Sherman assault gun (Playing with Grok)

[Nathan45]

I was bored and asked Grok to come up with a hypothetical afv for the US army in 1944

Historically the M36 was approved not as a tank destroyer per se, but the argument was the the 90mm would be highly effective against the heavier fortifications the US army expected to encounter, and indeed it was fairly effective at that role but was hugely appreciated for its ability to take on heavy German armor like Panthers at up to 1000 yards and change.

The point of departure here is that the army wants both heavier firepower and heavier armor to deal with these, but doesn't want to greatly exceed the Shermans weight, and also wants to use the same basic chassis to maximize mobility and reliability, so as a hypothetical, the army is willing to give up the turret.


Hypothetical M4A3-Based Casemate Armored Fighting Vehicle (AFV) Design for 1944
Document Created: May 2, 2025
Prepared by: Grok 3, xAI

1. Introduction
This document outlines the design process for a hypothetical casemate armored fighting vehicle (AFV) developed for the US Army, initially conceptualized for 1942 and refined for 1944. The AFV is based on the M4 Sherman (later M4A3) chassis, designed to enhance armor and firepower to counter fortifications and advanced German armor (e.g., Panzer IV, Panther, Tiger I/II) while sacrificing the turret for a fixed casemate. The design evolved through iterative adjustments to meet weight, firepower, and armor requirements, culminating in a final configuration specified by the user.

2. Design Requirements and Evolution
2.1 Initial Concept (1942)
Objective: Develop an AFV to improve armor and firepower over the M4 Sherman, addressing fortifications and German armor improvements, using the same chassis and engine.
Requirements:
• Weight: Within 5% of M4 Sherman (~30.3 tons, max 31.8 tons).
• Engine: Continental R975 radial, 400 hp.
• Chassis: M4 Sherman, welded hull, VVSS suspension.
• Design: Casemate (no turret) to save weight for armor/firepower.
Initial Design:
• Firepower: 3-inch M7 gun (~130 mm penetration at 500 m).
• Armor:
◦ Hull glacis: 76 mm at 60° (~150 mm effective).
◦ Casemate front: 90 mm at 45° (~127 mm effective).
◦ Sides/rear: 38 mm (vertical).
• Weight: ~31.8 tons.
• Crew: 4 (commander, gunner, loader, driver).
• Mobility: ~12.6 hp/ton, top speed ~40 km/h, range ~200 km.
Performance:
• Firepower: 30% better penetration than M4’s 75 mm M3 (100 mm).
• Armor: ~67-71% improvement over M4’s ~90 mm effective frontal armor.
• Role: Tank destroyer/assault gun, comparable to M10 Wolverine but better armored.
2.2 Adjustment for 1944
Objective: Enhance firepower beyond the 3-inch M7, incorporate the Ford GAA engine, and allow a 7% weight increase over the M4A3 Sherman.
Updated Requirements:
• Weight: Within 7% of M4A3 (~31.8 tons, max 34.0 tons).
• Engine: Ford GAA V8, 500 hp.
• Chassis: M4A3 Sherman, welded hull, VVSS suspension.
• Firepower: 90 mm M1 gun to counter Panther/Tiger I/II.
Intermediate Design:
• Firepower: 90 mm M1 (~150-170 mm penetration at 500 m).
• Armor:
◦ Hull glacis: 89 mm at 60° (~178 mm effective).
◦ Casemate front: 100 mm at 45° (~141 mm effective).
◦ Sides/rear: 38 mm.
• Weight: ~33.9 tons.
• Crew: 4.
• Mobility: ~14.7 hp/ton, top speed ~40 km/h, range ~200 km.
Performance:
• Firepower: ~15-30% better than 3-inch M7, ~40-70% better than M4A3’s 75 mm/76 mm.
• Armor: ~78-98% improvement over M4A3’s ~90-100 mm effective frontal armor.
• Role: Tank destroyer/assault gun, comparable to M36 but with heavier armor.
2.3 Armor Redistribution Iterations
Objective: Adjust frontal armor to redirect weight to side armor, refining based on user feedback.
Key Iterations:
1. Reduced Frontal Armor:
◦ Hull: 50 mm at 60° or 71 mm at 45° (~100 mm effective).
◦ Casemate: 71 mm at 45° (~100 mm effective).
◦ Sides: Increased to ~48-58 mm.
◦ Outcome: Improved flank protection but compromised frontal armor, vulnerable to 75 mm PaK 40 and 88 mm KwK 36 at <1000 m.
2. User Clarification:
◦ Hull: 100 mm at 45° (~141 mm effective).
◦ Casemate: 90 mm at 45° (~127 mm effective).
◦ Sides: Reduced to ~43 mm to stay within weight limit.
◦ Outcome: Stronger frontal hull armor, modest side armor improvement, but limited flank protection.
Challenges:
• Balancing frontal and side armor within the 34.0-ton limit was constrained by the heavy 90 mm M1 and casemate structure.
• User-specified hull armor (89 mm at 60°) required reverting side armor to baseline to meet weight constraints.

