Dave Unwin examines the ‘big gun’ variant of the
Having flown operationally as a fighter, night-fighter, fighter-bomber, bomber, photo-reconnaissance and in many other roles, the de Havilland DH98 Mosquito can probably be considered to be the first multi-role combat aircraft and one of the most versatile aircraft ever made. Possibly one of the least known variants was the FB Mk.XVIII, often referred to as the ‘Tsetse Mosquito’ as it possessed a particularly powerful sting!
Of its myriad roles, a task that particularly suited the Mosquito was anti-shipping patrol. Its combination of speed, range and agility, plus the twin-engine configuration made it ideal for the interdiction of axis shipping and for use as a convoy escort to help deter or destroy the U-boat scourge. It is generally accepted by historians that winning the Battle of the Atlantic was pivotal to ultimate victory and that the Kriegsmarine’s U-boat fleet posed the greatest danger to Britain. Royal Navy planners were particularly keen to destroy U-boats entering or leaving the Bay of Biscay and initially the Mosquito seemed perfectly suited to this mission.
However, its armament of four 0.303 Browning machine guns and four 20mm cannon – lethal against aircraft and thinly skinned trucks – soon proved ineffective against a submarine’s steel-plated pressure hull, and other weapons were examined. Although the RP (rocket projectile) seemed an obvious choice (the difficulty of hitting a small, moving target with bombs is evident) another idea was to fit Mosquitos with a larger gun, and the FB.XVIII was born.
The nose of a Tsetse-armed Mosquito XVIII. (Key Collection)
MOSSIES, AUTOLOADERS, AND CIGARETTES
In March 1943 the Ministry of Aircraft Production asked de Havilland to study the feasibility of fitting such a weapon into a Mosquito, principally a variant of the 6Pdr (57mm) anti-tank gun. de Havilland’s chief designer, Ronald Bishop, quickly calculated that carrying the gun and its ammunition were well within the aircraft’s capabilities – in fact the company had already initiated a feasibility study into fitting a heavily armoured Mosquito with a huge 94mm gun for dedicated ground-attack the year before. As the 6Pdr and its associated components and ammunition would weigh around 1,800lb, it was well within the Mosquito’s load-carrying capabilities, while the gun’s hydropneumatic recoil system lessened the recoil to the extent that it was not expected to create significant problems.
Selecting a suitable gun proved relatively straightforward, but making it automatic was more of a challenge, for although larger guns were fitted to aircraft during World War Two (for example, a few B-25 Mitchells were armed with a 75mm gun), these had to be loaded manually. The Mossie gun was intended to be a fully automatic weapon, capable of firing in bursts, (although in practice most pilots fired single shots). The 6Pdr chosen was a QF (quick-firing) anti-tank weapon with one-piece ammunition, but was still single shot.
The Molins Company of Peterborough (its stock-in-trade was building specialised machines for making and packaging cigarettes) had been contracted by the War Department to design and build an automatic feed mechanism for the weapon the previous year, envisaging that it would be fitted to armoured cars. The auto-loading system carried the ammunition stacked in columns, with either four or five shells in each clip (navalised versions used on motor torpedo boats loaded from the side and could hold six rounds). An electrically operated ‘charger arm’ moved another clip into place as the previous one was expended, allowing the heavy rounds to be fed automatically into the breech without links. The gun used a recoil action to cycle the mechanism, with the spent cases being retained in the fuselage (they could not be ejected overboard in case they struck the tailplane). This weapon became known as the M-Gun or simply the ‘Molins’, although its official title was the rather wordy ‘Airborne 6-pounder Class M gun’.
SEVEN MONTHS FROM PEN TO RANGE
It may seem astonishing when compared to the gestation time of a modern weapons system, but it took less than seven months from the original idea being mooted to the type becoming operational. Initial enquiries into the possibility of fitting a 6Pdr to a Mosquito began in mid-March, and during April de Havilland fitted a regular gun into the nose of a crashed aircraft to study the effects of blast on the fuselage. The following month a FB.VI, HJ732, was moved into the experimental shop to begin installing the gun and its associated components. The barrel was mounted just to starboard of the aircraft’s centreline and inclined slightly downward, with the recoil spring faired beneath it.
A tremendous advantage to the Hatfield team was the Mosquito’s primarily wooden construction, which not only allowed new components to be machined and installed very easily, but also ensured the airframe could absorb the considerable recoil. Work continued at a rapid pace and by early June the cannon was satisfactorily tested on the Hatfield butts, with an entire magazine being emptied in a single burst. This must have been a truly spectacular sight (and sound) as flames up to 30ft long spouted from the barrel with each round! Incidentally, research for this article revealed that there is real uncertainty around the size of the magazine, with 21, 22, 23 and 25 shells all being quoted. Having studied several photographs of the installation, the most likely arrangement appears to have been two columns of five rounds and three of four, with one round in the breech, for a total of 23.
STEADY STREAM OF FIRE
The cyclic rate of fire was just under one round per second. As no problems with blast or recoil had been discovered, on June 9 HJ732 was flown to Boscombe Down for air firing trials, where it was found that the feed unit would not work above 2.5g, a problem that had been predicted by the RAF Gun Section. The charger arm and other parts were strengthened and an automatic muzzle cap fitted to prevent the problem of ‘blow-through’ when the breech was opened. This was an important part of the design – as without it a 300mph gale would have blasted down the barrel – yet it needed to be fail-safe and was designed so that – if it did not open – a shell could safely pass through it.
The uprated charger arm cured the g-related issues, and (as long as sideslip was avoided while firing, which could cause a jam) the M-Gun was quite reliable. By July a second Mosquito had been converted. This machine featured an extra 900lbs of armour around the nose and cockpit, vital as U-boats carried several AA guns. By now HJ732 had fired hundreds of rounds and although the gun was proving very accurate and reliable, blast and recoil effects were beginning to show on the airframe and flaps. It was decided to delete the outer pair of 0.303 machine guns (the M-Gun having replaced the four 20mm cannons) and stiffen the nose area with tie-rods. The remaining 0.303s benefited from having enlarged ammunition tanks. The gunsight was not the usual reflector type fitted to fighters, but a Barr & Stroud Mk.IIIa, a model more typically fitted to turrets. It had two graticules (aiming points); one for the Brownings and one for the M-Gun, although in practice pilots often used the tracers from the machine guns as a sighting aid because the trajectories of the two different projectiles converged at around 400 yards. A steady stream of fire from the pair of machine guns also kept the U-boat’s gunners’ heads down. A final modification was the installation of a 65-gallon fuel tank in the fuselage, and the variant – now known as the FB.XVIII – entered service.
Read the rest of this exciting feature in the July issue of Britain at War – in the UK shops from today.