In- flight refueling refers to the transfer of fuel from one aircraft to another aircraft or helicopter in flight. This allows the fueled aircraft to stay in the air longer and thus increase its range and carry more payload. The aircraft that provides the fuel is usually a tanker aircraft specially developed for this task . A pressure refueling allows a flow rate of more than 3000 liters per minute.
History and Development
The first documented successful attempt took place on June 27, 1923. The pilots Lowell H. Smith and John P. Richter, both members of the United States Army Air Service , took off from Rockwell Airfield in San Diego with a de Havilland DH4 , in whose fuselage they had installed an additional tank and a funnel-shaped filler neck. A de Havilland DH4B-1, which was flown by pilots Virgil Hine and Frank Seifert, had an additional tank that held 500 liters. The crew of this aircraft let off a metal-reinforced hose with a quick-release fastener on a steel cable through a trap door. The crew of the receiving aircraft had to grab the end of the hose, insert it into the filler neck and open the quick release fastener. As soon as the fuel rose in the filler neck, they had to cut off the supply. The process was developed by Alexander Procofieff De Seversky two years earlier .
On August 27, 1923, the same crew managed to keep the receiver aircraft in the air for 37 hours and 15 minutes with five refueling maneuvers. The US Army's aviation group continued to investigate the possibilities of aerial refueling. In 1928 she planned to set an endurance flight record. At the Middletown base in Pennsylvania, the Fokker C-2 with the serial number 28-120 was converted into a receiver aircraft with additional fuselage tanks and receiver equipment. This included a cockpit behind the wings with a large funnel nozzle and a line to catch the tank hose. The pilots Ross G. Hoyt and Rudy C. Strickland as well as Odas Moon and Joseph G. Hopkins flew the tank aircraft RP-1 and RP-2 of the type Douglas C-1 . The abbreviation RP stands for Refueling Plane. Irvin A. Woodring and Andrew F. Salter were on board as mechanics. The receiving machine, which was named Question Mark, was crewed by Carl Spaatz , Ira C. Eaker , Pete Quesada , Harry Halverson and RW Hose.
On January 1, 1929, the Question Mark took off from Los Angeles Airport at 7:27 a.m. and was refueled for the first time at 8:15 a.m. Despite bad weather conditions, she continued to fly without any problems. The night rendezvous maneuvers turned out to be very dangerous, as the US Army pilots had not yet trained to fly blind. Due to the bad weather, the operation was moved to Imperial Valley. After forty-three deliveries of fuel, food, beverage, oil, batteries, and even mail, Question Mark landed at 1:50 p.m. on January 7, 1929, after having been in the air for nearly 151 hours. The crew received the Distinguished Flying Cross for this . The tanker crews only received letters of thanks.
This record did not last long. In 1930 the Hunter brothers stayed in the air for 553 hours. On June 4, 1935, Algene and Fred Key took off on a Curtiss Robin . They landed on July 1, 1935. The record was 653 hours and 33 minutes.
The Briton Alan Cobham dealt with the air refueling since 1930. He made contact with British Captain Richard Atcherley , who had developed a system in which a weighted cable was dragged from the tanker and a catch hook was fired from the receiving aircraft. Based on this patented solution, Cobham founded Flight Refueling in 1934 . In 1935, the Royal Air Force demonstrated successful in-flight refueling of a Hawker Hart by a Westland Wallace .
Air refueling at the German Research Institute for Gliding
Felix Kracht's research at the German Research Institute for Glider Flight (DFS) founded in 1933 under its director Walter Georgii laid the foundation for in- flight refueling as it is today . At first Kracht had developed a pure coupling mechanism. This mechanism was located at the end of an approximately 80 meter long tow rope that could be extended with a winch in flight from the stern of a towing machine. The tests and constant further developments were so successful that the machine to be towed could be taken from the tow tow into the short tow and finally into the rigid tow. The possibility of air refueling has now been derived from these developments. The first successful air refueling with a controllable tank harness was flown in 1943. On this first flight, a converted four-engine Junkers Ju 90 V-7 served as a tanker, which had to be controlled by a Focke-Wulf Fw 58 . Further successful attempts followed in Bohemia with a six-engine Junkers Ju 390 V-1 and in France . In July 1944 the program was discontinued due to a lack of fuel.
