> The F-16 has a hydrazine APU, and I would not be surprised if the F-35 does the same thing.
The F-16 has an EPU not an APU, i.e. an Emergency Power Unit vs an Axillary Power Unit. Lacking an APU is an exception when it comes to jet aircraft and necessitates a few changes. Relevant here is that an APU can provide power in an emergency, so the role is played by the hydrazine powered EPU for the F-16.
This makes hydrazine somewhat more of a rational choice for something you expect to use rarely. If it were expected to be frequently used then it would have to be frequently refilled and that would necessitate all ground crew wearing the full body hazmat suits you see in the photo for the X-37b here as well as many other complex safety precautions.
The F-35 has a conventionally jet fuelled APU[0] so it has no need for an EPU or hydrazine.
Interestingly Concord didn't have space for an APU (due to the shape of the empennage and placement of a rear fuel tank). I have seen pictures of a hydrazine powered EPU on one of the prototypes, but this was never going to make it into service as certifying carrying something as toxic as hydrazine - even back then - would be nigh on impossible. This is the question that starts the most epic thread about Concord[1] (warning - you may get sucked into reading all 103 pages!)
The EPU on the F-16 is actually quite interesting itself and is a good fit for what is needed with 70s technology.
Redundant systems are normal on aircraft and the F-16 has a particular need for hydraulic and electrical power as its inherently unstable airframe needs various onboard systems to keep it stable and flyable.
If the main generator disconnects from the bus or the hydraulic pressure drops[0] the EPU will automatically operate to provide electrical and hydraulic power. It can run from engine bleed air, but if the (single) engine has failed then pressure will be provided from the hydrazine system.
It achieves this using a 70% solution of hydrazine in water as a monopropellant, which is decomposed over an iridium catalyst. The water helps keep it stable, and cools the catalyst when it boils, keeping it around 850C and the high pressure gasses are fed into the EPU.
This has the advantage of being light (25L of hydrazine and simple as only a single liquid), reliable and fast acting (~2s to max power).
pprune is the goto site whenever anything interesting happens in aviation (and the pro's there are very aware that it's where the journalists will start looking when there's an incident).
The F-16 has an EPU not an APU, i.e. an Emergency Power Unit vs an Axillary Power Unit. Lacking an APU is an exception when it comes to jet aircraft and necessitates a few changes. Relevant here is that an APU can provide power in an emergency, so the role is played by the hydrazine powered EPU for the F-16.
This makes hydrazine somewhat more of a rational choice for something you expect to use rarely. If it were expected to be frequently used then it would have to be frequently refilled and that would necessitate all ground crew wearing the full body hazmat suits you see in the photo for the X-37b here as well as many other complex safety precautions.
The F-35 has a conventionally jet fuelled APU[0] so it has no need for an EPU or hydrazine.
Interestingly Concord didn't have space for an APU (due to the shape of the empennage and placement of a rear fuel tank). I have seen pictures of a hydrazine powered EPU on one of the prototypes, but this was never going to make it into service as certifying carrying something as toxic as hydrazine - even back then - would be nigh on impossible. This is the question that starts the most epic thread about Concord[1] (warning - you may get sucked into reading all 103 pages!)
[0] http://www.f-16.net/forum/viewtopic.php?f=60&t=12192
[1] https://www.pprune.org/tech-log/423988-concorde-question.htm...