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Expelled rocket fuel pushing against something the rocket is attached to?

  1. Oct 6, 2011 #1
    Sorry the title sucks. It's hard to be descriptive about this in a title. I did a cursory check of the other topics and I didn't see this and assumed I probably wouldn't.

    I get very most basic idea behind rockets, every action with an equal and opposite reaction and all that jazz. Something that I thought of today that kind of embarrasses me to not be able to figure out.

    If you had something that looked a bit like a satellite dish, but in place of the feed antenna there was a rocket motor with propellent exhaust facing into the dish. This entire setup is in the vacuum and weightlessness of space. Ignore energy lost to the exhaust heating up the dish, I'm just thinking about the motion and kinetic energy imparted. It would seem as though the initial acceleration of the thing would be in the direction that a rocket would normally go for a brief moment before the propellent then made contact with the dish. I guess, in a perfect transfer of energy, the thing would just sit there as the net momentum of these two reactions taking place cancels each other out. But something seems off about that, since the propellent is bounced from the dish out into space to not come back into contact with this machine again. It just seems odd that something would be sitting in space pushing matter away from it and itself not accelerating.

    I'm certain that I've made some error in my thinking one way or the other. Can someone please show me the light?
  2. jcsd
  3. Oct 6, 2011 #2


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    So your rocket looks like (===>, where the "(" is the "dish" the rocket's propellent will fire into? There might be a momentary movement forward, until the propellent strikes that dish but then there would be an equal movement backward with rocket stops firing. If the rockets blast were completly self contained, there would be NO net movement. However, it that dish is large enough that it will direct some of the blast back past the rocket, there will be net movement in reverse (to the left in my ASCII picture above).
  4. Oct 6, 2011 #3
    Yeah, that was my first impression and I guess you've basically confirmed it. I'm still having some trouble in my head with this. I guess imagining it another way would be something like a tank of compressed gas being opened, but the hose is bent at one point so that it's facing the opposite direction, or another way I had thought about it was an astronaut strapped in to a big box of some sort, with the back of it opened up and then throwing a bowling ball (or something) at the wall in front of him so that it bounces out of the back. I dunno. Obviously the model I have in my brain is flashing an error somewhere for some reason.

    I guess if you're having an epic space battle, bounce the projectiles off of the wall first to keep them from propelling you huh?
  5. Oct 6, 2011 #4


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    Similar devices to the are used as "clamshell door" thrust revesers for jet engines. See the picture at the top right of http://en.wikipedia.org/wiki/Thrust_reversal

    The easiest ones to see in operation are on military jets, because there are no cowlings etc to hide the moving parts. Apart from the obvious use when landing, they are often used at low power settings instead of the brakes, to control the aircraft's speed when taxiing.
  6. Oct 7, 2011 #5
    @ thomas ford: actually the rocket will move even if it shoots its exhaust gas right at the dish that is attached to it. Lets say we fire the engine up. Our exhaust gas is initially at rest and we give it an initial momentum, so now it is traveling towards the dish and when it hits the dish and bounces off, the change it momentum is greater when it bounced off the dish that the initial momentum we gave it. so it will move and the forces will not cancel.
  7. Oct 7, 2011 #6
    Your device reminds me of a cartoon I saw decades ago where a sailor had "brilliantly" mounted a fan on his boat deck to blow air at the sail. The boat went nowhere.

    Assuming no losses of momentum (caused by blow-by gases) in your dish configuration, the device should not move once it is up and running. It would be better to 1) lose the dish and 2) think about Newton's Third Law. Your device has no external forces acting on it so there can be no net acceleration.
  8. Oct 7, 2011 #7
    Mythbusters did a "http://www.youtube.com/watch?v=uKXMTzMQWjo"" episode which showed that the sailor would indeed have moved in the direction he intended. The point is that the sail doesn't just stop the fan or rocket exhaust, it directs some of it backward. The reaction is enough to move a boat, as the video demonstrates.
    Last edited by a moderator: Apr 26, 2017
  9. Oct 7, 2011 #8
    Most http://en.wikipedia.org/wiki/VTOL" [Broken] work by redirecting the exhaust downwards, thus thrusting the plane upwards against gravity. The best way to think of it is action/reaction. After the exhaust is directed, deflected, spinned, or whatever you want to do, what is the final direction of the exhaust? That is the action. The reaction is that the aircraft will feel a force in the opposite direction.
    Last edited by a moderator: May 5, 2017
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