C-4 Explosive notes

by Carl Rigney (cdr@amd.com)

I pulled my copy of the Ranger Handbook (ST 21-75-2) off the shelf (every GM should have one of these lovely pocketfuls-o-fun) and looked it up. C-4 is 1.34x as powerful as TNT; I’ll give the formulas so you can figure things for yourself with whatever new joys 2050 military tech has come up with.

The “minimum safe distance for personnel in the open” for demolitions is 77 meters times the cube root of the weight of the TNT in kilos. For C-4 its 85 meters times the cube root of the weight of C-4 in kilos. So for a 200kg C-4 explosion we’re talking 500 meters. Now, that’s just for people, and I assume it includes a healthy safety margin. Buildings aren’t going to be leveled all the way out.

Incidentally, cutting down a foot-thick tree takes 3/8 kg of C-4 if you do it the smart way; nearly 4x as much if you just tie it next to the trunk.

Now for the reinforced concrete wall. The handbook doesn’t give an explicit case for blowing a hole in the ceiling, probably because Rangers aren’t supposed to do anything quite that stupid. But for a wall the charge of TNT in pounds (divide by 3 for C-4 in Kg) is given by the formula P=KCR^3, where R is breaching radius in feet (thickness), K is the material factor, and C is the Tamping Factor. The number of charges used is N=W/(2R) where W is the width of the breach.

C ranges from 1 to 3.6 depending on how well tamped the explosive is; 2.0 is pretty easy. For a ceiling tamping’s not easy, so let’s call it 3.6. K ranges from 0.07 for earth, up to 1.76 for small holes in reinforced concrete. The values for reinforced concrete are:

    R               K
    1' or less      1.76
    1'-3'           0.96
    3'-5'           0.80
    5'-7'           0.63
    7' or more      0.54

So if it’s 1 foot of reinforced concrete the charge size is (1.763.61^3)/3 = 2kg/charge, and for a 4’ wide breach (no trolls in the party) you need 4/(21) = 2 charges. So 4kg of C4 will do it. If it’s 2’ of reinforced concrete you’ll need (0.963.62^3)/3 = 9kg in a single charge. Assuming (with typical Shadowrun logic) that C-12 is 3x as good as C-4 :-) then adjust to taste. Note that this only blasts away the concrete; it doesn’t cut the reinforcing steel, but depending on what mesh was used in creating the building you may be able to squeeze or torch your ways through that easily enough, although possibly not before someone comes to find out what the hell that incredibly loud noise is. Note that if you can place the charges *inside the concrete by inserting them through the pipe you can get a much better C-factor, like 1.0. So that would drop your charge size to a little over a kilo for 1’ of concrete, 2.5kg for 2’, 45kg for 6’.

Of course, in my 2040 the military has keyed resonating explosives that analyze the vibrations of the previous explosions and time their detonation to maximize the effect. There are also smart-charges that explain their operation the way medkits do. Ignore both; the PCs don’t need these kind of toys.

Moving on to mechanics, I find FASA’s demolitions skill absurd. “Simple” demolitions like blowing holes in walls should be straightforward. For someone with demolitions skill, I’d make the target number equal to the number of charges required, regardless of size, because the tricky part is the fusing. Setup for simple demo takes 1 minute per 1kg block, obviously more if they have to be propped up, less if it’s prewired in demo carrysacks. Tamping increases the time quite a bit. You can do things hastily, dividing the time it takes by the penalty you accept to your target number. If you have enough explosives and make your success test, the wall is breached; the more successes, the cleaner the results. If the test fails, it could range from a hangfire to simply not breaching. People with very high demolitions skills can probably get by with less explosive simply because generous margins are built into the rule-of-thumb numbers given here. Properly destroying things like roads, bridges, or (gack!) airfields take MUCH longer, because you have to a lot of work preparing the structure, you can’t just tie 6 sticks of dynamite to the railroad track and blow up the bridge, no matter what you see in the movies.

In your example, if your demo engineer has all the time she wants she needs to roll skill 4 vs. target 2, although I might up it to a 4 on the basis that its a ceiling and suchlike. In other words, a routine task.

Commercial demolition is somewhat different: 30kg was used to bring down a 10-story hotel by putting it in the critical places and weakening load-bearing structures, etc. That takes MUCH longer, but it also comes down safely by toppling in on itself.

Disclaimer: I’m not a combat engineer so if anyone out there has real experience in these matters I’d love to be corrected. Of course, don’t try this stuff at home. :-)

“5 grams of this explosive will level this building.” “How much you got?” “A truckload.”