The U.S. Air Force recently successfully tested a X-51 hypersonic jet. Well, it was a partial success, but more of a success than this technology has ever enjoyed before. The 8 meter (36 foot) long, cruise missile-like X-51 aircraft was boosted to 3,300 kilometers an hour, using a solid fuel rocket, at which point the scramjet engine took over, and successfully operated for over two minutes, achieving speeds of nearly 6,000 kilometers an hour. This was the longest a scramjet had ever operated (the previous best was ten seconds).
What makes scramjets work is the compression of incoming air, without the use of a fan system (as in conventional jet engines). But while scramjets have been in development for half a century, the lack of adequate materials (that can handle the high heat and pressure), and adequate design tools, frustrated attempts to build workable, and reliable, scramjets. Scramjets have few moving parts, but must cope with very extreme conditions and the design challenges have proved very frustrating. The recent X-51 test, like all previous ones, ended with the aircraft crashing. The next step is to get longer hypersonic engine use, de-acceleration, and landing via parachute (and eventually an auxiliary engine.)
The difficulty in producing a useable scramjet has caused many aeronautical engineers to call the concept the "scamjet" and deride it as an ineffective line of research. But the recent X-51 test indicates that there is some possibility of scramjet becoming a cheaper and safer way of getting satellites, and people, into orbit. The next test will try to reach the goals set for the original test (five minutes of powered flight and top speed of 6,700 kilometers an hour.