Mark Gubrud in Bulletin of the Atomic Scientists:
Hypersonic missiles fall into two distinct categories. In what is known as a boost-glide weapon, the hypersonic vehicle is first “boosted” on a ballistic trajectory, using a conventional rocket. It may cover considerable distance as it flies to high altitude, then falls back to Earth, gaining speed and finally, at some relatively low altitude, pulling into unpowered, aerodynamic, horizontal flight. After that, it glides at hypersonic speed toward its final destination.
Hypersonic cruise missiles, on the other hand, typically are launched to high speed using a small rocket, and then, after dropping the rocket, are powered by a supersonic combustion ram jet, or scramjet, for flight at five times the speed of sound (some 3,800 miles per hour) or greater.
The recent failed Chinese and American tests were of boost-glide systems; the X-51 WaveRider, which the US successfully tested last year after a string of failures, is an example of the scramjet cruise missile. The boost-glide test failures were probably caused by issues with booster rockets rather than with the hypersonic gliders themselves. Regardless, these systems didn't work, demonstrating that both boost-glide and powered hypersonic cruise missiles require testing. Such tests are easily observable from space and via radar and signals intelligence gathering (not to mention old-fashioned human spying). A test moratorium would throw a huge obstacle in the path of hypersonic programs, and a permanent test ban would make it clear that they aren’t going anywhere. And that would be a good thing, because where they are going is nowhere good.