According to orders from the Artillery Directorate given to engineer D.I. Ilyukhin, the feasibility of the aforementioned vehicle must be investigated, with the following requirements:
- Ability to shoot on the move.
- 20-25 shells carried on board.
- Possibility of using a muzzle brake.
- Independent use in battle.
- If possible, use the 25 caliber gun with a muzzle brake.
- Leave room in the hull for a telephone and radio.
- If possible, leave the center of mass in the same place.
- Minimize the changes required to the gun and tank.
Based on the technical requirements, the following solutions can be used:
- 76 mm mod. 1927 gun with a muzzle brake, reducing the recoil length to 0.8 meters. The maximum elevation would be 12 degrees, which achieves the requirement for a range of at least 3800 meters.
- 76 mm mod. 1927 gun without a muzzle brake, with a maximum recoil length of 1 meter. The maximum elevation is 8 degrees, which limits the range to 2800 meters.
The 0.8 meter recoil length is the greatest we can allow with the current design. A 1 meter recoil length would mean that the engine compartment cover would have to be redesigned.
The bore axis height is finalized as 1800 mm, which gives a stability reserve of K=1.173.
Moving on the design of the SPG, the following must be individually examined.
- A 76 mm mod. 1927 gun, adapted to the T-18.
- The T-18 tank's equipment.
1. The gun can use the same oscillating part, with the following changes:
- The muzzle must be threaded to accept a muzzle brake.
- Attach the muzzle brake according to blueprint #2687 of the AU design bureau.
- Re-cut the counter-piston to account for the reduced recoil length.
- The mass of the tank is calculated as follows:
- Mass of the tank (empty): 4700 kg
- Mass of the gun: 400 kg
- Total: 5100 kg, without accounting for the reduction of mass achieved by removing the turret.
- Limit the elevation mechanism range to prevent knocking against the platform. Alternatively, cut slots in the platform to increase range, keeping in mind that the driver's view may be obstructed.
- Remove the trigger mechanism from the gun mount and fire using a cord (like in mod. 1902).
- Cut the gun rails by 50 mm in order to prevent it from knocking against the back of the crew compartment when recoiling.
- The mount must be specially cast from steel, rotating on an axle attached to the platform, held at the top by a round nut. In order to resist rear and forward movement, three pins are screwed into the platform at each end. The pins will move their cylindrical parts through the openings in the mount and hold the latter with bolts or splints.
The mount uses the elevation mechanism from the mod. 1927 gun. The turning mechanism is new. Maximum elevation is 12 degrees with a muzzle brake, 8 degrees without.
The oscillating section is fixed to the platform during marches with a travel lock. The travel lock has full and half rollers, which enables it to lock the elevation and traverse in one movement.
The sight is used from the mod. 1927 76 mm regimental gun.
Make the following changes in order to install the gun:
- Remove the cupola and central turret.
- Remove the driver's hatch and its frame, replace it with a solid 15 mm thick plate.
- Attach a chrome-nickel steel box under the front plate with rivets to make a foundation for the gun.
- Replace the front angle bar with a more robust one.
- A special platform must be made to hold the gun. For this, 40x40x6 angle bars are attached to the front plate to hold the left and right trails. Cut openings in them to reduce weight and, if possible, maintain the driver's field of vision. Above the trails is a 15 mm thick plate with an axle for the mount.
The top plate is attached to the front angle bar.
3. Gun shield
In order to protect the crew, a gun shield is used, composed of a front-top plate, two side plates, and a rear plate with a two piece hatch.
Parts of the shield are attached together with angle bars and rivets. The front angle bar provides rigidity and connects the gun shield and platform. It is attacked to the front and side plates, and is bolted to the platform. The rear hatch is to only be closed when under fire, and needs to be open to allow for longer recoil.
The SPG will have a 3.5 mm thick gun shield, which will weigh 136.96 kg. In order to improve protection, a second variant with a 16 mm front shield was developed, increasing the weight to 438.15 kg.
4. Crew locations
The SPG will be crewed by three people:
- Loader, also setting the detonators
The driver's location is unchanged. The gunner will have a special seat. The third crewman does not have a seat and is placed on the floor of the tank. In order to protect the crew when shooting in cramped conditions, the project includes a flip up shield which is placed around the path of the recoil when shooting, preventing the crew from being hit by the recoiling breech.
5. Ammunition location
The place for ammunition has not been defined in the new project. It can be placed in the same place as before, or on the bottom of the tank in boxes.
6. Mass and center of gravity
If the parts of the tank that have been removed weigh 500 kilograms, the mass of the vehicle is as follows:
- Tank mass: 4700 kg
- Removed components: 500 kg
- Tank mass without turret or cupola: 4200 kg
- Gun mass: 400 kg
- Mount mass: 197.5 kg
- Gun shield, 3.5 square meters: 183.3 kg
- Total mass: 4980.8 kg
The tank is approximately 280.8 kg heavier, and the center of mass moves 15.5 cm forward. The reserve of stability is K=1.6
7. Notes on using the 76 mm mod. 1927 regimental gun
The 76 mm mod. 1928 gun with a muzzle brake has a recoil length of 1000 mm and can only have a maximum of 8 degrees of elevation in this tank. This will give it a range of 3800 meters, which is the same as with the mod. 1927 gun at 12 degrees of elevation. Using the mod. 1928 gun reduced the tank's characteristics and made it much heavier. Consider it unsuitable for this purpose.
Senior engineer, department 11, Ivanov"