On 15 July 1954, Boeing's famous Model 367-80 prototype took to the air for the first time. The four-jet, swept-wing transport provided the basis for the prolific Model 707 civil airliner and Model 717 (C-135) tanker/transport families, and also set the design philosophy which extended throughout Boeing's hugely successful airliner dynasty. Note: The Model 717 differs primarily from the Model 707 by having a smaller-diameter fuselage, deletion of cabin windows, and reduced size and weight.

In September 1955, following successful trials with the "Dash Eighty" prototype configured with a Boeing-designed flying boom under the rear fuselage, Boeing received the first order for the KC-135A tanker. The first aircraft (#55-3118) flew on 31 August 1956. The KC-135A entered service with the U.S. Air Force on 28 June 1957. The last tanker was delivered to the Air Force in 1965.

A total of 732 KC-135s were built in a long and efficient production run. The first 582 aircraft were built with a short tailfin, but from then on a taller fin was introduced to make the aircraft more stable during takeoff. This feature was eventually retrofitted to all aircraft in the production line.

Over 500 of the tankers — all built by Boeing at its Seattle facilities — remain in service today. In addition, Boeing built 88 aircraft in over 30 different models for other USAF uses, such as cargo transports, flying command posts, electronic reconnaissance and photo mapping. The last of these special-purpose aircraft was delivered in late-1966. Responsibility for the KC-135 was transferred to Boeing in Wichita in 1969.

During aerial refueling operations, nearly all internal fuel can be pumped through the tanker's flying boom, the KC-135's primary fuel transfer method. A special shuttlecock-shaped drogue, attached to and trailed behind the flying boom, is used to refuel aircraft fitted with probes. In the early-1990s, a feasibility study was undertaken to fit the KC-135R with wing pods for the refueling of probe-equipped aircraft from U.S. and foreign services.

The Multi-Point Refueling System Program is an effort to enhance the efficiency and flexibility of the Air Force’s air refueling fleet. Forty-five KC-135R Stratotankers are being outfitted to accept wingtip hose-and-drogue and air refueling pods for refueling NATO and U.S. Navy aircraft. With the number of worldwide joint and combined military operations on the rise, the Department of Defense directed the Air Force to outfit part of its KC-135 fleet with the capability of refueling both probe-and-drogue and boom receptacle aircraft on the same mission. This also the capability of refueling up to two probe-and-drogue aircraft at the same time.

While in flight, the aircraft's large flyable boom is controlled by a boom operator, commonly called "boomer". Performed from a prone position in a fairing under the rear fuselage, there is an excellent view of the boom and the receiver aircraft. Normally, the boomer is in radio contact with the receiver aircraft. A small control column is provided which is linked to control surfaces on the end of the boom. Further controls are provided for extending and retracting the boom and the operation of director lights which provide positional information for receiver pilots. The hook-up is made by directions given to the receiver aircraft through a system of lights located on the belly of the KC-135, just behind the nose gear (the same system was used by the KC-97).

Internally, the KC-135 features integral wing tanks between the spars, and further tanks in the lower lobe of the fuselage, making a total of 22. The fuel cells are made of nylon fabric less than one-sixteenth of an inch thick — a cell weighing 80 pounds will hold seven tons of fuel! The fuel system is managed from the cockpit by the co-pilot. Fuel can be offloaded at a rate of 6,500 pounds (2,948kg) per minute. This is enough fuel in one minute to operate an average family car for one year.

The main cargo hold provides enough room for six standard 463L-type cargo pallets, which are loaded through a side cargo door. Depending on fuel storage configuration, the KC-135 can carry up to 83,000 pounds (37,648kg) of cargo or 80 passengers. A crew of four is comprised of two pilots, a navigator and a boom operator.

Re-skinning

Between 1975 and 1988, Boeing replaced the lower wing skins of all surviving KC-135s to extend their useful lives to beyond the year 2020. The work involved replacing about 1,500 square feet of aluminum on the underside of the wings, which carry most of the wing load in flight, with an improved aluminum alloy. The original wing surface consisted of a type of aluminum more susceptible to fatigue. Skin panels were milled, machined and contoured at Boeing. The wing then went into a rivet assembly jig where stiffeners and skins were joined. The bottom section of the old wing was cut away and replaced by the new sections. In addition to the skin panels, engine strut fittings were also replaced. Each unit required a total of 564 parts, 32,200 steel fasteners and 19,500 aluminum rivets.

Re-engining

In 1981, work also began on re-engining a large portion of the C/KC-135A fleet with turbofans to replace the noisy, smokey, and inefficient J57-P-59W turbojet engines. Of the two programs, the least ambitious was that to upgrade over 160 KC-135A tankers of the Air National Guard and Air Force Reserve (together with a small number of special mission EC/NKC/RC-135 aircraft) with refurbished TF33-PW-102 engines taken from used, commercial 707 airliners. After modification, the airplanes were redesignated as KC-135E.

Fitment of the TF33 provides several important improvements over the original J57. The greater thrust allows the aircraft to operate on far greater safety margins than previously possible, and to use shorter runways. Secondly, the fan engines are more efficient, offering cost-saving and greater fuel offloads on similar mission profiles. Also, noise and air pollution is considerably reduced, a major factor for Guard units operating from civilian airports, and lastly the TF33s incorporate thrust-reversers for enhanced safety during landings.

In 1980, Boeing announced a major upgrade program for the KC-135 involving the fitment of CFM International's CFM56 (military designation F108) high bypass-ratio turbofan engine to offer far greater fuel efficiency, noise reduction and operational flexibility than the E-model's TF33s. Also included was the addition of a Quick Start Auxiliary Power Unit (QSAS), characterized by intake and exhaust ports on the port side of the rear fuselage, which allows the aircraft to undertake autonomous operations from austere locations. Under the company designation KC-135RE, the first conversion took to the air on 4 August 1982.

Designated KC-135R in service (the second time this designation had been applied, having previously been used by a reconnaissance variant of the KC-135A), the first delivery took place in July 1984. Boeing has delivered approximately 400 re-engined KC-135Rs and is under contract for about 432 re-engine kits. Each kit includes struts, nacelles, 12.2 miles of wiring, and other system modification components.

The KC-135R can offload 50 percent more fuel, is 25 percent more fuel efficient, costs 25 percent less to operate and is 96 percent quieter than the KC-135A. This re-engined and enhanced version of the venerable KC-135 tanker is now the mainstay of the U.S. Air Force's tanker fleet.

Future Upgrades

The U.S. Air Force will continue its effort to improve the KC-135 with the Multi-Point Refueling System Program which adds wingtip, hose-and-drogue refueling pods to the tanker. The Air Force has also acknowledged that the cockpit of the KC-135 must be modernized. It has issued a solicitation for new Compass, Radar And Global positioning systems, known as "Pacer CRAG", in the cockpit.

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During the war in Vietnam, KC-135 Stratotankers made the air war different from all previous aerial conflicts. Aerial refueling brought far-flung bombing targets within reach. Combat aircraft, no longer limited by fuel supplies, were able to spend more time in target areas.

The KC-135 tanker fleet also made an invaluable contribution to the success of Operation Desert Storm (1991) in the Persian Gulf, flying around-the-clock missions to maintain operability of allied warplanes. The KC-135s form the backbone of the Air Force tanker fleet, meeting the aerial refueling requirements of bomber, fighter, cargo and reconnaissance forces, as well as the needs of the Navy, Marines, and allied nations.