Monday 17 July 2023

A Big Insect from Alkett and BMM

The German army felt a dire need for self propelled guns early in the Second World War. The highest priority items were a motorized anti-tank gun more powerful than the 3.7 cm Pak and a more mobile 149 mm sIG 33 gun. This was a versatile weapon that could serve in several roles thanks to variable propellant loads, although the SPG would be used in direct fire. A 38 kg HE shell carrying almost 8 kg of explosives could demolish a brick house in a few hits. This ability was widely used in May-June of 1940 when the 15 cm sIG 33 (mot S) auf Pz.Kpfw.I Ausf.B went into battle. The first attempt at an SPG had issues. The vehicle was too tall and the Pz.Kpfw.I Ausf.B chassis was overloaded. Nevertheless, the tankers (as the 15 cm sIG 33 (mot S) auf Pz.Kpfw.I Ausf.B formed artillery batteries in tank divisions) appreciated this vehicle.

Geschützwagen 38 für sIG 33/1 (Sf.), the most common German SPG with a 149 mm sIG 33 gun.

Successful application of the 15 cm sIG 33 (mot S) auf Pz.Kpfw.I Ausf.B pushed German command to continue the work on light SPGs. The next step was the development of the 15 cm sIG 33 B Sfl. Initially the development used a stock Pz.Kpfw.II chassis, but trials of a prototype showed that this was too cramped. A converted chassis was introduced. The 15 cm sIG 33 B Sfl turned out to be a poor vehicle whose engine was wholly inadequate. This failure did not stop work. There was another chassis in reserve: the Pz.Kpfw.38(t). It was more suitable for this task, and so the Geschützwagen 38 für sIG 33/1 (Sf.) was born, becoming the most common German SPG with a 149 mm sIG 33 gun.

Czech chassis for a German weapon

It was clear that the Pz.Kpfw.II was too small for the 15 cm sIG 33 back in the summer of 1941. The 15 cm sIG 33 B Sfl was quite heavy, about 16 tons. This was the mass of a medium tank, not a light one. The Germans had no tank in this weight class, but its newly acquired Protectorate of Bohemia and Moravia did. The Pz.Kpfw. Škoda T-22 also showed good results in June of 1941 and it was considered for mass production. The T-22 was also seen as a possible chassis for the 15 cm sIG 33.

The Voss II SPG could have looked like this if it was built.

Škodawerke was tasked with building this vehicle in July of 1941. It would have an open topped fighting compartment with 50 mm of front armour. The mass was estimated at 16 tons and the top speed at 50 kph. This made the vehicle quite promising. It could easily keep up with tank units and this armour would allow it to resist hits from 45 mm anti-tank guns, although considering the quality of Czech armour that might not have been entirely true. In any case, it is not possible to give any evaluation of the vehicle called Voss II (in honour of Wilhelm Voss, the new head of Škodawerke as of 1941). The Pz.Kpfw. Škoda T-22 stalled and the Voss II died at the development stage.

Geschützwagen 38 für sIG 33/1 (Sf.) prototype built at Alkett in November of 1942. Only a prototype was built in Spandau, production was never meant to happen here.

Work on the Voss II died completely by the summer of 1942. The development of SPGs armed with the 15 cm sIG 33 split. On one hand, there was a need in medium assault guns with a closed fighting compartment and armour on the order of magnitude of 80 mm. The need for mobile light SPGs remained. Thickly armoured assault guns also usually only fired directly, while light SPGs could be used for indirect fire. This led to Alkett and BMM being tasked with the development of a similar SPG but with the use of the Pz.Kpfw.38(t) chassis. BMM prepared the chassis and overall development fell to Alkett. The final decision to use the Pz.Kpfw.38(t) chassis was made on September 1st, 1942, which delayed the development of the vehicle.

The fighting compartment. The designers squeezed everything out of the base chassis, resulting in a more or less roomy fighting compartment.

