Aerial refueling
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 to take off with a greater
payload. Usually, the aircraft providing the fuel is specially designed for the task. The requirement to refuel during a flight is primarily a military requirement, and there are no known regular civilian in-flight refueling operations.
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. It was a very dangerous process until
1935 when brothers
Fred and Al Key demonstrated the first spill-free refueling nozzle, designed by A. D. Hunter. 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
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.
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.
Boom and receptacle
|
USAF C-5 approaches a KC-135R |
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). The receiver maneuvers to 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 crewman operating the boom, called a
boomer or
boom operator (in the
USAF, usually an enlisted aircrew member). 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, up or 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, he or she extends the nozzle at the end of the fuel tube into contact with the 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. 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, the boomer effects a disconnect by sending an electrical signal through the boom to the receiver aircraft disengaging the toggles in the receptacle. The boomer then retracts the fuel tube from the receptacle and flys 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) â€" a necessity when refueling the many large aircraft, such as
strategic bombers, operated by the US Air Force. 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.
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) â€" all operating US-designed aircraft.
Probe and drogue
 |
Tornado GR4 with probe attached to an RAF VC10 tanker over Iraq |
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-drouge method allows non-tanker aircraft to be temporarily converted into tankers by attaching a refueling pod. The pod contains a drouge 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.
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.
Wing-to-wing
In this method, 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).
Strategic uses and considerations
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 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 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 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 potential enemies, it was necessary to fly the craft to forward bases on Okinawa or in Europe. Hence, there were lots of trans-Pacific and trans-Atlantic flights. Also, for best performance, it was necessary for the SR-71 to take off with less-than-full jet-fuel tanks. Next, the SR-71 would immediately rendezvous 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 KC-135Q variant was necessary because the SR-71's fuel,
JP-7, was different from the fuel that was standard for other USAF aircraft.
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.
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 damaged fighters to hook up to the tankers and be towed to 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 was 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.
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 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 lead 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.
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 U.K. 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.
The
USAF provided nearly 90 percent of the NATO tanker force, 112 active and 63 Reserve-component KC-135 and KC-10 tankers.[
1] 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.
Boom and receiver
*
KB-29P** adapted from the
B-29 Superfortress*
KC-97 Stratotanker** adapted (heavily) from the
B-29 Superfortress*
KC-135 Stratotanker** adapted from the
Boeing 707** also used by the
French Air Force,
Republic of Singapore Air Force and
Turkish Air Force ** can also use drogue adapter. French Air Force KC-135 Stratotankers use the probe and drogue system
** MPRS models will have two drogue hose reels (pods) at the wingtips
** Modified Boeing 707 tankers users, past and present, include the
Royal Australian Air Force,
Brazilian Air Force,
Israeli Air Force,
Italian Air Force and
South African Air Force*
KC-10 Extender** adapted from the
McDonnell Douglas DC-10** (also has a retractable hose and drogue)
** The
Royal Netherlands Air Force operates two KDC-10s - former civil aircraft modified to a similar standard to the KC-10
*
KC-767** adapted from the
Boeing 767** used by the
Italian Air Force and
Japan Air Self Defense Force; not yet in U.S. military service
** All US models, if ordered, will have fitting for MPRS
*
Airbus A330 MRTT** development of Airbus A330 - 5
Australian aircraft will be equipped with both a flying boom and probe and drogue units. (UK aircraft probe and drogue only.)
Probe and drogue
*
Airbus A330**
RAF Future Strategic Tanker Aircraft, due in service around 2008.
*
Airbus A310 MRTT**4 for
Luftwaffe**2 for
Canadian Forces (as
CC-150 Polaris)
*
Avro Vulcan** specially modified for operations during the
Falklands War** no longer in service with the RAF or elsewhere
*
Blackburn Buccaneer No longer in service.
** Equipped for buddy tanking
*
Boeing 707** Used by the
Royal Australian Air Force,
South African Air Force and others
**
French Air Force KC-135 Stratotankers use the probe and drogue system.
*
KB-29M** adapted from the
B-29 Superfortress; earlier versions used a "grappling hose" system; later models used a true probe-and-drogue. No longer in service.
*
B-50 Superfortress No longer in service
** improved model of the
B-29 Superfortress*
HC-130 Hercules and
KC-130 Hercules, especially in
United States Marine Corps service as well as KC-130s in
Argentine Air Force,
Brazilian Air Force,
Israel Air Force,
Republic of Singapore Air Force and
Spanish Air Force service.
** variants of the
C-130 Hercules *
Lockheed L-1011 Tristar
** K1 and KC1 variants deployed by the
Royal Air Force*
Vickers Valiant No longer in service.
*
Vickers VC-10*
Handley Page Victor No longer in service.
*
KA-3 No longer in service.
*
KA-4** Equipped for buddy tanking
*
KA-6*
KA-7*
S-3 Viking ** The current primary carrier-based tanker
*
F/A-18E/F ** equipped for buddy refueling as "Strike tankers"
*
Il-78 Midas** Standard Russian tanker, adapted from
Il-76** the MKI variants currently deployed by the
Indian Air Force (IAF)
*
Myasishchev M-4-2** adapted from the M-4 bomber
*
Myasishchev 3MS-2** adapted from the 3M bomber
*
Tu-16N (and Tu-16Z with wing-to-wing system)
*
Su-24M ** equipped for buddy refueling with the UPAZ container as "Strike tankers"
*
Photo aerial tanking 1929Regarding
spacecraft: see
Docking maneuver.
*http://www.centennialofflight.gov/essay/Evolution_of_Technology/refueling/Tech22.htm
