Tank Gun Accuracy

by Major Bruce J. Held and Master Sergeant Edward S. Sunoskl The Accuracy Problem Operation DESERT STORM showcased the technological capabilities of the M1-series 'hk. Its combination of unmatched mobility, lethality, and survivability proved unstoppable. Additionally, the accuracy of the MlAl 120-mm main gun exceeded all expectations. First round hits at two to three thousand meters were common. However, the Army cannot afford to be satisfied with the MlAl's current level of accuracy. The M1-series itself is an example of what superior technology means on the battlefield. We can expect the next generation of tanks, to include those belonging to potential adversaries, to be more accurate than even the M1 series. This article will discuss some of the pmb-lems that scientists, engineers, and tankers must overcome in order to achieve the accuracy needs of the future.

The average NATO tank target is approximately 2.3 meters by 3.4 meters. At 3,000 meters, this equates to an angular measure of only 0.4 mils from the target's center of mass to its upper or lower edge. Therefore, with an aimpoint in the middle of this standard target, the tank's total system accuracy must be less than half a mil in order to hit consistently at 3,000 meters.

To emphasize how small half a mil is, picture the second hand of a watch. The angle it sweeps in five one thou-sandths of a second is about a half of a mil (see Figure 1). At longer ranges ur against defhde targets, accuracy must exceed even this.

Accuracy Error Sources Perhaps the easiest way to describe sources of accuracy error is to proceed chronologically through the engagement process and identify the potential problems. Once a target is identified, this process proceeds from finding an aimpoint on the target to the projectile hitting or missing the target. Potential accuracy errors occur during this entire process. Some of these can be minimized by the tank'screw, and this article will point out crew actions that will help minimize accuracy problems. Finally, this article only de& with a stationary firing tank versus a stationary target. The accuracy equation gets much more complicated when the fuing tank, target, or both are maneuvering. Laying the Reticle on the Target Once a target is identified, the next step in the engagement process is lay-Second= 1 Mil equals 30 Mils 1/64UUth of a 360-degre Figure 1. circle ing the reticle on the target. An integral part of the fm control system in any tank is the gunner. Even with a perfect system and perfect conditions, accuracy can still be poor if the gunner uses his system ineffectively. Factors that affect gunner performance include: fatigue, fear, inexperience, and excitement. Fortunately, the effects of these problems can be reduced through good training and pctice. While it may seem boring at times, repetition is what makes correct action automatic for the tank crew. Crew drills and the COFT are the skill builders that best integrate the men and machine so that they operate as a single, efficient system, even under harsh training or combat environments.

One basic assumption that we make in aiming any direct fire weapon is that the light path to the target is straight. Unfortunately, this is not always true. Just above ground level, heat is exchanged between air and soil. The result of this is that the air tempenture at the surface of the ground is not constant. As light passes through air of varying temperature, its path bends. This means that the line of sight between a tank and its target is not always a straight line. This phenomenon is known as optical path bending, and when it happens, the cannon cannot be effectively laid on its target since the line of sight to the target is no longer a straight path (see Figure 2). Optical path bending is at its worst after the ground has been heated up and the air is still and cool,

The gunner aims along the apparent line b the target. However, since the actual light path is bent, the line of fire misses the actual target.

