As is my usual weird habit, I’ve started wondering: would it be possible to build a functioning cannon out of concrete, or carved out of stone? And what kind of rock would be ideal (or at least, the least-awful) for the latter?
People have built wooden cannons, and even leather cannons—the latter being more of a relatively thin metal gun tube, reinforced and protected by a leather casing—neither exactly measuring up to a metal gun, in terms of overall quality.
But hey, I’m always up for variety, and for getting input on a weird-ass thought experiment. So, could you make a stone gun (say, for the purposes of argument…a six-pounder)—especially without “cheating” and using an inner metal barrel, a la the Leather Cannon above? And how thick would it have to be? Could one be made anything close light enough to be practically aimed—or even transported?
Stone and concrete is poor in tension and tension is what you have to resist when creating an explosion in a confined space. A lot of the newer fiber reinforced concretes are decent in tension, compared to your standard mix, but they’re still not great.
Is rebar allowed? Can it be tensioned? What about using way more rebar than is usual in a concrete structure?
Think about how they blast rock? They bore a hole, pack some explosive in, bung up the exit, and BANG! The rock shatters. Ok, that’s a relatively small hole with a load of explosive - cannon used to explode if the gunners got a bit overenthusiastic with the gunpowder.
I guess that if you judged it right, you could send a projectile out of the hole instead of breaking it. Drill a hole, use a modest charge, and ram a round ball in after some wadding. That would do it.
Like someone else said, concrete is poor in tension, that’s why steel is used, because it is strong in tension. During combustion, the barrel is like a gas cylinder, and the hoop stress on the cylinder is a form of tensile stress. Imagine the molecules being stretched out (tension) rather than being squished together (compression). You could make one with concrete and steel reinforcement but it would be big and heavy. Plus, I imagine the inside would deteriorate rapidly with the changes in heat and moisture, not to mention any adverse affects from the residual chemical components used for explosives.
All depends the nature (size, power, speed of burn, etc) or the propellant charge relative to the projectile, also, wrt to eg. duct tape etc cannons on ‘Mythbusters’, does the chamber have to retain its shape after the first shot, or is it a success if the whole thing just doesn’t burst? If you relax the criteria sufficiently, you can certainly make a stone or concrete device that ejects a projectile under force of a chemical propellant. Can you build a stone or concrete cannon that could repeatedly, safely, accurately fire say a standard 155mm projectile with standard full propellant charge? That would surely require stretching the definition of ‘stone’ and/or ‘concrete’ if possible at all.
Concrete, as it is conventionally made, is poor in tension, as you said. However, there must be ways around that. After all, they make bridges out of it. How about using chopped up old tires for the aggregate?
I don’t pretend to be a structural engineer. If I’m wrong, you needn’t go to a lot of trouble to bury me in physics.
The heck with speculation (informed or not). Submit the question to Mythbusters. If it involves shooting or blowing something up, they’re generally happy to give it a try.
No, a rubber aggregate wouldn’t work. It would only make the concrete lighter and weaker. The concrete would simply crack all around the rubber pieces under load and smash into a million pieces. What does work is steel reinforcement. You are right–they do make bridges out of concrete and the amount of concrete is huge, volumewise, in comparison to steel. It’s the same thing with concrete beams rather than steel beams. That’s why I said you could make a cannon out of concrete but it would be big and heavy.
Mythbusters did it with ice. Aaaaannd- it didn’t work. They had to scale back the charge a lot, then it blew up on the second shot.
Basically the first shot cracked the ice so the second shot blew it up. I suspect the same would happen with concrete. Of course, you could over build it to the point where it’d be safe- say 3 feet of concrete and a 3 inch bore. But that’s pointless.
Interestingly they did find that ice cannon balls do work, even tho ice bullets do not.
Mythbusters was overly limited by the size of the tubes and the weight of the ice in that episode. Yes, clearly ice in those quantities doesn’t make a workable cannon, but if you made it far thicker would it work ? I mean with a standard sized cannon ball and charge, but basically unlimited wall thickness. Yes, I realize that is impractical in southern california, but what about in say, northern alaska in the winter ? It probably would be possible to build the cannon to any arbitrary large size - but would the inside then shatter after the first shot, or could you fire it several times ?
Basically the same deal with stone, except way more materials choice - is there a type of stone strong enough to withstand the blast and not so brittle that shatters the inside on the first shot, even if the walls are 10 feet thick ?
I think the key is that duct tape, brass, iron and steel have much greater strength, no brittleness and some small degree of flexibility. Think of a gun barrel being made from hardened knife-edge steel and the reason a brittle, inflexible material won’t work should be clear.
I wonder how a cannon made of pykrete would do. Pyke-what? It’s fiber-reinforced ice. About 15% sawdust and 85% water.
Back in 2009, Mythbusters determined it was significantly stronger than plain ice, and even managed to build a boat-like thing that floated for a while. IIRC, their pykrete was able to survive being shot, but plain ice shattered.
To answer the question, we need a definition of “functional.”
How big a shell? How far does it have to go? How fast must it go.
I’d bet serious money you could design a concrete or stone “cannon” that would shoot a marble-sized projectile a few feet. But I wouldn’t bet it could do damage to anything more substantial than toilet paper.
Newer high performance concrete can have around 4 or 5 ksi tensile strength, about ten times your standard concrete. Still quite a bit below steel.
I think if you pre-stressed the concrete you’d be alright. Wrap it in bands and put the whole thing in compressing like is done in bridge girders. You could probably get 30 ksi usable stress out of it. I’m not sure about dimensions for a cannon, but I bet it’s doable in a reasonable size.
Years ago, some Big Science company (Dupont? Union Carbide? Dow?) had an ad campaign showing a fabric layer in between layers of road-building stuff. Their point was that such pavement would last longer.
In real life, such a thing falls in the category of “the old way is much, much cheaper.”
Watched this last night. I was really surprised that after the first breech test they didn’t also make a pykrete cannon for comparison. Under their rule of “simulate the effect”.
Ice isn’t really a fair comparison to stone/concrete as per the recent thread on the hardness of ice.