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 Post subject: Rocket motors
PostPosted: Mon Oct 31, 2005 06:11 
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I understand two effects by which rocket motors appear to derive thrust.

1) Pressure differential in the combustion chamber. Pressure from the rapidly expanding fuel pushes on the 'front face' of the combustion chamber, and no such pressure exists at the exit port.

2) Mass ejection. By throwing gas out the back, an equal and opposite force arises (Newton's third law).

Are these effects actually one and the same? Or not?

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PostPosted: Mon Oct 31, 2005 10:16 
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The action of ejecting the fast moving gasses through the propelling nozzle results in the reaction force we know as thrust. To give the phenomenon its correct name it is called 'momentum thrust' because it is the result of the change of momentum in the gas stream (0mph to ,say, 1000mph for example).
Now, within the vehicle itself, various sections of the engine will be contributing to the acceleration of the gas stream in different ways. Some parts such as the combustion chamber (the primary contributor) and the divergent section of the propelling nozzle will feel forward pushes, other parts such as the convergent section of the propelling nozzle will actually experience negative pushes (or drag) and reduce the value of thrust produced. However, without the combined action of all the component parts the gas wouldn't accelerate and thrust wouldn't be produced.
There is a second type of thrust (known as pressure or nozzle thrust) which is caused by the static pressure in the exhaust stream being above ambient pressure. This creates an additional forward push on the rear of the propelling nozzle.

So, in sum, the reaction to throwing the propelling gasses rearwards produces the thrust value of the motor, this force is felt internally by the various section contributing to the acceleration of the gas.


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 Post subject: Re: Rocket motors
PostPosted: Mon Oct 31, 2005 11:06 
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SafeSpeed wrote:
Are these effects actually one and the same? Or not?

I believe they are. I have also confirmed this with a colleague who specialises in physics.

1 causes 2 - 1 provides the thrust, 2 is the reactive element.

It is the pressure build-up at the front face of the combustion chamber which forces the expanding gaseous matter rearwards, this force providing the forward thrust.

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 Post subject: Re: Rocket motors
PostPosted: Mon Oct 31, 2005 12:10 
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smeggy wrote:
It is the pressure build-up at the front face of the combustion chamber which forces the expanding gaseous matter rearwards, this force providing the forward thrust.


Thats true, but it isn't the pressure per-se which creates the thrust- this is a common misconception. It is the massflowrate of the propelling gasses and the velocity to which they are accelerated; the latter does depend to a certain extent on the pressure that can be generated within the motor but the shape (convergent/divergent) of the propelling nozzle is equally if not more important.


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