Notable missions after the Second World War
As part of Operation Black Buck during the Falklands War in 1982 bombed Avro 698 Vulcan - bomber from Ascension from 6250 km away airfield Port Stanley . The flights required for this, sometimes over more than 13,000 km, were made possible by a very complex process of multiple refueling in the air, especially the tankers themselves. That was the longest distance of a bomb attack by an air force until then.
The operation El Dorado Canyon in 1986 required an in-flight refueling of used F-111 Aardvark . Since France, Italy and Spain denied overflight rights, the bombers launched on bases in Great Britain could not fly directly to Libya via mainland Europe and therefore had to choose a route via the Strait of Gibraltar . This increased the total distance flown to over 6,400 miles (approx. 10,300 km), which required eight to twelve refueling maneuvers with Boeing KC-135 and McDonnell Douglas KC-10 aircraft for each mission .
It was not until 1991 that an attack was carried out over an even greater distance than in 1982. B-52 bombers of the US Air Force launched from the US mainland, bombed targets in Iraq and then returned to the air base in Mildenhall back. A chain of regionally stationed tankers for aerial refueling was used for these attacks.
In the meantime, aerial refueling using unmanned aerial vehicles is also being tested, for example in the US KQ-4 program.
For the first time in April 2015, a Northrop Grumman X-47 off the coast of Maryland and Virginia was refueled in the air with around 1,800 kg of fuel from a Boeing 707 operated by Omega Aerial Refueling Services .
Air refueling systems
The two basic approaches to establishing the connection between the two aircraft today are the boom system and the probe and funnel system .
The boom is a long pipe that can be moved horizontally and vertically within narrow limits and is usually attached to the rear of the tanker (easy to see in the picture on the right). It often has a telescopic end, a valve at the end to regulate the flow of fuel, and small blades , which, depending on the design, are also known as ruddervators (made up of “rudder” and “elevator”) (in the picture below as “V "Visible). With them, the boom can literally be "flown" to its destination - the receiver's refueling opening. This refueling opening is usually located on top of the aircraft on its longitudinal axis and either behind or just in front of the cockpit . The refueling port is connected to the fuel tanks and has a valve that keeps the fuel in the tank out, as well as dust and debris. The end of the boom fits into this opening.
During a refueling operation, the tanker aircraft maintains constant altitude and speed and the fuel receiver takes a standard position behind and under the tanker aircraft. Modern tankers have headlights that illuminate the area outside this point. The pilot has to approach the dark point at night and can correct his attitude accordingly when he flies into the light area. As soon as he is in position, the pilot flies in formation with the tanker (this can be made more difficult by turbulence ). The crew member who controls the boom , the boomer or boom operator (in the USAF usually in the rank of sergeant ), then unlocks the boom from its rest position and directs it towards the tank opening by he "flies" it to the target with the attached wings. The telescopic end is then extended hydraulically until the end hits the tank opening. If successful, an electrical circuit is closed between the tanker and the recipient aircraft, causing the two valves in the boom and tank to open, and pumps on board the tanker pump fuel through the boom and the tank opening into the recipient aircraft (some types of tankers may use pure gravity refueling) . Once both planes are connected, additional lights will show the pilot whether he is drifting in one direction. This is regulated by switches in the boom. Once the refueling process is complete, the valves will automatically close and the boom will automatically retract.
Probe and funnel
The drogue , sometimes called a basket , resembles a shuttlecock with no feathers. At its narrow end, which points in the direction of flight, there is a valve. From there a flexible hose leads to the tanker. The receiving aircraft is equipped with a probe , a type of forward-facing tube. This is either rigidly attached or, for the purpose of better aerodynamics , retractable for normal flight operations.
During the refueling process, the tanker flies straight ahead, the catch funnel is dragged behind the tanker by its hose and is thus located below the fuselage. The funnel cannot be controlled directly, as is the case with a boom, but only slight movements are possible when the tanker changes course. Therefore, the pilot of the receiving aircraft must steer it so that the probe gets into the basket. If this is achieved, the connection between the probe tip and valve is established by the draft on the basket. The valve opens and the pumping of fuel can begin. The aircraft to be refueled must now maintain its position in relation to the tanker until the refueling process has ended. Then the pilot of the aircraft that has just been refueled reduces his speed and the probe separates from the funnel.
