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WW2 German SPGs

15 cm sIG 33 auf Panzerkampfwagen I ohne Aufbau Ausf.B Sd.Kfz.101

German Reich (1940)
Self-Propelled Artillery – 38 Built + At Least 5 Modified

The concept of mounting a heavy infantry gun on a tank chassis was born out of the need for providing the German infantry formations with more mobile artillery support. In order to test the whole concept, the German firm Alkett designed and built a small series of 38 self-propelled vehicles. These consisted of a Panzer I Ausf.B chassis armed with a 15 cm sIG 33 infantry support gun. Despite its primitive construction, this vehicle, named 15 cm sIG 33 auf Panzerkampfwagen I ohne Aufbau Ausf.B, would see extensive action up to 1943.

A 15 cm sIG 33 auf Panzerkampfwagen I ohne Aufbau Ausf.B. Source: http://www.panzernet.net/panzernet/stranky/samohybky/spanzer1.php

Rise of the Infantry Support Guns

The First World War brought a series of military reforms and the introduction of new technologies and weapons. One of these was the concept of artillery units put directly under infantry control, which were meant to provide close-up fire support. On the Western Front, the use of such artillery was highly desirable, but their employment was heavily hindered by the extensive entrenched lines. On one hand, the use of close support fire at key points on the battlefield would offer a chance to break the enemy line. On the other hand, the difficult terrain greatly reduced their mobility during advances. Another major problem was that these guns were not specifically designed for this role. Any available small caliber gun or mortar would be reused for this purpose. Because of this, these early ‘assault guns’ were too heavy or had insufficient mobility to be used more directly during assaults.

After the First World War, the Germans especially showed interest in designing and producing infantry support guns. Despite being forbidden from doing so by the Treaty of Versailles, which prohibited the development of such weapons, the Germans bypassed this by often simply adding fictitious design years to their guns, misleading the Allies into believing that they were old World War One designs. After some time spent testing and perfecting the design, two new infantry support guns would be developed. These Infatteriegeschutz (Eng. Infantry guns) included the lightweight 7.5 cm leIG 18 and the much heavier, larger caliber 15 cm sIG 33. Both of these proved to be excellent designs, serving the German infantry up to the end of the war. The 15 cm sIG 33 proved to be especially effective in combat. It had good elevation, was easy to maintain, and had excellent firepower. The range of both of these guns was rather limited but, given the specialized role that they were to perform, this was not seen as an issue. The divisional artillery formation armed with 10.5 and 15 cm guns was to provide long-range fire support.

The 7.5 cm LeIG 18 light infantry support gun. Source: Wiki
The larger 15 cm sIG 33. Source: http://panzerserra.blogspot.com/2021/04/15-cm-sig-33-sf-auf-panzerkampfwagen-i.html

Panzer I Ausf.B

The Panzer I was the first mass-produced German tank and entered service in 1934. The first version, known as Panzer I Ausf.A, while vital in providing experience regarding tank design and crew training, had many shortcomings. A year later, another improved version was introduced to service, the Ausf.B. It had a more powerful engine and an improved suspension, while the armor and armament remained the same. By the time production ended in June 1937, some 1,500 of both versions were built. While additional versions would see service in the following years, these would be built in limited numbers only.

Despite its obsolescence as a combat tank, with its weak armament of two machine guns, the Panzer I remained in frontline service up to late 1941. The German industry could not sufficiently produce an improved design, so the Germans were forced to use the Panzer I as a frontline tank. Due to its obsolescence, the Panzer I chassis would be reused for a series of adaptations for other roles. This included a small production run of a self-propelled version armed with the 15 cm infantry support gun.

A Panzer I Ausf.B. Source: https://warspot.net/579-pz-kpfw-i-ausf-b-all-grown-up

Self-Propelled Version

While the 15 cm sIG 33 offered great firepower to the German infantry, its weight severely limited its mobility. Infantry units of the German Army were not very mobile formations given the general lack of towing vehicles, mostly relying on horses to pull their equipment. In rarer cases, a Sd.Kfz.10 or 11 half-track was used to tow this gun. Depending on the means of towing, different road wheels were used. For example, when towed by horse, all-metal wheels were used. When towed by a half-track, metal wheels with solid rubber tyres were used. Despite this, moving a heavy gun was tiresome and took some time to set up properly. In addition, during retreats, the guns were often abandoned, as they could not be moved fast enough.

These shortcomings became obvious after the Polish campaign in 1939. Shortly after that, WaPrüf 6 issued orders to develop a self-propelled version armed with such a gun. According to original plans, this vehicle was to be a completely new design. These envisaged a fully tracked vehicle with 50 mm frontal and 20 mm side armor. The total combat weight was to be slightly over 12 tonnes and the maximum speed was to be 67 km/h. The last but most important requirement was that the gun could be easily dismounted and used in its original configuration.

