The scramjet engine design has no moving parts. The forward speed of the aircraft itself, enhanced by the shape of the nose, compresses a stream of air that is channeled into the engine, where it mixes with gaseous hydrogen fuel — there are no fan blades that compress the air, as in a normal jet engine. In a scramjet engine, the airflow through the whole engine remains supersonic. NASA says such a design may be capable of flying at Mach 15 or more. Scramjets have an advantage over rockets in that they use oxygen from the atmosphere, so no heavy liquid-oxygen tanks are required. Also, scramjets can be throttled back and flown more like an airplane, unlike rockets, which tend to produce nearly or full thrust all the time.
