Originally posted by gunnut
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So the keys are mobility and visibility. Never expected to be operating in close proximity to enemy infantry, an open topped turret increased visibility and spotting for the crew, allowing for quicker target discrimination and acquisition. Also not that TDs had no bow or coax MG (except for the M36B1 that kept the full M4 hull). Armor was not considered to be that big a deal because the TDs were going to be operating with "shoot & scoot" in mind - shoot from cover a couple of times, then maneuver quickly to an alternate position and re-engage before the enemy tanks have a solid idea of where you are. And with the M10, the first purpose-designed full-track TD for the U.S., the armor values actually compare favorably with those of the early-war welded-hull Shermans.
The M18 Hellcat was the ultimate design in this direction - VERY thin armor and VERY high speed - as maneuverable as they could get in those days on tracks.
The correct comparison on the German side would be the Marders and other open-topped SP TDs - the StuGs were originally simple and cheap assault guns intended to give direct HE support to the infantry. But with a much better appreciation of the gun-armor race that WWII created, the Germans kept to bigger guns on thicker, turretless hulls for their preferred SP TD solution. The German Hetzers (and the never-produced and similar E-10) were examples of this, although Germany's increasingly defensive stance meant anything with track was getting an AT gun stuck on it, and turrets are expensive and complex compared to a simple casemate mount.
-dale
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