The way I do it is by limiting my climb rate. NMT 500 m/sec up to 25 km altitude, then NMT 100 m/sec up to 32 km altitude. Once at 32 km, maintain altitude until periapsis is established, then use just enough throttle to maintain it. It sounds like you're climbing too rapidly, which will cause you to be too slow at altitude, which runs you out of air. You need to have low enough wing loading to climb efficiently at terminal velocity (about 1.0 Cl per tonne), enough intake area (about .018 m^2 per engine) and enough engine (about 1 turbojet per 15 tonnes or 1 RAPIER per 10 tonnes. As an aside... lifting 300 tonnes with a spaceplane sounds like a nightmare. A vertical turbojet lifter would make a lot more sense for that job IMO. Best, -Slashy