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such as in the desert at night. The same kind of atmospheric conditions also cause heat shimmer, which also has a detrimental effect on accuracy. Under extreme conditions, the error can be greater than a mil.’ Typically, though, this error is less than a tenth of a mil, so even at long range, it is not a big problem under normal conditions. Even when conditions are right for optical path bending, its effects CM still be minimized. When the tactical situation permits, positioning of the firing tank so that the line of sight to potential targets is well above ground level will reduce the problem. Generally,’ it does not take much. Keeping the line of sight as little as three or four meters above the ground for most of its path will Virtually eliminate any problems!?om optical path bending. In addition to the optical path bending problem, there is an error associ- ated with finding the aimpoint on the target. Generally, tankers are trained to use the center of presented target mass as the aimpoint. The problem is that estimating the center of mass is not a precise process. To demonstrate this to yourself, stand a few meters from a blank piece of butcher paper. Have someone mark what you think is the center of mass on the paper. While you can get reasonably close, it is difficult to be exact. The same is true for reid targets. Without some detined aimpoint on the target, there will be some small error associated with estimating the center of target mass. Fortunately, this is an error that the gunner can reduce. The key, of course, is practice. During any slow time, such as in the motor pool or waiting to enter a range, the gunner can find ‘targets’ and practice estimating their centers of mass. Use of the reticle lines as a guide will also help. After some practice, gunners should have little difficulty quickly and precisely estimating the target’s center of mass. Laying the Gun Once the aiming circle has been placed on target, a whole series of complex events occur that result in pointing the cannon dong a trajectory that intersects the target. The fmt potential error source at this point is one that probably occurred long before a target was ever seen boresighting. Boresighting procedures are established in order to align the sighting system with the cannon’s muzzle axis. Error is introduced in these procedures if the boresight is inherently in- accmte, if it is out of calibration, or if the procedures for using the device are not correctly followed? For these reasons, boresighting does not always perfectly align the cannon and sighting system. Additionally, for various maintenance. environmental, and equipment reasons, it is often difficult to maintain the calibration between the cannon and fire control system once it is established with boresighting procedures. For example, tempera- ture changes across the cannon from environmental factors such as sun, wind and rain or from firing can change the shape of the gun in just seconds. Muzzle reference systems (MRS) and thermal shrouds are designed to minimize the calibration er- rors between the muzzle axis and the sighting system between boresighting occasions. Unfortunately, MRSs are themselves subject to some error and may not always perfectly realign the system? They also depend on being correctly boresighted themselves and on being used often enough between boresighting events. Finally, while thermal shrouds help a great deal, they cannot perfectly manage thermal bending of the gun tube! Tests conducted at Ft. Knox and Aberdeen Proving Ground have shown that the boresight devices used by the Army are quite accmte. This means that the tank crew is the key to reducing boresight emr. By following the boresighting procedures precisely and by carefully maintaining and caring for the boresight device, a tank’screw can assure itself that it will have an accmte boresight. By conducting a boresighting exercise whenever possible and performing frequent M R S u p dates between boresighting events, the crew will help ensure that the m u zle/sight alignment error is small. On older tanks, the cannon and the sights used to aim the cannon were hard-mounted together. Superelevation was applied through the use of stadia lines and ballistic m s, while other ballistic corrections were applied with the application of ‘Ken- tucky Windage.’ On modem tanks, except in degraded mode, the fire control computer uses the factors that went into ‘Kentucky Windage,’ along with a precise measurement of range, and knowledge about the ammunition being fired and calculates the direction to point the cannon. To do this, the fire control computer depends on various data inputs (cant, propellant temperature, ammunition type, etc.). It applies these inputs to mathematical algorithms and calculates the projectile’s ballistic path. Errors occur in this calculation for two reasons. First, the mathematical algorithms only approximate models of reality. The ap-proximations are very good, but they are not perfect. Some of the factors that affect the ballistic path are imperfectly understood, or there may not be a satisfactory way to measure them. This means that some factors that influence the ballistic path may not be included in the mathematical algorithms, or are not modeled completely. Second, the math algorithms depend on the various inputs mentioned above. If the measurement of these variables is incorrect, or if they are incorrectly entered into the computer, error is introduced into the fire control solution.

The tank crew can ensure that the fm control computer operates most effectively by carefully following published procedures. Like all other computers, the fire control computer goes by the maxim of ‘garbage in, garbage out.’ Data that is manually entered into the computer, such as barometric pressure, should be as current as possible. Tankers should demand such data on a routine basis and units need to have SOPS in effect that update this information as often as possible. Additionally, good preventive maintenance checks and services (PMCS) will ensure that sensors on the tank, such as the crosswind sensor, are in good operating condition and are not providing bogus input to the fire control computer. Once a fm control solution has been calculated, it must be applied to the various motors, hydraulic pumps, and other mechanisms that control the motion of the gun. Since no electronic or mechanical system is perfect, implementation errors between the calculated fire control solution and what is possible in the machinery occur in this process. Complete PMCS by the tank crew, to include special gunnery checks, will ensure that.implementation emrs are minimized. If the system is well maintained and problems are quickly and effectively corrected, the fire control system errors should be very small.

