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B How do rockets propel themselves up?

  1. Jun 18, 2017 #1
    Well, first thing, I am new to this forum and it looks pretty good and I'm looking forward to reading more from it and post more questions.

    Well rocket propeling (or anything that uses some material to propel up when shooting it down) seems pretty straight forward at first, you have some gas and make a combustion putting a force downwards thus pushing the rocket upwards and it is explained by Newton's Third Law.
    Now the thing that I don't understand, for example, when you throw a ball against the ground it pushes the ground downwards with a force and then the ground pushes the ball upwards with an equivalent and opposite force making it go up and it is clear that the ground is making the ball go up again while in a rocket you are pushing down through a combustion of gas, you are like "throwing" millions of particles downwards (this interpretation may not be correct) making a force to the ground, but what exactly is pushing the rocket upwards?

    In my head it is something like this, you are pushing lots of particles into the ground or the are or whatever and once these particles are released they are "free" from the rocket and they don't have anything to do with it. So, is it these particles that really are related to the rocket that push it upwards? Is it the air? What is it?

    Sorry for any bad interpretation of events, I just want to know what is happening exactly and I don't have much idea about it, in the future I'd like to study aerospace engineering and learn a lot more about it.

    Thanks in advance to everyone.
     
  2. jcsd
  3. Jun 18, 2017 #2

    CWatters

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    It's the act of accelerating the rocket exhaust that propels the rocket (conservation of momentum). The rocket doesn't need anything to push against.
     
  4. Jun 18, 2017 #3

    Merlin3189

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    There are different ways of thinking about this.
    Newton's 3rd tells us that the force pushing the gas backwards is the same as the force pushing the rocket forwards. No ground, no air, just the rocket and the gas.
    Conservation of momentum is the same. If the rocket engine gives the gas a backward momentum, it must give the rocket an equal and opposite forward momentum. Again, no need for air or ground.
    Both give you the mathematical results you need. But I think you ask about how they manage it.

    Inside the rocket engine - combustion chamber and nozzle - there is gas at high pressure pushing on the surfaces. If the engine is radially symmetric, the radial forces cancel each other, leaving the axial forces. The engine is not symmetric axially, so the total of forces on the backward facing surfaces is greater than the total of forces on the forward facing surfaces.
    If you imagine an oversimplified engine which is a tube sealed at the front end and open at the back, there will be a force on the closed end, but not on the open end and the tube will be pushed in the direction of the closed end. The pressure which would have caused a force on the back end, had it been there, is now just pushing the gas out of the open end and accelerating it to high speed.
    The complexity of the chamber and nozzle shape (about which I know little) is to try to ensure that the pressure generated by the combustion is used to maximally accelerate the gas leaving. If it leaves the nozzle still at high pressure, that pressure will accelerate the gas radially and that momentum will not contribute to the forward momentum of the rocket.
     
  5. Jun 18, 2017 #4

    PeterDonis

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    Yes, but the force of the ground on the ball is separate from the force you yourself exert on the ball. When you throw the ball against the ground, the ball pushes back on you--before it even hits the ground. (I'm assuming you actually throw the ball instead of just dropping it and letting it free-fall.) If you were floating freely in space instead of standing on Earth, and you threw a ball, you would be pushed in the opposite direction.
     
  6. Jun 18, 2017 #5

    russ_watters

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    Forget the ground (clearly it cannot play a role if the rocket isn't near the ground): when you throw a ball you apply a force to it to accelerate it. And when the rocket throws the gas particles downwards, it applies a force to them. That force accelerates the gas down and therefore the rocket up.
     
  7. Jun 18, 2017 #6

    rcgldr

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    Details - in the case of a typical liquid fuel rocket, a turbine pump is used to pressurize the intake gases to a bit more than the pressure inside the combustion chamber in order for the gases to flow into the combustion chamber. The combustion process maintains the pressure as the spent fuel accelerates to a high velocity out of the rocket engine.
     
