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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?

Mr X
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  • 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. – Adwaenyth Oct 08 '19 at 13:34
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    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. – DLH Oct 08 '19 at 19:10
  • Cost per gallon, toxicity end that dream. But it does inspire looking into putting catalytic elements into a hydrocarbon turbine to reduce NOx emissions. – Robert DiGiovanni Dec 27 '21 at 12:09
  • @Adwaenyth but it’s terrible for energy density by volume which is at a premium too in an aircraft. Also risk of explosion. – Antzi Dec 30 '21 at 11:17

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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

tsg
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    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". – Jörg W Mittag Oct 08 '19 at 07:28
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    The high energy density of coal isn’t really a surprise, it’s just that having a crew to shovel it into a rocket nozzle is difficult to organise, especially in an expendable first stage. – Frog Dec 27 '21 at 07:05
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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.

Hobbes
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It has roughly 50% lower energy density than jet fuel, so obviously worse, but still much better than any rechargeable batteries. If we could use it directly in fuel cells, which generally have efficiencies approaching 60%, which is twice as good as piston and turbine engines, then we could actually get a system performance similar to that of jet fuel.

A big problem with hydrazine is toxicity. It is comparable to ammonia, but somewhat mitigated by the fact that hydrazine has a much higher boiling temperature, comparable to water. Hydrazine fuel system would need to be engineered to avoid an vapor emissions. Another problem is the fact that hydrazine can decompose exothermically in presence of catalysts, so material compatibility becomes a big issue.

This is theory. It seems doable from the technical standpoint. The question marks are economy and safety. If we were to stop using fossil, bio and synthetic hydrocarbon fuels today hydrazine is a much better option than batteries and than ammonia.

Gustav_L
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  • Exothermic decomposition in presence of a catalyst was the deciding factor for all practical cases where hydrazine was used in the past. I wouldn't call this a problem. Without this characteristic, nobody in their right mind would accept all the downsides of hydrazine. – Peter Kämpf Dec 27 '21 at 07:08