Jet engines plus water injection: questions

[Daryl J.]

In searching around for information on the F-4 Peace Jack project, water injection in mentioned as a key, amongst other changes, to getting the Phantom to Mach 3+.   

But it took two 2500 gallon water tanks.


So the questions are this:  how long would that volume of water last?   Were there significant design changes to materials within the engine to tolerate the different mixture's behavior?    Could, in theory, water injection be carried along, say with a bit of MeOH or EtOH to prevent freezing at altitude, for short 5-10 minutes of high speed either over target or as an escape from danger.

I'm not, however, asking about shape revisions for the intakes as my modeling skills aren't advanced to that level yet.

Yes, these questions partain to both the Draken and Phantom projects at hand.

[elmayerle]

The water is used for pre-cooling the intake airflow and allowing a greater mass-flow through the engine (hence more power).   The "one shot" avionics cooling system on the MiG-25 does much the same thing with a distilled water/alcohol mixture (hence it's nickname, "the flying vodka bottle").  You'd want to reserve this for when you really needed the extra power as I suspect engine life is reduced as hot-section temperatures would go up (I suspect this is one reason the recce MiG-25's doing Mach 3.2 always had their engines replaced afterwards).  I can't say for certain how long those tanks would last, but I wouldn't use them until "go fast" time was upon me.

Note, basic water injection to improve performance was a staple of the original KC-135's J57 engines for hot/high conditions on takeoff; it adds a very distinct crackle to the exhaust sounds.  You don't necessarily need material changes to the engine but I could see going to higher-temp materials for the turbine section, assuming such are available, for use of such a system at the upper right-hand corner of the envelope.

HTH,
Evan

[GTX_Admin]

I have a detailed article on this including photos and other details.  If you like I will scan it for you.

[Weaver]

Just as an aside, Harriers have water injection to boost power/cool the intake air in the hover.

On a related subject (apologies for the hi-jack), I've often wondered if it would be practical to boost the performance of jet engines at very high altitude by injecting LOX (or some other oxidiser) into the intakes. It wouldn't last long of course, so it would be more for zoom-climb profiles as a potentially simpler/safer alternative to a full booster rocket.

[elmayerle]

Just as an aside, Harriers have water injection to boost power/cool the intake air in the hover.

On a related subject (apologies for the hi-jack), I've often wondered if it would be practical to boost the performance of jet engines at very high altitude by injecting LOX (or some other oxidiser) into the intakes. It wouldn't last long of course, so it would be more for zoom-climb profiles as a potentially simpler/safer alternative to a full booster rocket.

You'd probably have to tweak the fuel control to provide enough extra fuel to properly utilize the LOX, but I won't say it's impossible.  On the other hand, a booster rocket burning LOX and jet fuel wouldn't be that much more complicated to install.  That is, after all, part of the basic Black Horse concept.

[Weaver]

Just as an aside, Harriers have water injection to boost power/cool the intake air in the hover.

On a related subject (apologies for the hi-jack), I've often wondered if it would be practical to boost the performance of jet engines at very high altitude by injecting LOX (or some other oxidiser) into the intakes. It wouldn't last long of course, so it would be more for zoom-climb profiles as a potentially simpler/safer alternative to a full booster rocket.

You'd probably have to tweak the fuel control to provide enough extra fuel to properly utilize the LOX, but I won't say it's impossible.  On the other hand, a booster rocket burning LOX and jet fuel wouldn't be that much more complicated to install.  That is, after all, part of the basic Black Horse concept.

If you're installing a rocket on an existing airframe then it's thrust needs to be absorbed by the structure and it's thrust line has to pass through the CofG to avoid excessive trim issues. That, plus it's weight and fuel plumbing seems significantly more complex to me. The LOX-boosted fighter on the other hand, is a perfectly ordinary jet fighter with a bit of extra plumbing and a wet hardpoint with an extra connector. take the LOX tank off and the penalty is pretty small.

[elmayerle]

If you're installing a rocket on an existing airframe then it's thrust needs to be absorbed by the structure and it's thrust line has to pass through the CofG to avoid excessive trim issues. That, plus it's weight and fuel plumbing seems significantly more complex to me. The LOX-boosted fighter on the other hand, is a perfectly ordinary jet fighter with a bit of extra plumbing and a wet hardpoint with an extra connector. take the LOX tank off and the penalty is pretty small.

Well, if you're good, you design the rocket to tie into existing load structure for the engines.  With regard to plumbing, given the packed nature of modern fighters, I suspect one would be about as challenging as the other.  I do see your point, though, and can't disagree in principle (I have physically dealt with aligning booster thrust vectors through the cg of the combined air vehicle for a boosted ground-launched vehicle - it gets more "interesting" when the vendor doesn't adequately inspect the indexing pin location ).