Some boom equipped tankers have special hoses that can be attached to the top of the boom. This makes it possible to refuel aircraft with probes.
Another equipment variant is the combination with a boom as well as with one or more catch funnel systems.
Comparison of the two systems
After the Second World War, the US Air Force converted B-29 bombers into tanker aircraft: they were perfect for this because they were fast, could climb high and had a high payload. Both of the above-mentioned systems were tested and tried out in use. By comparing the two systems, the following can be summarized: The system of probe and drogue exerts increased air resistance on the tanker when it is extended, which reduces its speed during refueling. The decrease in speed is particularly noticeable when fueling is carried out with three catch funnels at the same time and the tanker is driven by piston engines. Jet drive tankers can compensate for the increased air resistance more easily because they have more power reserves. The probe and catch funnel system can in principle be installed in any aircraft, from a fighter jet to a transporter, as a retrofit system, for example as a pod under the wings, with relatively little conversion effort. Among other things, this was an introduction to regular in-flight refueling at the Federal Air Force: three fighter jets flew as an association, two of them carried weapons and one with more additional tanks instead of weapons. This third jet gave the remaining fuel to the two ammunitioned jets at a pre-calculated point, thus increasing their range. The “little tanker” turned off after the fuel had been delivered and started the flight home. This method, albeit with Boomer, was also proposed by North American in the mid-1960s to extend the range of the XB-70 .
With the probe and catchment funnel system , the pilot of the receiver aircraft usually has to train more in order to make contact safely, and the dropout rate is also higher. The establishment of a contact also takes longer than with a telescopic boom system and is therefore an additional burden for the pilot of the receiving machine. The boom operator, on the other hand, can control the boom in two axes and thus relieve the pilot of the receiver considerably. The remainder of the work in making contact is essentially that of the boom operator. Basically, the pilot of the receiver only has to establish the formation and (despite the weight gain during refueling) hold the position. However, refueling with a telescopic system is not without risk: A series of serious accidents in the US Air Force, some of which have been total losses, document the risk. Thanks to the tube of the telescopic boom, which has a larger cross section, more fuel can be transferred in a shorter time. This is more important with the large numbers carried by large strategic bombers like the B-52 and B-2 and the C-5A and C-17 transports than with smaller tactical fighter jets. It reduces the refueling time and thus ultimately the risk of accidents. At night these effects intensify again. Overall, the system is a little cheaper due to various factors. The main advantage is also in the refueling of helicopters, for which the telescope system in its current form is not very suitable: The safety distance between the helicopter and tanker is much greater with the hose, and the hose, which is almost horizontal, has a significantly lower risk of collision with the helicopter rotor. Since the drip funnel system has a slight cost advantage, can refuel fighter jets, transporters and helicopters over a large speed range, and can ultimately cover a large area, it is very popular in European air forces. The telescope system shows its advantages when refueling the largest aircraft and is therefore rather necessary in the USA.
Strategic and tactical implications
The early development of the Boeing KC-97 and Boeing KC-135 was stimulated by the need in the United States to make up the strategic air command (SAC) B-47 and B-52 strategic bombers during the Cold War in the United States To hold air. The aim was to maintain the ability to make a second strike in the event of an attack, or - if necessary - to be able to carry out the first strike against the USSR . The bombers circled above their assigned positions from which they were to enter Soviet airspace if ordered to do so. The tankers made sure that the tanks of the bombers were always full to keep a squadron in the air for 24 hours so that they could always reach their destinations in the Soviet Union. A first strike against the bases of the bombers could not prevent a second strike by the USA (→ Chrome Dome ). Famous examples of this refueling practice can be found in the opening credits of the film Dr. Strangelove or: How I Learned the bomb to love (a fictional film, but showing scenes real B-52 refueling maneuvers) and in the film A Gathering Of Eagles (dt title. The commodore , who appeared in the US in 1963 and) almost documentarily describes everyday life in a unit of the SAC.
