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Aerial refueling

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Boom and receptacle: USAF KC-135R Stratotanker, two F-15s (twin fins) and two F-16s, on an aerial refueling training mission
Image:Helicopter aerial refueling.jpg

Aerial refueling, also called in-flight refueling (IFR) or air-to-air refueling (AAR), is the practice of transferring fuel from one aircraft to another during flight. This allows the receiving aircraft to remain airborne longer, and enables a takeoff with either a greater payload or a shorter/safer takeoff roll (as with the SR-71). Usually, the aircraft providing the fuel is specially designed for the task. The economic costs associated with refueling during a flight (requiring a separate specialized aircraft) have resulted in a practical restriction to only military operations, and there are no known regular civilian in-flight refueling operations.

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[edit] History and development

Image:Refueling, 1923.jpg

An 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

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 hand-held fuel tank on one aircraft and placed into the usual fuel filler of the other. A DH-4B biplane remained aloft with mid-air refueling for 37 hours on June 27, 1923.

Development was rapid. In 1930, the Hunter brothers set an endurance record of 553 hours 40 minutes over Chicago. Aerial refuelling remained a very dangerous process until 1935 when brothers Fred and Al Key demonstrated the first spill-free refueling nozzle, designed by A. D. Hunter. They exceeded the Hunters' record by nearly 100 hours in a Curtiss Robin monoplane, staying aloft for more than 27 days.<ref name="HistAv19">History of Aviation, Part 19, 1938</ref>

In the UK, Alan Cobham pioneered research on the probe and drogue method, and gave public demonstrations of the system. In 1934, he founded Flight Refuelling Ltd. (FRL), and by 1938 had used an automatic system to refuel aircraft as large as the Short Empire flying boat Cambria from an Armstrong Whitworth AW.23<ref name="HistAv19"/>. FRL still exists as part of Cobham plc.

Nowadays, specialized tanker aircraft have equipment specially designed for the task of offloading fuel to the receiver aircraft, based on Hunter's design, even at the higher speeds modern jet aircraft typically need to remain airborne.

In 1949 from February 26 to March 3 an American B-50 Superfortress "Lucky Lady II" flew non-stop around the World in 94 Hours, 1 Min., a feat made possible by 3 aerial refuelings from 4 pairs of KB-29M tankers. The flight started and ended at Carswell Air Force Base in Fort Worth, Texas with the refuelings accomplished over West Africa, the Pacific ocean near Guam and between Hawaii and the West Coast. This first nonstop circumnavigation of the globe proved that vast distances and geographical barriers were no longer an obstacle to military air power, thanks to aerial refueling.

[edit] Aerial refueling systems

The two most common approaches for making the union between the two aircraft are the boom and receptacle system and the probe and drogue system. Much less popular was the wing-to-wing system, which is no longer used.

[edit] Boom and receptacle

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Image:KC-135Boom-operator-521.jpg
The boom is a long, rigid, hollow shaft around a telescoping fuel tube, usually fitted to the rear of the aircraft. (It almost connects the two aircraft in the picture at right.) The telescoping fuel tube incorporates a nozzle at the end which mates to the receptacle in the receiver aircraft. A poppet valve in the end of the nozzle prevents fuel from flowing until contact is made between the nozzle and receptacle. Mounted on the hollow shaft surrounding the fuel tube are small wings, or ruddevators (visible in picture below, in the "V" shape), allowing the operator "fly" the boom to enable alignment between the nozzle and receptacle. Once aligned, the fuel tube is hydraulically extended to effect contact. Toggles in the receptacle engage the nozzle holding it locked in place during fuel transfer. The receiver's receptacle is fitted somewhere on the top of the aircraft, usually on its centerline. Installations are design specific for each aircraft resulting in procedural peculiarities for each receiver.

