Mosquito FB Mk XVIII Tsetse Molins 6-pounder Class M cannon (57 mm)
The Molins-equipped Mosquito Mk.XVIII (“Tsetse”) was a unique adaptation of the de Havilland Mosquito, designed to carry a 6-pounder (57mm) Molins anti-tank gun. This modification resulted in a powerful, specialized aircraft aimed primarily at anti-shipping and ground-attack roles during World War II.
Development and Installation of the 6-Pounder:
- The 6-pounder gun, originally introduced in 1941, was widely used in British tanks and as a standalone anti-tank artillery piece. Installing it into an aircraft like the Mosquito required significant engineering.
- A major challenge was that the gun could not be manually reloaded during flight. This was overcome by an automatic loading system developed by Desmond Molins, which was capable of holding 23 rounds in five racks.
- The modified gun system, branded as the Molins gun, underwent various ground trials before it was adapted for aircraft use.
Aircraft Modifications:
- The prototype for this design, a Mosquito FB Mk.VI (serial number HJ732), was modified at de Havilland’s Hatfield factory to accommodate the Molins gun.
- To fit the large gun and its ammunition into the forward fuselage, the four 20mm cannons typically found in the lower forward fuselage were removed. However, the four .303 machine guns in the extreme nose were retained, although some later conversions carried only two of these.
- The large Molins gun required the fuselage to be modified, with the gun mounted slightly offset to the starboard side due to its feed and ejection system. The right-hand access door in the gun bay was also enlarged.
- The gun’s recoil was significant, necessitating additional struts and internal reinforcements. A large oil-filled recoil buffer was installed beneath the gun, requiring a blister fairing on the underside of the nose.
Recoil and Airframe Stiffening:
- Early test firings showed that the gun’s recoil caused rivets and fasteners in the forward fuselage and engine nacelles to shear or loosen. These problems were addressed by strengthening various sections of the aircraft’s structure, including the underside of the crew compartment.
Performance and Designation:
- The new variant, designated Mosquito FB Mk.XVIII, was nicknamed “Tsetse” after the biting tsetse fly, referencing the aircraft’s enhanced firepower.
- The Mk.XVIII used more powerful Rolls-Royce Merlin 25 engines, rated at 1,635 hp, to compensate for the additional weight from the gun, armor, and airframe stiffening. The aircraft’s two wing internal fuel tanks were removed to make space for the modifications, so underwing fuel tanks became standard.
Operational Use:
- The prototype HJ732 underwent successful trials at the Aeroplane and Armament Experimental Establishment (A&AEE) at Boscombe Down in mid-1943. Despite its effectiveness, the “Tsetse” Mosquito was never mass-produced, partly because of the increasing use of unguided rockets, which became the RAF’s primary anti-shipping and ground-attack weapon.
- By late 1944, conversion work was discontinued after only 17 Mosquito Mk.XVIIIs were produced.
Summary:
The Mosquito FB Mk.XVIII “Tsetse” was a highly specialized variant equipped with the Molins 6-pounder gun, primarily intended for anti-shipping operations. Despite its firepower and success in trials, the introduction of 3-inch rockets with 60-pound warheads overshadowed its role, limiting production to a small number of aircraft.