In-flight refueling was one way to set airborne endurance records. The Atlantic-Fokker X-2A "Question Mark" stayed aloft for seven days in January of 1929, refueling 42 times in the air. Because radio communication was unreliable at this time, the pilots in the two planes communicated by notes dropped to the ground and by hand signals, flashlight signals, ground panels, and messages written on blackboards carried in the planes.
A Boeing KB-29P refueling in flight using the flying boom system. This is the most common method for in-flight refueling.
In the rigid flying boom system of aerial refueling, the pilot of the receiving aircraft flies behind and below the tanker for refueling.
A B-47 Stratojet taking on a load of fuel. The B-47 Stratojet was the first jet-to-jet refueling tanker.
A Boeing B-52H being refueled by a Boeing KC-135A.
The C-130 Hercules was originally designed during the 1950s as an assault transport but was adapted for a variety of missions including mid-air refueling of helicopters.
Aerial refueling expanded helicopter rescue capability. Here a U.S. Air Force CH-3C simulates refueling from a Marine Corps KC-130F tanker during a test on December 17, 1966.
Two Northrop B-2As and a McDonnell Douglas KC-10A Extender.
Aerial refueling allows aircraft engines to receive fuel while in flight and today is common for many large air forces. It is the equivalent of refueling your car by connecting it to a tanker truck while driving down the highway at high speed.
In 1917, a pilot in the Imperial Russian Navy, Alexander P. de Seversky, proposed increasing the range of combat aircraft by refueling them in flight. De Seversky soon emigrated to the United States and became an engineer in the War Department. He applied for and received the first patent for air-to-air refueling in 1921.
The first actual transfer of fuel from one aircraft to another was little more than a stunt. On November 12, 1921, wingwalker Wesley May climbed from a Lincoln Standard to a Curtiss JN-4 airplane with a can of fuel strapped to his back. When he reached the JN-4, he poured the fuel into its gas tank. Needless to say, this was not the most practical way of refueling an airplane in flight.
In 1923, the U.S. Army undertook tests at Rockwell Field, San Diego, California, to test a more practical way to lower a hose from one airplane to refuel another in flight. In its tests, a DH-4B biplane outfitted as a tanker and equipped with a 50-foot (15-meter) length of hose and a quick-acting shutoff valve would fly above the receiver and lower the hose. The person in the rear seat of the receiver aircraft would grab the hose and connect it to the aircraft. If the hose became detached, the valve would immediately cut off the flow, preventing it from spraying fuel over the receiving aircraft and its pilot.
The first flight was made on April 20, 1923. The aircraft remained attached for 40 minutes but intentionally passed no fuel. The equipment was tested over the next several months with numerous fuel transfers. On June 27, the pilots made an attempt on the aircraft flying endurance record. By August 27, using this technique, one of the DH-4Bs established 14 world records with a flight lasting more than 37 hours.
This achievement prompted many private pilots to attempt aerial (or in-flight) refueling, primarily to establish long duration flying records. By June 1930, the record surpassed 553 hours in flight (requiring 223 refueling contacts). In July, the record was 647.5 hours in the Curtiss Robin monoplane Greater St. Louis—nearly 27 days in the air. Pilots lived in the noisy, cramped, smelly confines of their airplanes for weeks at a time without ever touching the ground, occasionally climbing out on special scaffolding to service the engines in flight.
Despite all this activity, the technology for aerial refueling had not advanced significantly and pilots still used the clumsy and dangerous dangling-hose method. In 1930, a Royal Air Force (RAF) squadron leader, Richard L.R. Atcherly, developed a safer and simpler method, called the looped hose method. In this method, the receiving aircraft trailed a long horizontal line with a grapnel at the end. The tanker trailed a weighted line and approached the receiver from behind and to one side. It then crossed to the other side, causing the two lines to cross and touch. The receiver aircraft then hauled in the lines and the hose from the tanker. The RAF continued to refine this system, including adding a drogue to the hose that created drag and assisted in unwheeling the hose in flight. (A drogue is a special type of parachute that, in this instance, was used to ensure that the hose trailed behind the airplane and did not flop around.)
By 1934, Alan Cobham of Britain had established the firm Flight Refueling Limited (FRL) to develop the small but important fittings and hose connections that enabled aerial refueling to be performed routinely. Cobham thought that aerial refueling would have great advantages for commercial aviation. He was wrong though, for commercial aircraft never did use his techniques, but he was later knighted for his contributions to this field.
World War II brought about a hiatus in aerial refueling technology development as combatants sought to develop extremely long-range aircraft with large internal fuel capacity. In 1942, representatives of FRL visited the United States to fit their equipment to a B-24 Liberator tanker and a B-17 Flying Fortress receiver. The Army Air Forces planned to develop fleets of tanker and receiver aircraft. However, aircraft with large internal fuel capacity, such as the B-29 Superfortress, alleviated the need for aerial refueling.
