The RB199 was, I thought, the first engine to double its thrust when in reheat.

However, MTU are showing 41Kn to 72Kn Max Dry to Reheat, quite propulsively useful I'd say ;)

The propulsive usefulness of reheat (as a measure of the increase in thrust gained through its use) is a function of the change in temperature that is achieved between the dry value and the wet value, not the amount of flame coming out of the exhaust :}

Of course reheat increases thrust...

What I'm talking about is the fuel that's being burned outside the engine, which is presumably not doing much.

Phil

The fuel is actually being burned at the reheat manifold. It is the velocity of the exhaust gases that carries the flame outside of the propelling nozzle. Yes, it would probably be more efficient (in terms of heat transfer) to retain the flame inside the exhaust system, but I'm not sure its possible simply because of the way the system has to work.

It's well known that reheat isn't an efficient means of developing thrust. Around 10 times the fuel flow to double the thrust. Handy when required though.

And why do we see the 'dancing Diamonds' i think they are called, is that a photo or video effect.

More Geek Speak
 

PR does have a point. Most FJ aircraft just have convergent nozzles meaning much of the expansion energy is wasted outside the engine. It would be most efficient to have a large convergent-divergent nozzle (like the main engine nozzles on the back of the shuttle) which would capture the thrust generated by the full expansion of the exhaust gases behind the aircraft. However, the drag and weight would be excessive when not in reheat (90%+ of the time). Latest generation FJ engines have (variable) con-di nozzles but even then, the limited diameter means that they still can't recover all of the expansion energy.

N Joe

More geek speak!
 

The diamonds are due to the shock-waves in the expanding supersonic exhaust flow. Gases at different pressures/temperatures have different refractive indexes so bend the light differently. Same principle as shimmer in air over hot tarmac etc, except that the changes though the shock-waves are much sharper.

N Joe

Shock diamonds:

http://www.aerospaceweb.org/question...on/q0224.shtml

To confuse matters, if you watch a Tornado take off during the day it's AB is orange, at night it appears to be blue...:O

Or striations.:cool:

http://i165.photobucket.com/albums/u...toID214029.jpg

Orange, blue....Blue or orange.

It's all related to Vodka quality.

:}

> at night it appears to be blue

Yes. What you're seeing is the blue flame after it's exited the engine and encountered a whole load more oxygen. It's very dim. During the day it's invisible and all you can see is the rich-mixture area inside the, er, whatever you call the adjustable nozzle.

Phil

Watch an F-15E take off - you can tell if it's an older one with Dash 220 engines (yellow a/b) or a newer one with 229 IPEs (blue a/b).

I'll get my anorak.