Tankers - History
The idea of refueling aircraft in flight was born nearly 80 years ago, and, within a few decades aviation refueling moved from the age of dare devils and barnstormers to the routine refueling of all types of aircraft, in all conditions, all over the world. In the 1920s when aviators experimented with attempts to snag gasoline containers from propositioned floats with grappling hooks. In 1921, stunt pilot, Wesley May, put on a "refueling" demonstration for a crowd at Long Beach, California. May strapped a gas can to his back and walked out to the wing tip of his Lincoln Standard biplane, stepped onto the wing skid of a Curtis JN-4 and poured five gallons of fuel into the Jenny's tank. This feat was proclaimed the first "air-to-air" refueling.
Although these dare-devil feats were labeled barnstorming gimmicks, military aviators were the ones who recognized the value of air refueling. The basic complaint of aviators during World War I was that they could not stay aloft for more than 20-40 minutes before they had to return to base for refueling. Army Air Service Lieutenant John Richter sought help from Major Hap Arnold, then commander of Rockwell Field in San Diego to test the idea of using the force of gravity. They tested this refueling idea with success. In 1923, he and Lieutenant Lowell Smith flew their DH-4B aircraft aloft for more than 37 hours. They contacted each other fifteen times to receive oil, supplies and 75 gallons of gasoline by means of a fuel hose.
This exchange was followed by others, including a few that ended in tragedy. But, it was a Fokker C-2 trimotor monoplane called the Question Mark and two Douglas C-1 biplanes which demonstrated the value of mid-air refueling. The Question Mark was fitted with additional tanks to receive fuel; the biplanes, dubbed RP #1 and RP #2, were configured with two 150 gallon fuel tanks and a 50 foot hose with a lead weight attached to the end. The hose would be lowered through a trap door in the bottom of the C-1's fuselage. Despite numerous problems, which in themselves are a great story, the refueling was a great success. The Question Mark was kept aloft for more than six days during which time it had received 40 tons of material, including 5,660 gallons of gasoline, 245 gallons of oil, meals, water, batteries and other supplies.
Even with this success, refueling progress was limited. When the US was drawn into World War II, to reach their German targets bombers had to be based in England and the Mediterranean area. With no refueling capability, fighter escorts with limited range had to return to base for refueling. The Luftwaffe waited until the escorts headed for home and then attacked the bombers. On the Pacific front, inadequacies of long-range bombing also became clear. A proposed attack on Tokyo from Hawaii was scrubbed because of lack of refueling crews and equipment. Even though tests during 1943 and 1944 proved successful, the end of the war thwarted further development.
By 1946, when the Strategic Air Command (SAC) was activated at Bolling AFB, planners realized that in order for SAC to fulfill its global mission, in-flight refueling would have to be part of the program. The Air Materiel Command (AMC) contracted with Boeing Airplane Company to study transferring fuel by hose between two B-29s. The process called for the tanker and bomber to fly in formation at just under 200 knots and about 20 feet above and 75 feet behind, with the tanker above, ahead and to the left. A cable called a "contact line" with a 50 pound lead weight on the end trailed from the tanker; the receiving B-29 trailed a cable called the "hauling line" from its refueling receptacle. A grappling hook at the end of the hauling line caught the tanker's contact line during the crossover and a winch in the tanker wound in both lines. The tanker operator then attached the hauling line to the hose, another winch in the receiver aircraft pulled the hose out of the tanker. Once the hose nozzle was in place, it was locked with hydraulic toggles. A "contact made" signal was sent to the tanker and fuel transfer was started.
The process went through trials, and ultimately, the method proved successful. So much so, that the Air Force authorized 92 B-29s converted to hose-equipped tankers and another 74 modified to act as receivers. In June 1948, the 43d and 509th Air Refueling Squadrons, based at Davis-Monthan and Rosewell AFBs respectively, became the first air refueling units in the United States Air Force. The KB-29 was to become the work horse of the Air Force through the mid-1950s.
