50 Years of Probe and Drogue Flight Refuelling cover signed Air Chief Marshal Sir Michael Knight KCB AFC FRAES, Director of Flight Refuelling Limited

50 Years of Probe and Drogue Flight Refuelling cover signed Air Chief Marshal Sir Michael Knight KCB AFC FRAES, Director of Flight Refuelling Limited

RAF cover to commemorate fifty years of probe and drogue flight refuelling with artwork by Tony Jackson showing a Vickers VC 10 of No 101 Squadron refuelling a BAe Eurofighter (Typhoon) and beneath the very first probe and drogue refuelling attempt using a specially modified Avro Lancaster and a Gloster Meteor.

The cover bears a 1/6 VC10 engine stamp and a 1/9 TV stamp with cachet for the 50 years of Probe and Drogue flight refuelling 11 November 1999. The covers have been flown in a VC10K of No 101 Squadron from RAF Brize Norton to commemorate the fifty anniversary.

This cover has been hand signed by Air Chief Marshal Sie Michael Knight KCB AFC FRAeS a highly respected Royal Air Force commander and when he retired became a director of Flight Refuelling Limited formerly known as Cobham PLC set up by the famous flight refuelling pioneer Sir Alan Cobham.

Official special numbered and certified on the reverse.

50th Anniversary of Probe and Drogue - The Early Years

BACKGROUND

The birth of the United States Air Force (USAF) on 18 September 1947 was vigorously opposed by the US Navy, fearing they would lose control of their aircraft; disagreement over the employment of atomic weapons added to the bitter rivalry.  USAF requested defence funding of its new B-36 strategic bomber force, whereas the Navy backed building the USS United States, a 70,000-ton super carrier carrying a 45-ton atomic bomber world-wide.  When the Russians blockaded Berlin on 31 March 1948 it became clear that America’s enemy would likely be the Soviet Union, but the USAF did not have the range to reach targets in central Russia, and the giant B-36 was proving to be sluggish in early trials. The US Navy was clearly ahead in atomic weapon delivery and the USAF could only win the funding battle by proving their Strategic Air Command (SAC) bombers were capable of reaching their targets from bases in the USA.  Unsurprisingly, although the British invented and perfected practical use of air-to-air refuelling (AAR), the United States had greater need of the technology and were the first to apply it to military use.  In 1949, AAR evolved from a flying circus stunt into a military force multiplier using the probe and drogue technique and this major development took place over southern England; clearly it is appropriate to mark the 50th anniversary of this major achievement.

FIRST USAF TANKERS

Flight Refuelling Ltd (FRL) had been formed by British aviation pioneer, Sir Alan Cobham, in 1934 to develop AAR commercially; by 1939 the first regular flight refuelled transatlantic airmail flights, using the looped hose method, were inaugurated, but the outbreak of war terminated the service.  The surplus equipment was used in a series of successful wartime trials in the USA, and in 1944 Sir Alan was awarded an air ministry contract for 600 sets of looped hose flight-refuelling equipment, for the RAF Tiger Force Lancasters.  After WWII, Flight Refuelling continued the transatlantic trials, leading to a successful flight refuelled passenger service to Bermuda in 1947.  In April 1948, a USAF delegation visited the FRL Tarrant Rushton base, and an order for 100 sets of their looped hose equipment subsequently materialised.  Fortunately, the Air Ministry had not proceeded with its order for Tiger Force tanker and receiver equipment, enabling Cobham to purchase it at scrap value and meet the American tight delivery schedule with ease.  Boeing began a rapid conversion programme, predominantly using B-29 aircraft, destined to produce 92 looped hose tankers and 131 receivers. 

B-36 RECEIVER PUBLICITY

The first B36A entered service with SAC in June 1948, conversion of the giant bomber into a looped hose receiver began and by December 1948 the USAF was ready to try out the system.  However, the political battle with the US Navy was approaching a climax and the USAF needed positive publicity to assert their case.  The anniversary of the Japanese attack on Pearl Harbour gave them a perfect opportunity.  On 7 December 1948, a B-36A flew from Carswell AFB, Fort Worth, carried out an undetected mock attack on the US Navy’s Pacific Fleet in Pearl Harbour, dropped a dummy 10,000lb atomic bomb and returned, completing a round trip of 9,400 miles using looped hose flight-refuelling.  This stunt demonstrated the vulnerability of the US Navy fleet bases to atomic attack by long range bombers and greatly assisted the case for SAC and the B-36.  As a follow-up, the USAF planned to fly non-stop around the world, demonstrating its new global strike capability beyond any doubt.  On 26 February 1949 a Boeing B-36 named ‘Lucky Lady II’ departed from Carswell AFB and flew 3,800 miles east towards the Azores; at Lajes two KB-29 tankers performed the first refuelling.  Lucky Lady II then flew 5,200 miles to Dhahran for a second bracket, 4,900 miles to the Philippines for the third and 5,300 to Hawaii for a final top up before landing at Fort Worth on 2 March.  The flight covered 23,452 miles and took 94 hrs 01 min.  The around-the-world flight involving 16 tankers, was final proof that bombers were capable of striking anywhere in the world and the USAF won its case for the B-36 in Congress.  Although construction had already begun on the USS United States, it was cancelled in April 1949; despite efforts by the US Navy to reverse the decision, the B-36 became the mainstay of SAC in the early 1950s.

