I'm having problems getting m SSTO plane to maintain altitude. Usually I can get it up to at least 20,000 m, but that's about it. I've noticed craft seems to start slow down at high altitudes and follow a parabolic arc. Even I have to point the craft at about 45 degrees up to just to maintain a level trajectory, but with that attitude, I slow down even faster. Switching off the turbojets and switching to the rocket doesn't help. The damage is done. I've tried shifting my fuel from the front to the back of the jet, but that doesn't help. Any thoughts?