Orbiter Add-ons
While the source is not editable, an extensive API enables Orbiter users to contribute by creating add-ons. Many spacecraft are available for download as add-ons, ranging from the Soviet Vostok spacecraft to Project Mercury and the Apollo program. Another popular category of add-ons are modifications of the standard spacecraft of Orbiter, ranging from simple visual changes to complex simulations of the internal subsystems of these fictional crafts. Examples of these more advanced default spacecraft include the DeltaGlider IV and the XR Series of vessels.
Add-ons are also available for new surface bases, MFD modes, extensions of the simulation menu, space stations, planets, and even other solar systems. Since Orbiter 2006, a scenario editor is included, which can also be extended for supporting the special attributes of add-on vessels.
Read more about this topic: Orbiter (simulator)