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Rocket Propulsion

  1. Oct 15, 2014 #1
    Hi i am doing a project on the future of solar energy. While reading up on rockets and how they are propelled to escape velocity i noticed all references only tell us that the hot gases propel the rocket up. Could someone explain exactly how this happens. What i mean is does molecule collision cause the propusion or what?
  2. jcsd
  3. Oct 15, 2014 #2


    Staff: Mentor

    At the microscopic level, I think you are right. In general though, it is explained as an application of Newtonian physics via the conservation of momentum where the exhaust from the rocket burn is mass ejected and so the rocket goes forward as a consequence. It you look at it microscopically, a chemical reaction occurs, energy is released, molecules move faster and temperature and pressure builds with some molecules escaping out the nozzle and others pushing against the rocket chamber pushing the rocket forward.

    There are other forms of propulsion though as described in the wikipedia article:

  4. Oct 15, 2014 #3
    So then if the propulsion is caused by the molecules of gas with high kinetic energy pushing the inside of the rocket, is it possible while in space to heat the gases inside the rocket and release them to cause further propulsion in space
  5. Oct 15, 2014 #4

    Simon Bridge

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    Although just having a high speed (=high temperature) gas is not enough - you also have to let it out.

    I take it you are thinking of using solar power to heat the gas?
    This will work - and light-propulsion systems working on that principle have been developed.

    Note: if you throw anything at all one way, you will move the other way (unless something stops you.)
    This is how we'd normally think about how rockets work ... burning the fuel results in hot gasses, the rocket tube only has one exit - so the gasses escape that way very fast. The rocket goes the other way.
  6. Oct 15, 2014 #5
    Yes Newtons law, but is this also possible in space where there are no particles present. I am thinking of a concept of an electric propulsion system which would obviously include the concepts of magnetism and such.
  7. Oct 15, 2014 #6


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    Do you mean that there are "no particles present" in the sense that there are no particles to eject from the rocket? If you don't move anything out of the back of the rocket, the rocket won't move forward. This seems to obvious and fundamental that I think maybe I don't understand your question.
  8. Oct 15, 2014 #7


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    He's thinking of "molecule collision causing propulsion" but in the sense of molecules of the propellant pushing on the molecules in the air - a common misconception of how rocket propulsion works.

    @Siddharth Menon: the molecules of the gas in the propulsion chamber collide with the sides of the chamber(i.e., with the rocket itself), exerting force through pressure. The lack of anything to collide with on the exhaust side of the engine means that there is an imbalance of forces, and the rocket is propelled forward. Any ambient gasses present on the exhaust side reduce the force imbalance, reducing thrust - rocket propulsion is more efficient in the vacuum of space than it is in the atmosphere.
  9. Oct 15, 2014 #8


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    Like that physics prof 100 years ago who said that in the New York Times (that rockets in space couldn't work because there would be nothing for them to push against). This is so fundamentally wrong that I didn't think it was what he meant.
  10. Oct 15, 2014 #9
    Alright i understand that, so it is possible to change direction etc. using hot gases. It may seem like i am asking stupid questions but i just want to be sure.
  11. Oct 15, 2014 #10
    Now that everyone :) is "up to speed" on the fact that reaction engines don't need anything to push against, and considering that OP is interested in solar powered electric propulsion it should be noted that it is a large field with considerable promise. Many types exist and are presently in use though currently mostly in batteries of small thrusters for minor orbit corrections for satellites. The concepts are commonplace enough that wikipedia actually has a fairly decent article on them.
    As is rather common with wiki the best part is references and external links so don't miss those.
  12. Oct 15, 2014 #11
    Haha, i see what you did there. Thank you all this really helps me.
  13. Oct 15, 2014 #12

    Simon Bridge

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    No worries and have fun ;)
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