A plane is any craft which flies horizontally in an atmosphere utilizing lift primarily generated by wings, winglets, or control surfaces. Such flight involves lift-induced drag, but reduces the total thrust required to traverse a distance at a given speed.
Spacecraft can be launched more efficiently in terms of both fuel and recoverable parts by designing craft which fly as planes while lower in the atmosphere or having an aerodynamic mothership for air launch to orbit rockets, though sufficient altitude at separation is required to avoid the mothership being deleted.
As a rule, planes are built in the Spaceplane Hangar and takeoff horizontally (STOL) from the Runway. Planes can be built in the Vehicle Assembly Building, but the Launch Pad is inferior for horizontal takeoff and offers no advantage for vertical takeoffs.
Airplanes are planes designed to remain within an atmosphere. Jet engines are by far the most common choice due to efficiency. If an airplane uses rocket engines instead, it is a rocket-plane.
A plane able to land on and take off from bodies of water is considered a seaplane. Given the nature of KSP water physics, this can prove more difficult than it sounds.
Applicable to any kind of vehicle, the ability for “Vertical Take-Off and Landing” is accomplished by having engines which point downward and are balanced around the Center of Mass. These may or may not be the main engines used in horizontal flight. Care and skill are required to brake forward momentum in flight to achieve a landing. Most consider it necessary for engines rather than parachutes to be used for landing.
A plane without thrust is a glider. This may be a plane designed without engines which detaches from another lifting vehicle, or an airplane possessing a stable glide path after exhausting its fuel similarly to the real-life NASA Space Shuttles. Gliders exchange altitude for velocity. As there is no weather in KSP, no updrafts exist, so gliders can't ascend without losing speed.
A mothership plane is one which is used to carry another vessel, typically a rocket, to higher altitudes before it launches. The real-world Virgin Galactic is an example of this launch profile. The daughter-craft need not reach orbit, but typically air-launch-to-orbit is the objective.
Planes usually take advantage of jet engines, which require air intakes to breathe in atmospheric oxygen, allowing them to forego carrying the weight of oxidizer needed by rocket engines. Jet engines are fuel-efficient, thus great distances can easily be covered within atmosphere.
For a given set of engines at a given throttle level, there is a minimum amount of intake air which must be constantly collected by the air intakes. Collecting more air does not improve performance, but falling below the minimum amount results in one or more engines suffering a “flameout” and shutting down. Planes often have jet engines paired in bilateral symmetry, but it's rare both engines flameout simultaneously. Thrust then becomes asymmetrical about the plane's center of mass, applying turning torque that can easily cause a “flat spin” like a frisbee. Planes often have difficulty regaining control and thus may land in multiple pieces, if any.
Altitude is vitally important to operating jet engines. At lower altitudes, intake air is more available but drag also is greater, limiting top speed. As altitude increases, there is decreasing intake air available as well as less drag. Decreasing drag allows higher speeds which allows more intake air to be collected. However, increasing throttle position to gain speed increases the total required intake air to avoid a flameout. There is also decreasing lift, requiring a plane to pitch higher to avoid falling, which may decrease the efficiency of air intakes. A jet plane's ability to balance these competing demands defines its operational ceiling. On Kerbin, the highest that even supersonic jets can meet minimum intake requirements is around 40 km.
Planes operating above the range of jet engines require secondary propulsion, typically rocket engines. Such planes are called “spaceplanes” as in the name of the Spaceplane Hangar. Atmospheric drag still prevents achieving orbit unless and until the plane is able to fully clear the atmosphere. On Kerbin this is around 70 km.
The fastest known plane demonstrated, in version 0.23, a top speed of about 2377 m/s relative to Kerbin's surface — just short of Mach 7. At this speed it is possible to circumnavigate Kerbin in about 29 minutes at an altitude of 40 km. For comparison the speed of a stable Low Kerbin Orbit at 70 km (outside the atmosphere) is only 2296 m/s. Because the craft at that speed is faster than the orbital velocity of that height the craft naturally tends to rise.
- “The MachingBird Challenge!” in the forums.