A co-worker made this suggestion this morning. I don't think it's viable but don't know enough to be sure. What does everyone think?

Here's the scenario: A shuttle has leading edge damage similar to Columbia. Instead of firing the retros enough for a normal reentry, slow down just enough to go into a decaying orbit. Use aerodynamic and/or reaction controls to maintain attitude to slow gradually enough to avoid damaging heat. Slowing would take place over a number of orbits.

My co-worker thought this might take a couple days. I suggested a couple of years would be more like it, if it was possible at all.

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I don't think that would work. You're quickly going to reach a point where the atmosphere is slowing you down significantly while you're still going at near-orbital speeds. There's no way of slowing down gradually while staying in orbit or above the atmosphere, at least not with the shuttle's on-orbit propulsion system. Supposedly the shuttle re-entry is already as gentle as they can make it.

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An orbit is a balancing game--you have the pull of gravity pulling you down and your inertia trying to keep you going straight (or, if you like, the pull of gravity balancing the centrifugal force of your velocity. I know that's not particularly accurate, but it may help to explain the situation). In order to achieve this balance, the shuttle has to be travelling at orbital velocities--8 km/sec or so. Slow down a bit (which is what happens when the OMS engines take it out of orbit) and gravity starts to win--the shuttle starts heading down, encounters the atmosphere, slows down more, etc.

In order for the shuttle to remain above most of the atmosphere while you gradually slow it down, it's going to have to find something else to counteract the pull of gravity. The most obvious candidates are aerodynamic lift or rockets. Rockets are obviously out--it would take nearly as big a rocket to bring it in slowly as it did to put it up in the first place (actually, if you want to bring it in really slowly it would take an even bigger rocket). Which leaves aerodynamic lift. The problem with lift, though, is that you're still plowing into the atmosphere at 8 km/sec; if you encounter enough of the atmosphere to start producing lift, it will also be more than sufficient to start heating things up and will also produce enough drag to start the reentry.

Sanger's skip bomber reentry may be feasible--in this case the vehicle dives into the atmosphere fast enough and steep enough to produce enough lift to pull out again. The trajectory resembles somewhat a stone skipping across water. However, the shuttle isn't designed for such a trajectory, so this isn't a viable approach.

That's what usually happens when I screw up the re-entry of the shuttle in Orbiter. You get pretty high g-forces when doing this...

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I've simulated reentries and decaying orbits and they just don't work like you describe. What you are suggesting is slowing down the Shuttle to lower its perigee point so that is drips into the atmosphere slightly. This passage through the atmosphere slows it down a bit more but not enough to cause reentry. After passing through the perigee point it will rise again out of the atmosphere but its apogee point will be a bit lower because of the velocity it lost when moving through the atmosphere. Over time both the perigee and apogee points will continually lower. This will only go on for a limited time however. At some point the Shuttle will lose enough velocity that drag and gravity will overtake it as it tries to rise out of the atmosphere. It will pass through its perigee point, try to climb, and then drive back into the atmosphere and reenter. Even though it has lost some altitude and velocity, it is not nearly enough to make a difference. It will still generate tremendous heat and burn up if not protected.

Thanks, I'll pass this on and post a reply for him if he wishes.

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