Watch The Diesel Effect In Ballistic Gelatin

A striking video appears to demonstrate an explosion via the diesel effect in clear ballistic gel. The diesel effect or “dieseling” refers to when a substance ignites from the effects of pressure, and it’s the operating principle behind the gadgets known as Fire Sticks or Fire Pistons.

diesel-effect-ballistics-gelBallistic gel is a broad term referring to a large chunk of dense gel generally used in firearms-related testing to reliably and consistently measure things like bullet deformation, fragmentation, and impact. It’s tough, elastic, and in many ways resembles a gigantic gummi bear. Fans of Mythbusters (or certain DIY railguns) will recognize the stuff. Water-based blocks made with natural gelatin can be easily made at home, but end up with a yellow-brown color and have a limited shelf life due to evaporation. Clear blocks exist that are oil-based and don’t dry out like the water-based ones. It’s one of these that is in the embedded animation below.

Slow motion video capture is a natural companion to just about anything that you’d need ballistic gel for, and good thing — because the video captured what appears to be a diesel effect! The block is hit with a bullet, and as the bullet rapidly expands and dumps its energy into the gel, a cavity expands rapidly. During this process, some of the (oil-based) material in the cavity has been vaporized. After the expanded bullet exits (to the right of the gif above but easier to see in the video below), the cavity in the block begins to collapse. The resulting pressure increase appears to ignite the vaporized material, which explodes with a flash followed by some exhaust.

This effect has been observed in ballistic gel before, but this video shows a particularly clear ignition, followed by a secondary expansion of the cavity, then a flatulent-ish ejection of exhaust as the cavity collapses. If nothing else, it’s a very striking effect clearly captured on film. Slow-motion capture of destructive forces makes visible many things that would otherwise happen too quickly to perceive.

[via TheFirearmBlog]

41 thoughts on “Watch The Diesel Effect In Ballistic Gelatin

  1. It doesn’t ignite due to increasing pressure, but due to increasing temperature. To call it “dieseling” is incorrect. This is the less well known Le Petomane effect, and illustrates the results of a particularly difficult maneuver that has proven fatal for a number of would be competitors in The Great Crepetation Contest. Though not mentioned by name, it is the effect attempted, rather disastrously, near the end of the infamous blow by blow narration of the contest in 1946 when hosted by Canada.

    1. But the increasing pressure raised the temperature. Increasing either one of those parameters in an enclosed space (by external means) automatically raises the other.
      There is a direct correlation and the end result does not ‘care’ how the required conditions where reached/met.

      So, in this case, saying it’s not the pressure but the temperature is at least kinda strange – if not plain wrong.

      “The resistor is getting warm because of the ‘flowing’ current, not because of the supplied voltage.”

    1. What the ballistic gelatin is actually used for is testing how well firearms destroy living tissues and is has its properties calibrated to resemble human muscle tissue (or animal if that’s what you aim for..). The article says testing bullet deformation and fragmentation, but the purpose is actually to see what depth, shape, volume of a wound it will produce in the enemy at given distance to produce arms as lethal as possible.

        1. Well that and those incidental dead people from elsewhere that they’re supposed to figure out who killed them, with what, from how far away. So guess they might do the odd forensic reconstruction now and again, since they have the stuff around.

        2. Well, the REAL reason is that, when the FBI shoots somebody, that means that they REALLY want the person to stop doing whatever it is that they are doing that makes the agent want to shoot them in the first place. Killing REALLY is not the point, stopping them immediately is.

          If law enforcement shoots somebody about to commit a murder, what advantage is it for the potential murder to have enough time to to finish their nefarious purpose and then bleed out an hour later?

      1. You’re quite right that one of the results of penetration in ballistic gel is to infer the effects on and in living tissue. Amount of penetration, how much the particular bullet expands, whether or not fragmentation occurs, and even things like whether the bullet is stable when it hits are all things that can be observed and measured consistently when using ballistic gel. It resembles flesh and blood enough that those factors (penetration, expansion, etc) are pretty representative: if a bullet deforms in a certain way consistently in ballistic gel, then it (probably) will in a similar amount of flesh and blood — barring other factors like bone, etc..

        The biggest value ballistic gel has in firearms testing is not that it’s a flesh simulator (though it is that in some ways) – the biggest value is that it can be standardized and produces consistent, repeatable, and observable results.

        And sometimes an orange-yellow flash followed by a smoke fart on film.

      2. Yeah that sounds real cute and mythbusters rammed it in your brains but it’s bullshit, that gelatin is totally not like muscle (ever cooked meat?) not like a body with lining and organs separated by fluid, organs of various type and density.

        1. No-one is saying that ballistic gelatin is exactly like a body. It’s an _approximation_ of body that is useful for e.g. comparative testing. So, say, 12″ of penetration (as FBI specifies) in gel doesn’t translate to 12″ in a human body – it translates to “enough” penetration. And you can shoot two different rounds side by side to see which one penetrated and expanded/fragmented better, and be reasonably sure that it will also penetrate and expand better in flesh, even if absolute penetration depth and expansion/fragmentation effect will be different.

