How do rocket engines account for backflow?

In summary: U.S., failed because of this.In summary, the conversation discusses the concern of exhaust gases flowing back through the fuel and oxidizer injector tubes in a rocket engine and the potential risks and decrease in efficiency this could cause. The solution is to maintain higher pressure in the injector lines than in the combustion chamber. The specific impulse of a thruster also varies depending on the pressure and temperature of the fuel, and preheating the fuel can increase efficiency. The conversation also mentions a past failure due to fuel burning back through the fuel lines.
  • #1
Mr. Barracuda
3
0
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)?
 
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  • #2
How can it flow back, if new fuel flows through there and pushes everything out?
 
  • #3
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.
 
  • #4
Mr. Barracuda said:
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).
 
  • #5
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
 
Last edited by a moderator:

1. How do rocket engines prevent backflow?

Rocket engines prevent backflow by using a one-way valve system. This valve only allows the flow of propellant in one direction, preventing any backflow into the combustion chamber.

2. Why is backflow a concern for rocket engines?

Backflow can cause instability and decrease the efficiency of the rocket engine. It can also lead to damage or failure of the engine if the backflow is too strong.

3. How does the design of the combustion chamber account for backflow?

The combustion chamber is designed to create a high pressure and high temperature environment, which helps to push the exhaust gases towards the nozzle and prevent backflow. The shape and size of the nozzle also plays a crucial role in controlling backflow.

4. Are there any other methods besides one-way valves to prevent backflow in rocket engines?

Yes, there are other methods such as using baffles or screens to prevent backflow. These devices create turbulence and redirect the flow of propellant, preventing any backflow into the combustion chamber.

5. Can backflow be completely eliminated in rocket engines?

No, backflow cannot be completely eliminated, but it can be greatly reduced. Engineers must carefully design and test the rocket engine to minimize the effects of backflow and ensure its proper functioning.

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