Why does the military use hydrazine to power the F-16's Emergency Power Unit (EPU)? What is so unique about Hydrazine fuel that it's needed to power the EPU? Why not use a less volatile and less hazardous fuel?

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    $\begingroup$ Hydrazine is used so that if one is stranded on Mars, you have a single source for power, water, and oxygen! ;) $\endgroup$ – abelenky Jan 15 '18 at 19:48

The EPU of F-16 is fueled with a monopropellant hydrazine mixture, H-70, which contains 70% hydrazine ($N_{2}H_{4}$) and 30% water, by weight.

The main requirements for the EPU are that it should be simple, maintenance free, supply power immediately and consistently for the required time. Use of Hydrazine assures this while requiring careful handling.

Basically, the catalytic decomposition of hydrazine produces ammonia, nitrogen and hydrogen. Exhuast Gases from EPU Turbine contain 40% Ammonia, 17% Nitrogen, 15% Hydrogen and 28% Water.

$3N_{2}H_{4} \rightarrow 4NH_{3} + N_{2}$

$4NH_{3} \rightarrow 2N_{2} + 6H_{2}$

$3N_{2}H_{4} \rightarrow 4(1-x)NH_{3} + 6xH_{2} + (2x+1) N_{2}$

where x is the fraction of the $NH_{3}$ disassociated.

The water modifies the decomposition temperature (the EPU reaches temperatures of ~870 $^{\circ}C$), preventing thermal damage to the catalyst bed and the turbine parts. As the water removes heat, it is turned into steam which aids in powering the EPU.

The EPU, using hydrazine spins up to approximately 75,000 rpm in 2-3 seconds (The F-16 EPU starts within 2 sec.). It would take a much greater time if another fuel, like JP-8 were used. When required (EPU runs normally on engine bleed air), hydrazine is forced into decomposition chamber by nitrogen pressure, where the above reactions produce the gases to run the turbine/gearbox. The decomposition of hydrazine produces enough pressure, eliminating the need for a compressor, thus saving weight and also eliminating the need for an igniter, reducing complexity.

For the given weight, it provides continuous operation for the required time. In F-16, the EPU carries ~25l of hydrazine, which permits operation for about 10 minutes under normal load conditions and 15 minutes if the loads are less (i.e. in ground). If any other form, (like battery or cartridge) were used, it would be difficult to have a long operational time without heavy increase in mass.

For a combat aircraft, RAT is not an option. Also, a hydrazine powered EPU would work in any altitude or during maneuvering as it does not need an external oxidizer supply.


Exhaust Gas Composition of the F-16 Emergency Power Unit by Harry J. Suggs et al.

Technical Order 00-105E-9, USAF

AFR 110-14 USAF Aircraft Accident Investigation Report.

F-16 manual

Thanks to @Peter for pointing out errors.

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  • $\begingroup$ And why is a RAT not an option for combat aircraft? They’re used all the time in this role. $\endgroup$ – Carlo Felicione Apr 5 at 14:05

To fill the voids in aeroalias' answer:

Hydrazine is a monopropellant, something which does not need to be mixed and burned to free up the energy contained in its chemical bonds. This energy is freed by letting the hydrazine stream over a catalyst, in case of the F-16 EPU that is iridium. This breaks the chemical bond, producing ammonia, nitrogen gas, hydrogen gas and heat which converts the water into steam. Heating increases the volume of the gasses and the steam so they pick up speed, just like the gasses in the combustion chamber of a jet engine. In the catalyst chamber of the F-16's EPU the temperature reaches more than 800°C in just a couple of milliseconds, and no combustion is involved! This high-pressure, high-speed gas flows through a small turbine which in turn drives a generator and supplies hydraulic pressure.

Compression is not needed because pressure is supplied from the bottle containing the hydrazine-water mixture. Using a monopropellant also avoids any ignition problems and makes the setup very simple. Since hydrazine is quite stable at high pressure and ambient temperature, the shelf life of a hydrazine engine is high, and the liquid state of the hydrazine-water mixture makes its storage very compact. Just what you need in a fighter aircraft.

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  • $\begingroup$ Are special measure taken to keep the failure probabilities very low under normal operations low? e.g. some failure modes I can think of (A) Leaks of toxic hydrazine during ground ops & maintenance (B) Combat damage to Hydrazine tank (C) Accidental release during flight (D) Detonation stimulated by external fire $\endgroup$ – curious_cat Dec 5 '15 at 12:42
  • $\begingroup$ Are special decontamination procedures needed in case of F16 crashes? Or even after an aircraft that has needed EPU deployment returns to base? The lethal dose of Hydrazine I recall is in ppm quantities? $\endgroup$ – curious_cat Dec 5 '15 at 12:45
  • $\begingroup$ @curious_cat: Related: Exhaust gas composition of the F-16 Emergency Power Unit, see page 12 and F-16 Aircraft Hydrazine Procedures. $\endgroup$ – mins Dec 5 '15 at 19:17
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    $\begingroup$ @curious_cat Technically anything that has a lethal concentration could be said to have a lethal dose in ppm. For example, it could be 0.000001 ppm or 900,000 ppm. Or 2,000,000 ppm. :) $\endgroup$ – reirab Jan 15 '16 at 19:13
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    $\begingroup$ @DavidRicherby Perhaps the amounts in pharmaceuticals are measured in ppm relative to the mass of the pharmaceutical (whereas LD50 is measured relative to test subject body mass?) $\endgroup$ – reirab Jan 15 '16 at 19:17

Hydrazine use is related to aircraft stability. Without power to the computers, the F-16's preferred flight attitude is flying tail first. The basic aircraft design is aerodynamically unstable, intentionally, to achieve high performance. The flight control computers maintain stability.

If electrical power goes down, the pilot suddenly needs wading boots. To prevent such unfortunate circumstances, the APU was specified to be on-line at full power 0.25 seconds after power failure.

Apparently, in the late 60's to early 70's (design years) nothing else could meet the rapid start, weight, and power endurance requirements.

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