3. Final Design Specifications (1944)
3.1 Overview
The final AFV is a casemate tank destroyer/assault gun based on the M4A3 Sherman, optimized for 1944 to counter fortifications and heavy German armor. It uses the Ford GAA engine and 90 mm M1 gun, with specified armor thicknesses to maximize frontal protection within the weight limit.
3.2 Technical Specifications
Parameter Specification
Weight ~33.9 tons (within 7% of M4A3’s 31.8 tons, max 34.0 tons)
Engine Ford GAA V8, 500 hp (~14.7 hp/ton)
Firepower 90 mm M1 gun (~150 mm M77 AP, ~170 mm M82 APCBC at 500 m), ~45 rounds
Armor
- Hull Glacis 89 mm at 60° (~178 mm effective)
- Casemate Front 90 mm at 45° (~127 mm effective)
- Sides 38 mm (vertical, ~38 mm effective)
- Rear 38 mm
- Top 19 mm
Crew 4 (commander, gunner, loader, driver)
Dimensions ~5.9 m (L) x 2.6 m (W) x ~2.2 m (H)
Mobility Top speed: ~40 km/h (road), range: ~200 km
Suspension VVSS (M4A3 standard)
3.3 Weight Breakdown
• Base (sans turret): 28.3 tons (31.8 tons - 3.5 tons for turret).
• Casemate + 90 mm M1: 1.8 tons.
• Armor:
◦ Frontal (hull + casemate): ~4.3 tons (3.0 tons hull, 1.3 tons casemate).
◦ Sides/rear/top: ~0.5 tons (baseline 38 mm sides, no increase).
• Total: ~33.9 tons.

4. Performance Analysis
4.1 Firepower
• Gun: 90 mm M1.
• Penetration: ~150-170 mm at 500 m.
• Performance:
◦ Defeats Panther (80 mm at 55°, ~120 mm effective) and Tiger I/II (100-120 mm) at 500-1000 m.
◦ Effective against fortifications with HE rounds.
• Improvement:
◦ 40-70% better than M4A3’s 75 mm M3 (100 mm) or 76 mm M1 (~120 mm).
◦ 15-30% better than initial 3-inch M7 (130 mm).
• Limitation: Fixed casemate (~15° traverse) reduces flexibility vs. turreted tanks.
4.2 Armor
• Hull Glacis: 89 mm at 60° (~178 mm effective).
◦ Improvement: ~78-98% over M4A3’s ~90-100 mm (63 mm at 47°).
◦ Protection: Resists 75 mm PaK 40 (140 mm penetration) at all ranges, 88 mm KwK 36 (150 mm) at >500 m; vulnerable to 88 mm at <500 m.
• Casemate Front: 90 mm at 45° (~127 mm effective).
◦ Improvement: 43-67% over M4A3 turret front (76-89 mm).
◦ Protection: Resists 75 mm PaK 40 at 500 m, 88 mm KwK 36 at ~1000 m.
• Sides: 38 mm (unchanged).
◦ Limitation: Vulnerable to 50 mm PaK 38 (~60 mm), 75 mm PaK 40, and infantry weapons (e.g., Panzerfaust, ~140 mm).
• Overall: Strong frontal protection for hull-down or defensive roles, but weak side armor requires avoiding flank exposure.
4.3 Mobility
• Power-to-Weight: ~14.7 hp/ton (vs. M4A3’s ~15.7 hp/ton).
• Performance: Top speed 40 km/h, range ~200 km, comparable to M36 tank destroyer (15.8 hp/ton at 31.7 tons).

5. Comparative Analysis
5.1 M36 Tank Destroyer (1944)
• Weight: 31.7 tons.
• Gun: 90 mm M1 (~150-170 mm penetration).
• Armor: 38 mm at 55° hull (~60 mm effective), open-top turret, sides 25-38 mm.
• Engine: Ford GAA, 500 hp (~15.8 hp/ton).
• Comparison:
◦ AFV has superior frontal armor (~178 mm hull, ~127 mm casemate vs. ~60 mm hull, open turret).
◦ Same side armor (~38 mm).
◦ Same firepower, but casemate vs. open-top turret.
◦ Heavier (33.9 vs. 31.7 tons), similar mobility.
5.2 Jagdpanzer IV/70 (1944)
• Weight: 25.8 tons.
• Gun: 75 mm PaK 42 L/70 (~200 mm penetration).
• Armor: 80 mm at 50° (~120 mm effective), sides 30 mm.
• Engine: 300 hp (~11.6 hp/ton).
• Comparison:
◦ AFV has better frontal armor (~178 mm hull, ~127 mm casemate vs. ~120 mm).
◦ Better side armor (~38 mm vs. 30 mm).
◦ Less powerful gun (90 mm M1 vs. PaK 42).
◦ Heavier (33.9 vs. 25.8 tons), better mobility (14.7 vs. 11.6 hp/ton).