During refueling operations the tanker aircraft will fly at a constant altitude and speed with the boom unlatched and flying in the trail position (see the photo at right). From an initial rendezvous point, the receiver maneuvers into the precontact position, a standard position behind and below the tanker allowing contact within the movement and extension limits of the boom. Receiver positioning is aided with either voice or visual commands from the crewmember operating the boom, called a boom operator or boomer (in the USAF, usually an enlisted aircrew member). The boom operator faces toward the rear (i.e., toward the other plane) during refueling, lying prone in the KC-135 or sitting in a rear-facing seat in the KC-10. For radio silent operations, visual commands are effected by the use of pilot director indicators (PDIs) on the bottom of the tanker which the boomer can illuminate directing the receiver pilot to move fore/aft and up/down into the correct position. Once in position, the receiver pilot flies formation with the tanker, although this can be complicated by turbulence or poor tanker pilot technique (normally the tanker's autopilot is engaged for refueling, but may be disengaged if malfunctioning or for training). Once the receiver is in the contact position, the boomer flies the nozzle into alignment with the receptacle by positioning the ruddevators with a control stick. At the same time, the boomer extends the nozzle at the end of the fuel tube into contact with the conical receptacle using the extension control lever. Pumps operated by the pilot on the tanker drive fuel through the fuel tube into the receiver. While in contact, the PDIs reflect the boom's position to aid the receiver pilot in remaining within the air refueling envelope; in addition, the inner extensible tube of the boom has color-coded stripes to aid in keeping it extended to a proper length. The air refueling envelope is different for each receiver; it is based either on the boom's mechanical limits, or to prevent the receiver from moving into a position where any portion of the boom might contact the receiver outside the receptacle while in contact. When fueling is complete, either aircraft (typically the boomer) can effect a disconnect by sending an electrical signal through the boom to the receiver aircraft disengaging the toggles in the receptacle. A pressure disconnect can also trigger this disengagement when the receiver's fuel system has been filled to capacity. The boomer then retracts the fuel tube from the receptacle and flies the boom clear of the receiver. While not in use, the boom is flown up to the bottom of the tanker and latched in position.

The primary advantage to this method of refueling is that a high volume of fuel can be transferred in a short time (up to 1000 gallons per minute for the KC-135 series tanker). This aspect was considered a necessity by the large bomber fleet of the Strategic Air Command when this system was selected because pilot fatigue was a major safety concern. Receiver pilots are tasked with intense concentration in order to fly precise close formation throughout the procedure, and the lower flow rates from the lower pressure limited by the flexible hose used in the probe-and-drogue system resulted in unnecessarily long refueling times with corresponding unnecessarily long exposure to flight safety hazards. (Fighter pilots are not subjected to as much fatigue during refueling because their fuel tanks are much smaller, therefore the lower flow rates of the simpler and cheaper probe-and-drogue system is typically preferred by military organizations that do not operate a large fleet of strategic bombers, such as the US Navy.)

Although tankers equipped with rigid refueling booms can only service one properly equipped aircraft at a time, the high transfer rate allows rapid cycling of receiver aircraft. Another obvious disadvantage of this system is the cost associated with requiring the additional specialized crewmember to operate the boom.

In addition to US Air Force, the boom and receptacle system is in use by the Netherlands (KDC-10), Israel (modified Boeing 707) and Turkey (ex-USAF KC-135R) — all operating US-designed aircraft. Iran also operates a modified Boeing 747 - quite possibly the largest aerial refueling tanker anywhere - equipped with a boom and receptacle system. [1] [2] [3]

[edit] Probe and drogue

Tornado GR4 with probe attached to an RAF VC10 tanker over Iraq

Image:S-3 Viking in-flight refueling.jpg The drogue (or para-drogue), sometimes called a basket, is a fitting resembling a plastic shuttlecock, attached, at its narrow end, with a valve, to a flexible hose, running from the hose drum unit (HDU) or boom to drogue adapter (BDA). The receiver has a probe, which is a rigid, but sometimes jointed and retractable, arm placed usually on the side of the aircraft's nose.

Again, the tanker flies straight and level, and the drogue is allowed to trail out behind and below it. The drogue can be flown by the boom operator but the receiver pilot must fly his probe directly into the basket to make contact. The boom operator holds the BDA as motionless as possible. After the receiver states "contact" the boom operator triggers contact which allows the tanker pilot to start the air refueling pumps and offload fuel. 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 attached to the wing tips known as the Multi-Point Refueling System or MPRS. The U.S. Navy, Marine Corps, and other NATO nations use this system, rather than the boom system. Unlike the boom-and-receiver system, multiple aircraft can be refuelled simultaneously with the probe-and-drogue system.

