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The initial conditions of the thought experiment is (very) LEO / reentry. capsules, space shuttle and other spacecraft can generate lift in upper atmosphere during reentry, in order to reduce deceleration maximum G force loading and maximum heating.

Since capsules and space shuttle are not the best lift producing devices ever made, what would happen if we try to deorbit a hypothetical heat resistant/unbreakable high lift-to-weight ratio (let's say) Nimbus4 glider?

Subsidiaries: At which (maximum) altitude & speed would the variometer tell 0 m/s vertical speed? What would be max temperature reached? Have there ever been real tests of deorbiting high finesse / low wing loaded gliding devices? What would angle of attack be during the whole descent, until subsonic speed? How does Coanda effect work at hypersonic speeds?

Illustrations found here: https://www.quora.com/In-regards-to-atmospheric-reentry-what-exactly-is-a-ballistic-reentry

capsule lift

ballistic vs atmospheric

heating

Nimbus 4, google images

Nimbus 4 glider

Muze
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user721108
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    This falls apart at "hypothetical heat resistant". The hypersonic heating is a huge thing and you need to lose velocity fast for it not to become an insurmountable issue. See related questions: 1 2 – SF. Apr 26 '17 at 10:18
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    @SF. I think OP mean that the glider is made of unoptainium; so heat is not an issue. – Antzi Apr 26 '17 at 10:20
  • @Antzi: Yes. And then the concept is sound, nice, everything works and everyone's happy, sipping champagne from flute glasses while gently gliding into the atmosphere. As soon as we get the shipment of unobtainum. – SF. Apr 26 '17 at 10:46
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    It's an interesting question as a thought experiment; "Ignoring heating for a moment, what might a gradual re-entry trajectory of a high lift-to-weight ratio craft look like?" I'd say Gedankenexperimente are on-topic. It worked for Einstein - nobody poked fun at his sub-light train. Worked well for Schrödinger too! But less well for his cat, though the jury is still out on that. – uhoh Apr 26 '17 at 10:55
  • I'm not sure what your first three images are supposed to illustrate. The fourth is relevant, because at least I had no idea what a Nimbus4 glider is, and would otherwise have had to look it up in order to answer at all; the image provides some relevant context there. Could you edit the question to indicate what the first three illustrations are meant to, well, illustrate? – user Apr 26 '17 at 11:02
  • @uhoh Most trains I am aware of move at sublight speeds, which is probably a major part of the reason why nobody poked fun at Einstein when he suggested the concept. ;-) – user Apr 26 '17 at 11:03
  • @MichaelKjörling I'm referring to the insurmountable hypersonic heating. Einstein just asked about approaching c from below; the image I have in my mind is a train and a flashlight, but that may not have been part of the original thought experiment. Anyway, nobody said "that's stupid, your wheels would melt!" – uhoh Apr 26 '17 at 11:14
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    Both lift and drag are proportional to the same v^2, so as long as you have good speed, you have good lift, even in very thin air, and you can stay high enough that drag won't be slowing you down excessively. But heating is also proportional to v^2 and with a very nasty multiplier factor. So, yes you can glide in very, very thin atmosphere; your lift-to-drag coefficient changes significantly around 1 mach but then remains quite stable at higher speeds; aerodynamics of gliding works about the same at 2 mach and at 20, at 0.1 bar and 0.001 bar, but heating becomes prohibitive. – SF. Apr 26 '17 at 11:23
  • @MichaelKjörling first 3 images show the importance of lifting entry for manned vehicules. – user721108 Apr 26 '17 at 11:45
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    @SF. I like your answer, thanks. If I understand correctly, there is a compromise to find between beeing cooked for a long time at high temperature, and being cooked for a short time at Very high temperature. – user721108 Apr 26 '17 at 11:48
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    @qqjkztd: Yes, especially that with being cooked for a short time, you cook a lot of air and a little of ablator, which you then immediately leave far behind - heat doesn't get to penetrate deep. Cooking slowly, you cook yourself - heat penetrates into the craft. – SF. Apr 26 '17 at 11:54
  • I think this is a good question, I think that the answer will be something along the lines of - with an (theoretically) arbitrarily high lift-to-weight ratio, you could ‘glide’ on the interstellar medium. I’d be interested to see an answer on the given aircraft though! – Jack Jun 23 '18 at 15:23
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    It only goes to Mach 3 but this report gives a lot of practical information about how lift and drag work relate in the supersonic region: https://www.nasa.gov/centers/dryden/pdf/87877main_H-913.pdf It’s about the XB-70 Test program. I’m looking for a later report that goes to much higher Mach number that I’ve got here somewhere... – Bob Jacobsen Jul 02 '18 at 04:50
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    Here’s the X15 data I was looking for: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660010056.pdf. The relation of lift and drag is a bit complicated – Bob Jacobsen Jul 04 '18 at 04:19
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    @SF Indeed it is true that "aerodynamics of gliding works about the same at ... 0.1 bar and 0.001 bar" but somewhere before you get to ~10E-6 bar that all changes: the gas becomes non-collisional, i.e the mean free path of the molecules or atoms is much larger than the scale of the vehicle. Then continuum aerodynamics doesn't work anymore. Notably, the mechanism for generating lift is very different, relying on momentum exchange from molecules or atoms impacting the surface of the vehicle. The usual mechanism of impact-adsorption-reemission at thermal velocities is very inefficient. – Tom Spilker Jul 08 '18 at 01:00
  • @TomSpilker: ...and that border corresponds to around 90km altitude. – SF. Jul 08 '18 at 01:08
  • @BobJacobsen: Past Mach 1.2 or so? Fig. 11, page 40 of your X15 paper; I'd say not at all. – SF. Jul 08 '18 at 01:17
  • @SF. can your comments be made an answer? – Muze Dec 29 '18 at 18:24
  • @Muze: Tom Spiker made a strong point. I'm no longer confident my comments are correct in that context. – SF. Dec 31 '18 at 05:14

4 Answers4

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For a starting point Falcon 9's Fairing is going 8700 kph (Mach 7.7 sea level) and it has no ablative shell. The returned fairings come back water logged but un-scorched. The interesting point about this is that the booster was going 8650 kph, but suffers scorching inspite of the re-entry burn since it does not slow down in the wispy upper atmosphere and Block 5 has protective covers. SpaceX plans to recover the upper stage with a ballute. Presumably without a re-entry burn.