6. Suitability and Feasibility
6.1 Tactical Role
• Anti-Tank: 90 mm M1 defeats Panther and Tiger I/II at 500-1000 m. Strong frontal armor (~178 mm hull, ~127 mm casemate) resists 75 mm PaK 40 and 88 mm KwK 36 at typical ranges, ideal for hull-down or defensive engagements. Vulnerable to 88 mm at close range and flanking due to 38 mm sides.
• Anti-Fortification: 90 mm HE rounds excel against bunkers; frontal armor withstands defensive fire (e.g., small arms, lighter anti-tank guns).
• Limitation: Weak side armor requires tactical positioning to avoid flanking, limiting versatility in mobile or urban combat.
6.2 Doctrinal Fit
• Aligns with US tank destroyer doctrine (e.g., M36), prioritizing firepower and frontal armor for anti-tank/fortification roles.
• Casemate design sacrifices flexibility for enhanced protection and firepower within weight constraints.
6.3 Feasibility
• Components: 90 mm M1, Ford GAA, and M4A3 chassis are in production by 1944.
• Manufacturing: Casemate design leverages M36 and M4A3 experience; hull (89 mm at 60°) and casemate (90 mm at 45°) are achievable.
• Timeline: Prototype by mid-1944, fielding by late 1944, similar to M36.

7. Conclusion
The final M4A3-based casemate AFV is a robust tank destroyer/assault gun for 1944, balancing:
• Firepower: 90 mm M1, ~40-70% better than M4A3’s 75 mm/76 mm, ~15-30% better than 3-inch M7, effective against Panther, Tiger I/II, and fortifications.
• Armor: ~178 mm effective hull (89 mm at 60°), ~127 mm effective casemate (90 mm at 45°), ~43-98% improvement over M4A3; weak 38 mm sides limit flank protection.
• Weight: ~33.9 tons, within 7% of 31.8 tons.
• Mobility: ~14.7 hp/ton, comparable to M36.
The design evolved from a 1942 concept with a 3-inch gun to a 1944 vehicle with superior firepower and frontal armor, optimized for hull-down engagements and fortification assaults. Weak side armor necessitates careful tactics, but the AFV meets 1944 needs within specified constraints.

8. Notes
• Side armor remained at 38 mm due to weight limits; earlier iterations explored ~43-58 mm sides but were infeasible with the final frontal armor specifications.
• Further refinements (e.g., HVSS suspension, sloped side armor) could be explored if weight constraints are relaxed or alternative components considered.
End of Document


https://x.com/i/grok/share/S5CqNmqAMrh6upUIWCGXNyJo7


I know, it really wouldn't matter, the M36 did just fine without the heavy armor, the lack of a turret was a big deal, particularly on the offense, etc. Still its fun.

It does occur to me that the Cromwell chassis might be a better choice, it could neutral steer I believe.

[Pdf27]

Was actually done with the Churchill pretty early on - they fitted a WW1 3" 20cwt AA gun (very similar performance to the later 17pdr) in a casemate since the biggest thing they could fit in a turret at the time was the 6pdr.

image_2025-05-03_065339478.png

[warshipadmin]

What does neutral steer mean in context? TIA

[Craiglxviii]

What does neutral steer mean in context? TIA

Tanks could steer in two ways.

One way- skid steering- was to engage one track but not the other, which would be locked, and so would skid around that track (watch Kelly’s Heroes for a good example of this with the T-34 “Tiger” mockup).

The other was neutral steer, which required a different steering gearbox design. In this, one track can move forwards while the other moves in reverse (if stationary) so the tank can pivot on its centre point and spin in place; if driving the track on the inside of the circle moves to neutral so still rolls just without being driven.

Neutral-steer tanks are much easier & more pleasant to drive, chew roads and ground up far less (although this is a relative term when talking about 30-ton vehicles) than skid-steer, but are more complex to design and maintain.

[Nathan45]

In this context, a assault gun without a turret would benefit from that a lot, although the stug did fine without it.

One thing is if you limit it to Ford gaa engines you can use the lower hull of the m10 as the basis since you don't need a higher hull to accommodate the radial engine

[Craiglxviii]

In this context, a assault gun without a turret would benefit from that a lot, although the stug did fine without it.

One thing is if you limit it to Ford gaa engines you can use the lower hull of the m10 as the basis since you don't need a higher hull to accommodate the radial engine

The STuG did have iirc 10’degree traverse one way and 5 degrees the other.

[Nathan45]

I think that was the hetzer, which really is limiting, The Stug, honestly im seeing a lot of different sources online but it looks like about 10 degrees per side. A bigger deal for a attacking afv then a defending one, which is probably at least part of the reason there was limited interest in by the western allies. I'm curious how much traverse that Churchill assault gun had.

[Craiglxviii]

I think that was the hetzer, which really is limiting, The Stug, honestly im seeing a lot of different sources online but it looks like about 10 degrees per side. A bigger deal for a attacking afv then a defending one, which is probably at least part of the reason there was limited interest in by the western allies. I'm curious how much traverse that Churchill assault gun had.

Ah sorry. Brain fart, yes you’re right.

The Churchill Gun Carrier 3” had 5 degrees of traverse, but later British casemate assault guns all had either 15 or 20 degrees traverse either side of centreline.