Explosives As A Reflective Wall

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sardia
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Explosives As A Reflective Wall

Postby sardia » Mon Jan 29, 2018 4:52 pm UTC

We were playing GURPS, and I made a offhand comment that you could make a bomb directional by having parts of a bomb explode at slightly different rates. Say you have a bomb in front of a target, do explosives shockwaves only reflect off hard substances, like the ground or a back plate? I tried googling explosive lens, shaped charges, but I couldn't find anything.

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Re: Explosives As A Reflective Wall

Postby Izawwlgood » Mon Jan 29, 2018 4:57 pm UTC

I AM SO HAPPY I GET TO LINK THIS TO SOMEONE

FUCKING AWESOME

I think the tl;dr is that explosives can be shaped/directed, and absolutely used as directional barriers.
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Re: Explosives As A Reflective Wall

Postby p1t1o » Tue Jan 30, 2018 12:38 pm UTC

You can indeed focus explosive power in many different ways, shockwaves behave much like any other wave, with constructive/destructive interference, diffraction and reflection, resonance effects, propagation rates etc. The detonation wave that moves through the body of the explosive itself is also under the influence of these effects.

The "Misznay–Schardin effect" describes how a planar sheet of explosive will produce a planar blast wave parallel to the explosive surface, which allows us to manipulate the shape+propagation of some detonations.

Shaped charges take advantage of this effect and use a detonation wave travelling up a hollow cone to collapse a liner inwards and forwards.

The "SIMON" ballistic breaching device produces a neat little explosion tailored to be "just enough" to blow a door open.

Nuclear weapons utilise shaped blocks of different explosives that detonate at different velocities to shape the inwards-travelling detonation into a near-perfect spherical implosion.

Shockwaves will reflect off objects, and yes, harder objects will reflect more of the waves energy with more or less of the energy absorbed into the structure depending on many things.
Shockwaves can also be diffracted through different densities of air/gas as well.
These effects can even be pronounced on a large scale, causing increased destruction in unexpected areas, and less in others.

It should be noted that these are still bombs, and whilst a blast can produce focussed or shaped force, they still explode in all directions (equal and opposite reactions and all that). A claymore mine (which uses Miznay-Schardin to produce a specific spread of fragments) has the words "front towards enemy" on the front, this is just where the shrapnel is, it will still kill you if you are too close in any direction.
You can take measures to change this, but eventually you start constructing a gun.

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Re: Explosives As A Reflective Wall

Postby Izawwlgood » Tue Jan 30, 2018 2:39 pm UTC

p1t1o wrote:You can take measures to change this, but eventually you start constructing a gun.
This whole post was awesome.
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Re: Explosives As A Reflective Wall

Postby sardia » Tue Jan 30, 2018 6:13 pm UTC

Izawwlgood wrote:
p1t1o wrote:You can take measures to change this, but eventually you start constructing a gun.
This whole post was awesome.

Hmm, I googled most of these, and I think we could construct a directional bomb pretty easily with a moderate skill roll. However, I still think my initial ruling was wrong, or at least much harder without a backing plate or liner to focus the explosion. I'll need to diagram this out for my party. (we rotate story tellers, so I have to convince them when it's my turn to play )
They're mostly engineers or stem graduates, so I need to do my homework. GURPS is too realistic sometimes.

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Re: Explosives As A Reflective Wall

Postby jewish_scientist » Tue Jan 30, 2018 7:09 pm UTC

Two waves can travel through each other without effecting one another. If you use an explosive and a solid, you can get the result you are looking for. Otherwise, the best you can hope for is to create deconstructive interference, which will always fail to completely cancel the original explosive, because the center of its detonation is a different point. If you have a shell of one explosive material surround a ball of another explosive material and explode them with just the right timing, you can negate the ball's shock-wave, but not the shell's shock-wave.

In short, you need a solid in between the two explosions to get a shape charge. You might, might, be able to create a shape charge using 2 explosions in 2 different fluids. This is because how waves move from one fluid to another is complex and weird.
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Re: Explosives As A Reflective Wall

Postby p1t1o » Wed Jan 31, 2018 8:56 am UTC

sardia wrote:
Izawwlgood wrote:
p1t1o wrote:You can take measures to change this, but eventually you start constructing a gun.
This whole post was awesome.

