Author Topic: Rocket engine turbopumps Q&A  (Read 2826 times)

Offline cube

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Rocket engine turbopumps Q&A
« on: 09/18/2022 04:31 pm »
Hello, several rocket engines are equipped with turbopumps to pressurize the propellants before injecting them into the combustion chamber but I was wondering if the turbopumps help in addition, to pull the propellants out of the tanks towards them or if they just push the propellants towards the combustion chamber ?

Thanks !
« Last Edit: 09/18/2022 04:33 pm by cube »

Offline Proponent

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Re: Rocket engine turbopumps Q&A
« Reply #1 on: 09/19/2022 12:26 am »
You cannot really pull a liquid. Liquids will tend to flow from high pressure to low pressure. A suction pump might seem to be pulling a liquid, but actually it is creating a region of low pressure into which atmospheric pressure pushes the liquid.

Offline Nomadd

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Re: Rocket engine turbopumps Q&A
« Reply #2 on: 09/19/2022 04:10 am »
Hello, several rocket engines are equipped with turbopumps to pressurize the propellants before injecting them into the combustion chamber but I was wondering if the turbopumps help in addition, to pull the propellants out of the tanks towards them or if they just push the propellants towards the combustion chamber ?

Thanks !
You really don't want to pull the fuel. You'd be begging for cavitation, which would not end well. The fuel at the inlet needs to be kept at a certain pressure through tank pressure, a seperate boost pump or gravity/acceleration in certain parts of the flight.
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Online edzieba

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Re: Rocket engine turbopumps Q&A
« Reply #3 on: 09/19/2022 08:32 am »
Hello, several rocket engines are equipped with turbopumps to pressurize the propellants before injecting them into the combustion chamber but I was wondering if the turbopumps help in addition, to pull the propellants out of the tanks towards them or if they just push the propellants towards the combustion chamber ?

Thanks !
Additional turbopumps push propellants. not pull. Some stages have had booster bumps at the tank outlet in addition to the ones in the engine(s), such as the early Centaurs.

Offline cube

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Re: Rocket engine turbopumps Q&A
« Reply #4 on: 09/19/2022 02:46 pm »
Thank you everyone !

So if I understand correctly, the turbopumps are designed not to take propellants faster than can arrive by the pressure of the tank to avoid created a vacuum in the piping which could turn some of the propellants into gas. I imagine the propellant is coming to the turbopumps at a pressure lower than that inside the tank but high enough to avoid the creation of gas in the propellants.

I have another question that comes to mind when a rocket engine is on the launch pad on earth and it starts, i imagine there is air in the piping of the engine, is this gas a problem when starting the engine?
« Last Edit: 09/19/2022 02:57 pm by cube »

Offline Jim

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Re: Rocket engine turbopumps Q&A
« Reply #5 on: 09/19/2022 03:34 pm »
Thank you everyone !

So if I understand correctly, the turbopumps are designed not to take propellants faster than can arrive by the pressure of the tank to avoid created a vacuum in the piping which could turn some of the propellants into gas. I imagine the propellant is coming to the turbopumps at a pressure lower than that inside the tank but high enough to avoid the creation of gas in the propellants.


No, the tanks are pressurized with the head pressure to prevent cavitation




I have another question that comes to mind when a rocket engine is on the launch pad on earth and it starts, i imagine there is air in the piping of the engine, is this gas a problem when starting the engine?

The pipes have valve to ensure no gas and only liquid at the inlets.

Offline cube

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Re: Rocket engine turbopumps Q&A
« Reply #6 on: 09/20/2022 01:51 am »
What I meant is if the fuel in the tank is at 6 bar pressure, when you open the valve and let out
the fuel, the pressure of the fuel flowing out of the tank will decrease and when it will arrive at the inlet of the turbopump,
i imagine (but I'm not sure) that it will be at a pressure a little lower than that of the tank, for example 4 bar at
instead of 6 bar, (depending on how fast the fuel is flowing through the turbopump).
« Last Edit: 09/20/2022 01:53 am by cube »

Offline Jim

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Re: Rocket engine turbopumps Q&A
« Reply #7 on: 09/20/2022 02:03 am »
What I meant is if the fuel in the tank is at 6 bar pressure, when you open the valve and let out
the fuel, the pressure of the fuel flowing out of the tank will decrease and when it will arrive at the inlet of the turbopump,
i imagine (but I'm not sure) that it will be at a pressure a little lower than that of the tank, for example 4 bar at
instead of 6 bar, (depending on how fast the fuel is flowing through the turbopump).

