Rocket exhaust flowback

[Xd the great]

How does a rocket keep on injecting fuel into a high-pressure combustion chamber? Wouldn't the gas flow back into the tanks and cause problems?

Edited January 13, 2019 by Xd the great

[magnemoe]

You have to have over pressure in the tank or use an pump 

[Steel]

Simply, they inject it at a higher pressure than the combustion chamber pressure. It's why most engines rely on turbo pumps to pump the fuel.

[Reactordrone]

The turbopumps on a raptor engine, for example, make and use about 100,000 hp to maintain propellant flow.

 

The design of the combustion chamber also helps. Pressure wants to go out through the big hole at the bottom rather than the tiny injector nozzles at the top.

Edited January 12, 2019 by Reactordrone

[Xd the great]

So thick fuel lines supply the pump, fuel comes out nice and pressurized, and gets burnt in the chamber. 

How much stress does the pump take?

[kerbiloid]

http://www.lpre.de/energomash/RD-170/index.htm

RD-170/171

Combustion chamber: 24.5 MPa
Turbopump: 50..60 MPa.

 

NK-33 pump

Spoiler

 

 

Edited January 12, 2019 by kerbiloid

[YNM]

6 hours ago, Xd the great said:

How does a rocket keep on injecting fuel into a high-pressure combustion chamber?

Make the upward pressure even higher.

You can achieve this by additional compression (turbochargers) or just pressurize them high enough (ie. in simple monopropellant or hypergolic engines), then help it with some inert gas pressurant (ie. Helium).

Yes, rocket science isn't the "safest" one.

Edited January 12, 2019 by YNM

[KSK]

4 hours ago, Xd the great said:

So thick fuel lines supply the pump, fuel comes out nice and pressurized, and gets burnt in the chamber. 

How much stress does the pump take?

I think the technical term is ‘a metric shed load’. Historically, I believe that most rocket engine failures have been down to the turbopump going kablooey.

[Starman4308]

16 hours ago, Xd the great said:

So thick fuel lines supply the pump, fuel comes out nice and pressurized, and gets burnt in the chamber. 

How much stress does the pump take?

A lot. A modern, high-performance rocket engine is basically one of the most efficient and powerful turbopumps ever produced, with some other bits attached.

I'm exaggerating a bit, but turbopumps are probably the most complicated bit on a modern engine, as they have to feed low-pressure propellant into a very high-pressure combustion chamber (the higher pressure, the better), while being extremely lightweight, reliable, and sometimes throttleable and restartable... in mid-flight.

[DDE]

16 hours ago, KSK said:

Historically, I believe that most rocket engine failures have been down to the turbopump going kablooey.

Combustion instabilities are a contender. They hit even the smallest thrusters (as low as 5 ibf) but become something truly vile when you’re testing, say, an F-1 - even if it’s just a TC with no turbopump but a pressure-feed test stand.

[KSK]

20 minutes ago, DDE said:

Combustion instabilities are a contender. They hit even the smallest thrusters (as low as 5 ibf) but become something truly vile when you’re testing, say, an F-1 - even if it’s just a TC with no turbopump but a pressure-feed test stand.

Good point. Do you think it would be fair to say that most in-flight failures have been down to pump problems? Combustion instabilities are definitely bad (the F1 example being the one that I'm aware of) but I would have thought they mostly get taken care of during development?  Although I can well imagine that some unknown unknowns during flight could lead to combustion instability.

[Xd the great]

On 1/12/2019 at 7:32 PM, Reactordrone said:

The design of the combustion chamber also helps. Pressure wants to go out through the big hole at the bottom rather than the tiny injector nozzles at the top.

Won't gas try to shoot out of the tiny injector holes by bernoulli's principle?

[DDE]

4 hours ago, KSK said:

Good point. Do you think it would be fair to say that most in-flight failures have been down to pump problems? Combustion instabilities are definitely bad (the F1 example being the one that I'm aware of) but I would have thought they mostly get taken care of during development?  Although I can well imagine that some unknown unknowns during flight could lead to combustion instability.

Not by that logic. A full-duration burn on the teststand would be even more effective at testing the turbopump (less altitude-induced effects than the TC); a hotfiring of the actual stage would seal the deal.

1 hour ago, Xd the great said:

Won't gas try to shoot out of the tiny injector holes by bernoulli's principle?

Quote

The principle is only applicable for isentropic flows: when the effects of irreversible processes (like turbulence) and non-adiabatic processes (e.g. heat radiation) are small and can be neglected.

Furthermore, in most gas generator engines the propellant doesn’t get a chance to evaporate and is instead reduced to shall droplets moving at a considerable velocity when they hit the combustion zone.

[IncongruousGoat]

16 hours ago, KSK said:

Do you think it would be fair to say that most in-flight failures have been down to pump problems?

Yes, although I would like to point out that this is only the case because there were a lot of problems with turbopumps in the early days. If you look at the early flight tests of the Jupiter IRBM, for example, about half of them failed, and almost every failure was due to the turbopump going kablooey. Turbopumps are a mostly solved problem nowadays.

[p1t1o]

On 1/12/2019 at 10:00 AM, Xd the great said:

How does a rocket keep on injecting fuel into a high-pressure combustion chamber? Wouldn't the gas flow back into the tanks and cause problems?

The "turbopumps" used in major rocket applications are extremely powerful. Powered by partial combustion of the fuel/oxidiser mix itself, they can generate thousands of horsepower.

What you are thinking is correct though, total thrust available is dependent on inlet pressure, the chamber is designed around this and if chamber pressure rose above inlet pressure, yes, you would get damaging (probably catastrophic) backwards flow.

[DDE]

9 hours ago, p1t1o said:

What you are thinking is correct though, total thrust available is dependent on inlet pressure, the chamber is designed around this and if chamber pressure rose above inlet pressure, yes, you would get damaging (probably catastrophic) backwards flow.

Isn't that what a check valve is there to prevent?

Also, that's an RS-25, do I get a cookie?

[p1t1o]

49 minutes ago, DDE said:

Isn't that what a check valve is there to prevent?

Also, that's an RS-25, do I get a cookie?

Im sure its possible to take measures, but its not going to be a healthy engine!

And yes, because I just googled "rocket turbo pump diagram" or something

 

 

 

 

[magnemoe]

On 1/14/2019 at 2:57 AM, IncongruousGoat said:

Yes, although I would like to point out that this is only the case because there were a lot of problems with turbopumps in the early days. If you look at the early flight tests of the Jupiter IRBM, for example, about half of them failed, and almost every failure was due to the turbopump going kablooey. Turbopumps are a mostly solved problem nowadays.

Yes, look like most fails noways are software, structural or separation related. 
Not sure that happened to the falcon 9 who lost trust on one engine during flight. 

[Xd the great]

4 hours ago, magnemoe said:

Yes, look like most fails noways are software, structural or separation related. 
Not sure that happened to the falcon 9 who lost trust on one engine during flight. 

I thought that mission was a partial success, just that a secondary payload was lost...

[magnemoe]

2 hours ago, Xd the great said:

I thought that mission was a partial success, just that a secondary payload was lost...

Yes, it lost trust so it was not able to reach required orbit, add that NASA requirements was very strict, the falcon 1 had enough dV to reach ISS but not the upper stage. 
The interesting part is that Falcon 9 is supposed to be resistant to turbo pump catastrophic fails. The sane way airliners are not to be damaged by an engine catastrophic fail. The the engine is armored so the parts who can break trough and damage fuselage or wing is stopped.