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Simple question regarding rocket

  1. Dec 15, 2007 #1
    I was looking on wiki about different froms of propulsion and I have this very general question.

    When a rocket produces thrust from its exhaust, what exactly is it 'pushing on' ?

    I mean on Earth, when a jet plane uses its engines to take off, what is going on...is the exhaust 'pushing' on the particles in the atmosphere or against its own exhaust?

    I think the answer to the plane will satisfy a rocket in space too..so I'll wait to ask.


    p.s. Sorry if this is a stupid question....it just occured to me that I don't know the answer and I now need to!
  2. jcsd
  3. Dec 15, 2007 #2


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    It's pushing on the inside of the rocket engine.
    Imagine a simple rocket engine that is just a tin can with fuel burning inside.
    The exhaust is pushing on all sides equally so there is no net force and the can goes nowhere.
    Now remove the bottom. The exhaust is stillpushing on the top of the can pushing the whole can up - but there is nothing pushing on the bottom so there is no balancing force and so the can goes up.

    A jet is similair - it takes in air at the front, compresses it and burns it. This heats the air which expands, some of it pushes against the engine which pushes the plane forward. A modern high bypass turbofan like you see on an airliner actuully gets most of it's thrust from the large fan at the front which is driven by the jwet engine turning trubine blades - it's more like a propeller plane than a jet engine.
    Last edited: Dec 15, 2007
  4. Dec 15, 2007 #3


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    Good explanation, mgb.
    It's all based upon Newtonian physics. Equal and opposite reaction (although that's not technically the right way to state it). The combustion products exert equal force in all directions, but most of those directions are blocked. The nozzle end is open, so the exhaust runs out in that direction. The force that would be exerted in that direction is therefore not contained within the engine, so there is no counterbalance to the force pushing in the opposite direction. Therefore, the net result upon the engine is that it has to move toward that imbalanced force (ie: onward and upward).
  5. Dec 15, 2007 #4
    Alright, I think I get it now. I guess I was thinking if it incorrectly. I was thinking that it was more like if a person squatted down holding a box over their head (I can see already this analogy is going to suck)....then the box is the ship and the person is the engine.

    When the box needs to go up the person pushes on it AND the ground extending his body the way I had pictured the gas expanding.

    But unlike the person, the gas has nothing but the 'box' to push on, that is nothing opposite the boxes direction (like the ground for the person) except for air (unless it is in space, which is why I started this thread to begin with) and ITSELF.

    Now, does the latter (itself) even apply here...I feel like it must. That is, the gas heats and expands and then pushes on the rocket and itself (like the body between the box and the ground).

    Am I still envisioning this incorrectly?

    Thanks for your patience,
  6. Dec 15, 2007 #5


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    A good way of picturing equal and opposite reactions is a gun recoil.
    Imagine you were sitting on a skateboard and fired a rifle - the recoil would push you along, the force is acting between the cartridge charge and the chamber on the gun, where the bullet goes and after it left the gun doesn't matter.
  7. Dec 15, 2007 #6


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    There is another way to think about it...The engine, through compression and then combustion accelerates the gases out the back of the engine. For that to happen, there must be a force to create that acceleration. The engine accelerates the gases out the back and the equal and opposite is the force that we call thrust.
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