Given that designing a brand new chassis would take time, the Germans went for the simplest possible solution. In parallel with this vehicle’s development, an anti-tank vehicle armed with a 47 mm gun was being developed using the Panzer I Ausf.B chassis. The choice to use this chassis was made in order to maximize the efficient utilization of existing resources. Larger chassis, such as the Panzer III or IV, would be more suited for this task, but, given their limited numbers and the lack of production capabilities of the German industry, there was no other choice than to reuse the Panzer I tank for this project. Another fact is that the Panzer I was, by that point, an obsolete combat vehicle, but it remained in service, as there was nothing available to replace it.

In any case, the overall design of the new self-propelled vehicle was quite simple. The Panzer I’s upper superstructure was removed and replaced with the entire 15 sIG 33 gun, which was provided with box-shaped armor protection, open to the rear and top. While this simple approach left much to be desired, it made the whole construction easy to repair and maintain, and any damaged part could be easily replaced or salvaged.

The first working prototype was completed at the start of 1940. It was designed and built by Almarkische Kettenfabrik G.m.b.H from Berlin. Following successful testing, a production order for 37 (chassis numbers 10456 to 16500) such vehicles was given. These were completed by the end of February or March 1940, depending on the source.

According to some sources, this is the prototype vehicle during testing in early 1940. Source: https://warspot.ru/9376-malye-da-udalye

Name

This vehicle was designated as 15 cm sIG 33 auf Panzerkampfwagen I ohne Aufbau Ausf.B Sd.Kfz.101. As with other German armored vehicles of WWII, other sources use slightly different designations for this vehicle, such as 15 cm sIG 33 PzKpfw I Ausf.B. For the sake of simplicity, this article will use sIG 33 auf Pz. I, although this was not an official name.

The nickname ‘Bison’ is often associated with this vehicle, but the Germans never referred to it as such. It is also sometimes referred to as Sturmpanzer I, which was another false name, given the incorrect belief that the vehicle was designed to be used as a direct fire support weapon. While occasionally it did happen, it was not its primary purpose.

Design

Hull

The sIG 33 auf Pz. I’s hull consisted of a frontal-mounted transmission, a central crew compartment, and a rear-positioned engine. Its overall design was unchanged from the original Panzer I Ausf.B.

Suspension

The suspension was another element that remained unchanged the original Panzer I Ausf.B. It consisted of five road wheels per side. The first wheel used a coil spring mount design with an elastic shock absorber in order to prevent any outward bending. The remaining four wheels were mounted in pairs on a suspension cradle with leaf spring units. There was a front drive sprocket, rear idler, and four small return rollers. Given the added weight, the sIG 33 auf Pz. I’s suspension was very prone to malfunctions and breakdowns.

The suspension of this vehicle consisted of five road wheels, four return rollers, a front-drive sprocket, and the rear idler. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/
Due to the extensive weight of the gun, crew, and the gun shield, the sIG 33 auf Pz. I’s suspension was prone to breakdowns. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

Engine

With the added crew and the gun, the weight increased from the original 5.8 tonnes to 7 tonnes. Depending on the source, the weight of this vehicle is sometimes described as being 8.5 tonnes. The Panzer I Ausf.B was powered by a new water-cooled Maybach NL 38 Tr, which was able to supply 100 [email protected] 3,000 rpm. The maximum speed, depending on the source, ranged between 35 to 40 km/h, while the cross-country speed was only 12-15 km/h.

The fuel was located in two tanks, with one holding 82 liters and the other 62 liters. Both were located to the rear right side, separated from the crew compartment. The operational range was 170 km on good roads and 115 km/h cross-country. Sources, such as Tank Power Vol.XXIV 15 cm sIG 33(Sf) auf PzKpfw I/II/III, mention that the operational range was only 100 km. Author W. Oswald (Kraftfahrzeuge und Panzer) mentions that the operational range was 160 km on good roads and 120 km cross-country.

Superstructure

While the chassis remained unchanged, the original superstructure and turret were removed. A small frontal driver plate was retained. While it kept its driver visor port located on the left side, another smaller vision port was added next to it. Despite this, the driver’s vision would be severely limited by the added upper superstructure, which protruded slightly on both sides in front of the driver.

The driver’s front plate with its two visor ports is visible here. Source: Digital Collection of Armin Freitag

On top of the vehicle, a simple three-sided box-shaped armored superstructure was added for crew protection, which was open from the back and the top. The sides and lower parts of the front armor were flat. The upper front plates were slightly angled inwards at a 25º angle. On the front armor, there was a large U-shaped opening to allow the gun to be placed there. It was enclosed by the gun shield itself, so it did not leave the crew exposed. There was a large hatch located to the front upper left, which served to provide a clear view for the gunner’s sights.

