7.5 cm StuK 42 L/70 auf StuG III

German Reich (1942)
Assault Gun – Wooden Mock-Up Built

The introduction of the long-barrel StuG III (Sturmgeschütz III, Eng. Assault gun vehicles) gave the Germans an excellent anti-tank vehicle that was cheap, combat effective, and was slowly being produced in ever-growing numbers. But its overall design was not perfect and there was room for improvement. Adolf Hitler took this to the next level by urging that further StuG III development had to include the installation of the 7.5 cm L/70 gun that was specially developed for the new Panther tank. While a wooden mock-up was constructed, it was eventually decided that the larger Panzer IV chassis was better suited for this purpose and the work on the rearmed StuG III was canceled.

History

During the Great War, German infantry formations were supported by towed artillery. For German Sturmtruppen (Eng. Stormtroopers), which depended on mobility, the necessary towed artillery proved to be slow and inadequate for the supporting task of taking more fortified enemy positions. After the war, the great German Army tactician, General Erich von Manstein, proposed using highly mobile, well-protected, and armed self-propelled artillery. They were to provide infantry with mobile close-fire support during combat operations. This concept would eventually evolve into the StuG III. Due to Germany’s general lack of production industrial capacity during the 1930s, it would take years before the prototypes were completed. The actual production of these vehicles began just a month before the German invasion of the West in May 1940.

Once pressed into service, the StuG III proved to be an excellent infantry support vehicle. It featured a low silhouette, thick frontal armor (it was the best-protected German vehicle in the early stages of the war), and effective armament. The armament, in particular, consisted of a short barrel 7.5 cm gun. Its primary purpose was to deal with enemy entrenched positions at medium-to-close range. When using high-explosive rounds, it could easily destroy enemy anti-tank or machine gun emplacements. The Germans were not ignorant of the possibility that these vehicles would encounter enemy tanks. High-explosive rounds, while not designed for this role, could still deal significant damage to lightly protected vehicles. For dealing with more heavily protected vehicles, proper armor-piercing rounds were needed. These could pierce some 40 mm of armor at distances of 500 m. This was more than enough to deal with most enemy tanks encountered in the early stages of the war.

However, things changed after 1941, when the Germans began encountering better-protected enemy tanks in ever greater numbers. Most notable were the Allied T-34, KVs, and the Matildas. The German tanks struggled to deal with the new threats so, in desperation and due to a lack of any other alternatives, the StuG III was often put into the role of an ad hoc tank hunter. Despite the short gun, the StuG III performed well in this role. For example, during the German attempt to capture Crimea in March 1942, the StuG IIIs from the 197th Assault Gun Battalion saw action against Soviet Armor. From 13th to 19th March, they claimed to have destroyed 70 Soviet tanks, including KV-1s. This offensive was followed up by the Trappenjagd (English: Bustard Hunt) operation to dislodge the Soviet Forces in Crimea. The main spearhead of this operation consisted of the 22nd Panzer Division, supported by the 197th Assault Gun Battalion. The German operation lasted from 8th to 20th May 1942. During that time, these two units claimed to have destroyed 250 Soviet tanks for the loss of only 3 StuG IIIs and 8 Panzers. This success was partially achieved thanks to the use of newly developed high-explosive anti-tank rounds. While they could penetrate the thick armor of the Soviet tanks, their effectiveness was quite limited by the rather poor ballistic characteristics of such rounds.

To deal with the enemy armor threat, the Germans developed the 7.5 cm PaK 40 anti-tank gun. Being a towed gun, it was not suited in its original form for mounting inside armored vehicles. Thus, several changes were needed to be done before it could be reused for this role. This led to the introduction of the 7.5 cm StuK 40 L/43 and the slightly longer L/48 guns.

The installation of such a gun was tested on a modified StuG III Ausf.E in April 1942. While the gun’s performance was overall satisfactory, the ejection of spent cartridges proved somewhat problematic, but in time, this problem would be resolved. The first StuG III to enter service with this gun as standard armament was the Ausf.F version. The production began in late March and ended in September 1942. During this period, some 366 Ausf.Fs would be built by Alkett. It was followed by the Ausf.F/8 (250 built) and the final form known as the Ausf.G. The latter was one of the most produced German vehicles, with some 8,500 being built.

