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storm_soldier2377

Spaceplanes lose control on reentry.

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"Spaceplane loses control on reentry" is a classic problem in KSP, and the answer is almost always the same thing:  your CoM is near the back of the plane.  (This problem often compounded by having all the control surfaces at the back of the plane, too, which makes it not only unstable but uncontrollable as well.  Control surfaces right next to the CoM have no lever arm to work with and are therefore almost completely ineffective.)

That's almost never going to be a stable configuration.

Note that "CoM is behind CoL" isn't really the important thing to watch.  Many folks will give advice such as,

1 hour ago, Gojira1000 said:

CoM is behind the CoL

...but that's actually rather misleading.  Yes, having CoM behind CoL is a problem... but you can have a plane with the CoL behind the CoM that is still really unstable.

That's because what actually matters is whether the CoM is behind the center of dynamic pressure, not CoL.  Basically, if your CoM is in the back of the plane, there's practically nothing you can do to make it stable.

To offer any specific advice, we'd need to see a screenshot of your plane.  But broadly speaking, the solution will probably  be some combination of the following:

  • Move your CoM way far forward.  (95% of the time, this is the issue, and solving this solves the whole problem.)  You may need to reposition your engines-- often the engines are by far the heaviest part of an empty spaceplane, and putting them way back at the back causes CoM issues.  One configuration that often works well is to have the engines located amidships, side-mounted to the fuselage, like Skylon.
  • Make sure you have control surfaces far away from (i.e. far in front or behind) the CoM.
  • Be sure to disable everything on your vertical stabilizer except yaw, and make sure it's well positioned:  as far, far aft as possible, and not sticking up too high above the plane's centerline.
  • Control surfaces located in front of the CoM (e.g. canards) should be disabled for everything except pitch.

Easy way to test for stability:  try taking off from the runway, with empty fuel tanks (you can turn on the infinite-fuel cheat) and then see whether it's flyable (including keeping stability during turns, pitch up/down, etc.).  If you can fly the plane when the tanks are empty, then it should be fine for reentry.

 

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Do you have a screenshot of the craft showing CoM and CoL? And remember to check it with the fuel tanks empty to simulate re-entry. 

Usually either CoM is behind the CoL due to mass shift or you're trying to maneuver at trans-sonic speeds and get a flat spin as a reward. But that's guessing.

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Does it have a pilot, or is it remote controlled?
Because you can toggle commnet to give limited communication to remote controlled ships, while reentering the atmosphere, due to the plasma forming around your ship. You can turn it off in settings.

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Drag has become more of a factor now.   and most space plane designs don't take this into account.   the rear oriented com will cause it to want to fly engine first.   by moving the com  closer to the center point of the craft,  you can  then bring your wings forward so it resembles a more common plane design.   this will also increase the lever for pitch control while reducing  the lever of drag force against the nose.   hope that helps some.

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If you don't already have it, get the RCS Build Aid mod. It will show you full, empty, and average CoM at the same time, which is great for balancing space planes.

Now that fuel drains evenly, I've had a pretty easy time of making craft where the CoM doesn't move at all as the tanks empty just by re-arranging the fuselage segments (using "short" versions gives you more flexibility and makes this easier - when you're done, you can replace any adjacent ones with long segments)

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Expanding on what Snark is saying,

 Center of pressure is essentially the center of drag. It's not shown in the VAB, so you have to visualize it. When the plane is at a high pitch angle relative to it's direction of flight (angle of attack), your plane must create more drag behind the center of mass than in front of it, and that drag must have more leverage.

 Delta canard configurations look all futuristic and cool, but conventional layouts are much more stable at high angles of attack.

Best,
-Slashy

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