Spandau already had experience with developing the 15 cm sIG 33 (mot S) auf Pz.Kpfw.I Ausf.B and 15 cm sIG 33 B Sfl, but the new chassis was a serious challenge. Experience with the 15 cm sIG 33 B Sfl showed that lengthening the chassis leads to a negative result. This also introduced many issues into the production process. The Pz.Kpfw.38(t) chassis was left as is. Unlike the tank destroyers on the same chassis, the turret platform was not preserved. This allowed the designers to get rid of a pointless "step" and join the fighting compartment with the driver's compartment.

Production of the Geschützwagen 38 für sIG 33/1 (Sf.) at BMM. The Pz.Kpfw.38(t) Ausf.H chassis was converted to make them.

In part, the new Alkett design was linked to the SPGs developed by Alfred Bekker. Bekker had close ties with Alkett. The self-taught designer was the first to make the most out of a vehicle's chassis by overlapping the engine deck. The same technique was used on the Geschützwagen 38 für sIG 33/1 (Sf.), the name given to the self propelled gun on the Pz.Kpfw.38(t) chassis. The vehicle got a casemate with a 25 mm thick front and 15 mm thick sides. The casemate partially covered the engine deck, which complicated servicing the engine. This was not critical, since all one had to do to get to the engine was remove the rear plate and flip open the sides of the casemate. Another issue was the height of the vehicle, although at 2400 mm it was still lower than the tank destroyer on the same chassis.

A production Geschützwagen 38 für sIG 33/1 (Sf.).

This fighting compartment layout had more advantages than drawbacks. For starters, the vehicle carried 15 rounds for the gun. They were placed all over the place, but it was a lot more ammo than the 15 cm sIG 33 B Sfl, which carried only 10. The much larger Hummel also carried just 18 rounds. The reworked gun indexed 15 cm sIG 33/1 took up much less space in the fighting compartment. This was achieved in part by converting the pedestal mount, which became smaller and lighter. The crew grew in comparison to the 15 cm sIG 33 B Sfl to 5 men, although the commander was very poorly placed at the beginning. Finally, the mass of the Geschützwagen 38 für sIG 33/1 (Sf.) barely grew at all compared to the stock chassis to just 11.5 tons. The mobility remained on par with the basic vehicle.

The vehicle looks tall, but its dimensions are not that different from the Marder tank destroyers.

The result was an optimal light assault SPG. Of course, such a project would ideally get its own special chassis, but the Germans didn't have one. In addition, light SPGs were a low priority subject compared to the tank destroyers. BMM was also producing the Sfl.38(t) mit 7,5 cm Pak 40 and a contract for 200 SPGs didn't mean that they would get built. Production only began in February of 1943. As with the Sfl.38(t) mit 7,5 cm Pak 40, existing chassis of Pz.Kpfw.38(t) Ausf.H tanks were used, which also meant that the vehicles had an inconsistent range of serial numbers.

The fighting compartment was altered. The loader moved to where the commander used to be.

Production Geschützwagen 38 für sIG 33/1 (Sf.). A key difference was that the commander and loader changed places. The commander was clearly not in his element in the front right of the hull and so he was moved to the back along with the radio. The second loader was moved to where the commander was along with the ammunition racks. This was a correct decision. The commander now sat in his own area and didn't get in anyone's way. 

The commander was moved to the left rear corner.

Production of the SPG nicknamed Grille (grasshopper) was sluggish. The problem lay in BMM's limited manufacturing capacity. Nothing was stopping Škodawerke from helping, but Pilsen stayed out of the project. The plans called for 25 vehicles in January of 1943, 15 in February, 40 in March, and 45 per month in April through June. Actual production went differently. Nothing at all was delivered in January of 1943, 25 vehicles were delivered in February, 40 in March, 52 per month in April-May and 31 in June. The last 10 vehicles built outside of the main contract were delivered in October of 1943 for a total of 210 vehicles with serial numbers in the 1950-2637 range. This doesn't seem like much, but this was the most numerous German SPG with the 15 cm sIG 33.

