The aerospike engine is a type of rocket engine that maintains its efficiency across a wide range of altitudes. 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 in commercial production, although there are some large-scale aerospikes in testing phases.
Variations[]
Toroidal aerospike nozzle
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, 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 uses several small (small = suffer from friction) underexpanded, wrong pointed bells. This leads to shock waves, whose energy can mostly be converted to thrust with the help 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 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.
Further progress came in March 2004 when two successfull 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 26,000 ft and speeds of about Mach 1.5.
See also[]
- plug nozzle
- expanding nozzle
- LASRE
External links[]
- Aerospike Engine
- Advanced Engines planned for uprated Saturn and Nova boosters - includes the J-2T
- Linear Aerospike Engine - Propulsion for the X-33 Vehicle
- Dryden Flight Research Centre
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