Suppose we get attacked by Unicron in the not so distant future and need to build something like the Sister Ray to defend the Earth. Of course in the real world, we don’t have mako energy, but we do have nuclear energy. So let’s ignore the economic and engineering hurdles it would take to build this. All I want is a design that would work. Here are the requirements. The cannon can be on land or in space. (I don’t know if shooting a projectile at such extreme speeds through the atmosphere would be such a good idea.) It has to fire a pretty massive projectile, at least 1000 lbs or 500 kg. The projectile has to be moving at extreme speeds, 1 km/s or greater. It has to be pushed through the barrel by a nuclear explosion, fission or fusion. It’s acceptable if the cannon has to be rebuilt after every shot, but the projectile obviously has to stay mostly solid.
I was making some calculations today. If they’re correct, a steel cylinder 8" wide and 17" long would be roughly 1000 lbs. With a barrel 300 yards long, it would take a constant pressure of less than 3,000 psi to push the projectile out at 1 mile per second. That’s not really a lot of pressure so a nuclear explosion should easily surpass this. So I guess it all comes down to whether or not the pressure can be kept low enough not to blow the cannon up and if temperatures can be kept low for that split second.
Projectile weight= 1900 to 2700 lbs, muzzle velocity= 700 to 800 m/sec.
Admittedly, they use chemical, not nuclear propellent. (You can make chemical propellent work in an airless eniornment, with the oxydiser in the propellent charge.) Heh.
Neither the size nor speed of the projectiles you mention is “extreme”. 16" battleship guns fire shells that way around 2000-3000lbs.
Given that the Orion spaceship concept demonstrated the feasibility of launching large vehicles into space using nuclear bombs (without vaporizing the ship) I think that you could do away with the idea of a “cannon” (in the sense of a tube that contains the explosion and shoots a projectile out the open end) and build something more like an Orion fragmentation grenade - you take an atomic bomb and put a projectile near it, aimed so that when the bomb goes off the projectile gets launched at the target. You don’t want to be anywhere nearby when someone pulls the trigger of course!
If you want to contain the atomic blast that’s a little tougher. The only way I’ve heard of it being done so far is to bury the thing under a thousand feet (+/-) of rock, and it’ll still vaporize a chamber a few hundred feet across. You could do something like bury the bomb deep underground in a chamber with hundreds of independently aimed gun barrels leading from the chamber to the surface, each loaded with a single projectile. Aim all barrels at the Invading Alien Horde and then light off the bomb. The mechanism would certainly be destroyed but you’d get quite the claymore effect.
Since you’re allowing fission or fusion energy, you could also consider some form of coilgun or railgun. A reactor hooked to a bank of supercapacitors could fire some hefty slugs at the target velocity, given the lack of engineering constraints you’re allowing. From the Wikipedia railgun article:
I didn’t know we could fire such massive slugs to such speeds already using chemical propellants. That’s pretty amazing. Well let’s make it simpler and say the theoretic projectile has to have significantly more kinetic energy than any actual projectile fired via chemical propellants. It can’t be an open explosion, but it can use the ground as a firing chamber. Anyone have any idea how much kinetic energy we could get from a projectile?
While interesting, that doesn’t use a nuclear explosion as a propellant which is what I’m going for here.
The pressure wave in a nuclear weapon is created when gamma radiation superheats the air around the bomb, with an energy exchange that increases the wavelength of the photons. The heated air then does what heated air always does. What’s remarkable is how quickly this can happen - just a few milliseconds is enough to superheat a pretty good chunk of air.
So all you want out of your gun is a good way to create a large pressure differential. A nuke is probably a pretty wasteful way to achieve this and it’s kind of “inside the box” thinking.
I think there’s a lot more potential in rail guns. One of their key advantages is the ability to push the projectile through the entire barrel length. A particle accelerator is really nothing more than a rail gun for very small projectiles, and the circular path means you can accelerate things to virtually as fast as you want.
Build a large enough accelerator and your only limit on muzzle velocity is the atmosphere, since you don’t want the projectile to fragment and explode in your face.
Put it in space and your only limit on velocity is your ability to keep curving the projectile’s path with magnets. You could do as much damage with a handful of sand as all the nukes in the world put together.
All explosives, even low explosives like black powder, are self-oxidizing. You can fire a bullet underwater or in space or deep inside a rock, as long as the moving parts of the gun work.
