Big Men and Tanks

W - The height range of US. tankers is obvious in this photograph of 5-fOOt. 3-inch SP4 Curtis Johnson, right, and 6-foot, 4-inch SSG Derick McElveen, both of 5-73rd Armor at Fort Knox. i Big Men and Tanks Military designers are constantly striving to maximize the battlefield survivability of army materiel. In the area of tank design, conventional wisdom has dictated that one way of achieving increased survivability is to present the smallest target area to the enemy. Accordingly, designers have for some time looked for new ways to reduce tank heights. It has been suggested that one way to achieve a reduction in the height of combat vehicles is to limit the maximum allowable size of crewmen. The assumption is made that smaller crewmen will allow for smaller tanks. However, restricting the crewmember height would have little effect on the size of tank silhouettes. Rather, it would serve only to decrease the size of the pool from which capable crews could be drawn. Army combat vehicles are designed to accommodate crewmembers who range in size from the 5th to the 95th percentile of male soldiers. The 95th percentile U. S. Army male is 73 inches tall. Soviet crewmembers are, on the by Ronald W. Cammarata other hand, smaller than their American counterparts; Soviet tankers have a general height of 66 inches.' Despite the generally shorter height of the Soviet Army tank crewmembers, that height differential is not reflected in the height differences of the crew compartments of Soviet and American tanks. Though the U. S. Army MI Abrams tank is 6.5 inches greater in height than the Soviet T-72 main battle tank, the crew compartment in the Abramsat 65 inches is only l-inch higher than the 64-inch high crew compartment of the Soviet T-72. Most of the 6.5 inches difference in the heights of the T-72 and MI tank. can be accounted for elsewhere. The ground clearance of the MI is 2.5 inches greater than the T-72 and it has 3 inches more space under its turret to accommodate a rotating turret floor. These differences account for 5.5 inches of the total 6.5 inches difference in the height of the two vehicles. Limiting the height of tank crewmembers to men smaller than the 95th percentile would influence the height of the crew compartment only if there is a significant difference in the seated heights of crewmen and then only if the seated height of a crewman were the only factor that determines the required height of the tank. Conventional seats with straight backs used most often in army vehicles tend to maximize the height differences of seated crewmen. The sitting height of an arctic-clothed 95th percentile Army male is 39.9 inches. The same dimension for a 50th percentile arctic-clothed Army male is 37.3 inches, a difference of only 2.6 inches. (Most proponents for reducing the allowable size for a tank crewman suggest the 75th percentile U. S. Army male, or larger, as the maximum. The 50th percentile army male size is used here as a worst case.) For conventional seating arrangements, MIL-HDBK-759 (military handbook) recommends a seat height of 6 inches minimum and 15 inches maximum and a minimum seat-to-ceiling distance of 42 inches for all crewmen. If the space necessary to 18 march-aprill985 accommodate the seated crewmen were the only criterion that determined the height of the crew compartment, then the suggested minimum height of 48 inches -6 inches plus 42 inches -is a full 17 inches less than that available in the M1 tank -and 16 inches less than in the T- 72 tank. The supine position reduces the height, as well as the height differences, of seated crewmen compared to the more conventional upright seating arrangement. Although MIL-STD-1472C (military standard) and MIL-HDBK- 759 provide no guidelines for the supine position, U. S. Army Human Engineering Laboratory Letter Report 243 recommends dimensions for a supine seating arrangement.* Restricting the size of crewmen to percentiles less than the 95th when employing the supine position reduces the required height of a crew compartment less than the same restriction when employing conventional seating arrangements. If a tank were to be designed to achieve the absolute lowest silhouette possible and tank crewman height were the only criterion used to determine the overall height of the tank, then the maximum reduction in height that could be achieved by limiting the size of crewmen to the 50th percentile male soldier, as opposed to the 95th percentile, would be the difference in the seated height of the two percentiles, about 2.6 inches. The reduction would be only 1.5 inches if the limit were the 75th percentile male soldier. If the turret floor is necessarily 20 inches above the ground because of other restrictions, then the lowest ceiling height of a tank that could accommodate a 50th percentile arctic-clothed Army male soldier using conventional seating would be more than 63 inches high. The same tank, designed for a 95th percentile male soldier, would be nearly 66 inches high. (The lowest ceiling heights that would accommodate each percentile soldier are determined by adding the soldier sitting height to the tank’s floor height of 20 inches and the minimum seat height of 6 inches.) This means that designing for the smaller man would reduce the total height of such a tank by only 2.6 inches, or, at the most, 3.9 percent. The differences between the 95th percentile male soldier and soldiers of percentiles greater than the 50th (like the 75th or 80th) would affect the height of such tanks by even smaller percentages. The firing height of the MI tank (the centerline of the gun tube when the gun is level) is 74.4 inches above the ground. The turret floor is 24 inches above the ground. When the gun is level, there are 50.4 inches from the turret floor to the gun breech. As a result, the breech of the level gun sits higher than the absolute lowest minimum height required for a seated 95th percentile male crewman. Additionally, U. S. Army requirements demand that a tank gun must be able to be depressed 10 degrees and fired. For a gun whose breech extends back 50 inches from its trunnion, the height of the crew compartment ceiling would need to be 7.8 inches higher than the gun’s center line for the gun to be depressed 10 degrees. To load around at 10 degrees depression, like the 105-mm HEAT which is 39 inches long, an additional 6 inches in the height of the crew compartment is necessary. If, for example, the center gun line of a hypothetical tank is 74 inches from the ground and the turret floor is 24 inches above the ground, to load a 39-inch round into a gun whose breech extends 50 inches from the trunnion, the height of the crew compartment would have to be 64 inches. That is sufficient to accommodate a seated 95th percentile U. S. Army male. Theoretically, the ideal combat vehicle should be as low to the ground as possible. In practice, however, it is sometimes not advantageous to have a low tank. Field Manual 17-95, “Cavalry,” states that, because the larger silhouette of American tanks enables their main guns to be depressed more than Threat tanks with smaller silhouettes, the American tanks can be concealed better when shooting at targets below them. In addition, a tank commander who sits too low to the ground may have severely restricted visibility. Although theoretical line-of-sight must exist simultaneously between any two points, the search capabilities of a taller tank in an environment with many low obstacles will be greater than those of a shorter tank. Crewmen within the taller vehicle might be able to spot a portion of the smaller tank while barriers would prevent crewmen in the shorter vehicle from acquiring the taller tank. The ideal height of a tank will be determined by the terrain on which it is operated, and by the tactical use of this terrain. The most advantageous height for a tank in use in the desert would not necessarily be best for one operated on hilly or forested terrain. The ideal height of an overwatching tank in hull defilade may be different from that of an ambushing tank in turret defilade or a tank attacking through high brush. Although it would be unrealistic to build a different height tank for each battlefield condition, these are the types of considerations, and not crewman height, which should determine the height of tanks. Footnotes ‘Details of the Soviet T-72 battle tank. (1980). International Defense Review, Special Series -11,31-34. ZKeesee, R. L., Dickinson, N. F., C Wiegand, D. B. (1978). Prone and supine crew positions in tracked vehicles (Letter Report 243). Aberdeen Proving Ground, MD: U. S. Army Human Engineering Laboratory.

is a mechanical engineer on the Combat Vehicle Systems Team in the Close Support Directorate at the U. S. Army Human Engineering Laboratory, Aberdeen Proving Ground, MD. He received h i s BS degree in engineering from The Johns Hopkins University, MD in 1981.

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