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#1
Old 02-19-2013, 10:03 PM
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Why Is There An Angle of Re-entry?

Sure, the angle of re-entry seems logical at first. It is spoken about in Apollo 13 and (IIRC) The Right Stuff. At the wrong angle, you'll bounce off the atmosphere like a stone skipping across a lake. OTOH, with all of these rogue asteroids and now meteorites as of late...how come they can enter our atmosphere any old way they please? (Why don't THEY skip like a stone?)
#2
Old 02-19-2013, 10:05 PM
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Lots of them do.
#3
Old 02-19-2013, 10:47 PM
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Right, asteroids aren't particularly interested in gently landing several delicate water balloons at a particular location.

Actually, I never was very clear what was meant by "skipping" when talking about reentry. Is there actually some sort of aerodynamic effect other than ordinary air resistance? Or is "skipping" just a fanciful way of describing an orbit that passes through the atmosphere a little bit?
#4
Old 02-19-2013, 10:57 PM
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Maybe astronauts who re-enter at too steep an angle will land too hard, like that meteor is Russia last week.
#5
Old 02-19-2013, 11:11 PM
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Quote:
Originally Posted by lazybratsche View Post
Right, asteroids aren't particularly interested in gently landing several delicate water balloons at a particular location.

Actually, I never was very clear what was meant by "skipping" when talking about reentry. Is there actually some sort of aerodynamic effect other than ordinary air resistance? Or is "skipping" just a fanciful way of describing an orbit that passes through the atmosphere a little bit?
Sort of. It means that you are going too fast for atmospheric braking to reduce your speed below the level at which Earth's gravity will pull you to the surface.
#6
Old 02-19-2013, 11:33 PM
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The angle of reentry for an object that is hitting the atmosphere at a speed greater than orbital speed, like an Apollo capsule returning from the moon or a passing asteroid, is critical for two reasons. If the angle is too shallow, the object will not slow down enough, will pass through the thin upper atmosphere and then head back out into space. This is what is being referred to, somewhat inaccurately, when they speak of the capsule 'skipping' off the atmosphere. If the capsule or asteroid comes in too steeply, it will decelerate so quickly that the force of deceleration will crush or shatter it. That is exactly what happened to the meteor in Russia - it came in too steeply to survive and shattered into pieces.
#7
Old 02-19-2013, 11:38 PM
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Originally Posted by Senegoid View Post
Maybe astronauts who re-enter at too steep an angle will land too hard, like that meteor is Russia last week.
Actually, the meteor didn't land. It exploded a goodly ways up.
#8
Old 02-19-2013, 11:39 PM
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Meteors will skip if they come in too shallow. I believe this is a video of one doing just that:

https://youtube.com/watch?v=KcsJ6e8S5lA

If not, there have been plenty of documented meteors that did. It's not just astronauts that have this ability.

If an astronaut comes in too steep, they convert their speed into heat via friction at a faster rate. This makes the spacecraft's heat shield get even hotter. The heat shields in man-made spacecraft have their limits and if you exceed these limits they will burn through and fail. This is generally considered to be a very bad thing for astronauts. Most astronauts prefer to aim for somewhere in between "skip off and miss" and "burn up and die". Meteors aren't so picky.
#9
Old 02-19-2013, 11:44 PM
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Something that comes in at a shallow angle will build up compressed air on the side closer to earth, and this air will push back, slightly deflecting the trajectory. This is, in fact, somewhat analogous to what happens when a stone "skips" on the water.
#10
Old 02-20-2013, 03:56 AM
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Originally Posted by Bob X View Post
Something that comes in at a shallow angle will build up compressed air on the side closer to earth, and this air will push back, slightly deflecting the trajectory. This is, in fact, somewhat analogous to what happens when a stone "skips" on the water.
A re-entry trajectory for a returning spacecraft is a finely judged balance between gravity, friction, heat and aerodynamics. The capsule itself is designed as a lifting body, to manage the process of descending through the atmosphere, ensuring that the balance of velocity and air density does not exceed the ability of the capsule to shed frictional (yeah, I know, it isn't strictly friction) heat. Too steep an angle of attack, and there isn't enough lift, the capsule descends too steeply, and hits the denser air travelling too fast. If the capsule cannot cope with the heat, it will burn up or impact the ground. If the angle of attack is too shallow, there is too much lift and not enough slowing down, and the capsule will exit the atmosphere again with no hope of recovery.

