"Bouncing" off on re-entry?

[magnemoe]

Note that you do not need to skip on the atmosphere, its pretty easy to get this effect in KSP even if you have no lift. 
Its also very useful if you plan for it as it let you do multiple passes aerobrake. 
If your Pe is too high you just end up lowering your Ap you are not deorbiting in one pass. 

[magnemoe]

16 hours ago, mikegarrison said:

I don't know why that wouldn't be "skipping off". What's the definition of "skipping off" anyway?

Yeah, there's really not much difference in velocity here. It wouldn't be all that hard to add enough velocity when coming back from the moon to put yourself in a situation where you could in fact end up going fast enough to bounce out into solar orbit.

How can you raise Ap with aerodynamic? yes you can do plane changes but don't see any way to raise Ap. 
If you know how to do it NASA and others would be very interested. 

Granted if they did not aerobrake at all like an engine failed at end of return burn from moon leaving them with an Pe of 400 km, they would encounter moon an second time some months later and would likely be kicked out into solar orbit like the early saturn 5 3rd stages. One of them came back into earth orbit for some months. 

[mikegarrison]

7 minutes ago, magnemoe said:

How can you raise Ap with aerodynamic?

When you "skip" a rock in the water, you don't raise the rock's total energy. Nevertheless, the rock does skip.

The comment in question postulated a ship that already had enough energy to escape from Earth (an amount that isn't all that much more than what it takes to get to Lunar orbit). In that circumstance, if you hit the atmosphere but don't scrub off enough energy, you could end up exiting to solar orbit. The poster then said, "but this isn't skipping". And I wondered, if this isn't skipping, what is skipping?

[Terwin]

2 minutes ago, mikegarrison said:

When you "skip" a rock in the water, you don't raise the rock's total energy. Nevertheless, the rock does skip.

The comment in question postulated a ship that already had enough energy to escape from Earth (an amount that isn't all that much more than what it takes to get to Lunar orbit). In that circumstance, if you hit the atmosphere but don't scrub off enough energy, you could end up exiting to solar orbit. The poster then said, "but this isn't skipping". And I wondered, if this isn't skipping, what is skipping?

To me 'skipping' sounds like aerodynamically raising your periapsis with minimal loss of energy so that it gets a lot harder to reenter in a timely fashion.

No need to go out to solar orbit if your periapsis is high enough that you will run out of life support before you reenter.

[mikegarrison]

1 hour ago, Terwin said:

To me 'skipping' sounds like aerodynamically raising your periapsis with minimal loss of energy so that it gets a lot harder to reenter in a timely fashion.

If that's the definition of skipping (and I disagree that it is), it can never be possible. It is literally impossible to raise your periapsis when you are at your periapsis.

[wumpus]

1 hour ago, magnemoe said:

How can you raise Ap with aerodynamic? yes you can do plane changes but don't see any way to raise Ap. 
If you know how to do it NASA and others would be very interested. 

Granted if they did not aerobrake at all like an engine failed at end of return burn from moon leaving them with an Pe of 400 km, they would encounter moon an second time some months later and would likely be kicked out into solar orbit like the early saturn 5 3rd stages. One of them came back into earth orbit for some months. 

The idea is that "skipping off" will avoid flying down to Pe (and slowing the spacecraft enough for re-entry).  Raising Ap is impossible, but it is entirely possible that you can gain enough velocity escaping the Moon to have a hyperbolic trajectory (Ap outside of Earth's "sphere of influence") and still having that after the "skip".  I wouldn't expect to have such velocity, but if you don't have the life support for a second, lower orbit (that will only go as low as the original "skip") than it doesn't really matter what your final trajectory is.

- pedantic note: this assumes that the spacecraft will enter Earth's atmosphere near Pe.  If you somehow managed to concoct an orbit where the ellipse is sufficiently narrow to enter it on the sides, it just might be possible to transfer some velocity in a way that increases Ap (at the cost of other velocities).  I can't think of any offhand.

[magnemoe]

45 minutes ago, mikegarrison said:

When you "skip" a rock in the water, you don't raise the rock's total energy. Nevertheless, the rock does skip.

The comment in question postulated a ship that already had enough energy to escape from Earth (an amount that isn't all that much more than what it takes to get to Lunar orbit). In that circumstance, if you hit the atmosphere but don't scrub off enough energy, you could end up exiting to solar orbit. The poster then said, "but this isn't skipping". And I wondered, if this isn't skipping, what is skipping?

An rock skipping is the same as an ball bouncing after hitting the ground, next trajectory don't go as high, you loose energy. 
Not saying its impossible, you can trade climb for speed and it might be theoretical possible in something like an space shuttle returning from the moon. Not for an capsule with the aerodynamic of an short cone with an rounded bottom. 

