Jet engine

Used for atmospheric aircraft, as well as for near-surface maneuvering of spacecraft. For an in-system run, it is actually possible to get by without using anything else as long as you plan your route well (slingshot around a planet, leaving the atmosphere with enough momentum to carry you the rest of the way). Such slingshot craft do not need a reactive craft license, but are legally required to have at least vents to get out of each other's way in an emergency.