In 1942, development of an armoured vehicle for use by the Royal Engineers (RE) began. This was the famous Churchill AVRE (Armoured Vehicle Royal Engineers), which was armed with a 290mm Spigot mortar. This mortar, known as the ‘Petard’ (a 16th-century word of French origin describing ‘a bomb to breach’) was capable of firing a huge, 28lb (12.7kg) projectile nicknamed the ‘Flying Dustbin’. The weapon was designed as a demolition tool that would breach defenses and crack open enemy bunkers, a role which it performed extremely well. However, there were a couple of quite dangerous problems with the operation of the Petard.
Reloading the mortar was a hazardous endeavor, as the mortar had to be reloaded externally. Not ideal in combat situations. To begin loading, the turret would be traversed so the Petard was over the bow gunner’s position. This man would then slide open his hatch (which replaced the two-part hatch on standard Churchills) and reach up to the barrel of the Petard. Like a giant shotgun, the barrel would be broken in half, and a fresh round inserted.
Range was another issue. At maximum, the Petard could only throw one of these ‘Flying Dustbins’ 100 yards (91 meters). This wasn’t ideal, as the tank would have to get extremely close to a target to fire. More often than not, AVREs would advance under the cover of regular gun-armed tanks to engage any enemy posing a threat to the AVRE.
The British Military began looking for a solution to these issues. Interest grew in a new mortar being designed and developed by Imperial Chemical Industries Limited at their factory at Ardeer, North Ayrshire in Scotland. This new weapon would be tested on the hull of a Mk. III Churchill, and would prove to be a powerful weapon, perhaps a little bit too powerful…
The prototype vehicle with the new giant mortar. Photo: The Tank Museum
The Churchill Mk. III
Officially designated as ‘Tank, Infantry, Mk.IV, A22’, the Churchill entered service with the British Armoured forces in 1941. It was named, contrary to popular belief, after an ancestor of the famous Winston Churchill, not the man himself. It was the last ‘Infantry Tank’ to serve in the British Military.
The specific model used in the tests was the Mk. III Churchill, which was produced from late 1942. It had armor of up to 102mm thick over the frontal arc. The turret was a welded type and mounted the tank’s usual main armament, the Ordnance Quick-Firing 6-Pounder (57mm) Gun.
Secondary armament consisted of a coaxial and a bow mounted 7.92mm BESA machine gun. The tank was crewed by 5 men. These were the commander, gunner, loader, driver, and bow machine-gunner/wireless operator.
The Churchill was not fast. A lumbering beast at approximately 40-tons, its top speed was only 15 mph (24 km/h). It was powered by a Bedford 12-cylinder engine producing 350 hp. The tank was supported on a complicated suspension with 11 small wheels per side, each one attached to an independent coil spring. The drive wheel was at the rear with a sprocketed idler at the front. Though it was slow and heavy, the Churchill made a name for itself as being one of the best cross-country tanks ever built and could climb higher gradients or cross harder obstacles than most other tanks then in service.
The weapon that garnered so much interest was a large mortar originally designed to be placed on a towed mount. There was even a prototype of the gun tested on the mount of a towed 6-Pounder anti-tank gun. This mortar had a 300mm (11.8 in) bore, 10mm larger than the Petard of the standard AVRE which had a 290 mm (11.4 in) bore. It was ten calibers (L/10, 3 meters) long, and fired a 20kg (44lb) High-Explosive filled projectile, almost twice as much as the 12.7 kg (28lb) ‘Flying Dustbin’ fired by the Petard. Range was also drastically increased from the Petard’s 100 yards (91 meters). This new mortar could lob a round to an effective range of 410m. (450 yards).
The trial mortar on the towed 6-pounder mount. Photo: Ed Francis Personal Collection
The gun was an early endeavor into the idea of ‘Recoilless’ guns. This type of gun operates on the principles of Isaac Newton’s Third Law of Motion; “For every action, there is an equal and opposite reaction”. These guns are not truly ‘Recoilless’, but they have drastically reduced recoil compared to a regular gun. When they are fired, another charge is fired backward from the rear of the gun, canceling out the recoil effect of the projectile leaving the barrel.
The gun required a huge gunpowder load which was packed in behind the projectile. Firing produced clouds of acrid smoke and fumes. This is where the “Aggie” received its name. The mortar was named after a local bus that ran people around Ayrshire (where the gun was made), which was famous for producing great clouds of smoke as it traveled. For loading, the gun broke in half with the rear portion sliding backwards (it would protrude from the rear of the tank). The projectile and gunpowder load would be placed in the fixed front half. The two haves were then reunited and locked in place prior to firing.
