Linear aerospike rocket engines have been around for more than 30 years. Based on a concept developed by the Air Force's Propulsion Directorate in the early 1960s, Rocketdyne (now Boeing North American - Rocketdyne) developed the technology for both linear and annular aerospike engines during the mid-1960s, ground-testing various designs into the 1970s. Rocketdyne proposed the aerospike engine for use on the Space Shuttle, but the engine was turned down because the technology was considered too immature at the time. Since then, Rocketdyne has accomplished 73 laboratory and ground test firings, with over 4,000 seconds of operation of this type engine. Rocketdyne has spent over $500 million over the years to test and improve aerospike engine technology. Recent improvements funded by the Air Force in the early 1990s made it possible to improve the manufacturing of aerospike engine thrust cells, while modern performance sensors and monitoring controls enable split-second engine control.
The linear aerospike engine is very similar to normal rocket engines in its plumbing and accessories, utilizing similar components. However, one of the major differences, and the most notable, is the absence of a bell-shaped nozzle. The linear aerospike engine uses the atmosphere as part of its nozzle, with the surrounding airflow containing the rocket's exhaust plume. This keeps the engine at optimum performance and efficiency along the entire trajectory of ascent to orbit. Traditional rocket engines cannot compensate for atmospheric changes, from low altitude and high atmospheric pressure, to high altitude and low atmospheric pressure. So, they are designed for a particular performance range in an effort to get the best performance from them.
A regular bell shaped nozzle's efficiency is based on the outside air pressure. That means that its peak efficiency is only at one altitude. The aerospike has the gases flowing on the outside of a nozzle. So as the altitude increases, the shape of the exhaust plume is automatically changed. This concept is illustrated by following the links below:
The steering of the X-33 and VentureStar was to be done with the engines. The throttle of each engine is adjusted to make the vehicle move.
As a side note, CSULB students are also developing a 1000 lbf aerospike rocket engine as part of the California Launch Vehicle Education Initiative (CALVEIN). If successful, the engine will be flown on one of CSULB's liquid-propelled rockets for the first powered flight of an aerospike rocket engine in the history of space propulsion. For more information, including pictures and videos, click here.