The issue of range extension was especially important early in the Korean War when Far East Air Force [FEAF] aircraft were operating out of airbases in Japan. The distance to North Korea left the fighters and bombers with less than a half hour of combat flight time before they had to return to base. Even once the North Korean Army was pushed back across the 38th Parallel and FEAF was operating from Korean bases, the fighters desperately needed a method to extend their time in the combat zone.
To sustain this new level of global flexibility at the outset of the Korean War, the Air Force needed air refueling procedures fitted for combat aircraft. The KB-29M tested the probe-drogue system. One KB-29 trailed refueling hoses from pods on both wing tips and one from a centerline position The remaining tankers had only one centerline hose. Fighter aircraft flew into position behind the tanker so that its probe could be inserted in the drogue at the end of the tanker's hose.
The system was good, but too limited. Boeing was hired to build an aerodynamically controlled, swiveling and telescoping arm known at the "Flying Boom." The most practical method up to the Korean War was the “flying boom,” which involved a remote-controlled rigid telescopic boom containing a tube through which fuel flowed. This boom was extended by an operator in the tanker aircraft and then engaged by a probe-equipped receiver aircraft. The boom was controlled by an operator positioned in the B-29s tail turret. The operator "flew" the boom nozzle by means of airfoils called ruddevators into a receptacle, usually atop the receiver's fuselage. The pilot stayed in contact by following a series of colored lights along the base of the boom to indicate "right," "left," "fwd," "aft," etc.
The highest rate of fuel transfer was 600 gallons per minute while the aircraft flew 250 miles per hour. This system was dependent on an uninterrupted communications link between operator and receiver. It was extremely dangerous because an accidental sudden or rapid extension of the boom while the two aircraft were coupled could cause a mid-air explosion as the tanker “pitched down.” Engineers at AMC tested the “boom” method on a B-50D receiver and a KB-29P tanker to determine the need for extra power while flying in the downwash of the tanker during the refueling mission. Additional tests evaluated the most suitable altitude for refueling, the best means of approach, and the best position. By late 1952, the flying boom method was installed on or planned for 116 KB-29P and 750 KC-97 tankers and 213 B-50s, all B-47s and B-52s, and all F and G models of the F-84.
By September 1950, an additional in-flight refueling system, called the “probe and drogue,” had been developed in Great Britain. This method involved the use of a flexible hose that was unreeled from the tail, bomb bay, or wing tips of a tanker. The receiver pilot directed a specially designed probe tip into the cone-shaped drogue attached to the hose, thereby allowing the fuel supply to be replenished. The high-rate-of-flow hose reel unit (the Mark 14) was capable of dispensing 600 gallons of fuel per minute at speeds of 185 to 300 miles per hour. By late 1950, the new method was receiving much attention by the United States Air Force, and it was being studied by personnel in the laboratories at AMC for possible use in Korea. In December 1950, HQ Air Force requested the Wright Air Development Center [WADC] Engineering Division to project the time and effort required for installing this method for refueling the external tanks on the F-80s, F-84s, and F-86s in Korea. Republic provided cost and schedule estimates to the Air Force on converting F-84G flying boom receivers to probe receivers. Boeing also converted the B-47 into a bomber-tanker with probe and drogue equipment. The Air Force also chose to install the system on B-29 and C-119 aircraft, making them quickly convertible to tankers. WADC’s Flight Test Division began a series of aerial demonstrations to prove to skeptical Strategic Air Command officials that the drogue system was superior to the boom in both fighters and bombers.
