Boeing KC-135 Stratotanker

**A tanker for the jet age**

In the early 1950s the US Air Force's aerial refuelling relied on the Boeing KC-97 Stratofreighter, a piston-engined derivative of the Stratocruiser that already carried the rigid "flying boom" developed by Boeing. The arrangement had two basic flaws. The first was fuel: the KC-97 ran on aviation gasoline and needed separate kerosene tanks for its receivers, an awkward mix that a jet tanker would solve by using one fuel for its own engines and for the offload. The second was speed: with a cruise of about 370 km/h, the KC-97 forced the new jet bombers and fighters to slow right down to hook up to the boom.

The answer came from an aircraft Boeing had built at its own risk: the 367-80, the "Dash-80", the jet transport proof-of-concept demonstrator that would also lead to the commercial 707. In 1954 Strategic Air Command (SAC) held a competition for a jet tanker and in 1955 Lockheed's entry, a tanker version of its proposed L-193 airliner with engines on the rear fuselage, was declared the winner. But Boeing's design was already flying and could be delivered about two years earlier, so Air Force Secretary Harold E. Talbott ordered 250 Boeing tankers while the Lockheed design matured. In the end the Air Force chose not to support two tanker types and the Lockheed aircraft was dropped; Lockheed never built its jet airliner and Boeing went on to dominate that market with the 707 family.

The first order, for 29 KC-135As, was placed in 1954. Boeing designated the type Model 717 (the first 29 were Model 717-100A and the following batches of 68 and 635 aircraft were 717-146 and 717-148), a number the company would later reuse for a different aircraft. Although it looks very much like the 707 and 720 from outside, the KC-135 came first and is structurally different: a narrower and shorter fuselage, without the row of cabin windows, and a main deck able to take about 80 passengers or the equivalent cargo, while all the tanker equipment sits on the lower deck. It shares with the family the 35-degree swept wing, the four engines in underwing pods, the horizontal tail with positive dihedral, and the distinctive high-frequency antenna that projects forward from the top of the fin.

The first KC-135A flew on 31 August 1956 and the first production aircraft was delivered to Castle Air Force Base, California, on 28 June 1957. Production moved fast: 732 KC-135As were built, and the last member of the family reached the Air Force in 1965, by which time the C-135 variants as a whole (Stratotanker tankers, Stratolifter transports and special-mission aircraft) exceeded 820 airframes.

**How it works: the boom and the prone operator**

The primary transfer method is the rigid boom. The boom operator rides in the rear of the aircraft lying face down, looking through a window in the underside of the tail while steering the boom into the receiver's receptacle; both the boom and the operator's station descend directly from those of the KC-97. Nearly all of the aircraft's internal fuel can be pumped through it. For probe-equipped aircraft, such as those of the Navy and Marine Corps, a shuttlecock-shaped drogue adapter is fitted to the end of the boom. That adapter is far less forgiving of errors by the receiving pilot than a conventional trailing hose and, while it is fitted, the tanker cannot refuel by boom. That is why the MPRS (Multi-point Refueling System) modification was added later, hanging two Flight Refueling Mk.32B pods under the wings: with them the aircraft keeps its boom and can serve two probe-equipped receivers at the same time. Above the refuelling system there is a cargo deck that can carry a mix of passengers and freight.

**From Strategic Air Command to Vietnam**

The KC-135 was born for SAC: its job was to give the B-52 fleet the reach of its strategic mission. In the late 1960s, in Southeast Asia, the aircraft found another trade. Refuelling F-105 and F-4 fighter-bombers, as well as B-52s, put distant targets within reach and let fighters spend hours over the front rather than the few minutes their fuel reserves allowed. Crews served Air Force, Navy and Marine Corps aircraft, switching to the drogue adapter as the mission required, and more than once they shepherded aircraft with punctured tanks, feeding them fuel while still connected all the way to a runway or to a point where the crew could ditch. That force-multiplier experience was repeated in Desert Storm and every campaign since.

Service with regular SAC units ran from 1957 to 1992; from 1975 aircraft also began to pass to the Air National Guard and the Air Force Reserve. When SAC was inactivated in 1992, most went to the new Air Mobility Command (AMC), and a few to the air forces in Europe and the Pacific and to the training command. In May 2018 AMC managed 396 Stratotankers, of which the Reserve and the Guard flew 243. The KC-135 is one of the few fixed-wing military aircraft with more than sixty years of continuous service with its original operator. The oldest of all, 57-1419, built in 1957, was still flying in 2025 with the 161st Air Refueling Wing of the Arizona Air National Guard at Goldwater, and was the oldest active aircraft in the US military.

**Two engine lives**

Every KC-135 left the factory with Pratt & Whitney J57-P-59W turbojets, giving about 44 kN dry and around 58 kN with water injection. That "wet" thrust came from injecting some 670 US gallons of water into the inlet and diffuser over roughly three minutes of take-off: the water cooled the air, made it denser and lowered the turbine temperature, allowing more fuel to be burned for a short time, much like the war emergency power of piston engines.

