There are two models of Boeing's (Rocketdyne's) linear aerospike
engine. The XRS-2200 will propell the X-33 Technology Demonstrator currently being
built by Lockheed Martin Skunk Works. The upgraded RS-2200 is intended to power
VentureStar, Lockheed Martin's next generation RLV.
The linear aerospike engine uses a common
turbo-pump and a bank of liquid oxygen / hydrogen thrusters alligned along the top edge of
a wedge shaped thrust structure. The metal wedge acts as one half of the rocket
nozzle and the ambient atmosphere acts as the other. The key feature of the
Aerospike engine is that, as the launch vehicle ascends during its trajectory, the
decreasing air density allows the effective nozzle area ratio of the engine to increase.
As a result, an aerospike engine's performance is very high along the entire
trajectory and is more efficient than it's "fixed nozzle" counterparts.