Once you get to mach 5+ hypersonic speed, then a scramjet works and it is by far the most efficient type of engine for hypersonic speeds. A scramjet needs some other form of propulsion to get it to Mach 5. As a result, scramjets have become something of a well-studied technology in search of a practical application.
To reach these hypersonic speeds, Michael Smart, professor of hypersonics at the University of Queensland in Brisbane plans to combine an uncrewed scramjet with conventional rockets. He believes his Spartan launch system could radically reduce the costs of blasting satellites into orbit.
“All conventional satellite launch systems use different stages,” says Smart. “There’ll be a first stage rocket that normally gets up to Mach 5 or 6, you’ll have a second scramjet stage that goes two thirds of the way to space and you’ll have a final upper stage that takes the satellite into orbit.”