Aerospike engines will use 25–30% less fuel at low altitudes. Rocketdyne did a lot of tests in the 1960s on this technology. NASA tried to make aerospikes work for the X-33 program in the 1990s.

Three XRS-2200 engines were built during the X-33 program. The XRS-2200 produced 204,420 lbf (909,300 N) thrust with an Isp of 339 seconds at sea level and 266,230 lbf (1,184,300 N) thrust with an Isp of 436.5 seconds in a vacuum.