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) Liquid H2 Slush H2 Methane JP-7 Kerosene 116.7 116.6 50.0 43.9 42.8 Energy/Volume (MJ/Lt) 8.2 9.8 20.8 34.7 34.2 Density (kg/m3) 71 82 424 790 800

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 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 810 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 scramjetpowered vehicles, Journal of Propulsion and Power, vol. 17, no. 6, 2001, pp. 1199-1204

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