The Shot Process -In Bore The firing of modem tank ammunition is an exceptionally violent process. Once.the cannon, target, and sighting system are ballistically aligned, the gunner starts what is essentially a controlled explosion. In the space of less than five meters and in less than a hundredth of a second, the projectile accelerates to a velocity of 1,600 meters per second: roughly Mach 5 (over 3,500 mph). The pressure needed to push the projectile to these velocities in such a short time span approaches 100,OOO pounds per square inch for some ammunition types. Precise control of this process is necessary if accuracy is to be maintained. Understandably, perfect control is very difficult to achieve in this violent environment.

Cannon systems are not completely rigid. Unleashing the energy that pro-pels the projectile can also cause the cannon to rotate about its trunnion, recoil along its longitudinal axis, shake in its recoil mechanism, and bend and vibrate in all directions. This all starts to happen before the round exits the muzzle, so that by the time it does, the muzzle is not pointed in the same direction as when the trigger was pulled?

The cannon’s dynamic action would not hurt accuracy if all shots and all tanks were the same. If that were the case, a fire control computer correction could correct the problem. In fact, computer correction factors do correct some of the gun dynamics errors. Unfortunately, a cannon’s dynamics are a little different on each shot,6 and there are significant differences in the gun dynamics between tanks? This means that the change in muzzle pointing angle at the time of shot exit from the cannon cannot always be accurately predicted and accounted for. While the muzzle pointing error that this lack of predictability creates is relatively small, the cannon’s dynamic response to the shot process also ties in with some other phenomenon, such as an accuracy dependence on ammunition temperature and the projectile aerodynamics. Because of this, controlling the cannon dynamics is significant to improving the tank‘s accuracy.

The tank crew’s ability to influence the gun tube dynamics depends, once again, on good PMCS. By ensuring that the cannon and recoil system are constantly inspected for problems, and that identified problems are corrected, the tank crew will be doing its part toward ensuring that the gun dynamics remain as constant as possible for every shot.

As the round travels down the length of the cannon, it not only gets pushed from behind by the burning propellant, it also gets pushed sideways, up and down. This occurs for several m-sons. It was already noted that the cannon is vibrating before the round exits. Imagine quickly shaking a tube up and down while a tennis ball rolls from one end to the other. The same thing happens to the projectile as it moves down the cannon.

Additionally, no tube is perfectly made. Small bumps and bends are introduced in the manufacturing process that the projectile must ride over as it travels down the cannon.

Also, thermal bending of the gun tube occurs. This creates more curves for the projectile to negotiate. Finally, the gun tube is along, heavy structure that is only supported at one end. This

- -Figure 3.

Photograph captures the moment when the sabot petals of an M865 training round break free from the penetrator.

Note the visible shock waves generated by the discarding sabot petals. means that gravity causes it to droop toward the muzzle?

These two factors, the gun dynamics and the static tube shape (caused by manufacturing irregularities, thermal distortions, and gun tube droop), do two things that affect accuracy. First, they cause a projectile to follow a crooked path down the gun. Second, they cause the projectile to vibrate in bore. This is known as projectile balloting. The crooked path the projectile follows in bore, and balloting, are important to xcuracy because they can influence the direction of flight with which the projectile leaves the muzzle. Also, they cause the projectile to flex and vibrate, which affects later portions of the shot process. The Shot Process -Transition to Free Flight During the transition from in-bore travel to free flight, the projectile is subject to several more processes which can affect accuracy. As the round exits the cannon, the projectile and sabots undergo rapid decompression from the in-bore flexure and compression. This can slightly change the direction of flight and later affects the sabot discard process and the aerodynamic properties of the round. Also, as the projectile's obturator seal exits the muzzle, the hot gases behind the projectile expand and accelerate around the projectile. This means that the air flow over the projectile is the reverse of normal, free flight air flow. In this flight configuration, the projectile's fins actually destabilizing. The muzzle blast can thus cause deviations from a perfect launch and magnify round-to-round variations in the launch?

