Use of the Pressure Setting In the Ml/MlAl Fire Control Computer

Use Of The Pressure Setting In the Ml/MIAl Fire Control Computer by Mr. David H. Tofsted and SFCWakeland Kuamoo Introduction Several atmospheric conditions influence the flight of tank rounds, among them air pressure.

Pressure can present a large problem in tank gunnery because pressure slowly decreases with altitude above sea level. Decreasing pressure also means decreasing air density; and for the high-explosive antitank (HEAT) round, this influence can be large at ranges of 1,500 meters and beyond. Crucial to the problcm is the use of inaccurate barometric pressure settings (BPS). Recently, the U. S. Army Atmospheric Sciences Laboratory reviewed the method for adding pressure corrections into the Ml/ML41 ballistic solution. The review showed that though the fire control computer (FCC) correctly treats pressure effects, an accurate pressure value is often difficult to obtain.

In this instance, the MIIMLA I operator's manual instructs the gunner lo index 29.92 as standard pressure. This practice can lead to large vertical errors when firing the main gun.

Analysis of Current Method In a training environmcnt, the value of local pressure may be obtained from range control, a weather station, or by other accessible means. However, in a tactical environment, to gct accurate weather information to the units may be extremely difficult. Therefore, they will have to use the standard pressure correction value. Since air pressure decrcases stcadily with Use of a "standard" barometric pressure setting in the M1 fire-control computer can lead to considerable gunnery error when the tank is not firing at sea level. The chart at right provides accurate corrections. height, large errors can result if the area of operations is significantly higher than sea level.

An example of this type of error can occur at 1,2(K) melcrs above sea level. At this height, normal pressure is roughly 26 inches of mercury. Use of the standard pressure correction (29.92) at 1,200 meters would result in a.64 mil error in elevation when firing an M456 HEAT round at a target at 2,000 meters. The round flies higher because the air is 13 percent thinncr. In addition, the effects of pressure errors are non-linear so that at 3,000 meters, the error could increase to ovcr 2 mils with an increase in range of only 1,CXn) meters.

Additionally, evcn if a valid pressure is entered at one time, atmospheric pressure changes by about one perccnt ovcr every 100 mcters of altitude change. These changes mean that in irregular terrain, pressure at one location could be incorrect lor another location. I 30 ARMOR - I

"... A second source of error is associated with the need to update the BPS when moving to anew elevation. Due to today's rapidly moving battlefield, continuous updating of the BPS may not be feasible

Table 1 Ml/MlAl Pressure Setting Corrections for Various

Setting

29.92 29.57 29.22 28.87 28.53 28.19 27.85 27.52 27.19 26.86 26.54 26.22 25.90 25.59 25.28 24.97 24.66 24.36 24.06 23.77 23.47 23.19 22.90 22.61 22.33 22.05 21.78 21.51 21.24 20.97 20.71 0 Determine elevation using 0 Read across to determine a map. the correct pressure setting.

To provide a more accurate barometric pressure setting (BPS) for the FCC, we have composed a simple table (Table 1). This table is based on height information from a standard topographic map. Tahle 1 will provide a BPS for the FCC, based on elevation (to the nearest 100 meters), that would be readily available to the tanker in the field. Errors in Proposed Solution A source of error in the elevationbased proposed solution appears in terms of pressure changes due lo the weather. Since only a standard pressure at each elevation would be used, any variations due to weather would be unaccounted for. However, the degree of this error appears to be minimal. A sccond source of error is associated with the need to update the BPS when moving to anew elevation. Due to today's rapidly moving battlcfield, continuous updating of the BPS may not be feasible. Tank crews should identify the most likely terrain (elcvation) in which they expect contact and index the appropriate BPS into the FCC before entering this area. Summary Errors due to the current method (standard pressure value) are larger than those in the proposed solution. The proposed solution avoids the possibilities of severe degradation which could occur when using the current method. It also allows the tank crew to determine the pressure immediately and to update the FCC as needed. This update can be accomplished easily during normal unit training and more important, when it may really count, in combat. Since this proposed solution would not entail the modification of any hardware, it could be implemented immediately. Mr. David H. Tofsted Is a 1979 graduate of Pennsylvania State University with a bachelors degree in physics and a 1980 graduate of the Signal Officer Basic Course. He served as a physicist with the U. S. Army Atmospheric Sciences Laboratory during his time in service and currently continues work there as a civilian physicist. His research work has covered various topics related to atmospheric effects on tank gun accuracy, Including refraction, crosswind, and pressure effects.

First Class

K. Kuamoo entered the Army in December, 1973. He is a graduate of the Armor NCO Basic and Advanced Courses, the Air Assault Course, and the Master Gunner's Course. He has sewed in key positions with the 1st AD, 9th ID, 25th ID, and 2d ID. He has been a senior instructor at the USAARMS Master Gunner's Branch and is currently assigned as a master gunner in the Gunnery and

Branch of the Weapons

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