LOOPED HOSE/PROBE & DROGUE/BOOM

Cobham continued to believe that his flight-refuelling system had a future in commercial aviation but the military application now drove developments with the USAF, rather than the RAF, as its main proponent.  Although FRL’s looped hose was proven and reliable its complexity and the low speed required for the operation limited its use.  In 1947 the Air Ministry had concluded that the system was impractical and of little value in future military operations and that any further development of flight refuelling equipment would be discontinued.  Without Cobham’s persistence and the lucrative USAF contract, development in Britain might have ceased altogether.  In the USA Boeing began investigating the boom method for refuelling high speed jet bombers but the USAF were also interested in any new AAR technique that could be used for single seat jet fighters.  When Cobham was told of this interest late in 1948, during a meeting with the USAF he mentioned that his company was already working on such a system.  The USAF was most impressed and arranged to visit Tarrant Rushton in spring 1949 for a demonstration.  Having promised a new system, Sir Alan had just over 4 months to come up with results.  The new project was allocated top priority with his design team, who investigated several options before concentrating on a receiver aircraft equipped with a probe mounted nozzle which would make contact with a receptor coupling in a tapered funnel or drogue on the end of a tanker’s hose.  Various designs of drogue were tested by towing them at high speed behind a car down the runway at Tarrant Rushton.  In principle the technique looked promising but a problem remained on how to prevent the tanker hose from whipping and looping when contact was made.  Peter MacGregor, one of FRL’s top design engineers, came up with the solution after considering the method of retraction used by measuring tapes and roller blinds, whilst lying in bed one Sunday morning.  If a spring or equivalent could take up the slack then the problem of hose whipping would be eliminated.  MacGregor opted for an electro hydraulic fluid drive motor attached to the hose drum unit (HDU) and fitted it to an Avro Lancaster tanker. All that had to be found was a suitable receiver aircraft.

INITIAL TRIALS OF PROBE & DROGUE

Cobham enlisted help from a generally unenthusiastic Air Ministry by obtaining Meteor Mk 3, EE397, on loan from the RAF.  Time was running out and Flight Refuelling had just over a month to refine the coupling system and modify the Meteor with a nose-mounted probe and internal fuel lines.  The system was first flight-tested just 2 days before the demonstration that had been set up for the USAF.  On 4 April 1949, FRL test pilot Pat Hornidge took off from Tarrant Rushton in the Meteor and carried out the first series of dry contacts with the Lancaster tanker flown by Tom Marks.  Throughout the weekend trials, they refined the technique of the probe and drogue tanking.  Not only had a completely new method of AAR been developed in a matter of months, but also for the first time a jet aircraft had refuelled in flight and this heralded a new era of possibilities.  The USAF delegation was most impressed with the demonstration but technical problems delayed a repeat performance for over a month.  Subsequently, a series of 7 demonstration flights were planned for VIPs from the RAF and aviation industry; Sir Alan claimed that his probe and drogue system could transfer fuel at 4,000lbs a minute whilst flying 400kts at 35,000ft.  However, one officer noted that the demonstrations were restricted to contacts at 130kts and 2,000ft and little fuel appeared to be transferred to the Meteor.  Cobham explained that a temporary snag had curtailed that demonstration but in truth, a more capable tanker would be needed before high altitude trials could be conducted.

JET FLIGHT DURATION RECORD

To demonstrate the clear operational advantages offered by the probe and drogue method, test pilots Hornidge and Marks organised an attempt on the world jet endurance record.  On 7 August 1949, Hornidge took off from Tarrant Rushton in the Meteor and flew a continuous circuit down the Bristol Channel and along the south coast to the Needles to rendezvous with the HDU-equipped Lancaster orbiting around the Isle of Wight.  Hornidge remained airborne for 12 hrs 03 mins, flying about 3,600 miles; he made 10 refuelling contacts and received a total of 2,352 gallons of kerosene.  The record breaking was a great success and gained considerable international publicity for FRL’s revolutionary new probe and drogue AAR technique.

