[UT] Plasma Torch vs. big metal door on a spaceship?

[Curmudgeon]

Originally Posted by vicky_molokh

Err, we have rules for 'cut out a 10ft² section' rules, and they have been cited, but they're mostly calibrated to wood and stone. I'm looking to getting some of those clarified in contexts of metal and/or different-sized holes. Even HT drills are described in such a context, but some things aren't quite clear in it.

For context on metals, I'd suggest ignoring DR entirely or effectively reducing it to no more than 1. This is based on layman's articles on real-life lasers and assuming that they would apply to both laser and plasma torches.

As I understand it, lasers initially heat up the surface of a material and as the surface heats, that heat penetrates more deeply as well as spreading laterally. Continuous beam lasers encountered a problem in that the surface continued absorbing heat even after it's transformation into the gaseous state because the laser beam effectively pressurized the gas preventing its escape and thereby degraded the efficiency of the laser. Pulse lasers were developed as a solution to the problem by briefly interrupting the beam to allow the heated surface gas an opportunity to "boil off" before resuming.

Assuming that laser and plasma torches have a pulsing interruption feature that allows the surface gas to boil off, it seems reasonable to say that the laser/plasma torch doesn't engage more than 1 mm of depth at any given instant, so it never has to deal with more than DR 1 due to thickness, so most of its burning damage is expended directly in attacking the HP of the wall.

[Joseph Paul]

Originally Posted by GodBeastX

Lowering DR makes no sense of the equation because you are lowering DR per location on the object. What I mean is, you cut a foot long slice in the door. If someone shows up with an assault rifle and shoots at it, the bullets are still going to bounce off.

Exactly this. I am not following the logic of the 'must reduce DR' argument. The materials that this applies to are ones that generally lose material adjacent to the area being hit until a larger piece of it fails - block wall, stud wall, fracturing a rock etc. That doesn't really happen with a heavy plate of metal.

Yet without lowering DR you will never be able to damage HP with a plasma torch.

Unless I have missed something this is only true for some thickness of metal material. Take that TL 9 4D+1(5) torch to a half inch thick plate and you are doing avg dmg of 15 vs28DR/5-~6DR or 9 HP damage per second to it. [edited to correct DR of .5" steel]


As I stated earlier all plasma torches (and lasers, gas-axes, and water jet cutters) have a limit on how thick a material they will cut. Just letting it sit there and arc into the plate doesn't work once you've reached that limit. With plasma torches that limit seems to be closely associated with the amperage the rig can handle. It looks to me (still) like the examples given are pretty light weight ( the Heavy Plasma Torch is all of 40 pounds and works on Dp battery) and the real answer is get a bigger rig with more power. To me this is the equivalent of asking 'Why won't my 37mm AT penetrate the front of that Panther?"

As for cutting speeds we may have to do what we do for vehicles - look at actual stats and punt. There are graphs of cutting speed vs material thickness.

And yes it may very well be that on a damage per second basis that looks like the torch isn't doing as well as its damage would suggest it should. I would chock that up to the operation needing a certain amount of 'overkill' to maintain the arc or waste in needing to control the motion and not kill the arc etc. Remember what you are getting out of it is a hole right where you want it with out a lot of shrapnel flying around.

[Varyon]

Originally Posted by Joseph Paul

Fred - here is the video I saw - https://www.youtube.com/watch?v=vsqFS7bbaMc

I would like to know how GURPS is supposed to stat something like this without it being a high damage paradigm.

Going off the YouTube timer, piercing starts at 0:13 (this is when the sparks start flying) and stops somewhere around 0:16 or 0:17 (when the sparks stop), for around 3-4 seconds there (not certain why the video says 2 seconds). The circle that gets cut out looks to have a diameter of approximately 2 inches (tough to say - the hole looks about as wide as the tool, which looks about as wide as the block is thick), and takes roughly 20 seconds (from around 0:18 to around 0:38 or 0:40) to cut all the way through. The second pierce is around 4 `seconds again (0:48-0:52), and that cut takes around 50 seconds (0:54-1:44). That shape is harder to quantify, but it looks like it could be approximated as a 3"x6" rectangle.

Assuming that chunk of steel were RHA (it probably isn't) or similar, that's 3-4 seconds to get through Cover DR 140, which the plasma torch from HT will do at maximum (or just consistently above-average for 4 seconds) damage if we just let things add up from round to round. Beyond that, we're looking at 20 seconds for a 2*pi in linear cut (the circumference of that circle), 50 seconds for 15 in. The first is 3.18 s/in, the second is 2.78 s/in. Considering the circle was done with two rotations - possibly just for added precision - I'd be tempted to go with 3 seconds per inch. That basically works out as something like piercing time per inch, which makes things a bit easier - work out pierce time, and multiply it by the length of the cut (or perimeter, if cutting out a section) +1 for the initial pierce. Realistically, torches have a maximum thickness - or perhaps just maximum DR - that they can reach. Looking at the specs for that particular model, that 2" pierce is actually around the best it can do, so something like a 3-second maximum may be appropriate. I don't know if the laser/plasma torches in UT work the same way, but it may not be horribly inappropriate to assume so.

