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?
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asked Dec 27, 2017 at 9:34
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2$\begingroup$ Could you narrow down your question on a fuel type? I guess you are talking about liquid fluid? Rocket-motors might use hydrogen or even kerosene. Scramjets also use a wide spectrum of fuels. Or are you talking about hypergolic fuels? $\endgroup$– rul30Commented Dec 27, 2017 at 10:34
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$\begingroup$ "High energy fuel" might be more about what you are trying to ask. $\endgroup$– user3528438Commented Dec 27, 2017 at 16:29
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$\begingroup$ Not sure what you are asking here. What do you mean by "rocket fuel"? Liquid hydrogen? You propose it as an "alternative " what do you think current scramjets are using as fuel? Can you provide more context or background on what you are trying to get at? $\endgroup$– Daniel KCommented Dec 27, 2017 at 16:36
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$\begingroup$ If your idea is to boost the power of a scramjet by using rocket fuel, then you'd have to include the oxidizer that all rocket fuel has. In that case, you wouldn't have a scramjet, which is an air breathing engine. You'd have a rocket. $\endgroup$– tj1000Commented Dec 27, 2017 at 17:36
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1 Answer
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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.
answered Jan 4, 2018 at 23:49
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$\begingroup$ Kampf How can flamefront ( 364cm/s) speed more than flow speed? That speed is 0.1 speed of sound. And speed of flow in scramjet is supersonic. $\endgroup$– AuberronCommented Jun 25, 2021 at 17:58
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1$\begingroup$ @Auberron You're right, but this value is for room temperature. Due to the high compression, gas temperature in a scramjet is around 1000 K, and then the flame speed is sufficiently high if flame holders are provided. $\endgroup$ Commented Jun 25, 2021 at 22:33