Aerial Tanker Refueling Aircraft
The usefulness of air refueling became apparent to the military almost as soon as they started using aircraft. The main advantage of air refueling is obvious: it enables aircraft to stay airborne longer. Since most aircraft are incapable of taking off with maximum fuel and full payload, without in-flight refueling there is always a balance to be struck between range, payload, and fuel. Air refueling is more than just a range stretcher: it allows one to carry out missions with a smaller number of sorties, or alternatively, fewer aircraft.
Common refueling aircraft have a plurality of wing fuel tanks and a central wing tank. Auxiliary fuel tanks can also be provided within or proximate to a fuselage of the aircraft. Fuel is commonly transferred to the boom or hose via a single wall header which is isolable by one or more shut-off valves. Common refueling systems include pumps to pressurize the fuel for transfer from one or more of the tanks, and valves which are controlled between an open and closed condition by simple on-off switches normally positioned on a refueling system panel and manually selected by a trained refueling operator.
Several systems are used by aircrafts in order to aerially refuel other aircrafts. One known type of refueling system is the so-called "boom deploy" system. In this type of system, a boom extends from the tanker or fuel-source aircraft to the fuel-receiving aircraft, whereby fuel is conveyed from the former to the latter aircraft. The boom is generally pivotally mounted beneath the tail of the fuel-source aircraft and must be deployed (i.e., extended) downwardly from the fuel-source aircraft to a fuel-receiving aircraft positioned behind and beneath the fuel-source aircraft.
The other approach to in-flight refueling is the hose-and-drogue system, in which a drogue attached to a fuel hose is extended from the refueling aircraft's belly or wings. The receiver aircraft is equipped with a fixed or retractable probe and the receiving aircraft's pilot flies the probe into the drogue. The advantages of the hose-and-drogue system are the following: (a) up to three receivers can take fuel simultaneously; (b) if one hose/drogue unit (HDU) becomes unserviceable, the tanker still can offload its fuel; (c) the HDU is inherently safer than the heavy, rigid boom, which is restricted in its movements; (d) it is easier to install on non-purpose-built aircraft; and (e) it is compatible with most receivers, e.g., fixed wings, as well as rotorcraft. There were, however, two disadvantages to early hose and drogue system: it has lower fuel transfer rates than the boom system, and the drogue is uncontrollable and is susceptible to winds and gusts. In bad weather conditions and particularly in low level refueling situations the hookup process was very difficult and demanded excessive receiving aircraft pilot maneuvers.
Aerial tankers represent an important augment to air operations in general since they can significantly expand the range and payload envelope of attack aircraft. Experiments with aerial refueling began in the early 1920's, and by the end of the Second World War the air forces of the United States and United Kingdom had achieved mastery of the technique. Soviet efforts began in the late 1940's, and resulted in deployment of a fleet of tanker aircraft. Even such simple expedients as adding external fuel tanks can produce a significant improvement in range.
When the strategic bomber Tu-4 was found to be too 'short-legged' to deliver strikes against the main potential adversary the USA, both Tupolev and Myasishchev OKBs began the task by creating turbine-engined strategic bombers. During the late 1940s and early 1950s the Soviet Union also experimented with aerial refueling. The Soviets experimented with a strange wingtip-to-wingtip system as well as a more conventional probe and drogue system. But the Soviets lost interest in the technology until the early 1960s. Even then, they never became as enthusiastic about aerial refueling as the US Air Force, preferring to operate smaller aircraft closer to their bases.
- Several variants of the Tu-4 were made. An experimental tanker was developed in 1952. Three Soviet Tupolev Tu-4 bombers (a virtual copy of the American B-29 Superfortress) were equipped as tankers and three were equipped as receivers.
- To increase range, the TU-16 subsequently received an air refueling system. Some TU-16 were converted into tanker aircraft, which were first tested in 1955 and received the designation TU-16E [some Western sources suggest the designation was Tu-16Z]. However, they could still be used as bombers. This initial inflight refuelling tanker version used a novel "wingtip-to-wingtip" method, with a hose trailed from the right wingtip and snagged by grapnel trailed by receiver, then winched into fitting in receiver's port wingtip.
