on reentry

[ravener]

this is the wikipedia article on skip reentry, but i have seen this one note before and it doesent seem to make sense.

Skip reentry is a reentry technique involving one or more successive "skips" off the atmosphere to achieve greater entry range or to slow the spacecraft before final entry, which helps to dissipate the huge amount of heat that is usually generated on faster descents. The range modulation made possible by skip entry allows a spacecraft to reach a wider landing area, or to reach a designated landing point from a wider range of possible entry times, which is especially important in abort situations. Like aerocapture, skip reentry requires precise guidance. An overly shallow entry angle will result in the spacecraft retaining too much of its velocity, possibly escaping into space permanently if this is more than escape velocity. An overly steep entry, on the other hand, results in more intense heating and stress that could exceed the design limits of the spacecraft, potentially destroying it.

ok, here it seems to make sense as it puts in an IF, but i have seen this said about shuttle reentry and reentry of soyuz craft as well during documentaries. it just doesent seem to add up as none of those craft has the nessecary energy for escape.

[ravener]

can it be that they dont consult rocket scientists when making documentaries on rockets?

[FanaticalFighter]

I think they mean it as in if you came in at an interplanetary trajectory, and you didn't hit the right angle, you won't lose enough speed to be able to capture into an orbit.

[Nibb31]

Normally, if your reentry is too shallow, you will go through the atmosphere and come out on the other side. If your spacecraft generates lift, then you can actually "skip" and raise altitude.

However, you will not reach escape velocity unless you were already on an escape trajectory. If you are orbiting Earth, you will continue to orbit and necessarily lose some energy during the skip. This will bring you down deeper on your next perigee.

The risk for Apollo wasn't that it would "skip off into outer-space" as the media sometimes said, but that the CM would fly through the atmosphere and go around another orbit (potentially for several days) for which it did not have enough power or supplies to keep the crew alive. It would eventually come back, but as a dead weight.

Edited March 12, 2014 by Nibb31

[DonLorenzo]

Lift or no lift, interaction with the atmosphere is going to decrease your orbit. So if you're coming in from the moon for instance, there's no way you're going to escape out into a solar orbit. What could happen though is that you fail to slow down significantly and zoom all the way out to (nearly) lunar height before coming back to earth again (just keeping your orbit, really). In the real world this might as well be 'lost in space forever' since your life support (or at least, Apollo's life support) wouldn't last long enough to try a second reentry

edit: yeah, what nibb said

[lajoswinkler]

The "skipping" is a frequent disinformation in the media and the films. They even did it wrong in Apollo 13 movie. The capsule doesn't bounce like a flat rock on a surface of a lake and it won't gain speed, that would be a violation of the physical laws of conservation of energy.

Nibb31 explained it correctly.

[K^2]

This actually makes me wonder if it's absolutely impossible under all circumstances. I seem to recall that a particularly fast asteroid can generate significantly more energy on entering atmosphere than it carries in kinetic energy due to atmosphere undergoing fusion. Ok, yes, at this point we aren't talking about a re-entering ship, because lets face it, this is not a survivable scenario. And yet, there is a definite surplus of energy, and not an insignificant one. So the question is, is it possible in principle, for an asteroid to hit atmosphere just right, generate massive explosion in atmosphere, and for some of the fragments to leave at higher velocity than they came in at. And again, I realize that in order to be anywhere close, the thing already has to move at much faster than escape velocity, so this isn't about skipping to escape. Just about gaining velocity via "aerobraking".

[Ravenchant]

This actually makes me wonder if it's absolutely impossible under all circumstances. I seem to recall that a particularly fast asteroid can generate significantly more energy on entering atmosphere than it carries in kinetic energy due to atmosphere undergoing fusion. Ok, yes, at this point we aren't talking about a re-entering ship, because lets face it, this is not a survivable scenario. And yet, there is a definite surplus of energy, and not an insignificant one. So the question is, is it possible in principle, for an asteroid to hit atmosphere just right, generate massive explosion in atmosphere, and for some of the fragments to leave at higher velocity than they came in at. And again, I realize that in order to be anywhere close, the thing already has to move at much faster than escape velocity, so this isn't about skipping to escape. Just about gaining velocity via "aerobraking".

But the explosions would probably be happening on the front end of the asteroid... energy released there, as opposed to behind it, would slow it down further. And if fragments get ejected, they would probably be quickly stopped due to the square/cube law (higher drag per unit of mass).

[Spenzor]

This actually makes me wonder if it's absolutely impossible under all circumstances. I seem to recall that a particularly fast asteroid can generate significantly more energy on entering atmosphere than it carries in kinetic energy due to atmosphere undergoing fusion. Ok, yes, at this point we aren't talking about a re-entering ship, because lets face it, this is not a survivable scenario. And yet, there is a definite surplus of energy, and not an insignificant one. So the question is, is it possible in principle, for an asteroid to hit atmosphere just right, generate massive explosion in atmosphere, and for some of the fragments to leave at higher velocity than they came in at. And again, I realize that in order to be anywhere close, the thing already has to move at much faster than escape velocity, so this isn't about skipping to escape. Just about gaining velocity via "aerobraking".

It might be possible for part of an object to achieve escape following an explosion on reentry, but I seriously doubt it would be possible for the entire object to do so. At the point that it's generating significant shock heating, it's already lost a good chunk of its energy. Might fusion caused by that heating impart energy on the object? Sure, but logically if it's applying a force on the object, that force would be more likely to oppose the object's velocity than aid it.

[Motokid600]

The best 25min you'll probably spend all day. Skip to 6:40 if you like.

Edited March 12, 2014 by Motokid600

[Nibb31]

I suppose if you were coming in from an interplanetary trajectory, you could do a gravity assist and slingshot away from the Earth faster than you came in. This wouldn't apply when reentering from the Moon though.

[Lukaszenko]

I was meaning to ask the same question, as the "skip off the atmosphere like a rock off a lake" scenario didn't sit well with me. Glad to have it confirmed that indeed it shouldn't have.

[magnemoe]

You can easy get the skipping effect in ksp too, return from mun to arund 29 km, you will go up again to 40-50 km before final decent. This is nice with deadly reentry

[Lukaszenko]

Right, I've experienced it as well. But it really has nothing to do with "skipping like a rock". You're simply going through the atmosphere like a hot knife through butter.

[Brethern]

Right, I've experienced it as well. But it really has nothing to do with "skipping like a rock". You're simply going through the atmosphere like a hot knife through butter.

I'm fairly certain it's an analogy for people who don't care to learn to technical side.

[Rickenbacker]

Very interesting video! I've seen some strange reentries from the Mun in KSP, some that took several orbits to finally lose enough energy to drop to the ground, and one that came in at a shallow angle, resulting in almost an entire orbit in the upper atmosphere, before finally slowing down.

[Idobox]

You can easy get the skipping effect in ksp too, return from mun to arund 29 km, you will go up again to 40-50 km before final decent. This is nice with deadly reentry

That's not technically skipping, just an orbit that isn't aerobraked enough to bring you down.

To truly skip, you need aerodynamic lift.

[goldenpeach]

And you can do that in KSP(with mod): just install FAR and do the same thing as the previous video by tilting your capsule.

I suggest you to use the test weight mod(an old mod, but it still work) to move your COM:it will help you tilting the capsule.

[vger]

I don't know where I was when this happened, but as a child, I vaguely remember the 'skipping' being described to me in a discussion about space shuttle re-entries.