This document reviews hydrogen and hydrocarbon fuels for hypersonic engine applications. Hydrogen fuels are generally used for high-speed space entry due to their high specific impulse, allowing flight up to Mach 15. However, hydrogen has low density, requiring larger fuel tanks. Hydrocarbon fuels have lower performance but higher density, making them suitable for hypersonic missiles up to Mach 10. Future research may blur the lines, with active cooling extending hydrocarbon engine operation and supplementing hydrogen fuels to increase thrust.

1. HYDROGEN VERSUS HYDROCARBONS
A LITERATURE REVIEW OF HYPERSONIC ENGINE FUEL OPTIONS
Introduction
Scramjets offer many advantages over conventional rocket design, principally the fact
they are air-breathing engines and do not require the need to carry an additional oxidiser
(generally in the form of liquid oxygen) [1]. This allows the physical size and mass of the
scramjet powered vehicle to be considerably smaller in comparison, significantly
reducing the cost of the vehicle. However, there is a major decision to be made in the
combustion processes utilized for hypersonic flight, the selection of the base fuel to be
used.
This combustion product selection focuses on two main types, being hydrogen based
fuels or hydrocarbon alternatives. The choice in fuels is generally made with careful
consideration to the mission type of the vehicle. Of the two main areas for currently
envisaged for hypersonic flight, hydrogen fuelled scramjet engines are more commonly
associated with high speed engines required for entry into space and orbit. The
hydrocarbon fuelled alternatives are currently the choice for military applications and the
development of missile projectiles at slower speeds [2].
This report will briefly describe the background of fuel choice in hypersonic engine
development. It will also explain the principal uses of each of the types of combustion
products, their advantages and disadvantages and where the technology is heading for the
future.
Background and historical fuel uses
Air breathing engines capable of reaching hypersonic flight speeds have been
investigated and studied since the early 1960s [3] (albeit somewhat sporadically with
changing policy and funding resources). Extensive research was conducted in during the
period to 1973 into the use of hydrogen fuelled scramjet as a means for a single stage to
orbit (SSTO) space vehicle. Most of the tests conducted in these early periods were
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2. conducted with space entry in mind and as such, made use of hydrogen as their principal
fuel source. However, there were smaller projects funded by US Air Force and Navy into
hydrocarbon fuelled engines as a means of missile delivery [3].
The period from 1973 to 1994 was initially one of low activity in hypersonic research due
to funding limitations, before the initialising of the National Aerospace Plane (NASP)
program in the United States in the early 1980s. This project began a reinvigoration of
hypersonic research. After this period and continuing into the new century, different
directions of research have been pursued by different parties. NASA and other
international agencies continued to pursue the use of hydrogen fuelled research vehicles.
These concepts are aimed at developing sufficient engine technology for vehicle access to
space via either a single or two stage to orbit system. [2] This said, there is still vast
interest by many parties, particularly the US Air Force, in investigating and developing
technology in an effort to design liquid hydrocarbon fuelled scramjet missiles.
Hydrogen fuelled scramjets
The performance of scramjet engines, principally measured by its specific impulse, is far
greater than rocket designs. As can be seen in figure 1 below, for both common types of
fuels within the industry, the use of scramjets lends itself to reaching high Mach numbers,
of particular interest in space and orbit programs as well as high speed missiles.
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3. Figure 1: Performance by engine type
(courtesy Fry [3])
Some basic properties of liquid hydrogen and common hydrocarbons are listed in
Table 1, adapted from work by Lewis [4].
Table 1: Basic properties of hydrogen and some common hydrocarbons
Fuel Energy/Mass
(MJ/kg)
Energy/Volume
(MJ/Lt)
Density
(kg/m3
)
Liquid H2 116.7 8.2 71
Slush H2 116.6 9.8 82
Methane 50.0 20.8 424
JP-7 43.9 34.7 790
Kerosene 42.8 34.2 800
This clearly shows the much higher energy per unit mass of cryogenic hydrogen, but its
smaller density causes a significantly lower energy per unit volume when compared to
various hydrocarbon derivatives. This creates difficulties in that a hydrogen powered
scramjet vehicle will be comparatively larger than a hydrocarbon equivalent as it must
carry a greater volume of fuel.
The vastly higher specific impulse attainable by hydrogen propulsion makes this the
option of choice when considering the use of hypersonic vehicles in space entry. Its fast
burning rate and high specific energy content enable it to be ones of the only fuels able to
produce the required thrust at orbital velocities [1,4]. It is believed hydrogen fuelled
scramjets engines are capable of operating with the range of Mach 5 to 15. In a recent
study by Tetlow and Doolan [1] into the comparison of hydrogen and hydrocarbon
fuelled scramjet engines for orbital delivery, the hydrogen variant was found to be the
clear choice. The higher specific impulse and extended operating range of the hydrogen
vehicle was found to vastly outweigh the increased fuel storage density of a hydrocarbon
equivalent when considering orbital type missions.
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4. The use of hydrogen as the fuel source also provides benefits with respect cooling
abilities. As the hydrogen is stored at very low temperatures, this can be used as an active
cooling area on temperature critical parts of the vehicle body [5]. This procedure is
generally not available to standard hydrocarbon fuels.
The principal disadvantages of the use of hydrogen as the fuel of choice in scramjet
combustion lie with its very low density and lower energy per unit volume compare to
hydrocarbon variants. The added size and mass from the extra fuel and storage containers
within the vehicle design are major factors in fuel selection. This added weight has a
compounding effect as although there have been significant attempts to develop single
stage to orbit scramjet vehicles, most still rely on some additional mechanism to achieve
the speeds required for the engine combustion to be sustainable. The main methods for
this are currently through use of a low speed engine being incorporated into the design of
the vehicle, or some form of booster rocket or assistance to reach the high speeds. This
means any added weight in the vehicle itself will require a larger secondary engine or
platform to propel it, compounding any efforts for minimum size. Additional design
implications arise with the need to store the fuel cryogenically, in liquid or hydrogen
slosh form and the complexity and weight to achieve this.
