Starship Flight 7

If you cooled the mixture at low enough temperature, you'd stop it from burning (like when you pour water on top of a camp fire), but it's not clear how you're supposed to do that in a spaceship where you can't carry a few tons of water for your sprinklers.

There are other methods too, e.g. fire inhibitors (like Halon or whatever is allowed now) or shockwave to disrupt fire boundary. But I doubt they are very practical on a spaceship.

> If you cooled the mixture at low enough temperature, you'd stop it from burning (like when you pour water on top of a camp fire), but it's not clear how you're supposed to do that in a spaceship where you can't carry a few tons of water for your sprinklers.

Also water would make it hotter, given this is liquid oxygen.

First stage (Super Heavy) is flushing the engine bay with massive ammounts of CO2.

It's not liquid at the point of ignition, that's the thing: if you mixed liquid oxygen and fuel nothing would happen expect the fuel would freeze. For a fire to take place the temperature must reach the fire point temperature, and if you manage to get your fire below this temperature then the fire stops. I don't know how low this temperature can be when the oxidizer is pure oxygen and maybe it's so low water wouldn't be enough, but then you can imagine using other fluids. The problem being the mass burden it adds to a spacecraft, I'm not it'd make any sense given that such q leak should happen in the first place.

Unless I'm misunderstanding you, it's not the same thing at all: in the case you're talking about you're shielding against nominal heat, which is not the same thing as contingency planning to extinguish a fire that shouldn't be there in the first place.

Not an expert but I'm not too sure about shockwave in a confined space.

How does Halon works?

I believe LOX is injected into the engine as a liquid, it gets atomised rather than boiled?

And you can have fires where both fuel and oxidiser are solid: thermite reactions.

"Fire point" seems to be more of a factor for conventional fire concerns, albeit I'm judging a phrase I've not heard before by a stub-sized Wikipedia page: https://en.wikipedia.org/wiki/Fire_point

It's not about the state itself, but about temperature. For things to burn you need to have three elements:

- a fuel

- an oxidizer

- enough heat

It's the fire triangle.

> but about temperature

Which is why I said water would raise the temperature because the oxygen is liquid oxygen — i.e. very cold.

I mentioned solid phase because you were saying "if you mixed liquid oxygen and fuel nothing would happen". There are also videos of people starting fires by pouring LOX onto things.

The fuel and oxidiser in a rocket are often pumped around the outside of the engine bell before reaching the injectors, in order to keep the engine bell itself from melting due to the heat of combustion. I'm not sure exactly what temperature the fuel and oxidiser are at when they hit the injector, but I've seen ground tests where there's frost on the outer wall while the engine is running.

Also, one of the (rare) Falcon rocket failures was due to ice (IIRC oxygen ice) building up around the plumbing during flight: https://www.adastraspace.com/p/spacex-falcon-9-grounded

> Which is why I said water would raise the temperature because the oxygen is liquid oxygen — i.e. very cold.

Water is hotter than liquid oxygen indeed, but it's still much colder than a fire with fuel and oxygen.

> I mentioned solid phase because you were saying "if you mixed liquid oxygen and fuel nothing would happen". There are also videos of people starting fires by pouring LOX onto things.

Do you have an example of such a video? Because in all liquid oxygen I could find on YouTube right now they are always setting the thing on fire (which gives it the initial heat). As I said, it's possible that some mixture of pure oxygen and some fuel have a fire point that is below 0°C, in which case water wouldn't work. But then you could use another (non-flamable) fluid that would be at a temperature below that fire point.

> The fuel and oxidiser in a rocket are often pumped around the outside of the engine bell before reaching the injectors, in order to keep the engine bell itself from melting due to the heat of combustion. I'm not sure exactly what temperature the fuel and oxidiser are at when they hit the injector

The temperature when they hit the injector is irrelevant, what matters is the temperature at the fire boundary.

Imagine a gas stove: the gas reaches the injector around room temperature, but as soon as it reaches the flame of the previous gas (or a match, or a spark from the piezo igniter), it starts burning. But if you pour a glass of water on it (or if you just turn the gas on without igniting it), it will stop burning, and the gas is now just going to spread into the room without burning (don't do this at home, you'd blow up your house)