Aerospike engine
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Image:Twin Linear Aerospike XRS-2200 Engine.jpg
The aerospike engine is a type of rocket engine that maintains its 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 25-30% 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. However, no engine is operational. The best aerospike is still only in the testing phase.
The terminology in the literature surrounding this subject is somewhat confused- the term aerospike originally was used for a (very roughly conically tapering) truncated plug nozzle with some gas injection to form an 'air spike' to help make up for the absence of the tail of the plug. However, frequently, a full length plug nozzle is now described as being an aerospike.
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Variations
Image:Annular-Aerospike.jpg Several versions of the design exist, differentiated by their shape. In the toroidal aerospike the spike is bowl-shaped with the exhaust exiting 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, something similar can be achieved. In the linear aerospike (see picture at top) 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.
Principles
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 4,400 N·s/kg in space, but only 3,500 N·s/kg 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 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 – 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.)
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 base pressure drops, but the recirculation zone keeps the pressure on the base up to a fraction of 1 bar, a pressure that is not balanced by the near vacuum on top of the engine; this difference in pressure thus gives extra thrust at altitude, giving the altitude compensating effect (effectively increasing the size of the nozzle at altitude by the area of the base).
In theory the aerospike is slightly less efficient than a bell designed for any given altitude, yet it outperforms that same bell at almost all other altitudes. The difference can be considerable, with typical designs claiming over 90% efficiency at all altitudes.
Performance
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; 200,000 lbf (890 kN) in the J-2T-200k, and 250,000 lbf (1.1 MN) 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 NASA's X-33 project. In this case the slightly upgraded J-2S engine machinery was used with a linear spike, creating the RS-2200. After more development and considerable testing, this project was cancelled when the X-33's composite fuel tanks continually failed.
Although this was a setback for aerospike engineering, it is not 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 liquid-propellant powered 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 (4.4 kN) LOX/ethanol aerospike engine. Image:Non-truncated toroidal aerospike nozzle.jpg
Further progress came in March 2004 when two successful tests were carried out at the NASA Dryden Flight Research Centre. The two rockets were solid-fuel powered and fitted with non-truncated toroidal aerospike nozzles. They reached an apogee of 26,000ft and speeds of about Mach 1.5.
Additional images
 RS-2200 linear aerospike test firing |
 CSULB aerospike engine |