Explosion Basics

Explosions – So What?

Explosions fascinate most people – they are either drawn to or horrified by the display of power and destruction explosions can produce. Explosion phenomena also draws my attention, but as an experienced explosives engineer and professor, the science behind explosions fascinates me more than the simple spectacles of light, noise and destruction. Explosives engineering combines science and art to change the world around us – shock hydrodynamics and material sciences on one hand, and an innate feel for dynamic balance and situational effects on the other. For example, I spent much of my time and energy as a researcher at the university testing protective schemes created by civil engineers, exploring the design limits of those creations. The art/science combination eventually allowed me, with few exceptions, to tell the civvies how their designs would perform before we tested them.

Barrier shot w 50 lb TNT at 8 inch standoff #1

First frame capture from a video of the detonation of 50 pounds, TNT equivalent, of ANFO at 8 inches away from a steel-reinforced concrete T-barrier during testing

Barrier shot w 50 lb TNT at 8 inch standoff #2

Second frame capture from a video of the detonation of 50 pounds, TNT equivalent, of ANFO at 8 inches away from a steel-reinforced concrete T-barrier during testing

Barrier shot w 50 lb TNT at 8 inch standoff #3

Final frame capture from a video of the detonation of 50 pounds, TNT equivalent, of ANFO at 8 inches away from a steel-reinforced concrete T-barrier during testing

Outside of nuclear reactions, we either grow or mine the materials for everything we make or use, mining requires breaking rock, and explosives are the most efficient and economical way to break rock. That is, of course, as long as lawyers aren’t involved!

If an online source or one of your friends has convinced you that you should be making your own explosives (or exploding targets), don’t do it. Even with proper training and education, pursuing my profession is hazardous, and it is unlikely you have either one of those. As with using firearms, always consider Safety First!

Regarding online sources, I hope everyone knows not to trust reloading information posted to a forum without independent verification from a reliable source. What about explosives concoctions presented in the same way? Even the “Anarchist Cookbook” is not to be trusted. Many people have misconceptions about explosives that I’ll try to correct.

Background

In this post, I’ll restrict my discussion to chemical explosives, which release pressurized gas and give-off light and heat when the explosives initiate. Nuclear explosives are a different kettle of fish, even though the effects on its surroundings of a nuclear detonation can be similar, when scaled down, to the effects of a chemical explosive detonation. This detonation requires a source of oxygen (an oxidizer), a fuel, and some stimulus (energy) to set off the process, just like burning. In a manner similar to burning, the chemical explosive reaction is an oxidation type that liberates energy by breaking chemical bonds; contrary to burning, though, the reaction in an explosion completes itself nearly instantaneously.

The type of chemical exothermic (fancy word for “energy comes out”) reaction varies according to the speed at which the reaction takes place. Rusting of iron-based metals is an example of the slower end of the exothermic reaction range; burning or combustion is a faster reaction than rusting, and deflagration is combustion that propagates through the fuel very rapidly but at subsonic speed. An explosive detonation, however, is a reaction that takes place in and propagates through the fuel at greater than supersonic speed. Detonation, since it occurs faster than the speed of sound, produces a shock wave in the material just like a supersonic aircraft produces a sonic boom in the air through which it travels. Its ability to detonate is the property that sets a chemical high explosive apart from other materials that “explode.”

Let’s Detonate

Unfortunately, the mass media misuses the term “detonate” and its derivatives when they should use “explode,” and this adds to the general confusion about what exactly is an explosive. Different from the rapid gas release and high-pressure generation by a propellant such as smokeless powder, or the explosion of a vaporized combustible fuel such as propane or gasoline, the detonation shock-wave shattering effect of a high explosive such as TNT or C-4 tears apart any surrounding materials.

C-4 demolition high explosive in bulk form, rather than the green plastic-wrapped 1-1/4 pound blocks typically shown in the media

Notice that I slipped the term “high explosives” in on you; in the explosives business we had to add “high” in order to separate detonatable explosives from “low explosives,” an old and unfortunate term for materials such as black powder designed to deflagrate but not detonate, much in the same way as propellants. Chemists designed low explosives to produce large volumes of high-pressure gas, and their reaction speeds go nearly to supersonic velocity but don’t quite get there. This is fortunate – otherwise, we wouldn’t have propellants for firearms and rockets, since the detonation shock wave from the reaction would destroy the gun’s chamber and the rocket’s engine. Therefore, an event is not a detonation unless there is a shock wave, and it isn’t a high explosive unless it detonates.