For cutting into a thick metal door, then, a Fusion Torch (8d+2(5), or 50 damage vs 1/5th DR) has a piercing time of 0.2 seconds for a DR 50 door, 1 second for a DR 250 door, and at maximum can pierce through a 750 DR door in 3 seconds. Cutting a 2-yard by 1-yard "doorway" (to get through) takes P*217 seconds (perimeter of 216 inches, +1 for initial pierce), where P is Piercing Time - so around 45 seconds against DR 50, 217 seconds (~3.5 minutes) against DR 250, and 651 seconds (~11 minutes) for DR 750. Cutting a 1-yard diameter circle (to crawl/dive/fall through) takes P*114 seconds instead.

Originally Posted by Joseph Paul

So could vicky_molokh and safisher do worked examples of figuring out how long it takes to cut a 1'x3' hole in a 2" thick airlock door?

With the HPR400XD (the device from the video) and an airlock door made of steel, that's a 96" perimeter, for around 291 seconds. Using the Fusion Torch from UT, that 96" perimeter in a DR 150 or so airlock has a pierce time of 0.6 seconds, so you're looking at 58.2 seconds - about a minute to cut that hole.

These are all just rough estimates, but would probably work well enough for a game.

[vicky_molokh]

It has been recently pointed out to me that Thermite (High-Tech 188) does turn metal armour semi-ablative for the purpose of cumulative damage (but only for a direct burn).
So what I'm thinking: should the two categories of TL9 torches be comparable to TL6 thermite for the purposes of cutting through metal? The question applies both to the 'normal' mode of functioning, and to some hypothetical higher-power mode of the devices.

[Nereidalbel]

That sounds reasonable enough for both realism and playability.

[vicky_molokh]

Originally Posted by Nereidalbel

That sounds reasonable enough for both realism and playability.

The question becomes how does it line up with the amount of energy spent by the torch when working from a power cell. E.g. a C-cell at TL9 seems to have an energy-per-unit-of-mass ratio that is slightly better than that of thermite; it would last about 20 seconds if it were to expend energy at the same rate as a stick of thermite from High-Tech, but a C-cell in a regular-sized torch lasts 15 minutes while doing ×2/3 the damage per second (counting by dice only) or ×3½ the DPS (counting dice multiplied by the armour divisor, since by Kromm ADs do affect the rate of armour ablation). How plausible is it for efficiency to be so much higher? How much higher can the efficiency be at TL9ish? I mean, it's 3 TLs of difference, but still.

(Other source of numbers: the regular Plasma Torch seems to require about ¼ the power compared to some modern plasma torches people tried bringing up.)

[Nereidalbel]

Originally Posted by vicky_molokh

The question becomes how does it line up with the amount of energy spent by the torch when working from a power cell. E.g. a C-cell at TL9 seems to have an energy-per-unit-of-mass ratio that is slightly better than that of thermite; it would last about 20 seconds if it were to expend energy at the same rate as a stick of thermite from High-Tech, but a C-cell in a regular-sized torch lasts 15 minutes while doing ×2/3 the damage per second (counting by dice only) or ×3½ the DPS (counting dice multiplied by the armour divisor, since by Kromm ADs do affect the rate of armour ablation). How plausible is it for efficiency to be so much higher? How much higher can the efficiency be at TL9ish? I mean, it's 3 TLs of difference, but still.

(Other source of numbers: the regular Plasma Torch seems to require about ¼ the power compared to some modern plasma torches people tried bringing up.)

Remember that thermite is radiating heat in all directions, while a torch is directing the majority of its energy into a very small area. That's what makes it more efficient than just putting thermite on what you want to cut through.

[Fred Brackin]

Originally Posted by Nereidalbel

Remember that thermite is radiating heat in all directions, while a torch is directing the majority of its energy into a very small area. That's what makes it more efficient than just putting thermite on what you want to cut through.

It's a little bit more complicated than that. The "ash" from burning thermites is actually molten iron so the greatest damage from thermite is usually inflicted on the surface _below_ the burning mass.

Molten iron contains an enormous amount of energy by itself but when circumstances are right you get a layer of molten iron between the burning thermite and the surface below. That can transmit heat energy by conduction rather than radiation.

Your torch is having to work hard to be better at damaging materials than thermite.