Given the difficulty and complexities of storing liquid hydrogen for commercial flight and the high velocity of air passing through a scramjet, do other types of fuel provide a feasible alternative or solution for scramjets to achieve their maximum theoretical speed of Mach 24? are there any better alternatives?
To list possible alternatives, it is helpful to know why hydrogen is used for scramjets today. The key is the speed of propagation of a combustion in the hydrogen-oxygen mixture.
In order for combustion to take place, the fuel needs to mix with the air and then the ignition needs to propagate fast enough through that mixture in order to allow the combustion to stay stationary relative to the engine. Combustion which happens after the fuel-air mixture has exited the nozzle of the engine is not helping in propulsion.
Therefore, to be suitable as a scramjet fuel, the candidate has to
- mix with air really quickly,
- have a flame front speed higher than the flow speed in the scramjet, and
- achieve complete combustion as quickly as possible.
In all points hydrogen is hard to beat. It is so good that scramjet designers have decided to accept it as their favorite fuel despite of all its disadvantages, which are well known:
- Low volumetric efficiency, even in liquid form.
- Storage in a cryogenic or high pressure tank.
- Small molecular size, so it diffuses into and leaks through many materials, even metals.
All bullet points can more or less directly be traced back to the small size of the hydrogen molecule. Any other scramjet candidate would also benefit from a small molecular size, and acetylene would be the next best candidate, with a flame front speed of 168 cm/s at room temperature (compared to the 364 cm/s of hydrogen).
The difference in speed makes clear that no other fuel comes close to hydrogen.