In atmosphere, planes cover lateral distances with lower thrust than rockets, thus less fuel and less weight. Planes are great for traversing Kerbin and, after an interplanetary journey, other celestial bodies with an atmosphere.
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. However, drag at low altitudes reduces both efficiency and top speed. On Kerbin, the altitude with an optimal balance for minimizing drag and availability of intake air occurs between approximately 20 km & 24 km.
A hazard to jets, especially at high altitudes, is engine flameouts caused by lack of intake air. 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. The higher the altitude, the higher the speed required for an intake to collect a given amount of intake air. On Kerbin, the highest that even supersonic jets can meet minimum intake requirements is around 40 km.
Planes operating above the range of jets 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.
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.
Center of lift and its position relative to center of mass is crucial to aerodynamics. Positioning and angling of the wings and other aerodynamic parts can be a complex process. Also, fuel consumption during flight tends to shift the center of mass. Care must be taken during construction to ensure proper fuel flow so the relation between lift and mass don't become undesirable. Keeping the total amount of fuel
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 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, no updrafts exist, so gliders can't ascend without losing speed. 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 the Sepratron 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 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.