Doomsday machine
The Doomsday Machine (sometimes called the "planet killer") is a planet destroyer of extra-galactic origin that appears in TOS "The Doomsday Machine".
Characteristics
The machine is an automated warship designed to destroy planets and then consume the debris to refuel itself, presumably converting the mass directly into energy by unknown means. It would also react to starships that approached too close, attacking and disabling them before resuming its primary mission.
According to Spock, the hull of the vessel was composed of "pure neutronium", making it opaque to Federation sensors and essentially invulnerable to Federation weapons. The Constitution-class starships USS Constellation and USS Enterprise both attempted to destroy it with phasers, with no effect.
The machine attacked with a beam weapon that Commodore Decker of the Constellation described as "pure antiproton", although its visible effects were inconsistent with a beam composed entirely of negatively charged antimatter particles. Shots from this beam defeated the shields of the Enterprise with just a few shots, but it did not instantly annihilate the ship even after the shields failed. This same beam was apparently cable of blasting large chunks out of planets.
The machine had a tractor beam which was capable of overcoming the propulsion systems of the Enterprise (although the Enterprise had sustained some damage by that time). The primary purpose of the tractor beam was presumably to bring pieces blasted out of planets into the machine's "maw" for consumption.
The machine generated subspace interference that prevented long-range communications in its vicinity.
The machine was finally destroyed by flying the crippled USS Constellation into its "maw" and detonating the impulse engines. The resulting 98-megaton explosion caused internal damage that destroyed the vessel.
Implications
It is interesting that the Enterprise made no attempt to launch photon torpedos into the mouth of the machine after learning that its internal mechanisms could be damaged that way. If torpedos have the multi-megaton yields often claimed by Trekkies, a few torpedos into the machine's mouth should have been sufficient to disable it.
Trekkies however note that Warsies miss (or deliberately ignore) important facts in the episode.
Relevant dialog:
SCOTT: Captain, Washburn has our report.
KIRK: Go.
WASHBURN: We made a complete check on structural and control damage, sir. As far as we can tell, something crashed through the deflectors and knocked out the generators. Somehow the antimatter in the warp drive pods has been deactivated.
KIRK: Deactivated? Scotty, could some kind of general energy dampening field do that, and would the same type of thing account for the heavy subspace interference?
We learn that the machine is able to remotely "deactivate" antimatter, this may be some form of spin reversal the charge that renders antimatter into normal matter. If the Enterprise's antimatter was similarly effected the way the Constellation's was, then photon torpedoes would likely be rendered useless since their warheads use antimatter as reactants.[1][2]
Later on:
KIRK: Spock, listen. Maybe Matt Decker didn't die for nothing. He had the right idea but not enough power to do it. Am I correct in assuming that a fusion explosion of ninety seven megatons will result if a starship impulse engine is overloaded?
[Bridge]
SPOCK: No, sir. Ninety seven point eight three five megatons
Kirk clearly asks about a starship impulse engine, not engines. If Kirk had made an error, Spock would not only have corrected him on the yeild, but also the number of engines necessary to generate such an explosion.[3][4]