Quick gun for tank divisions

Nothing changed about the users of the Geschützwagen 38 für sIG 33/1 (Sf.). From the start, these SPGs were intended for use in heavy gun batteries attached to tank divisions. TO&E K.St.N. 1120 (Gp). was prepared back on February 1st, 1941. It was initially prepared for the 15 cm sIG 33 B Sfl since the plan was to build a whopping 481 of them, but they didn't materialize. Later, the TO&E K.St.N. 1120 (Gp) changed. K.St.N. 1120a (Gp) appeared in January of 1943 and was approved on May 1st, 1943. This TO&E called for 3 platoons of 2 SPGs per battery.

Grille in Italy.

Such a small battery size meant that the limited production run did not impede the completion of batteries. It was even frequently possible to attach a battery to tank regiments as opposed to the tank division. Deliveries began on April 20th. The 1st, 16th, 24th, and 26th tank and 3rd and 29th motorized divisions each received 2 batteries. Motorized division Feldherrnhalle received six vehicles. A number of these vehicles went to divisions being rebuilt after their destruction at Stalingrad.

The super-caliber HEAT shell could knock out any tank of the era, provided it hit the target.

The second wave of Geschützwagen 38 für sIG 33/1 (Sf.) deliveries took place in May-June of 1943. Motorized division Grossdeutschland was among the recipients. They got a whole 16 vehicles. The rest of the divisions got the standard two batteries of 6 vehicles each. Only two tank divisions got SPGs this time around: the 17th and Hermann Goering. Most of the vehicle went to SS units: Leibstandarte, Das Reich, Totenkopf. The 2nd, 4th, and 5th tank divisions for theirs in July. After that, deliveries were of singular vehicles as they were issued to replace losses. The last 4 vehicles that came out of refurbishment were issued to the 1st Ski Brigade.

The Grille made its debut during the Battle of Kursk.

The Grille, as it was officially called as of May 1943, made its debut at the Battle of Kursk. Leibstandarte and Grossdeutschland were the first to use them, but the 2nd and 4th tank divisions as well as Das Reich and Totenkopf also used them in July. Initially losses were minimal since they were rarely used as assault guns, at least in the SS. The tankers continued to treat everything with tracks and a gun as a tank. The first total losses came from the 2nd Tank Division in July of 1943. The 4th TD also lost 6 vehicles in August-September of 1943. These were the biggest losses among these vehicles in this period.

This photo shows the dimensions of the vehicle. It's big, but not that big.

Losses began to climb during the retreat following the failure of Operation Citadel. Leibstandarte began to record total losses in December of 1943. That is also when one of these vehicles, called SU-150 by the Soviets, turned up at the NIBT Proving Grounds. The vehicle's history does not mention the conditions of its capture. Nevertheless, the Grille were quite resilient, in part because they were mostly used in indirect fire. It sat somewhere between the Wespe and Hummel, even though it had a lower range than either of them.

Captured Grille at the NIBT Proving Grounds.

This intermediate status played a role in production volumes. On one hand, the tall silhouette alongside the thin bulletproof armour did not leave many chances for survival if the Grille was used in a direct support role. On the other hand, the limited range made indirect fire difficult. Further investigation of the 15 cm sIG 33 SPG showed that a direct fire assault gun was needed. On the other hand, it's hard to call the Grille bad. It wasn't bad, just niche.

Soviet sources called it SU-150.

Only one Geschützwagen 38 für sIG 33/1 (Sf.) survives today. It drowned in the Enns river in Austria in the spring of 1945. In 2000 it was raised and can now be seen among the collection of the Museum Fahrzeug-Technik-Luftfahrt. No one knows when it will be restored. The vehicle was found in poor condition and the museum is in no hurry to tackle the project.

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