Air-to-air refueling, also known as aerial refueling
or in-flight refueling
, is the practice of transferring aviation
fuel from one aircraft (typically called the tanker
) to another while still in flight. So far, so simple. It was the idea of a Russian naval pilot, Alexandr de Seversky
, who left Russia around the time of the Revolution and emigrated to the United States. He continued to develop his ideas. It is notable that de Seversky was a naval
pilot; long range flights over water, with no convenient places to stop and refuel, prey much on the naval aviator's mind.
Originally, the first attempts at this practice were a bit manpower-intensive. Literally. Wing walkers would climb from one aircraft to another with containers of fuel strapped to them, and then pour the fuel into the other airplane's tanks. This, however, wasn't practical for any real volume of fuel, and by 1923 the U.S. Army Air Corps was practicing refueling by means of lowering a hose from the tanker aircraft to an aircraft below. One of the lower aircraft's crew would catch the hose (sometimes with the assistance of a boat hook) and then connect the hose to their craft's tank. A quick-shutoff valve assured that if the airplanes deviated too sharply and the hose detached, the tanker's supply wouldn't empty all over the airplane below.
This was still not ideal; however, it led to flurry of aerial endurance records as pilots of all walks of life competed to spend longer and longer in the air using the dangling-hose refueling system. Records of over 600 hours straight were achieved. This required regular engine maintenance, with crew wing walking to oil their machines and repair fouled spark plugs. In the meantime, the RAF came up with a safer method, albeit one still involving a hose. The refueling aircraft would drag a line with a hook or grapnel at the end. The tanker flew above, behind, and to one side, dangling the hose, and then 'dragged' the hose laterally so that the lines came in contact. The refueling aircraft would then haul in the hose. This was safer because it kept the hose as far as possible from propellers.
Although usually war tends to accelerate the development of military techniques and technology, the tech of aerial refueling languished during the Second World War. There are several reasons for this. The first is that generally the air war was fought (in the early years, at least) at ranges easily achievable by airplanes using internal fuel, and in addition mostly within reach of land. Given the ferocity of the air battles that developed, tanker aircraft would have been far too vulnerable to send up and would have made their customers vulnerable as well. In an environment where no combatant could achieve absolute air superiority over the disputed regions (and AAA made them more lethal for fragile airplanes) the technique made no sense. Even the one place it might have helped, in carrier battles, it was difficult to implement. Rendezvous were problematic enough between flights of aircraft and their base ship; adding intermediate rendezvous with tanker airplanes without the benefit of modern electronic navigation was problematic at best. Finally, the ranges of aircraft kept on improving. By the end of the war, the best piston-engined bombers such as the B-29 Superfortress and most modern fighters such as the P-51 Mustang, using the newly-developed drop tanks, could easily reach the most distant targets.
So it was left for the jet age for air-to-air refueling to come into its own. Jet aircraft, especially high-performance turbojets such as were used intially, are terribly thirsty. General Curtis LeMay, of Strategic Air Command, realized at the outset that his mission (being ready to and capable of bombing the Soviet Union's main cities from the CONUS) would require very, very long-range flight. Given the massive nature of early atomic weapons, it was much more efficient to use air-to-air refueling to increase the range of his bombers rather than spend more of their capacity on fuel. Given the expectation that they would be expected to receive go/no-go orders while loitering over Northern Canada, it made sense to refuel them there as well. Given the risk perceived to U.S. air bases, the faster the bombers could be flown away the better; if they needed fuel, fueling them in the air away from their vulnerable bases made sense. This became even more true when ICBMs made their bases even more vulnerable.
By this time, the Boeing corporation had a new approach to air-to-air refueling. Since it was their airplanes that needed the gas, it made sense for them to push for building the tankers as well. The main innovation that Boeing brought to the table was the use of a rigid boom for refuelling, rather than the flexible hoses which had been used up to that point. The boom was given small airfoils on the far end, and operators in the tanker could use controls to 'fly' the boom to where the refueling aircraft's fueling probe was rather than force the refueling aircraft to maneuver. Thus, both pilots could concentrate on keeping station relative to each other and let the boom operator concentrate on connecting their airplanes. The boom allowed a five-fold or more increase in possible fuel flow due to its heavier construction, and the reduction of workload on the pilots made the whole thing safer. The RAF weren't sitting still; in 1948 they pioneered the use of a pointed fueling probe on the client aircraft and the use of a drogue-attached hose (which trails directly behind the tanker, rather than below it) to 'socket' onto it. This was quickly licensed by the US, who added it to their fleet to complement Boeing's boom, and the systems that are still in use today were born. The boom is useful on large airplanes built specifically as tankers for transferring large amounts of fuel (i.e. to larger airplanes). The hose-and-drogue, on the other hand, while not as fast, is more easily adapted onto airframes and can be used with 'buddy tanking' systems.
