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Insensitive Munitions Testing: Protecting Ourselves From Our Ammunition

Everyone knows that ammunition is dangerous. It is designed to inflict damage, usually by hitting a target with great force, exploding, or both. When we look at the bare-bones theory behind ammunition, we see that it revolves around energy. When using ammunition, our objective is to throw “balls of energy” (projectiles, missiles, and bombs) at bad guys and hit them—hard.

To achieve this goal, we somehow have to get these balls of energy from the factories that manufacture them to Soldiers and other military personnel who will use them to protect and defend their units and themselves. Unfortunately, until we perfect ammunition teleportation technology, ammunition is vulnerable throughout the supply system. An article in the March–April issue of Army Logistician, “Preserving Readiness Through Ammunition Packaging,” described the lengths to which packaging engineers go to protect ammunition from problems created by the transportation system and the environment. Readers of that article may be prompted to ask: “What’s being done to protect us from our ammunition?” After all, energetic materials such as propellants and explosives are not discriminating. Give ammunition a good spark, a little fire, or a hot fragment, and most of the energy it has stored up for the bad guys will be hurled at the good guys instead.

Insensitive Munitions

So what are the Army’s engineers and scientists doing to keep us safe from our own ammunition? The answer is that they are working to make ammunition insensitive. The goal is to develop ammunition that will react in a dangerous way when we want it to and not before.

A reaction is normally most dangerous if it is a “high order” detonation event. In “techno-speak,” this means an event in which a chemical reaction produces high-pressure, high-temperature shock waves that consume the explosive material nearly instantaneously. Shock waves from high-order detonations can travel faster than a mile a second and cause a lot of damage. If we put these already potentially dangerous energetics into a closed container, such as a shell, an armored vehicle, or any other tightly enclosed space or structure, we introduce the effect of confinement to the explosive reaction. Confinement often increases the violence of an explosion because of a buildup of pressure, which eventually bursts the container that encloses it and creates what is essentially a bomb. So not only do we have fire, heat, and a shock wave, we also have flying fragments.

Propellants and Explosives

The ideal approach to making munitions insensitive is to use propellants and explosives that do not react unless they are hit with a specific stimulus. Unfortunately, this is probably the most difficult way to make explosives insensitive. We still want munitions to pack a punch and explode on impact with a target. This means we have to come up with new chemical mixtures that pack similar amounts of energy but react only when we want them to. Scientists and engineers have developed several new materials that are powerful but hard to set off by accident. A word of caution, though: These materials still have more than enough stored energy to do real damage if mishandled, no matter how insensitive they may be.

Containers

One solution to this problem is the use of melt-away panels to protect munitions during transport. In the event of a fire, the panels melt before the ammunition has a chance to explode, leaving behind huge gaping holes in the container. Munitions may react much less violently if their containers are designed so that the munitions cannot build up pressure from confinement. They may burn, but they are not likely to explode. However, a container designed with insensitivity in mind still must be able to protect the munitions and pass stringent handling and environmental testing. This balancing act between insensitivity and ruggedness can be tricky.

Contained munitions can build pressure so fast that common solutions such as pressure-relief valves will not work. One solution to this problem is the use of melt-away panels. These panels protect munitions during transport, but, in the event of a fire, they melt away, leaving huge gaping holes in the container before the ammunition has a chance to explode. When the ammunition finally explodes, the pressure has somewhere to go; it does not turn the container into a bomb.

Several other techniques also are being tried. Most of them offer some way to weaken the structure of the container so that it will vent at precise spots under pressure. Ideas such as scoring the wall of the container or weakening the welds have been studied, but these approaches pose challenges for quality control and manufacturability. It is difficult to develop a container that is strong enough to pass all packaging tests but strategically weak enough to pass all IM tests.

