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 instantly 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 ship's jets. The maximum speed possible with the Bergenholm drive 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 the matter in intergalactic space is less dense than in interstellar space.
To maintain conservation of momentum, ships under Bergenholm drive retain a (temporarily suspended) 'intrinsic velocity' as well as the actual velocity they move at. When a Bergenholm is switched off (or disabled), an inertialess ("free") object resumes its 'intrinsic velocity', the velocity it had when the Bergenholm was activated. As a result, a ship must use its jets for some time while in a non-inertialess ("inert") condition to match speed with destination planets. Matching velocities is usually and more easily done after arrival, but if the velocity of the destination is known it may be done before engaging the Bergenholm. 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 Bergenholm drive. Once the Bergenholm is deactivated, 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.
The Bergenholm also acts as an unconventional form of defense: an inertialess ship will be instantly 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.
The Bergenholm appears to violate conservation of energy. In tall buildings, elevators are replaced with open vertical shafts in which people travel up and down using personal-sized Bergenholms. A person descending a shaft activates their personal Bergenholm and falls down the shaft, instantaneously attaining terminal velocity, and decelerating instantaneously on reaching the ground at the bottom without shock or injury. The person must lose gravitational potential energy in so moving from higher altitude to lower but this energy just seems to disappear; no mechanism for its dissipation is ever mentioned.
Neutralisation of inertia would have severe unwanted effects which are not mentioned in the novels. For example, all thermal motion would cease and chemical reactions would be halted. One effect of this would be at the very least to put living beings into a state of suspended animation and possibly even to kill them. No mention is made of how this and other untoward consequences are avoided.