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One of the problems with space flight is the speed required to achieve orbit; 17,500 mph. One of the ways I've always thought to achieve this was to launch at a low speed to a very high altitude then let the rocket fall back to earth, being speed up by gravity until it reaches orbital velocity and begins orbiting.

Question: Are there any programmes for this by the major space agencies, or papers written about the practicality of it?

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    $\begingroup$ I would guess it wouldn't save any energy. $\endgroup$ – gerrit Jul 19 '13 at 10:11
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    $\begingroup$ I am lost here you realize that you dont just automatically start orbiting when you hit a magical number right? you have to have the right trajectory otherwize you just make a large crater. The escape velocity refers to the slowest possible speed where an object can escape the pull of gravity and achieve orbit any thing less and the object will return to earth with a spectacular ending. Most objects in orbit go much faster. $\endgroup$ – Chad Jul 19 '13 at 18:36
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    $\begingroup$ If Kerbal Space Program is anything close to a legitimate simulator, this just wouldn't work ;) $\endgroup$ – JohnB Jul 19 '13 at 19:09
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One of the major show stoppers for slow ascent to orbital heights and beyond is that "slow" by necessity means "longer time", i.e. increases the time needed to achieve this, thus increasing your overall energy output needs (in a nutshell, W = PT). While we could probably be able to achieve this, it's just not economical. It might be practical to combine a bit of both, and release the rocket off an airplane carrying it till the heights airplanes can operate, and maybe require a rocket stage less, but the total energy required will still be greater than doing it as fast as possible.

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  • $\begingroup$ We're using earth's gravity to accelerate the object, so shouldn't that reduce the power requirement? (I understand that as we lift the body up above the earth we're effectively storing kinetic energy, which is released in the fall to earth.) $\endgroup$ – user39 Jul 19 '13 at 11:58
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    $\begingroup$ "We're using earth's gravity to accelerate the object..." Yes, but it's the same gravity that's working against you while you're boosting the thing into orbit. Gravity's not going to give you any energy other than the potential energy you gave it with your rocket engines. No free lunch, sorry. $\endgroup$ – Don Branson Jul 19 '13 at 19:20
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    $\begingroup$ Right, this makes sense; I understand. $\endgroup$ – user39 Jul 19 '13 at 20:50
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    $\begingroup$ @DonBranson: Don't you mean "no free launch"? ;-) $\endgroup$ – Sean May 10 '19 at 3:37
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There are several issues with this approach, but three in particular come to mind:

  • Slow ascents by a rocket (I know you might be considering other modes) are counterproductive due to gravity drag. This is why rockets accelerate as fast as their payloads can withstand. Unfortunately, with humans this isn't very fast.
  • Atmospheric drag. If you were doing this around a planet with no atmosphere, and you were able to avoid the gravity drag issue, you'd be golden. Unfortunately, without an atmosphere, you are probably using rocketry and therefore gravity drag will bite you. Even then, if you could attain slow ascent, why not just keep going...
  • Impact. Your "dive" would need to follow a path that doesn't intersect the planet's surface -- which means you will probably need to ascend a very long ways or find a way to provide significant velocity perpendicular to the line bewteen you and the planet's center of gravity.
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