When talking about non-rocket space launch, (specifically, launching things from earth into space) one idea that comes up often- and then gets quickly dismissed- is the idea of a Mass Driver- a rail gun, a coil gun, or a light-gas gun.
On the one hand, the idea is nice because it can be built on earth, where we already have people, resources, and manufacturing equipment.
On the other hand, it's basically unworkable because the g-forces involved would kill a person, and destroy most equipment, unless you were building a staggeringly long system- hundreds of kilometers. The sheer cost of this is basically why no-one ever seriously talks about making a Mass Driver on earth.
But recently, we've been seeing progress in the 3d printing in general, and 3d printing in space in particular. The point where we will be able to actually build things via remote-controlled 3d printers is looking quite close- IF we can feed resources into them. (And, more importantly, if we can get those resources to the 3d printer cheaper than if we shipped the assembled product from earth instead.)
Which brings me back to the idea of Mass Drivers in earth. What if, instead of launching people or satellites, a Mass Driver was constructed specifically to launch small, solid masses- say, 10 kilograms of steel feedstock for a 3d printer? Launched towards a 3d printer that was already in space?
With smaller payloads and higher g-force tolerances, do Mass Drivers become a realistic option? Are they actually viable, both in the sense of 'how much do they cost to build?' and in 'can they actually get stuff into space?'