The probe-and-drogue method allows non-tanker aircraft to be temporarily converted into tankers by attaching a refueling pod. The pod contains a drogue and hose reel that can be extended to refuel other aircraft from the temporary tanker's fuel supply in a process known as "buddy tanking". While fighter, attack, and anti-submarine aircraft lack the fuel capacity of dedicated tankers, buddy tanking provides a great deal of flexibility.

This system was first used on late models of the KB-29M Superfortress. Its first use in combat occurred on May 29 1952 when twelve F-84s were refueled during a mission from Itazuke, Japan to Sariwon, North Korea.

The inability for a probe-equipped receiver aircraft to refuel from a non-drogue tanker boom (likewise, a boom receptacled receiver from a drogued tanker) has been a longstanding frustration for military planners who have had such mismatches to deal with (let alone the frustration of the aircrew who are caught in such a situation). This problem was acutely experienced during the Gulf War of 1991, leading to consideration by the USAF of the enormous cost required to convert all boom systems to probe-and-drogue. Advancements of the probe and drogue system now allow a fuel flow comparable to the boom and receptacle method. In some cases, such as the KC-135FR in service with the French Air Force, refueling-boom equipped tankers can be converted to an all probe-and-drogue system. The KC-135FR retains its articulated boom, but has a hose at the end of it instead of the usual nozzle.

[edit] Wing-to-wing

In this method, similar to the probe and drogue method but more complicated, the tanker aircraft released a flexible hose from its wingtip. An aircraft, flying beside it, had to catch the hose with a special lock under its wingtip. After the hose was locked, and the connection was established, the fuel was pumped. It was used on a small number of Soviet Tu-4 and Tu-16 only (the tanker variant was Tu-16Z).

[edit] Grappling

The primitive systems used for pioneering aerial refueling used the grappling method, where the tanker aircraft unreeled the fuel hose and the receiver aircraft would grapple the hose midair, reel it in and connect it so that fuel can be transferred either with the assistance of pumps or simply by gravity feed. This was the method used on the Question Mark endurance flight in 1929, and also the first ever non-stop around-the-world flight by Strategic Air Command's B-50 nuclear-capable bomber nicknamed the Lucky Lady II in 1949.

[edit] Strategic and tactical implications

An F-15 Eagle disengages from a KC-10 Extender
A KC-10 Extender from Travis Air Force Base, California, refuels an F-22 Raptor

[edit] Strategic uses and considerations

The development of the KC-97 and KC-135 Stratotankers was pushed by the Cold War requirement of the United States to be able to keep fleets of nuclear-armed B-47 Stratojet and B-52 Stratofortress strategic bombers airborne around-the-clock either to threaten retaliation against a Soviet strike for Mutually Assured Destruction, or to bomb the U.S.S.R. first had it been ordered to do so by the President. 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 would refill the bombers' fuel tanks 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 by bomber. A noted example of refueling used in this manner in the 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 air-ballet of refueling scenes are from actual B-52s refueling from KC-135s).

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 matériel, supplies, and personnel to Vietnam without landing to refuel. KC-135s were also frequently used for refueling of air combat missions from air bases in Thailand.

The USAF SR-71 Blackbird strategic reconnaissance aircraft made frequent use of air-to-air refueling. Its home base was at Beale AFB in central California, but to make actual reconnaissance missions over enemy territory, it was necessary to deploy the craft to forward bases in Okinawa or in Europe. Hence, there were lots of trans-Pacific and trans-Atlantic flights. Also, for safe takeoff performance, it was necessary to launch the SR-71 with less-than-full jet-fuel tanks. The SR-71 would start the mission by rendezvousing with a specially modified KC-135 to top off its tanks for full extended-range. Then the SR-71 was capable of flying for many hours on its own. This tanker variant was necessary because the SR-71 used a special fuel, JP-7, with a very high flash point (needed to withstand the high skin temperatures of Mach 3+ cruising flight) which could not be used in other aircraft engines and the KC-135Q was equipped with a separate internal bladder system to carry and deliver this non-standard fuel.

During an actual nuclear SIOP certification for a SAC bomber crew (required before they are authorized to pull alert duty), the commanding officer (himself a bomber pilot who had previously been in the tanker community) quizzed the new crew, "What three expendibles do you have to defend yourself?" The crew was puzzled, being able to only come up with the two standard defensive expendibles that are designed to draw enemy missile fire: chaff and flares. When the crew finally admitted to being stumpped, the commanding officer came back with his answer: "Chaff, flares, and the tanker!" Laughter ensued at this highly unexpected joke that was told by a top ranking officer at what is otherwise an extremely serious event, but this story serves to indicate some of the rivalry that exists between such communities within an air force.