The frontal gun shield that covered the large opening on the front superstructure armor. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

The side armor plates had few noticeable features. On top of them, small observation ports were added. The rear sections of the two side walls could, if needed, be opened to provide the crew with more working room and some protection during ammunition resupply. Lastly, on the sides of the armor plates, there was a noticeable bulge that protruded out of the vehicle. Its purpose was to provide slightly more working space during the mounting or dismounting of the main gun.

The rear sections of the side walls could, if needed, be opened to provide the crew with more working room. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

With this new superstructure, the sIG 33 auf Pz. I was a fairly large target. There are conflicting recollections on the precise turret dimensions. For example, D. Nešić (Naoružanje Drugog Svetskog rata-Nemačka) mentions a length of 4.42 m, width of 1.68 m, and height of 3.35 m. According to T.L. Jentz and H.L. Doyle (Panzer Tracts No.10 Artillerie Selbsfahrlafetten) the length was 4.42 m, width of 2.17 m, and height of 2.7 m. Lastly, Tank Power Vol.XXIV 15 cm sIG 33(Sf) auf PzKpfw I/II/III gives slightly different numbers of 4.42 m length, 2.65 m width, and 3.35 m height.

The sIG 33 auf Pz. I’s large size in comparison to one of its crew members. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

Armor

The original Panzer I Ausf.B’s armor was quite thin. Its front hull ranged between 8 to 13 mm. The side armor was 13 mm, the bottom 5 mm, and the rear 13 mm. The armor was made of rolled homogenous hardened plates with a Brinell hardness of 850. It was welded and formed the body of the superstructure and hull. Although not protected from even small caliber anti-tank guns, it could provide protection against small arms fire and SmK bullets (steel-cored rifle bullets).

An illustration showing the armor thickness around a Panzer I Ausf.A. Source: Panzer Tracts

For crew protection, the vehicle received a large box shaped superstructure. Its armor thickness was only 4 mm thick according to T.L. Jentz and H.L. Doyle (Panzer Tracts No.10 Artillerie Selbsfahrlafetten). Authors, such as D. Nešić (Naoružanje Drugog Svetskog rata-Nemačka), mention it being 10 mm thick. This basically provided the crew protection only against regular small caliber ammunition. Armor-piercing ammunition of the same caliber could easily penetrate this armor. With its open rear and top, its large silhouette, and a powerful gun, the sIG 33 auf Pz. I was a magnet for enemy return fire. This alone indicates that using this vehicle in a close support role was dangerous for the crew and completely suicidal.

The sIG 33 auf Pz. I’s armor did not offer any real protection besides small caliber bullets and shrapnel. Being hit by any properly designed anti-tank weapons could easily take it out of action or completely destroy it, like in this picture. This also shows the round platform on which the 15 cm sIG wheels were placed. Source: Digital Collection of Armin Freitag
This sIG 33 auf Pz. I’s upper superstructure armor was completely shattered, possibly being hit by an enemy artillery round. This photograph clearly shows how its armor protection was quite poor. Source: Achtung Panzer. 150-мм САУ Bison I Военные машины 62 Торнадо
Additional spare parts would sometimes be added to the front to act as extra armor. Their effectiveness is questionable. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

Armament

The main armament of this vehicle was 15 cm sIG (schwere Infanteriegeschutz – Heavy infantry gun) 33. Rheinmetall began its development in 1927 and it entered service in 1933. With a total weight of approximately 1,700 kg, it was one of the heaviest guns ever to be used for infantry support. It was a reliable and robust gun that was easy to build and required very little maintenance.

In terms of construction, it was a quite conventional design. It had a two-wheeled carriage and the older type of box trail equipped with a hydropneumatic recoil system placed under the gun barrel. The gun possessed a high elevation and used a horizontal sliding-block breech mechanism. To help counteract the muzzle weight, two balancing springs (one on each side) were installed. The 15 cm sIG was considered a satisfactory weapon by the Germans, but the greatest issue was its weight. It would remain in use throughout the whole of World War II in both its original form and as the main weapon of many German self-propelled guns.

A gun crew loading a 15 cm sIG 33. Source: Wiki

The gun installation was quite simple. The gun was not modified in any way and was simply placed on top of the modified Panzer I chassis. To hold the wheels in place during firing the gun, they were placed in a metal housing located on top of the mudguards and held in place by two large screws. In addition, there were two large metal rings that housed the two gun wheels, providing further stability. The gun was also secured by a vertical pin connected to the gun tow hitch (towing eye) and a triangular-shaped armored plate that was welded to the engine compartment.

