“GUNNER, MPAT...”

by Greg Kolasa, Wakeland Kuamoo, and Michael Bono The 120mm M830A1 high-explosive, multipurpose tracer round, generally referred to by tankers as MPAT, was first used in combat during Operation Iraqi Freedom (OIF). The MPAT round proved to be quite versatile during OIF, even though there were no real air threats. MPAT quickly became the “round of choice” against ground-type targets. The round has been available for more than 10 years; however, there seems to be some misunderstanding on exactly how it functions. The combustible cartridge case is built to specifications similar to all other 120mm rounds, no surprises there. The warhead consists of a point-initiating, base-detonating (PIBD) fuse, along with a chemical explosive content encased in a steel cylinder approximately 80mm in diameter. This steel case is surrounded by three sabot petals, which discard shortly after exiting the muzzle during firing. At the top of the warhead is an electronic proximity sensor with a ground/ air selection switch. When firing MPAT, the loader can use ground mode (the normal setting) or switch the proximity sensor to “air” if instructed to do so during the fire command. The round is then loaded into the gun and fired. This is the easy part; the tough part is understanding how the fuse, switches, and proximity sensor on the MPAT round function. We will start with the M774 PIBD. When the round is fired, “setback” occurs. Setback is similar to sitting in a quickly accelerating car or airplane. As the vehicle moves forward, it feels as if you are being pushed further back into your seat. As the round starts to accelerate, components within the PIBD are pushed further back toward its base. During this setback period, two things happen: the setback generator is activated and produces a voltage, which is stored on a capacitor and used later to detonate the explosive composition; and the latches within the base fuse are set in motion to begin lining up the firing mechanism. When the round clears the muzzle end of the gun tube, it will start to decelerate and “set forward” occurs. When the speed decreases, you feel yourself moving forward in your seat, as in the example of the speeding car or airplane. This set-forward action allows further movement within the base fuse and completes alignment of the firing mechanism. The round is now fully armed and awaits the voltage that was initially stored on the capacitor. This setback and set-forward action assures the round is bore safe out to a minimum of 11 meters from the muzzle, which poses the question: is the round fully armed after 11 meters from the gun? As with any mechanical device, not all things happen at exactly the same time. By design, no fuse can be armed below 11 meters, and at 60 meters, all fuses must be armed. In between the two ranges, fuses can arm at different points with most armed at approximately 35 meters from the gun. We will now look at the M74 proximity switch located at the front of the round. When ground mode is used, on impact with a target, the crush switch is closed. This action allows the voltage stored on the capacitor to flow to the firing mechanism and the warhead detonates. Approximately three inches behind the front crush switch is the shoulder switch. When closed on impact at an angled surface (such as the upper glacis of a BMP), it will function the same as the front crush switch. The “air mode” selection causes a couple of other things to occur before detonation. First, during setback, a small battery is activated in the nose, which powers up the proximity sensor. As with the base-detonating fuse, there are variations in range when the proximity sensor would be fully activated. Four-hundred meters is the minimum range for activation of the proximity electronics, and at 1,000 meters, all sensors must be fully powered up. Generally, a significant amount of the sensors would be fully active in the 600- to 800-meter range band. A radar-like procedure is used by the proximity sensor to detect a target and receive a significant radio frequency return. This target is generally a mass of a certain size and density, which could be a real target or ground clutter. Once a sufficient return is sensed, the proximity sensor closes a circuit, allowing the voltage stored on the capacitor to flow to the firing mechanism, detonating the warhead. The radar 32 — May-June 2005

Radar Searching Limits Radar Searching Limits Area which will return signal Area which will return signal

