why JFS+EPU instead of APU

[twistking]

I don't udnerstand how it can be more efficient to have both  a JFS and an EPU instead of having one part that does both... so an APU that can start the engine and run a generator.
I understand that an EPU that's only there to run a small emergency gen for a short time can be very small and leightweight compared to a proper APU, but i do not really understand why you could not put a small gen on the JFS and use that for emergency power.
Can someone point me in the right direction?

Thanks!

[Frederf]

Because when the fuel is exhausted a generator that runs off fuel is useless. For a FBW airplane both electricity and hydraulic power are needed or it's a rock. The EPU can run off JP-8 as well.

[mvsgas]

20 minutes ago, twistking said:

I don't udnerstand how it can be more efficient to have both  a JFS and an EPU instead of having one part that does both... so an APU that can start the engine and run a generator.
I understand that an EPU that's only there to run a small emergency gen for a short time can be very small and leightweight compared to a proper APU, but i do not really understand why you could not put a small gen on the JFS and use that for emergency power.
Can someone point me in the right direction?

Thanks!

I don't know the answer to your question, but in the F-117, there is and APU that runs the entire flight and the same EPU that is used in and F-16 ( except it does not have fuel/oil cooler and does not use hydrazine). So and APU would not preclude an EPU. F-15A to C does not even have a battery, no APU nor an EPU...not related, just thought is interesting.

Edited August 11, 2022 by mvsgas

[mvsgas]

13 minutes ago, Frederf said:

The EPU can run off JP-8 as well.

It can't run on JP-8, it has no way to do so. It can run with bleed air from the engine, but it will always use hydrazine when it initially starts. Only time it only uses bleed air is while being testing on the ground... and some time the jet decides to use hydrazine when doing that, that is why pilots go to a 100% oxygen when testing the EPU during launch.

[Frederf]

My mistake. It's possible for EPU to not be using hydrazine in operation. This isn't JP-8 but engine bleed air. Why there isn't a one-item-for-all. There would have to be a big APU to provide all that power which could be a packaging challenge and it would have to run on both fuel and some emergency fuel. Big APUs tend to be slow and when emergency power is needed it's needed very quickly in the F-16.

[twistking]

1 hour ago, Frederf said:

Because when the fuel is exhausted a generator that runs off fuel is useless. For a FBW airplane both electricity and hydraulic power are needed or it's a rock. The EPU can run off JP-8 as well.

The argument with the fuel running out doesn't make sense to me. You could just bring more JP-8 for the size and weight of the EPU.

*edit* Did not see your latest post before posting this.

Edited August 11, 2022 by twistking

[twistking]

Thanks for the replies to both of you.

I agree that it has probably to do with the time it takes to spool up an APU compared to that speedy, little hydrazine EPU thingy... (The hornet f.e. has an APU, but it also has multiple levels of reversion modes for the FCS, so is still flyable without any power at all...)

The Viper's EPU does also deliver the hydraulic power, right?

[Raptor9]

It can also come down to the intended operation of the aircraft during initial design. Often times, when aircraft are equipped with a robust APU setup, it's to allow them to sit on alert for extended periods of time with all of their systems initialized and ready to go, short of the main engines running.

Not to say that's why the F-16 has a JFS versus an APU, just that the intended or expected employment of an aircraft can drive such decisions in any design.

[Dragon1-1]

I remember hearing, some time ago, that it came down to size. An APU is a bulky thing that needs its own exhaust pipe, while the EPU is dinky (even with the hydrazine tank), and so it JFS. The Viper is small and tightly packed, so this was a real concern. These solutions fit well with it being a cheap, very lightweight fighter it was originally intended to be. 

Interestingly, some non-US Vipers supposedly did have an APU installed, at least AFAIK. Presumably, advances in technology allowed it to fit. I'm not sure which country it was, but it was one of the newer blocks.

[twistking]

13 minutes ago, Dragon1-1 said:

I remember hearing, some time ago, that it came down to size. An APU is a bulky thing that needs its own exhaust pipe, while the EPU is dinky [...]

Doesn'T the JFS also needs it's own exhaust pipe? Couldn't you just put a generator on the JFS and call it an APU?

[Raptor9]

36 minutes ago, twistking said:

Doesn'T the JFS also needs it's own exhaust pipe? Couldn't you just put a generator on the JFS and call it an APU?

It does have a separate intake and exhaust, just like an APU. Regarding the generator, it depends on how much space is available like Dragon 1-1 said, and how much power you need to output from the generator. Any generator that is powerful enough to run the various electronics, even if it was just the critical ones, might have been too bulky in the engine bay where the JFS is located to accommodate such a generator.

Keep in mind that it's not just all the electronics in the flight control servos themselves that need to be powered, but also the flight computers to control it based on inputs from the stick and pedals. The warning lights next to the BATT/MAIN PWR switch are even set up this way to warn when the main generator, standby generator or the EPU isn't providing enough power to the FLCS.

[mvsgas]

12 hours ago, Frederf said:

It's possible for EPU to not be using hydrazine in operation.

IIRC, back when I work on them (10 years removed now). So if the jet loses electrical power or hydraulic pressure gets below a certain number the EPU will activate. The pilot could also choose to turn it on. Any way, the EPU will initially use hydrazine to achieve operation speed, after switching to bleed air if it can.

 

12 hours ago, twistking said:

The Viper's EPU does also deliver the hydraulic power, right?

Yes

 

8 hours ago, twistking said:

Doesn'T the JFS also needs it's own exhaust pipe?

Yes

8 hours ago, twistking said:

 Couldn't you just put a generator on the JFS and call it an APU?

You don't need and generator to call it APU. Again, using the only APU I ever work with as an example, F-117 APU was not connected to a generator nor a hydraulic pump. It simply ran an air compressor, which in tern is used for several things ( Air starting system call P.A.S.S, supplement environmental control, emergency power unit, etc.)

 

Anyway, the thing that looks like it has tin foil around it, is the JFS, very compact, even tho the engine is removed..where would fit the generator?

JFS bay photo

Keep in mind, above the JFS is the JFS controller (basically a fuel pump), behind the JFS (inboard) there is hydraulic pump, middle is where the engine connect to the Accessory Drive Gearbox (ADG) and on the right side there is Main gen, stand other hydraulic pump and stand by gen and on the door there is a frequency converter for the stand by gen.