- From 1963 on the TU-16 was converted into TU-16N tanker aircraft. This tanker version featured a 'Probe and drogue' system with a Yakovlev-built centerline fueling unit in the bomb bay and ARK-5 beacon. It was mainly used to support probe-equipped Tu-22 and Tu-22M Blinder bomber regiments.
- The Myasishchev M-4 Bison today serves primarily as a tanker. The 3MS2 tanker aircraft included refueling equipment in the bomb bay. The tanker variant used the "drogue and probe" aerial refueling technique.
- The Il-78 'Midas' is an air-to-air refuelling development of the Il-76, built to replace Myasishchev 'Bison' tankers. Service entry was in 1987
- An aerial tanker version of the MiG-29K has been developed. large internal fuel capacity, two drop tanks and the UPAZ unified refueling pod are good prerequisites for a MiG-29 tanker version;
In 1954 Western intellgence had no intelligence that the USSR was actually employing in-flight refueling. However, it was noted that in flight refueling techniques would not impose serious technical problems and the USSR had access to the wartime techniques and equipment employmed by the US In this field. The USSR was known to have evinced interest in Western of refueling techniques, and methods had been discussed in literature. It was therefore neccessary to consider the effect of in-flight refueling in extending the range of Soviet bomber aircraft. a fleet of tanker aircraft, appreciable in their operational use, and the modification of mission aircraft fuel systems, would be necessary before two-way missions US could be conducted on a large scale. The establishment of tanker units would the conversion of TU-4's or new tanker aircraft. Since TU-4 operational units was estimated by Western intelligence to decrease from the 1,100 in 1954 to about 700 aircraft in mid-1957, sufficient TU-4's could be available for conversion to tanker aircraft. With one refueling the combat radius/range of USSR's aircraft could be increased appreciably.
There were a number of errors in estimated Soviet military force projections in the 1950s, but these did not involve disputes within the US Intelligence Community, nor did they become public or policy issues. Some developments in the last half of the 1950s were more consequential. A serious overestimation of Soviet heavy bomber production and a corollary overestimation of refueling tankers from 1955 through 1957 led to a public outcry over an alleged "bomber gap" to Soviet advantage, imperiling US superiority and security. The error was a compound of insufficient information on the current situation in Soviet aviation development and mistaken assumptions as to Soviet military "requirements" for the future. CIA originally joined the other intelligence agencies in accepting Air Force projections, but later, detailed CIA analysis of Soviet aircraft production facilities--information obtained from U-2 reconnaissance flights--and other technical intelligence led to deflating the feared bomber gap by 1958.
Instead of the 700-800 heavy bombers projected from 1955 to 1957, the Soviet Union never fielded more than 150 (plus 50 others configured as tankers). In parallel, the large Soviet medium bomber force (much larger than initially expected) was, in US intelligence estimates, expected to enhance its possible intercontinental capability by aerial refueling from a tanker force estimated in 1954 to reach 850 aircraft by 1959 (even though the Soviet Air Force at that point had no experience in aerial refueling); as late as 1956 the estimate was 350-400 tankers by 1960-61. In fact, the Soviets did not acquire more than about 70.
Problems with Western estimates of Soviet medium bombers and medium range missiles were not primarily shortcomings in information, but error in estimating the Soviet view of their requirement for forces deployed to strike targets in Eurasia, especially forward-based US strike forces. Similarly, the erroneous assumption that the Soviet Union must be intending to build up a large aerial tanker force to enhance the weak intercontinental capabilities of the Bison and Badger aircraft was a case of projecting onto Soviet military planners the US view of priorities.
As late as NIE 11-3/8-88 (the most recent substantially declassified strategic forces estimate), the Western projections for the future continued to be inflated. NIE 11-3/8-88 in December 1988 was still estimating deployment of some 80 to 120 Blackjack (Tu-160) heavy bombers, and up to 150 heavy jet tankers, by the late 1990s. The key judgments section of NIE 11-3/8-91--the next after the 1988 NIE to be available in part in declassified form and the last of the series, issued in August 1991-- noted that the Soviets were reducing the estimated future number of Blackjack bombers, down from the 80-120 by the late 1990s previously estimated to some 40 by the year 2000. (There was no longer even mention of heavy tankers.)