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-   -   How to get an obect to stop rising? (http://www.physicsforums.com/showthread.php?t=581535)

jehan60188 Feb26-12 11:10 AM

how to get an obect to stop rising?
 
once a rocket is launched vertically, how does one stop it's rising, and start it into an orbit around earth?
actually, the only thing I can really fathom is launching a rocket straight up, and having it fall back to earth, or having it leave earth's gravity, and drift off into whatever trajectory the universe selects for it
so, how about getting it to the moon, or steering a probe to jupiter?
thanks!

DaveC426913 Feb26-12 11:18 AM

Re: how to get an obect to stop rising?
 
Quote:

Quote by jehan60188 (Post 3785315)
once a rocket is launched vertically, how does one stop it's rising, and start it into an orbit around earth?

It takes very little of a rocket's energy to get it from 0 alt to 100 miles. The vast majority of the energy is put, not into altitude, but into horizontal velocity. A satellite needs to be moving at 16,000 mph to achieve and remain in orbit.

When you watch rockets lift off, you will see them go straight up only for a very short period, they almost immediately tilt and gain speed over ground. That's the key to orbit.

jehan60188 Feb26-12 11:24 AM

Re: how to get an obect to stop rising?
 
how does that happen? thrusters? fins?

once an object is in orbit, it's my understanding that it takes very little energy to keep it there (since there's negligible friction). is that an inaccurate statement?

what about getting to the moon or jupiter? is it just another degree of tilting?

russ_watters Feb26-12 02:46 PM

Re: how to get an obect to stop rising?
 
Quote:

Quote by jehan60188 (Post 3785333)
how does that happen? thrusters? fins?

Thrust vectoring nozzles: The nozzles move.
Quote:

once an object is in orbit, it's my understanding that it takes very little energy to keep it there (since there's negligible friction). is that an inaccurate statement?
It is accurate.
Quote:

what about getting to the moon or jupiter? is it just another degree of tilting?
And additional speed, yes.

DaveC426913 Feb26-12 02:54 PM

Re: how to get an obect to stop rising?
 
Quote:

Quote by jehan60188 (Post 3785333)
how does that happen? thrusters? fins?

Yes and/or yes. Fins rapidly become useless so usually it's a matter of angling the main propulsion.

Quote:

Quote by jehan60188 (Post 3785333)
once an object is in orbit, it's my understanding that it takes very little energy to keep it there (since there's negligible friction). is that an inaccurate statement?

It is an accurate statement. Except for the occasional compensatory burn for orbital decay due to atmo friction, it takes zero energy to keep in orbit.

Quote:

Quote by jehan60188 (Post 3785333)

what about getting to the moon or jupiter? is it just another degree of tilting?

An interplanetary journey is about firing up your engines to achieve escape velocity - your orbit spirals outward. You must time your burns and do corrections so that you hit your next target, which might be a gravity-assist from another planet. Depending on whether you want to optimize time or fuel, you choose different journeys. Some journeys can visit more than one planet, or even the same planet on several successive passes before ending upon a final path to your destination.


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