Explosives

with-a-glass-of-mimosa  asked:

Hello! I'm writing a Harry Potter fic (if there is no answer to my question, I can always say it's because of magic) where one of the characters is killed by potion experient gone wrong. What kind of chemicals (and the following question which is for a different script blog: where in the nature can I find them?) are needed for an explosion big enough to throw off a human too close to it?

A disclaimer up front – this post is going to deal with the topic of an individual being killed in an explosion-type accident. I’m definitely not going to show you any pictures of that kind of injury, and I won’t even be giving detailed descriptions (that’s @ScriptMedic’s gig, and the topic has already been covered), but I wanted to give everyone a fair warning before proceeding.


This is quite an interesting ask because the scenario you’re aiming for is actually fairly difficult to produce; you need enough oomph™ to get a person in motion, but it needs to be a controlled and directed oomph™ because I’m assuming that after the character is thrown you’d like there to be a body, a building, and potentially even some witnesses left to tell the tale. First we should take a moment to discuss what an explosion is, and the different sorts of explosions you can get from chemical reactions; then we’ll move on to how they might affect the surroundings and an unfortunate individual who happened to be nearby. Finally, I may be able to offer some advice as to how to throw your character and still have them be recognizable at the end of the scene, though it’s going to take some careful planning to do it in a realistic fashion.  

Just so we’re all on the same page, an explosion is a very fast release of stored potential energy, with most of it being released as heat, light, sound, and pressure. Chemical explosives are usually compounds that decompose to release a lot of heat (energy from chemical bonds) and large volumes of gaseous products like nitrogen; the rate of this decomposition plays a large part in how useful a material is as an explosive. So-called ‘low explosive’ compounds decompose by deflagration, meaning that the reaction travels through the material slower than the speed of sound; low explosives include things like gunpowder, pyrotechnics, propellants (propane/gasoline), and many other mixtures of fuels and oxidizers. If the reaction propagates faster than the speed of sound, you have a ‘high explosive’ material that decomposes by detonation. As an example, let’s take a look at something called detonation cord (det. cord), a thin flexible line filled with the high explosive pentaerythritol tetranitrate (PETN). Below is a setup where a bullet is fired from right to left, activating an electronic trigger connected to the end of 16 feet of det. cord. (FullMag’s full video can also be found here.)

PETN detonates at 24-28,000 ft/s, so in the time it takes the bullet to travel the remaining 2.25 feet to the target (moving around 2800 ft/s), the reaction front of the explosive travels 16 feet to catch up with it. If you look closely, you can also see the blast wave from the newly formed gases expanding outward after the explosion – look for the ripple in the air at the top of the frame, or for the wave of dust knocked off the right-hand cinder block as the concussive force of the shock wave moves past it. This high-pressure, high-velocity wave of compressed gas is what causes most of the damage associated with an explosion, but unfortunately we’re going to run into a slight problem if we try to use an explosive to throw a person – humans are relatively small and squishy, which makes them extremely resilient to pressure waves and able to survive much more than you might expect. Here’s one more explosion gif to demonstrate how this works (and this time it’s a splodey-melon):

There are a few things to take note of here, besides the complete lack of eye protection – if the chunk of watermelon rind that struck his head was two inches lower down, that eye may very well have been lost. First and foremost, the individual pictured was completely unharmed by the pressure wave, and the melon shrapnel luckily only caused a welt; you can see the full clip here. However, if you watch the edge of the table you can see it flex down with the pressure of the explosion, and if you look really carefully you can see chunks of debris knocked off the bottom side of the table at very high speed (through a process called spallation). When a pressure wave encounters a solid object, it deposits some of its momentum as kinetic energy and is then reflected off the surface; that energy must either be absorbed or dissipated by the solid, and if the solid is rigid it will crack and crumble. If the solid is squishy and flexible (like a person), it can deform slightly to both absorb and dissipate energy without shattering and falling apart. This table compares damage to structures and humans at various peak blast overpressures in the frame of mining explosions; at peak overpressures of 5 psi, only 1% of humans exposed will even rupture an eardrum, but at this pressure most buildings will collapse. Real-world mining explosions that reach 5 psi overpressures do in fact cause many injuries and fatalities, but it isn’t the pressure that kills – it’s the shrapnel and the blast wind that accompanies large-scale explosions. The other factor protecting a person is the fact that humans have a relatively small surface area when compared to things like tables or walls or buildings, so only a fraction of the explosive energy from a pressure wave can even be absorbed by a person to begin with. In order for enough energy to be transferred to a person to throw them across a room, the explosion needs to be massive.

So what does this mean for your character and their exploding potion? If you want the actual explosion to throw them, you’re going to need something huge; it’s going to take out the room, probably the floors above and below it, and maybe even the entire building/wing of the dungeon. An explosion of this scale involves forces far greater than those holding the body together, so if the character is near the center of it then there isn’t going to be much left at the end; to get this effect from something the size of a potion would also require military-grade high explosives, and they’re not the sort of thing you make accidentally.

There is perhaps another way to achieve the same effect, but with a much smaller explosion – it’s even a plausible accident that could occur in the real world. Consider that fact that the amount of energy in a small firework, which can turn a watermelon into a vapormelon without injuring a person sitting a few feet away, is more than an order of magnitude larger than the energy required to fire a bullet from a gun. The difference here is how the explosion is contained; with the melon it expands in a spherical wavefront and can bounce around and reflect off of things, but with the bullet the explosion is funneled down the barrel, propelling a single piece of shrapnel to a very high velocity. If you can contain the explosive energy of your potion and channel it into a heavy, solid object, it could easily strike your character and carry them across a room, killing them in the process through blunt force trauma.

Perhaps your character was preparing something in an iron cauldron over a small open fire, and instead of grabbing that vial of Horklump juice they accidentally grabbed the hydrochloric acid. Iron (and a number of other metals) will react with hydrochloric acid to produce iron chloride and hydrogen gas; the reaction isn’t particularly fast or violent, and the gentle bubbling and yellow color of the solution might not even be noticed in the bottom of a black cauldron. If your character were to put a heavy iron lid on top and let it simmer for half an hour, quite a lot of very flammable gas would build up, but as long as the lid remained in place it wouldn’t be able to come into contact with an ignition source.  Your character returns and grabs the next ingredient, but as they start to lift the lid off, hydrogen can escape into the room and oxygen from the air can diffuse into the cauldron. The escaped hydrogen is ignited by the small open fire, and it quickly flashes back towards the cauldron, snaking down under the lid where it meets an ideal mixture of hydrogen and oxygen. This results in a powerful explosion with almost all of the force being directed straight up into the iron lid; it takes off towards the ceiling and strikes your character’s head or torso on the way, causing them to fly back and collapse in a heap.

This is just one way to spin this unfortunate tale, but it would give you a plausible potion accident with a readily available material, and it could cause an explosion that (indirectly) kills your character and sends them flying across the room. You could even have any number of people standing around with ringing ears who are otherwise uninjured, and besides that dent in the ceiling you haven’t done much structural damage.

Of course it goes without saying that you are always free to exclaim “MAGIC” at any point to either augment or supplant chemistry and physics, but going that route is entirely beyond the purview of my expertise.

~J

Disclaimer

Robotic spinach

Credit: MIT

A research team at MIT has embedded carbon nanotubes in spinach leaves, turning them into sensors that are able to detect harmful chemicals found in explosives wirelessly and relay the results to a smartphone.

These plants could be used to monitor groundwater for signs of nitroaromatics, with the embedded carbon nanotubes emitting a fluorescent signal when the chemical is detected.