Planetary engine
The planetary engines are technological features of Cybertron in the Generation 1 continuity family
Those who desire total conquest of Cybertron rarely want to stop there, instead envisioning the creation of a vast intergalactic imperium with Cybertron as its throne. To further this conquest and enslave the rest of the heavens, many a would-be dictator has attempted to either create or use Cybertron's unfathomably powerful planetary engines that can actually repurpose the world as an unstoppable mobile dreadnaught.
In theory.
Fiction
Generation 1 cartoon continuity
Galvatron's master plan to punish humanity for having helped the Autobots involved the creation of planetary engines. With the Autobots themselves largely defeated on Cybertron, Galvatron ordered the Combaticons to extract their defeated victims' power packs, then construct gigantic planetary engines--"the biggest rockets ever built!"--and use the power packs as fuel. This tactic succeeded, allowing Galvatron to pilot the planet into an Earth-orbit, at which time he attempted to use the power of the Plasma Energy Chamber to destroy the entire solar system. Luckily for everybody (including himself), his plan was foiled. The Rebirth, Part 3
Marvel Comics continuity
Megatron's plans to create the planetary engines were seen as so radical and extreme that most of Cybertronian intellectual society dismissed him as a meaningless kook. One who did not do so was the egotistical Autobot scientist Flame, who liked the idea so much he chose to appropriate it for himself. Acting covertly, Flame discovered the immense fusion engines Megatron had tried to use, and endeavored to reactivate them himself--disregarding all warnings from Xaaron that engaging the reactors would destroy the planet. Flame would be undone by his own ambition, destroyed in the reactor explosion. Meltdown!
Dreamwave Comics continuity
As he built his underground army, Megatron somehow came to glimpse the ancient nature of Cybertron and its potential ability to traverse the heavens. Using attacks against Autobot cities as a diversion, Megatron attempted to activate the giant planetary engines and turn Cybertron into a warworld--but Optimus Prime was able to defeat him. The War Within issue 4
Notes
- The various comic treatments of Cybertron's planetary engines tend to vaguely align with one another, which makes sense since they were all written by Simon Furman and there are some themes he is fond of revisiting. Though the cartoon's portrayal lacks the ancient historic tie-ins, it is noteable (albeit almost certainly coincidental) that once again the idea for the giant engines comes from Megatron--or at least, the guy who was built from him.