A sIG 33 auf Pz. I without its armored superstructure. Source: Digital Collection of Armin Freitag
Close-up view of the metal housing located on top of the mudguards, which held the wheels in place. Source: https://warspot.ru/9376-malye-da-udalye
The gun was also secured by a vertical pin connected to the gun tow hitch and a triangular-shaped armored plate that was welded to the engine compartment. Source: Digital Collection of Armin Freitag

If needed, the gun could be easily removed, which made performing maintenance and repairs much simpler. For removing the gun, a sufficiently strong crane (either specially designed or a simple improvisation) would be needed. As mentioned earlier, for this purpose, on both sides of the armored superstructure, there were two oval projections. The right-hand side oval was slightly closer to the front of the vehicle, in contrast to the left side. This allowed the gun to be turned anti-clockwise and, with the help of some kind of crane or a winch system, the gun could be removed without dismantling the rest of the superstructure.

The gun could be easily replaced using any kind of sufficiently strong crane. Source: Digital Collection of Armin Freitag

The 15 cm sIG 33 fired a 38 kg heavy high-explosive round at a maximum range of 4.7 km. This high-explosive round, during explosion, created a lethal area of around 100-120 m wide and 12-15 m deep. While the 15 cm sIG used several different ammunition types, on the sIG 33 auf Pz. I configuration, only the high-explosive rounds were used. The main gun elevation was -4° to +75, while the traverse was 5.5° in both directions. These numbers differ depending on the source used. The rate of fire was low, at only 2 to 3 rounds per minute. This was due to the heavy weight of the shells and the use of separate two-part ammunition (shell and charges). The 15 cm sIG 33 used the Zeiss Rblf 36 gun sight.

Close-up view of the Zeiss Rblf 36 gun sight. Source: Achtung Panzer. 150-мм САУ Bison I Военные машины 62 Торнадо

Due to its cramped interior and the large size of the ammunition required for the gun, only three spare rounds were transported in the vehicle. This would limit the effectiveness of the sIG 33 auf Pz. I if the supporting ammunition carriers could not reach them for whatever reason. It is possible that the crew would store additional rounds inside the vehicle. There are photographs that show that an additional fourth round was carried under the gun itself, making it difficult to spot. Spare rounds were held in woven containers. Two were usually located on the left rear mudguard and one on the right side, plus the one under the gun. Author D. Nešić (Naoružanje Drugog Svetskog rata) states that the ammunition load of this vehicle consisted of up to 18 rounds. This is highly unlikely given the limited internal space.

Besides the crew’s personal weapons, no secondary armament was provided for self-defense. As this vehicle was designed to fire from a distance, this was not a significant issue.

The round that was placed under the gun can be seen here. In addition to this, the right round is also visible. Two more round containers (possibly empty) are held on top of the gun’s box trail. Source: https://www.tapatalk.com/groups/missinglynx/sig-33-auf-pz-i-ausf-b-ohne-aufbau-4th-ammo-basket-t42129.html
The crew of this vehicle appears to have stored at least 6 spare rounds. Source: T. Anderson Panzerartillerie
The crew of this vehicle have stockpiled additional rounds and various spare parts on their vehicle. Source: Achtung Panzer. 150-мм САУ Bison I Военные машины 62 Торнадо

Using a lighter gun may seem to have been a better choice due to the overburdened chassis, but the Germans chose not to for a number of reasons. The two main infantry support guns used by the German Army were the 7.5 and the 15 cm guns. The much smaller 7.5 cm leIG 18 was lighter. On the other hand, the gun was highly mobile, so placing it on a self-propelled chassis was not seen as urgent or providing enough advantages. The 15 cm sIG 33 was very heavy and mobility was a major issue. Using it in a more mobile configuration was deemed more important. In addition, it had much stronger firepower than its ‘smaller’ ‘cousin’. Using other artillery pieces, such as the 10.5 cm howitzer, may also have been an alternative. However, as it was not directly under the control of the infantry, the use of this caliber would have caused some logistical problems.

Crew

According to authors T. L. Jentz and H. L. Doyle (Panzer Tracts No.10 Artillerie Selbsfahrlafetten), this vehicle was operated by a crew of five. During relocation to other positions, three of them were stationed inside the vehicle. The two remaining crew were transported by the auxiliary vehicles belonging to the unit. Somewhat confusingly, the same sources later mentions a crew of four including the commander, two gun operators, and the loader in the specifications table. It is unusual, but the driver is not mentioned, which might explain the difference.

Sources such as Tank Power Vol.XXIV 15 cm sIG 33(Sf) auf PzKpfw I/II/III mention a crew of four: commander/gunner, driver, and two loaders. This is a significant difference between sources, especially regarding the commander’s role. To complicate the matter, older photographs show this vehicle with both four and five crew members.

The driver was positioned on the vehicle’s left side and was fully protected. As there was no hatch available for him, the driver would have had to squeeze around the gun to get to his position. The gunner was positioned to the left of the gun and the loader right of him. The last crew member was likely positioned behind them, ready to assist the gun’s loading.

The crews from the supply vehicles (usually three people per vehicle) would also help with delivering the ammunition. As there was little room for all of the crew members, on long marches, the crew, with exception of the driver, were usually transported by the support vehicles (but this depended on the situation in the field). As an open-topped vehicle, the crew was also exposed to the weather. A canvas cover could be placed over the vehicle, but it limited the crew’s view of the surroundings.

