Planetary engine

The planetary engines are technological features of Cybertron from the Generation 1 continuity family

Classics War Planets.

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 first experimented with planetary engines on a comparatively small scale, arranging to have a set built on a comet. He intended to use them to crash the comet into Metroplex, but an attack by the Autobots destroyed the engines, and the comet was then eradicated by the sonic weaponry on the nearby planet Eurythma. Carnage in C-Minor

Galvtron later revisited the idea of planet engines when devising his master plan to punish humanity for having helped the Autobots. 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), the planetary engines actually provided the means to foil Galvatron's plans, as Spike Witwicky had the Autobots' Nebulan allies reverse the rockets' polarity. With its mechanism thusly reversed from "output" to "intake" (we know, we know, just go with us, okay?), the engines were used them to absorb the excess solar energy and channel it straight to Vector Sigma, preventing the explosion of the sun and revitalizing Cybertron and restoring its Golden Age. 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

The planetary engines were constructed by Primus himself as he adjusted to his new body. Transformers: The Ultimate Guide

Eons later, when Megatron built his underground army, he somehow came to glimpse the ancient nature of Cybertron and its potential ability to traverse the stars. 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 notable (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.
• Galvatron was really into planetary engines in the Generation 1 cartoon—in addition to their appearances in "Carnage in C-Minor" and "The Rebirth", an aborted plotline from "Fight or Flee" involved planetary engines being built on Paradron in order to turn the planet into a mobile fortress.