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I have spent a lot of hours with Kerbal Space Program recently and I am curious about one thing.

I got into orbit around the Moon and then I was able to get back to an orbit of "Kerbin" (the home planet in this game, similar to Earth) and land successfully.

I did not have much fuel left, therefore I wanted to save as much as possible for anything that might happen during landing. When I was at my Apoapsis (which was as far away as the Moon) from Kerbin I used fuel to get my periapsis to only 50km, while Apoapsis remained the same. The trajectory was very "ellipse-like".

What happened? When I was aerobraking at 50km with more than 3000m/s speed, the apoapsis was decreasing, while periapsis remained almost the same. Then I was catapulted "back into space", but with a shorter trajectory around Kerbin.

I did this multiple times and after some time, my speed at periapsis decreased to 2400m/s and after that, the apoapsis got as low that I stayed in the atmosphere and landed.

The point is - I did not have to use as much fuel to get low orbit, I slowed down a lot by repeating "slow a little with aerobreaking and then go back to space".

I am curious - why this is not used in reality? At least I did not hear about it. I am thinking that probably real materials do not take lightly to "burning and freezing" multiple times...?

PS : This is a copy of an astronomy question This site is more relevant.

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    $\begingroup$ MRO used such maneuvres when inserting into its final orbit around Mars. It performed 445 aerobrakes in 5 months $\endgroup$
    – MadBender
    Feb 11, 2016 at 12:04
  • $\begingroup$ The circularization of your orbit that you observed is also typical of a real reentry. $\endgroup$
    – Rikki-Tikki-Tavi
    Feb 11, 2016 at 12:33
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    $\begingroup$ Install a life support mod. Watch how Kerbals starve/suffocate to death while waiting for the 137 cycles of aerobraking to be over. $\endgroup$
    – Luaan
    Feb 12, 2016 at 12:50
  • $\begingroup$ @Luaan - It didnt take that much time, it took about 10 cycles which was about 10 days long. $\endgroup$
    – libik
    Feb 12, 2016 at 12:56

3 Answers 3

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This is used in reality, although it is not as easy to do in real life as it is in Kerbal Space Program. Such passes dramatically increase the heat that the capsule is forced to deal with, and as a result, it might simply fail. The only time it is done is when orbiting bodies with an atmosphere, specifically Mars and Venus. What is done is that the spacecraft enters a highly elliptical orbit around the body. Repeated passes through the very upper part of the atmosphere will slow down the satellite, dragging the apoapsis down. This is then continued until a short burn will bring it above the atmosphere, circularizing the orbit, and saving a massive amount of fuel. To do this safely takes a long time: the Mars Reconnaissance Orbiter took almost 6 months to complete the aerobraking process.

To a lesser extend this was used in Apollo's re-entry. The re-entry was aimed at such a point where after the lowest part of the atmosphere was reached, it continued to rise, although it slowed down before a complete orbit was accomplished. This allowed for extending the period of time which the speed was dropped, improving the re-entry process.

The limiting factor, which does not apply to KSP, is that the higher atmosphere of bodies isn't well known, and in fact is quite variable over time. If you push aerobraking too far, you've lost the spacecraft, so one has to be cautious. But it can, and it has, been used.

It should be noted that in reality the first spacecraft to use aerobraking for a real mission was Magellan, in 1993. 4 spacecraft have since used it around Mars, and there was a demonstration mission around Earth in 1991.

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    $\begingroup$ And Venus Express demonstrated aerobraking during the end of its mission 2014. (Very overly dramatized music in the videoclip in that link. The PR department at ESA could learn from NASA). I suppose that aerobraking at Venus, Earth and Mars now are pretty mature. $\endgroup$
    – LocalFluff
    Feb 11, 2016 at 13:08
  • $\begingroup$ 6 months to aerobrake is mature. Until we can do it in less than a month, I don't think we can call it mature overall... $\endgroup$
    – PearsonArtPhoto ♦
    Feb 11, 2016 at 13:13
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This works well in reality too, and helps to reduce heat loads.

The reason this is not used is the fact that you must then pass through the Van Allen radiation belts multiple times, where you ideally want the crew to spend as little time as possible due to the high levels of radiation.

Another reason is redundancy. If the heat shield is not able to the whole deceleration in one go, how can you be sure it is safe enough? The final part of the aero-braking from orbit that you must do in one take is close to 8000 m/s, and that is not so much smaller than the 11000 m/s you have when hitting the atmosphere when returning from the Moon.

A less extreme variant of multiple re-entries is skip re-entry, which uses aero-dynamical lift to pass through the atmosphere multiple times to bleed of speed, but also radiate heat in-between.

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  • $\begingroup$ I'd like a heat shield that can slam into the atmosphere in one go just in case I, for whatever reason, and hurtling at the planet head on. $\endgroup$
    – corsiKa
    Feb 11, 2016 at 21:08
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    $\begingroup$ @corsiKa: You can't survive 30 Gs so no. You need to come in edge on. $\endgroup$
    – Joshua
    Feb 11, 2016 at 22:29
  • $\begingroup$ I feel this answer misses an important point. A heat shield that can protect against heating is significantly easier to make than something that can protect against cycles of heating-colling-reheating. $\endgroup$
    – Taemyr
    Feb 12, 2016 at 12:29
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This works just fine. This is not normally used for manned returns to Earth because of the amount of life support equipment required to be in the return module, making it much heavier (and slower return -- as you probably discovered the first elliptical orbit may well have a period of > 1 day). Also, this makes recovery annoying due to the low precision involved.

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