The Firefly is in the upper tier of the most popular WWII tanks, mostly because it was one of the few Allied vehicles capable of knocking out heavy German tanks with its 17-pounder gun. However, despite its fame, there are still a number of interesting things about the Firefly many do not know.
Such lesser known details include the origin of its name, and its hidden connection to an Australian tank.
The Sherman Firefly, or, to use its other less common names, ‘Woodcock’ and ‘Mayfly’, came about during the Second World War from the desire to mount the 17-pounder anti-tank gun into a tank to increase the Armored Corps’ capability against German tanks.
Contrary to popular belief, it was to be used as a tank and was never intended to fulfil the role of a tank destroyer.
The UK had issued specifications for a 17-pounder armed tank in early 1942. One of the first tested was the A30 Challenger Mk1, using a modified Cromwell hull by the Birmingham Railway Carriage and Wagon Company (BRC&W). It was ready by September of 1942, but due to uncertainty from the war office on which guns to use and some interference from the US, it wasn’t accepted until early 1943.
Meanwhile, the UK had begun to receive Shermans from America. They were well-liked and offered adequate firepower for most of the panzers they would come across, but they would be inadequate against newer German designs such as the Tiger, or the next generations of German tanks. Thus, the idea to mount the 17-pounder onto the Sherman was raised.
Two key men were involved early on in this project. First was Major George Brighty of the Royal Tank Regiment, while he was stationed at Lulworth Armoured Fighting School in early 1943. Major Brighty had not been happy with the early A30 trials, and felt that the 17-pounder would be better mounted on the Sherman’s hull rather than the Cromwell’s.
The second was Lieutenant Colonel George Witheridge, who had retired from active service after he was injured when his M3 Grant was knocked out in North Africa. After spending time at Fort Knox in the US in January 1943, he was sold on the Sherman.
Upon returning to the UK Witheridge met up with Brighty to work on mounting the 17-pounder on the Sherman in June 1943.
However, neither officer could work out a suitable solution for how to mount the 17-pounder inside the Sherman’s turret. Accommodating the gun’s 40 inch recoil was especially puzzling.
The first attempt was to install the gun rigidly and test it on a rail mount with the recoil energy absorbed directly into the turret face, but this was far from desirable as fixed guns like this take a high toll on the vehicle themselves.
The project was nearly derailed when the Ministry of Supply (MOS), who was still in turmoil over what guns various tanks should have and who should use them, felt that such a project was impossible, a sentiment shared by the Americans as well.
The MOS thus asked work on the Firefly to be stopped. However, Witheridge had connections inside the industry; Major General Raymond Briggs and the Director-General of Weapons and Instruments Production at the MOS, Mr. Claude Gibb.
He argued that it was still possible, but the project would require expert advice.
This allowed them to take on expert engineers. One such man was William Kilbourn. Kilbourne had been initially an engineer at Vickers, working on Airships in 1914 under Barnes Wallis. He was moved over to tanks in 1922.
He was sent to the Fighting Vehicles Design Department (FVDD) at Chertsey in 1929, where he worked on amphibious tanks and redesigned the turret for the Valentine into a three-man configuration before being transferred to the Directorate of Tank Design.
In 1942 he became responsible for the design of turrets, sights, and gun mountings.
In 1943 he received a phone call from the MOS, who at that point were despairing. They called him to say that they didn’t think the 17-pounder would ever fit the Sherman turret, and that the US had said such a notion was unrealistic.
So, that night, he sat down with the plans and the following morning phoned the MOS back to say it could be done, and that he would need a dedicated team.
Kilbourne was able to modify the 17 pounders 40-inch recoil by redesigning the recoil system using shorter recuperators on either side, allowing the ergonomics of the Sherman to be used to the best effect.
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Kilbourne was also credited with altering the gun cradle and the shape of the gun itself as well as the choice to remove the radio into a new bustle. This resulted in what was effectively a new gun, which had to be custom-built specially for the Firefly. This took place in October 1943, and final testing was completed in February 1944.
Now all of this is fairly well known to those with more than a smattering of knowledge in British tank design. While this is very grand and British, it’s also missing some vital information: Australia’s connection to the Firefly’s development.
The Sentinel Link
On the other side of the world, the Australians had their own problems. They had seen Britain’s defeat in France and desired their own armored vehicles, yet knew that supplies from the British following Dunkirk were unlikely. Japanese aggression was becoming a real threat, and it was looking extremely unlikely that they would receive any essential supplies from their allies.
So with the determination so characteristic of their nation, they resolved to design their own tanks and vehicles to face what seemed to be an inevitable assault by Japan, and any German tanks encountered abroad.
Although they had no previous experience in building tanks, they were able to acquire some British and American samples early on. They had also sent an engineer over to Britain in 1940: Colonel William Watson.
Watson himself was a British officer who had been transferred to the Australians as a technical advisor in October 1940. He had also been involved in the design of the Covenanter previously.
