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| Artist's concept of the T90 Mortar Motor Carriage, with the mortar at its maximum elevation. |
Research at the Franklin Institute developed several types of 155 mm mortars for use against different types of field fortification. These were all conventional muzzle-loaded weapons, except for the T9, which was breech-loaded and had a rifled bore. This was essentially a conventional tank gun design, making it suitable for turret installation. It was initially proposed to mount the mortar inside the turret of the T24E1 light tank. However, the close-support nature of the design required heavier armor and more internal space for the large mortar rounds.
On 2 January 1945, Ordnance approved the development of a self-propelled mortar based on the M4 medium tank chassis and designated the new vehicle as the T90 mortar motor carriage. After some delay, the Birdsboro Steel Foundry and Machine Company recieved a contract to build a mock-up of the T90 turret and 155 mm mortar mount.
Initial tests showed the developed mount used up too much room. To conserve space, the T9 mortar was modified to use a concentric recoil system and was redesignated as the 155 Mortar T36. Given the nature of the weapon, for it to be used effectively the elevation mechanism was designed with three overlapping stages, giving a total range of +70 to -10 degrees, and the turret was made to accommodate this wide range. The appropriate stages could be locked and a quick release mechanism allowed for the mortar to be lowered horizontally for loading, and then returned to the original setting. Given the projectiles weighed around 60 pounds, this was an important feature.
Ordnance directed that M4 tanks ill-suited for overseas shipment be used to construct all T90 pilot models. It was estimated the tank could carry a total of 26 rounds. However, while a wooden mock-up of the T90 would be completed, the end of the war killed any interest in heavy self-propelled mortars. While the tank was initially based on early M4 designs, had it reached production it would have surely utilized all of the modern M4 components. On 23 August 1945, the project was formally terminated.
So, the T90 MMC would probably have the highest firing arc of any SPG in the game, but also one of if not the worst firing ranges of its tier. However, this is easily compensated for by having an awful reload speed (even relative to SPGs) and a bad platform.
Contrary to practically every other SPG in the game, the T90 MMC is more at home in dense urban environments, rather than large open fields.
Guns: 155 mm Mortar T9; 155 mm Mortar T36
Engines: Continental R-975-C4 (460 hp); Ford GAA (500 hp)
Turrets (Turret Armor (mm)): T90 (??/??/??)
Turrets (Turret Armor (mm)): T90 (??/??/??)
Suspensions/Alternate Hulls, Total Weight (metric tons): T90 MMC Early (??); T90 MMC Improved (??)
Hull Armor (mm): ??/??/??
Top Speed: 34 km/h
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