‘Per Ardua Ad Astra’ – the N.F. Mk XV, a Mosquito worthy of the motto!

The motto of the Royal Air Force might well be said to apply to the de Havilland Mosquito. If it were not for the struggles of Air Chief Marshal Sir Wilfrid Rhodes Freeman, the Air Member for Research and Development and Geoffrey de Havilland, Snr., the Mosquito might well have been still-born. However, there was one particular Mark of Mosquito that really did live up to the RAF’s motto, ‘Through struggle, to the stars’.

No matter what the various competing ‘kill totals’ say, the Luftwaffe lost the ‘Battle of Britain’; if they had won, Hitler would have been driven up Whitehall, have no doubt! In January, 1942, No. 157 Squadron received its first Mosquito, a T.III, to be shortly followed by the first N.F. Mk II night fighters, which by mid-May had begun to take an ever-increasing toll of the reduced number of German night raiders over the British Isles, as well as ranging further afield over Occupied Europe. With the Luftwaffe mostly occupied in supporting Adolf Hitler’s ill-advised invasion of the Soviet Union, it must have seemed that both civilian and military targets in Britain were becoming safer from aerial attack with each passing month.

The first Mosquito ‘night kill’ came on the 30th May, 1942, when a Dornier Do 217 of KG2 was shot down. Just 11 days later, on the 9th June, the first unit of the 8th Air Force, USAAF, the 97th Bombardment Group equipped with Boeing B-17Es, arrived at RAF Polebrook, and what could be described as the build-up for the eventual invasion of Europe had begun. Just two short years later, on the 6th June, 1944, the Allies stormed ashore on the beaches of Normandy in great force, and the defeat of Adolf Hitler became almost certain.

However, those two tumultuous years from 1942 to 1944 saw heavy action for all versions of the Mosquito – fighter, bomber, fighter-bomber and reconnaissance, and on almost all fronts, and it could have been said to have exceeded every expectation that the Royal Air Force and its manufacturer might have had of the aircraft. Despite the overall excellence of all versions the aircraft, there was one particular Mosquito that can only have been regarded with awe – the Mosquito F. Mark XV/N.F. XV.

By early 1942, the British Isles was fairly secure against reconnaissance by the Luftwaffe, and that was all to the good, given that more and more American units, both Army and Air Force were pouring into what was rapidly becoming an ‘unsinkable aircraft carrier’ just off the shores of ‘Festung Europa’. Then, any complacency that the authorities might have had was shattered. It started with single contrails, with a tiny silvery speck at the end of them, crawling across the summer skies. The aircraft making them was not overly fast, but was impossibly high, sometimes being tracked at altitudes in excess of 41,000 feet! Radar tracked them across Southern Britain (sometimes as far as Bristol), then watched them exiting British airspace. Obviously, strenuous efforts were made to intercept these enemy machines, which were photographing all the military targets they could reach. Not only that, but single 250kg or 500kg bombs were being dropped with impunity (the Ju86P could carry up to 1,000 kg of bombs or 3 cameras). These versions of the Ju86 operated not just over the UK, but over Russia and the Middle East. Spitfires could only get to around 5,000 or 10,000 feet lower than these twin engine machines, and something had to be done or the build up to the coming invasion would be compromised.

It turned out the high-flying enemy aircraft were special versions of the Junkers Ju86, a pre-war German design that was now obsolescent. In 1939, a standard Ju86G was heavily modified by increasing the wingspan from 74 to 84 feet, adding a pressure cabin for the reduced crew of just two (a pilot and a radio operator), and taking the unusual step of fitting two Junkers Jumo 207A1 compression ignition engines. These six-cylinder Diesel engines only offered 865 hp at take-off, but maintained a much higher percentage of that than the average petrol engine when operating at altitudes over 30,000 feet.

The first flight of the ‘new’ type took place in January, 1940, and results were so good that orders for forty conversions were issued, some as Ju86P-1 bomber versions, and some as Ju86P-2 photographic aircraft.

To counter this new threat, the Ministry of Aircraft Production – who had fought so hard against the adoption of the Mosquito – turned in desperation to de Havillands. It so happened that the Hatfield works had just completed the prototype of a new, pressurized version of the Mosquito – the PR.VIII – serial number ‘MP469’, powered by two-stage, two-speed supercharged Merlin 61 engines producing 1,565 hp, and driving experimental 12 foot diameter, four-bladed DH Hydromatic propellers.

A Marshall supercharger blower on the port engine, controlled by a Westland valve, pressurized the cabin to + 2 lbs/square inch, slowly building up, starting from 15,000 ft to 30,000 ft. The twin-walled cockpit canopy was vented to three ‘galleries’ containing silica gel desiccant, so that ‘fogging’ of the vital areas would be prevented, or minimized at least.

The de Havilland design staff and Hatfield workforce made a ‘round the clock’ effort, and in the remarkable time of just seven days had rolled out a high-altitude fighter prototype! MP469 was fitted with extended wingtips, giving a span of 59 feet (increased from 54 feet 2 inches). On top of this the aircraft was lightened by over 1,000 pounds, and only the inboard and outboard (No.5) fuel tanks were used for a total of 287 gallons, thus giving MP469 the shortest range of any Mosquito at 1,050 miles. As to armament, it so happened that F.II ‘DD715’ had just had its nose sawn off, so that the AI Mk VIII could be fitted, converting this into the first Mosquito nightfighter equipped with the revolutionary ‘centimetric’ radar. This nose section – complete with its 4 x 303” Browning machine guns, and their wooden ammunition boxes – was grafted onto MP469. As no radar was fitted at the time, MP469 was at this stage, an ‘F’ not an ‘NF’. Still wearing its intended colours of Dark Earth and Dark Green over PR Blue, the aircraft was rolled out of the Experimental Shop at Hatfield, but the standard Mosquito wheels and were soon swapped for smaller and lighter units.

