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guardiangod

It must be the engine burning its internal alloys then. Probably the turbopumps' due to the fuel running rich?

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The exhaust was engine rich :)

It's probably the engine bells, since they are known to contain copper, and copper burns green.

Source: https://mobile.twitter.com/elonmusk/status/11773871411160023...

copper gets used because it conducts heat very well and so assists in cooling. In one of the first test stand videos of a raptor run you can see a little bit of green and I think Musk even tweeted it was running "a little engine rich". the first time I watched a night flight of a falcon9 there was a green glow right at ignition which caused me to hold my breath but that was the hypergolic ignition fluids which also burn green. Raptor uses a torch ignition so any green from a Raptor is very bad news.

Probably oxygen, since the fuel tank had low pressure

That's my suspicion as well. If it's methane rich then the exhaust would have a lot of soot due to incomplete combustion. If it's oxygen rich then the burn temperature would be much higher than designed, and every internal metal surface would get oxidize in a hurry.

Once you hit a propellant bubble, the combustion temperatures are the least of your worries. The turbines are designed to ramp up gently, work fully loaded and transfer the energy from the hot side to the liquid propellants; the power density is enormous, 100.000 horsepower in the space of a truck engine 100 less powerful. Without an energy sink this immense power is directed at the turbine itself, which spins out of control and will dismantle or shatter when hitting liquid again.

Your engine is now full of solid projectiles that take out other parts, fuel lines and injectors, damage film cooling nozzles and coolant canals, and in general transform your precision machine into a smoldering pile of gunk.

heh describing the ramp up of a turbo pump on a rocket engine as "gentle" seems funny. zero to 100k HP in about 500ms or so right?

also, from the pictures I've seen, the turbopumps are about the size of a propane tank, even smaller than a truck engine.

Well, half a second is quite gentle compared to ramming a fluid with a speed in the tens of m/s into a surging turbopump, especially one that is designed to minimize mass and exactly handle normal operating load and not more.

Why would oxygen rich combustion be hotter than stoichiometric combustion?

I know very little about rocket engines, but I would imagine that the fuel pre-burner, which appears to be intended to run extremely fuel rich, might have some serious problems if it didn’t have enough fuel coming in. Aside from just generally not working, it could internally burn much less fuel rich and therefore much hotter than intended, resulting in any number of failure modes.

The adiabatic temperature peak is stochiometric. However dynamics can shift the peak temperature lean or rich.

However, SpaceX is also tuning for engine temperature control and specific impulse. Raptor runs ~3.55 lox/methane (very rich), 4.0 is stoich.

https://www.researchgate.net/publication/255632584_On_the_Of...

Running fuel-rich is cool since the fuel won’t find extra oxidiser to burn.

Excess hot oxidiser on the other hand can often fuel. Like the combustion chamber wall.

This is the answer that the GP was looking for, but it's phrased funny.

sitharus means that the excess oxygen will oxidize (burn) things other than methane if there is enough oxygen present at the temperatures and pressures found in a rocket engine combustion chamber. It will oxidize (burn) things that you wouldn't normally think of as "fuel", such as the copper in the metal alloys.

This line of thought is very reasonable for normal engines, but I wonder if it’s not quite right for the raptor.

I’m not 100% on this, but I think the raptor actually runs really oxygen rich. According to Elon’s tweet and Wikipedia, it runs 3.5-1 O2-CH4. It runs so lean that the unburnt o2 cools things.

If it were running closer to 1-1 (low ch4) then it would run much hotter and burn things like we saw.

Edit: reading more I’m not positive they run O2 rich anymore

Minor correction: 4.0 lox/methane is stoichiometric. 3.8 is rich. 3.55 is very rich. It runs fuel rich to keep cool and possibly increase specific impulse.

You were probably thinking for the turbo pump? There is a lox rich condition somewhere to get full flow. Just not the overall ratio

To clarify this point. The Raptor uses two turbopumps. One turbopump hangs off the side and runs fuel rich, and this fuel-rich exhaust is the fuel input for the combustion chamber.

The second turbopump, to which tdy721 is referring, sits right above the injectors and runs oxygen rich. This oxygen rich exhaust is the oxidizer input for the combustion chamber.

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