January 24 is a landmark day at Fly By Wire Air. For on this date ten years ago, we began our mission of making aviation accessible to all. Though we've encountered some turbulence at times, our goal has always been to lead our viewers to a smooth flight and a greater understanding of aviation as a whole. Because of this, our blog for this month will be the same as our first blog ten years ago-the story of inflight refueling. We hope our viewers will enjoy the flight.
In-flight refueling got off to a rather crude start in the early 1920's when men standing on fast cars would hand cans of gas to aviators as their slow moving aircraft passed overhead. However, the first actual mid-air refueling occurred on June 27, 1923 in which two Airco DH-4B biplanes of the United States Army Air Corps flew in formation with a hose running from a hand-held fuel tank of the top aircraft to the port of the receiving aircraft. Based upon a design by Alexander Seversky, the refueling was successful and was followed by two other refueling tests in August and October of that year. During the test of August 27-28, 1923 some 687 gallons of fuel were transferred to the receiver plane over nine refuelings-setting an endurance record of 37 hrs. aloft for the receiving plane. The October flight was conducted by the same DH-4B and crew, in which the flight originated in Sumas, Washington with the Airco landing in Tijuana, Mexico, using the same refueling technique with mid-air refuelings at Eugene, Oregon and Sacramento, California. News of the successful tests of the Seversky system spread to Europe, where similar experiments were conducted by the Royal Air Force and the Armee de l Air in France. While these tests were largely successful, both services considered the concept impractical.
By the late 1920's long distance flights became national events, in which pilots engaged in intense competition to win prizes for new records. A key player in the long distance flights was Sir Alan Cobham. Cobham was an RAF pilot during World War I and made a number of flights to both Australia and Africa. As a result of these efforts, Cobham began experimenting with the possibilities of in-flight refueling and later became one of the directors of Airspeed Limited, an aircraft company which produced a specially designed plane, the Airspeed Courier. The Courier, an experimental aircraft for in-flight refueling tests, was flown by Cobham from London to India, using in-flight refueling along the way. In the United States, a group of Army Air Corps pilots led by Major Carl Spaatz, who later commanded the Eighth Air Force in World War II, set an endurance record in excess of 150 hrs., flying the Question Mark over Los Angeles in 1930. Later that year, the Hunter brothers set a new record of 553 hrs., 40 min. over Chicago, flying two Stinson SM-1 Detroiters as refueler and receiver. Perhaps the primary rationale for conducting such tests was the desire of the Postal Service to expedite the delivery of mail from Europe to the United States by the use of aerial refueling.
Two advances in the 1930's made midair refueling a more practical activity. The automatic valve was perfected in 1931, which controlled the flow of fuel between aircraft. First used on British flying boats servicing the empire, the automatic valve would also cut off the flow of fuel in the event the refueler aircraft lost contact with the receiving aircraft. The introduction of automatic valves drastically increased the safety of the refueling process. In 1934, another major innovation, the crossover line, made aerial refueling more practical. The crossover system was developed by Richard Atcherley, an officer in the Royal Air Force. While stationed in the Middle East, Atcherley patented the system, in which the tanker plane trailed a large hooked line that would reel in a similar dropped line from the receiving plane, allowing the refueling to proceed. After further modifications in the late 1930's, the crossover system became the first practical means of aerial refueling.
Though aerial refueling was not utilized during World War II, the demands of the Pacific Theater forced USAAF planners to consider methods from which to gain the maximum range from existing aircraft. During the early part of the war, the Air Corps engaged the services of civilian contractors to develop new concepts of aerial refueling. Using modified B-24 Liberators as tankers, the USAAF was able to extend the Boeing B-17 Flying Fortress's range to approximately 1,500 miles. Though such experiments demonstrated the potential of in-flight refueling, the demands of wartime production would not allow aircraft manufacturers to produce enough tanker aircraft to make aerial refueling practical, as well as a lack of available pilots trained to fly the planes. After the war, the USAAF became interested in the British developed looped-hose refueling system, perfected in 1938 to refuel flying boats for transoceanic crossings. The looped-hose system operated by having the receiving aircraft trailing a steel cable, which was then grappled by a line shot from the tanker. The line was then drawn back into the tanker, where the receiver's cable was connected to the receiving hose. The receiver could then haul back its cable bringing the hose to it. Once the hose was connected, the tanker climbed sufficiently above the receiver aircraft to allow the fuel to flow by gravity. In 1948, the USAF bought a number of looped-hose units, fitting them on B-29 tanker planes to refuel specially equipped B-29s and B-50s. The USAF system had only one major change from the loop hose used by the RAF. The USAF version had auto-coupling of the refueling nozzle, where the leader line with the refueling hose is pulled to the receiver aircraft and a refueling receptacle on the belly of the plane, allowing high-altitude aerial refueling and doing away with the aircraft having to fly to a lower altitude to be depressurized so a crew member could manually do the coupling. The looped-hose system was used on a global flight by the B-50 Superfortress Lucky Lady II in 1949. The flight took ninety-four hours and one minute with the B-50 refueling four times from four pairs of KB-29M tanker aircraft, setting a new benchmark for aerial refueling.
