If skipping off the atmosphere during re-entry runs a real risk of of the spacecraft "escaping" from orbit, why don't deep-space missions use this method instead of using power to reach escape velocity?

(I am not a physicist and my knowledge in this field is limited so I apologize if the question was asked wrong or is based on a false premise)

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    $\begingroup$ You can skip stones off the surface of water. Why can't stones under water escape by skipping off the surface, too? $\endgroup$ Dec 20 '15 at 23:09

Skipping off the atmosphere can only lead to an escape if your speed going in was above the escape speed. The skip changes your direction, and it reduces your speed a bit. So if you enter a skip below escape speed, skipping will send you into an elliptical orbit.
So for a deep-space mission there's no advantage to doing a skip. You need to build up velocity some other way.

  • $\begingroup$ Won't the perigee remain in the atmosphere therefor slowing you more and more each orbit ? $\endgroup$
    – Antzi
    Dec 20 '15 at 20:38
  • $\begingroup$ @immibis What do you mean by that? I understood Antzi's question as referring to (for example) a low-earth-orbit spacecraft that hits the atmosphere at a bad angle, causing it to skip. In such a case (according to this answer), the craft would not escape the planet's influence. In such a case (where the craft is not escaping the planet's influence) wouldn't (as Antzi asked) the perigee remain in the atmosphere slowing down the craft with each subsequent orbit? $\endgroup$ Dec 20 '15 at 21:18
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    $\begingroup$ Yes, perigee remains in the upper atmosphere and you will come back down. The reason they were worried about it with Apollo is that you can skip high enough that their life support would run out before they came back down. $\endgroup$ Dec 21 '15 at 2:52
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    $\begingroup$ Your statement "The skip changes your direction" got me to thinking about an exception. Lift will be perpendicular to the velocity direction, with only drag in the velocity direction. So the statement seems true, that lift can only change your direction and reduce your speed. But the drag and lift are with respect to the atmosphere-relative velocity, not the inertial velocity. So if the planet and atmosphere are rotating, you could have a lift vector contribute in the inertial velocity direction. $\endgroup$
    – Mark Adler
    Dec 21 '15 at 4:35
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    $\begingroup$ So you should be able to come up with a case with a nearly-escaped elliptical entry that a high L/D turns into a barely hyperbolic exit. $\endgroup$
    – Mark Adler
    Dec 21 '15 at 4:36

During reentry, spacecraft like Apollo run a risk of skipping due to an excess of lift. (The blunt body shape doesn't generate a lot of lift, but it does generate some at hypersonic velocities.) But lift generates drag: you can't get lift for free, you have to have extra speed to create it. Apollo in particular reentered with a lot of extra velocity, since it was generally returning from a translunar orbit, which turns all the potential energy from a high altitude into velocity.

So yes, you can use lift to get mostly out of the atmosphere. It's not a free lunch, though, and generally the mass of making wings that will give enough lift without falling apart from drag stresses is a serious problem for spacecraft, so there are at present no orbital spaceplanes. Ultimately, too, that's only mostly. A stable orbit has to be quite some distance above any concentration of air thick enough to provide measurable lift, or else it will decay from drag and reenter early.


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