The term ‘skipping’ was popularized during the television coverage of the early space program. It was a simple way of describing the complexities already discussed that affected spacecraft re-entry to the television audience. I recall hearing more detailed discussions that indicated ‘burning up’ to the major problem to avoid in re-entry.
This 25-minute NASA video seems to do a good job explaining the whole return and reentry process for the Apollo missions, in a fair amount of depth. I don’t know how rigorous it is, but at least now I can visualize the process beyond “the capsule goes down, then things get really hot.”
Actual reentry stuff starts around 6:40, and an explanation of the aerodynamics involved starts around 10:20. It’s worth noting that you’ll be going a lot faster returning from the Moon than from Low Earth Orbit, 11 km/s versus 7.8 km/s according to Wikipedia, so I’d guess that you’ve got much smaller margins for error in the Apollo missions than elsewhere.
Well, the Sanger was suborbital, so could never have skipped into an eccentric orbit. The problem for the Sanger was drag - it cannot “skip” until air density is sufficient to provide lift, but that point is reached well into the atmosphere, and drag from the less dense air that it is going through will still slow it down. The best the Sanger could do was about half-way round the world (thus the designation of antipodal long-range glider).
Thanks for the video. I didn’t realize quite how significant lift was during the Apollo reentry. I guess I just thought that a blunt cone had terrible lift compared to any sort of proper air foil, and therefore lift was not very significant. My assessment of lift wasn’t terribly off: lift to drag ratio of the Apollo command module is 0.37, vs 10-20 for actual airplanes. But even that little fraction of lift is pretty substantial when drag is 7 g.
Still, it seems to me that a too-shallow reentry doesn’t “skip” the way most people think of it. The linked video explains that the lift vector can be controlled by rolling the command module, so that you could have entirely negative lift if necessary.
If I’m understanding things correctly, then: before reentry, the orbit has an apogee in the vicinity of 400,000 km, and a perigee somewhere below 100 km. After a too-shallow re-entry and subsequent exit of the atmosphere, the new orbit would have apogee << 400,000 km, and perigee < 100 km, so the vehicle would certainly re-enter on the next orbit (or two). But as previous posters have pointed out, that would still be a disaster for the Apollo mission since there were no provisions for another several days in orbit, nor any way to control the second re-entry.
(Disclaimer: most of what I know about orbital mechanics and aerobraking comes from Kerbal Space Program.)
Years ago I used to play around alot with X-Plane about a decade ago. While it wasn’t as pretty as the other flight simulators it was to supposed to have some of the best modeling when it came to actual physics of flight. Some folks even supposed used it to model airplanes they were building.
Anyhow, they had a simulation of the Space Shuttle and the X-15 as well.
Let me tell you, I spent hours and hours trying to get the Shuttle safely through the atmosphere. I think only once did I suceed, but it was close and the kicker was I ended up out over the ocean so even my one hard earned almost success ended in failure.
I burned up most of the time (or probably broke up first due to excessive G forces). But I seem to recall a fair number of times where it sure felt like I was skipping off the atmosphere and after the skip the next reentry was even worse and a sucessful rentry didn’t even seem possible.
I can’t recall for sure if the X-15 did the skipping thing so I’ll just go with a weak I think maybe it did.
No, I see that in the case of any current craft it wouldn’t be possible but my “theoretical” comment was purely that. One could have a craft that did carry additional fuel for such an eventuality but it would have to be designed with that in mind from the outset and…as has been amply described in the thread,it would be horrendously wasteful and still very difficult and dangerous.
It all hammers home the underlying peril of space travel. Screwing up a superficially simple thing like a re-entry angle dooms you. No second chance.