This vehicle appears to have a crew of five, with the driver and three more crew inside the vehicle, and one more further back. Some of them may have not been part of the crew, but rather part of the auxiliary personnel. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/
Another vehicle with a crew of five. Source: F. V. de Sisto Early Panzer Victories
This vehicle appears to have had four crew members. Source: Bobban-Militaria via Ebay

Organization

With the completion of all 38 vehicles, it was possible to begin creating the first units equipped with this vehicle during spring 1940. These were allocated to six vehicle strong Schwere Infanteriegeschütz (Motorisiert) Kompanie – s.I.G.(mot.) Kp (Eng. self-propelled heavy infantry gun companies). With the available vehicles, six such companies were formed, numbered 701 to 706. The remaining two vehicles were allocated for training, but also acted as replacement vehicles.

These companies were attached to the Schütze-Brigade (Eng. rifle brigade) of various panzer divisions just prior to the German invasion of the West in May 1940.

s.I.G.(mot.) Kp Panzer Division
701st 9th
702nd 1st
703rd 2nd
704th 5th
705th 7th
706th 10th

The structural organization of each company consisted of a command unit that was equipped with four military cars, such as the Kfz.15 Horch, as well as four motorcycles. One of these motorcycles was provided with a sidecar. The companies were divided in three two-vehicle strong platoons. These were supplemented by four Sd.Kfz.10 half-track vehicles with two trailers and two motorcycles. Additional trucks would be used to transport ammunition, fuel, and spare parts from designated army storage bases to the unit at the front. After 1941, some structural changes were made to address this unit’s lack of radio equipment, enlarging the command unit with additional vehicles and radio equipment. In later years, some vehicles may have been provided with radio equipment, like some photographs indicate.

Each of the 6 self-propelled heavy infantry gun companies received 6 sIG 33 auf Pz. I vehicles. These appear to be undergoing training, as some vehicles lack the upper armor protection. At least one even lacks its gun, or is acting as an ammunition carrier. Source: Digital Collection of Armin Freitag
Due to their low ammunition capacity, additional auxiliaries vehicles were needed to transport ammunition, spare parts, and even some of the crew members. Source: Digital Collection of Armin Freitag
While the sIG 33 auf Pz. I was initially not provided with radio equipment, later in the war, some of them appear to have been equipped with such devices. Source: http://www.panzernet.net/panzernet/stranky/samohybky/spanzer1.php

In Combat

Conquest of the West, May 1940

The sIG 33 auf Pz. I self-propelled heavy (motorized) infantry gun companies, numbered from 701 to 706, were allocated to six panzer divisions that were preparing for an attack on the Westin May 1940.

During the German offensive in France, the vehicle proved to be an effective weapon, but was not without its flaws. While the firepower was deemed excellent, other characteristics, such as mobility, armor protection, and reliability were deemed insufficient. The mechanical breakdowns, especially of the transmission, were common, and many vehicles were put out of action because of this. For example, the 703rd Company only had one operational sIG 33 auf Pz. I after the first week of fighting. In total, only two were lost due to enemy fire during this offensive. One of these two was hit by an artillery shell and destroyed.

In a report about the sIG 33 auf Pz. I’s performance by the 706th Company, it was noted that:

“ … The sIG 33 auf PzKpfw I Ausf B, in its present form, has not performed well. However, the gun, if it could be used in action, was very effective and fully met our expectations. The PzKpfw I chassis proved to be too weak. The running gear, although overhauled at Alkett, was still considerably worn out after many years in service. Most failures (up to 60 percent) occurred with the clutches, the brakes and the tracks. Most sIG companies could not keep pace with the marching speed of the tank division, which often exceeded 30 kph. For this reason, we advise to attach the sIG companies to infantry divisions for the coming combat… Combat usually involved a single sIG 33 gun firing from a concealed position at ranges varying from 50 to 4,000 m. But, the front gun shield was repeatedly penetrated by armor-piercing infantry ammunition…. “

A sIG 33 auf Pz. I during the advance in the West. Somewhat unusually for the German standards, the crews of these vehicles often added various painted names on them. Source: T. Anderson Panzerartillerie
Due to its weak armor protection and sheer size, the sIG 33 auf Pz. I was an easy target for enemy gunners. At least two vehicles were lost during the invasion of the West in 1940. Source: T. Anderson Panzerartillerie

In the Balkans

During the Axis invasion of Yugoslavia in April 1940 and later the invasion of Greece, only the 701st, 703rd, and 704th companies would see action. There is little to no information about their use in this campaign, as the war reports of the panzer divisions they were attached to (2nd, 5th, and 9th) barely mention them. Yugoslavia was defeated very quickly (the war lasted less than two weeks) with minimal German losses. The sIG 33 auf Pz. I may have seen some limited action there. Following the capitulation of Yugoslavia, the Germans successfully invaded Greece. They were likely used to bombard the heavily defended Greek Metaxa Line. As with their service in the Balkans, due to lack of information, it is unknown whether these units suffered any losses, but it is likely none were lost.