In 1943, a prototype combining the gun with a Churchill Mk. III was constructed. The Churchill Mk. III’s turret was drastically modified to accept this new large gun. The standard main armament of the 6-Pounder anti-tank gun was removed, a slot carved out of the turret face along with a small section of the turret roof. Inside, the gun ran the length of the entire turret with the blast-vent protruding through the rear of the turret. This could be covered by a sliding panel. A rudimentary mantlet was welded to the turret face around the gun barrel, bent at the top to cover the part cut out of the turret roof. A small hole was made in this for the gun sight.
Side view of the modified Churchill. Photo: Scalemates
Conditions inside the turret would have been harsh, with the 300 mm Mortar taking up most of the room from the back to the front. It did incorporate internal loading, however, one of the issues with the AVRE that need to be changed.
Luckily for the crew in the turret, most of the smoke and fumes were ejected out of the barrel and blast-vent at the rear. The mortar, when fired, still produced horrendous recoil though, jarring the whole tank. A simple ballast was tested. This is believed to be a heavy sandbag that was placed at the blast vent and would be propelled off the tank to dampen the recoil when fired. This somewhat helped, but, as one, can imagine, this was not a popular solution with the crew, as a man would have to exit the tank to replace it. This would somewhat undo the work of trying to keep everyone inside the tank when reloading. The sandbag was later replaced by a purpose-built ballast block, but replacing it would be done via the same method.
This scale model provides us with an excellent view of the blast vent at the back of the turret. Photo: SOURCE
The turret retained the ability to rotate through a 360-Degree arc, but elevation or depression was extremely limited. Looking at photos, it is hard to say if it had any range of motion at all as it had to stay in line with the blast vent in the rear of the turret. Unfortunately, we don’t have any documents to give such detail.
The crew remained the same as regular Churchills with 5 personnel. There were three men in the turret and two in the hull. Positions were also the same with the commander at the rear right, loader on the left and gunner on the right. In the hull were the driver on the right and the bow machine gunner on the left. It is possible that the bow gunner position may have been removed to allow stowage of ammunition.
In the end, the project was rejected, most likely due to cramped conditions in the turret and the weapon being generally hazardous to operate. The turret became very cramped, not only did the mortar take up at least 50% of the space inside, but it also had to carry projectiles, charges, and the counterweights.
Firing the mortar was dangerous. Even with the counterweight at the back the amount of recoil and concussive forces generated would have been extremely unpleasant for the crew. It also made a deafening sound and became very hot after firing.
It was also dangerous for any personnel outside of the tank. Especially if there were infantry behind the tank as the back-blast and propelled counter-weight could easily end up in fatal injuries. Attempts were made to assuage this issue by the installation of a blast shield, but this was unsuccessful.
With the rejection of the project, work on it ceased. Though too late for service in World War Two, the military would eventually find a replacement for the Petard in the Ordnance BL 6.5″ Mk. I Demolition Gun. The gun fired a 64 lb (29 kg. It also contained a 40lb charge of C-4) High Explosive Squash Head (HESH) shell at up to 2,400 m (2,600 yd). This was a vast improvement over both the Petard and the ‘Aggie’.
|Dimensions||24ft 5in x 10ft 8in x 8ft 2in
(7.44 m x 3.25 m x 2.49 m)
|Total weight||Aprox. 40 tonnes|
|Crew||5 (driver, bow-gunner, gunner, commander, loader)|
|Propulsion||350 hp Bedford horizontally opposed twin-six petrol engine|
|Speed (road)||15 mph (24 km/h)|
|Armament||‘Ardeer Aggie’ 300mm Mortar
1 x 7.92mm (0.3 in) BESA machine gun
|Armor||Up to 102mm|
Links & Resources
Haynes Owners Workshop Manuals, Churchill Tank 1941-56 (all models). An insight into the history, development, production, and role of the British Army tank of the Second World War.
Osprey Publishing, New Vanguard #7 Churchill Infantry Tank 1941-51
Article on the vehicle (Russian)
The Tank Museum, Bovington
Royal Engineers Museum, Kent
The modified Churchill Mk. III with the ‘Ardeer Aggie’ mortar. Illustration by Tank Encyclopedia’s own AmazingAce, based on work by David Bocquelet.