Saboted ammunition has additional accuracy problems. As the sabot petals begin to fly free from the penetrator (Figure 3, above), they can interfere with the penemtor both mechanically, by striking the rod," and aerodynamically, with the shock waves that they create." The mechanical and aerodynamic interactions between the sabots and the projectile vary with every shot. Therefore, accuracy is affected because the effect of sabot discard cannot be predicted precisely and accounted for. The Shot Process -Free Flight Finally. once a projectile enters free flight, it is subject to aerodynamic forces. These forces can alter the projectile's line of flight even further from the one originally intended. This is known as aerodynamic jump. A projectile's aerodynamic chmcteris-tics depend on its shape and its pitch-ingyawing motion. I2 Pitching describes the up and down rotation of the projectile and yawing is the side to side rotation (Figure 4). This motion is imparted to the projectile during earlier phases of the shot process. Since pitchindyawing motion varies from round to round, the aerodynamic jump will also vary from round to round.

During its free flight phase, crosswind affects every projectile to some degree. HEAT ammunition is much more susceptible to crosswind effects than saboted ammunition bemuse of its lower initial velocity, higher retar-dation (loss of velocity with range), and larger cross sectional area Cross-; winds can vary to a large degree in both magnitude and direction over the flight of the projectile, and thus cannot necessarily be compensated for1 by the measurements made at the tank'by the crosswind sensor. This is particularly true when the tank is dug into a defensive position and the crosswind sensor is shielded from the wind. In general, the ability to estimate crosswinds diminishes with range, so crosswind effects increase with range. At short range, crosswind errors for high-velocity ammunition can be negligible, but at long range, the effect can be significant. As mentioned ear-

VELOCllY/DIRECTION OF FLIGHT

Amazingly, the earth's rotation can aIso have a noticeable effect on the accuracy of tank ammunition. Imagine riding a merry-go-round and trying to throw a ball at someone else sitting on the other side of the ride. The rotating paths on which you and your target are traveling make this a much more difficult problem than it would seem at first. This effect, known as coriolis acceleration, is particularly noticeable at long range with slower projectiles, such as HEAT, but even high-velocity ammunition is affected at very long range. While the mathematical solution for this effect is easily calculated, it depends on knowing the tank's location and the absolute pointing direction of the cannon. Since this information is not available to the fire control computer of MlAls and Mls, there is currently no compensation for the rotation of the earth effect. Finally, improper storage and h a -dling of ammunition can cause damage that will seriously impair the accuracy during every phase of the shot process. Damage to cases and improper storage can introduce moisture into the cartridge and cause uneven propellant burning. This causes the in-bore time and muzzle velocity of the ammunition to vary. Damage to ob-turators, sabots, and sealing rings can cause problems in-bore and during the projectile's disengagement from the tube. Damage to a projectile's tip and fms will affect its aerodynamic performance. Any one of these various factors will cause accuracy problems. Taking care to protect ammunition is also critical from a safety standpoint. The key here is to inspect and correct. If ammunition looks bad, it probably is, and should be carefully checked by a certified ammunition specialist. Not qualifying on Table VI11 because of improperly maintained or inspected ammunition is a tank crew's nightmare. The effect in combat could be worse.

Concluding Remarks At the crew and unit level, the key to tank gun accuracy is constant training and effective, continual PMCS of both the engagement system on the tank and the ammunition it will fue. Dedication to and enforcement of these two principles will reduce or eliminate the element of human error in the accuracy equation. On the material side of the accuracy equation, research and testing of new ideas and new technologies must continue in the areas of fire control, cannon systems, and ammunition. The ideas and technologies that are developed need to be evaluated and, where appropriate, incorporated in existing and future tanks. As mentioned at the beginning of this article, we have achieved an amazing level of accuracy with the M1-series tank. Crews feel confident about hitting targets at ranges in excess of two or three kilometers. This creates a danger that we may become complacent. To win in the future, our tanks must continue to be able to fmd and hit targets before they find and hit us. From the lab to tank tables, constant attention and correction of accuracy problems is critical to future success.