The Need for a Bigger Gun

Despite the effectiveness of the 7.5 cm L/48 guns, Adolf Hilter was adamant that the StuG III’s firepower had to be further improved. After 1941, Hitler increasingly interfered in major military decisions, including vehicle designs. He was obsessed with installing the strongest possible armaments and armor to new vehicles, which often led to overly ambitious and too often unrealistic projects. Rearming the StuG III was one such project that was rather unrealistic. With the development of the Panther tank project, a new gun, the 7.5 cm L/70, would be made available. Thus, in September 1942, the Waffenamt (Eng. Army Weapon’s Office) discussed the development of the so-called Sturmgeschütze Neue Art’, Stu.Gesch.n.A. (Eng. Assault Gun New Type).

The Germans essentially had a few different options. They could either continue the development based on the existing Panzer III chassis or create a completely new vehicle. Both of these approaches had advantages and problems. One of the earlier works was an attempt to mount the L/70 gun on the chassis of a modified VK9.03, essentially a light tank intended to replace the Panzer II series. Given that it was based on a light chassis, it would have serious trouble supporting the weight of the new weapon and superstructure. Thus, the project was doomed from the start. It did not help either that the development of the VK9.03 was stopped. The work on this assault gun version was obviously canceled and did not go beyond some basic drawings.

A simpler and cheaper solution would be to reuse the already existing Panzer III chassis. While not perfect, it was already in production, so it would speed up the development time and reduce the overall costs.

According to Hitler, the new StuG III had to have improved overall drive performance by increasing the ground clearance from 39 to 50 cm and ground pressure from 1.04 to 0.7 kg/cm². The frontal armor protection was to be increased to 100 mm and the sides to 40 to 50 mm. To further increase the vehicle’s survivability, it was to use angled armor plates. The overall weight was expected to rise to 26 tonnes. In general, the Germans tried to avoid reducing the speed of the armored vehicles, and that was one of the reasons why they implemented lighter armor protection. For the new StuG III project, however, Hitler was completely fine with reducing the speed from 40 to 25 km/h. Lastly, the main armament was to consist of the 7.5 cm L/70 gun.

Alkett, which was responsible for the StuG III’s production, responded by delivering a wooden mock-up in 1943. The new gun could not be installed in a standard StuG III’s superstructure given its sheer size. Alkett’s engineers instead developed a completely new fully enclosed superstructure that had a simpler design and would have had better structural integrity.

Name

The whole attempt to improve the assault gun design was referred to as Sturmgeschütze Neue Art. This did not necessarily refer to a single project. Unfortunately, this particular project was short-lived and there is no specific name for it in the sources. Given the German praxis in meaning their vehicles, this would likely have been designated as 7.5 cm StuK 42 L/70 (sometimes referred to as PaK 42) auf StuG III. It is also possible that, if this vehicle had entered production, the Germans would simply have used the Ausf.H designation, following the previous Ausf.G. For the sake of simplicity, this article will refer to it as the StuG III L/70.

Design

Chassis

The StuG III L/70 chassis would remain the same as that used on the previous versions. The front part of the hull served as a housing point for the transmission, followed by the crew compartment, and the engine. The front hull, where the transmission and steering systems were located, was fully enclosed, protecting the vital components housed within. The positions of the two transmission hatches would likely have remained unchanged. It provided easy access for repairs but also acted as an auxiliary entry point to the driver.

Suspension

Given that it was a further development of the StuG III series, the suspension would have remained the same. It consisted of six small road wheels, three return rollers, the front drive wheel, and the rear-positioned idler. It is possible that some parts would have been additionally strengthened to cope with the extra weight.

Engine

The sources do not specify the precise engine that would have been used for this proposed version. It can be assumed that it would have remained the same. This assumption is based on the fact that the Germans would want to press this vehicle into production as soon as possible. Leaving most of the available components unchanged was one way to ensure this.

If this was the case, then it would have been powered by the twelve-cylinder, water-cooled Maybach HL 120 TRM engine providing 265 hp @ 2,600 rpm. With this engine, a regular StuG III was able to achieve a maximum speed of 40 km/h and a range of 160 km (on good roads). Given the added weight of the gun and the superstructure, the overall drive performance was surely to decrease but to what extent, it is impossible to tell with certainty.

Superstructure

The StuG III L/70 received a completely new pyramidal-shaped superstructure. It consisted of four highly angled armor plates. Somewhat surprisingly, the wooden mock-up showed that the side plates were connected using what appears to be bolts. It is unclear if this feature would have been carried over to the production vehicles or if it was simply put to hold the side wooden panel. The latter option seems more plausible. The use of bolts may have been a simpler option for construction but it came with several disadvantages. Using bolts meant that the structural integrity of the superstructure was much weaker in comparison to a welded one. A particular example of this were the Italian tanks that were built using a metal frame on which bolted plates were placed. This approach proved ineffective during the war.

