This leads to some interesting gun and bullet design possibilities.
First off, you go smooth bore. Without an atmosphere, there is nothing to cause the bullet to be unstable, or to drift. Spinning the bullet costs energy that is better put into more velocity.
Your bullet doesn’t need to be streamlined in any way. You probably want a point to make it feed well, but a plain cone is fine. If that isn’t a concern, round balls are fine, and will minimize mass for the diameter, which will give maximum energy. You won’t lose any velocity due to drag, so balls it is.
It doesn’t need to be lead. Lead is used in an atmosphere to maximize the stored energy needed to overcome the air drag due to a given projectile size. Light projectiles start faster, but quickly lose speed in air.
The muzzle blast won’t look like on earth, and you want to steer it upward so you don’t kick up moon dust.
You probably want crimped primers and bullets, as trapped air might push a primer or bullet out when exposed to vacuum, causing feeding and reliability problems.
I remembered that story, but I couldn’t remember the title or author. Just the premise a shooting war on the surface of the moon ends up with hundreds of high-velocity satellites orbiting the moon ready to collide with anything standing in the periluna area (the bullets returning to the original “target” of the shot).
The image (if I’m remembering correctly) of zones marked out on the ground to avoid being hit by the next return of the bullets stuck with me.
While it’s true (I think) a smooth bore shot will be faster, you cant assume accuracy. It’s unlikely the bullet will fire absolutely straight and parallel to the barrel, so it will randomly carom off the barrel walls and be travelling somewhat off course as it leaves the barrel.
And they also have alarms when the computer says the next volley will hit, so everyone hides as the bullets rip through the buildings, and then repair crews come in and patch the holes. Life goes on.
But they’re secretly building a wall that will stop ALL the bullets. Then they’ll show those rooskies what for!
Any bullet you fire at the Earth is just going to burn up in the atmosphere.
But that got me thinking about an awesome science fiction weapon. In The Moon is a Harsh Mistress, the Loonies used mass drivers to hurtle huge rocks at the Earth, to pulverize ground targets.
But then I thought, how about making making a lunar ‘shotgun’ that just keeps firing small rocks in quantities enough to pollute Low Earth Orbit and deny the Earth the ability to use space? Can we make that work theoretically?
I was thinking of a metal cylinder full of rocks in a linear accelerator. it just launches like a sabot round - the rocks fly out the end and the cylinder just goes along. Or maybe it’s designed to explode in a way that launches the rocks into many different orbits. Maybe it stays intact until more than halfway to earth, then it explodes and sprays the rocks all over the place.
If it was cheap enough, you could just keep flinging rocks, thousands at a time, for days, months, years. The threat of that would be a pretty big hammer to threaten earth in a science fiction story.
One big problem is that the rocks aren’t going to orbit the Earth - they’re going to be in an elliptical orbit coming back to the moon, and any kind of shotgun explosion along the way risks having some of those rocks rain right back down on the moon.
I do like the idea of a rifle that you can use to plink at Earth. We might not have one available now, but if you really had a need for a 5 km/s rifle on the moon, it could be done. Without atmosphere to pressurize the front of the bullet, you could just make a bigger charge and use a longer barrel. It shouldn’t be that hard to get there. It might even be hand-holdable in 1/6 G, so long as it’s braced.
I can see it as the new Lunar recreation - try to shoot the Earth.
So, how do you miniaturize a light gas gun into a rifle? How do you design the ammunition so you can replace the consumable elements in one go? (the burst disk and projectile assembly. You’d fuel the gas gun with a hydrogen/oxygen explosion behind the piston, which would need some mechanism to auto-retract.
No, because the bullet is not traveling at 2380m/s throughout the trip. If you fire it correctly, at 2381m/s, the moon’s gravity will cause the bullet to decelerate to the edge of the moon’s hill sphere, until it reaches 1m/s and crosses over into the earth’s hill sphere. At this point, it will accelerate steadily until it reaches pretty close to earth’s escape velocity (11.2km/s). You have to use the old “d=1/2at[sup]2[/sup]” formula, or t=sqrt(2d/a) to calculate the travel time (once for outbound from the moon and again for inbound to the earth). This is additionally complicated by the fact that the earth/moon system rotates around a barycenter that is 1700km below the surface of the earth, so the relative position of the earth will not resemble a solid rock sitting in space but, given the bullet being fired in the correct retrograde direction, the earth will rise toward the bullet. And if that is not enough to make your head hurt, you must adjust your calculation to account for the local effects of time dilation due to general relativity, which are pretty small but not quite irrelevant.
It seems to me that you could make the time arbitrarily long, by nudging closer and closer to the threshold of minimum possible speed (assuming sufficient precision, at least).
Have your cylinder spin about its axis and then fractureas it approaches Earth, creating a cloud of debris. The rocks don’t need to be large; they can range from the size of pea gravel to coarse dust, which at Lunar return speeds would be sufficient to destory any orbiting object. And you don’t need your rocks to keep orbiting; after demolishing enough satellites, the debris field will become self-sustaining with debris in permanent orbits occupying a wide array of altitides and inclinations, denying space access for the foreseeable future. This would be frighteningly easy using Earth-based weapons, much less something cheap like a practical mass driver on the Moon.
Good idea! It seems to me that this would be a really credible threat for a nascent moon colony or any rogue power to use to threaten the Earth and force concessions. Unless a lunar mass driver for huge loads which would be harder to camouflage, you could make small ‘guns’ that are portable and be almost impossible to find from orbit.
I don’t know that I’d call it a “good idea” but it is certainly plausible. Denial of orbital space by deliberately spawning debris is shockingly easy given basic orbital launch capability.
Stranger will probably be along to completely contradict this, but I don’t think that temperature in a vacuum (or near vacuum) is going to have any effect on the gun powder or primer (it wouldn’t have that much effect in atmosphere either, assuming it remained dry), if that’s what you are asking. It should ignite and fire pretty much exactly like it would in atmosphere at normal temperatures.
The rifle will be whatever temperature it is when you haul it outside. Vacuum, as it were, has no temperature. It is an insulator (as noted in an earlier post). It is not “cold” on the moon, the way Antarctica is cold. If you keep the rifle in a heated case, it will remain that temperature when you take it out, very, very slowly radiating heat. If it is in the sun, it will absorb heat (radiated from the sun) and get hotter, but the only real temperature on the moon is how hot the ground is. If the ground is, being n shade, -385°F, it will not be making the space around it colder, because there is no medium by which it might carry off heat. If it is 250°F, it will be radiating heat, but not really intensely enough to matter. Whatever temperature your rifle is when you take it out, that will not change very much, until you shoot it.
The strategic problem with this is that a nascent moon colony is going to be far more dependent on space travel from earth than earth is. The landlubbers would say, “You want to destroy the transportation that makes living in your tin cans tolerable? Go ahead. We’ll figure out a way to clear the debris eventually, but it will most likely take us till after you starve to death. In the meantime, we’ll be fine down here.”
They’re much better off threatening to drop big rocks on people’s heads.