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I've been wondering how large a payload could be launched & if it's viable to launch those payloads directly to the earth for splashdown & collection.

Obviously the payload would need to be designed not merely for reentry but also to float but I'm not overly worried about that side of things, the cost of the implied project & necessary on site support industries also aren't considerations I'm interested in for the time being.

Here's a YouTube video showing the technology I'm referring to.

U.S. Military Electromagnetic Railgun

The US Navy cannon in this video is advertised as firing a projectile at speeds exceeding Mach 7 (so faster than 2.401 km/s as well as the 2.38 km/s escape velocity of the moon). So we know we have the technology to do this with the 23-pound projectile the Navy uses with that gun.

All I'm interested in for the purpose of this question is how large a payload could reasonably be launched from the moon to the earth using this technology & the size of the gun needed for that.

However those two in combination may be too broad so lets refine it down to a specific payload.

  • Can this be scaled to deliver a 24,000 pound payload to space from the moon.

And if it can.

  • How large is the rail-gun that can do that going to be.
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    $\begingroup$ @Dragonreek (etc) "put on hold as primarily opinion-based" please explain how this is opinion based, the answer is going to be a function of the available technology & the math. $\endgroup$ – Pelinore Apr 6 '18 at 13:38
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    $\begingroup$ The math is fine, it's the available technology that is the issue. It doesn't exist, it would have to be developed. We know it can be done, but, for instance, what is the budget for this? How can we really anticipate how much it would cost, even knowing a budget? How can we anticipate the engineering difficulties of the lunar environment for large projects, with no experience? There are approximate solutions for all of this, but after a few approximations, it's really just opinion. One way to refine the question would be to ask about the known difficulties of doing it. $\endgroup$ – kim holder Apr 6 '18 at 14:01
  • $\begingroup$ @kim holder : thanks for that, I'll give some thought to that, budget is something I want to ignore though I was thinking the materials for this would be mined & refined on site (because getting them all up there for a canon of the size I was thinking of is surely not something I'd want to foot the bill for). $\endgroup$ – Pelinore Apr 6 '18 at 14:13
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    $\begingroup$ @Pelinore grain barges, perhaps? en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress $\endgroup$ – Organic Marble Apr 6 '18 at 14:29
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    $\begingroup$ @Organic Marble : Perhaps, I can't deny a certain amount of inspiration for this question has come from a certain author, I see you've edited the link in now :) $\endgroup$ – Pelinore Apr 6 '18 at 14:31
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Short answer:

Yes. Big.

Long answer:

There's no law that says this technology doesn't scale. Railguns functionality is dead simple. You apply a current to two rails and bridge across which produces a large perpendicular force with magnetic fields. The reason I originally voted to close this question was because asking if something is feasible is pretty broad and easy to answer (does it violate the laws of physics? no --> It's feasible).

The math is pretty simple, I made a spreadsheet which uses the formula off of Wikipedia and uses their suggested value for the "inductance gradient", which is very optimistic.

Spreadsheet

As you can see, we can shoot 10mt off of the moon using a 10km long ideal railgun which has around 100000 amps running across it. It draws a ton of power which would need to be stored somewhere to be able to discharge quickly and the projectile would accelerate at around 30G.

This calculation is a careful balance between rail length and the current across the rail. Actually constructing this would have a lot of engineering problems though such as:

  • Projectile heats up and melts in the railgun
  • Transferring large amounts of current over a sliding contact is difficult
  • Storing this much energy is difficult
  • Designing manned or even unmanned payloads to withstand 30G or higher is difficult
  • Many, many more problems

Overall verdict: Magnetic based launcher designs (rail or coil) have enormous advantages for launching however are so expensive that estimating their cost today, especially for a moon based launcher is basically impossible. Building something like this would require a large launcher tube (probably in the 10's or 100's of km in length), large power storage capabilities, and large power generation capability.

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  • $\begingroup$ "expensive" that doesn't seem to match with the US military's opinion, around 1:24 on the video he appears to be saying that it's far cheaper than the rocket alternatives, they may be (very, extremely even) expensive but still for comparable payloads potentially far cheaper than our more traditional methods (rockets) surely. $\endgroup$ – Pelinore Apr 7 '18 at 15:36
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    $\begingroup$ The difference is where the cost is. In a rocket system the launcher costs essentially nothing whereas in a railgun system the projectile costs essentially nothing. Using a railgun launcher system is dirt cheap--but only once the thing has been built. After that you're essentially only paying for electricity costs. The reason the military says it's cheap is because when you've got a 22.5 billion dollar boat with nuclear reactor and a 500 million dollar railgun, a small 25,000 dollar shell is nothing (also compare this to the traditional costs of missiles often in the million dollar range). $\endgroup$ – Dragongeek Apr 7 '18 at 16:56
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You're going at this the wrong way. What you are showing is meant for mobile use. That means it runs at very high power densities and you're going to get some nasty scaling problems due to the square-cube law. I would consider your mission probably impossible because of this.

Your requirement to float is almost certainly impossible--given the extreme acceleration it's going to be hard to protect any open space inside the projectile. To have any chance of this you would need a block of metal foam--and that's something you make in space, not on the surface of a body.

However, since you seem to be describing a cargo transport rather than a weapon you don't need it to be portable. Don't think railgun, think magnetic levitation train. It will be very long but is certainly possible (moving 24,000 pounds is no big deal. Any problems with the square-cube law can be solved by lowering the acceleration.) You also have the advantage that you don't need specialized projectiles--you can accelerate some sort of bucket, release the payload and then catch the bucket again. Making it float is no big deal.

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