Despite defense spending getting heavily cut after WWI, work to perfect existing vehicle didn't stop. One of the directions which British engineers developed was mechanization of artillery. The war showed that the army needs tracked tractors capable of quickly transporting artillery on the march and on the battlefield. Wheeled tractors were unsuitable for the harsh terrain of a WWI battlefield. Tracked tractors like the Holt 75 were also suboptimal due to their low speeds.
Two relatively new players faced off on this arena. Vickers began work on the 18-pdr Transporter, and Sir W.G. Armstrong Whitworth & Co. Ltd. designed a vehicle called Dragon (from "drag gun"). The concept of the vehicles was similar. The gun rolled into the transporter during the march, and the crew sat inside, facing each other. Both vehicles were ready in 1922. Further work on the 18-pdr Transporter led to the creation of the Light Tank Mk.I, which later evolved into the Medium Tank Mk.I.
The Light Artillery Transporter, the first British SPG created after WWI.
As for the Dragon, its further development took a different path. The designers rejected the idea of rolling the gun into the vehicle, and a long evolution began. After the unification of Vickers and Armstrong Whitworth into Vickers-Armstrong, the vehicle was mass produced, exported, and even saw battle. Dragon family vehicles took many unusual forms, one of which turned out to be an SPG.
The ammunition racks are open from all sides, hinting at a brief lifespan on the battlefield.
In 1924, Armstrong Whitworth developed the Light Artillery Transporter using elements from the Dragon Mk.II. This was a fully fledged SPG without even a hint of being able to remove the gun. The modernized 76.2 mm QF 13-pdr 9 CWT AA gun was used as the armament. Judging by the placement of the gun, it was not going to be used in an AA role. The gun would fire on ground targets directly. The gun was installed in the front part of the hull, with ammunition containers behind it and then a 48 hp AEC engine.
The Birch Gun in its original configuration.
The Light Artillery Transporter was a new kind of SPG: small, cheap, and much more mobile than the same AA gun on a towed chassis. However, those were the only advantages. The design didn't even have a hint of armour, including a shield for the crew. The idea of using this SPG for direct fire was madness, since any infantryman with a rifle or even a tribesman with a spear could be a deadly enemy for the crew. It was hard not to hit the negligibly vulnerable ammunition racks, and the result of such a hit is not hard to imagine. The Light Artillery Transporter was rejected. Nevertheless, the novelty was interesting, and its story continues, albeit in a different company.
In the Name of the Ordnance Master
In 1923, Sir James Frederick Noel Birch was appointed to the office of Master General of the Ordnance. A former cavalryman, he was closely involved with issues of artillery during WWI and had hands on experience with problems of mechanization. It's no wonder that Birch's appearance coincided with a blossoming of self propelled artillery.
The SPG turned out to be noticeably lower than the tank whose chassis it used.
It is unknown what Birch had to do with the Light Artillery Transporter, but another SPG bore his name. Work on it began in 1923, just as Birch took his post, and Vickers received an order to develop a tracked SPG back in 1921. It is often said that it used the hull of the Medium Tank Mk.II, but this is only partially true. In 1923, then work on the Birch gun began, the Medium Tank Mk.II that allegedly served as the chassis was still in development. The SPG was also built in 1924, before the "base" Medium Tank Mk.II.
Some doubts about whether or not the Medium Tank Mk.II was used also arise when looking at the chassis. It is similar, but there are some notable differences. For example, the Medium Tank Mk.II had its suspension covered with armour skirts. Meanwhile, the experimental Birch Gun entered trials in 1925 without any skirts. The number of road wheels was increased to 13 per side, with 5 return rollers. The carriers for the drive sprocket and idler were also different. The Birch Gun received tracks made from the No.9 Link Track.
Birch Gun Mk.I in travel mode.
Conceptually, the SPG developed at ROF Woolwich differed considerably from the Light Artillery Transporter. While the Armstrong Whitworth SPG was designed to fire directly, the Birch Gun was a universal SPG. A new version of the 18-pdr (83.8 mm) field gun was created for it. Unlike the regular 18-pounder, the Birch Gun could shoot at airborne or ground targets. Effectively, the only part from the gun was the barrel, the other components were radically redesigned. The design resembled the universal 18-pdr Mk.3 which was developed during WWI but never entered production. According to video footage, the universal gun mount had good maneuverability of fire.
Same vehicle from the rear.
The hull and suspension had noticeable differences when compared to the Medium Tank Mk.II. Since one of the requirements was 360 degree horizontal traverse, the height of the hull had to be reduced by lowering the engine compartment. At the same time, the driver lost his cabin and only a movable shield remained. Due to a lack of turret, the fighting compartment height was also reduced. The gun was positioned in the center of the vehicle, so the bore axis was even lower than that of the tank. The gun was mounted to a much larger turret ring, and the hull had to be enlarged at the top. The crew of the Birch Gun consisted of six men. Despite the limited room, they were positioned comfortably.
More Armour isn't Better
The experimental Birch Gun was sent for trials to the 28th Field Battery of the IX Field Brigade of the Royal Artillery. A list of improvements was made as a result of the trials. The artillerymen mostly did not like how the vehicle was devoid of armour. They did not ask for complete enclosure, and a gun shield would have been enough. The use of skirt armour to protect the suspension elements like on the Medium Tank Mk.II was also suggested. It was strange to request armour for the suspension considering the purely symbolic protection for the crew, but the designers took it into consideration.
The Birch Gun could act as a SPAAG due to its high gun elevation angle.