So what – why all this detail about detonations? At times, after news reports of an accidental natural gas or other fuel explosion I am asked by members of the press to compare the explosion to TNT or dynamite.

“Sticks” of gelatin dynamite mining and excavation high explosive

In each case, the reporters found it hard to believe me when I told them there was no way to make that comparison. They really weren’t interested in the details of my answer (and they never attempted to print the correct information), because they only knew the Hollywood visuals leading them to believe that all explosions are created equal. This is similar to the movie lie that someone can shoot another person with a handgun or long gun (from the shoulder) and knock him off his feet.

In fact, a typical wood fire releases over three times more BTUs of energy than the detonation of the same mass of TNT or dynamite. This seems to be counter-intuitive, and Hollywood and the press don’t like comparisons that have to be complex, but if you’ve read this far you are well on your way to understanding what they won’t tell you.

What Good are Explosives?

If a fuel-air deflagration releases more energy than the detonation of a high explosive, why do we use explosives? Well, the detonation shock wave, not the high-pressure gas or the intense heat from the chemical reaction, shatters the target of a military high explosive shell or bomb. A detonation shock wave also fractures rock when we use explosives in mining; the high pressure gas then pulverizes the fractured rock and heaves it so we can dig it with machinery (this isn’t a small matter – we use over 6 billion pounds of explosives every year for mining and construction in the US). This shattering plus pressurization effect is an enormously more destructive (and efficient) process than any other chemical or manual way of breaking rock. We used blasting powder (“low explosives”) for mining in the beginning, but gave it up when more effective (i.e., detonable) blasting explosives were invented. We now have an entire suite of rock-blasting explosives, tailored to the requirements, from low shock and high gas volume to more shock and less gas volume.

Bad from the Good

Nevertheless, a bad guy can use a low explosive (black powder, for example) or a propellant to make bombs. After the Boston Marathon terrorist attack in April 2013, you may have read or heard about the pressure cooker bombs those terrorists used. This sort of bomb is similar to a pipe bomb, in that a high explosive is not necessary to make it work. The bomb maker takes a strong vessel and fills it with an energetic material that can create enough pressure to burst the vessel. The result is similar to a boiler explosion. It’s an explosion, but it doesn’t involve high explosives. If the terrorists use high explosive charges instead of pressure bombs for an attack, the high explosive charges add a destructive, high-energy blast shock immediately around each bomb, and hurl bomb fragments and pieces of surrounding materials (sidewalks, roadways, parking meters, etc.) at several times the velocity of fragments generated by a pressure burst.

Our government attempted to make it difficult for terrorists to obtain explosives in the US via the Safe Explosives Act of 2002, which introduced restrictions on all purchases of explosive materials. Prior to the Act, one could purchase and use explosives in his State of residence without any Federal restrictions. Some of you may remember those days; I remember being able to purchase dynamite, detonators, and safety fuse at the local hardware store to use shooting post holes in the rocky Ozarks ground where I live. It’s questionable whether the Act made us any safer; similar to gun control laws, when a law restricts our activities only law-abiding people obey the laws. For example, had it been in place at the time, the Act would not have prevented Timothy McVeigh’s attack against the Murrah building in Oklahoma City in 1995 since he stole the items from a rock quarry needed to initiate the blast and purchased the fertilizer and racing fuel required for the main charge without any requirement for a license.

Exploding Targets

How about exploding targets?

Commercial exploding target product components. After the materials are properly mixed, they will explode after being shot with the specified firearm.