    Last edited: Jun 19, 2017
  8. Jun 18, 2017 #7
    It can be evaluated as conservation of momentum. Think this like an explosion of a particle. While a piece of particle is going in the +x direction the other part goes on -x if it seperated 2 pieces.

    In the Earth atmosphere, the force exerted on the rocket is also arised from the reaction force that is caused by thrust force on air. But it's not valid on the space.

    There was a russian scientist whose name is weird. He explained the motion of rockets on space with some equations. I'll edit when I found.
     
  9. Jun 19, 2017 #8
    I've searched for it and I think you mean the "Tsiolkovsky rocket equation" which from what I´ve read it describes " the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself (a thrust) by expelling part of its mass with high velocity and thereby move due to the conservation of momentum".
     
  10. Jun 19, 2017 #9
    Yes, it was Tsiolkovsky. So, you have answered yourself I hope you found what you were searching for.
     
  11. Jun 19, 2017 #10

    Merlin3189

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    I agree this is an unhelpful description. But I wonder whether the conclusion may be correct?

    It seems to me that, the effect of surrounding the rocket by air is to impede the rearward motion of the exhaust. So the gases exit more slowly and the pressure in the chamber rises. Then the pressure view says there will be more thrust.

    But if the exhaust gases exit more slowly, what does the momentum view say?
     
  12. Jun 19, 2017 #11

    CWatters

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    You could increase the pressure by totally blocking the exhaust nozzle :-)
     
  13. Jun 19, 2017 #12

    russ_watters

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    You are correct: thinking the rocket is pushing against the air leads you in the wrong direction, incorrectly concluding that a rocket is more effective when it is pushing on air. It isn't.
     
  14. Jun 19, 2017 #13
    Yes, from an efficiency perspective the ideal for steady level flight is that the exhaust speed relative to the rocket equals the rocket's air speed so that the individual particles of exhaust are accelerated from rest within the fuel tank with respect to the rocket to at rest with respect to the air... perfect would be laying out the exhaust motionless in the air behind the speeding rocket. Using greater exhaust speed means wasting energy - the amount of energy of the still air slowing the particles to rest. Likewise for an exhaust speed less than the rocket's air speed; the exhaust particles would be placed into the air with a forward momentum to be slowed by the air, so wasted energy.

    But, that is for horizontal flight or extremely local vertical flight... For a rocket going from pad to orbit, the exhaust throttle needs to take into account the air's density and the rocket's air speed. At launch the air density is maximum but the air speed of the rockets is initially zero and virtually all exhaust is being slowed by the air (and pad). At low elevation the air is thick but the launch is still early and the air speed is relatively low, but later when the elevation is high the air is thin and the rocket's air speed is fast. The two curves for air density and rocket speed typically don't overlay, and the air resistance to the rocket varies with speed and altitude (dynamic load), so the throttle is adjusted accordingly. There will be a period where the rocket encounters maximum dynamic load (worst case combination of density and air speed) where more thrust is required after which the throttle is backed off proceeding to higher thinner air at greater speed (I think the Shuttle flights did "throttle back" at about 70 miles up).
     
  15. Jun 20, 2017 #14

    jbriggs444

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    In a chemical fuel rocket, the exhaust velocity is largely a function of the fuel. Maximum "efficiency" (*) is obtained when all of the chemical energy in the unburnt fuel is converted to kinetic energy in the burnt fuel expelled out the back. You cannot throttle back the exhaust velocity, nor would you want to. In a liquid-fueled rocket you can throttle back on the mass flow rate by reducing the rate at which fuel is introduced to the combustion chamber. Ideally, this leaves the exhaust velocity unchanged.

    The exhaust stream is typically supersonic. That means that any "push" from the atmosphere behind the nozzle cannot propagate upstream in the flow to affect the rocket. Being in atmosphere does not help. Atmospheric pressure in front of the nose cone still has full effect. Being in atmosphere hurts.

    (*) There is more than one way to measure "efficiency".
     
    Last edited: Jun 20, 2017
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