A sIG 33 auf Pz. I during the German offensive in Greece. Source: J. Engelmann Bison und andere 15 cm-Geschutze auf Selbstfahrlafetten

Although not a single vehicle is recorded as having been lost in direct combat, as many as six were lost in a highly unusual accident. After the successful campaign in the Balkans, the Germans began to withdraw their forces in preparation for the upcoming attack on the Soviet Union. On 19th May 1941, the 703rd Company, together with other armored vehicles of the 2nd Panzer Division, was meant to be transported by the ships Kybfels and Marburg from Patras (Greece) to Taranto (Italy). On 21st May, unbeknown to the Axis forces, the British HMS Abdiel (M39) minelayer secretly laid down some 150 mines near the planned route. Close to Cape Dukato, Kybfels struck a mine at around 14:00. The damage was so extensive that the whole ship sank very quickly. Shortly after, the Marburg would also run into a mine, and the explosion caused a huge fire. The ship did not sink immediately, but it also shared the fate of the first Axis ship. In this action alone, the Germans lost 226 personnel, including all the equipment and weapons stored inside these two ships. While not completely clear, it appears that at least some vehicles and equipment from the 2nd Panzer Division may have been unloaded a few days before this accident. Whatever the case, the 703rd Company lost all of its six-vehicles. As a replacement, it received towed 15 cm sIG guns instead.

The sIG 33 auf Pz. I from the 703rd Company being loaded on ships to be transported to Italy. All six were lost when the ships carrying them were sunk by mines. The 703rd Company was not disbanded, but was instead equipped with towed 15 cm sIG 33. Source: https://www.forosegundaguerra.com/viewtopic.php?t=20226

In the East, 1941-1943

By the time of the German invasion of the Soviet Union in June 1941, there were at least 30 sIG 33 auf Pz. I available for action. Although the German forces did not expect any serious resistance from the Soviet Army, the first month of the invasion proved to be different from their expectations. All six self-propelled heavy infantry gun companies were heavily engaged in combat.

In a combat effectiveness report (as found in T. Anderson Panzerartillerie) made by the 702nd Company dated from 18th September 1941, there are few quite interesting comments regarding the use of these vehicles.

“.. Despite being thoroughly aware of the shortcomings of the type, the sIG (mot S) [referring here to sIG 33 auf Pz. I] has performed very well as an assault gun during the campaign in Russia. This is in contrast to the first combat deployment in France, when the unit was afflicted by a number of serious problems. However, our troops have benefited from the long training phase and become accustomed to all mechanical vagaries and adept at correcting any faults.

The sIG (mot S) is the ideal assault gun for the lead echelon of a Kampfgruppe in the Panzer division. The low trajectory of the gun, when firing with a No.4 charge, is very effective for attacking a point target, such as a bunker or dug-in artillery, or machine-gun nests and mortar positions, with a minimal expenditure of ammunition. Due to having a fully tracked chassis, the vehicle can be moved quickly to attack a fresh target. Also, by being armored, the sIG (mot S) can also be deployed in an open position, and this can have a demoralizing effect on enemy forces; many cease firing and give away their positions. The gun is not suitable for use as an anti-tank gun, but it must be emphasized that in an emergency situation, it can be used to attack enemy armor. Just the massive effect of a 15 cm high-explosive shells detonating near enemy tanks will normally cause the attack to turn away; this was even applicable to the 52 tonnes heavy tank [KV heavy tank] . A stationary tank or any that approaches head-on can be destroyed at 300 to 400 m range by firing two or three shells propelled by the No.4 charge. In most instances, a combat company was supported by a single self-propelled gun, but any action involving the platoon would be the exception: the guns would be concealed in a covered position: Some 80 percent of all rounds were fired from an open position… “

This report shows that the crew of some units prefered to use the sIG 33 auf Pz. I in a role for which it was not designed for. Thanks to its excellent firepower, enemy targets, such as fortified positions, could be easily taken out. The report even mentions the possibility to use it as an improvised anti-tank vehicle in desperate situations

This report also mentions the logistical problems, something that the Germans always faced during the war and which were never fully solved.

