Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Jet propulsion in space?

  1. Jan 26, 2006 #1
    Jet propulsion in space???

    If space is "nothing" then how does rocket propulsion used by space shuttles work? What exactly do those blasts push off of?
     
  2. jcsd
  3. Jan 26, 2006 #2

    I'm not sure what you mean.

    What do the blasts "push off of" in an atmosphere? Nothing. When you burn propellant, you get an explosive reaction, the energy from which is directed out the nozzle. By conservation of momentum , if you have hot gas propelled backwards, something must be propelled forwards to conserve momentum. That something, is the rocket/shuttle.
     
  4. Jan 26, 2006 #3
    So you are saying that the rocket/shuttle is pushing off itself? I mean if you cant "swim" in space, then why can a rocket move in it? Can a person somehow push off himself to move in space?
     
  5. Jan 26, 2006 #4

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Action and reaction are equal and opposite.

    A lot of mass is ejected at high speed from the rocket engine; the force necessary to accelerate that mass reacts on the engine, pushing it 'forwards'. Push your brother and see what happens to you, - that is before he thumps you!

    Garth
     
  6. Jan 26, 2006 #5

    Pengwuino

    User Avatar
    Gold Member

    You can't swim in space becuase you're not pushing anything away from you. You can swim on Earth because you are pushing off of something but pushing off of something is not the only way to move. In fact, the water you push away is the same thing as the exaust on a rocket! You're just pushing something away and the equal and opposite reaction pushes you off.
     
  7. Jan 26, 2006 #6

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    If you were in space and you threw an object, the object would move, and so would you. That's how a rocket works except it expels gas rather than throwing an object.
     
  8. Jan 26, 2006 #7

    Astronuc

    User Avatar

    Staff: Mentor

    Adding to what Garth and others mentioned, a rocket is not swimming in space. The exhaust moves in one direction, and the rocket moves in the opposite direction. It is conservation of momentum.

    It would be analogous to some on skates throwing/pushing a mass, e.g. 10-20 kg. The skater would roll in the opposite direction. So in effect, the rocket is pushing against the propellant exhaust.

    Space in not quite empty, there are still a few particles per cubic cm. One could deploy a solar sail and capture the light and particles from the sun. Again the momentum of the light/particles would be transfered to the craft.
     
  9. Jan 27, 2006 #8
    Right. Another way to put it is that a rocket isn't a moving system at all, but rather it's an expanding system... a controlled explosion (if you will) of energy and mass. The center of mass (including the exhaust) of a rocket essentially remains at the starting point.
     
  10. Jan 29, 2006 #9
    My understanding was on earth a jet works by pushing against the atmoshere, in space i thought rockets worked mainly by directly pushing against the exhaust gases previously expelled. you learn something new everyday:)
     
    Last edited: Jan 29, 2006
  11. Jan 29, 2006 #10

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Hopefully your (mis)understanding has now been corrected. Rockets are propelled by ejecting exhaust gases, in accordance with the conservation of momentum.

    - Warren
     
  12. Jan 29, 2006 #11

    russ_watters

    User Avatar

    Staff: Mentor

    ...and a jet engine need-not push against the atmosphere either. You could send a jet engine into space, pipe-in some air to the inlet, and it'd work just fine.
     
  13. Jan 30, 2006 #12
    To get a good idea of the principle on which rockets work (newton's 3rd law btw) sit in a swivel chair and chuck a heavy object across the room (preferably not something prescious :tongue: )
     
  14. Jan 31, 2006 #13
    So I could push off a mass in space and I would move. If I pushed off a mass a lot smaller than me, would I still move? If I tied a rope to an object, pushed off it and pulled the rope, would it come to me or would I it?
     
  15. Jan 31, 2006 #14

    russ_watters

    User Avatar

    Staff: Mentor

    If you push off an object smaller, you'd still move - just not as fast as the object you push off of (which is why rocket exhaust gasses move very fast). If you tied a rope to it, and pulled-back, you'd meet it where you started.
     
  16. Jan 31, 2006 #15

    Astronuc

    User Avatar

    Staff: Mentor

    The air in the atmosphere provides the oxidant for the fuel. The thrust is still provided by the expanding hot gas blasting out the back, as well as the thrust provided by the fan/compressor. The thrust is not due to pushing on/against the atmosphere.
     
  17. Jan 31, 2006 #16
    Thankyou to everyone

    Astronuc last question

    The atmosphere must create some form of resistance against the hot gases coming out of the jet engine. and i would have thought any form of resistance against the flow of gases coming out of the engine would result in the plane being pushed if you get what i mean.
     
  18. Jan 31, 2006 #17

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In terms of a rocket engine operating in the atmosphere lowers the efficiency of the engine.
    And in this I don't just mean due to the friction of moving through the air, but that the actual thrust produced by the engine will decrease.
     
    Last edited: Jan 31, 2006
  19. Feb 1, 2006 #18

    russ_watters

    User Avatar

    Staff: Mentor

    ....because, as said before, the thrust comes from the action-reaction of the acceleration/velocity of the exhaust gases. Increasing the pressure behind the engine decreases the amount those gases can accelerate when coming out of the nozzle.
     
  20. Feb 1, 2006 #19
    Upon reading all the replies in this thread, I've come to a new understaing of "rocket science". (Thanks everyone) And like always, the new understanding leads me to new questions.

    So if something expands, it does so by "pushing" away from a center point. Say a shuttle wanted to travel forward. I gives off a burst of thrust to get it going. The shuttle has [x] amount of mass. Would the rocket also have to eject [x] amount of mass to move?
     
  21. Feb 1, 2006 #20

    russ_watters

    User Avatar

    Staff: Mentor

    No - it's conservation of momentum. If you stand on a skateboard and push against another person, half your weight and also standing on a skateboard, they will move at twice the speed you do.

    m1*v1=m2*v2
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Jet propulsion in space?
  1. Anterior Propulsion (Replies: 1)

  2. Superluminal jets (Replies: 1)

  3. Superluminal jets (Replies: 2)

  4. Outer space propulsion (Replies: 13)

Loading...