The wooden mock-up generally lacked many smaller but important details, such as the driver vision port and hatches. It can be said with certainty that a driver’s vision port would have been positioned on the left side of the vehicle. On top of the superstructure, it can be assumed that at least two hatches would have been installed, one for the commander on the left side and one on the opposite side, for the loader. This configuration was employed on the StuG III and on later anti-tank vehicles, such as the Jagdpanzer IV and the smaller Jagdpanzer 38(t). Another feature missing was a commander’s cupola. Lastly, it is unclear from the available photographs if the new superstructure fully enclosed the engine compartment.

Armor Protection

The armor protection of this vehicle is unspecified in the sources. The StuG III Ausf.G’s hull armor was an 80 mm thick armor plate at the front. The sides and rear were weaker, at 30 mm and 50 mm. The front armor plate of the angled superstructure was to be 100 mm thick, according to Hitler’s orders. How realistic this was is another matter. With this armor, the new StuG would have been almost immune to the enemy from the front.

Armament

The wooden mock-up was shown being armed with the 7.5 cm L/70 gun. An immediately noticeable feature is that the gun assembly was bolted down to the front superstructure. This indicates that the gun cradle mount had been fixed to the front superstructure plate. On a standard StuG III, the gun cradle was fixed to the bottom of the vehicle. The former installation was implemented on vehicles such as the Jagdpanzer IV and 38(t)’s. This way, the gun could be easily removed for repairs and the vehicle’s overall height could be reduced.

The L/70 gun offered greater penetration power than the L/48, as presented in the following table (Source. T. Anderson Sturmartillerie Spearhead Of the Infantry).

Range (m)	100	500	1,000	1,500	2,000
7.5 cm L/48	99 mm	91 mm	82 mm	72 mm	63 mm
7.5 cm L/70	138 mm	124 mm	101 mm	99 mm	88 mm

Given these numbers, it makes sense that the Germans wanted to install the longer gun. The problems would have been the increased weight, longer recoil, and larger ammunition which had to be taken into account when designing the vehicle.

The gun was protected by the round-shaped gun mantlet. The mock-up gun used a muzzle brake. This feature was present on all StuG IIIs excluding the early version and the latter one armed with the 10.5 cm howitzer. Interestingly, the later built Jagdpanzer IVs also initially had a muzzle brake. The crews that operated these vehicles quickly noticed that, during firing, the muzzle brake would create extensive dust clouds in front of the vehicle due to the Jagdpanzer IV’s small height. This reduced visibility, but, more importantly, also gave away the vehicle’s position to the enemy. As a result, crews began removing the muzzle brakes from their vehicles. It is possible that the StuG III L/70 would at some point have had the muzzle brake removed, either at the factories or at the frontline, if the recoil mechanisms were able to take that. Besides that, the overall characteristics of this gun on the StuG III L/70 are unknown.

Given the longer length of the gun barrel, an external travel lock had to be provided. Its purpose was to help stabilize the gun during traveling. This, in turn, would help avoid damaging or misaligning the gun sight. In addition, this was necessary to avoid accidentally hitting the ground when driving on uneven terrain. While this seems unlikely to happen, the StuG III’s lower height and longer barrel meant that this was a real possibility. Even some of the standard StuG IIIs received front-mounted travel locks during the later stages of the war.

Crew

This vehicle was likely to retain the four-crew configuration, consisting of the commander, the gunner, the loader/radio operator, and the driver. The driver’s position was on the front left side, with his overall awareness of the surroundings being limited. Due to the position of the gun, the driver had a huge blind spot to the right. Just behind him was the gunner’s position, and behind him the commander. Lastly, the loader was probably seated on the opposite side of the vehicle.

Fate

After a wooden mock-up was constructed, the work on installing the 7.5 cm L/70 on a Panzer III chassis was stopped. This project encountered several obstacles that ultimately led to its cancellation. For a start, the Sturmgeschütze Neue Art project included too many different proposals, without arriving at a final design. In January 1943, Albert Speer informed Hitler that this problem had to be resolved shortly, as constant changes in the overall design would ultimately lead to a huge delay in its eventual production.

After examining Alkett’s proposal, it quickly became apparent that this chassis was unsuited for the given task. Considering that the Germans barely squeezed the L/43 and 48 guns into the StuG III, an even larger superstructure would have been required to operate this gun effectively. Thus, the StuG III L/70 gun project met a rather abrupt end.

