiles (JP-10)

cyclopropane: C3H6

• formerly used as a medical anesthetic
• very expensive (cost unknown, but probably >$100 per kg) and probably unreasonable as a candidate for large scale use unless a new and cheaper synthetic route were developed
• considerable strain energy stored in 3 membered ring
• the former USSR has used “sintin” as a high performance hydrocarbon fuel for upper stage use - one reference suggests that this may be a cyclopropane derivative

UDMH : (CH3)2NNH2, or C2H8N2

• unsymmetrical dimethyl hydrazine : widely used rocket fuel, but expensive and toxic
• a 50/50 mixture of UDMH and hydrazine is even better performing (not considered here)

methylacetylene: C3H4

• pure methylacetylene is non-explosive as a bulk liquid, but vapors will explode if strongly heated
• highly positive heat of formation - methylacetylene has been considered for use as a monopropellant
• methylacetylene can be shipped with the same shipping restrictions as propane
• methylacetylene not commercially available in bulk, but could in principle be available if there were a market
• cost unknown, but unlikely to exceed $10 per kg, based on cost of MAPP gas (see below)

propargyl alcohol: C3H4O

• propargyl alcohol (ignoring proper chemical terminology) could be called “methylacetylene alcohol”
• propargyl alcohol is a common industrial chemical, liquid at room temperature, high density
• cost about $6 per kg in 1991, shipped by tank car load
• toxic, suspect carcinogen

MAPP Gas:

• methylacetylene spontaneously isomerizes to propadiene, and crude methylacetylene often contains up to 30% propadiene
• propadiene has a very slightly more positive heat of formation than methylacetylene - for calculation purposes, treat any propadiene content as if it were methylacetylene
• a mixture of methylacetylene and propadiene, diluted with propane, propylene and butane, is sold as MAPP® gas
• retail cost in cylinders to the welding industry is about $4.00 per kg in 1996
• available in tanker truck loads for distribution to welding supply companies, who refill MAPP cylinders
• MAPP gas specifications are below
• for computational purposes, a 40:60 mix of methylacetylene:propylene would be a reasonable approximation to MAPP gas

Compound	Concentrations in Commercial MAPP
methylacetylene/propadiene	40-48%
propane	15% maximum
saturated C4 hydrocarbons	4-10 %
1,3-butadiene	1% max
propylene	balance

Return to Home Page