The first re-engining came in the 1980s. Reserve and Air National Guard aircraft received TF33-PW-102 turbofans taken from retired 707 airliners: 157 according to the account of the programme, 161 according to the list of variants. These KC-135Es were 14% more fuel-efficient than the A, could offload 20% more fuel on long sorties and were the only aircraft in the fleet with thrust reversers. Their career was short: the Es were either converted to R standard or put into storage, and the last one, 56-3630, was delivered by the 101st Air Refueling Wing to the Davis-Monthan boneyard in September 2009.

The second re-engining was the decisive one. Around 500 aircraft of the family received high-bypass CFM56 turbofans (F108 in military designation), built by General Electric and Safran, with 98 kN of thrust, nearly double the J57. KC-135As and some Es became KC-135Rs (more than 417 conversions) and the KC-135Qs became KC-135Ts. The gain was huge: up to 50% more fuel offloaded on a long sortie, 25% better fuel efficiency, 25% lower operating cost and take-off noise cut from 126 to 99 decibels. The R's operational range is 60% greater than the E's for a comparable offload, which widened the choice of bases from which it could operate.

**A new cockpit for an old aircraft**

The avionics were renewed as well. The Pacer CRAG programme (compass, radar and GPS), from 1999 to 2002, modified the whole fleet and eliminated the navigator position; the fuel-management system was also replaced. Rockwell Collins handled development and BAE Systems the installation at Mojave Airport. Block 40.6 brought the aircraft into line with global air-traffic management, and Block 45 added a glass cockpit with digital displays, a radio altimeter and a digital autopilot and flight director; the first 45 upgraded aircraft had been delivered by January 2017, with the work done at Tinker.

**Variants with a trade of their own**

The KC-135Q was modified to carry the JP-7 fuel of the Lockheed SR-71 Blackbird separately from its own fuel: the fuselage tanks carried JP-7 and the wing tanks JP-4 or JP-8. There were 56 of them; the survivors, once fitted with CFM56 engines, became the 54 KC-135Ts, distinguishable from the R only by a clear window under the tail housing a remote-controlled searchlight. Eight KC-135Rs received a receptacle so that they could themselves be refuelled, the KC-135R(RT), all with the 22nd Air Refueling Wing at McConnell, Kansas, in 1994. Four RC-135As returned to tanker configuration in 1979 as KC-135Ds, and 17 KC-135Bs were built as turbofan-powered airborne command posts. For France, 12 dual-role tanker/transport C-135Fs were built with a drogue adapter on the boom; 11 survivors received F108 engines between 1985 and 1988 as C-135FRs. Many KC-135As and Bs were also converted into command, electronic reconnaissance and observation aircraft of the EC-135, RC-135 and OC-135 families.

**Test bed and "Vomit Comet"**

As the NKC-135, the aircraft also served science. Between 1979 and 1980 an NKC-135A lent to NASA tested at Edwards the wingtip winglets designed by Richard Whitcomb of the Langley centre: they reduced drag and increased range by up to 7% at cruise speed, and today they are found on most new commercial and transport jets. NASA also used several KC-135s without tanker equipment as its zero-gravity aircraft, the famous "Vomit Comet"; the longest-serving, from 1973 to 1995, was 59-1481, named Weightless Wonder IV and registered N930NA. Two KC-135As also served with the Federal Aviation Administration to check navigation aids across the country.

**The long search for a successor**

Keeping such a veteran fleet flying grew ever more expensive: between 1993 and 2003 depot maintenance work doubled and the overhaul cost per aircraft tripled. The Air Force calculated that only a few aircraft would reach their flying-hour limits by around 2040, when some would be about 80 years old, and a 2005 study put 2030 as the horizon for upgraded aircraft. The replacement turned into a saga: the KC-767 lease agreed in 2003 (80 bought and 20 leased) was frozen that same year and cancelled in January 2006 after irregularities in its award came to light; the KC-X competition, opened in January 2007, was won in February 2008 by the EADS/Northrop Grumman KC-30, but the Government Accountability Office upheld Boeing's protest in June 2008. Relaunched in February 2010, the competition was won in February 2011 by Boeing's 767-based tanker, the KC-46, the first of which was delivered in January 2019. France and Singapore, the other users, replaced their Stratotankers with Airbus A330 MRTTs.

**Still at war**

Far from retiring, the KC-135 stayed in the front line. On 28 February 2026 it refuelled aircraft in the joint US-Israeli operation against Iran, for which more than 30 KC-135 and KC-46 tankers were deployed to Israel. On 12 March 2026 a KC-135 crashed in western Iraq, killing all six crew members, in what is thought to have been an incident involving another tanker. According to a Pentagon inspector general report cited by The Intercept, Iranian attacks on US bases in the Middle East destroyed or damaged 12 KC-135s during that conflict. Almost seventy years after its first flight, the Stratotanker remains the backbone of the American tanker fleet while the KC-46 gradually replaces it.