FIRST USAF PROBE AND DROGUE AIRCRAFT

The old looped hose method was having mixed results in service with the USAF and, by the end of 1949, four B-29 bombers were sent to Tarrant Rushton for conversion to HDU-equipped tankers.  One of the aircraft was converted into the world’s first three-point tanker, designated YKB-29T with Mk11 HDUs mounted in the bomb bay and on each wing tip.  The momentous events of 1949 truly marked the birth of military AAR with the British invention (backed by American money) of the probe and drogue technique.  Britain’s invention of the world’s first practical in-flight refuelling system can, with hindsight, be seen as a major milestone in the development of air power.  Yet far from bringing glory and profit for Flight Refuelling Ltd, the technology continued to be regarded as unnecessary circus stunt by the Air Ministry and the small Dorset based company now found itself in direct competition with the most powerful aviation industrial giant, Boeing.  With the advent of the Vietnam War, the USAF standardised its AAR operations using boom equipped KC-135 tankers but, perhaps unsurprisingly, the US Navy opted to retain the probe and drogue technique. However, profits from the successful looped-hose deal with the USAF barely covered losses incurred by Flight Refuelling when its Berlin airlift contracts were suddenly cancelled earlier than expected in August 1949.  To save the company Cobham was forced to sell a majority shareholding to a merchant bank who promptly began an asset stripping operation.  The arrival of 4 Boeing B-29 bombers and 2 Republic F-84 Thunderjet fighters at Tarrant Rushton for probe and drogue conversion had potential for easing the financial crisis.  This project represented a huge leap forward from the Lancaster/Meteor trials of early 1949.  The first B-29 was fitted with a single Mk8 HDU in the rear fuselage and 2 others were fitted with a probe extending forward above the cockpit for the first bomber trials with the new system.  The single point tanker was re-designated the KB-29T and the receivers became B-29MRs.  The most significant conversion was made to the second B-29, when it was fitted with 3 electrically driven Mk11 HDUs (2 in wing-tip pods) becoming the world's first three-point tanker, designated YKB-29T.  Both F-84s were fitted with wing mounted probes.  The Superfortress and Thunderjet were considerably more advanced than the Lancaster and Meteor; this made conversions far more difficult than expected, with costs significantly more than agreed in the fixed price contract.  Unfortunately, Cobham was forced to sell the manufacturing rights for the probe and drogue system to the Americans, in order to cover his losses.  To add to its problems, FRL also lost effective monopoly in practical refuelling techniques.  Boeing, who had benefited from FRL’s work with huge USAF contracts to convert B-29s into KB-29M tankers, had been working in secret since November 1947 on a new in-flight refuelling method for its future swept-wing B-47 jet bomber.  On 19 October 1949 Boeing publicly unveiled the flying boom refuelling system.  Boeing claimed that the rigid boom permitted higher fuel flow than a hose and operation at greater altitudes and speeds than had been demonstrated with probe and drogue.  Also, receiver pilots needed no extra training in formation skills because the boom operator in the tanker aircraft made the coupling.  The Boeing challenge threatened FRL’s AAR lead with probe and drogue.  As 1949 drew to a close the future adoption of the probe and drogue technique on a large scale by the USAF was still in doubt and a firm commitment from the RAF was not evident.

FIRST NON-STOP TRANSATLANTIC JET FLIGHTS

At the beginning of 1950, flight refuelling was a well-established technique in the USAF.  Air refuelling squadrons in Arizona and New Mexico had operated the looped hose system since June 1948, and Boeing was making real progress with its flying boom tanker.  FRL’s probe and drogue was now a proven system and the Tarrant Rushton plant was converting two B-29 bombers to HDU tankers and fitting F-84 Thunderjets with probes.  However, without British government support, FRL’s attempts to compete on equal terms with Boeing were doomed to failure.  The conversion work on the American aircraft was much more complex than anticipated and it brought FRL to the edge of bankruptcy to meet the contract deadline.  Commander SAC, General Curtis Le May, saw only the need to air refuel bombers and favoured Boeing’s flying boom.  FRL needed to re-affirm the advantages that probe and drogue could give to jet fighters and did so publicly with the Lancaster and Meteor at the 1950 Farnborough Air Show.  Shortly afterwards the USAF authorized a non-stop flight refuelled crossing of the Atlantic by F-84s.  The plan was for Colonels Schilling and Ritchie to fly a pair of fighters from Manston to Mitchell Field, New York, refuelling from Lincoln and KB-29 tankers over Prestwick, Keflavik and Goose Bay.  The first attempt on 19 September 1950 but was aborted, due to a Lincoln HDU malfunction, so a second try was made on 22 September 1950.  The first RV went well but poor weather over Iceland led to problems joining up with the second Lincoln tanker; Schilling made successful contact but Ritchie damaged his probe by disconnecting before the valve closed.  Despite this, both F-84s pressed on for the third tanker, the KB-29, near Goose Bay.  Ritchie was unable to take on fuel so climbed before gliding towards Goose Bay; he survived bailing out, 30 miles from the airfield, having run out of fuel.  Schilling continued on to Loring AFB, Maine after a flight time of 10 hrs 8 mins.  Although Ritchie had bailed out, emphasizing the need for an automatic probe valve, the first non-stop transatlantic crossings had been made by jets and had proven that flight refuelled deployments were practical.

