Aerospike engine

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XRS-2200 linear aerospike engine for the X-33 program being tested

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 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 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.

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.

1 Variations
2 Principles
3 Performance
4 RS-2200
5 Additional images
6 See also
7 External links

[edit] Variations

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 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 while augmenting steering performance with the use of individual engine throttle control.

[edit] Principles

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 – 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 – 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 fixed 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.

The disadvantages of aerospikes seem to be extra weight for the spike, and increased cooling requirements due to the extra heated area.

[edit] 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 XRS-2200. After more development and considerable testing, this project was cancelled when the X-33's composite fuel tanks continually failed.

Three XRS-2200 engines were built during the X-33 program and underwent testing at NASA's Stennis Space Center. The single-engine tests were a success, but the program was halted before the testing for the 2-engine setup could be completed. The XRS-2200 produces 204,420 lbf thrust with an I_sp of 339 seconds at sea level, and 266,230 lbf thrust with an I_sp of 436.5 seconds in a vacuum.

Although the cancelling of the X-33 program 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 lb_f (4.4 kN) LOX/ethanol aerospike engine.

Toroidal aerospike nozzle

Small-scale aerospike engine development using a hybrid rocket propellant configuration has been ongoing by members of the Reaction Research Society. Another new aerospace group and future company called StoffelCorp Aerospace (Research and Development) had recently developed and static tested an aerospike nozzle hybrid rocket configuration with success [July 2006]. Further aerospike hybrid rocket motor tests are scheduled for 2007.

Further progress came in March 2004 when two successful tests were carried out at the NASA Dryden Flight Research Centre using small-scale rockets manufactured by Blacksky Corporation, based in Carlsbad, California. 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.

[edit] RS-2200

The RS-2200 is the design for the larger aerospike engine derived from the XRS-2200. The RS-2200 was to power the VentureStar single-stage-to-orbit vehicle. In the latest design, seven RS-2200s producing 542,000 pounds of thrust each would boost the VentureStar into low earth orbit. The development on the RS-2200 was formally halted in early 2001 when the X-33 program did not receive Space Launch Initiative funding. Lockheed Martin chose to not continue the VentureStar program without any funding support from NASA.