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Thread: What are the physics behind a rocket being propelled?

  1. #1 What are the physics behind a rocket being propelled? 
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    Hi forum!

    What are the physics behind a rocket being propelled ?

    How does it exactly work,
    how is it that it travels? (does it push off of particles?)

    and how is it possible that a rocket propels itself in space (vacuum) if there's little or scarce resources of matter/particles in it?

    Id appreciate your input! thanks.

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  3. #2  
    Brassica oleracea Strange's Avatar
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    Oct 2011
    Conservation of momentum.

    Imagine you are in a wheeled office chair in the middle of a frozen pond (so there is minimal resistance) with a collection of heavy objects. As you throw these away from you, you will move in the opposite direction.

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  4. #3  
    Moderator Moderator Janus's Avatar
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    Here's another way to look at it. A rocket works by burning fuel which produces expanding gases.

    Imagine that you burn this fuel in a spherical chamber. the gas tries to expand and as it does so, pushes outward against the walls of the chamber. Now since it pushes in all directions equally, it won't move the chamber.

    Now poke a hole in one side of the chamber. The gases pushing in this direction will escape the chamber, and as they do so they are now longer supplying a push on the cylinder in that direction. The gases on the other side of the chamber are still pushing on the inside of the wall. There is now an imbalance of forces on the chamber and it is pushed in one direction harder than the other so it moves in that direction.

    Of course as the gas escapes, the pressure and push goes down. So what a rocket does is keep burning new fuel to make more gases to replace the ones being lost.

    Now, it turns out that a rocket actually works better in a vacuum then in does in an atmosphere. This is because the air outside has a pressure that pushes against the gasses trying to escape. This reduces the push the rocket gets.

    A real rocket has some addtional features like a bell shaped housing at the back which helps to get the most out of the escaping gases. But the basic principle is the same as above.
    Implicate Order and Muto-kun like this.
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  5. #4 Newton's Third Law Of Motion 
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    Sir Isaac Newton's third law of motion says "Every action has an equal and opposite reaction." You can think of an accelerating rocket or burning jet engine as throwing lots of high speed matter to its rear. The principle is the same for jet engines, the difference is they get gaseous oxygen from the atmosphere, while rockets carry their own oxygen, so no atmosphere is required for a rocket.

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  6. #5  
    Time Lord
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    Rockets are often likened to a balloon or chamber with air escaping. Cute illustrations show thrust arrows pushing in all directions from inside, the one arrow that pushes nothing gets lost out the back. Propulsion can work like this but in actual rockets it's different: combustibles are pumped through an engine, blended and ignited so they essentially explode behind the rocket, knocking it forward.
    A pong by any other name is still a pong. -williampinn
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  7. #6  
    Forum Masters Degree Implicate Order's Avatar
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    Quote Originally Posted by Muto-kun View Post

    What are the physics behind a rocket being propelled ?

    If you want to get deep down and dirty then here. :-))
    Quidquid latine dictum, altum videtur
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