Notes 'Tolfsted. D. and D. Snider. Refroction Up dute, paper pmented at the Tank Gun Accxl-racy Committee Meeting, Aberdeen Proving Ground, Md.. 20 May 1986. 2Held. B. J. and D. W. Webb, A Cornprison of Muzzle Boresightr For Tank Cannon, BRL Memorandum Report. BRL-MR-3977. U. S. Amy Ballistic Research Laboratory. Aberdeen. Md, June 1992.

3Bundy. M., Thermal Dirtortion of fhc MlAl Muzzle Reference System Collimator. BRL-lR-3107. U. S. Amy Ballistic Research Laboratory. Aberdeen Proving Ground, Md., June 1990. 4Bundy, M.. Thermal Distortion Protection by Caddate Metal ond Composite Thermal Shrouds for 120-mm Tank Cannon, BRL-TR-2807. U. S. Army Ballistic Research Laboratory. Aberdeen W i g Ground. Md, June 1987. 'Gay, H. P. and A. S. Elder, The Lderal Motion of a Tank Gun ami Its Effec? on the Accuracy of Fire. BRL Report No. 1070, U. S. Army Ballistic Research Laboratory, Aberdeen Proving Ground. Md., March 1959.

6Held, BJ., Variability in Tank Gun Accuracy D m to Recoil Variation. BRLTR-3309. U. S. Army Ballistic Research Laboratory. Aberdeen Proving Ground. Md.

January 1991. 'Bornstein.

J., T. Erline.

B. T. Haug. and D. Hopkins, "Investigations on the Dynamics of Tank Guns," Eleventh In?erna?ioml Symposium on Ballistics, Brussels, Belgium, May 1989. and J. Bornstein. "The Effed of Sabot Front Borerider 8Plostins. P.

I. cehins.

1977.

"Hostins, P. Launch Dynamia of APFSDS Ammunition, Proceedings of the Eighth Intema-June 1978. pp.162-167. American Institute of tional Symposium on Ballistics, American De-Aeronautics and Astronautics. 370 L'Enfant feme Preparedness Association, Arlington, Va. Promenade S. W., Washington D. C. October 1984.

12Murphy. C.. Free Fligh? Motion of Symmet- "Sdunidt, E. M. and D. Shear, "Aerodynamic ric Missiles, BRL-TR-1216, U. S. Army Ballis-Interference During Sabot Discard," Journal of tic Research Laboratory. Aberdeen Proving Spacecraft and RocRpts, Vol. 15, No. 3, May-Ground, Md.. July 1963.

ARMOR - Major Bruce J. Held was commissioned in 1980. He holds a BS degree from the United States Military Academy and earned an MS in aerospace engineering from Stanford University in 1989. His military education includes MIOBC, the EW/ Crypt0 Course, AOAC, Airborne, and CAS3. He is currently assigned as the Armor Technology Manager at the Army Research Laboratory. Previous assignments include S2 positions in the 4th Infantry Division, Ft. Carson, where he was also the scout platoon - leader of 6/32 Armor. He was also the S4 and a troop commander with 2/2 Armored Cavalry Regiment in Bamberg, Germany. Master Sergeant Edward

S. Sunoski is currently assigned as the NCOIC of the Weapons Technology Directorate, Army Research Laboratory, Aberdeen Proving Ground.

Previously, he served as first sergeant of Troop B, 5-12 Cavalry; Troop A, 1-12 Cavalry; and Company B, 1-77 Armor.

Other assignments include master gunner, 3d Infantry Division; platoon sergeant, Company B, 3-64 Armor; platoon sergeant, Company D, 3-63 Armor; platoon sergeant, Company A, 2-34 Armor; and master gunner, 1-77 Armor. He is a 1978 graduate of Adams State College with a BA in joumal-ism. He is a 1978 and 1981 graduate of the Master Gunner Course and is a graduate of Class 37, United States Army Sergeants Major Academy.