The aerospike is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency across a wide range of altitudes. For this reason it is sometimes referred to as the altitude-compensating nozzle as well, although that is a more general term that refers to a class of such designs. It has been studied for a number of years and is considered as the baseline for many SSTO designs, but remains a test article only.

A normal rocket engine uses a large "engine bell" to direct the jet of exhaust from the engine from the surrounding airflow and maximize its acceleration – 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 and more tapered than one designed for low altitudes. The losses of using the wrong design can be significant, for instance the Space Shuttle engine can generate a specific impulse of just over 450 seconds in space, but that number falls to just over 360 at sea level. 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 a small hole in the middle of a bell, it instead exits on one side of a cone-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 – thus the aero-spike.

Annular aerospike test firing
Several versions of the design exist, differentiated by their shape. In the toroidal aerospike the spike is bowl-shaped with the exhaust existing in a ring around the outer rim. In theory this requires an infinitely long spike for best efficiency, but by blowing a small amount of gas out the center of a shorter truncated spike, you can achieve something fairly similar. In the linear aerospike the spike consists of a tapered wedge-shaped plate, with exhaust exiting on either side at the "thick" end. This design has the advantage of being stackable, allowing several engines to be placed in a row to make one larger engine.

The "trick" to the aerospike design is that as the spacecraft climbs to higher altitudes, the air pressure holding the exhaust against the spike decreases. This allows the exhaust to move further from the spike, and the virtual bell automatically expands in just the right way. In theory the areospike is slightly less efficient than bell designed for any given altitude, yet it vastly outperforms that same bell at all other altitudes. The difference can be considerable, with typical designs claiming over 90% efficiency at all altitudes.

RS-2200 linear aerospike test firing
Numbers like these are hard to argue with, and Rocketdyne conducted a lengthy series of tests in the 1960s on various designs. Later models of these engines were based on their highly reliable J-2 engine machinery and provided the same sort of thrust levels as the conventional engines they were based on; 200,000lbs in the J-2T-200k, and 250,000 in the J-2T-250k (the T refers to the toroidal combustion chamber). Thirty years later their work was dusted off again for use in NASAs X-33 project. In this case the slightly upgraded J-2S engine mechinery was used with a linear spike, creating the RS-2200. After more development and considerably testing, this project was cancelled when the X-33 encountered massive cost overruns.

Although this was a setback for aerospike engineering, it wasn't the end of the story. A milestone was achieved when a joint academic/industry team from California State University, Long Beach (CSULB) and Garvey Spacecraft Corporation successfully conducted a flight test of a powered liquid-propellant aerospike engine in the Mojave Desert on September 20, 2003. CSULB students had developed their Prospector 2 (P-2) rocket using a 1,000 lbf LOX/ethanol aerospike engine.

N.B. when used with regard to engine technology the term aerospike is unrelated to the frontally mounted drag reduction aerospike as fitted to the Trident missile.

See also:

plug nozzle
expanding nozzle

External links:
Images, video, and information from the Prospector 2 rocket flight test
Aerospike Engine
Advanced Engines planned for uprated Saturn and Nova boosters
- includes the J-2T
Linear Aerospike Engine - Propulsion for the X-33 Vehicle