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Is there a specific reason that they built the F-16 with an EPU fueled with Hydrazine? Why couldn't they have made an APU with JP-8, isn't this possible? Why take a risk with hydrazine?

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Because an APU wont come online as fast as an EPU will. Ideally it should be instantaeous. The EPU takes 2-3 secs.

bealio wrote:

Is there a specific reason that they built the F-16 with an EPU fueled with Hydrazine? Why couldn't they have made an APU with JP-8, isn't this possible? Why take a risk with hydrazine?

I've wondered the same thing. Hydrazine is a whole kettle of fish I'd rather not have to mess with. Why not a RAT? Ram Air Turbine that pops into the slipstream to provide hydro and/or electric power. How about an EPU that uses a cartridge, like the old engine start systems on B-52/B-57/KC-135 aircraft that would burn a pyrotechnic cartridge to propel an EPU. I've only worked on this jet for four months and have already been around two EPU's that inadvertantly fired. It's a pain to go for a week of follow up's at the hospital!!!

Eggroll135R wrote:

bealio wrote:

Is there a specific reason that they built the F-16 with an EPU fueled with Hydrazine? Why couldn't they have made an APU with JP-8, isn't this possible? Why take a risk with hydrazine?

I've wondered the same thing. Hydrazine is a whole kettle of fish I'd rather not have to mess with. Why not a RAT? Ram Air Turbine that pops into the slipstream to provide hydro and/or electric power. How about an EPU that uses a cartridge, like the old engine start systems on B-52/B-57/KC-135 aircraft that would burn a pyrotechnic cartridge to propel an EPU. I've only worked on this jet for four months and have already been around two EPU's that inadvertantly fired. It's a pain to go for a week of follow up's at the hospital!!!

RATS are heavy, complex, costly to maintain, and dependent on a given airspeed. As for the cartridge it would have to be HUGE to give 15 minutes of burn. The old BUFF one is the size of a big coffee can and only burned for what - 15 seconds or so? So it would need to be 60 times larger?

Remember that the Viper was designed not only to be lightweight but also inexpesive to buy and maintain. I know people hate EPU's but from a maintenance reliability and life cycle cost standpoint they have been very successful.

Why couldn't the EPU run off of JP-8, i realize that JP-8 is not as combustable as hydrazine, but is it too far fetched to design an EPU that could burn off of JP-8, you wouldn't have to worry about running out of hydrazine in 10 minutes with no bleed air, you could use the jets' gas, ofcourse if the jet ran out of gas, you'd be screwed. I guess you're pretty much screwed no matter what if you run out of gas, EPU or not. Has an F-16 ever landed after running out of gas thanks to it's EPU?

Same reason you couldn't use an APU, JP-8 would take too long to atomize, combust and spin the turbine up. Food for thought, the JFS spins at approximately 61,500 rpm at 100% using JP-8 and taking up to 30 seconds to reach that 100% rpm.

The EPU, uzing hydrazine (only 6.7 gallons/56 pounds on the acft.) spins up to aprroximately 75,000 rpm in 2-3 seconds. The key factor is the ability to provide backup power with little or no interruption to essential electrical and hydraulic components (FLCS, FFP...)

JP- would require a high-energy igniter - oops, EPU supplies electrical power when the main system is down. Yes you could use the battery but I guarantee the engineers would want the igniter firing on high the whole 15 minutes. In addition just dumping JP-8 into an ambient pressure combustion chamber doesn't do much towards spinning a turbine. It just burns the blades from an overtemp. The hydrazine supplies it's own oxygen and pressure in the combustion chamber - thus negating the need for a compressor wheel. Case in point - dump raw fuel in the main engine combustion section without turning the compressors and see what happens. Lots of burn but no turn....

falconfixer860261 wrote:

Remember that the Viper was designed not only to be lightweight but also inexpesive to buy and maintain. I know people hate EPU's but from a maintenance reliability and life cycle cost standpoint they have been very successful.

It just seems like a lot of hassle for an emergency system. IMHO, the Keep It Simple Stupid (KISS) should apply double for back up systems. In theory, the simpler the back-up, the more reliable. And maybe our lack of experience is working against us also. EPU's provide us with about 10% of the work we have to do in the shop and it just gets frustrating to have to do mono check after mono check after mono check.

Eggroll135R wrote:

falconfixer860261 wrote:

Remember that the Viper was designed not only to be lightweight but also inexpesive to buy and maintain. I know people hate EPU's but from a maintenance reliability and life cycle cost standpoint they have been very successful.

It just seems like a lot of hassle for an emergency system. IMHO, the Keep It Simple Stupid (KISS) should apply double for back up systems. In theory, the simpler the back-up, the more reliable. And maybe our lack of experience is working against us also. EPU's provide us with about 10% of the work we have to do in the shop and it just gets frustrating to have to do mono check after mono check after mono check.

Well when you look closely at the system it does follow KISS. Certain mx elements might not be so simple but from a weapons system standpoint it is. How long to do you want to be without flight controls on a FBW a/c? A RAT takes a certain amount of time to deploy and come up to speed - in the meantime your a/c might be flipping ends or keeping you out of the optimal ejection envelope.

Elite 1K Joined: May 26, 2005 Posts: 1152

Is the EPU needed because a Viper only has one engine? Other FBW jets (F-117, F-22) are twins and to the best of my knowledge don't have an EPU, they just have generators on each engine, right? So I wonder what they're doing with the JSF? I'm guessing it won't fly worth a crap without electrical power either...

Guysmiley wrote:

Is the EPU needed because a Viper only has one engine? ...

Thats the reason I was always told which explains why the F-15 doesnt have an EPU, and why an aircraft like the F-4 with it's 2 engines could have a RAT which is not as instant as a hydrazine powered EPU.

Pete

Yo Ho!

The hydrazine EPU doesn't need electric ignition.

The EPU can run at any altitude, any gee, anywhere. If the motor is running, it runs off of bleed air.

A RAT loses power when you land, so you might runoff the runway. Also takes airspeed to crank up, so at low speed you could be outta hydrualic power for too long. RAT will also have trouble if you are supersonic when things go Tango Uniform.

Two engines are a waste of weight, fuel and volume (see my thots on that elsewhere).

Besides, without that hydrazine, how are we gonna keep the lawyers in business with their lawsuits concerning exposure to the poisonous gases, huh?

out,

There's also another thread along these same lines. "The EPU - how does it work and why hydrazine?" has some other information that might help Wink

.

Guysmiley wrote:

Is the EPU needed because a Viper only has one engine? Other FBW jets (F-117, F-22) are twins and to the best of my knowledge don't have an EPU, they just have generators on each engine, right? So I wonder what they're doing with the JSF? I'm guessing it won't fly worth a crap without electrical power either...

Only one engine and FBW. In twins the second engine is your backup power supply but eh F-22 also has an APU that can run in flight that will also supply power. If I remember correctly the F-35 also has an APU.

Newbie Joined: Jan 20, 2006 Posts: 2 Status: Offline

Been said above back to the LFP weight is the critical issue along with almost instant spin-up.
As an aside, many years ago i was unfortunate to have a AVPIN (raf eqiv of hydroBen) shower when I was dismantling a component that had become filled with pressurized AVPIN........it don't half make your eyes and skin burn!!

Cheers

MoJo.

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Why Hydrazine? Why not JP-8?