no.

a.  the engine is lower than the tank.  Head pressure at the engine will always be higher.
b.  Pressurant is added to the tank to maintain pressure in the tanks
c.  The lines are filled up to the engine

Offline cube

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Re: Rocket engine turbopumps Q&A
« Reply #8 on: 09/20/2022 01:25 pm »
no.

a.  the engine is lower than the tank.  Head pressure at the engine will always be higher.
b.  Pressurant is added to the tank to maintain pressure in the tanks
c.  The lines are filled up to the engine

There is just a. which I'm not sure I understand, do you mean that the pressure at the entrance of the turbopump will be higher than at the tank because of gravity or the thrust of the engine or both?
« Last Edit: 09/20/2022 01:25 pm by cube »

Offline Jim

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Re: Rocket engine turbopumps Q&A
« Reply #9 on: 09/20/2022 02:38 pm »

There is just a. which I'm not sure I understand, do you mean that the pressure at the entrance of the turbopump will be higher than at the tank because of gravity or the thrust of the engine or both?

Both.  Head pressure is what dams use to generate power.

Offline Nomadd

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Re: Rocket engine turbopumps Q&A
« Reply #10 on: 09/20/2022 07:07 pm »
no.

a.  the engine is lower than the tank.  Head pressure at the engine will always be higher.
b.  Pressurant is added to the tank to maintain pressure in the tanks
c.  The lines are filled up to the engine

There is just a. which I'm not sure I understand, do you mean that the pressure at the entrance of the turbopump will be higher than at the tank because of gravity or the thrust of the engine or both?
Gravity and thrust are the same thing. Think of the water tower in your town. No pressure at the top of the water  but 80psi at the bottom of the outlet pipe.  If you launch the tower (actually a real thing) you get more pressure at the bottom because the acceleration makes the water weigh more. The effect of the acceleration from the thrust isn't just like gravity getting stronger, it is the same in physics.

 Rockets are complicated because gravity while on the ground, acceleration and occasional freefall keeps changing pressure at the pump inlet. Tank pressure (and a little negative effect from air drag) is all you have when you're in the air and the engines aren't firing.
« Last Edit: 09/21/2022 05:50 am by Nomadd »
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Offline cube

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Re: Rocket engine turbopumps Q&A
« Reply #11 on: 09/24/2022 02:13 am »
Thank you ! I understand now

I imagine that the propellant flow is the same at the tank outlet as at the turbopump outlet?

Offline Nomadd

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Re: Rocket engine turbopumps Q&A
« Reply #12 on: 09/24/2022 02:40 am »
Thank you ! I understand now

I imagine that the propellant flow is the same at the tank outlet as at the turbopump outlet?
Never exactly. Take some time and watch a video of how the different engines work. Tim Dodd did a good one.

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Offline Jim

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Re: Rocket engine turbopumps Q&A
« Reply #13 on: 09/24/2022 01:55 pm »
Thank you ! I understand now

I imagine that the propellant flow is the same at the tank outlet as at the turbopump outlet?

It has to be (mass wise, not velocity).  Propellant is not created or destroyed.

Offline Nomadd

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Re: Rocket engine turbopumps Q&A
« Reply #14 on: 09/24/2022 05:52 pm »
Thank you ! I understand now

I imagine that the propellant flow is the same at the tank outlet as at the turbopump outlet?

It has to be (mass wise, not velocity).  Propellant is not created or destroyed.
No, but it can leave via an exhaust port and the turbine has more than one input. In ffsc the fuel turbine will put out more mass than the fuel tank and the oxidizer turbine will put out less mass than the oxidizer tank.
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Offline Jim

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Re: Rocket engine turbopumps Q&A
« Reply #15 on: 09/24/2022 06:18 pm »

 No, but it can leave via an exhaust port and the turbine has more than one input. In ffsc the fuel turbine will put out more mass than the fuel tank and the oxidizer turbine will put out less mass than the oxidizer tank.

wrong.  it might not all go through the nozzle and go out like the  "exhaust port", which the turbo pump exhaust from the gas generator.  The gas generator is still fed by the turbo pump.

And, we are not talking turbines and what feeds them, we are talk about the pumps themselves.  How the pump discharge is used, split up, burned, heated, dumped, etc is not part of the discussion.   All the propellant goes from the tanks to the pumps, not all of the propellant goes from the pumps to the combustion chamber/nozzle.
« Last Edit: 09/24/2022 06:19 pm by Jim »

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