Monday 4 June 2018

Object 240 Ergonomics

There is a common misconception that the Soviet army had no regard for the conditions of its crew. This is provably false. Just as with the US army, the Red army had a medical service, which would evaluate the user-friendliness of its equipment. Tanks were no exception. For instance, the Object 240, the precursor of the IS-2 tank, was put through thorough testing by the medical service, and the faults that were discovered were corrected by the time the tank reached mass production.

"Conditions of servicing of the 122 mm D-25 gun in the IS tank

1. The D-25 gun is installed in the turret of the IS tank.
The fighting compartment has the following dimensions: turret ring diameter: 1800 mm, height in the rear part: 1520 mm, height in other parts: 1580 mm.
The height of the fighting compartment is insufficient for free work by standing people. The turret crew stands in a slouched over pose.
The short height of the fighting compartment is due to the metal crates with ammunition on the floor. The crates are covered in rubber and used as the floor of the fighting compartment.
To better evaluate the dimensions of the fighting compartment, they should be compared with those of the IS tank armed with a D-5 85 mm gun. With the same sized turret, the gun takes up much more space, which reduces crew comfort.
The width of the gunner's station is reduced by 60 mm (to 650 mm) and the breech operator by 30 mm (to 660 mm). It is narrow, but possible to work.
The distance between the deflector and the rear of the turret was 450 mm for the D-5 and is 200 mm for the D-25. It is not possible to pass behind the gun. It is also uncomfortable to load, since the casing does not fit into this opening and catches onto the rear ammunition rack. When inserting it the loader has to aim (make additional careful motions).

2. The conditions for the gunner are as follows.
  1. The gunner works sitting down. The seat is soft, on a carrier. It can be collapsed underneath the gun when necessary. The seat can be adjusted vertically. The pipe with electrical wiring in front of it somewhat impedes sitting.
  2. The gunner uses the following to aim:
    1. Telescopic tank sight.
    2. Hand operated elevation and traverse mechanisms.
    3. Electric traverse mechanism for rapid turret traverse.
      The gunner did not use the periscopic sight due to its unfinished state and low precision. The electric traverse mechanism was not used for aiming, since it is also very "raw" and it is impossible to alter the turret traverse speed. 
    4. The following table contains the characteristics of the hand traverse and elevation mechanisms:

      Elevation mechanism
      Turret traverse mechanism
      Distance to the flywheel axle from the fighting compartment floor
      830 mm
      850 mm
      Flywheel radius
      100 mm
      95 mm
      Elevation effort
      5-10 kg going up, 1-5 kg going down
      From 3 to 25 kg (average 10 kg)
      Maximum angle
      +21 degrees 30 minutes to -3 degrees
      360 degrees
      Number of turns to traverse the entire range
      24 ¾
      Time to traverse the entire range
      7 seconds
      570 seconds
      Traverse speed per second
      3.5 degrees/second
      0.6 degrees per second
      Somewhat excessive and uneven effort was required to use the traverse and, in part, the elevation mechanism. The hand turret traverse mechanism speed is insufficient.
      This data is satisfactory for towed guns of this caliber, but not enough for a tank gun, as the strict need to fire on the move and to fire at moving targets requires a certain ease of aiming.
    5. The telescopic gun sight is uncomfortable to use, as it is positioned too close to the gun (40 mm distance between the center of the pupil and the gun) and the bulge of the trigger mechanism. This distance should be at least 80 mm for comfortable use.
    6. The hand trigger mechanism is conveniently positioned close to the aiming flywheels. The effort to fire is 25 kg, which is high. There is no foot trigger.
    7. The fact that the aim is somewhat offset by opening of the breech, inserting the shell and propellant casing, and closing the breech slows down aiming. Because of that, the gunner must adjust his aim after firing. 
  3. The ammunition racks are inconvenient. It is hard to open racks on the floor of the turret. To open a crate, one must clear the floor, take off the rubber mat, open the metal lid, and extract the heavy ammunition while standing on one's knees from the small box. All of this takes time. People in the turret and spent casings that litter the floor impede loading. It is hard to find the necessary crate when the turret rotates. The ammunition racks in the floor are therefore deemed inconvenient.
    The rear rack for 24 shells, places in the turret bustle, also has it downsides. It is unsuitable for the HE shell, which is shorter (by 10 mm) and has a smaller diameter. When loading them into the rack, the lock has to be held at the correct height, which is difficult. The clasps get tangled. The clasps in the middle impede loading, since they have to be pushed aside when inserting the casing.
  4. When the breech opens, the screw and handle move 350 mm to the right, which blocks the breech from the breech operator. Therefore the gun commander must perform most of the loading, and the breech operator can only open and close the breech. Involving the commander in the loading process prohibits him from observing the battlefield and correcting fire.
  5. The loading assistance mechanism is bulky. The loader is impeded by the bulge on the right of the device. It is desirable to remove this bulge.
  6. The inconvenient racks and cramped position of the breech operator after the breech is opened leads to a low rate of fire. At trials on October 22nd, 1943, a rate of fire of 2-3 RPM was achieved (20-30 seconds per shot).
  7. During firing, the location of the recoil indicator proved inconvenient. It is placed in a dark place behind the machinegun. The measurement scale is inconvenient, as the runner obscures the number.
  8. The commander cannot correct fire, as he only has a periscopic sight without a grid. The commander must have a commander's periscope with a grid. It is also necessary to free the commander from the loader's duties.
  9. Firing was performed with closed hatches and without the fan and engine, as well as with the fan and engine.
    In both cases, 10 shots were made at a rate of 1-2 rounds per minute.
    Number of shots
    Time taken
    Rate of fire
    CO concentration
    Closed hatches, no ventilation
    Closed hatches, engine running, fan on
    As you can see from the data, the concentration of fumes did not surpass 0.224 ml/L. The crew did not suffer any ill effects. The personnel felt healthy. It is possible to fire for some time with closed hatches and no ventilation.
    With the ventilation running, the concentrations do not surpass 0.1 ml/L. This ventilation is sufficient.
Major of the Medical Service, Aleksandrov"

Diagram of crew positions in the IS tank armed with a 122 mm gun.

Diagram of the 122 mm gun operator's station in the IS tank.

Turret of a production IS-2 tank. The complaints made by the medical service were corrected.

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