How do rocket engines account for backflow?

[Mr. Barracuda]

When combustion occurs in a rocket engine, most of the high speed fluid passes out the back as exhaust. But surely some of it flows back through the fuel and oxidizer injector tubes? Wouldn't this decrease efficiency (and potentially even lead to safety risks)?

 

[mfb]

How can it flow back, if new fuel flows through there and pushes everything out?

 

[berkeman]

I don't know much about rocket engines, but it would sure seem like the designers would have to make certain that the fuel pump pressure has to exceed the ignition chamber pressure by a fair amount. That would prevent combustion gasses from being able to flow back into the fuel system.

 

[BobG]

When combustion occurs in a rocket engine, most of the high speed fluid passes out the back as exhaust. But surely some of it flows back through the fuel and oxidizer injector tubes? Wouldn't this decrease efficiency (and potentially even lead to safety risks)?

Yes, it is a concern. The exhaust gases flow out the nozzle because there's nothing on that side to stop it. But the exhaust gases apply pressure to the entire combustion chamber (in fact, that's why the rocket moves the opposite direction of the exhaust gases), including to the injector lines. And, yes, it could be very dangerous to have monoprellant fuels flowing back into the fuel lines. The solution is to keep the pressure in the injectors higher than the pressure in the combustion chamber, if possible - or at least high enough to minimize the loss of efficiency.

That's just part of the difficulty in building a rocket engine and part of the difficulty in calculating the specific impulse for a thruster. Different fuels have different specific impulses, but even with a given fuel, the specific impulse varies by the pressure and temperature of the fuel injected into the combustion chamber (and why one trick for increasing the fuel efficiency of a thruster is to preheat the fuel using some sort of electrical means before it's injected into the combustion chamber).

 

[BobG]

And what happens when the pressure in the fuel lines is lower than the pressure in the combustion chamber?


(embedding isn't allowed for this video)

Actually, it's a little hard to determine for sure what caused the failure, but the most likely cause was the turbopump took long to reach full pressure, allowing burning fuel to come back through the fuel lines. Vanguard TV3