Would it be possible to avoid the heat-from-friction problem when re-entering the atmosphere by performing (vertical) "U-turns"?

Question

If you lift a swing at a children's playground and release it will swing back and forth for a while, losing some altitude in each swing, mostly (?) due to drag. Eventually it has lost all energy and stops.

Couldn't that principle be used when a spacecraft is re-entering the atmosphere? That is, go downwards into the atmosphere and then steer upwards - converting kinetic energy to potential energy but also lose some energy to friction - when the speed/friction-heat is too high, and cool down a bit (when you lose speed by going upwards the friction is reduced). Rinse and repeat until you lost enough energy to avoid the heat from atmospheric friction being an issue.

This would of course require some kind of wings with control surfaces but the space shuttle had exactly that.

Answer 1

It isn't possible to avoid heat from friction in re-entry, you have to deal with it in some way. What you are describing is called a skip-reentry, and it doesn't require wings. This technique was used by the soviet Zond spacecraft and Apollo spacecraft. The Zonds used the technique to alter trajectory, Apollo used it to avoid heat loads by extending re-entry. It is possible that the technique will be used again for returning missions from the Moon or Mars due to the high re-entry speeds.

Note that this technique helps to reduce friction loads, it doesn't eliminate them, you will still have an atmospheric reentry with significant thermal loads. This isn't an issue as we know how to deal with it.

Answer 2

What you're suggesting is an aerodynamic reentry, where aerodynamic surfaces are used to slow the rate of descent into the lower atmosphere. In the real world, heat shields are often shaped in such a way as to generate lift. But wings are almost never used. That's because it's very hard to make an aero-spacecraft that's able to maintain level flight at hyper sonic velocities. It's a common misconception that friction is the primary source of heat on reentry. In actuality, the vehicle creates a zone of extremely compressed air ahead of itself. This increase in pressure super heats the atmosphere. The problem with efficient (level-flight capable) aerodynamics at near-orbital velocities is that sharp edges are needed at the front of the wings. The flatter a surface is, the further away it can hold the reentry plasma. That means significantly less conduction, and less overall heating. But near sharp edges, that plasma can inch much closer to your craft. This is why the space shuttle's leading edges had to use a much tougher, heavier carbon-carbon structure to maintain rigidity. All and all, conventional reentry methods are usually more cost effective. Check out Scott Manley's video here for more information:

Answer 3

What you're talking about is a skip-reentry, where you cut a series of passes through the high atmosphere to bleed off speed before making the final descent. But note that there's no need for wings, and not really a U-turn happening here -- from the perspective of an outside observer floating in space above the north pole, your orbit is just a curved path through the atmosphere that curves less than the surface of the earth does. If you watch your altimeter as you do this, you will appear to descend and then rise back into the sky, but that's just because your altitude is being measured relative to a ball instead of a flat surface. The vessel doesn't have to do anything to make this happen, it's just their orbital path, which happens to pass through some air along the way.

To explain that point more, consider an eccentric orbit that doesn't even touch the atmosphere -- at one end of the orbit it's 30,000 miles above the surface of Earth, and at the other end it's 80,000 miles away. Is the spacecraft "making a U-turn" on each orbit? Not at all, it's just going around in an ellipse. But if you watch the radio altimeter, it says you're going up and down all the time.

Anyway, there are several reasons we don't use skip-reentry paths. Skip-reentry is a trade-off. You can reduce the immediate heating but you have to stretch it out into a low-and-slow bake, which is actually harder to deal with for a spacecraft. Contrary to popular belief, space is not cold (at least, not the way we would think of the word), and things in space (or the extremely high atmosphere) cool very slowly, so if you give the hot shield time to cool, the easiest place for heat to go is into the cabin, and you're going to have to plan for how to manage that. You really want your reentry to be as fast as you can make it without causing damage to the ship or crew. It's usually better to just build a more robust heat shield that can power through the worst of it and get down into the thick air that will convect the heat away, instead of a lighter shield that requires you to tippy-toe into the atmosphere.

There's also a safety issue with a long reentry path. It requires your vessel to stay stable and operational for longer in between between "safely in orbit" and "safely on the ground". You need more air, more battery capacity (since you had to ditch or stow any solar cells before reentry), and potentially more fuel for the control jets. That makes it more risky in general -- there's more time in the critical zone for something to fail -- and it makes the vessel less useful in emergency situations. If something has gone wrong on orbit and you need to get down as soon as possible, a lazy multi-hour reentry path that swoops through the atmosphere multiple times is probably a lot less desirable than just pushing through and getting to a place where you can get assistance.

Answer 4

Another way of braking on atmosphere re-entry is to think how to recuperate the energy.

Imagine one space vehicle destined for re-entry, and another just launched and just arriving above the atmosphere, but far below orbiting speed, so it would soon fall down again. We could imagine connecting them with a very long elastic tether system between them, allowing them to exchange impulse. Not easy, but feasible.

The fast orbiting vehicle will lose a lot of its impulse, while the slow orbiting one will gain a lot. If the vehicles can remain connected for enough time, the reentry speed of the incoming vehicle could get low enough to seriously reduce the need for a very high temperature heat shield, while the outgoing vehicle gets a serious sling so that its needs for fuel to get into orbit are seriously reduced.

Of course, synchronisation is not evident. But the concept can be tested on a small scale, first with a rather short sling tether, and limited energy savings, and gradually having more ambitious tests.