Slingatron - build a railroad to space

Ballistic launch to orbit is... problematic. Getting something going fast enough is tough (G loads, aero loads, material in the launcher, etc) but doable. A really really big light gas gun should do, if the "Slingatron" won't. Thing is, you cannot shoot something from the planet's surface to orbit. Just can't.

If energy is too low, you're normal ballistic, and the projectile falls somewhere downrange. If energy is too high, you escape from Earth's gravity well, and you never see the thing again. (This might be fine for sending materials to the Moon, though.)

What if the energy is just right? Well, you go into orbit with closest approach at exactly your altitude. Which is you one orbit later... if aero effects haven't already caused enough energy loss to run the projectile into the surface, as they would on Earth.

(Side bar: even on the Moon or Mars, it wouldn't work well. Orbit at the height of the highest lunar mountain would be very unstable, though it would work better than on Earth! Might get a few orbits out of it before crashing, if you can find a path without huge mascon changes, and if it doesn't hit your launch facility. The top of Mons Olympus still has enough atmosphere to be problematic.)

So you have to have high-altitude propulsion to alter your trajectory once you get there, so you can orbit at a reasonable altitude. Which is why they note that they need an onboard rocket. And that's unfortunate, because it throws away the appealing simplicity of ground launch: passive payloads (and thus low marginal costs). Now, alongside your space-gun you've also got to design a mass-produced probably-expendable rocket system (with guidance) that can survive wicked G loads. And so here come a lot of extra marginal and development costs and a lower payload fraction. Does your gun-and-rocket system still beat a 2/3-stage rocket? With enough volume it might, but where do you get those customers?

I kind of like the idea of shooting your massive space-gun at an active target that can do the orbital circularization for you; either a tug that refuels itself from the projectile, or more exotically a rotating momentum exchange tether. That on-orbit system adds additional development and capital expense, in exchange for very low marginal cost. Which would be nice if you had the volume--but nobody needs that sort of volume in space now (nor is likely to for quite some time.)

But once some system with reasonable development costs and acceptable marginal costs has paved the way, it might be a lovely system for bulk material transport (probably mostly volatiles, like water, but maybe also metals for construction).