On the other side of the coin (literally a coin) tanker crews have been known to have coin medallions manufactured with inscriptions including the letters: "NKAWTG", which stands for the unofficial tanker motto, "Nobody Kicks Ass Without Tanker Gas".

[edit] 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.

[edit] Vietnam War

It was common for USAF fighter-bombers flying from Thailand to North Vietnam to refuel from KC-135s en-route to their target. Besides extending their range, this enabled the F-105s and F-4 Phantoms to carry more bombs and rockets. Tankers were also available for refueling on the way back if necessary. In addition to ferrying aircraft across the Pacific Ocean, aerial refueling made it possible for battle damaged fighters, with heavily leaking fuel tanks, to hook up to the tankers and let the tanker feed its engine(s) until the point where they could glide to the base and land. This saved numerous aircraft.

The US Navy frequently used carrier-based aerial tankers like the KA-3 Skywarrior to refuel Navy and Marine aircraft such as the F-4, A-4, A-6, and A-7. This was particularly useful when a pilot returning from an airstrike was having difficulty landing and was running low on jet fuel. This gave him fuel for more attempts at landing for a successful "trap" on an aircraft carrier. The KA-3 could also refuel fighters on extended Combat Air Patrol and E-2 Hawkeye Airborne Electronic Warning aircraft on extended patrol. USMC jets based in South Vietnam and Thailand also used USMC KC-130 Hercules transports for air-to-air refueling on missions.

[edit] Falklands/Malvinas War

Aerial refueling played a vital role in all of the Argentine successful attacks against the Royal Navy. The Argentine Air Force had only 2 KC-130H Hercules available ("Berlin" and "Paris") and were used to refuel both Air Force and Navy A-4 Skyhawks and the Navy Super Etendards in their Exocet strikes, with the two models achieving almost all of the British ships sunk. The Hercules on several occasions approached the islands (where the Sea Harriers were in patrol) to search and guide the A-4s in their returning flights. On one of those flights (callsign jaguar) one of the KC-130s went to rescue a damaged A-4 and delivered 39,000 lb of fuel while carrying it to its airfield at San Julian. On the other hand, the Mirage IIIs and Daggers lack of air refuelling capability prevented them from achieving better results. The Mirages were unable to reach the islands with a strike payload, and the Daggers could do so only for a 5 minute strike flight.

On the British side, Aerial refueling was carried out almost exclusively by the Handley Page Victor. These aircraft aided deployments from the UK to the Ascension Island staging post in the South Atlantic, and further deployments south of attack, transport, and maritime patrol aircraft. The most famous refueling missions were the "Operation Black Buck" sorties which involved Victor tankers refueling Avro Vulcan bombers to attack the Argentine-captured airbase at Port Stanley on the Falkland Islands. Despite only one direct hit, the raids were the longest ranged bombing raids in history until surpassed by the B-52 in the 1991 Gulf War. The almost total commitment of RAF tankers to the Falklands War led to a shortage of tankers to support UK-based RAF air defence aircraft. This gap was filled by USAF KC-135s.

The Victor tankers were shortly afterwards retired from service, and they were replaced in RAF service by Lockheed L-1011 and Vickers VC10 transports which were bought economically on the second-hand airliner market. The L-1011s, converted by Marshall Aerospace, and VC10s, converted by British Aerospace, have the capability of refueling jets from the RAF, the Royal Navy, and NATO aircraft.

[edit] Libya

During Operation El Dorado Canyon, several F-111 Aardvark fighter-bombers stationed in the United Kingdom utilized aerial refuelling to enable them to operate non-stop against targets in Libya. Since the aircraft were allowed to cross neither French nor Spanish airspace, they should make a detour around the Iberian Peninsula and stay above International waters during all transit.

[edit] The Persian Gulf War

During the time of Operation Desert Shield, the military build up to the Persian Gulf War, US Air Force KC-135s, McDonnell Douglas KC-10As, and USMC KC-130 Hercules aircraft were deployed to forward air bases in England, Diego Garcia, and Saudia Arabia. Aircraft stationed in Saudi Arabia normally maintained an orbit in the Iraq-Saudi Arabia neutral zone, informally known as "Frisbee", and refueled Coalition Aircraft whenever necessary. This 24-hour air-refueling zone helped make the intense air campaign during Operation Desert Storm possible.

