Sherman Firefly - Design

Design

It was W.G.K. Kilbourn, a Vickers engineer at the time working for the Department of Tank Design, who transformed the prototype into the tank that would serve the British forces from D-day onwards. The first thing Kilbourn had to fix was the lack of a workable recoil system for the 17 pounder. The 17 pounder travelled 40 in (1.0 m) back as it absorbed the recoil of the blast. This was too long for the Sherman turret. Kilbourn solved this problem by redesigning the recoil system completely rather than modifying it. The recoil cylinders were shortened to allow the turret to take the gun and its recoil, and the new cylinders were placed on both sides of the gun to take advantage of the width of the Sherman's turret rather than be hindered by its height.

The gun breech itself was also rotated 90 degrees to allow for loading from the left rather than from on top. The radio which was mounted in the back of the turret in British tanks had to be moved. An armoured box (a "bustle") was attached to the back of the turret to house the radio. Access was through a large hole cut through the back of the turret.

The next problem encountered by Kilbourn was that the gun cradle, the metal block the gun sits on, had to be shortened to allow the gun to fit into the Firefly, and thus the gun itself was not very stable. Kilbourn had a new barrel designed for the 17 pounder that had a longer untapered section at the base, which helped solve the stability problem. A new mantlet was designed to house the new gun and accept the modified cradle. The modifications were extensive enough to require that 17 pounders intended for the Firefly had to be factory built specifically for it.

Kilbourn had to deal with other problems. On the standard Sherman tank, there was a single hatch in the turret through which the tank commander, gunner and loader entered and left the tank. However the 17 pounder's larger breech and recoil system significantly reduced the ability of the loader to quickly exit from the tank if it was hit. As a result, a new hatch was cut into the top of the turret over the gunner's position. The final major change was the elimination of the hull gunner in favour of space for more 17 pounder ammunition, which was significantly longer than the 75 mm shell and thus took up more room.

The Firefly had no armour or mobility advantages over the normal Sherman tank, although the gun mantlet was some 13mm thicker.

By October and November 1943, enthusiasm began to grow for the project. The 21st Army Group was informed of the new tank in October 1943. Even before final testing had taken place in February 1944, an order for 2,100 Sherman tanks armed with 17 pounder guns was placed. This reaction was understandable, as the Challenger program was suffering constant delays and few would be ready for Normandy, and even worse, the realization that the Cromwell tank did not have a turret ring big enough to take the new High Velocity 75mm gun (50 calibres long), so the Cromwell would have to be armed with the general purpose Ordnance QF 75 mm. Thus the Sherman Firefly represented the only available tank with firepower superior to the QF 75 mm gun in the British Army’s arsenal. Not surprisingly, it was given the ‘highest priority’ by Winston Churchill himself.

The nickname "Firefly" is not found in wartime official documents. It was sometimes used at unit level (Brigade/Regiment) war diaries from March 1944, with another nickname being 'Mayfly'. During the war, Shermans with 17 pounder guns were usually known as '1C', '1C Hybrid', or 'VC', depending on the basic mark of the vehicle. In British nomenclature, a "C" at the end of the Roman numeral indicated a tank equipped with the 17 pounder.