The aircraft, flown by Geoffrey de Havilland, Jnr, managed to reach 43,000 feet on 15th September, and the following day was sent to the RAF High Altitude Flight at RAF Northolt, to the west of London. In November, a new nose, complete with AI Mk VIII centrimetric radar had been fitted, and the four Brownings had been moved to a blister under the fuselage. As such, it became known as the NF. Mark XV. Shortly afterwards a letter from the Ministry of Aircraft Production sent it to the Aircraft & Armament Experimental Establishment for testing.. The A&AEE Report No. S.B.39800/DD/RA gave its top speed as 408 mph at 27,800 ft in full supercharger, and a service ceiling of 43,000 feet (achieved in just over 32 minutes), although it was thought that another 800 feet might be possible. Initial climb rate was a sprightly 2,995 ft/min, and still 2,255 ft/min at 26,000 ft asl. Handling was described as satisfactory, and the aircraft did not drop a wing when stalled. As an aside, the outside air temperature at 43,000 ft was measured as minus 53 degrees Centigrade; despite this, the cabin temperature was kept relatively warm by moving the heater controls to ‘Cold Air’! It was found impossible to achieve the designed pressure differential of 2lbs/square inch at full height – 1.75lbs/sq in being the best recorded – and this was thought to be due to poor cabin sealing.

Radar trials were undertaken at RAF Defford, and then MP469 was sent to the Fighter Interception Unit on the 4th February, 1943. Just three weeks later the NF.Mk XV was issued to No. 85 Squadron who were stationed at RAF Hunsdon, and commanded at the time by the legendary Wing Commander John Cunningham, who along with his navigator Flight Lieutenant C. F. ‘Jimmy’ Rawnsley, formed one of the most potent nightfighter teams in the Royal Air Force. By this time MP469 had a ‘fighter stick’ fitted, rather than the bomber’s ‘control wheel’.

The ‘proof of concept’ aircraft having proved successful, a small batch of four more high-altitude bombers were converted to N.F. XV standard, with even longer wings (62 feet 6 inches) giving an increase in wing area from 454 to 479 square feet. However, the Merlin 60 Series were discarded in favour of a pair of two-stage 70 Series engines producing 1,710 hp. These were a Merlin 76 and 77*, the only difference between them being that the 77* had the cabin blower and associated ‘plumbing’ fitted. These aircraft were issued to No. 85 Squadron, and seem to have been finished in overall PRU Blue or the related Deep Sky colour, with black spinners and radome, e.g. ‘K – King’, DZ385. All of these machines retained their ‘bomber canopies’, with the distinctive ‘V’ fronts, thereby making the Mark XV the only armed Mosquito to do so.

In April, 1943, Cunningham and Rawnsley took MP469 on a trial flight. They passed 30,000 ft in 10 minutes – an altitude which would take 40 minutes in their regular Mosquito! At 40,000ft Rawnsley reported a pounding in his ears, and frosting of certain areas of the canopy, but other than taking in a lot more oxygen than usual, the flight was manageable. Indeed, MP469 recorded the highest altitude by ANY Mosquito – 44,600 feet.

Fortunately (for the Ju86P’s), there were no combats between the Mk XV and the Luftwaffe machines. Indeed, the most common sortie appears to have been a practice interception between two of the Mark XVs. The Ju86 threat had abated, due to their increasing vulnerability. Indeed, in August, 1942, a stripped Spitfire Mk V had managed to intercept and destroy a Ju86P over the Nile Delta at the phenomenal height of 49,000 feet; after this, the Luftwaffe machines were withdrawn from that area.

Also in August, MP469 was sent north to Scotland, and based at RAF Turnhouse, near Edinburgh, to offer cover from possible high-altitude raiders over that region. Eventually withdrawn from use, she ended her days as an instructional airframe (No. 4882M) at the School of Aeronautical Engineering at RAF Henlow. Sadly, no remains of this, or the other ‘production’ machines, exist.

Other ‘solutions’ to the high-altitude raider problem were sought, including the Westland Welkin. This very large aircraft carried 4 x 20mm cannon and employed the same Merlin 76/77* engines as the Mark XV, and had an almost identical service ceiling of 44,000 feet. It entered service in May, 1944, but was never issued to a squadron. Similarly, the intriguing Vickers Type 432 (a single prototype using a pair of Merlin 61s, like MP469), and unofficially called ‘Mayfly’, first flew in December 1942. Unfortunately, the Type 432 – which carried the extremely heavy armament of no less than six 20mm cannon – could not reach more than 38,000 feet, and its top speed was no more than 380 mph.

With the N.F. Mark XV, the Mosquito had, once again, proved itself infinitely adaptable – and in the fastest possible time. It even out-performed two other ‘special purpose’ designs intended to counter the same threat. It may have been born in haste, but this Mark of Mosquito really did live up to the motto of the Royal Air Force.

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