On January 16/17th, 1991, the first combat sortie of Desert Storm, and the longest combat sortie in history, at that time, was launched from Barksdale AFB, Louisiana. Seven B-52Gs flew a thirty-five hour mission to the Persian Gulf region, and back, to launch Boeing Air Launched Cruise Missiles (ALCMs) with the surprise use of conventional warheads. All of this was made possible by in-flight refueling, and by the secret switch away from nuclear warheads on the ALCMs.

An extremely useful aerial tanker in Desert Storm was the USAF KC-10A Extender. Besides being larger than the other tankers, the KC-10A is equipped with the USAF "boom" refueling and also the "probe-and-drogue" system. This makes it possible for the KC-10A to refuel USAF aircraft, and also USMC and US Navy jets that use the "probe-and-drogue" system, and also allied aircraft, such as those from the UK and Saudi Arabia. KC-135's may be equipped with a drogue depending on the mission profile.

The KC-10A was originally designed for the support of NATO in Europe by the USAF. In the case of armed conflict, with a full jet fuel load, the KC-10A is capable of flying from a base on the east coast of the US or Canada, flying nonstop to Europe, transferring a considerable amount of fuel in air-to-air refueling, and then returning to its home base, all without landing anywhere. This could have been very useful in the case when numerous European bases become disabled by Warsaw Pact strikes in Germany, Holland, France, and Great Britain.

[edit] Kosovo War

The USAF provided nearly 90 percent of the NATO tanker force, 112 active and 63 Reserve-component KC-135 and KC-10 tankers.[4] Tankers were also provided from Britain’s RAF (Tristars and VC-10s), French Air Force and Turkish Air Force KC-135s, Spanish Air Force KC-130 Hercules and Royal Netherlands Air Force KDC-10s. Although some European nations provided air-refuelling aircraft, the conflict highlighted the problem Europe has with a lack of such aircraft and dependence on the United States for tanker support during a major operation.

[edit] Aerial Rearming

In 2003 the U.S. Air Force and Far Technologies applied secretly for patents on mid-air rearming of aircraft. The technique proposed is similar in many respects to airborne refueling, with a number of notable modifications. The air borne rearming system is comprised of a rearming plane with an internal bomb storage area and loading device consisting of a large aft door and a modified remote-driven robotic arm (boom) equipped with a day-night camera as well as sensors. On the attack aircraft, a special pylon to receive the arms from the boom. At present financial and technological problems stand in the way of aerial rearming; mainly the need for an automatic system to perform the rearm currently under development for aerial refueling [5].

[edit] Media

  • F/A-18 Automated Aerial Refueling (AAR) (file info)
    • Engineers at NASA's Dryden Flight Research Center are evaluating the capability of an F/A-18A aircraft as an in-flight refueling tanker to develop analytical models for an automated aerial refueling system for unmanned air vehicles (UAVs). (9.38 MB, ogg/Theora format).

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  • Problems seeing the videos? See media help. </li> </ul> </div>

    [edit] HIFR (Helicopter In-Flight Refueling)

    Image:HIFR Practice.jpg
    A variation of aerial refueling is when a naval helicopter approaches a warship (not necessarily suited for landing operations) and receives fuel through the cabin while hovering.

    Note: Alternatively, some helicopters equipped with a probe extending out the front can be refueled from a drogue-equipped tanker aircraft in a similar manner to fixed-wing aircraft by matching a high forward speed for a helicopter to a slow speed for the fixed-wing tanker. Therefore a less ambiguous meaning for the abbreviation HIFR would be HOVER In-Flight Refueling.

    Note also: The transfer of cargo while an aircraft is hovering is known within the US Navy and the US Coast Guard as Vertical Replenishment (VERTREP).

    [edit] Tanker aircraft by refueling system

    [edit] Boom and receiver

    A B-2 Spirit prepares to refuel from a KC-135R

    [edit] Probe and drogue

    Image:33 Sqn.jpg

    [edit] External links

    [edit] See also

    Regarding spacecraft: see Docking maneuver.

    [edit] References

    <references/>

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