# Challenging the Kármán line from above

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

Nimbus 4, google images

• 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
• @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
• 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
• 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
• @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

## 2 Answers

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.

• This does not address the high l/d premise of the question at all. – Organic Marble Dec 29 '18 at 22:14

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

Source: Russian Foundation for Advanced Studies (FPI) via russianspaceweb

• It is more than obvious that gliders can return from space; the shuttle did it over 100 times. This doesn't answer the question. – Organic Marble Dec 29 '18 at 22:09
• @OrganicMarble the booster is a glider on the return trip empty of fuel. – Muze Dec 31 '18 at 19:39