A ship must have an engine in order to move. Ships without engines can still be moved between docking bay, hangar and shipyard facilities using Logistics Drones via a Logistics Hub, but cannot move on their own.
Ship engines can be installed from any connected Cargo Bay. You can uninstall a ship's engine to a Cargo Bay on the ship while it is launched, as long as it is not currently moving.
As of v?, a flight consists of two symmetrical phases: a continuous maximal acceleration for the first half, followed by a maximal decceleration for the second half. There is no maximum speed for ships; no relativistic effects are implemented.
The maximum acceleration a Ship Engine can provide is the lower of two factors:
The force-limited acceleration af=mF, where F is the maximum force the engine mounting brackets or the ship superstructure can sustain, and m is the gross mass (the mass of the ship, equipment and cargo).
The power-limited acceleration ap, which is the rate at which the engine can deliver energy to increase the ship's kinetic energy. ap=mE˙v−1 where mE˙ (the engine power divided by the gross mass) is a constant, and v is the ship's current velocity.
All flights are initially force-limited, then become power-limited as the velocity of the ship increases. The transition velocity v∗=v(ap=af)=FE˙ is usually less than 1ms−1 and the corresponding distance s∗ less than 1m; so for all but the most powerful engines flight can be approximated as purely power-limited.
The distance covered by the ship during an acceleration (or decceleration) burn is s(τ)≈9m8E˙τ23. Since the maximum allowed flight is 3 weeks (1.8Ms), the maximum range smax of a ship with a given engine is smax≈2.3×106mE˙km or ≈0.0154mE˙ astronomical units.