With Cold War tensions beginning to form in the late 1940's, the newly formed United States Air Force was in need of a more efficient manner of refueling Strategic Air Command (SAC) bombers. Because of the number of aircraft involved, General Curtis LeMay, commander of SAC, asked Boeing to develop a more efficient fueling system, capable of transferring fuel at higher rates than existing flexible hose systems. In less than a year, Boeing designed a system which could transfer fuel at a significantly higher rate than previous flexible hose units. The flying boom system is a solid telescoping tube with flexible flight control surfaces (winglets) operated by the boom controller in the tanker aircraft. The boom operator extends the boom, inserting it into a receptacle on the receiving aircraft. Refueling tests conducted by B-29s in 1950 proved the boom concept a success with Boeing converting 116 B-29s to tankers in 1951. Though the B-29s were adequate, the USAF realized they would soon need a dedicated tanker plane. Boeing met this need, as well, by producing the KC-97 Stratofreighter modified with a flying boom and extra jet fuel tanks replenishing the boom. The Stratofreighter was developed from the piston-engined Boeing Stratocruiser airliner, a civil derivative of the B-29 Superfortress. While the KC-97 was the first mass produced tanker aircraft, it had its share of problems. The two major problems in using the KC-97 as a fueling platform were a mixed fuel system with the Stratofreighter carrying gasoline for its own operation in addition to kerosene for the jets it was refueling, a potential safety hazard, as well as a slow speed relative to the jets it was refueling. Many of the supersonic jets entering service in the 1950's had to slow down to make contact with the tanker's refueling boom, approaching the stall speed of the jet. To remedy this, the USAF initiated plans for a jet tanker aircraft capable of both refueling at higher speeds, in addition to carrying a single fuel for its own engines and the receiver aircraft. It wasn't long before Boeing began receiving contracts from the USAF to build jet tankers. Boeing's response was a modified Boeing 707 jetliner with a flying boom system. The USAF designation, KC-135, entered service in 1957 and was both a capable and versatile aircraft, built for a number of missions. With 732 examples built, there are still nearly 400 planes in service today by the USAF, as well as the USAF Reserve and State Air Guard units.
Two other notable refueling systems came into service in the early 1950's, the buddy store or buddy pod and the probe-and-drogue. The buddy store is an external pod loaded on a hardpoint underneath the wing of the tanker plane containing a hose-and-drogue system (HDU). The buddy store concept allows reconfigured bomber or fighter planes the flexibility of refueling other combat aircraft of the unit, extending the range of a strike without the need of dedicated tanker aircraft. Tankers utilizing the buddy store system are also capable of refueling multiple aircraft, if necessary. Aircraft attempting to refuel from the buddy store will ideally approach from behind and slightly below the drogue but need to be careful when approaching as winds may shift the position of the drogue, since the basket is made of a canvas webbing. Once initial contact is made the hose and drogue is pushed forward by the receiver aircraft, the hose is then reeled slowly back into its drum in the HDU. This action opens the tanker's main refueling valve, allowing fuel to flow to the drogue under the appropriate pressure, assuming the tanker crew has energized the pump. Tension on the hose is aerodynamically adjusted by a motor in the hose-and-drogue system so the hose extends and retracts with movement of the receiver aircraft, which prevents bends or kinks in the hose, causing undue stress on the probe. Fuel flow is typically indicated by activation of a green light near the HDU. If the hose is pushed either too far or not far enough, a cutoff switch will inhibit fuel flow, which is typically indicated by an amber light. Disengagement is commanded by the tanker pilot with a red light. The probe-and-drogue refueling method utilizes a flexible hose trailing from the tanker aircraft. The drogue, also called a basket, is a fitting resembling a shuttlecock, attached at its narrow end with a valve to a flexible hose. The drogue stabilizes the hose in flight and provides a funnel to aid insertion of the receiver aircraft probe into the hose. The probe, a rigid pivoting or protruding arm, placed on the receiving aircraft's nose or fuselage to make connection with the hose from the tanker aircraft. A valve placed at the tip of the probe allows fuel to pass from the tanker to the receiver aircraft. More recent jet aircraft are equipped with a retractable probe when not in use due to their high speed performance. Standardized drogue and probe valves allow drogue-equipped tanker aircraft the ability to refuel probe-equipped aircraft from other nations. For example, the NATO standard probe system incorporates shear rivets that attach the refueling valve to the end of the probe in case a large side load develops while in contact with the drogue, the rivets shear and the fuel valve breaks off, rather than the receiver aircraft sustaining structural damage. Unlike the boom system, both the buddy store and probe-and-drogue refueling methods can refuel multiple aircraft.
Aerial refueling has come a long way since those first experiments in the early 1920's. There are conversion kits available today to convert tanker planes from one fueling system to another. Both the KC-10 and the recently developed KC-46 incorporate both the boom and probe-and-drogue refueling systems. For aviation, this is better than having a gas station on every corner.