Just a wild guess here, maybe they’re using a variant of a ramjet, skipping the inefficient speeds, and then propelling it to LEO speed once it’s launched.
Ramjet would still require atmosphere. If you look at the Japanese SS-520 or the RocketLab's Electron, you don't need a giant rocket to get a small payload to orbit from a standing start. If you give your payload a Mach 5 kick you can eliminate a lot of the propellant mass and get by with smaller rocket motors optimized for high altitude operation.
Elon Musk wants to put 12,000 satellites into orbit. It would be very strange to me if he doesn't have someone looking into the feasibility of a giant subterranean launch tube to do something similar. Get the payload up to Mach 10 on terrestrial power and complete the orbit with a small single stage.
The Hyperloop is a rehashed vacuum-train idea originally from the 1800s (reduce air and rolling resistance as the primary forces counteracting speed). [1]
A launch tube would be very different, if for no other reason then the fact that you need to actually jettison the payload at one end at high speed.
so to put something in orbit you have to do 2 things:
- get it high out of the atmosphere
- give it a sideways kick so that it's going fast enough as it falls back that it misses the earth
Going straight up minimises the velocity lost by drag, but means you have to pull a right angled turn, going straight sideways grossly increases drag but makes the energy for insertion less
A railgun can only put one velocity vector on a launch, you still need a rocket of some sort to pull that turn
All unthrusted orbits eventually return to their starting points - so without a rocket on the payload there would be absolutely no way for a railgun to orbit anything. (Even if the railgun was on a tower on the moon, the satellite would eventually hit the back of the railgun.)
