attaining orbit

Discussion in 'Physics & Math' started by bestofthebest, Nov 29, 2010.

  1. bestofthebest Registered Senior Member

    Messages:
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    according to the equations of a stable orbit the velocity required to maintain orbit at a distance r from earth is v = SQRT(GM/r)

    but lets say you are already in a stable orbit and you want to acheive a higher orbit, common sense says you should increase your velocity but according to this equation, if you doubled your distance from earth you would need to be travelling at a velocity about 1.5 times SLOWER to attain orbit, so surely, to attain a higher orbit, you would first have to increase speed to break orbit and then deccelerate to a speed less than you started with... but is this really what the space shuttle does to achieve an orbit??? i know it has retroburners but are these really enough to do what i just described? or am i completely off with what actually happens?

    i cant find any information on the motion of a rocket AFTER the launch...
     
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  3. Janus58 Valued Senior Member

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    Moving to a higher orbit is a two step process.

    First you fire your engines to increase velocity. This puts you in an elliptical orbit with your initial orbital distance being the perigee and your new orbital distance at apogee. If you do nothing else, you will stay in this elliptical orbit moving between the two distances every orbit.

    While you are climbing from perigee to apogee you are gaining gravitational potential. To compensate, you lose kinetic energy and speed. By the time you reach apogee, you will be moving slower than you need to in order to maintain orbit at that distance (this is why you start to fall back to perigee). So you fire your engines again to accelerate up to circular orbital speed in order to insert yourself into the higher circular orbit.
     
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  5. bestofthebest Registered Senior Member

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    ah that makes sense, thanks
     
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  7. James R Just this guy, you know? Staff Member

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    This all goes to show that flying a spaceship in orbit is a counter-intuitive process. The effects of firing your engines in a particular direction can be quite different from what you'd expect if you didn't understand the physics.
     
  8. John Connellan Valued Senior Member

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    3,636
    So you fire your engine twice. Aren't you still increasing your velocity in the end so that you are travelling faster than the lower orbit? How does this make sense with the equation provided by OP?
     
  9. cosmictraveler Be kind to yourself always. Valued Senior Member

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    Or you can use this methold...

    Railgun: As Future Space Vehicle

    While exotic propulsion technologies viz. traversable wormholes, kroniskov tubes,macroscopic casimir effects etc are quite pessimistic, railguns may still be paramount for future commercialization of space. As NASA studies possibilities for the next launcher to the stars, a team of engineers from Kennedy Space Center and several other field centers are looking for a system that turns a host of existing cutting-edge technologies into the next giant leap spaceward. An early proposal has emerged that calls for a wedge-shaped aircraft with scramjets to be launched horizontally on an electrified track or gas-powered sled. The aircraft would fly up to Mach 10, using the scramjets and wings to lift it to the upper reaches of the atmosphere where a small payload canister or capsule similar to a rocket’s second stage would fire off the back of the aircraft and into orbit.

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    http://weirdsciences.net/2010/09/20/railgun-as-future-space-vehicle/
     
  10. Janus58 Valued Senior Member

    Messages:
    2,394
    No, As I explained in my post, as you climb from the lower orbit to the higher orbit distance you lose speed.

    For example, the circular orbital speed for an orbit with a 300 km altitude is 7.657 km/sec. To move to a 600 km altitude orbit, you first have to accelerate to 7.827 km/sec. You now enter a new trajectory that reaches its peak at 600 km. When you get to 600 km, you will have lost velocity climbing against gravity and will be moving at 7.491 km/sec. However, the circular orbital velocity at 600 km is 7.574 km/sec, so to remain at 600 km, you have to add more velocity.
     
  11. John Connellan Valued Senior Member

    Messages:
    3,636
    OK, I was confused about the fact that when it falls from apogee back to perigree, it should convert all that potential energy back to kinetic energy and the average velocity will still be higher. But I think I understand it from your last post.
     

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