Total conversion drive was a type of propulsion system that was found to be fitte on the planet killer doomsday machine devices.
This drive system operated on a principle of absorbing large amounts of solid planetary matter to be converted to energy inside the machine. Although starships can easily be absorbed, the drive is vulnerable to overloads of energy that can be generated by a vehicle's exploding impulse drive.(TOS episode & novelization: The Doomsday Machine; TOS video game: Shattered Universe)
Cadet Jean-Luc Picard of Starfleet Academy once had a discussion in one of his Starfleet Academy classrooms regarding the fuel needs of this total conversion drive. He theorized that the device would be unable to move through inter-galactic voids, as it was previously thought to have done, because there was no planetary mass to provide fuel. (TNG novel: Vendetta)