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It is true that hydrazine is highly toxic and explosive, but it has tremendous energy density. The decomposition of hyrazine into hydrogen and nitrogen gases AFAIK produces enough heat to ignite the resultant hydrogen when it occurs in the presence of air. But to make it useful for low speed, subsonic jet propulsion would involve tightly controlling the decomposition and combustion processes. Is this possible? And has there been any attempt to design jet engines that use it?

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  • $\begingroup$ Liquid Hydrogen would be preferable in terms of energy density (eg. 6 times more energy density) but is terrible to store for prolonged periods without maintenance, thus hydrazine or comparable fuel is used for maneuvering rockets. $\endgroup$ – Adwaenyth Oct 8 '19 at 13:34
  • $\begingroup$ Hydrazine is only advantageous as a rocket fuel in that it does not require an oxidizer to be stored on the rocket as it is a monopropellant. Not having to store an oxidizer on the rocket can save some weight but even then rocket designers prefer other fuels due to their higher energy densities. $\endgroup$ – DLH Oct 8 '19 at 19:10
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A quick look into wikipedia (https://en.wikipedia.org/wiki/Energy_density) shows hydrazine (in full combustion to N2 and H2O) at 19,5 MJ/kg while Jet-fuel is listed at 43 MJ/kg.

So => no

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    $\begingroup$ Indeed, pretty much anything that is or was at some point used as fuel has a higher energy-density than Hydrazine: Methanol, Ethanol, Diesel, Gasoline, even coal! In fact, Hydrazine is only slightly better than cow dung. Not sure why that would be considered "tremendous". $\endgroup$ – Jörg W Mittag Oct 8 '19 at 7:28
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Hydrazine is sometimes used to power an APU (e.g. for the F-16). APUs are generally turbine engines, so hydrazine turbines exist.

For rockets, there's a movement away from hydrazine and other nasty propellants toward less toxic alternatives. There's little chance aviation will move in the opposite direction.

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