A plane is a craft which starts horizontally and utilises the lift generated from wings or winglets. Because planes usually use jet engines they are more efficient than rockets but normally don't leave the atmosphere. Therefore, long distances can easily be covered in-atmosphere, but on the other hand the atmosphere induces drag at all times, preventing a stable orbit.
Planes are build in the Space Plane Hangar and start from the runway. It is possible to build planes in the Vehicle Assembly Building but from there planes would start on the launch pad, whose small size makes a horizontal takeoff difficult. There are various stock planes available in the Space Plane Hangar.
The fastest known plane moved at about 2377 m/s relative to the ground a bit short of Mach 7 in 0.23. At this speed it is possible to circumnavigate Kerbin in about 29 minutes at an altitude of 40 km. For comparison the speed required to orbit Kerbin at 70 km (outside the atmosphere) is only 2296 m/s.
Another hazard to planes are flameouts, which occur when not enough intake air is available, especially when there are engines generating thrust off the center of mass. This could lead to an unstable flight, when only engines on one side fail and engines on the other side continue to generate thrust applying torque.
Another form of planes are gliders. Those don't use any fuel to propel them and simply exchange height with velocity. As there is no weather, no updrafts exist, so gliders can't ascend without losing speed. Launching gliders can also be problematic. However, the aerodynamics system can also be exploited to produce infinite gliders, which accelerate without the use of engines.
Air launch to orbit
- → See also: Air launch to orbit on Wikipedia
Rockets could be carried higher into the atmosphere by planes to use the more efficient jet engines for part of the way. Because the rocket then starts higher up, the drag by the atmosphere is lower and the engines do work more efficient. A plane also imparts momentum on the rocket prior to the rocket launching. The plane must also carry the rocket which limits it size and mass. Also releasing the rocket is tricky, although Sepratrons I can assist in pushing the rocket from the plane.
Airplanes should reach altitudes between 15 and 20 kilometers to be effective and reach speeds up to 1000 m/s at release to reduce the speed which is required by the rocket. To keep the rocket's mass and size low only one or two stages are recommended.
- “The MachingBird Challenge!” in the forums.