The SdKfz 166 "Sturmpanzer IV", better remembered as the "Brummbar", was an early German World War 2 (1939-1945) heavily armored and armed tracked fighting vehicle built upon the chassis of the Panzerkampfwagen IV (Panzer 4) Medium Tank series. The main gun was a 15cm (150mm) field howitzer and the vehicle's primary mission was in support of ground troops, mainly in urban areas where its armament could be used to devastating effect against structures. The Sturmpanzer IV was not a "true" tank as it lacked traversing turret - instead, its powerful howitzer armament was fitted within a fixed superstructure giving the combat vehicle the proper classification of "assault gun" (additionally, the weapon only fired High-Explosive (HE) projectiles which were generally unsuitable against armored vehicles).
Such modifications of outgoing tank lines like the Panzer 4 series proved common for the German Army throughout the war, particularly by its end, and proved cost effective measures to help fulfill battlefield needs and attempt to shift the balance of a fight.
The desire for a new self-propelled assault gun - in part driven by Adolf Hitler himself - was determined in early 1942 with Germany now fully committed to war along several major fronts. The existing self-propelled assault guns then in service were doing their part but lacked armor to protect both crew and critical components. An opportunity grew with the arrival of the new "Panther" Medium and "Tiger I" Heavy tanks beginning to grow in number, forcing the outmoded Panzer III and Panzer IV series medium tanks to secondary roles or modifications to continue life in other needed roles.
Details of the new assault gun were ironed out in late 1942 with the Alkett concern as the primary developer and, following a short period of evaluation, first deliveries of the "Sturmpanzer IV" commenced in May of 1943. However, wartime shortages and resource commitments elsewhere forced quantitative production to lag for several months before eventually picking up speed in November of that year. The Sturmpanzer IV originally called for the use of rebuilt ex-Panzer IV tanks but even new-build chassis were eventually optimized to keep up with demand. Contrary to common German wartime production practices of such modified vehicles, Sturmpanzer IVs were in fact produced at German Army facilities (first in Vienna, then Duisberg after June 1944) and not in any specialized company factory facilities.
The Sturmpanzer IV was quickly fielded as more Panzer IV hulls became available and were modified for the role. These early forms required four operating personnel but they lacked the any sort of self-protection in the form of 7.92mm defensive machine guns found on later production models. The short-barreled 15cm (150mm) StuH 43 L/12 series guns proved heavy on recoil for what the superstructure was designed to manage and the sheer weight of such a system also proved to be taxing on the selected engine installation which hampered reliability and operational ranges.
Armor on the superstructure was sloped on all sides including the roof with front plate armor completed at 100mm (2.54in) thick. Side armor along earlier models managed only 30mm (1.18in) of armor, making them somewhat vulnerable to Allied anti-tank weapons, particularly at close ranges in which the Sturmpanzer IV was intended to fight. The stout 15cm (150mm) heavy gun was mounted in a ball-type socket and measured only 12 calibers long. Power was supplied from a single Maybach HL 120 TRM series 12-cylinder engine of 300 horsepower. Later models introduced the Maybach HL 120 TRM 112 series 12-cylinder engine, again, of 300 horsepower. Range was out to only 130 miles so her strategic usefulness was somewhat limited.
The Sturmpanzer IV crew enjoyed the inherent protection from the superstructure's design though the fighting compartment was relatively spacious for the required crew of four (later increased to five). The commander was seated to the rear of the superstructure directly behind the gun mount and used a periscope mounted on the rooftop to mark targets for the gunner and direct field fire as needed. A pistol port at the rear sides of the superstructure (added during initial production) allowed the crew to fire on enemy infantry from within the vehicle. One or two operators served to handle the ammunition while a forth crewmember fired the gun. The fifth served as the driver whose position was at the left front of the vehicle. There were 38 x High-Explosive (HE) 43.5kg projectiles stored in the crew area with 600 x rounds of 7.92mm ammunition for self-defense machine gun(s) (if equipped). Machine guns could counter both infantry and low-flying aircraft but the Sturmpanzer IV crew generally relied upon supporting infantry squads to stave off enemy anti-tank attacks against the vehicle as Sturmpanzer IV actions were usually slow, plodding and operated through calculated street-by-street approaches.
Sturmpanzer IV missions were generally in support of infantry units, against enemy bunker fortifications or in the elimination of small- to medium-sized artillery concentrations. First use of these vehicles came along the East Front versus the Red Army at Kursk, Kharkov and Dnepstroy as part of the Sturmpanzerabteilung 216. Despite the 50 fielded in the ensuing actions, many were lost to enemy fire as the Red Army picked up steam and began to put the German Army on their heels in many areas.
Attacking the stout Sturmpanzer IVs usually involved tank-killing infantry squads utilizing "magnetic" mines. These mines could be affixed to the metal sides of the vehicle and set to explode, at least knocking the vehicle out of commission for lengthy periods or at most destroying them outright. To counter these mine attacks, the Germans coated armored vehicles with "Zimmerit" paste designed to resist the contact of such magnetic mines. Additional protection was also added through side armor "skirts" (known as "Schurzen") to help protect the more vulnerable sides of the lower hull and track systems from enemy fire.
By late 1943, the Sturmpanzer IV design was modified, particularly along her superstructure, to help reduce overall operating weight. A lighter form of the 150mm main gun - the StuH 43/1 L/12 - was also fitted. The driver position was addressed with periscopic vision blocks over the original Tiger I-style vision port. These new redesigned vehicles went into production in December of 1943 and eventually sent to war. Another series redesign occurred the following year as operation action dictated. It was about this time that the aforementioned Zimmerit anti-magnetic paste was being used and a new front-facing, Kugelblende 80 ball-mounted 7.92mm MG34 machine gun was fitted in the front superstructure panel for a self-defense measure. The key design feature of these new Sturmpanzer IVs was its commander's cupola atop a redesigned superstructure emplacement that shortened the overall height of this inherently tall vehicle. Sturmpanzer IVs fought through to the end of the war but losses mounted. One of the last recorded actions involving these vehicles was near Budapest, Hungary of the Sturmpanzerabteilung 219 as part of the 23rd Panzer Division. Like other fighting groups before them, these Sturmpanzer IVs were useful for their intended role but fared no better in combat. Regardless, German authorities (including Hitler himself) were still impressed with such weapons simply because they provided devastating fire support to fulfill a variety of mission requirements - often softening target areas ahead of a main thrust.
Troops called the Sturmpanzer IV the "Sturmpanzer" or "Stupa" for short and the name "Brummbar", meaning "Grizzly Bear", became tied to the machine in the post-war years (the name never formally used by the German Army itself). One true negative of the design was her limited off road capabilities due to her operating weight (approximately 28 tons), making her extremely heavy and slow (with a top listed speed of just 25 miles per hour in ideal conditions) and giving the vehicle a terrible tendency to get bogged down on wet terrain. Unfortunately for the German Army, such terrain was in large supply on the East Front. Steel-tired road wheels also replaced later rubber-tired types to help offset the Sturmpanzer IV's excessive weight on stable ground but only had a limited effect on operation. She was a tall vehicle that proved difficult to conceal against the terrain and limited space within the fighting compartment meant a limited ammunition supply.
At least 313 Brummbar examples were produced in all including a command vehicle version fitting additional communications equipment and identified by their antenna installations. These were designated in German Army nomenclature as the "Befehlsturmpaner IV". The British and Americans both evaluated captured Brummbar's in the years following the end of the war.