Ten years ago, NASA launched the first test flights of solid-fuelled rockets with aerospike nozzles. These inside-out rocket engines trade bell-shaped exhaust manifolds for a narrow spike, building the rest of the bell out of air.
Test flight of a rocket equipped with an aerospike nozzle. Photograph credit: NASA Photo / Steve Thomas
Aerospike engines are altitude-compensating engines that expels exhaust gases along the outside edge of a spike. The spike shapes air into a bell shape that contributes to forward thrust. The bell increases in size with decreasing air pressure, producing additional thrust as altitude increases. Overall, it's a bit like an inside-out rocket engine, one that produces the exhaust bell on the outside of the engine instead of funnelling exhaust through a bell-shaped nozzle.
At low altitudes, ambient pressure compresses air wake against the nozzle. The overall impact is neutral: no thrust is produced at the base of the nozzle, but it also avoids the creation of a partial vacuum that would reduce thrust.
At high altitudes, air pressure decreases, reducing wake compression. As the pressure drops, the pressure at the base of the nozzle is fractionally higher, producing extra thrust.
Close-up of aerospike nozzle on the test rocket. Photograph credit: NASA/Carla Thomas
On March 30th and 31st in 2004 in Fort Stockton, Texas, NASA ran two consecutive test-flights with 10-foot solid-fuelled rockets equipped with aerospike nozzles. Both flights were flown successfully, exceeding Mach 1.5 at over 8,000 meters altitude. These flights were used to collect real flight data on aerospike nozzle performance, and how that performance was impacted by transsonic speeds.