The first dedicated tanker aircraft for the U.S. Air Force, the KC-135 Stratotanker, was based (like so many other U.S. Air Force planes) on Boeing's 367-80 aircraft, also precursor to the wildly successful Boeing 707. Many of these aircraft are still in use today. Later aircraft modified for this role included the KC-10 Extender, a militarized version of the McDonnell Douglas DC-10 which could carry fuel, cargo, passengers, or mixes of all three (fuel is fairly heavy, and most tankers don't use near their full volume). By the time of the Vietnam War, U.S. Air Force and Army helicopters were flying longer and longer search and rescue operations as pilots went down in North Vietnam as well over the surrounding waters after nursing wounded birds away from enemy territory. After initial experiments using an Air Force CH-3 'Jolly Green Giant' helo, a C-130 Hercules with a hose-and-drogue added various types of helicopters were outfitted with refueling probes and were regularly being gassed up in midair in what was termed HIFR, for Helicopter In Flight Refueling. In addition, the use of air-to-air refueling was invaluable in saving aircraft which would otherwise have been lost; airplanes losing fuel could, if they could make it as far as the tanker, fly home attached to it and feeding fuel from the larger airplane's tanks. This worked for other leaky airplanes too - the SR-71 Blackbird and its sibling the A-12 both leaked fuel horribly while on the ground. This was because their fuel tanks and fuselages had large gaps in them, intended to seal as the metal of the airplanes expanded under the extremely high temperatures the skin would reach in flight. As a result, they would be fueled up, taxi onto the runway and take off leaking fuel heavily. This would sometimes mean a fiery trail of burning fuel behind them. Once they were airborne, they would 'warm up' and then be refueled by tanker for their mission.
All manner of large and small aircraft have been pressed into service as tankers. The KC-135 and KC-10 have been joined by the KC-130, a tanker version of the C-130 Hercules, the medium-range cargo airplane. The U.S. Navy used variants of their carrier-based airplanes, with the KA-3B Skywarrior and the KA-6D Intruder equipped with hose-and-drogue systems, eventually joined by the adapted S-3B Viking. These were used both for pre-planned refueling and to perform emergency refuelings of airplanes which either couldn't make it back to their carrier or made it back only to find a traffic jam or poor weather and were forced to remain aloft.
The Soviet air force also used air-to-air refueling for its long-range bombers and interceptors. The RAF maintained a fleet of converted Avro Lancaster bombers immediately after the Second World War, and eventually settled on the K1/KC1 variants of the civilian Lockheed TriStar for their refueling needs. The Italian and Japanese air forces are purchasing KC-767 aircraft - Boeing 767s modified with boom and receiver fueling systems. Even regular combat aircraft have been so equipped with so-called 'buddy tanking' capabilities - equipment to trail a drogue and transfer fuel to other airplanes. The U.S. Navy's F/A-18 Hornet fleet contains several 'strike tankers' which have come online as the S-3s age.
It's not enough to simply have the airplanes to do this. The aircraft intended to receive fuel must have a fuel system which can be fed from a single point - the refueling probe. This is not usually required, and usually needs to be designed into the systems from the start. As an interesting aside, Japan's constitution restricted its military forces from being used for anything other than defense of the Japanese Islands for many years. As a result, Japanese versions of fighters were deliberately built without the piping required, in order to limit their range from base.
Although air-to-air refueling was never put into place in commercial aviation, which saw internal fuel as much safer and more efficient (not having to operate tanker fleets), the militaries of the world find it invaluable to this day. Given the unpredictability of missions in the military, the ability to extend the range of your aircraft for either transfer or combat is priceless.
Hatshepsut points out that I forgot the most important phrase of all involving this concept: AIRPLANE SEX.