It is interesting that they claim to have got to Mach 5 conditions in only 9 months, using their proprietary pre-cooler (no details on that). I suppose there is more than one way to skin a cat. It also seems that they have a ramjet incorporated rather than a rocket engine, however I assume that means that they use the usual method of slowing intake air to subsonic speed (with cooling) and also some form of bypass for use at higher speeds when the ramjet could become effective.