“ … The workshop facilities for the company must be improved and enlarged. The lack of an 8-ton Zugmaschine [Sd.Kfz.7] and a flatbed trailer to enable the recovery of damaged guns or repair, is a very serious problem. Recovery services at divisional level were insufficient and extremely slow at recovering any damaged gun. This was observed when two were lost during the advance and, despite of their exact positions being reported (as required in regulations), neither vehicle was recovered immediately. However, both were later recovered by crews from the Speer organization, but were not returned to the company. If a tractor and flatbed trailer had been available, these precious guns would have been returned directly to our workshop unit for repair and be available for service after seven days. The resupply to our guns worked satisfactorily, this was due to the dedication to his task by the leader of the ammunition squad. However, it has become obvious that the standard Opel Blitz truck has poor cross-country mobility and a lack of cargo capacity. Since a large stock of ammunition cannot be carried on the gun, the resupply team has to follow in close proximity. As a consequence, the delivery of heavy cross-country trucks must be considered a vital necessity.’’

Lastly, the unit report also mentions the numbers of destroyed enemy targets during the advance toward Leningrad. The enemy losses, at the cost of some 1,640 rounds, were 24 bunkers, 31 guns, 13 anti-tank guns, and 6 tanks. Interestingly, the report also mentions the list of repairs undertaken during a four-day march, which included the replacement of 68 road wheels, 392 track links, 1,057 track bolts, 8 idler wheels, 2 drive sprockets, 5 return rollers, 9 leaf springs, etc. The source for this report is T. Anderson (Panzerartillerie).

While the use of the sIG 33 auf Pz. I by the 702nd Company may indicate that this vehicle could be quite effectively used in more aggressive and direct combat actions, a few things should be remembered. In the early period of the war in the East, the Soviet forces were often poorly led and trained. This affected their overall combat performance greatly, to the point that they would often run from the Germans, thinking the enemy was superior to them. As the war progressed and the Soviet soldiers became more experienced with fighting the enemy, the German advance slowed down. The sIG 33 auf Pz. I’s armor was minimal at best, and the Soviets possessed a great number of anti-tank guns and anti-tank rifles that could easily take out this vehicle.

To prevent the inadequate use of the lightly protected self-propelled artillery, in late 1942, the Waffenamt (German Army Weapon Agency) issued a series of orders that essentially banned their use in more direct attacks. With the rising losses, the surviving crew members were often relocated to the infantry school at Döberitz. There, they trained and helped with the formation of new units. The last unit to operate this vehicle was the 5th Panzer Division (704th), which still listed two operational vehicles on 30th June 1943.

A sIG 33 auf Pz. I am from the 702nd Company somewhere in the Soviet Union, possibly during the winter of 1941/42. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

New Vehicles

While the sIG 33 auf Pz. I chassis was overburdened and prone to malfunctions and breakdowns, its simplicity allowed an easy replacement of damaged parts. Another benefit of its overall design was that, in case the chassis was damaged beyond repair, the gun could simply be dismounted and used in its original configuration, or the other way around. Easy removal of the gun allowed the German crews to fabricate additional vehicles, if the components for it were available, with minimal equipment. This was the case of the 701st Company, which received 5 Panzer I chassis and two 15 cm guns. While, officially, only 38 vehicles were built, additional conversions were undertaken in the field. At least a few were documented, although others may have not. This somewhat complicates determining the precise number of such conversions used, but it is highly unlikely that many additions were completed.

The Germans somewhat overcomplicated their self-propelled artillery design by having the possibility to use the gun in its original towing configuration. The sIG 33 auf Pz. I actually achieved this with minimal work needed on its overall design.

A pair (or even more) sIG 33 auf Pz. Is without their armament. These were under repair or even possibly newly rebuilt vehicles. Source: Bobban-Militaria
Thanks to its simple mechanical nature, the replacement of the gun was relatively easy. Source: T. Anderson Panzerartillerie

Was This Vehicle Used as an Assault Gun?

There is some confusion about the precise role of this vehicle. Often, in some sources, this vehicle is described as the performing role of an assault vehicle. The well-known and only footage of this vehicle in action shows it firing at French buildings at close range supported by infantry. It is important to note that this was likely a propaganda film made by the Germans. In general, the self-propelled artillery guns developed and used by the Germans, such as this vehicle and later models (Wespe, Hummel, and the 15 cm armed Grile based on the Panzer 38(t) chassis) were provided with limited armor protection, giving more priority to their mobility and firepower. Their role was not to charge at the enemy and fire at close range, but instead to follow up fast motorized German units and support them from a distance. For close firing support, the German employed the well-known StuG III series, but also the 15 cm armed Sturmpanzer III and IV. These were much better protected and able to resist enemy return fire. While the sIG 33 auf Pz. I could be and probably was used as an assault gun on some occasions, this kind of deployment was highly risky for its crew. If the enemy had any kind of anti-tank weaponry or were even positioned above, they could easily take out the sIG 33 auf Pz. I’s crew or the vehicle itself. The effectiveness of its firepower can be seen in a contemporary German propaganda video, where it is filmed destroying a house during one German offensive action (possibly somewhere in France).