A second engineer, Alan Chamberlain, was sent to the US to study their tanks and gather what information he could.
The Australians had one redeeming factor for building tanks: a reasonably decent railway industry. Because of this, they had suitable foundries that produced high-quality steel, something that would be essential in any tank building enterprise.
When the engineers returned they set to designing a new cruiser-style tank. Looking like a hybrid of the M3 Lee and Cruiser tanks, she clearly had influences from both the US and Britain in her.
She was not thin-skinned like the cruisers, nor slow and sluggish like the infantry tanks, having a good all-round balance of features.
They decided to use a modified version of the M3’s suspension, and a very respectable 50 to 65 mm of well-angled cast steel armor. Its main armament was a 40mm or 3-inch CS gun, which for its time was very good.
The AC1 Sentinel, as it came to be known, had a four-man crew, and although the engine proved problematic, they were able to fit three Cadillac V8 car engines into a clover formation, which produced 330 horsepower for a top speed of 30mph.
The whole process took 22 months from start to finish, and the tank was by all accounts a good vehicle. But where the Australians led the field was in foresight: they realized that while the gun itself was more than adequate from what they knew about German tanks at the time, this could quickly change.
They designed it with a huge turret ring from the outset, so they could easily update the vehicle to counter any new threats.
The AC1 was ready in 1942, but they immediately began working on up-gunning and improving the vehicle, notably in regards to its firepower.
At the forefront of this effort was Colonel Watson, who wanted to mount a 17-pounder gun onto the tank. However at the time the British could not spare any for testing, as all available guns were being sent to North Africa where they were effectively slapping Rommel’s panzers back into the shadowlands.
Thankfully, Australia had the data and plans for the gun and knew its behaviour, particularly its fierce recoil force and ungainly dimensions.
The Australians had been working on putting a 25-pounder gun into the AC1, which would be known as the AC 1B. The first version tested had the recuperators below the gun as standard, but this would inevitably affect its gun handling, and so the recuperators were placed above the gun and built into the mantlet.
The testing of the 25-pounder was completed by July of 1942. But, as mentioned, the Australians were aware of the rapid increase in the development of Germans tanks and wished to have a better anti-armor weapon.
There were proposals to mount the 6-pounder, an effective anti-tank weapon, but it was felt that by the time they did this the Germans would have up-armored again. Thus it made more sense to leap straight to the 17-pounder, which at the time was the best Allied gun available. The 25-pounders would be retained as an interim solution.
As they were not going to be given any large amounts of 17-pounders for the foreseeable future, they set about building their own to the plans provided.
Before trying to mount it in a tank, they decided to test the recoil by fitting two 25-pounders side-by-side inside an AC tank, rigged to fire simultaneously to generate enough force for evaluation.
The test showed that the system was able to handle the recoil, so the Australians set about building a turret and fitting a 17-pounder gun in it. This was done and proved successfully in November 1942 at Fort Gellibrand, Victoria.
The new 17-pounder tank would have been called ACIV Thunderbolt, however, the platform tested on was not an ACIV hull, and was in fact an older model with temporary fittings to test the idea. The real Thunderbolt was never made.
Colonel Watson’s secondment as a British officer to Australia ended in late 1942. He was then flown back to England, where he met with Claude Gibb, who, ironically, was Australian by birth and was now the Director-General of Armoured Fighting Vehicles.
Gibb enquired as to whether the Australians had been able to achieve the mounting of a 17-pounder, to which Major Watson was able to confirm they had done so in 1942. He supplied the drawings and schematics to Mr. Gibb, who had received a lot of criticism from the MOS who said that it was impractical if not impossible to do so.
Gibb was able to explain to the detractors that not only was it possible but, it should be doable in England. Colonel Watson was able to meet with those who were involved in the efforts and show them the changes they had made and how they had gone about it.
With this new information, the British, along with Kilbourne, were able to begin making the first Sherman Firefly.
The Firefly’s Name
Before we go, one of the more exciting connections was the name of the Firefly, which is often attributed to the flash of the gun, yet the names Mayfly and Woodcock also crop up as names for the Firefly, and there may be more to this.
As we mentioned, Willaim Kilbourne originally started his career at Vickers working on Rigid Airship No. 9r, the second Vickers giant Airship.
Their first large airship, the largest of its time, was nicknamed “Mayfly,” but she broke her back in a strong wind, and the wreckage was often visited by Kilbourne during his work. The second name may be Attributed to H.L Woodcock, who was a pilot for many of the dirigibles of the time and was involved in the Mayfly project being friends with both Kilbourne and Barnes Wallis, and later Inspecting Commander of Dirigible Airships.
Claude Gibb would be recognized for his work, with Sir Winston Churchill acknowledging Gibb’s contribution, at least for a short time, while Brighty and Witheridge would be paged into the history books.
Sadly though, Watson and his essential contributions seem to have been erased from common history, despite the fact that without him the Firefly would not have been ready in time for Normandy or made the contribution it did.