This "skip" is unlikely to occur for a rock from space - it may pass through part of the atmosphere (as noted above) due to angle/velocity/size, but the aerodynamic features of the skip are not going to occur (stable flat surface presentation to the atmosphere). However, it was a plot-point in the novel Moonfall by Jack McDevitt, where the moon is hit by an extrasolar impactor. A large chunk of moon is deflected from a major earth impact by using nuclear rocket engines to rotate and present the rock so that it does skip off the atmosphere, then the rockets are used to stabilise the orbit to prevent subsequent risk.
#11
Old 02-20-2013, 06:12 AM
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Question: if the capsule/shuttle etc. "skips" off due to a shallow angle of re-entry, is there any chance of another re-entry attempt?
I'm assuming that the craft is not going fast enough to escape earth orbit completely and so will approach the earth again (though in an altered orbit) So can they burn again and adjust for another entry? Would they not have enough fuel for such jiggery-pokery?

(Please note above my use of the term "burn" which is inserted in order to give the impression that I know what the hell I am talking about, and also the term "jiggery-pokery" which confirms that I don't)
#12
Old 02-20-2013, 06:46 AM
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Quote:
Originally Posted by Bob X View Post
Something that comes in at a shallow angle will build up compressed air on the side closer to earth, and this air will push back, slightly deflecting the trajectory. This is, in fact, somewhat analogous to what happens when a stone "skips" on the water.
Ditto.

This seems to be a point that many of the posters here don't realize, or at least didn't mention. If one doesn't like the word "skip", I'd suggest that a good synonym might be "bounce". Watch what happens when someone gets thrown out of a boat, or a water skier lets go of his line -- he is moving so fast that the water can't get out of his way, and the water acts more like a solid than a liquid, so he literally bounces off until his speed slows to a point where the water can get out of his way.

That is what happens when something hits the atmosphere at too shallow an angle. It is NOT simply that the surface of the atmosphere is curved, and the object comes out the other side. It bounces off of it.
#13
Old 02-20-2013, 06:50 AM
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Originally Posted by Novelty Bobble View Post
Question: if the capsule/shuttle etc. "skips" off due to a shallow angle of re-entry, is there any chance of another re-entry attempt?
I'm assuming that the craft is not going fast enough to escape earth orbit completely and so will approach the earth again (though in an altered orbit) So can they burn again and adjust for another entry? Would they not have enough fuel for such jiggery-pokery?
No. The subsequent orbit would be highly eccentric and may take an extended time to return, and the next approach will be too fast and at the wrong angle. They wont have anywhere enough fuel for the adjustments, needed - they used most of that losing velocity to drop into the atmosphere in the first place.
#14
Old 02-20-2013, 07:01 AM
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There is an intermediate approach, that for re-entry to the Earth might be pretty difficult, but real. With exactly the right approach you can enter a very eccentric orbit, one that grazes the atmosphere, and then with each pass into the atmosphere you can lose a bit of orbital energy, and with some additional control, circularise the orbit, or get to a point where you can safely take the dive down to full re-entry. This isn't the "skip-off" trajectory, which would probably dump you into a solar orbit to quietly die, with no nearby pass to the Earth in eons. Rather the aerobraking entry takes a very measured pass though only the more tenuous atmosphere. It may be the only viable way of getting a manned mission back from Mars.
#15
Old 02-20-2013, 07:40 AM
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Quote:
Originally Posted by si_blakely View Post
No. The subsequent orbit would be highly eccentric and may take an extended time to return, and the next approach will be too fast and at the wrong angle. They wont have anywhere enough fuel for the adjustments, needed - they used most of that losing velocity to drop into the atmosphere in the first place.
An Apollo capsule would have been undocked from its service module by then anyway, and would have had no propulsion unit attached to it. Not that, as you point out, it would have been much comfort to them if they had. Fuel reserves tend to be tight on space missions; when Apollo 11 landed on the Moon it had 25 seconds of fuel remaining.
#16
Old 02-20-2013, 08:12 AM
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Originally Posted by Jinx View Post
OTOH, with all of these rogue asteroids and now meteorites as of late...how come they can enter our atmosphere any old way they please? (Why don't THEY skip like a stone?)
Frankly, I do not understand the question. What do you mean by "any old way they please"? It's not like they make a conscious choice to enter this way or that way.

