There is another type of propulsion device but it can only be use in a Vacuum. They say it will/does well for interplanetary, uses no fuel or oxidizers either.
Considering the fact that MACK III is sixties tech. It would be naive to dismiss MACH X piloted air travel . The technology and equipment to support exists on the shelf . The old saying is that when you find out about the next big thing , they've had it for ten years plus .
Looks alot like the long fabled "aurora" spyplane i heard about years ago in flight school.
will be nice to see how this developes