Those Mach numbers are for the relevant speed in air. I would have written that differently, but that’s how the cited paper worded things.
Mostly-sealing against part of the tube before cutting it is less problematic than dealing with a large pressure difference.
Aerodynamic support and propulsion in hydrogen is less expensive than magnetic propulsion and support in a vacuum-filled tube. Building an unpressurized tube is cheaper than a tube that doesn’t buckle under compressive forces. And so on.
Those Mach numbers are for the relevant speed in air. I would have written that differently, but that’s how the cited paper worded things.
Mostly-sealing against part of the tube before cutting it is less problematic than dealing with a large pressure difference.
Aerodynamic support and propulsion in hydrogen is less expensive than magnetic propulsion and support in a vacuum-filled tube. Building an unpressurized tube is cheaper than a tube that doesn’t buckle under compressive forces. And so on.