The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. For this reason the nozzle is sometimes referred to as an altitude-compensating nozzle. A vehicle with an aerospike engine uses 25â€“30% less fuel at low altitudes, where most missions have the greatest need for thrust. Aerospike engines have been studied for a number of years and are the baseline engines for many single-stage-to-orbit (SSTO) designs and were also a strong contender for the Space Shuttle main engine. However, no engine is in commercial production. The best large-scale aerospikes are still only in testing phases.
A normal rocket engine uses a large engine bell to direct the jet of exhaust from the engine to the surrounding airflow and maximize its acceleration â€“ and thus the thrust. However, the proper design of the bell varies with external conditions: one that is designed to operate at high altitudes where the air pressure is lower needs to be much larger than one designed for low altitudes. The losses of using the wrong design can be significant. For instance the Space Shuttle engine can generate an exhaust velocity of just over 4,400 m/s in space, but only 3,500 m/s at sea level. If a large bell (designed for high altitude operation) were used near sea level, the extra weight of the bell might not overcome the additional thrust gained. Tuning the bell to the average environment in which the engine will operate is an important task in any rocket design.
The aerospike attempts to avoid this problem. Instead of firing the exhaust out of a small hole in the middle of a bell, it is fired along the outside edge of a wedge-shaped protrusion, the “spike”. The spike forms one side of a virtual bell, with the other side being formed by the airflow past the spacecraft â€“ thus the aero-spike.
The trick to the aerospike design is that at low altitude the ambient pressure compresses the wake against the nozzle. The recirculation in the base zone of the wedge can then raise the pressure there to near ambient. Since the pressure on top of the engine is ambient, this means that base gives no overall thrust (but it also means that this part of the nozzle doesn’t lose thrust by forming a partial vacuum, thus the base part of the nozzle can be ignored at low altitude).