If we have a Safe Explosive Act that restricts most people from buying explosives, why can we buy Tannerite™ from a store or online? I’m sure many of you use the targets – seeing and hearing an explosion thrills almost everyone, and the targets are safe if used according to the directions that come with the packaging. Some people, when they find-out that I’m an explosives engineer, ask me why the government does not regulate those targets. “They explode, so aren’t they explosives?” In many cases, the questioner believes those targets should be regulated by the ATF, and cannot understand how any reasonable person could think otherwise. State regulations may be coming, because a couple of State Fire Marshalls have asked me to help them answer legislators’ questions about regulating exploding targets. Nevertheless, the Feds do not regulate these targets because the sellers separate the reacting materials, the oxidizer and fuel components, into different containers. Common materials used for many industrial and commercial purposes other than exploding targets, the Feds don’t have explosives regulatory power over the separated materials. For some formulations with particular oxidizers, the exploding target components are not explosives until the fuel and oxidizer are mixed. For others, after mixing the fuel and oxidizer one must also combine the mixture with a small amount of a so-called catalyst (a small portion of the fuel that reacts with the oxidizer under shock loading from a projectile impact more easily than the remaining fuel). Once you mix one, you must use the now-detonable target; you cannot store the mixture unless you possess a Federal Explosives License, the requisite State permits/licenses, and a magazine approved for explosives storage. Another area the Feds have restricted with regard to use of exploding targets is by banning them from US Forest Service and Bureau of Land Management lands in some states, claiming that explosions may start wildfires. If this is the case, why not just ban exploding targets, as they ban campfires, in specific locations when wildfire conditions are present? I’ll leave the answer to you.

Resources

If you are curious about some of the myriad of Federal rules regarding explosives, try looking at atf.gov/explosives and the submenus found at that website. Strangely, Federal regulation of explosives use and storage depends on how the government categorized them for shipping. Therefore, the US Department of Transportation governs transportation (and therefore categorization) of hazardous materials such as explosives (see websites such as phmsa.dot.gov and fmcsa.dot.gov). The Institute of Makers of Explosives (ime.org), and the International Society of Explosives Engineers (isee.org) are non-government organizations that are good sources for information about explosives, the application of explosives, and explosives-related training and education.

The Bottom Line

One of my former bosses was an explosives chemist, and used to say that he was one of the few of his colleagues who still had all his fingers. It’s one of those self-deprecating jokes that is funny because there is a foundation of truth to it. If the folks trained to work with energetic materials still face on-the-job injuries or death, what are the chances that you, the untrained and internet-educated sort, is going to mess with hazardous materials without being injured? Don’t even try it.

Also, don’t be a dead hero – if you spot something that looks like an explosive material or device that is obviously not where it should be, don’t fool with it! I’ve include photos of some of the devices and materials you may see if you run across blasting materials.

Non-electric detonator on a spool of shock tube lead-in line

A non-electric detonator (blasting cap) with its shock tube lead-in line, still in its wrappers, placed atop a spool of shock tube lead-in line

A detailed view of a non-electric detonator, or blasting cap, showing the detonator cup (silver) containing sensitive high explosives and the lead-in shock tube (yellow) that carries the initiation pulse to the detonator

Pentolite boosters, used in mining with blasting caps (detonators) to initiate the detonation of blasting agents such as ANFO (Ammonium Nitrate + Fuel Oil).

Don’t be like the sheriff’s deputy attending one of my courses, who told a story one morning before class about responding to a domestic dispute. After he and his partner handcuffed the wife abuser who was the subject of the call, the wife said the old man had some bad stuff in the garage. So, the deputy checked it out. When he peeked into a cooler, he spotted a grenade sitting there. I asked the deputy what he did – what do you think he said? “I stuck the cooler into the back of the squad car and took it to the jail.” Bad idea! Particularly, since he couldn’t tell a live grenade from one rendered inert.

Sadly, a similar incident recently killed three Los Angeles Sheriff’s Deputies who belonged to the agency’s bomb squad. According to reports, law enforcement personnel recovered two “military-style” grenades from a garage in Santa Monica and Sheriff’s personnel took them to the squad’s training facility in East Los Angeles, where they stored the grenades overnight. Deputies x-rayed the devices and apparently believed the grenades were inert, but one or both detonated during disassembly by the three men killed in the explosion. The event is still under investigation as I write this, but you can bet officials will take a hard look at training and procedures for bomb squads in the future.

As I said up front, working with explosives is hazardous, and although the hazards cannot be eliminated they can be reduced by proper education, training and a Safety First attitude.