Rather, each one enters at a particular angle based mostly on where it is coming from, and partially based on other influences like the earth's gravity. But the bottom line is that if the angle is too shallow, it will bounce off, and if it comes in too steep it will burn up or explode or whatever. Only in a rather narrow range will it come in for a not-too-disastrous landing.
#17
Old 02-20-2013, 08:14 AM
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Originally Posted by si_blakely View Post
No. The subsequent orbit would be highly eccentric and may take an extended time to return, and the next approach will be too fast and at the wrong angle. They wont have anywhere enough fuel for the adjustments, needed - they used most of that losing velocity to drop into the atmosphere in the first place.
Thanks for that.

So theoretically they could try again but practically.......nope. I suspected fuel margins would be too tight.

I wonder what the options would be for a lightly-fuelled craft that found itself in such an eccentric orbit?
Not enough fuel to correct for re-entry but perhaps enough to ensure a quick burn-up? Time to break out the cyanide pills? enter hypersleep in hope of landing on a monkey planet?
#18
Old 02-20-2013, 08:47 AM
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Originally Posted by Novelty Bobble View Post
Thanks for that.

So theoretically they could try again but practically.......nope. I suspected fuel margins would be too tight.

I wonder what the options would be for a lightly-fuelled craft that found itself in such an eccentric orbit?
Not enough fuel to correct for re-entry but perhaps enough to ensure a quick burn-up? Time to break out the cyanide pills? enter hypersleep in hope of landing on a monkey planet?
As pointed out above by Malacandra, there was no fuel (or engines, for that matter). Even the shuttle carried limited fuel, and didn't (couldn't) use main engines after launch.

I think the consensus is to open the airlock and breathe space - the air will last a bit longer than the food.
#19
Old 02-20-2013, 12:19 PM
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Relative to the idea of "Skipping" off the atmosphere, is it then not possible for the proposed Nazi Sanger spaceplane to have "skipped" off the atmosphere all the way around the world back to its base?
#20
Old 02-20-2013, 12:49 PM
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I would think no, for the reasons quoted by si_blakely above - namely that this would result in a wild, random orbit after bouncing off and a huge amount of fuel burned at each attempt.
#21
Old 02-20-2013, 12:55 PM
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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.
#22
Old 02-20-2013, 01:37 PM
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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.
#23
Old 02-20-2013, 02:54 PM
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Originally Posted by Mr. Milton View Post
Relative to the idea of "Skipping" off the atmosphere, is it then not possible for the proposed Nazi Sanger spaceplane to have "skipped" off the atmosphere all the way around the world back to its base?
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Originally Posted by Ferret Herder View Post
I would think no, for the reasons quoted by si_blakely above - namely that this would result in a wild, random orbit after bouncing off and a huge amount of fuel burned at each attempt.
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).
#24
Old 02-20-2013, 03:30 PM
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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.)
#25
Old 02-20-2013, 03:39 PM
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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.

Last edited by billfish678; 02-20-2013 at 03:42 PM.
#26
Old 02-20-2013, 03:59 PM
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Originally Posted by si_blakely View Post
As pointed out above by Malacandra, there was no fuel (or engines, for that matter). Even the shuttle carried limited fuel, and didn't (couldn't) use main engines after launch.
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.
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