How high could an X15 fly

Ammonia could again be an alternative to aviation fuel

A British aerospace company has carried out a feasibility study on the use of the gas as a low-emission fuel for aircraft. It has happened before.

Reaction Engines, based in Abingdon in the English county of Oxford, and the British Science and Technology Council (STFC) have researched the use of ammonia as a fuel for aircraft. The heat exchangers developed by Reaction Engines are to be coupled with STFC catalysts in order to produce sustainable and low-emission aviation fuel.

Modern jet engines burn various kerosene-based fuels with a very high energy density, which bring aircraft to supersonic speeds and transport passengers across the globe like heavy cargo. The disadvantage: conventional aviation fuel is made from fossil energy sources and is responsible for a high proportion of global CO2Emissions. The aviation industry has set itself the goal of radically reducing carbon dioxide emissions by 2050.

Most alternative fuels for aircraft have a significantly lower energy density than conventional kerosene. Batteries for electric aircraft are still too big and heavy. Hydrogen is a possible alternative, but aircraft would have to be designed significantly different to use it, which also affects the infrastructure.

Ammonia as aviation fuel is not a new idea. Even if the gas has only a third of the energy density of diesel, it is relatively easy to liquefy and store. The famous North American X-15 rocket aircraft used ammonia fuel for its space flights in the 1950s and 1960s. In addition, ammonia is free from CO2.

The reason why gas has not yet established itself as a fuel is likely to be due to the difficulty in making it economical to use in aircraft propulsion. The system developed by Reaction Engines and STFC uses cooled ammonia stored in the wings in pressure tanks, very similar to kerosene.

The heat exchangers take the heat out of the engine and use it to heat the ammonia, while some of it is converted into hydrogen in a catalytic converter. The mixture of ammonia and hydrogen is then used as fuel and burns like kerosene, only with emissions that consist only of water vapor and nitrogen.

According to information from Reaction Engines, the energy density of ammonia is high enough that no noteworthy modifications to the aircraft would be necessary and the propulsion system for use with ammonia could be converted quickly. The first test flights should be possible in a few years.

According to James Barth, chief engineer at Reaction Engines, ammonia for low-emission aviation should be possible on short-haul routes well before 2050, and this with aircraft and engines that only cause low retrofitting costs.

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