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Rockets and inverted pendulums

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  1. Jan 31, 2016 #1
    Hi, I can´t understand how a rocket is not flipped over by the thrusters (especially without finns). I know this is a general misconception of how a rocket works, but it seams intuitive that it would behave like a inverted pendulum, since the force works below the center of mass, of the rocket.

    Does someone have a dumbed down explanation?
     
  2. jcsd
  3. Jan 31, 2016 #2

    Hesch

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    The rocket motors can be turned some degrees. Hence the rocket can be balanced, giving the lower end of the rocket some horizontal force.
     
  4. Jan 31, 2016 #3

    Hesch

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    Have a look at this video:



    After about 1:42 min., you can see the motors turn slightly.
     
  5. Jan 31, 2016 #4
    Ok, thanks for the response.
    So the stability is caused på some kind regulator, f. instance a PID? Meaning a fixed motor will be un-stable?
     
  6. Jan 31, 2016 #5

    Hesch

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    I don't think the term "PID" ( first order filter ) can be used. I think that a system of higher order filters are used for the purpose.

    Yes, with a fixed motor, the rocket will be unstable.
     
  7. Jan 31, 2016 #6
    oh i see, thank you.
     
  8. Jan 31, 2016 #7
    But at sea level, a rocket may be stabilized with finns on the body? (Fixed motor)
     
  9. Jan 31, 2016 #8

    Nidum

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    Several types of aircraft have been built with rocket engines - mostly for research purposes . These had control surfaces .

    One of the most successful was the X15 .



    https://en.wikipedia.org/wiki/North_American_X-15
     
  10. Jan 31, 2016 #9

    Hesch

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    A rocket, that is launched at sea level, has no speed. Thus wings/finns will not work. But a missile flying horizontally at sea level at high speed may be stabilized by means of wings/finns.

    New year rockets are stable with a "fixed motor", because the motor is placed at the top og the rocket. ( Front wheel drive ).
     
  11. Jan 31, 2016 #10
    I don´t follow when you say rockets at sea level has no speed. What do you mean ?
    And I believed that the center of pressure is further to the ground than the center of mass, in new years rocket.

    I saw an old picture of a ¨Front wheel´´ driven rocket, but it was highly unstable, and crashed shortly after launch. Should think a rocket like that, would be stable.

    http://history.info/wp-content/uploads/2015/03/Modern-rocket.jpg
     
  12. Jan 31, 2016 #11

    Hesch

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    Well, I assume that the rocket is not launched submerged, and if the astronauts are to be accelerated by say 5g, it will take some time before the rocket has achieved a speed whereby it can be controlled by wings/finns.

    I believed that the center of pressure is further to the ground than the center of mass, in new years rocket.

    Well, it has this rod at its tail. Try to remove/shorten the rod next new year, and test the result.
    ( Keep distance )
    .
     
    Last edited: Jan 31, 2016
  13. Jan 31, 2016 #12
    Oh yeah. I did not think of the rod. Sounds interesting to try tough (remive the rod)
     
  14. Jan 31, 2016 #13

    Hesch

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    It is not interesting, it is dangerous.

    The rocket will be completely out of control.
     
  15. Jan 31, 2016 #14
    Yeah, obviously :p
     
  16. Jan 31, 2016 #15

    anorlunda

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    All the more reason to applaud the early rocket pioneers who made rockets work without the benefit of today's control systems. And don't forget the Chineese who were the first (I think) to make practical skyrocket fireworks. It makes on think again about the role of the stick in a bottle rocket.
     
  17. Feb 1, 2016 #16

    jbriggs444

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    Although it is attractive to view a rocket standing on fixed rear-mounted engines as an inverted pendulum and a rocket hanging from fixed front-mounted engines as a normal pendulum, both views are incorrect. Let us neglect the influence of air resistance for the moment...

    In an inverted pendulum, the unbalancing torque increases as the tilt angle increases. In a rocket with rear-mounted motors, the unbalancing torque does not increase. As the rocket tilts, the motors tilt along with it. Their relative angle remains fixed and the unbalanced torque remains unchanged. The result is a fixed angular acceleration for the rocket.

    In a normal pendulum, the restoring torque increases as the tilt angle increases. In a rocket with front-mounted motors, the restoring torque does not increase. As the rocket tilts, the motors tilt along with it. Their relative angle remains fixed and the unbalanced torque remains unchanged. The result is a fixed angular acceleration for the rocket.
     
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