In 1952, while tests were continuing at WADC, a Weapons Systems Division report stated, “this method of in-flight refueling is feasible for heavy aircraft under conditions of high speeds and rapid fuel transfer rates.” According to Captain E. B. Owens, a project officer for the probe and drogue, the system could refuel three receivers at one time, at altitudes up to 25,000 feet, in smooth or turbulent air, day or night. Furthermore, the receiving and tanker aircraft needed little modification to allow them to use the system, and the conversion of a bomber to a tanker could be completed in the field in less than 8 hours. Early in July, the Air Council recommended that the probe and drogue method become tentative-standard for in-flight refueling, and HQ Air Force confirmed this decision, directing that the method be the only refueling equipment installed in convertible bomber-tanker and cargo-tanker aircraft. HQ also requested immediate steps be taken to outfit specified B-29s and B-50s as tankers. WADC monitored contractor conversions of several prototype tankers, including two B-29s, a B-36, a B-47, a KC-97, and two C-119s. In 1953, the B-52 was scheduled for flying boom refueling equipment to be used in conjunction with the KC-97 tanker, but flight tests conducted near the Boeing plant in Seattle, Washington, revealed a pilot preference for the probe and drogue with the B-47 as tanker. 39
By the summer of 1951, the probe and drogue system was in use by the 116th Fighter-Bomber Wing in Korea. On 6 July 1951, a KB-29M tanker outfitted with the probe and drogue system refueled four RF-80s over North Korea during a reconnaissance mission. This was the first time the system was used in an actual combat situation. On 28 September of the same year, a second refueling mission in Korea allowed an F-80C to fly for over 14 ¼ hours. This plane, heavily loaded with armament, spent over 10 hours in the combat area and refueled six times from a KB-29M tanker.
The new system was not without problems; however, these were minor and fixed easily. Tests revealed buffeting in the bomb bay of the tankers and trouble with the hose lines and reels. The Power Plant Laboratory investigated the probe and drogue system failures in the Far East and concluded that the different aromatic content of fuels adversely affected the probe kits sent to FEAF. An improved probe kit was sent and this solved the problem.
WADC engineers in the Aircraft Radiation Laboratory also worked to develop electronic homing systems that would allow receiver aircraft to automatically navigate to a tanker from a distance up to 200 miles. This system involved the AN/APN-69 radar beacon, which underwent testing at Air Proving Ground, Florida, in mid-1952. The production drawings were approved and released in the fall of 1952, and the manufacturer, Sperry Gyroscope Company, continued to fabricate and test the subassemblies for the beacon.
The benefits of in-flight refueling extended well past combat use. Fighters no longer had to be transported via aircraft carriers, which was a slow process. Not only was the in-flight trip shorter, but the planes were not exposed to corrosion from sea air and they were not grounded for several weeks upon arrival for maintenance and repairs. The practicality of transoceanic flights using the boom method was proven in July 1952, when 58 F-84Es of the 31st Fighter-Escort Wing took off from Turner AFB, Georgia, en route to Misawa Air Base, Japan. The trip took 11 days.
Although the flying boom method seemed adequate, pilots and engineers alike were partial to the probe and drogue. WADC believed that the method had greater potential and better performance than the flying boom. By 1953, the B-36, B-47, and B-52 were proven to work sufficiently as probe and drogue tankers. WADC was studying the Mark 15 and Mark 17 hose reels, the latter of which would be capable of transferring between 1,000 and 1,200 gallons per minute. The British-designed Mark 15 reel was the prototype for an American-engineered and built A-16 reel, expected for production in April 1954. Strategic Air Command continued to favor the flying boom simply because the only logistics tanker in service or in development was the KC-97, which was only equipped with the flying boom equipment. WADC was planning a replacement for the KC-97 for 1958.
Various other methods of range extension were attempted during this time period. These included fabrication of a fuel wing, wing tip coupling, parasite fighters carried within bombers, and the possibility of towing aircraft, specifically helicopters, to the field of operations. Tests proved all three methods, however impractical, were feasible. Slow equipment deliveries, contractual problems, and fatal crashes during testing plagued these programs throughout the early 1950s. Simple in-flight refueling or the attachment of accessory external fuel tanks remained the best means of range extension.