          1. Excellent! You have clearly stated the mistake almost everyone one makes about all modeling. A model can never exactly match reality, because reality is too complicated. But a well designed model – exactly because it is simpler than reality – can simulate some important aspect of reality and thus permit standardized testing! If only everyone – or at least many more people understood this!

      3. Terminal ballistics is a well-studied field; as you can see, while the _temporary_ wound cavity is sizeable, the _permanent_ wound cavity is only the track of the bullet’s path. Many studies have been done that show that bullet fragmentation is the primary source of tissue disruption in bullet wounds; a bullet will flip 180° on its path through a body, and the size of the permanent wound cavity depends on the shape of the bullet and whether and how it deforms or fragments. Ammunition designers have put a lot of work into designing projectiles that will deform reliably to create a predictable permanent wound cavity.

        1. If that is what was meant, then statement is incorrect. Very well established that cast majority of heat comes from oxidizing the food we eat. That is why we measure the calories of food. Remember 1 kilocalorie = 3.966 BTUs. Just like a furnace for your house or a hot water heater has specified BTUs (based on the energy in the type of fuel it burns and how efficiently it burns it). For us has to do with ATPs generated in our mitochondria. And search for mitochondria and/or ATP will give you plenty of links.

  2. Correct me if I am wrong but isn’t the function of Ballistic Gelatin to tell how much damage a projectile causes, So gelatin that has a secondary explosion just because a bullet passed through would be pretty useless.

      1. Not so much – you can see from the mechanical displacement of the gel that the “explosion” is not much more than a little pop – partly because the bullet has already passed through the gel block, so there is an open pathway to the outside of the block through which the pressure is discharged, and partly because there really isn’t much fuel to “explode” inside the block – it’s just a momentary flash, and doesn’t even generate much in the way of heat. If you cut the gel block open immediately afterward and feel the inside of the cavity, it’s ~the same temperature as the rest of the block.

  3. Cool, thanks for posting. A month or so ago there was a new TV doc on the JFK
    assassination with ballistic gel slow motion footage showing exactly this flash. I was
    wondering if I actually saw that or if it was some lighting artifact.

  4. Now I read the links I wanna make a diesel-punk atlatl … stick, hinged to cylinder, projectile close fit to cylinder, squirt in a touch of WD-40 or something, set projectile loosely in mouth of cylinder, raise it up and whip it forward….. and theoretically you lose your ear in a freak accident… no, I’m going for theoretically the projectile slams back in the cylinder as you whip it forward and gets a boost from compression ignition… obviously, this is a don’t try this at home, but round your buddy’s house when his wife is out is a maybe.

    1. You are right, simple cavitation bubble, filmed in a spectacular way. The temperature reached upon collapse can be comparable to the surface of the Sun. This is the source of the flash (sonoluminescence) and burning the oil based ballistic gel.

      1. It may well be that the bubble collapsing is causing the pressure, and ignition, but that doesn’t mean that the light is produced by the same mechanism (whatever that is) as sonoluminescence.

        The fact that the flash is yellow/orange, and that there’s smoke coming out the hole afterwards suggests that it’s heat/pressure induced combustion going on. It looks a lot like fire, there’s air in there, and the gel is combustible. I’ll plead Occam’s razor.

    1. Sonoluminescence (or somnoluminescence, if you’re sleepwalking) and piezoluminescence are both light-generating phenomena, but are not the mechanism here. Here, the phenomenon is triboluminescence – the discharge of light by the flow of imbalanced valence electrons ripped from molecules on either side of a violently torn substance. The discharge has a secondary effect of igniting any vaporized hydrocarbons or unburned gunpowder in proximity to the flash, which serves as a “spark plug” when the two sides of the cavity come back close enough that the electrons can jump the gap. There are pressure waves within the cavity, but think about the physics – the cavity is created by the mechanical energy of the bullet entering the gel and forcing the gel to separate. That creates a LOW pressure cavity – not a high pressure cavity. The low pressure cavity is why the cavity collapses again – atmospheric pressure on the outside of the gel, along with the gel’s natural elastic rebound, pushes it closed. In such a cavity, there’s no pressure sufficient to cause dieseling of the hydrocarbons – only pressures below atmospheric, except for limited pressure waves within the cavity. Dieseling requires pressures around 16 atmospheres (=500 feet down in the ocean) to initiate combustion from compression-based thermal temperature rise. That (235 psi) doesn’t happen in a gel block – and the combustion pressure post-ignition would blow the block apart if it did. (Think – dieseling can crack a cast iron block; that’s why diesel engine castings are so robust). Ignition, however, of hydrocarbons – by the electrical discharge – can cause a secondary pressure spike, resulting in the exhaust of the smoke from the cavity as it collapses, through the bullet’s exit hole and backwards through the entrance hole.

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