Aerospike engine. Credit: Boeing.
An aerospike engine is a type of rocket engine originally developed for the X-33 space plane. Aerospike engines are unlike conventional engines that push a vehicle aloft by hot gases escaping from bell-shaped chambers. Instead, aerospike engines are designed so that one end of their exhaust plume runs along a ramp while the other end remains open to the atmosphere. This makes them more powerful and easier to steer than conventional engines.
The aerospike engine has two chief advantages over conventional rocket engines:
1. It automatically achieves close to maximum efficiency at both sea level and high altitude. A conventional engine would need to change the shape of its nozzle to achieve this.
2. The aerospike engine can change the direction of its thrust and steer by varying the rate at which fuel flows to different parts of the engine. No moving parts are needed, unlike in a conventional engine.