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Aerial refueling

Aerial refueling, also called in-flight refueling (IFR) or air-to-air refueling, is the practice of transferring fuel from one aircraft to another during flight. This allows the receiving aircraft to remain airborne longer, and to take off with a greater payload. Usually, the aircraft providing the fuel is especially designed for the task.

Table of contents
1 History and development
2 Aerial refueling systems
3 Strategic and tactical implications
4 Tanker aircraft by refueling system

History and development

Some of the earliest experiments in aerial refueling took place in the 1920s (?), when it was as simple as two slow-flying aircraft flying in formation, with a hose run down from a handheld gas tank on one airplane and placed into the usual fuel filler of the other. Nowadays, specialized tanker aircraft have equipment especially designed for the task of offloading fuel to the receiver aircraft without spillage, even at the higher speeds modern jet aircraft typically need to remain airborne.

Aerial refueling systems

The two most common approaches for making the union between the two aircraft are the boom and receiver system and the probe and drogue system.

KC-135R Stratotanker, two F-15ss (twin fins) and two F-16ss, on an aerial refueling training mission.

Boom and receiver

The boom is a long, rigid, hollow shaft, usually fitted to the rear of the aircraft. (It almost connects the two lower aircraft in the picture at right.) It usually has a telescoping extension, a valve at the end to keep fuel in and permit it to flow, and small wings, sometimes known as ruddevators depending on design particulars (visible in picture below, in the "V" shape), to enable it to be "flown" into the receiver of the aircraft to be refueled. This receiver is fitted onto the top of the aircraft, on its centerline and usually either behind or close in front of the cockpit. The receiver is a round opening which connects to the fuel tanks, with a valve to keep the fuel in when not being refueled, and dust and debris out. The boom has a nozzle which fits into this opening.

USAF C-5 approaches a KC-135R

During refueling operations, a tanker aircraft will fly in a straight and level attitude at constant speed, while the receiver takes a standard position behind and below the tanker. Modern tankers have lights which illuminate the areas outside this range, so that if the pilot can see them, he is directed to fly back towards the desired spot. Once in position, the receiver pilot flies formation with the tanker, although this can be complicated by turbulence in its wake. The crewman operating the boom, called a boomer or boom operator (in the USAF, usually an enlisted sergeant), then unlatches the boom from its stowage position, and directs it towards the receiver by "flying" it with the attached wings. The telescoping section is then hydraulically extended until the nozzle fits into the receiver. When an electrical signal is passed between the boom and receiver, both valves are hydraulically opened, and pumps on the tanker drive fuel through the shaft of the boom, and into the receiver. (Some may use gravity feed only, but I doubt it.) Once the two are mated up, additional lights on the tanker will be turned on if the receiver flies too far to one side, too low or too high, or too near or too far away, activated by sensing switches in the boom. When fueling is complete, the valves are closed and the boom is automatically retracted.

Probe and drogue

USMC AV-8B Harrier refuels from a KC-10 drogue.
The drogue, sometimes called a basket, is a fitting resembling a plastic badminton shuttlecock (not the feathered kind), attached to a flexible hose at its narrow end, with a valve where the two meet. It is carried by the tanker, as seen in the picture at right, near where the Harrier's left wing meets the body. The receiver has a probe, which is a rigid, sometimes jointed, arm placed usually on the side of the airplane.

Again, the tanker flies straight and level, and the drogue is allowed to trail out behind and below it. The drogue is not controllable other than by flying the tanker, so the receiver pilot must fly his probe directly into the basket, at which point wind drag on the basket forces the probe into the valve, which opens to allow fuel to be pumped through. The receiver maintains his position during refueling, keeping an eye on the hose to make sure he remains in a suitable position. When fueling is complete, he decelerates hard enough to yank the probe out of the basket.

Some boom-carrying tankers have special hoses which can be attached to the nozzle of the boom to allow them to also refuel probe-equipped aircraft. Others may have both a boom and one or more hose-and-drogue assemblies equipped.

Strategic and tactical implications

(This may take a while...)

Strategic uses and considerations

F-101A Voodoo (top right), B-66 Destroyer (top left) and F-100D Super Sabre refuel from a KB-50J tanker. Taken in the early 1960s.
The early development of the KC-97 and KC-135 Stratotankers was inspired by the desire of the United States to be able to keep fleets of B-47 Stratojet and B-52 Stratofortress strategic bombers aloft during the Cold War, either to retaliate against a Soviet strike and ensure Mutual Assured Destruction, or to bomb the U.S.S.R. first had it proven necessary. The bombers would fly orbits around their assigned positions from which they were to enter Soviet airspace if they received the order, and the tankers kept the bombers' fuel tanks full so that they could keep a force in the air 24 hours a day, and still have enough fuel to reach their targets in the Soviet Union. This also ensured that a first strike against the bombers' airfields could not obliterate the U.S.'s ability to retaliate. A famous example of refueling used in this manner in movies can be seen in the opening credits of Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (a fictional movie, but the scenes are from actual B-52 refueling).

A byproduct of this development effort and the building of large numbers of tankers was that these tankers were also available to refuel cargo aircraft, fighter aircraft, and ground attack aircraft, in addition to bombers, for ferrying to distant theaters of operations. This was much used during the Vietnam War, when many aircraft could not have covered the transoceanic distances without aerial refueling, even with intermediate bases in Hawaii and Okinawa. In addition to allowing the transport of the aircraft themselves, the cargo aircraft could also carry materiel, supplies, and personnel to Vietnam without landing to refuel.

(More about post-Vietnam deployment of aircraft to come; sadly, I don't know squat about other nations' use of strategic refueling.)

Tactical uses and considerations

The capability of refueling after takeoff conveys two considerable tactical advantages to those with tankers. Most obviously, it allows attack aircraft, fighters, and bombers to reach distances they couldn't otherwise, and patrol aircraft to remain airborne longer. Additionally, since an aircraft's maximum takeoff weight is generally less than the maximum weight with which it can stay airborne, this allows an aircraft to take off with only a partial fuel load, and carry additional payload weight instead. Then, after reaching altitude, the aircraft's tanks can be topped off by a tanker, bringing it up to its maximum flight weight.

Vietnam War

Falklands War

Gulf War

Kosovo War

Tanker aircraft by refueling system

Boom and receiver

Probe and drogue