Steam Bomb

[Gedrin]

The World: TL3^, transitioning to TL4^, low moderate magic using Threshold Magery.

The Prototype: A hollow iron sphere, filled with water. + The Heat spell.

Use: Sphere's are staged in the combat area and modestly heated using conventional means. The idea is to pre-heat them to conserve magical power. "Arming" the sphere consists of casting the Heat spell on it. Spheres are launched using whatever means is available, balista, catapult, angry ogre.

Theory: The sphere fails, likely on impact, and the superheated water creates a steam explosion.

The spheres, constructed using Shape spells, could be stockpiled in peacetime. A combat effective weapon would probably require developing a "Fast Heat" spell or extensive study of the impact characteristics and temperatures needed in combination to heat safely and detonate properly. While magic could produce similar explosions, in this game world, the number of Magery 0 people about 10 times that of those with Magery 1 (and access to Explosive Fireball). Additionally, I suspect the power to boom ratio is better for such a weapon.

I'm sure this isn't the first time it's come up. Is there a DF on this, or another resource that would be useful? Also, is this use of steam power appropriate at TL4? Pressure cookers are mid TL4, and mass produced in late TL4. I'm guessing pressure cooker explosions are also mid TL4, if not sooner.

[Dwarf99]

on a similar note, I wonder how Essential spells would change things up.

[Gedrin]

Originally Posted by Dwarf99

on a similar note, I wonder how Essential spells would change things up.

I'm sure it would make things more potent, particularly "Essential Metal" (higher contained pressure). I was really looking for an anti-personnel artillery weapon that integrated magic, but could be widely deployed using as little magical training as possible. The one that jumps out at me is: What if it wasn't just water in there? Aerosolized, steam heated, shrapnel filled, bad gas. I can think of some people who'd do this, but they wouldn't be the one's to invent the base steam bomb.

Using Heat gets me the weapon at 4 spells, Magery 0, and the associated spells have solid utility. In peacetime they'd have a head start on Extinguish and Fireproof, which would be good if they stay in the military or just want to join a civic fire-brigade.

[johndallman]

Your "Fast Heat" spell needs to give a lot more heat per FP than the basic Heat spell. Taking the temperature up to about 212F (about 8FP edit with ordinary Heat, which uses up one basic caster) will start the water boiling, but it needs to get a lot hotter to develop significant steam pressure.

A pressure cooker explosion is not much like a grenade explosion. The lid of the cooker yields and comes off, releasing the pressure. You don't get the shattering effect of an explosive, so you don't get lots of small fragments moving at high speed, which are what do the damage of a grenade.

[Turhan's Bey Company]

Originally Posted by Gedrin

Pressure cookers are mid TL4, and mass produced in late TL4.

Invented kinda late TL4 (circa 1680, IIRC, which is significantly closer to 1730 than to 1450), and not, to the best of my knowledge, mass produced until the middle of the 19th century. On the other hand, the problem with early pressure cookers was that they exploded messily, which is exactly what you want here.

That said, I agree with what has been said about needing a Heat variant that pumps a lot more heat into your water in much less time in order to get a useful explosion happening. You might get more mileage out of adapting the steam cannon from Fantasy-Tech, using a team of magicians with Heat to crank up the temperature of some sort of heating element which then gets water pumped in for the steam-flash propulsion.

[Running Wolf]

Wonder how some sort of "crushing" spell might work on the water? Would be a lot more bank for the buck. Compact 5 gallons in a 1 gallon metal shell. Would allow for a variant of a heavy duty metal shell. Or one could be a anti-troop device and the other could be a normal building bomber.

Just my two cents.

[Anthony]

Originally Posted by Running Wolf

Wonder how some sort of "crushing" spell might work on the water?

Under RAW, that implies allowing Shrink Object, at which point it's hard to justify a lack of Enlarge Object, which is a spell notable for the ability to destroy planets for fairly modest fatigue costs.

[Gedrin]

Thing is, I don't need to heat the bomb all the way to critical via magic. I can heat it to volatile but stable via plain old fire. The only reason Heat is needed (or it's new brother), is to push it over the edge after launch. There's probably a lot of "margin of safety" research in the R&D effort.

I'll be sure to check out the steam cannon. However, I want to avoid "teams of mages". They're beyond the resources of this idea. A large coal fire pit to pre-heat bombs is a better deal.

The shrapnel point is a good one, but I'm imagining projectiles of significant size, 300-400 pounds at the big end. RL trebuchet's could manage that without the fantastic materials available IMC. However, the point about shrapnel is well taken. I think this could be overcome by properly shaping the sphere to fragment. That would be a later innovation to the same weapon, but likely a significant one.

[whswhs]

Originally Posted by Running Wolf

Wonder how some sort of "crushing" spell might work on the water? Would be a lot more bank for the buck. Compact 5 gallons in a 1 gallon metal shell. Would allow for a variant of a heavy duty metal shell. Or one could be a anti-troop device and the other could be a normal building bomber.

You can't really compress water. Solids and liquids have a natural density that doesn't vary much. The only thing you can do is change them into a different allotrope, whose properties may differ as much from those of water and ice as they do from each other.

Bill Stoddard

[Anthony]

Your basic problem here is that your container will rupture long before you achieve any useful level of damage, unless the containers are very large. Sure, a steam boiler explosion is dangerous, but that's because it's huge.

If we consider a 1m sphere with 10mm thick walls, it has an external volume of 0.52 cubic meters and an internal volume of 0.27 cubic meters; the shell has a volume of 0.25 cubic meters and a weight of just under 1 ton, and it can hold about a quarter ton of water. The shell has a cross-section of 0.28 m^2, the inner cross-section is 0.5m^2. If we assume steel with a yield strength of 400 MPa, that means it will rupture at 224 MPa internal pressure (this assumes no weak spots, which is unlikely) and the total energy contained as pressure is 90 megajoules.

If we took the same design and filled it with TNT, it would detonate with an energy of 1,700 megajoules. Black powder would be maybe half that, and might not produce as efficient an explosion, but would still be vastly more powerful than the steam bomb.