Hydrazine is used a monopropellant in Satellite OMS rockets. But interestingly enough, it's highly exothermic decomposition into ammonia, nitrogen, and hydrogen provides heat for an endothermic reaction with additional hydrazine to form hydrogen gas. So a scramjet using hydrazine would have to reactions inside it's combustion chamber: The decomposition of hydrazine and the combustion of the resulting hydrogen gas with air. And unlike hydrogen gas, hydrazine has much higher propellant density. Has this ever been considered by aerospace engineers and even tested?
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3$\begingroup$ General Electric has a patent on it, granted 28 years ago. $\endgroup$– Ron BeyerCommented Sep 7, 2018 at 19:01
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$\begingroup$ Does it offer more energy density than conventional fuel? If no then why? $\endgroup$– user3528438Commented Sep 12, 2018 at 19:01
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3 Answers
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Technically yes, you could use hydrazine in a scramjet.
But there's no reason to do so. The singular purpose of building a scramjet is to get air into the combustion chamber, so that you don't have to use a monopropellant, which all have high cost and poor energy density.
Kerosene or H2 won't burn without air, so you need a scramjet or an oxygen tank to burn them. Hydrazine doesn't, so you can burn it in a rocket, at higher pressure with more efficiency and better aerodynamics.
Advantage: Rocket engines have a thrust:weight ratio of up to 133:1, scramjets about 2:1. So an equivalent rocket is up to 66 times lighter than a scramjet.
GE's patent refers to the use of a small amount of hydrazine in a catalyst-like role, or more accurately to act as a fuel/air mixer.
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$\begingroup$ What I was actually describing is hybrid engine of sorts: Part rocket engine, part scramjet. Initially the hydrazine would act as a monopropellant from startup to get the vehicle up to a sufficiently high speed. Once it reaches ~mach 5 it would work as a hybrid engine. The idea is that it would produce a lot more thrust than a typical scramjet and have a higher thrust-to-weight ratio. Making it much more efficient and giving it longer range. $\endgroup$– Mr XCommented Sep 8, 2018 at 0:13
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$\begingroup$ Once you reach such high speeds using a monopropellant would likely be thermally inefficient due to the mass flow of air through the combustion chamber in addition to much lower chamber pressure. $\endgroup$– jCiscoCommented Sep 10, 2018 at 1:13
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In addition to it's use as a monopropellant, hydrazine has been used as a fuel in bipropellant combinations, for example in a 30% by weight mix with 57% methanol and 13% water, with concentrated hydrogen peroxide as the oxidiser in the Messerschmitt Me163 rocket propelled interceptor. This suggests that it's use as a fuel in a scramjet engine might be technically possible, although it is chemically unstable and toxic in pure form.
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The Scramjet is an AIRbreathing engine. Thus there's no reason to incorporate hypergolic fuels because that requires it's own oxidizer onboard weight. Second, with an energy density of 19.5 Megajoules per kilogram; it's half energy density of diesel fuel. Basic jet fuel has a energy density 43MJ/kg both of which make a better fuel with NONE risky toxicity issues
