As well as the efforts to refine current fuels, manufacturers are thinking just as hard about what new fuels might be required for future aircraft, as well as planning for the day when oil reserves inevitably dry up.

The US Air Force has developed specialised jet fuels that can withstand higher thermal loads and have lower freezing points: the JP-7 and JP-TS standards developed for the Lockheed SR-71 Blackbird and Lockheed Martin U-2 respectively. These fuels cost about three times as much as standard JP-8, but were designed to fulfil specific mission requirements.

Further developments in fuel technology are likely if some of the high-speed concept aircraft being proposed ever make it to production. The US Air Force Research Laboratory (AFRL) has proposed the fuel property requirements of a hypothetical Mach 4 air vehicle. The fuel, labelled "JP-11" - the next available nomenclature - would be the most useful cooling resource on the aircraft and could be required to absorb 1,400kJ/kg (600BTU/lb) of heat, equating to a temperature increase of about 480°C (860°F). Similarly, the fuel would need to have very low volatility at such extreme temperatures. Fuel close to its boiling point can cause problems such as pump cavitation, boil-off losses and safety risks. Combining such properties with the low freeze points required to ensure practical operation in cold climates, aerial refuelling and safe high-altitude relights will be a real challenge.

Fuel providers are also looking to alternative kerosene sources for when the oil wells dry up, such as natural gas or crops, and the two major fuel specification bodies have already approved the use of semi-synthetic jet fuel derived from coal.

Although it is expected that there will not be any pressure to investigate alternative sources for at least 30 years, requirements to reduce the amount of aromatics - impurities - in kerosene may drive the process instead.

"Aviation fuels will not innovate, they will use developments created for the auto industry or elsewhere," says Mike Farmery, global technical and quality manager at Shell Aviation. "The last usage of kerosene may well be in aviation," he adds.

With more to be gained from allowing a larger cut of the barrel - the proportion of the total crude oil extracted - to be used for kerosene, he sees changes in other industries giving greater supply to aviation. "Currently 10% of the barrel goes to kerosene. If we could get it to 15%, then that's a huge difference in supply," he says.

Source: Flight International