RAF TENTATIVENESS v. USAF BOLD PROGRESS

Publicity from the USAF’s achievement encouraged the Air Ministry to convert sixteen Meteor Mk8 fighters of No 245 Squadron, RAF Horsham St Faith to probe receivers for refuelling trials with Lincoln tankers.  The trial, Operation Pinnacle, began on 8 May 1951 but, although completely successful, failed to convince the Air Staff who felt that provision of tankers would simply mean fewer fighters.  This conception was completely opposite to that of the USAF who had already succeeded with flight refuelling in combat.  By spring 1951, FRL had completed the conversion of the B-29s into a single-point KB-29T and a three-point YKB-29T; the latter, nicknamed the “Triple Nipple” made the first simultaneous 3 aircraft refuelling with the FRL Mk4 and two No 245 Squadron Meteors.  The Americans realized that large numbers of fighters were without the range to reach their targets in Korea from bases in Japan and flight refuelling was essential to their operations.  In June 1951 the single point KB-29T was flown to Yokota AB, Japan.  By fitting probes to their drop tanks the fighters did not need the additional internal plumbing.  Lockheed, Republic and North American were tasked with converting the tanks on the F-80 Shooting Star, F-84 Thunderjet and F-86 Sabre.  The swept wing and under wing tanks of the F-86 proved to be unsuitable but the straight wing and tip tanks on the F-80 and 84 gave no problems.   The first flight refuelled combat mission in history occurred on 6 July 1951, when 3 RF-80As took off from Taegu, South Korea to RV with the KB-29T tanker off Wonsan, North Korea.  After refuelling the 3 RF-80As were able to use their extended range to photograph targets in the far north of Korea.  Early on 28 September 1951, Major Harry Doris flew from Japan in an F-80A, armed with a full weapon load; he refuelled 8 times from the KB-29T tanker, carrying out a series of low level strikes on targets in North Korea, before landing back in Japan that evening.  He had established a new endurance record for jet aircraft of over 14 hrs and done it under combat conditions.  The USAF’s full-scale flight refuelling operation for the Korean War, Operation High Tide, involved ten KB-29M HDU tankers, converted by FRL and initially three F-84 squadrons fitted with tip tank probes.  The operation reached its peak on 29 May 1952 when twelve F-84Es flew from Japan to carry out successful full-scale ground attack strikes on North Korea, refuelling en-route.  The USAF concluded the value of flight refuelling to tactical aircraft operations was indisputable and whilst the boom system still continued in use, probe and drogue was now combat proven and the system of choice for tactical aircraft.

RAF PROBE AND DROGUE ADOPTION

It was not until November 1959, when No 214 Squadron became UK’s first fully operational tanker unit, equipped with single-hose Vickers Valiants, that the RAF adopted AAR.  A further 7 years elapsed before the first RAF three-point tanker entered service with Victors of No 57 Squadron.  However, probe and drogue has proved to be the most versatile method of AAR, capable of use with aircraft as diverse as helicopters and supersonic fighters. The difference in opinion between the British and Americans over the advantages of AAR could be explained by the very different wartime experiences of their air forces.  The RAF had fought mainly in the European theatre over what would be considered today as tactical ranges.  During WWII, Lancasters, Lincolns and even Mosquitoes could easily reach their targets in Europe from bases in England and aircraft underdevelopment promised even greater ranges.  However, the USAF had fought its campaign in the Pacific over ranges unimaginable a few years earlier.  It is 635 miles from RAF Brize Norton to Berlin but the USAF B-29 Superfortresses were tasked to fly 3128-mile round trips from Guam to Tokyo.  This vast difference in experience may partly explain why the British were reluctant, and the Americans ready, to embrace AAR.