A small part of the story in my Manticore TL involves technical defense cooperation between manticore and the United States in the late 1930s. This had the effect of enabling Manticoran production of artillery pieces like the 90mm anti-aircraft gun and the 155mm howitzer beginning in 1937, so more pieces are available when the war starts. Because I need to figure out what kind of Lend-lease armor Manticore will be able to receive, I essentially need to redesign US armored vehicle production throughout the war. In a very generalized summary, The M3 was produced in 1942, the M4 with 75 mm gun and M10 tank destroyer were produced in 1943, and the M4 with 76mm gun or 105mm howitzer was produced in 1944. I know there was a lot of overlap, because I have a document detailing monthly production numbers of every piece of military equipment that the United States used, but the important part for me right now is what would be available for Lend-Lease.
I've detailed Manticoran tank production before, but here's a quick recap.
1936-37: Basically the Strv m/42
1938-40: Medium 25-ton tank, with 3-inch L/40 field gun (comparable to Sherman 75 or T-34/76) and torsion bar suspension
1941-42: Medium 30-ton tank, with 76 mm Gun M1
1943-45: Medium 40-ton tank, with 90 mm Gun M3
Manticoran tanks use two-stroke diesel engines, so that won't be much help for the US if they want to stay with gasoline fuel. However Manticore is also sponsoring development of the Ford GG aero engine, which could make large numbers of the Ford GAA tank engine available for production in 1942. With a big V8 in service by the time M4 production begins, it might be possible to avoid some of the problems that resulted in the Sherman due to the use of radial engines.
Let's start with the M2 Medium. Manticoran tank design philosophy has always focused heavily on the use of a 3-inch field gun in a rotating turret. Hopefully, these kinds of discussions would be enough to break the cult of the machine gun and result in a more reasonable design for the M2. The result would be a tank with a larger turret, holding a three-man turret crew, and either a 37 mm anti-tank gun or a 75 mm light howitzer. The 3-inch Gun M1902 could also be a reasonable fit, but that is basically no longer a supported product at this point. The selected engine will be the Wright R-975, so the hall form will be basically the same as OTL. The transmission choices made for the T20 series were, generally speaking, selected to allow the use of a rear-mounted transmission without needing gear shift levers to run from the drivers compartment to the rear of the vehicle. Inaccuracies in manufacturing were probably the reason why the transmission on the T-34 was so difficult to shift, so the US wanted to avoid that. Therefore, the arrangement with a rear engine and a front transmission will remain the same. Because of the metallurgical complexities of torsion bars and extensive US industrial experience with volute springs as part of rail car suspensions, the VVSS system will remain.
My idea for M3 production in 1942 is to keep the same general hull form and increase the size of the turret ring to accommodate a 75 mm field gun. We entirely avoid the interim sponsor design, and the M3 tank is basically identical to the OTL M4. The VVSS system is retained, as is the rear-engine, front-drive arrangement. The issue here is hull height as a result of engine selection. If the Ford GAA is introduced, hull height can be substantially reduced, but that could be a complication considering the need to quickly design and produce a new tank in late 1941 and early 1942. On the other hand, very little of the upper hull architecture was retained between the OTL M2 and the OTL M3. The M2/M3/M4's hull between the tracks was too narrow to fit the 69-inch turret ring, so sponsons are required regardless of hull height. The alternate engine option is the GM 6046 (twin 6-71 diesels).
The bulk of 1943 tank production in the US was made up of M4 (75) Shermans and M10 TDs, both of which entered production in the second half of 1942. Engine options will definitely be the Ford GAA (gas, 450 hp) and the 12V-71 (diesel, 450 hp). The main difference in hull architecture between the OTL M4 and the T20 series was the rear-mounted transmission and lower-profile engine, so the entire T20 hull is roughly 1.5-1.7 meters tall. The hull was designed from the start to be wide enough for the 69-inch turret ring without sponsons. Considering the difficulties with the rear-mounted transmissions, staying with the front-mounted transmission may be necessary for 1943 production. Gun options will be the 76 mm Gun M1 for the gun tanks and the 90 mm Gun M3 for the tank destroyers. Suspension selection is a somewhat complicated choice. The US tested a torsion bar suspension on the M4A2E4 in summer 1943, but that wasn't much better than the VVSS or HVSS. ITTL, Manticore has extensive experience with torsion bars so that could be the selection for the 1943 tank. Stacking the prop shaft on top of hull-bottom torsion bars will add some hull height (about a foot) over the T20 series. The final hull might look like an OTL M10 hull with sloped front and sponson armor, but I have never been able to find exact dimensions on an M10 hull to actually compare it to a welded-hull Sherman.
By 1944, the need for a tank with a 90 mm gun is likely to be apparent. Manticore already has a 90 mm gun tank and the Germans have a new and much larger medium tank. My thinking is that the 1944 production tank taking the place of the OTL Sherman 76 will closely resemble the T25 tank. The Torqmatic transmission used on the M26 Pershing (from the T20 medium) is the likely choice. Armor will be roughly three inches on the frontal surfaces. Tank production numbers were significantly reduced from 1943 to 1944.
My preliminary plan for the tanks Manticore will be receiving from Lend-Lease in 1942, 1943, and 1944 is:
1942: M4A2 with 75 mm Gun M3, GM 6046 engine, VVSS suspension, and narrow tracks
1943: M4A2 with 76 mm Gun M1, 12V-71 engine, torsion bar suspension, lower hull, and wide tracks.
1944: T25E1 with 90 mm Gun M3, 12V-71 engine, Torqmatic transmission, torsion bar suspension, and wide tracks.
