Where does the thrust of a rocket engine actually acts?

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niggannivea
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Joined: 03 Jun 2019, 16:41

Where does the thrust of a rocket engine actually acts?

Post by niggannivea » 03 Jun 2019, 16:43

Few things I know (which may be wrong): The hot gases are accelerated from near zero velocity in combustion chamber to supersonic velocity at the nozzle exit, the rate of change of momentum is equal to the force. Assuming that the exit velocity is nearly constant, Force = mass flow rate times exit velocity.

Now my question is that where does this force actually acts on inside the rocket engine? What if I integrate the pressure on entire internal surface of the con-di nozzle to get a net force, and say that the vertical component of that force is thrust and the horizontal component is trying to rip the nozzle apart?

AlexC
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Joined: 23 Oct 2018, 09:18

Re: Where does the thrust of a rocket engine actually acts?

Post by AlexC » 03 Jul 2019, 11:11

This is indeed a very good question and I will try to answer it as logical as possible.

As you probably already know, in straight and level flight, Thrust equals Drag as Weight equals lift. The resulting forces act on the centre of pressure of the wing.

Now, looking directly into the Turbine engine, or Rocket Engine as you call it ( Hope you are not mentioning or Talking about supersonic Aircraft ) , cause then Thermodynamics are not in direct use, the total force produced by a Turbine ( Thrust ) acts upon the Bolts or Support he is holding to. This is what is producing the Thrust Force Vector acting on the Entire aircraft and resultant Fr on the Centre of Pressure.

As far as I know, Turbine Engines do not fly alone as they are not aerodynamically capable of doing so. However, a Rocket engine and provided he has unlimited power, can indeed fly without any relation to Aerodynamics as the Thrust he produces F, is far greater than the Mass ( mass of the Rocket Engine).

Taking Newton’s Second Law of Motion
If your Thrust FORCE ( F) keeps increasing = your mass remains the same, the acceleration will have/ will increase as well.
However if you thrust FORCE F remains the same ( FULL POWER) = Your mass remains the same (M) and obviously your acceleration will remain constant.

This obviously relates only to Thermodynamics and in Supersonic Speeds, then there are other factors like Temperature affecting your Supersonic Speed. But I guess I am way rusty on Thermodynamics do give you a better explanation.

Hope this Helps. :geek: :geek:
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Bracebrace
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Re: Where does the thrust of a rocket engine actually acts?

Post by Bracebrace » 03 Jul 2019, 13:40

Even in a jet engine, the combustion creates a flow speed increase that creates a net drop in pressure, so there is no high sideways force.

In a rocket engine, the exhaust is designed for a certain "atmospheric region". In the converging section, the pressure does not increase, the velocity increases. Once you hit the diverging section, the atmospheric pressure comes into play. If the created pressure is lower than the atmospheric pressure, you have an overexpanded nozzle and the exhaust gases will be "pushed" together by the atmosphere (so that's how low the internal pressure in the gas is). This is like an afterburner, which is preferred choice to protect the military aircraft from it's own engine exhaust gas.

If the pressure is higher than the atmospheric pressure, you have an underexpanded nozzle. This is like the space rockets we know when they lift-off. There will be some sideways pressure in the diverging nozzle, but not a lot. Once the gas leaves the nozzle design, it expands slowly.

Depending on the atmospheric pressure your rocket engine has to pass, the nozzle is adapted (hence multi-stage rockets with different nozzle designs).

As to where the actual force is transferred onto the rocket, I don't know honestly. This would be where the rocket engine is connected to the rocket itself? On a jet engine the fan is connected to the static aircraft through the thrust bearings, so the thrust bearings actually transfer all force from the rotating fans and compressor onto the static aircraft design. Pretty impressive bearings :-).

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