In 1948, U.S. Air Force General Curtis LeMay became head of the Strategic Air Command (SAC) and made aerial refueling a major goal for his new command. LeMay realized that the jet-powered bombers then entering service consumed far more fuel than piston-engine planes and also needed to fly farther—from the United States to targets deep in the Soviet Union and back. Existing aerial refueling systems had severe drawbacks. In particular, the hoses could not transfer large amounts of fuel and could not operate at higher speeds.
Around the same time that LeMay began pushing for better aerial refueling methods, the Boeing Company began testing the "Boeing boom" system, consisting of a large-diameter pipe connected to the rear of a B-29 and fitted with small wings at the end. The boom was lowered and "flown" to a connector on the receiver aircraft. This allowed fuel transfers to take place at higher speeds and, more importantly, allowed more than six times as much fuel to flow per minute. Another important development was the "single-point refueling system" on receiver aircraft, which allowed all of an airplane's several fuel tanks to be refilled from a single spot instead of from multiple nozzles around the airplane.
While the Air Force and Boeing were developing the flying boom, FRL was continuing its work in Britain, trying to develop a system that would enable a single-seat aircraft to refuel from a tanker. FRL engineers developed the "probe and drogue" system whereby a small plane was equipped with a probe that could be plugged into a drogue at the end of a refueling hose trailing behind a tanker. FRL conducted its first test on April 4, 1949, and soon the U.S. Air Force expressed interest in this technology.
Over the next several years the U.S. Air Force began converting existing bomber and transport designs, resulting in the KB-29 and KC-97 tankers. But the increasing demands of SAC led to the procurement of the Boeing KC-135, a dedicated tanker aircraft that was similar (but not identical) to the commercial Boeing 707 airliner. During the 1950s, under LeMay's strong advocacy, SAC built up a large KC-135 tanker fleet to support its B-52 bombers, which could not attack targets inside the Soviet Union without refueling.
During the late 1940s and early 1950s the Soviet Union also experimented with aerial refueling. Three Soviet Tupolev Tu-4 bombers (a virtual copy of the American B-29 Superfortress) were equipped as tankers and three were equipped as receivers. The Soviets experimented with a strange wingtip-to-wingtip system as well as a more conventional probe and drogue system. But the Soviets abandoned the technology until the early 1960s. Even then, they never became as enthusiastic about aerial refueling as the U.S. Air Force, preferring to operate smaller aircraft closer to their bases.
The U.S. Air Force operated for many years using both the flying boom and probe and drogue systems. It finally phased out the latter in favor of the boom, which could operate at significantly higher speeds and deliver fuel much faster. The Navy preferred the probe and drogue system, which could be mounted on smaller carrier-based aircraft. Navy engineers also developed a "buddy stores" system. This consisted of a fuel tank with a hose and drogue and enabled one aircraft to refuel an identical aircraft and did not require dedicated tanker aircraft.
The search and rescue mission in Vietnam increasingly required helicopters to fly great distances to rescue downed airmen. As a result, in 1965 the Air Force equipped a CH-3 Jolly Green Giant helicopter with a refueling probe at the end of a long boom extending from the helicopter's nose and conducted experiments on refueling it from a U.S. Marine Corps KC-130 aircraft (a variant of the C-130 Hercules) equipped with a hose and drogue system. The tests were successful and soon helicopters were regularly making flights deep into North Vietnam to rescue pilots, using aerial refueling to extend their range.
With the introduction of the KC-10 Extender aircraft in the early 1980s, the Air Force incorporated a number of new features, including both a boom and two hose and drogue systems, allowing it to refuel both Navy and Marine aircraft in flight.
Despite all the technological advances, commercial aircraft designers never adopted aerial refueling. They preferred to build aircraft with large internal fuel tanks because this was cheaper than operating a dedicated fleet of aircraft that simply served as flying gas tanks. Aerial refueling is now exclusively a military operation, despite Sir Alan Cobham's vision of using it for commercial aircraft.
--Dwayne A. Day
Sources and Further Reading:
Butowski, Piotr. Russian Flight Refueling, Part 1. Air International, August 1998.
___________. Russian Flight Refueling, Part 2. Air International, August 1998.
Hopkins, Robert S. Boeing KC-135 Stratotanker. Leicester, UK: Midland Publishing Inc., 1997.
Latimer-Needham, Cecil H. Refueling in Flight. London: Sir Issac Pitman & Sons, Ltd., 1949.
Smith, Richard K. Seventy-Five Years of Inflight Refueling. Washington, DC: Air Force History and Museums Program, 1998.