It's an X plane, and it's unmanned. If propulsion fails, it will presumably crash in the desert.

Some of the press on this talks about weight savings. If they're really serious about that, well... none of their flights are intended to go above Mach 0.7. Possibly they could deploy a ballistic parachute like Cirrus airplanes offer.

Presumably, this would be operated in hostile territory, so nobody would particularly care about it turning into a giant fireball upon crash landing. Or self-destructed when still airborne.

It's unmanned so a self-destruct mechanism is probably the straightforward solution - or disable power and use a parachute

Backup air reservoir?