Vickers received an order for four 18-pdr SPGs in September of 1925. They were ready in about a year. Each vehicle cost 12,250 Pounds Sterling, or about 1000 Pounds Sterling per ton. The 12 ton Birch gun had 5 mm of armour. The layout was similar to that of the Medium Tank Mk.II: the engine and transmission used on it were the same.
There were also differences, some of which were described above. Vehicles with WD numbers D.118 through D.191 had a modernized suspension. Aside from better designed skirts, they received the new No.3 Link Track, even earlier than the Medium Tanks Mk.I and Mk.II. Overall, their suspension was better than on the tanks.
As the requirements demanded, the SPG had a large shield that covered the crew from the front. A travel lock was also added. The gun elevation grew to 80 degrees, which improved the Birch Gun's ability as a SPAAG. The ammunition racks held 80 rounds, and, unlike in the Light Artillery Transporter, they were stored safely inside the hull.
Birch Gun battery during exercises in Kimberly. November 1926.
Formally, the letter D in the registration number classified the vehicle as an artillery tractor. However, the result of ROF Woolwich's work was obvious during the first exercises held on November 13th, 1926, to demonstrate new technology to governments of the British Commonwealth. The exercises were designed to demonstrate the full might and perfection of the new mechanization program. In pouring rain, the Birch Guns showed the difference between a towed gun and an SPG. It took seconds to convert the Birch Gun from travel mode to firing mode, while a towed gun of comparable power would take about a minute to do the same.
A more noticeable difference was revealed during maneuvers on hilly off-road terrain. While Burford Kegresse tractors showed difficulty on this terrain, the Birch Gun battery slipped through in an instant, climbing the hill without problems. The maneuverability of the SPGs was similar to that of tanks. The same thing happened in February of 1927 during exercises in Wool. The Birch Gun had no equals among other artillery mechanization projects. Vehicles of the 20th Field Battery of the IX Field Brigade of Royal Artillery reigned supreme once more.
Birch Gun during exercises, Wool, February 1927.
The Birch Gun didn't only receive positive feedback. Despite its designers trying to reduce the height as much as possible, the center of gravity was high for a 12 ton vehicle. Since the 18-pdr gun had much more kickback than a 3-pdr (47 mm) gun of the Medium Tank Mk.II, the tank shook more when firing.
The request to make the SPG universal also backfired. Often, the request to make something universal means it performed at most adequately in any given category, as was the case with the Birch Gun.
The armour was still insufficient to fight ground targets. The 360 degree rotation of the gun meant that the crew was completely unprotected if the gun turned to the side. The driver had it even worse, as he was only shielded by a hatch to the left.
Birch Gun Mk.IE, 1928
Weighing the pros and cons, the military ordered two more vehicles from Vickers, designated Birch Gun Mk.IE. They were built a year later, numbered D.416 and D.417. There are other designations (Mk.I, Mk.II, and Mk.III), but they should be viewed skeptically, as they were contained in the "Tanks" encyclopedia published in Munchen in 1925. Sadly, much of what was written in the book has little to do with reality.
The main difference from the Birch Gun Mk.I was a significant boost to the armament. The driver received a new cabin with a hatch. The gun armour increased notably. Now it was placed in a semi-enclosed cylindrical turret with a massive shield in the front. Since the armour limited the gun's elevation angles, the fighting compartment was redesigned, increasing the ammunition capacity to 88 rounds.
The modernization had many effects. The protection of the crew grew noticeably, but other characteristics worsened. The mass of the vehicle grew by almost a ton, which reduced the maximum speed on a highway from 29 to 25.6 kph, and off-road from 16 to 14 kph. The visibility also worsened. The center of gravity rose, and oscillations from firing increased. The Birch Gun was headed into a dead end.
Victim of Conservatism
On May 1st, 1927, all Birch Guns were transferred to the Experimental Mechanized Force commanded by Colonel John Fuller. After their construction, the Birch Guns Mk.IA were sent here as well. All guns were included in the 20th Battery, which migrated from the IX Field Brigade.
It is often said that the last two Birch Guns differed in design, but this is incorrect.
Despite the drawbacks, which were worsened by the extra armour, the concept of the Birch Gun was a good one. If the British had developed it further, they would have had a family of mobile SPGs which were capable of fighting alongside tanks and providing fire support. The problem was that the British army was dominated by conservatism. A towed gun and a tractor also turned out to be much cheaper than an armoured SPG. The gun was lower, and it was much easier to prepare a position for it. All of these arguments by opponents of mechanization prevailed. The fact that towed and self propelled artillery were made for different problems was of little interest to them.
The Birch Gun started to give in 1927 when Sir Birch left his post and transferred to Vickers. He was replaced by a more conservative Lieutenant General Sir Webb Gillman. His opinion was that the correct way to develop artillery is through tractors, especially the Dragon family. The negative feedback further sapped the Birch Gun's position, and the disbanding of the Experimental Mechanized Force in early 1929 was the end. Soon after, the Birch Gun program was closed.
Fighting compartment of the Birch Gun Mk.IE.
The victory of the conservatives echoed ten years later. It turned out that self propelled artillery plays a significant role on the modern battlefield. In 1941, work on SPGs had to start from scratch, repeating the same mistakes. All this could have been avoided if the self propelled artillery program from the 1920s was retained in even token volumes.
As a result, British SPGs of WWII often had an unconventional layout and weren't as good as foreign SPGs of the same class. The best designs were based on American tanks and SPGs. Such was the cost of "victory" achieved by military conservatives over their own tank designers in the late 1920s.