In 1917, a
pilot in the Imperial Russian Navy, Alexander P. de Seversky, proposed
increasing the range of combat aircraft by refuelling 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 refuelling 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 refuelling 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)
refuelling, primarily to establish long duration flying records. By June
1930, the record surpassed 553 hours in flight (requiring 223 refuelling
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 refuelling 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 Refuelling Limited (FRL)
to develop the small but important fittings and hose connections that
enabled aerial refuelling to be performed routinely. Cobham thought that
aerial refuelling 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 refuelling 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
refuelling.
In 1948, U.S.
Air Force General Curtis LeMay became head of the Strategic Air Command
(SAC) and made aerial refuelling 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 refuelling 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 refuelling 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
refuelling 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 refuelling 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.
During the
late 1940s and early 1950s the Soviet Union also experimented with aerial
refuelling. 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 refuelling 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 favour 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 refuelling probe at the
end of a long boom extending from the helicopter's nose and conducted
experiments on refuelling 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 refuelling 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 refuelling. 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 refuelling is now
exclusively a military operation, despite Sir Alan Cobham's vision of
using it for commercial aircraft.