A substance that doesn’t contain its own oxidizer is a fuel, not an explosive.
“A lot”. It’s limited by the size of the bomb and what percentage of the energy released you can turn into propulsion.
For example a tiny fission bomb yielding 10 kilotons is releasing the approximate blast of 10,000 tons of TNT. That is far, far more than any cannon charge ever used to fire a projectile. If you could get, say, 1% of that turned to kinetic energy of your slug that’s still 100 tons equivalent which if my math is right works out to around 400,000 megajoules. The last test of a railgun for the US Navy that I read about was on the order of 10 megajoules - that’d be for the main armament of a modern fighting ship.
If I were to design a reusable nuclear-powered cannon, I’d probably use a nuclear reactor to boil water into steam, and then feed it into a more or less conventional steam cannon. In fact, I suspect that this is probably what modern aircraft carriers use to launch planes: They’ve got a nuclear reactor or eight on board anyway, and if you need steam, that’s the simplest way to get it.
And note that while calculations indicated that the lid was traveling six times escape speed when it took off, it is unlikely that it kept going at that speed for long…
A few drops of steel probably left earth orbit though.
Ok, thanks. I was thinking that a lot of current explosives might not handle the extreme hot and cold of space. (Gun lubrication might freeze solid, for example.)
Another potential “advance” on the battleship type gun is rate of fire. Imagine the battleship sized gun firing a round every two or three seconds, instead of one to two rounds a minute.
What you might lack in accuracy, you make up for with a blizzard of shells.
The payload was small (94 kg/210 lb) but the muzzle velocity was 1600 m/sec (a mile a second). The gun was used in world War I to shell Paris from a distance of 120 km (75 mi).
The problem with railguns and Gauss guns (coilgun) is once you get up to the performance comparable to high end chemical propellants the stresses on and heat produced by the inductive elements will exceed material capabilities of those elements. This requires strong supporting structure and active cooling to increase performance, and at some point the internal stresses and thermal loads on the inductive elements will essentially vaporize them. So it’s not just power input that is required; the system also needs to hold together at least long enough for the projectile to clear the gun.
The problem is that (for a long cannon) the pressure will drop as the projectile moves and creates a larger internal volume. Unless you can continuously fill the chamber to maintain pressure then the acceleration of the projectile will slow, and even if you could contrive to pump the working fluid into the back of the chamber pressure transients and resonance feedback from natural modes (like resonance sound waves in a woodwind instrument) would prevent efficient force transfer. This is the benefit of using a chemical propellant; as the projectile moves down the barrel the combustion reaction is still ongoing and providing continuous pressure input; by controlling the grain size and burn rate you can keep the pressure reasonably constant between the ignition and egress transients and gain optimum energy benefit from the propellant.
The only way to effectively do this with steam is to stage the pressure input so that you inject gas just aft of the projectile as it transits the barrel. However, even that is going to have thermodynamic limits; once the projectile is moving faster than the expansion rate of steam at a given injection temperature and pressure the projectile will outrun the pressure wave and will never attain greater momentum; ditto for chemical propellants and explosives.
Regarding the question of the o.p., it is certainly possible to create a sacrificial nuclear fission cannon; check out Operation Plumbbob and search for “Pascal-B” to see where this was done accidentally. If you wanted to do this on purpose you’d place a tamper between the projectile and the Primary to convert x-rays and gamma rays to heat and kinetic energy, and then use the vaporized tamper to push the projectile. You’d still have to protect the projectile from the thermal pulse and blast effects to keep it from being destroyed.
However, this would almost literally be like attacking a mosquito with a nuclear sledgehammer; the amount of kinetic energy you’ll get into your projectile will be a tiny fraction of a percent of total energy output. This is why we (at least in concept, thankfully not commonly in practice) deliver nuclear weapons with large chemical rockets or bomber aircraft, although the Vought SLAM used the idea of a nuclear fission powered ramjet; the delivery vehicle was nearly as hazardous as the payload.
Perhaps instead of using a single nuclear charge to fire the projectile, using multiple sets of smaller nukes along the length of the barrel, like in the V-3 Cannon, might come in handy?
It might be easier to keep the projectile from disintegrating that way, at least. (Unless you have a Mithril/Adamantium slug ready; in which case you could use multiple Tsar Bombas and try to get it up to relativistic speeds. :D)