A by-product of this development and the construction of large numbers of tanker aircraft was the possibility of refueling transport aircraft , fighter jets and ground attack aircraft and thus being able to take them to distant locations. This was often practiced during the Vietnam War , when many aircraft could not have made transoceanic distances - even with stopovers at bases in Hawaii and Okinawa - without aerial refueling. In addition to the self-transport of the aircraft, they could of course also bring material, supplies and soldiers to Vietnam without having to land to refuel.
The ability to refuel after take-off has two major tactical advantages. On the one hand, it allows fighter jets, attack aircraft and bombers to reach targets that are usually out of range, and patrol aircraft to stay in the air longer. On the other hand, the maximum take-off weight of an aircraft is generally less than the maximum weight with which it can remain in the air. This allows an aircraft to take off with only a partial fuel charge and to carry more payload. After it has reached the operating altitude, the tanks can be filled up by a tanker, and the aircraft reaches its maximum flight weight there and thus a long range from there.
Classification of the tanker aircraft according to tank system, dimensions and production
|Aircraft type||Tank system||Working time||Production time||Length in m (from – to)||Span in m (from – to)||Number of items in production (until 2015)||Number of units ready for use (mid-2008)||country||image|
|Airbus A310-300 MRTT / CC-150 Polaris||Probe, drogue||current||2000–||47 m||44 m||8+||8th||Germany / European consortium|
|A330-200 MRTT / FSTA KC-30 / KC-45||Probe, funnel, boom||current||2006–||58 m||60 m||27+||27 (2015)||Germany / European consortium|
|Boeing KB-29 M||Probe, drogue||historical||1949–1951 (approx.)||30 m||43 m||92||-||United States|
|Boeing KB-29 R / P (conversions of the Boeing B-29 )||boom||historical||1949–1951 (approx.)||30 m||43 m||190||-||United States|
|Boeing KB-50 (variant of the Boeing B-29 )||Probe, drogue||historical||1947–1953 (approx.)||30 m||43 m||371||-||United States|
|Boeing KC-97||boom||historical||Early 1950s||36 m||43 m||816||1||United States|
|Boeing KC-135 Stratotanker||Boom; also catch funnel adapter||current||1954-1991||41 m||39 m||803||539||United States|
|Boeing 747-100 KC-33 (for the Iranian Air Force )||Probe, drogue||current||1970s||71 m||60 m||4th||1||United States|
|Grumman KA-6 (conversions of the A-6A / E)||Probe, drogue||historical||1962-1990||17 m||16/10 m||90 (approx.)||-||United States|
|Handley Page Victor B.1A / K.2P / BK.1 / K.2 (variant of the HP80)||Probe, drogue||historical||1952–1963 (approx.)||35 m||34 m||20 (approx.)||-||United Kingdom|
|Ilyushin Il-78M "Midas"||Probe, drogue||current||1984–1992, 2005–||47 m||50 m||40 (approx.)||26th||Russia (former Soviet Union )|
|Xian HY-6U (variant of the Tupolew Tu-16N "Badger" )||Probe, drogue||current||1975–||34 m||34 m||10 (approx.)||8 (approx.)||People's Republic of China|
|Lockheed HC-130 and KC-130 (variants of the C-130)||Probe, drogue||current||Mid 1950s -||35 m||40 m||210 (approx.)||102 (approx.)||United States|
|Lockheed L-1011 -KC1, -K1 (conversions for Royal Air Force )||Probe, drogue||current||1980s||50 m||50 m||9||9||United States|
|McDonnell Douglas KC-10 "Extender"||Boom, also retractable hose with catch funnel||current||1979-1990||55 m||50 m||60||59||United States|
|Vickers 667 Valiant Type 733/758 K||Probe, drogue||historical||1952-1957||33 m||35 m||57||-||United Kingdom|
|Vickers VC10 K / SuperVC10 K (conversions)||Probe, drogue||current||1962–1970, 1978 – mid-1990s (approx.)||49 m||45 m||28+ (?)||16||United Kingdom|
Air refueling tankers are mostly designed as combined cargo and tanker aircraft. Strictly speaking, these are mostly cargo planes with the technical facilities for air refueling. For example, the specification for the KC-45 does not provide for an additional tank compared to the Airbus A330-200 . Tanker planes simply refuel, which is rarely the case in commercial aviation. The standard tanks are used, which are located in the wings, in the tail units and at the bottom of the fuselage.