NOT TO SCALE

searches in a circular scan; however, it does have a blind spot, which prevents any type of return from an object in that zone. The good news is that this blind spot was put there intentionally, so that a round flying directly toward a target, such as a helicopter, would fly right into that target, closing the crush switch and det onating the round, which gives greater lethality than if the round airburst in front of the target. The blind spot does not affect the way a round is fired. There is a trembler switch located in the base fuse, which can also detonate the warhead. The trembler switch is similar to a tilt switch found in a pinball machine, except it requires more than just tilt ing the round for the trembler to function. When the round impacts a soft target or area, such as a sandy surface, which may not be solid enough to close the nose crush switch, the trembler switch closes and allows the round to detonate. The ability of the trembler switch to activate on all surfaces (ground self-destruct capability) ensures no duds are left on the battlefield. To further reduce duds in the field, the MPAT has a self-detonation feature. When the round is fired in the air mode, an electronic clock is activated. This clock provides a self-destruct capability if the round does not detonate after approximately nine seconds of flight. The MPAT round defeats enemy targets by penetration or fragmentation effects. When the round detonates, explosives within the body of the warhead form into a molten jet of plasma and copper. This jet, traveling at approximately 7,000 meters per second, literally “melts” its way into the object and defeats the target. As detonation occurs, the metal body of the warhead is fractured, exploding into many smaller pieces thus creating fragmentation. This fragmentation allows the round to defeat targets, such as a helicopter, without having to actually hit the target. Concerns from the field indicate there were times when the round did not appear to detonate when fired. Tankers must remember if you fire the round in ground mode, the minimum arming distance is 11 meters, maximum arming range is 60 meters, and most fuses should arm at around 35 meters. In the air mode, a target fired at less than 400 meters means the proximity sensor would not activate. However, if a target is impacted and one of the crush switches close, the round would detonate on the target, provided the fuse had fully armed. If air mode must be used, crews should engage targets at approximately 1,000 meters to ensure the proximity sensor will activate. However, if a reduced range is required, the 600- to 800-meter range band should provide a good confidence level of assurance that the proximity switch will activate as designed. Remember, the round could get a return from normal ground clutter or other objects, which could cause detonation before contact with its intended target. The MPAT round has proven to be extremely effective against all types of targets. The key here is to fully understand its functions and capabilities. Used correctly, the MPAT round is a great combat multiplier. There is currently another round being used in Iraq, which is very similar to the MPAT, called the M908 high-explosive, obstacle-reduction tracer (HE-OR-T). Gen erally, you will hear tankers refer to this round as the MPAT-OR or just the OR round. The major difference between the MPAT and the OR is that the OR round does not have a proximity sensor in the nose of its projectile. Instead, this sensor has been replaced by a yellow-colored steel nose. The steel nose allows additional penetration of the target by the warhead before detonation. The M774 fuse-safe and arming distances for the M908 are the same as for the MPAT round. In the current war, the OR round is being used against walls and obstacles. Tankers must remember to select MPAT on the fire-control system when firing this round; there is no separate selection for the OR round. Greg Kolasa is currently project leader for M830A1 MPAT, U.S. Army Research Development and Evaluation Command, Picatinny Arsenal, NJ. He received a B.S. from Newark College of Engineering at New Jersey Institute of Technology. Between 1980 and 1984, he served as project leader for various 105 and 120mm tank ammunition programs and has been involved with the MPAT program from its inception. Wakeland Kuamoo is currently an armament and munitions consultant for Program Manager-Maneuver Ammunition Systems, Joint Munitions Command, Fort Knox, KY. He also provides contractor support for tank main gun ammunition to the Program Manager-Maneuver Ammunition Systems at Picatinny Arsenal, NJ, and to the tank team of the Joint Munitions Command at Rock Island Arsenal, IL. During his military career, he served as senior live fire trainer, Bosnia and Herzegovina; chief, Master Gunner Branch, Fort Knox, KY; first sergeant, A Company, 2d Battalion, 35th Armor, Fort Carson, CO; senior armor trainer, Technical Assistance Field Team, Republic of Yemen; and division master gunner, 2d Infantry Division, Korea. Mr. Michael Bono is currently a tank ammunition production engineer, Picatinny Arsenal, NJ. He is currently involved with the XM1002 training round program, which was developed to simulate the firing of the M830A1 MPAT round on training ranges. He received a B.S. from Virginia Tech University. He has served in a variety of positions, to include lead test engineer for 120mm ammunition accident investigations; and as a member of the Tank Training Ammunition Production Team, Picatinny Arsenal. Figure1. MPAT in Air Mode searching for an enemy helicopter “The MPAT round defeats enemy targets by penetration or fragmentation effects. When the round detonates, explosives within the body of the warhead form into a molten jet of plasma and copper. This jet, traveling at approximately 7,000 meters per second, literally “melts” its way into the object and defeats the target.” May-June 2005 — 33