If this happens then you will have a real answer to your question. I'm pretty sure the answer is yes if the craft is light enough and has enough drag/lift then yes it will survive re-entry, like a paper plane.

Elden Crom
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Here is the reentry plan:

Control your altitude during re-entry such that

  1. There is enough air density to generate enough lift to equal 1g at your current speed. At an angle of attack that isn't too close to stall. Earlier in the flight you will not need anywhere near 1g of lift since you are somewhat in orbit, but it will not hurt to have it: just lower your angle of attack to lower your lift.

  2. There isn't enough air to get 2g of lift. This ensures aerodynamic forces don't grow too high.

Here are the issues:

  1. Supersonic handling. The glider as-is wouldn't be controllable at supersonic speeds without delta-wings or swing-wings, etc.

  2. Hypersonic lift:drag ratio is around 4 unlike a sailplane which can reach 50. At optimum L/D ratio the lift coefficient is very low but at lower L/D ratio (of around 2) we get a lift of around 0.3. Subsonic airfoils can have a lift coefficient of around 1.0 at a safe margin from stall. This would mean 3x the force of a slow-gliding sailplane (not too extreme), unless your delta-wing is 3x fatter. In which case the wind-force would be similar to the subsonic case. You have 0.5g or so of drag-deceleration thus less sight-seeing time but that means a peak g-force of about 1.1g. No big deal.

Even with these two issues you can could stick your (pressure-gloved) hand into the airflow all the way through reentry and the force is no worse than a car on a freeway. Although it would feel different since it is a hypersonic flow at very low air density instead of subsonic at high density. You could see the shock-wave glowing and enveloping your hand. It would look pretty.

  1. Heat! Heat! Heat! The force of the wind is ~ρv^2 and is kept just below hurricane force. But heat is ~ρv^3. When v is mach 20 you can't have enough wind to generate the needed lift without getting incinerated. Your hull is made out of muileh which is a material that cannot melt no matter how hot it gets. But the payload and people aren't, so you have the issue of thermal soak.

Ablative heat-shields saturate in terms of heat flux: Above a critical temperature the heat-shield starts to sublimate and the gas created prevents further heat flux to the shield. This means time is more important than temperature and you want to go in quick.

The shuttle has a non-ablative system and indeed reenters at a lower force. But there is still some advantage in not drawing reentry out too much (time still "wins" against temperature) even if there isn't as much of a saturation effect.

With this plan in mind:

At which (maximum) altitude & speed would the variometer tell 0 m/s vertical speed?

No limit here. You could (for a short time) maintain 0 m/s speed at any altitude/speed on this reentry.

What would be max temperature reached?

Probably around 1500C as that is the what the space-shuttle's relatively gentle reentry is. But for an even longer time than the space-shuttle.

Have there ever been real tests of deorbiting high finesse / low wing loaded gliding devices?

Not that I am aware of. Supersonic wings aren't "high finesse" (low chord). Keeping tensile strength high anywhere near 1500C is a real challenge and a long wing exposed to the airflow would be hard to heat-shield. As is protecting the avionics.

What would angle of attack be during the whole descent, until subsonic speed?

Near the zero-lift point in orbit (for my reentry plan), to highest in hypersonic but much below orbital speed and then lower again for the subsonic portion.

How does Coanda effect work at hypersonic speeds?

I am guessing it would be reversed. At low speed airflow tries to follow the curve of the ball. Air flowing over the top will end up getting deflected downward (lifting the ball upward) when it passes the upper-backside of the ball. Visa-versa for air passing the bottom of the ball. The Coandă effect uses skin-friction: a pitched baseball with backspin will slow down air on the lower side and speed up air on the upper side. This asymmetry generates a net upward lift. At supersonic speeds the air doesn't have time to follow the curve of the ball. It slams into the front of the ball as a shock-wave and is flung outward at great speed. It will eventually be pushed back inward and fill the void left by the ball but (enough above mach 1) this is too far downstream to affect the ball. A back-spinning ball will tend to push the air ramming into the front of it up due to skin friction and thus push itself down which is opposite to the subsonic case.

Kevin Kostlan
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Can't be done. The Karman line is defined as the point where you can't maintain enough lift without having orbital speed and thus aerodynamic flight is impossible.

Loren Pechtel
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If you could modify the glider's wings to fold to enter the atmosphere like an arrow then expand the wings a bit to gradually shed speed while creating lift then yes you could use a glider to enter from orbit to meet the Kármán line, glide and then land safely. The glider would still need a heat shield, modification and weigh more. As it is the wings would rip off to just to start.

This booster rocket kind of does this:

Baikal flyback booster with second stage
The flyback wing is stowed above and parallel to the fuselage
Baikal flyback booster with second stage
Source: Russian Foundation for Advanced Studies (FPI) via russianspaceweb

Muze
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