The total capacity of a tanker is heavily dependent on its size. One example is the KC-45, which can fill up with kerosene with a mass of 111 tons. He can donate most of this if the air refueling takes place near his own departure airport, or correspondingly less if the tanker first has to fly longer distances to the refueling area.
The sloshing of the fuel back and forth is avoided in tanker aircraft - just as in commercial aircraft, ships or trucks - with the help of baffles ( baffles ). These are half-height walls in the tank that reach down from above and thus divide the tank into compartments without obstructing the drain on the floor.
The size of the crew in a tanker varies from three to more. In any case, the commander and copilot are on board. The flight engineer is added to older aircraft . One or more operators for the boom are on board for refueling, and if there is cargo on board, then usually the load master as well. In the future, the number of people operating the refueling equipment should be saved and the process should run fully automatically.
In normal air refueling operation, the technology ensures that kerosene only flows if the connection (hose or boom) is securely and tightly closed. This should prevent environmental pollution. Even if air refueling is unsuccessful, it must be ensured that, as with fuel dumping , most of the kerosene evaporates on the way down and is therefore harmless to the people on the ground.
The same applies to landing with a full tank as to the corresponding civilian variant. Again using the example: An Airbus A330 has an approved maximum take-off weight of 238 t, but its maximum landing weight is only 182 t. If the aircraft was fully loaded or fully refueled when taking off, the difference of 56 t must either be used up by a correspondingly long flight, handed over to other aircraft or, in an emergency, lowered before landing.
Air refueling tanks
Air armament is a new theoretical concept for rearming combat aircraft in flight. A cargo aircraft is supposed to hang bombs and missiles on the mounts of the warplanes with a special boom system that extends from the stern . In this way, multiple attacks would be possible without returning and landing for rearmament. The US Air Force and the Israeli company Far Technologies filed a patent application for a patent in 2003. The practical implementation is still a long way off. Among other things, a robot arm for the cargo aircraft and new robust suspension mechanisms for the aircraft to be armed must be developed.
Air refueling of passenger planes
A study by the Zurich University of Applied Sciences showed that the use of air refueling in passenger jets could reduce fuel consumption by 11 to 23 percent, and noise pollution near airports from aircraft taking off could also be reduced. The take-off would take place with less fuel and after reaching an altitude of 10,000 meters, the refueling would take place. This also enables non-stop flights from Europe to Australia, which is already possible with some aircraft such as the Airbus A350-900 ULR .
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- K. Schwarz: Flying gas stations. In: Flug Revue No. 3/2018, pp. 42–47; with an overview of current tanker aircraft
- Aerial Refueling Systems Advisory Group (ARSAG International), military advisors from 15 nations (English)
- airpower.at: The Tanker War (the first page contains technical details)
- James K. Libbey: Alexander P. de Seversky and the Quest for Air Power . Potomac Books, 2013, ISBN 978-1-61234-179-8 ( limited preview in Google Book Search).
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- Internet source | url = http://www.joebaugher.com/usaf_bombers/b29_23.html
- not yet checked in detail
- The length and span dimensions are rounded to the nearest meter in order to keep the table clear. Different sizes for variants of a model: xy m, for swing-wing aircraft : maximum / minimum size.
- Overview in: Flight International : World Airliners, 21. – 27. Oct. 2008, pp. 31–43 and Oct. 28 - Nov. 3, 2008, pp. 51–63 (all aircraft produced up to mid-2008 and all aircraft still in use including temporarily decommissioned examples).
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- Archive link ( Memento from March 30, 2012 in the Internet Archive )
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- USAF Museum: picture of a KB-29 ( memento from April 21, 2007 in the web archive archive.today )
- as models KC-135E / R / T
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- IL-78MK refueling tanker. ilyushin.org, archived from the original on February 23, 2007 ; accessed on November 10, 2009 .
- Peter Simmons, Lockheed Martin Completes KC-130J Marine Corps Tanker Deliveries for 2001. (No longer available online.) Lockheed Martin Corporation, December 3, 2001, archived from the original ; accessed on June 16, 2019 (English, to upgrade to KC-130J by 2001).
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- Report on the study on air refueling in passenger planes