X-13 Series

The X-13 Series engine is used as the primary engines on all UNSC transatmospheric aircraft. It allows the vehicle to operate both in atmospheric conditions and in exoatmospheric conditions without seperate engines or fuels.

The engine is a variable ramjet engines featuring a variable inlet spike. During atmosphereic flight, these are left open, allowing air into the engine, where fuel is injected under pressure and combusted in the chamber. The inlet spike can move to make the inlet narrower or wider, conversely making the speed higher or lower. However, to achieve transatmospheric travel, where air is not availible, it closes the inlet spike then adds a oxidative to the fuel, turning the jet engine into a rocket engine. The performance of both configurations are roughly similar it both the atmosphere of a planet and in space.

This engine has been in use for almost 100 years, with its powerful design being unlikely to be replaced for another twenty years.

Uses

 * X-133 D77I-TC Pelican and variants
 * X-134 D03UH-TVMC Petrel and variants
 * X-135 F-406 Katana