Thrust enhancement

[Weaver]

Wasn't sure where to put this, so if the mods think it should be somewhere else, so be it.

Two conventional ways of boosting the thrust of a jet aircraft:

1. Afterburner. Nice and light/simple but very inefficient and limited in it's potential (potentially at high altitude).

2. Rocket Booster. The least mechanically complex choice would be a kerosene/LOX unit that shared the jet engine's fuel supply, but it's still a lot of extra hardware. Does work better at high altitude though.

This got me thinking: could you boost the thrust of the exisiting jet engine, at high altitude, thus:

1. Inject LOX (or some other oxidant) into the intake, thus cooling everything downstream of it and boosting the oxygen content of the intake air,

2. Boost fuel rate to the combustion chamber, to take advantage of the extra oxygen in the more efficient phase of combustion,

3. Add as much afterburner as is ineccessary to use up the remaining oxygen left by the combustion chamber.


You get lots of extra thrust, at altitude, for the cost of carrying a LOX tank and a bit of extra plumbing around, and you can continure to use the afterburner normally if the LOX runs out. I can't see anything wrong with this, but then I'm not an engineer, so opinions please?

[The Rat]

Wasn't there some method employed using just plain old water? I presume it was squirted into the exhaust where it would have instantly turned to steam, expanded incredibly, and gave the needed boost. But I might have been dreaming.

[rickshaw]

Wasn't there some method employed using just plain old water? I presume it was squirted into the exhaust where it would have instantly turned to steam, expanded incredibly, and gave the needed boost. But I might have been dreaming.

Its injected into the precombustion chamber.  Cools the airflow and lowers the detant temperature, thereby increasing thrust IIRC.

[PR19_Kit]

...and makes LOTS of smoke! 

B-47s and early 707s and DC-8s used that technique as I recall. The SAC even had to modify its massed take-off procedures because there was so much water injection induced smoke on the runway the following aircraft couldn't see well enough to take-off!

[The Rat]

Okay, so let's try my method instead. In my mad scientist universe it would work! 

[Mossie]

Plenum Chamber/Duct Burning.  PCB burns the fuel in the bypass duct rather than the jet efflux.  I know it was proposed for the HS P.1154 & advanced projects like the P.1216.  From what I've read, PCB seems to produce higher thrust values than reheat/afterburner.  So what are the reasons for it not being adopted over afterburner?

[Weaver]

Plenum Chamber/Duct Burning.  PCB burns the fuel in the bypass duct rather than the jet efflux.  I know it was proposed for the HS P.1154 & advanced projects like the P.1216.  From what I've read, PCB seems to produce higher thrust values than reheat/afterburner.  So what are the reasons for it not being adopted over afterburner?

Well on a jet-fighter type low-bypass turbofan all the bypass air goes through the afterburner, so it effectively is "plenum chamber burnt". What made it tricky on the Harrier was getting the burners to light and stay lit in the turbulent cold airflow in the short distance between the fan and the front nozzles. This isn't a problem in a normal afterburning turbofan, since the core flow acts as "pilot light" to keep ignition going no matter what.

[Weaver]

Wasn't there some method employed using just plain old water? I presume it was squirted into the exhaust where it would have instantly turned to steam, expanded incredibly, and gave the needed boost. But I might have been dreaming.

Its injected into the precombustion chamber.  Cools the airflow and lowers the detant temperature, thereby increasing thrust IIRC.

It's also 33% oxygen instead of the 21% of atmospheric air. So if that's a good idea, why not LOX, which is 100% oxygen and even colder. Aircraft use LOX for their on-board oxygen systems anyway, so it's not like you're introducing a strange new chemical, just a lot more of a familiar one.

[Hobbes]

You could probably do this, but at the cost of having to carry all that oxygen with you rather than scooping it up as you go along. Compare with NOS bottles on cars: a pretty big canister gives only a few minutes worth of boost, and that's with a small IC engine.
LOX also needs cryogenic storage which makes for more difficult handling.

[Weaver]

You could probably do this, but at the cost of having to carry all that oxygen with you rather than scooping it up as you go along. Compare with NOS bottles on cars: a pretty big canister gives only a few minutes worth of boost, and that's with a small IC engine.
LOX also needs cryogenic storage which makes for more difficult handling.

True, but the same could be said of a rocket booster, and my way involves less hardware AND you can still use regular afterburning when the LOX has gone. In other words, it has all the advantages of a rocket but less of the penalty.

What I'm thinking of here is the 1950s style interceptor requirements, where rocket+jet propulsion was under serious consideration to meet the altitude & climb rate requirements implicit in intercepting high-altitude supersonic bombers. The "victory" of afterburning is as much to do with that requirement going away as anything: if we were still looking at M2 bombers at 80,000 ft then something like this would be mandatory.

Another thought: if the LOX tanks were external, then:

a) They could be dropped once empty to decrease drag, although this would be more expensive than dropping kerosene tanks (and would probably need a parachute to avoid damage/hazard on the ground),

b) They could be left off for operational missions/ferry flights that didn't need them, thus reducing the penalty to the weight and volume of the extra plumbing.

[sagallacci]

I see a materials problem with pure O2, unless it is metered VERY well, will expose the hardware to an oxydizing burn. In effect a cutting torch-like flame. Not only hot, but chemically reactive.
Rocket motors avoid the issue with very good mixing, a slightly rich mix (both to avoid loose oxygen), and fuel barrier bleed into the chamber, which both helps cool the motor and protects the metal from the oxygen in the mix.

[The Rat]

With all this talk of thrust enhancement shouldn't this be in the Vickers Viagra thread? 

[Weaver]

I see a materials problem with pure O2, unless it is metered VERY well, will expose the hardware to an oxydizing burn. In effect a cutting torch-like flame. Not only hot, but chemically reactive.
Rocket motors avoid the issue with very good mixing, a slightly rich mix (both to avoid loose oxygen), and fuel barrier bleed into the chamber, which both helps cool the motor and protects the metal from the oxygen in the mix.

Well you wouldn't be feeding "pure" O2 to the engine, rather you'd be feeding it with air which had it's oxygen content boosted. What are the lower limits on this? How high does the oxygen content have to be to start causing a problem? How hot does it have to be: combustion chamber hot or compressor hot?

[Hobbes]

Thinking about it some more, I'm not sure it'd have a dramatic effect. Jet engines already have a surplus of oxygen; otherwise afterburners wouldn't work. So the only advantage you get from injecting LOX is the increase in density due to the temperature drop. Injecting water is a cheaper way to achieve that.

[Weaver]

Thinking about it some more, I'm not sure it'd have a dramatic effect. Jet engines already have a surplus of oxygen; otherwise afterburners wouldn't work. So the only advantage you get from injecting LOX is the increase in density due to the temperature drop. Injecting water is a cheaper way to achieve that.

But jet engine, and afterburner, performance falls off at altitude because the total amount of oxygen being ingested falls as the air pressure decreases. LOX injection puts that back. Effectively, you could be ingesting air at high altitude which has the same amount of oxygen as sea level air, not because the pressure's increased but because the percentage of oxygen in it's increased. In other words (I think) 40% oxygen air has the same oxygen content per unit volume as 20% oxygen air at twice the pressure.