To meet the more powerful aircraft being developed, the KB-29s were phased out with the development of the more powerful KC-97 class of tanker-transporter. The primary improvements were speed and a better boom position to place the nozzle more precisely into the receptacle. Despite propeller problems, the KC-97 went on to set many records. But its most important contribution to air flight was its ability to refuel the short range aircraft such as the B-47 to serve as long-range bombers. It served the military warfighters well from 1956-1965.
The Boeing Military Airplane Company's model 367-80 was the basic design for the commercial 707 passenger plane as well as the KC-135A Stratotanker. In 1954 the Air Force purchased the first 29 of its future fleet of 732. The Boeing KC-135 jet tanker was destined to dominate air refueling for decades thereafter, and continues to play a significant role in air refueling into the twenty-first century. Record-breaking flights were commonplace with the KC-135. They played an integral part in the SAC decision to keep a portion of its alert force airborne at all times. The mission of the B-52s was called "CHROME DOME." Missions averaged 24 hours in length and covered the southern route crossing the Atlantic, the Med and then return to the U.S.; the northern route up the eastern coast of Canada, across Canada west toward Alaska, then down the west coast of North America. This mission lasted seven years.
The other KC-135 historic achievement was its support of refueling missions in Southeast Asia. One of the most interesting accomplishments was the "Silent Refueling" missions of ARC LIGHT. Most missions consisted of between 18 and 30 tankers and an equal number of bombers. The entire process was accomplished without any form of vocal communication. Takeoff came by way of a green light from the tower. Tankers launched one minute apart with three minute spacings between formations. The tankers rendezvoused with the receiving B-52s by electronic beacon, often in bad weather, and usually at night. The tankers off-loaded about 12,256 gallons of fuel to each B-52, then returned to base. By 1972, there were 172 KC-135s flying from six bases. Monthly tanker sorties peaked at 3900 in support of fighter and bomber operations. From 1964 to the end of the war, KC-135s had flown 194,687 sorties, providing nearly 9 billion pounds of fuel in 813,878 refuelings. Truly representative of American mastery of the skies.
After nearly 25 years of service, the KC-135 was scheduled for a major modification to address metal fatigue in the wings and inefficiency of the J-57 engines. Even though tight budgets slowed down the reengineering effort, by 1990, a large portion of the fleet had been updated and return to service.
Responding to the need to be able to move air power into theaters of operation quickly, the Air Force looked to a concept known as ATCA - the Advanced Tanker/Cargo Aircraft. The demand for large cargo, as well as fuel transfer capabilities, led the military toward a wide body jet. McDonnell-Douglas had a DC-10 in operation. Work began on modifying this commercial aircraft to meet the military's specific needs. The most important logistical advantage to this aircraft was that the plane's components were interchangeable with the commercial DC-10. Since this plane was already used around the world, the Air Force could make use of an existing worldwide supply network.
Advantages over the KC-135 are notable. The KC-10 can be refueled using a universal receptacle atop the fuselage just aft of the cockpit. It can also refuel receivers using both the boom and probe-drogue methods on the same mission. With additional tanks added to the normal DC-10 tanks, the fuel capacity is nearly twice that of a KC-135 at 356,065. The boom area is also radically different. In the KC-135, the boom operator works in a prone position. In the KC-10, there is a small room with three seats and a large 26x54 inch window for the operator to see the receiver. A system of mirrors allows the "boomer" to monitor other aircraft in formation off the KC-10 wing tips. The boom is 10 feet longer than the KC-135s and features "fly by wire" flight controls and an increased fuel transfer rate of up to 1,500 gallons per minute. The KC-10 has served as an invaluable tool in the military forces. Perhaps too often , the tanker force goes unrecognized in the accomplishment of feats that attract media coverage to fighter aircraft. But, in the greater scheme of things, they have the satisfaction where it counts-from pilots who rely on them everyday to be there when they need refueling. And, they always are - refueling the forces.