(1) The X - 43A is an air-breathing supersonic ramjet , which ducts air directly from the atmosphere, mixing it with hydrogen before combustion.

(2) The engine, which will be provided by the Air Force, will be a dual-mode scramjet capable of running as a ramjet or scramjet.

(3) They've carried 400 lb of cargo, but the X - 15 carried a comparable amount of instrumentation, telemetry, flight experiments, data recorders, and on several occasions, a test model of a supersonic ramjet .

(4) The unpiloted vehicle's supersonic combustion ramjet , or scramjet, ignited as planned and operated for the duration of its hydrogen fuel supply.

(5) The two main contenders are hypersonic ramjets or scramjets and nanotube-based space elevators.