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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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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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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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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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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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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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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