However, this was not the end of the attempts to develop a new StuG that was better armed. Parallel to Alkett’s project, another company named Vomag proposed that the new StuG be based on the larger Panzer IV chassis instead. This proposal was presented to Adolf Hitler on 2nd October 1942. Hitler was impressed by what he saw and gave the project the go-ahead. To avoid further delays in the Sturmgeschütze Neue Art project, Hitler ordered that this vehicle was to be developed on the Panzer IV’s chassis in February 1943. This would eventually lead to the creation of the Jagdpanzer IV and its better-armed version, known as the Panzer IV/70 (V). The term Jagdpanzer (Eng. Tank hunter) in connection to the assault gun projects may be confusing at first. While the overall story is long, essentially what happened is that, at some point, the new Sturmgeschütze Neue Art project was hijacked by Heinz Guderian. While initially intended to perform the role of infantry support weapon, operated by the Artillery branch, it was instead allocated to the Panzer branch of the Army and allocated to anti-tank units. The proof that this vehicle shared the assault gun heritage can be seen in the fact that, throughout its history, it was designated as Stu.Gesch.n.A. auf Pz.IV (Eng. New Type Assault Gun on the Panzer IV Chassis) in November 1943 and Sturmgeschütze Neue Art mit 7.5 cm Pak 39 L/48 auf Fgst.Pz.Kpfw during the period of February to October 1944.

Given the lack of L/70 guns, it was initially armed with the shorter L/48 guns. When the production of the L/70 was increased and it became available, the Jagdpanzer IV was rearmed with it, changing its name to Panzer IV/70 (V).

Ironically, such vehicles finally reached assault gun units in the final month of the war. Three years after the Sturmgeschütze Neue Art project was initiated, it finally reached the units for which it was intended for, albeit in far too few numbers.

Potential Influence on the Jagdpanzer38(t)

The Alkett wooden mock-up superstructure design shares many similarities with the later-built Jagdpanzer 38(t) that was introduced in 1944. It is often said that the general shape of the Jagdpanzer 38(t) was inspired by the Romanian Mareșal tank hunter project. According to Romanian records, two German officers, Lieutenant-Colonel Ventz and Lieutenant-Colonel Haymann, inspected the work on the Mareșal tank hunter project in April 1944. Their report allegedly influenced the overall shape of the Jagdpanzer 38(t). However, Alkett’s wooden mock-up’s superstructure shares many similarities with the Jagdpanzer 38(t)’s. The installation of the gun and the overall shape of the superstructure are also visually the same. The wooden mock-up lacked many vital features. Of course, given the lack of any written sources on this topic, it can only be speculated if this was true, or just a quite lucky coincidence.

Conclusion

This project was used to test if the installation of a long 7.5 cm L/70 gun on a modified StuG III was possible. Theoretically, this would offer several advantages, such as reducing development time and overall cost. However, it became apparent that such an installation on the existing Panzer III chassis was impractical, and the project was canceled in favor of the Panzer IV’s chassis.

Finally, it must not be forgotten that, while the StuG III possessed many characteristics of an efficient tank killer, such as a low silhouette and good armor, it was not designed for this role, as it was primarily an infantry support weapon. Even the installation of the longer L/43 and 48 guns did not change its primary role. In essence, this meant that, even if the Germans managed to somehow fit the L/70 gun into a StuG III, its role to support the infantry would probably have remained the same, but the extra firepower would surely have helped deal with enemy tanks better. This is, of course, considering that the Panzer branch of the Heer would not have stolen this project from the artillery, as it did multiple times during the war.

7.5 cm StuK 42 L/70 auf StuG III Specifications
Crew	4 (driver, gunner, loader, and commander)
Weight	26 tonnes
Engine	Maybach HL 120 TRM
Speed	25 km/h
Armament	One 7.52 cm StuK 42 L/70
Armor	up to 100 mm

Sources

Walter J. Spielberger (1993) Sturmgeschütz and its Variants, Schiffer Publishing Ltd.
T. L. Jentz and H. L. Doyle Panzer Tracts No.20-1 Paper Panzers
Military Vehicle Prints 30 (1976) Panzerjager IV, Bellona prints
T. J. Gander (2004) Tanks in detail, Jagdapazner, Ian and Allan Publication
D. Doyle (2005). German military Vehicles, Krause Publications.
T.L. Jentz and H.L. Doyle (1999) Panzer Tracts No.8 Sturmgeschütz
T. Anderson (2016) Sturmartillerie Spierhead Of the Infantry, Osprey Publishing