Hydrocarbon fuelled scramjets
Hydrocarbon fuels are more readily associated with current aerospace and military flight
applications. Its relative ease of use and storage ability has made it the popular choice to
date for the majority of conventional aerospace applications. Principally, conventional
hydrocarbons fuels are grouped into two categories, methane and its derivatives or the
kerosene (JP) range of combustible fuels.
The popularity of hydrocarbon fuels lies somewhat in their packaging and storage
capacity. Hydrocarbon fuels are up to 11 times more dense than liquid hydrogen [4] and
although they posses far less energy capacity per unit mass, their overall energy per unit
volume is still up to 3 ½ times greater than that of hydrogen fuels. This greater energy
content lends itself to a greater volumetric efficiency of the hypersonic craft, allowing for
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5. a much smaller tank sizes. This will potentially lead into a significantly smaller overall
vehicle design [1].
In comparison to hydrogen fuelled scramjet cycles, hydrocarbon systems have a much
smaller specific impulse and cannot propel the vehicle to the equivalent velocities [5,6].
Waltrup states that there was early speculation as far back at the 1950s that suggested it
would be possible for hydrogen fuelled systems to be able to reach orbital speeds of up to
Mach 26, while hydrocarbon cycles could expect speeds in the Mach 14-16 range [6].
Continuing studies over the years reduced these figures, with Waltrup concentrating his
research on deducing the upper Mach limit of hydrocarbon powered vehicles. He found
there was significant modern research into the upper limits of hydrogen fuelled concept
vehicles for space entry, found to be in a range between Mach 12-16. During the study,
he investigated the use of a scramjet missile using JP-7 endothermic liquid hydrocarbon
fuel over as a function of mission acceleration and overall vehicle design [6]. Waltrup
found the upper limits on flight Mach number for such fuelled scramjets to be between
Mach 9 and 10, significantly less than that expected of a hydrogen fuelled variety.
Recent technology advances have occurred in scramjet engine development focusing on
missile design, created through the easier parameters and conditions under which they
operate. Due to the limitation on upper Mach number, the added effects past Mach 10 are
not of concern to these designs, as such concentration can be focused in other areas.
Lewis reported extensively on the optimal fuel selection for hypersonic vehicle range,
comparing cryogenic hydrogen combustion processes to that of hydrocarbon varieties [4].
He found that for an optimised case of a hypersonic cruiser, operating between Mach 8-
10 region, the choice of hydrocarbon as the combustion fuel had a positive effect on the
operational range of the vehicle. This showed that for this mission type, where long range
is a key requirement in preference to orbital entry, the hydrocarbon powered hypersonic
engine cycle is superior up to its upper limit of Mach 10.
Where to in the future
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6. Historically there has been a significant divide in the mission capability of hydrogen
versus hydrocarbon fuelled scramjets. With continuing research and technology
improvements there is an ever increasing blurring of the lines with respect to which fuels
is optimal in given situations. One developing area is the use of active cooling in
hydrocarbon fuelled systems. Through the use of endothermic fuels this method can be
used to greatly increase the performance capabilities of the slower burning hydrocarbon
fuels. It has been suggested by Curran that in order to maintain thermal balance within
the engine, active cooling methods will be essential on these particular variants for speeds
greater than Mach 6.5.
In an analysis by Pike [5] it has also been shown that there is scope to increase the thrust
of hydrogen fuelled scramjet vehicles by replacing the additional hydrogen in the
combustion process that is above the stoichiometric burning ratio with other materials.
This includes the supplementing with inert gases such as neon, hydrogen-oxygen
mixtures or even a hydrocarbon fuel. The advantage of this method is that it can
significantly increase the thrust of the engine, thus allowing for a reduced size and mass
for an equivalent payload.
Conclusion
This review has given a brief outline of the two main types of fuels available for
hypersonic engine combustion and their common purposes and mission variants. It is
generally accepted that hydrogen fuels are the optimum choice for designs and vehicles
where a high impulse and quick burning time is required, principally being space entry
mission types. Hydrocarbon fuelled scramjet engines are presently restricted in their
upper Mach levels when compared to their hydrogen counterparts and as such are better
suited to hypersonic missile programs. This complements their smaller size and mass due
to the greater density of hydrocarbon forms.
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7. References
[1] Tetlow, MR & Doolan, CJ, ‘Comparison of hydrogen and hydrocarbon-fuelled
scramjet engines for orbital insertion’, Journal of Spacecraft and Rockets, vol. 44,
no. 2, 2007, pp. 365-373
[2] Curran, ET, ‘Scramjet engines: The first forty years’, Journal of Propulsion and
Power, vol. 17, no. 6, 2001, pp. 1138-1148
[3] Fry, RS, ‘A century of ramjet propulsion technology evolution’, Journal of
Propulsion and Power, vol. 20, no. 1, 2004, pp. 27-58
[4] Lewis, MJ, ‘Significance of fuel selection for hypersonic vehicle range’, Journal
of Propulsion and Power, vol. 17, no. 6, 2001, pp. 1214-1221
[5] Pike, J, ‘The choice of propellants: A similarity analysis of scramjet second
stages’, Philosophical transactions: Mathematical, physical and engineering
sciences, vol. 357, no. 1759, 1999, pp. 2357-2378
[6] Waltrup, PJ, ‘Upper bounds on the flight speed of hydrocarbon-fueled scramjet-
powered vehicles’, Journal of Propulsion and Power, vol. 17, no. 6, 2001, pp.
1199-1204
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