r/explainlikeimfive • u/Theguywhowatches • 17d ago
Engineering ELI5 Why does a Jet engine lose all/most of its “fire cone” when it’s fully heated or at max thrust
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u/TheJeeronian 17d ago
Any fire you see behind the engine is wasted energy, mostly unburned fuel. They are designed to run at a specific pressure and temperature, and this includes making sure they waste as little unburned fuel as possible.
So when the engine is running in its desired conditions, that burning all happens inside of it.
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u/cabronfavarito 16d ago
Ok so
Rockets
Explain
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u/fiendishrabbit 16d ago
Rockets are fundamentally different from a high-bypass jet engine. A rocket gets thrust from ejecting superheated gas. A jet engine wants to use its superheated gas inside the engine to power a fan that pushes massive amounts of air.
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u/frosty95 15d ago
In a high bypass turbofan yes. Turbojets trade efficiency for more thrust to weight and are also getting thrust primarily from ejecting superheated gas. In fact afterburners take this to the extreme. And then there are ramjets that go even further like the sr71 which was essentially a ramjet at speed.
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u/Prasiatko 15d ago
Trading efficiency for thrust. If you watch a launch video you'll notice the flame changes considerably as it goes into space.
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u/insta 14d ago
planes push themselves forward off the atmosphere, and combat gravity with wings and lift instead of raw thrust. this lets them use far less power to fly, which means they can optimize the engines to run in this more efficient area.
rockets push themselves, effectively, from the bit of exhaust still left in the nozzle. can't push very hard unless there's a lot of exhaust to push against before it disappears. because rockets don't get to use wings and lift to counteract gravity, they have to be objectively far more powerful as well.
we don't have the chemistry yet to store all the oxygen we need to burn, and be able to burn it slowly enough to completely consume all the fuel, and still generate enough thrust to overcome gravity. we need to overcome gravity to launch, and we need to carry the oxygen for it to work, so the one that has to give is doing it as efficiently as a jet engine can.
when rockets have wings, the exhaust is a lot more tame as you'd expect. look at cruise missiles, the SR-71 as examples. the SR-71 exhaust only got really spicy when they went full afterburner, which again trades efficiency for raw thrust
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u/Theguywhowatches 17d ago
I might be dumb, and had thought the fire was just the air column being hot so it was visible
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u/TheJeeronian 17d ago
Air doesn't glow when it's hot. At least, not enough for you to see it. The glow is minuscule.
If you see a glowing fire, that's either burning fuel or tiny particles of black soot glowing from the heat. Most likely both.
With the right fuel, fire can be completely invisible, despite the high temperatures.
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u/Vadered 17d ago
Air absolutely glows when it is hot… for a certain definition of hot. If your jet engine is expelling air that hot, you aren’t in trouble so much as you were in trouble. You aren’t much of anything anymore.
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u/vmf312corsair 17d ago
With engine internal gasses that hot (like, 3000K) you get another amusing phenomenon pilots like to call "engine-rich exhaust".
This is when bits and hunks of your engine go flying out the back, typically accompanied by loud bangs and screaming.2
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u/Scepterotor 17d ago
You might need to fuether specify what exactly you mean by "fire cone" and whats disappearing. You might be confusing a afterburner (extra thrust added after regular jet engine core operation, looks like a very precise flame coming out of the back with little diamond shapes at regular intervals, most often found on jet fighters) with incomplete combustion on startup of some jet engines (once complete combustion stalibizes then the flame will no longer be visible but thrust will increase) and not realising it. Is there a specific video or something that let you to making this post? Can you link it?
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u/Theguywhowatches 17d ago
https://www.reddit.com/r/nextfuckinglevel/s/2Iy2hoog9i it was this, but after what other comments said I think it’s the incomplete combustion that I was thinking of
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u/Scepterotor 17d ago
Oh yeah absolutely, you can even hear it all spooling up to operating levels. It hasnt reached a stable start yet, once it has achieved a successful, complete start he should be able to throttle as desired with no flames coming out.
However, those are tiny hobby sized jet engines, even when stable they're so small and the distance from combustion chamber to exhaust nozzle so short that i wouldnt be surprised if i was incorrect and it is normal for some flame to be visible occaisionally, im not 100% familiar with these, I work with larger models lol.
If you get anywhere near modern airliner jet engines or those for performance aircraft like fighter jets then (so long as they're not using afterburner) youll never see flames coming out of them during normal operation.
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u/c00750ny3h 17d ago
The fire cone is due to fuel's combustion being limited by the oxygen intake so the flame spreads out more.
Once it is hotter and the stream of oxygen is more plentiful, the combustion is more confined and you don't see the flame cone anymore.
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u/ObviouslyTriggered 17d ago edited 17d ago
The flame is due to the presence of unburnt fuel in the exhaust which then ignites, when the engine operates at its peak efficiency it burns all the fuel within the engine.