IM Testing

All munitions acquired by the military services must be examined to determine if they meet established IM requirements. This is true whether the munitions are developed by the services or procured from commercial or foreign sources. This examination normally involves a series of six tests designed to assess the ability of munitions (typically in their shipping configuration) to withstand shock, heat, and impact. The specific tests are identified during a threat hazard assessment conducted by the acquiring service. The six tests normally include fast cookoff, slow cookoff, sympathetic detonation, bullet-impact, fragment-impact, and shape-charge jet impact tests. These test requirements, methods of conduct, and passing criteria can be found in Military Standard (MIL–STD)–2105, Hazard Assessment Tests, Non-nuclear Munitions, and in various North Atlantic Treaty Organization Standardization Agreements (STANAGs).

Both the fast and slow cookoff tests subject munitions to the threat of elevated temperature. In the fast cookoff, the munition in its container is placed over a huge vat of gasoline or jet fuel that is ignited. This raises the munition’s temperature very quickly and tests how it reacts when it is engulfed in fire. In the slow cookoff test, the temperature is raised again but at a much slower rate in a specially designed oven. The munition is placed close enough to a fire for its temperature to rise above the ignition point, but the munition is not necessarily engulfed in flames. In both tests, the violence of the reaction, the degree of fragmentation, and the debris throw are evaluated. If the munition’s reaction is no worse than burning and no hazardous fragments are projected, the munition passes these tests. For the purposes of IM, a hazardous fragment is one that produces 58 foot-pounds of energy out to a distance of 50 feet. This is calculated either with instrumentation during the test or by collecting and analyzing post-test debris.

A sympathetic detonation test involves several munitions that have been placed in their packaged configuration and stacked close together as they would be for transport or storage. The object of the test is to see if the explosion of one munition will cause a simultaneous, or nearly simultaneous, explosion in the surrounding munitions. In the test, one munition is intentionally detonated, and the rest are free to react. If one munition’s reaction is no worse than an explosion and the other munitions do not react, they pass this test.


Bullet- and fragment-impact tests are performed to check a munition’s reaction to small-arms fire and impact from high-speed fragments that may come from sources such as exploding bombs or artillery shells. Depending on the threat-level assessment (what the Army thinks might be fired at the particular type of munition being tested), various rounds are fired at the munition in its container. The projectiles vary from 5.56 millimeters up to .50 caliber for the bullet-impact test. Both armor-piercing and ball ammunition are used. The Department of Defense has developed more specific criteria for the fragment-impact test, which includes size, shape, and speed of the fragment. If the munition’s reaction is no worse than burning and no hazardous fragments are thrown, it passes these tests.

The final IM test is the shape-charge jet impact. A munition is hit with a shape-charge jet to see how it reacts. If the munition detonates, it fails this test. (A shape-charge jet is a hollow metal cone built into a projectile. Explosives packed around the outside of the cone detonate on impact, squashing the cone and forcing a fine jet of metal out of the front of the shell.)

All IM testing must be approved in advance by the Army Insensitive Munitions Board. After the tests, the results are presented to the board for evaluation. The board has the final say in test implementation, test procedures, and data analysis. In other words, the Army Insensitive Munitions Board is responsible for declaring whether or not an item is “insensitive” and ready for fielding.

The Army’s IM program has led the way in ensuring the safety of the munitions that Soldiers, Sailors, Airmen, and Marines use to protect and defend themselves, their units, and their country. After all, the enemy is the only one who should ever experience the power of our ordnance.
ALOG

Robert M. Forrester is an engineer in the Logistics Research and Engineering Directorate of the Armament Research, Development, and Engineering Center at Picatinny Arsenal, New Jersey. He has a bachelor’s degree in mechanical engineering from Virginia Polytechnic Institute and State University and a master’s degree in mechanical engineering from the Stevens Institute of Technology in New Jersey.

Kendal M. Duncan is an explosives logistics specialist in the Logistics Research and Engineering Directorate of the Armament Research, Development, and Engineering Center at Picatinny Arsenal, New Jersey. He has co-chaired the Army Insensitive Munitions (IM) Board, served as the Army representative on the Joint Services Insensitive Munitions Technical Panel, assisted in managing IM improvement projects for Army munitions, and developed the Department of Defense Insensitive Munitions Handbook and Army policies and procedures for the implementation and management of IM within the Army.