Notices
Results 1 to 11 of 11

Thread: Thrust

  1. #1 Thrust 
    Forum Junior
    Join Date
    Feb 2008
    Posts
    213
    If a rocket takes off, the thrust pushes against the air and for every action is an equal and oppisite reaction, so it is propelled upward. But once it reaches space, there is nothing for the thrust to push off on, so how would thrust even make a difference when in a vacuum?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Universal Mind John Galt's Avatar
    Join Date
    Jul 2005
    Posts
    14,168
    The exhaust gases do not push against the air. They are expelled. It is the reaction to that action that propels the rocket. In space, in a vacuum, with no air resistance, the rocket is more efficient.


    Edited to correct another of these interminable typographical errors.


    Reply With Quote  
     

  4. #3  
    Forum Junior Zitterbewegung's Avatar
    Join Date
    Dec 2007
    Posts
    217
    A rocket does not push against the air. NEWTON, hellooohoooooo. Repeat after me: every action (firing exhaust gasses with a certain speed out on one side) causes an opposite and equal reaction (spaceship moves in the other direction)
    I love deadlines. I like the whooshing sound they make as they fly by
    Reply With Quote  
     

  5. #4 Re: Thrust 
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,155
    Quote Originally Posted by Raymond K
    If a rocket takes off, the thrust pushes against the air and for every action is an equal and oppisite reaction, so it is propelled upward. But once it reaches space, there is nothing for the thrust to push off on, so how would thrust even make a difference when in a vacuum?
    Try looking at it this way:


    Start with a typical rocket engine, except for the fact that we have plugged up the open end. We ignite the fuel. Hot gasses are produced which attempt to expand. They produce an outward acting force on the sides of the rocket.

    Since the forces act equally in all directions, there is no net movement of the rocket.

    If we continue to burn fuel, we create more gasses and more outward force. Eventually, these forces will exceed the structural strength of the rocket and it will burst (we have created a bomb rather than a rocket.)

    But what if we punch a hole in one end of the rocket? The hot gasses still expanda nd they still exert force on the inside walls of the rocket, except where the hole is; these gases escape the rocket. Now the forces produced by the expanding gasses on the wall opposite the hole are no longer balanced by a force acting in the opposite driection. We have a net force pushing the rocket in the direction opposite that of the hole.

    This works if there is air outside of the rocket or not. In fact, as already stated, having air outside the rocket reduces its effectiveness. To see why, assume that the outside air pressure is equal to that of the inside expanding gas pressure. In this case, the outside pressure pushing in acts just like the plug did in the first situation, it stops the gases from escaping and we have equal forces acting in all directions again.

    The amount of force pushing the rocket forward will be proportional to the difference in the inside pressure and the outside pressure. Zero outside pressure gives us our best efficiency.
    Reply With Quote  
     

  6. #5  
    Forum Junior
    Join Date
    Feb 2008
    Posts
    213
    What does a rocket use for fuel in space? And how can it propel the rocket without oxygen?
    Reply With Quote  
     

  7. #6  
    Forum Ph.D. william's Avatar
    Join Date
    Jun 2006
    Location
    Wherever I go, there I am
    Posts
    935
    Conservation of momentum; mv<sub>backward</sub>=mv<sub>forward</sub>.


    Quote Originally Posted by Raymond K
    What does a rocket use for fuel in space? And how can it propel the rocket without oxygen?
    The spaceship takes its fuel with it.
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
    Reply With Quote  
     

  8. #7  
    Forum Junior
    Join Date
    Feb 2008
    Posts
    213
    Quote Originally Posted by william
    Conservation of momentum; mv<sub>backward</sub>=mv<sub>forward</sub>.


    Quote Originally Posted by Raymond K
    What does a rocket use for fuel in space? And how can it propel the rocket without oxygen?
    The spaceship takes its fuel with it.
    Thanks,
    So they burn the fuel with oxygen stored in the rocket?
    Reply With Quote  
     

  9. #8  
    Forum Ph.D. william's Avatar
    Join Date
    Jun 2006
    Location
    Wherever I go, there I am
    Posts
    935
    Quote Originally Posted by Raymond K
    Quote Originally Posted by william
    Conservation of momentum; mv<sub>backward</sub>=mv<sub>forward</sub>.


    Quote Originally Posted by Raymond K
    What does a rocket use for fuel in space? And how can it propel the rocket without oxygen?
    The spaceship takes its fuel with it.
    Thanks,
    So they burn the fuel with oxygen stored in the rocket?

    I'm not up to speed with what exactly rocket fuel consists of, but everything needed to burn it is brought with the spaceship. It probably wouldn't be hard to find the answers on the internet.

    Cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
    Reply With Quote  
     

  10. #9  
    Moderator Moderator Janus's Avatar
    Join Date
    Jun 2007
    Posts
    2,155
    Quote Originally Posted by Raymond K
    What does a rocket use for fuel in space? And how can it propel the rocket without oxygen?
    Solid rocket fuels contain their own oxygenators(substances that release oxygen to support combustion.

    Otherwise, one of the most common fuels is hydrogen. Hydrogen is stored in one tank and liquid oxygen in another. The two are fed into the combustion chamber in the proper ratio and ignited.
    Reply With Quote  
     

  11. #10  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    The thrust pushes against the Higgs Boson, thats what gives the object movement.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  12. #11  
    Forum Junior
    Join Date
    Feb 2008
    Posts
    213
    thanks
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •