by Mark L. Morgan
Hq. Air Mobility Command History Office
This is the fourth article in a series of articles highlighting the history of aerial refueling and the important role aerial refueling has played in American military history.
Strategic Air Command entered the 1950s on a roll. It operated a growing fleet of tanker aircraft, and the first jet bombers — commencing with the B-47 Stratojet — were coming on line.
The combination of tankers and bombers made SAC a truly global strike force, with mission duration only limited by crew endurance. However, one question remained: what was the proper role of SAC’s small escort fighter force?
During World War II, fighters, such as the legendary P-47 Thunderbolt and P-51 Mustang, escorted bombers over their targets. However, the postwar jet-propelled fighters, such as the F-80 Shooting Star and F-84 Thunderjet, used fuel at a much higher rate and were, therefore, range-limited. They could no longer escort the bombers.
To be sure, the Air Force regularly transferred fighter units overseas, particularly after the outbreak of the Korean War on June 25, 1950. The standard method involved using U.S. Navy or Navy contact vessels — primarily World War II-era escort aircraft carriers — to physically ship the aircraft. This took weeks, and — more often than not and despite protective efforts — upon arrival the fighters required extensive maintenance because of salt air exposure and corrosion.
The answer was simple: find a way to extend the range of fighter aircraft. Initial efforts included projects with the names of Tip-Tow, Tom-Tom and FICON (for “Fighter Conveyor”). These did not involve actual in-flight refueling, but instead involved literally “towing” fighter aircraft, albeit under rather unusual circumstances.
Project Tip-Tow employed a modified B-29 and two F-84Ds, which attached to the bomber at either wingtip through a clamping device. Project Tom-Tom, tested in 1953, was similar and involved the coupling of RF-84F reconnaissance aircraft to the wing tips of a modified B-36. The FICON proposal involved the actual carriage of a modified RF-84F in the bomb bay of a B-36, slung beneath a trapeze.
During testing, flying the fighters in close proximity to large bombers and hooking up proved supremely challenging, even in perfect weather. A fatal crash involving the Tip-Tow B-29 and one of the F-84s in April 1953 reinforced this and led to the cancellation of Tip-Tow. Doing such hook-ups operationally, possibly in combat and most likely at night and in bad weather, made the efforts even more risky. Fortunately, advances in air refueling of fighters made all three of these difficult and complex “towing” methods superfluous.
As during the early development of SAC tankers, the United Kingdom’s Flight Refueling Limited, or FRL, led the way. The company fabricated external drop tanks with integral refueling probes, suitable for using with the probe and drogue system. On Oct. 22, 1950 — barely four months after the start of the Korean War – U.S. Air Force Col. David C. Schilling used this system to make the first non-stop, air-refueled flight by a fighter across the Atlantic Ocean.
Colonel Schilling commanded the 62nd Fighter Squadron and later the 56th Fighter Group in the European Theater during World War II. In July 1948 he led the F-80s of the 56th Fighter Wing from Selfridge Air Force Base, Mich., across the Atlantic to the Royal Air Force installation at Odiham, England. Accomplished under the title of Fox Able One (“Fighters Atlantic, Operation No. 1″), Schilling’s pilots went over via landings and fueling stops at Bangor, Maine; Goose Bay, Labrador; Bluie West 1/Narsarsuaq, Greenland; Meeks Field, Iceland; and RAF Stornaway, the Hebrides. Because of stops and the weather, the 16 fighters took 10 days to get to Europe.
In October 1950, using aerial refueling, Colonel Schilling made the trip in the reverse direction in an incredible 10 hours and 8 minutes.
After launching from RAF Manston in two modified F-84Es, Schilling and Colonel William Ritchie refueled from FRL-operated Avro Lincoln bombers/tankers over Scotland and Iceland. Unfortunately, one of Ritchie’s probes sustained damage during the contact over Iceland. Unable to take on fuel, he literally ran out of gas and ejected. Fortunately, he was quickly rescued.
With the Korean War well underway and its high demand for fighter aircraft, the Wright Air Development Center at Wright-Patterson Air Force Base, Ohio, designed additional external drop tanks with fixed refueling probes and dispatched them to the theater. Republic Aviation, the manufacturer of the Thunderjet, concurrently started delivery of the F-84G with a refueling receptacle in the leading edge of the left wing compatible with SAC’s boom-equipped KB-29Ps.
On July 6, 1951, the first combat air refueling of fighter-type aircraft took place over Korea. Three RF-80As launched from Taegu with the modified tip-tanks and rendezvoused with a tanker offshore of Wonsan, North Korea. Through in-flight refueling, the RF-80s effectively doubled their range, which enabled them to photograph valuable targets in North Korea.
The big test came with plans for the movement of an entire fighter wing to the Korean theater. On July 4, 1952, 60 F-84Gs launched from Turner AFB, Ga., and flew the 1,800 nautical miles to Travis AFB, Calif., non-stop. Refueled en route by 24 KB-29Ps over Texas, this served as the rehearsal for the main event, designated Fox Peter One.
Organized by Colonel Schilling — who now served as the commander of Turner AFB’s 31st Fighter Escort Wing — Fox Peter One kicked off on July 6 when the first of the 31st FEWs three squadrons of F-84Gs headed west from Georgia to Travis. Throughout the following three days, each squadron refueled from KB-29Ps over Texas.
At 1,860 nautical miles and with no alternate landing sites or divert fields, the flight from Travis AFB to Hickam AFB (Territory of Hawaii), was the longest of the trans-Pacific flight. All of the fighters made it and then island-hopped the rest of the way to Yokota Air Base, Japan, via Midway Island, Wake Island, Eniwetok, Guam, and Iwo Jima. The arrival of the last aircraft in Japan on July 16, less than two weeks after leaving Georgia, marked Fox Peter One as a resounding success.
The following October, the 27th FEW from Bergstrom AFB, Texas, replicated the route and in-flight refuelings and relieved the 31st FEW. The 27th FEW’s commander was Col. Donald Blakeslee, another famous World War II pilot and ace.
More record flights followed, including Operation Longstride in October 1953, which saw Colonel Schilling’s wing — now designated the 31st Strategic Fighter Wing — dispatch eight F-84Gs to Nouasseur Air Base, French Morocco. The aircraft covered 3,800 miles in 10 hours and 20 minutes, thanks to in-flight refueling by brand-new SAC KC-97s in the vicinity of Bermuda and the Azores.
Concurrently, Col. Thayer S. Olds, commander of Turner AFB’s 40th Air Division, led 20 F-84Gs of the 508th SFW to RAF Lakenheath, England. Three of the fighters landed at Keflavik, Iceland, because of mechanical problems; however, the remaining aircraft successfully hooked up with the orbiting KC-97 tankers and made it to England in one flight.
In 1957, SAC’s fighter units transferred to Tactical Air Command or were inactivated as part of a reorganization of Air Force strategic and tactical assets. However, they set the standard; by the end of the 1950s, trans-oceanic flights became commonplace. The Air Force never bought another fighter aircraft without in-flight refueling capability; a capability which proved its worth a few years later when Vietnam heated up.
by Mark Morgan
Hq. AMC History Office
This is the third article in a series of articles highlighting the history of aerial refueling and the important role aerial refueling has played in American military history.
On Aug. 6, 1946, the world’s largest bomber took to the air from the Consolidated-Vultee plant on the south side of Lake Worth, west of Fort Worth, Texas. The huge, six-engine B-36 was a wartime design, built for non-stop round-trip flights from the United States to Germany in the event of the loss of bases in England.
The first B-36 production models entered service with Strategic Air Command in late 1948. The B-36 was big enough to carry the large nuclear weapons of the time and carried enough fuel – more than 183,000 pounds — to make the 10,000-mile round-trip flight to Europe and back.
However, the B-36 lacked speed and maneuverability and, during the early 1950s, the aircraft became highly vulnerable to enemy jet fighter aircraft. Fortunately, the Boeing Airplane Company had the B-36’s replacements in development: the jet-propelled B-47 and B-52. However, these new aircraft — unlike the B-36 — needed air refueling to hit targets in the Soviet Union. At the time of their development, no aerial refueling aircraft existed.
In January 1948, former bomber commander and the first U.S. Air Force chief of staff, Gen. Carl A. “Tooey” Spaatz, identified in-flight refueling as the young service’s highest initial priority. Two months later, Air Force personnel from Wright-Patterson AFB, Ohio, visited Britain’s Flight Refueling Limited, evaluated the company’s loop-hose air refueling system design, and bought two examples. The Air Force also ordered 40 additional sets and acquired manufacturing rights for the system.
Upon arrival in the states, the two refueling systems went to Boeing’s Wichita, Kan., plant for installation in B-29s. The subsequent conversion program resulted in the production of 40 KB-29M tankers and 40 B-29MR receivers. On June 30, 1948, SAC activated its first two KB-29M squadrons: the 43rd Air Refueling Squadron at Davis-Monthan AFB, Az., and the 509th Air Refueling Squadron at Walker AFB, New Mexico.
The introduction of dedicated tanker aircraft and crews allowed SAC to extend the range of its B-29 and B-50A bombers. Concurrently, SAC and the Air Force made the decision to equip all future bombers with an in-flight refueling capability. However, the loop-hose system proved unwieldy and difficult, particularly in bad weather. With a two-and-a-half-inch diameter refueling hose, the FRL-developed system transferred fuel at a rate of only 110 gallons per minute. With new high-speed, high-altitude jet bombers coming on line, capable of operating at night and in bad weather, it quickly became apparent something better was needed.
Interestingly enough, Boeing already had a better system in mind. The company developed a “flying boom,” which featured a telescoping pipe with fins at the nozzle end. The fins were termed “ruddervators” because they functioned as both rudders and elevators. The boom operator, sitting in the B-29’s converted tail turret, literally flew the boom into a receptacle on the upper fuselage of the receiver aircraft. This design allowed more positive control of the air-to-air refueling operation and, with the boom’s four-inch diameter, it offered much faster fuel transfer.
The Air Force responded by ordering more than 100 B-29s fitted with the flying boom system, designated the KB-29P. The first KB-29Ps went into service with the 97th Air Refueling Squadron at Biggs AFB, Texas, on Sept. 1, 1950.
In the meantime, training continued with the KB-29Ms, including periodic attempts at record-breaking flights. For example, in 1948, from Dec. 7 through Dec. 9, a 43rd Bombardment Group B-50A, commanded by Lt. Col. Michael N. W. McCoy, flew from Carswell AFB, Texas, to Hawaii, dropped a practice bomb, and then returned to Carswell. The flight was made possible by KB-29Ms assigned to the 43rd and 509th Air Refueling Squadrons.
The Air Force followed up with a non-stop, around-the-world flight. Again, the 43rd Bombardment Group got the call, although the first attempt on Feb. 25, 1949, came to a quick end when the B-50A “Global Queen” sustained engine problems and landed at Lajes Air Base, Azores.
The next day, the back-up plane, the “Lucky Lady II,” commanded by Capt. James Gallagher, launched from Carswell. The aircraft returned to Texas on March 2, having completed a 94-hour-1-minute flight of 23,452 miles with four in-flight refuelings.
Afterwards, Gen. Curtis E. LeMay, SAC commander, told the news media the obvious: SAC could now deliver an atomic bomb anywhere in the world, and tankers made it possible.
Subsequently, SAC converted its KB-29Ms to a probe and drogue system, using another design pioneered by Flight Refueling International. It featured a refueling hose mounted on an electrically-driven reel inside the tanker, with the receiver aircraft taking on fuel through a fixed refueling probe. While initially tested with bombers, the design later proved particularly useful with fighter aircraft.
However, the B-29’s aging airframe and limited fuel offload capability definitely made it an interim tanker (although the last B-29s didn’t retire from SAC until November 1957). In the meantime, Boeing came up with an improved tanker aircraft, the KC-97.
Based on the Model 377 “Stratocruiser” trans-oceanic airliner, the KC-97 featured a unique double-bubble fuselage with plenty of space available inside for fuel, cargo and passengers, combined with the wings and engines of the Boeing B-50.
The first prototype YC-97A transport served with the Military Air Transport Service during the Berlin Airlift in 1949 and went into full production that same year. In 1950, Boeing introduced the KC-97 variant, equipped with the flying boom system.
Dubbed the “Stratotanker,” the KC-97 quickly became the most numerous SAC tanker, with more than 800 built. The first aircraft went into service with the 306th Air Refueling Squadron at MacDill AFB, Fla., in 1951. By 1953, SAC operated almost 30 air refueling squadrons with 502 tankers, with the majority of the squadrons flying KC-97s. Nearly every B-47 wing had a KC-97 air refueling squadron assigned to it. When B-47s deployed overseas, their tankers went with them, enabling the mass deployment of entire wings of bombers to bases in Europe and the Far East under Operation Reflex.
However, even the new KC-97 operated with several limitations. While a single KC-97 could adequately refuel a B-47, it took two or more to refuel a B-52. Additionally, it took a long time for a fully laden KC-97 to get to its cruising altitude. This forced SAC to deploy its tankers for extended periods to locations in Alaska and Canada, strategically located along the routes the bombers would use to get to their targets. With adequate warning, the KC-97s would get to altitude in time to service the bombers coming from the United States.
However, speed disparity between the KC-97 and its receivers provided the biggest problem. During aerial refueling, the bomber had to slow down and drop to the KC-97’s altitude. Once the aircraft connected, the tanker went into a dive, allowing the bomber to maintain enough speed to stay in the air. As the receiver took on more fuel, it grew heavier, which made the maneuver — known as “tobogganing” — even more difficult. When done in poor or marginal weather, the experience proved even less enjoyable for the aircrews. Once the two aircraft completed the refueling, the jet bomber had to climb back up to its cruise altitude, which burned a lot of the fuel it had just taken on.
Fortunately, a suitable replacement for the KC-97 was already on the way. And this aircraft, still in operation to this day, would feature jet propulsion.
by Ellery D. Wallwork
Headquarters AMC History Office
This is the second in a series of articles highlighting the history of aerial refueling and the important role aerial refueling has played in American military history.
After the success of the Question Mark’s Jan. 1929 aerial refueling flight, and the operational shortcomings of the spring 1929 Army war-game maneuver, the U.S. Army Air Corps spent little time thinking about aerial refueling. This was not to say that nothing was done with the air refueling concept through the 1930s, but most was accomplished thanks to civilian aviators. The Question Mark also rekindled Britain’s interest in air refueling.
From 1930 until 1937, the Royal Air Establishment at Farnborough conducted a series of air refueling experiments. The Royal Air Force looked at air refueling not so much as a way to extend an aircraft’s reach, but more to help lighten take-off weights to reduce wear and tear on the aircraft and grass airfields. They also looked at it as a way to supplement the narrow bomber size restrictions being considered by the League of Nations — less fuel on take-off, meant more bombs could be loaded on the aircraft.
These experiments began with the Question Mark’s techniques (improved by U.S. barnstorming efforts) of the dangle-and-grab method. To accomplish this, the tanker aircraft would feed out a hose that someone in the receiving aircraft reached out and grabbed.
In September 1934, Flight Lieutenant Richard Atcherly introduced his newly patented looped-hose aerial refueling system.
This new technique put most of the operational effort on the tanker crew. Both the tanker and receiver trailed cables with grapnels on the ends. The receiver flew a straight line, while the tanker crossed its path from behind allowing the grapnels to catch. The receiver then reeled in the cables, along with a hose from the tanker. Once the two aircraft were connected with about 300 feet of hose, the tanker pilot would then maneuver to a higher position and let gravity take care of rest.
These experiments continued until 1937, but by then, even the Royal Air Force had decided that air refueling offered a limited application at best. Aircraft technology had surpassed any perceived need for air refueling. Before this date, the standard aircraft were bi-planes (although monoplanes had started becoming more frequent) using “doped” linen fabric and fixed landing gear, with only a little consideration given to aerodynamics.
By 1933, two American corporations built the first all-metal, low-wing monoplanes — the Douglas DC-1 and the Martin B-10 bomber. These aircraft, each about 17,000 pounds, had retractable landing gear, cowled engines, and high-lift devices to improve take-offs. They also used the new controllable pitch propeller. These advances didn’t do much for payloads, but they doubled the DC-1’s and B-10’s airspeed and operating range over their contemporary aircraft.
British commercial interests, however, soon returned to the idea of air refueling. Companies began looking at “flying boats” to connect the widespread British Empire, and they hoped air refueling would improve their operation.
Sir Alan Cobham and Flight Refueling Limited, or FRL, further refined the looped-hose system. In 1939, from Aug. 5 to Sept. 30, Imperial Airways took advantage of the first commercial air refueling operations. The company flew Short S.30 flying boats for weekly mail service flights between Southampton, England, and New York City. FRL used two obsolete Handley Page HP.54 Harrow bombers as tankers — one at Gander, Newfoundland, and the other at Rineana, Ireland. These air refueling operations were not intended to extend the flight times, but to allow the flying boats to take off with minimal fuel and more mail. Imperial Airways flew 15 of these transatlantic missions before the outbreak of World War II.
World War II offered many examples of how air refueling could be used. For example, Britain depended on shipping to stay alive, and aircraft technology provided only limited support. Bomber operational ranges early in the war meant they were not very useful in helping to suppress the German submarine threat. Still, in wartime, many innovations are examined and tested. During World War II, air refueling was among them.
Just after Pearl Harbor, the U.S. Army Air Forces began working on an air refueling solution. With the help of Hugh Johnson, the man who had been in charge of FRL’s Gander operations, they studied three primary concepts. First, planners looked at launching B-17 Flying Fortresses from Midway Island against Japan, with the idea of using modified B-24 Liberators as tankers. Second, they considered using B-24s from Hawaii with tanker support from U.S. Navy seaplanes. The third concept called for B-17s to tow fuel-laden gliders to serve as tankers.
Testing — using a variation of the looped-hose method — began in the summer of 1943 at Eglin Field, Fla. A B-17E served as the receiver and a modified B-24D as the tanker. The successful tests extended the B-17’s range (with three tons of bombs) from 1,000 to 1,500 miles.
The problem now was how would the country’s taxed manufacturers build the equipment for squadrons of B-24 tankers and B-17 receivers? Added to this dilemma was the time required for the aircraft modifications and crew training. Additionally, by mid-1943, Boeing began rolling out the B-29 Superfortress. The B-29 had a combat radius of 1,500 miles and carried twice the bomb load of the B-17.
In 1944, the U.S. Army Air Forces began studying the feasibility of equipping B-29s with an air refueling capability. The engineers at Wright Field, Ohio, determined it was possible to modify the aircraft, but the 1,500-gallon capacity of a B-24 tanker only
by Ellery D. Wallwork
Air Mobility Command History Office
This is the first in a series of articles highlighting the history of aerial refueling and the important role aerial refueling has played in American military history.
On Jan. 1, 1929, a tri-engined Fokker C-2 aircraft with a crew of five climbed into the southern California sky. This aircraft, dubbed the “Question Mark,” was not history’s first air refueling mission, but it played a crucial role in the beginning of air refueling efforts and the development of the U.S. Army Air Corps.
The flight, born from the ingenuity of Airmen through their experiences in World War I, lasted from Jan. 1-7, 1929; a total of 150 hours and 40 minutes. The crew flew a 110-mile racetrack from Santa Monica, Calif., to San Diego, Calif. They also flew over the New Year’s Day Rose Bowl football game. During the flight, they made 43 contacts with the tanker aircraft. Each contact lasted about 7.5 minutes, with the aircraft about 15 to 20 feet apart. Day-time contacts took place at an altitude between 2,000 and 3,000 feet, and the 10 night-time contacts took place between 5,000 and 7,000 feet.
The receiving aircraft, the Question Mark, a Fokker C-2, was a high-winged monoplane with two 96-gallon wing tanks supplemented by two 150-gallon tanks installed in the cabin. The two refueling aircraft were Douglas C-1 single-engine bi-planes with two 150-gallon tanks for offloading and a refueling hose passed through a hatch cut in the floor.
All told, the Question Mark received 5,700 gallons of fuel. During the contacts, the tanker crews also passed oil, food, water and other miscellaneous items, by means of a rope. Neither the Question Mark nor the two refuelers were equipped with radios because of the radios weight and unreliability,. The crews maintained communications via notes dropped to the ground, hand and flashlight signals, and written messages displayed by ground panels and both planes.
The Question Mark’s crew consisted of Maj. Carl Spatz (he later changed the spelling to Spaatz), Captain Ira Eaker, Lieutenant Elwood Quesada, Lt. Harry Halverson, and Staff Sgt. Roy Hooe. The crews of the tankers were Capt. Roy Hoyt and Lts. Auby Strickland and Irwin Woodring in the No. 1 aircraft, and Lts. Odas Moon, Joseph G. Hopkins and Andrew F. Salter were in the No. 2 aircraft. Capt. Hugh Elmendorf was in charge of ground operations and logistics for the mission.
Air refueling was still considered by many to be a modern marvel, and it had humble beginnings. The first attempts were in 1921 with the employment of five-gallon gas cans when a U.S. Navy lieutenant, in the back of a Huff-Daland HD-4, used a grappling hook to snag a gas can from a float in the Potomac River. In another attempt, a wing walker with a gas can strapped to his back, climbed from an airborne Lincoln Standard to a Curtiss JN-4 to pour gas into the aircraft’s tank.
While these two publicity stunts deserve mention, the first air-to-air refueling using a gravity-flow hose occurred in 1923. Earlier that year, the Army Air Service had equipped two de Haviland DH-4Bs with in-flight hoses. After installation, testing and preparation, the Army Air Service was ready to put it to use. On June 27, one of the DH-4s flew a six-hour-and-38-minute flight that included two air refuelings.
However, the early days of air refueling weren’t without danger. Navy Lieutenant P. T. Wagner, the pilot of the refueler was killed during testing in 1923when the refueling hose became entangled in the right wings of the two aircraft.
At that time, the Army’s budget was very limited, and the aviation branch in particular, had not recovered from the 1919 demobilization. The tests in 1923 attempted to show the practicality of air refueling with a flight demonstration that consisted of a 37.25 hour long record-setting 3,293 mile distance record set in August, and again with a border-to-border flight from Lamas, Wash., to Tijuana, Mexico, in October.
Between the budgetary constraints and the lack of an actual application, the air refueling testing slowly ground to a halt. The Nov. 18 accident caused the Air Service to stop it altogether.
The idea for the Question Mark flight started with Lt. Quesada. Years later, General (retired) Quesada recalled that the mission was actually an incidental thought rather than a planned objective.
Additionally, by 1928, Belgium had restarted air refueling experiments, picking up where other countries had left off. In the process, the Belgians set a new record of 60 hours and 7 minutes aloft. Also in 1928, a German aircraft, The Bremen, attempted to fly across the Atlantic.
However, it was forced to land in a barren area of Labrador. When the German government requested help from the U.S. State Department, the Army Air Corps accepted the task. Major Gen. James Fechet, head of the Air Corps, led a flight team which consisted of Quesada and Capt. Eaker. Despite poor weather and periods of heavy ground fog, they found The Bremen and her crew safe and sound. Quesada said he was surprised when Captain Eaker “decided to go over the ground fog. I said, my God, what are we going to do if we get caught up here. So then I began to think, my God, wouldn’t it be nice if we had a gas station. We could just pull in to a gas station and fill up with gas again.”
Captain Eaker took that idea a step further and began organizing the effort for a prolonged refueling technique, with a demonstration that would attract a lot of attention for the Air Corps.
The Question Mark’s mission portended little militarily. Based on the success of this air refueling mission, the Army Air Corps scheduled a formal demonstration in the spring of 1929 as part of an Army war game maneuver.
During the demonstration, a Keystone B-3A bomber was to fly, accompanied by a Douglas tanker, from Dayton, Ohio, on a simulated bombing mission over New York City’s harbor, and then return. Refueling was to occur over Washington, D.C., during both parts of the mission. However, a network of thunderstorms between Ohio and Washington caused the aircraft to become separated.
Icing conditions forced the tanker to make an emergency landing in Uniontown, Penn., where it lodged itself in the mud. The bomber successfully pressed on to New York City and returned to Washington without the tanker’s support.
With this disappointment, the U.S. Army Department shelved the idea of air refueling for another 12 years.
Still, in its primary objective, the Question Mark was a huge success.
“It got tremendous public attention, which is exactly what [we] had in mind,” said General Quesada. “The Question Mark had no noble purpose. It wasn’t going to create an operational procedure that would plunge the Air Force into a great superior power that would make it unnecessary to have an Army or a Navy. The purpose was to attract attention. I think it would be somewhat abusive not to recognize that.”
In fact, it captured the public’s imagination. American aviators were enthralled with the concept of air refueling. By May 26, 1929, a pair of commercial pilots in Texas, using a reconditioned Ryan Brougham monoplane, broke the Question Mark’s record with 172 hours and 32 minutes in the air.
For the next several years, the record continued to be extended. It also sparked more interest among British aviators. One of them, Flight Lt. Richard Atcherly, invented one of the first formal air refueling systems.