Feasibility of a small mass driver?

Question

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?'

Answer 1

I can see 2 problems with your idea.

First of all, if the object leaves the mass driver with anything like orbital speed, the driver had better be located very high. Otherwise, the payload will burn up as soon as it exits into the atmosphere. Note that I'm assuming the driver tube is kept in vacuum; if it isn't, atmospheric friction will destroy the payload even before it leaves the driver.

On top of that, a mass driver will never work without the ability to change trajectory after the payload leaves the driver. Without changing trajectory, the object will follow an ellipse, and hence it will eventually return to the exact location of the driver (ignoring the rotation of the earth underneath it). You need to give the payload an additional boost in order to change its orbit into a circle or ellipse that stays above the atmosphere. That implies it must have rockets and fuel on-board, and the result will be way more massive than your 10 kg.

A mass driver in orbit could be used to launch probes to other planets, but it is questionable whether the fuel savings would make it worthwhile, as the driver and its energy supply (solar? nuclear fusion?) would need to be in orbit in the first place. A mass driver on the moon makes more sense - as long as it could be built and fuelled with local materials. At least it would be in vacuum.

Answer 2

The general concept of leaving all the heavy parts of a launcher have been investigated several times and the notes here remain relevant. The actual firing into space is quite possible, the difficulty is that space is more than just going up, you also need 7kms sideways velocity so being fired to 200km straight up still leaves you needing most of the rocket a 9kms surface launch would have used and that rocket needs to survive being fired.

You can of course fire faster and at an angle, but this means more time suffering atmospheric drag unless you can also lift it to high altitude.

It would be nominally possible to reverse an orbital mass driver to 'catch' the largely stationary masses fired up from such a gun as it orbited past at 7kms and bring them up to orbital speeds but this will mean you need the equivalent amount of rocket thrust to the mass driver station after catching as the mass would have needed on a rocket anyway so you just have an incredibly dangerous game of catch for limited gain.

Answer 3

Gerald Bull's work on Project Babylon in the late 1980s was very promising and it seems likely that his system could have worked to get payloads into orbit. His systems were essentially very large conventional artillery pieces.

He was assassinated in 1990.

This has somewhat discouraged further research into this area.

Answer 4

If you launch a lump of metal from Earth's surface, then after less than or equal to one orbit it will re-intersect the surface of Earth. This is obviously the end for it.

Therefore any gun proposal to launch into orbit must have a way of applying a force to the object well after it has left the gun in order to circularize the orbit.

Quicklaunch solved this by launching a rocket out of a gun, the rocket would circularize the payload once it was up in space.

The effect of the atmosphere scales down as the payload gets bigger. For a sufficiently large pointed payload (e.g. 10,000 tons) it's almost as if the atmosphere isn't there. For a sufficiently small payload, getting to orbit from a gun is impossible - it will burn up in the atmosphere.

In summary, a small lump of metal with no plan for how to circularise is not feasible.

If you fix these two problems you get quicklaunch, which was technically feasible but apparently didn't make it off the drawing board.

https://en.m.wikipedia.org/wiki/Quicklaunch

Answer 5

A different approach to this topic is Skyhook assisted mass drivers. You don't have to get the ballistic container into orbit, just to the skyhook. From there the skyhook can grab it and fling it with pre-existing momentum. You would actually use another mass driver on the moon to fire the containers back toward the earth. The skyhooks would collect and slow the drivers while stealing much of the kinetic energy. Missing the skyhook from the moon would not be as big of an issue as the material could approach the earth at only a few km/second and the containers could slow their own decent on their own using oxygen mined from the moon, (40-45% of lunar regolith is oxygen. We will likely outstrip our storage of oxygen with the metals we will be mining which makes up most of the rest of the 55-60% of the regolith) This compressed oxygen would also allow limited maneuvering for whatever you project from the moon and whatever gas we choose to launch from earth, probably hydrogen due to its low weight.

Your idea of feeding lunar bases raw material is somewhat flawed however. While this could allow raw materials to be sent to the moon it is more likely that the moon will need specialized finished products or comforts and in return, the moon would send raw materials to the earth and materials that are easier to manufacture in zero-G or in an atmosphere that is effectively non-existent. The moon is an abundance of resources that may be far easier than mining on the earth while avoiding GHG emissions. While sending material to the moon may help initially you also have to have a method for collecting the material on the moon which is actually harder than on the earth where you can use the atmosphere to slow the projection and designated water locations for collection.