40 minutes ago, wumpus said:

The idea is that "skipping off" will avoid flying down to Pe (and slowing the spacecraft enough for re-entry).  Raising Ap is impossible, but it is entirely possible that you can gain enough velocity escaping the Moon to have a hyperbolic trajectory (Ap outside of Earth's "sphere of influence") and still having that after the "skip".  I wouldn't expect to have such velocity, but if you don't have the life support for a second, lower orbit (that will only go as low as the original "skip") than it doesn't really matter what your final trajectory is.

- pedantic note: this assumes that the spacecraft will enter Earth's atmosphere near Pe.  If you somehow managed to concoct an orbit where the ellipse is sufficiently narrow to enter it on the sides, it just might be possible to transfer some velocity in a way that increases Ap (at the cost of other velocities).  I can't think of any offhand.

You can do an high power return burns. In KSP adding 100 m/s to you budget get you from Minmus to LKO in less than 4 days. Works the other way to but then you also need to factor in the higer burn to get into orbit. 
As I understand later Apollo missions did more energetic burn to give more time on the moon. 
Apollo 11 played it very safe. 

[kerbiloid]

39 minutes ago, magnemoe said:

An rock skipping is the same as an ball bouncing after hitting the ground, next trajectory don't go as high, you loose energy.

If the rock orbit is originally hyperbolic, it can stay hyperbolic, not become.

[Terwin]

18 hours ago, mikegarrison said:

If that's the definition of skipping (and I disagree that it is), it can never be possible. It is literally impossible to raise your periapsis when you are at your periapsis.

Think of it as having a Pe of 20km in kerbal, but before hitting the atmosphere you angle up aerodynamically so that you do not go lower than 50km(at the cost of reducing your Ap).  As you never got to the thicker atmosphere, you did not slow down as much as planned and will end up coming around for another pass(but this time with a Pe of ~50km, possibly needing yet another pass)

If your Previous Ap was near Minmus, your current Ap may still be outside of the Mun, and if you ejected all of your life support before reentry, you may not have enough for that unplanned additional orbit.

And if you were on a return from Duna, you may not even stay in the Kerbin SOI.

[Guest]

On 12/16/2019 at 10:03 PM, mikegarrison said:

If that's the definition of skipping (and I disagree that it is), it can never be possible. It is literally impossible to raise your periapsis when you are at your periapsis.

Except you aren't at Pe. You're closer to it than to AP, but you still have a good bit of a way to go. 

It's much easier to understand when you realize Apollo was designed by people whose intuition came from aircraft. If you think in aircraft terms, skipping is simple: you use capsule's lift to alter your vertical velocity, and "skipping" is when your altitude starts to briefly increase. Basically, you move your Pe so that it's behind you, in a way similar to doing a radial burn. It is not the optimal way to think about how spaceflight works, but that's what the entire US space program did (Soviets were better in that regard). 

[Zeiss Ikon]

On 12/16/2019 at 11:15 AM, Dragon01 said:

If CO2 scrubbers lasted, they could perhaps pull through, though they'd be extremely dehydrated.

Even if the working CO2 scrubbers (which require power) hadn't been jettisoned with the LEM, there wasn't enough oxygen storage in the CM to last another several days if reentry went too shallow and they wound up back out in orbit.  Not to mention insufficient battery power to prevent hypothermia, keep the scrubbers warm enough to operate, etc.  If the CM had in fact been too shallow and wound up leaving the atmosphere, even a few hours after dumping both the LEM and SM would have meant a dead crew -- or just possibly (if the apogee was low enough and the period short enough and the crew drew straws or had two volunteers) a single hypothermic survivor who was no longer functional enough to fly the reentry a second time, even if it was possible to do so in a CM with dead batteries (unlikely in the extreme; I recall the attitude system being electrically controlled).

No question, if they'd failed to fully deorbit and gone back out, they'd have died, just more slowly than if they'd gone too deep and got cooked (or died from G overload with a prolonged 10G from being too low, too fast).

[Guest]

Quite frankly, over-G is probably the least lethal thing that could happen there. Prolonged 10G would most likely not be fatal, humans can take quite a lot of that "eyeballs-in" (which is how they were oriented with regards to it). A 10G reentry would have been much shorter than a nominal one. At worst, something in the capsule itself could fail.

[Rover 6428]

In KSP, you are most likely not reentering correctly. Apollo capsules, and all other spacecraft for the matter, actually dont just slam into the atmosphere, but come at an angle. This makes it so there are not 1 but 2 types of reentry: ballistic and aerodynamic. I guess that since Apollo would be coming at such a high speed, that the former type would be out of question (due to G's) and the aerodynamic one would decrease the ap a lot slower than your KSP simulation. As a result, their AP would still be very high and it is not that fun to almost go back to the moon again with no air.

Here is a video that explains the reentry types: