Help me stat a steam thrower weapon

[Varyon]

Originally Posted by Fred Brackin

The steam chamber is probably the msot dangerous thing to the user (specifically the part whe it builds up and then holds the pressure) but the weak link in the superheated steam projection system is the flexible hose connecting the chamber to the wand part. There's no way that's going to withstand the same pressures as the chamber.

Part of me feels it would be interesting to get rid of the hose and wand - rather, you've got a solid tube of (heavily-insulated) metal coming forward from the backpack with a release trigger on it, allowing the character to quite-literally shoot from the hip.

Alternatively, you might be able to merge this idea with that of a handheld steam-cannon. Have a backpack of lower-temperature steam, with a hose connected to a large "wand" that has part of it highly-heated (like the chamber of a steam cannon). Pull one trigger, and it injects steam from the backpack into a reservoir on the wand. Pull the other trigger, it opens the reservoir to the heat source and the exit, and you superheat the steam, either for a brief burst of water-cutting or to propel a projectile. The two-trigger system (necessary so the superheated steam doesn't flow back into the hose, destroying it) might give the weapon RoF 1(1), although sufficient training (I'd probably just make it a Perk) would let you fill the reservoir and attack in the same second, for RoF 1.

As for the general idea, it's really not a realistic weapon, but it's a freaking steam-powered water cutter. Provided the campaign is cinematic/quirky enough for it to fit, that's a very interesting and cool signature weapon for a post-apocalyptic character. You could also potentially create attachments to allow it to use the pressure to fling projectiles at enemies.

[Anaraxes]

Dictionary of Danger (GURPS Action) has some rules for heat. The 4d specified in the OP rates above "interior of industrial kiln" or "thermite fire" (3d/s) and below "interior of huge foundry furnace" or "unshielded reactor" (6d/s). So that seems overly optimistic for the parameters in the OP.

Fooling around with some online calculators, for the 1 qt bottle at 250 C in the OP, I get a heat loss about 35 C per hour, assuming the bottle is a squat cylinder about twice as high as it is wide and that it's wrapped in an inch (2.5 cm) of fiberglass batt insulation (because post-apoc salvage), and that it's full of water (a kilo). You could be issued your ammo before a battle, but not pack it around with you while hiking for a couple of days. Damage would decline over time; after six hours, it's down to at worst the "touching a hot radiator" level, 1 point of damage.

Also, that's about a 20% increase over the thermal energy radiated by a human, so the steamthrower will be pretty visible to thermal imaging devices. And probably extra uncomfortable on a hot day.

Not my area of physics, though, so those results are worth checking.

(I assumed the weapon-wielding isn't actually carrying the heat source with them, operating while they run around and fight. That would be more wacky steampunkish, for sure.)

[clu2415]

When you hear stories of a steam leak cutting a broomstick in half, that was a steam line pressurized to 1 000 psi. The question becomes how are you going to build a vessel and flexible tubing that can contain 200, 500, or 1 000 psi? Secondly, how much fuel will you need to heat and pressurize the water?

[johndallman]

Originally Posted by Green-Neck

My GM is going to run a post apocalyptic game and I'm trying to give my character an interesting weapon. The campaign is semi cinematic and set in near-ish future Australia.

I'm imagining a weapon that looks much like a ww2 flamethrower, except it uses solid fuels like wood and coal to heat water to superheat steam.

I'm thinking of a 30 pound backpack with wand. A brazier that preheats a water tank, then fills a one quart steel tank that heats to about 250 C/480 f, that's about 40bar/600 psi.

The tank would have a temperature solenoid and thermo-electric-generator powered fan to moderate the fire.

This is very cinematic. The things that stretch suspension of disbelief with a little knowledge of steam engineering include:

• Superheating is a process of getting more heat into steam, after it has become steam. It needs to be done separately from boiling the water, because if it isn't, you just boil more water. So you need more plumbing and chambers, and any backpack superheater is implausibly small.
• You're going to run through any carriable amount of fuel terribly quickly, if you're burning it fast enough for superheating. However, a thermo-electric powered fan will not have enough power to get air through the fire quickly enough for that. Using a small steam engine to blow the fire would work better.
• Corrosion. Very hot water under high pressure is horribly corrosive stuff, unless the water is really clean, as in distilled. Maybe distilled twice, in a post-apocalyptic setting. Classic steam locomotives didn't use very high pressure or temperature. Being next to a bursting boiler was deadly because there was enough steam to scald your whole body, plus flying chunks of metal, not because of high temperatures.

[Green-Neck]

Thanks everyone for your replies.
The proposed weapon is certainly cinematic and not realistic.

I'm not trying to convince people that it's realistic, I'm just trying to explore the functioning of the thing, and then making a 'more realisic' version of a fantasy item.

The proposal was for a backpack with a rocket stove style heater incorporated,burning 1KG coal or wood to heat a10 litre tank of water (with a pressure release valve) to 90 degrees C. A seperate 1 litre tank is heated by accelerating the fire with a TEG powered fan to 250C/ 600 psi just prior to combat. This is released through a wand with a waterjet cutting tip to collminate the stream.

Small non-fan assisted rocket stove can acheive a exhause temperature of 1000C or higher. The fan is to increase heat more quickly during use.

I never realised how dangerous superheated steam was until i talked to a retired worker from a 650psi turbine coal power plant. He said if you heard a high pitch shrill noise, the workers would take their pants off and walk slowlytowards the exit swinging thier pants around. if the pant got cut in half you found the leak. That was 1970's Austrlain health and safty for you.

600psi steam was piped around battleships during ww2 and leaks were detected by waving a broom around until it was cut in half by the invisible pin-hole steam leak.

The materials to contain 600 psi are non-problemic, ie tanks and hoses are common in our society, and a tour of an industrial estate will provide tanks to 3000 psi and hoses to 60,000.

1kg of coal contains 7000 kcal, 1L of water takes 250 kcal to heat. that means the sytem is about 35% efficent, which is optimistic but not crazy.

The campaign will feature black powder weapons and laser rifles with highly limited ammunition. I was trying to essentially have a flame thrower, but to fit the game world better with liquid hydrocarbons being too valueable to spray around. From a meta gaming perspective I used 4d based on having 1/3 the rate of fire of a flamethrower, and not wanting to hamstring my PC too much. This is probably too much.

The suggestion tha the weapon should be more based on cutting damage, with a smaller portion of heat damage is probably more realistic.