Despite being often portrayed and sometimes used as a close support vehicle, the sIG 33 auf Pz. I was best used from a relatively safe distance. Source: https://www.worldwarphotos.info/gallery/germany/artillery/sig33-bison/

Conclusion

The sIG 33 auf Pz. I self-propelled gun solved the problem of the low mobility of towed artillery guns. It could engage enemy positions and then disengage, change position, or retreat to safety much faster than towed artillery. But, due to its overburdened chassis, breakdowns were common, which led to a reduction of its mobility. The firepower of the main 15 cm sIG 33 guns was considered to be satisfactory, being able to destroy most targets and even, in rare cases, enemy tanks. The sIG 33 auf Pz. I also had a very simple design, which allowed for quick and easy repairs and replacement of any damaged or worn-out parts. The simple installation of the 15 cm sIG gun also allowed it to be dismounted (if the tank chassis was damaged beyond repair) and to be used as an ordinary towed gun.

The weight of the 15 cm gun and the additional armor plates was simply too much for the weak Panzer I tank’s chassis. This overloading often resulted in many transmission and suspension breakdowns. A common problem was the frequent malfunction of the shock absorbers fitted to the front road wheels. Breakdowns of entire wheels and tracks were also common. The main gun recoil during fire was so strong that the vehicle would wildly shake and could be thrown back. This also increased the chance of damaging the chassis.

The high profile was a big problem for the sIG 33 auf Pz. I too, making it an easy target for enemy artillery gunners. The armor was also quite light and offered only limited protection from small arms fire and shrapnel.

It must be noted that this vehicle was not designed to be used as an assault weapon. Instead, it was intended to be used in a supporting role from a distance, where the lack of armor was not so important. Regardless, heavy camouflage and a well-selected combat position were necessary for the crew’s survival, but this was not always possible or easy to achieve successfully.

Low ammunition capacity was a major issue, especially during prolonged fighting, as the gun could quickly run out, which limited its combat potential. The sIG 33 auf Pz. I, therefore, needed constant support from a supply vehicle for the delivery of additional ammunition, which were themselves in short supply. Moreover, the crew compartment was too cramped, meaning that some of the vehicle’s crew had to be transported by these ammunition vehicles.

Despite its flaws, the sIG 33 auf Pz. I would become an example of how the Germans would (especially in the later part of the war) reuse obsolete or captured tank chassis and combine them with the 15 cm sIG gun. Later models would use more fitting tank chassis, such as the Panzer 38(t), which would be built in much greater numbers. The sIG 33 auf Pz. I may not have been a perfect weapon, but it influenced the future development of similar vehicles used by the Germans during the war. It not only helped German designers and engineers gain experience in making similar vehicles, but also contributed to the development of adequate tactics.

The author of this article would like to thank Guillem Martí Pujol for providing valuable data and Smaragd123 for providing photographs.

15 cm sIG 33 (Sf) auf Geschüetzwagen I Ausf.B Sd.Kfz.101 of the schwere Infanteriegeschütz-Kompanie 701, France, May 1940.
15 cm sIG 33 (Sf) auf Geschüetzwagen I Ausf.B Sd.Kfz.101 of unit 704, attached to the 5th Panzerdivision.
Both illustrations were produced by Tank Encyclopedia’s own David Bocquelet.
sIG 33 auf Pz. I Specifications
Weight 7 tonnes
Crew 4 to 5 (driver, commander/gunner, loader, and radio operator)
Engine Maybach NL 100 hp ​​@ 3,000 rpm
Speed 35-40 km km/h / 12-15 km/h (cross-country)
Range 170 km / 115 km (cross-country)
Armament 15 cm sIG 33
Armor 4 to 13 mm

Sources

T. Anderson (2020) The History of the Panzerwaffe, Osprey Publishing
J. Engelmann, Bison und andere 15 cm-Geschutze auf Selbstfahrlafetten, Podzun-Pallas-Verlag GmbH
P. Chamberlain and H. Doyle (1978) Encyclopedia of German Tanks of World War Two – Revised Edition, Arms and Armor press.
Ian V. Hogg (1975) German Artillery of World War Two, Purnell Book Services Ltd.
T. L. Jentz and H. L. Doyle (1998) Panzer Tracts No.10 Artillerie Selbsfahrlafetten
H. Doyle (2005). German Military Vehicles, Krause Publications
F. V. De Sisto (2010) Early Panzer Victories, Condor Publication Company
Tank Power Vol.XXIV 15 cm sIG 33(Sf) auf PzKpfw I/II/III, Wydawnictwo Militaria
D. Nešić (2008) Naoružanje Drugog Svetskog rata-Nemačka, Beograd
W. Oswald (2004) Kraftfahrzeuge und Panzer Motor Buch Verlag
T. Anderson (2019) Panzerartillerie, Osprey publishing
D. Predoević (2008) Oklopna vozila i oklopne postrojbe u drugom svjetskom ratu u Hrvatskoj, Digital Point Tiskara

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