27 The X-15 was a rocket-powered aeroplane that, by flying incredibly high and fast, qualifies as the first spaceplane, and its pilots as astronauts. It was operated by the US Air Force and NASA between 1959 and 1968.
Its liquid-fuelled rocket engines were similar to those used in space launchers, but the X-15 flew horizontally rather than vertically. It was carried aloft under the wing of a B52 bomber, firing its engines when at the required altitude. Because it was designed to fly at the top of the atmosphere, where the air is very thin, it had a thruster-based control system rather than the usual control surfaces, which would be ineffective at such a high altitude.