Aerospike engine
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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 a test article.
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Variations
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 a small hole in the middle of a bell, it instead exits on one side of a cone or 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 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 aerospike is slightly less efficient than a 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.
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 considerably testing, this project was cancelled when the X-33 encountered massive cost overruns.
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 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 (4.4 kN) LOX/ethanol aerospike engine.
Additional images
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See also
External links
- Aerospike Engine (http://www.aerospaceweb.org/design/aerospike/main.shtml)
- Advanced Engines planned for uprated Saturn and Nova boosters (http://april.friends-partners.ru/partners/mwade/stages/satt200k.htm) - includes the J-2T
- Linear Aerospike Engine - Propulsion for the X-33 Vehicle (http://www1.msfc.nasa.gov/NEWSROOM/background/facts/aerospike.html)