Hmm, I googled most of these, and I think we could construct a directional bomb pretty easily with a moderate skill roll. However, I still think my initial ruling was wrong, or at least much harder without a backing plate or liner to focus the explosion. I'll need to diagram this out for my party. (we rotate story tellers, so I have to convince them when it's my turn to play )
They're mostly engineers or stem graduates, so I need to do my homework. GURPS is too realistic sometimes.


What exactly do you mean by "directional bomb"? What effect are you going for?

(Dear Secret Service, this is a hypothetical discussion, thanks)

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Re: Explosives As A Reflective Wall

Postby sardia » Wed Jan 31, 2018 12:33 pm UTC

Originally it was a thrown satchel that did damage in a circle but it was hard to throw or plant the satchel far enough away without getting hurt. That's when we started asking about how to make the bomb deal more damage in one direction, like a claymores. Then I made the guess that a smart enough person could design a similar effect, but without the metal wall to reflect the blast.

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Re: Explosives As A Reflective Wall

Postby speising » Wed Jan 31, 2018 12:36 pm UTC

You won't get around Newton's third law. You can't have a bomb that only exerts force in one direction.
You can have a bomb that has a tight, sharp blast in one direction (=towards enemy, ideally) and a wider, half-globular blast in the other direction, which will not reach as far.

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Re: Explosives As A Reflective Wall

Postby Soupspoon » Wed Jan 31, 2018 12:55 pm UTC

Alternately, reverse that. A spread, short-range2 broad damaging blast towards the enemy (enough to cause problems, without needing such precise rotational control upon release, just more enthusiasm to get it closer), and maybe two or more balancing1 spikey-jets of force facing mostly backwards but somewhat sideways to skirt the deployer.

1 Not necessarily totally balanced, either way, but experimentation in handling would have to be used to see how much over/under-push you can have before it causes more problems to the operator than worthwhile as you get more rocket-exhaust roughly towards them or exhausted payload 'matter' propelling itself back.

2 Technically, it's just when the inverse square drops it below damaging levels, for a given steradial cone-ish area of effect. But I think we can all agree it's not the cut-off-distance of the fictional explosive in (IIRC) George's Marvelous Medicine, whichever design and orientatiin we go for. ;)

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Re: Explosives As A Reflective Wall

Postby morriswalters » Wed Jan 31, 2018 12:59 pm UTC

Do what the military would do, use cluster munitions, contact activated. Use the body of your enemy to absorb the blast wave from small explosions rather than one large one.

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Re: Explosives As A Reflective Wall

Postby p1t1o » Wed Jan 31, 2018 2:46 pm UTC

sardia wrote:Originally it was a thrown satchel that did damage in a circle but it was hard to throw or plant the satchel far enough away without getting hurt. That's when we started asking about how to make the bomb deal more damage in one direction, like a claymores. Then I made the guess that a smart enough person could design a similar effect, but without the metal wall to reflect the blast.


I did say thats not how claymores work. They do not have a metal plate that reflects rearwards blast forwards. You do not want to stand behind a claymore.
And any metal plate will be very strongly propelled backwards unless it was very massive. You can get away with it if the plates mass is very much in excess of the explosive mass - and funnily enough, this is the exact situation that a gun represents, where the "plate" is the barrel, breach and other mass of the gun itself.

To be honest I think what you really want IS a gun.

You want to be able to project explosive force, at short-ish range, without injury to the person deploying the weapon.

Thats what they invented 40mm grenades for, and they even do a semi-armour-piercing shaped charge version.

But also, for reference: there is such a thing as a "ballistic blanket" which is essentially a very heavy, padded and reinforced shield made of various fabrics and fibres. It is precisely used for when soldiers want to be able to be closer to a detonation (like a satchel charge or wall breaching charge) than would otherwise be safe. This is, in effect, an extrapolation of a claymore with a very massive backing plate - only the "plate" can be much less massive because it is in contact with you, and not with the explosives.

tl;dr - no you really cannot have an explosive that only explodes in a single direction.

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Re: Explosives As A Reflective Wall

Postby thoughtfully » Wed Feb 14, 2018 3:23 pm UTC

Shaped charges?
Go nuclear or go home! :twisted:
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