Bergenholm space drive

In E.E. "Doc" Smith's fictional Lensman series, the Bergenholm generator is the key to faster-than-light transportation. It functions by rendering an object inertialess so that it instantaneously takes on an analogue of terminal velocity: that is, the speed and direction of motion in which frictional resistance from the medium it is travelling through is equal and opposite to the applied force, such as the force of a ships jets. The maximum speed possible with the Bergenholm drives depends on the force exerted by a ship's jets, the density of matter around the ship, and even the size and streamlining of the ship. Thus, intergalactic space travel is possible in reasonably short time, because intergalactic space is less dense than interstellar space.

To maintain both conservation of energy and conservation of momentum, ships under Bergenholm retain a (temporarily suspended) 'intrinsic velocity' as well as the actual velocity they move at. When Bergenholms are switched off (or disabled), the inertialess object resumes its 'intrinsic velocity', the velocity it had when they were activated. As a result, a ship must use its jets for some time while in a non-inertialess condition to match speed with destination planets. Matching velocities in usually and more easily done after arrival, but if the velocity of the destination is known it may be done before engaging the Bergenholms. Also, and for the same reasons, two ships that make a rendezvous must match intrinsic velocities. It can be very dangerous for any object or substance to come aboard a ship while the ship is under Bergenholms. Once the Bergenholms are deactived, the foreign object will resume its own intrinsic velocity, which may be very different from that of the ship. In such a case it will become a very dangerous projectile or projectiles.

The Bergenholm also acts as an unconventional form of defense; an inertialess ship will be instantaneously pushed away from an attacker by the slight force exerted on it by the weapon being used. Thus, tractor beams are required to hold a ship while it is fired upon in order to do any damage.