A Bigger Howitzer on a Smaller Chassis

The American Ordnance Committee standardized the HMC T47 as the HMC M8 on May 15th, 1942. This vehicle was accepted into service even before a prototype was built and became the first American light SPG to enter production. The HMC M8 was a good vehicle, but had its drawbacks. The fighting compartment was cramped and there was no room to improve the armament. Experience in combat showed that 75 mm was too small of a gun even for a light SPG. This prompted the development of a new SPG, also based on the Light Tank M5 chassis. The result was a number of experimental SPGs, including the HMC T82.

Light Tank M5: The Peak of Evolution

Rapid advances in armoured vehicles during WWII meant that even very good designs did not stay at the top for long. This was especially noticeable in American tank building. In 1939, the Light Tank M2 and Combat Car M1 were at the top of technical progress, but they were replaced with the Light Tank M3 by the time the USA entered the war in late 1941. In 1942, the Light Tank M3A1 entered production, but it did not last long as America's main light tank. At the end of 1942, the Light Tank M5 was there to replace it, the last of the descendants of the Light Tank M2.

American Ergonomics

Ergonomics in tanks is an important factor, one that I touched on previously in some detail. It turns out that despite certain prevalent stereotypes, the ergonomics of various tank schools are more complicated than many people believe. Having already applied Soviet ergonomics standards to a German design, let us hop across the pond and see what the Americans thought about the topic.

Thankfully, this time, the Fort Knox Medical Research Laboratory report "Adequate Head Room in Tanks" provides us proper measurements, which makes my job a lot easier. The adequate head room for a sitting crewman, excluding the upper and lower 5% of men, is stated as 34" to 38.25", or 86-97 cm. The Soviet "dimensions of an average man" define the same measurement to be 90 cm (35.4"), which falls pretty well in the middle of that range. The American tank helmet adds a whole 1.5" to the height of a tanker by his crash helmet, a thickness that is found to be excessive. At the time of the study (November 27th, 1942), a thinner helmet was already being tested.

Let's see how well average people could fit into American tanks.