Profile – Boeing’s Jet Heritage

The Boeing 367-80, or the Dash 80 as it is more popularly known, had its origins in a mix of Boeing piston engined transports and military jet bombers. Boeing had long been a leader in the field of transport aircraft, but more often than not its lead had been rapidly overtaken by unfortunate circumstances or other companies. In February 1933 for instance Boeing introduced its Model 247, which revolutionised air travel with its all metal, twin engined monoplane construction and performance greater than that of standard airliners of theday,such as the Ford Trimotor and a variety of biplane transports. But in June the same year Douglas unveiled its rival DC-1 prototype, which fast developed into the successful DC-2 of 1934, and then into the highly successful DC-3 in late 1935. The DC-2 so overshadowed the 247 that Boeing ended production at just 75 aircraft.

Boeing then developed the four engined Model 307 Stratoliner which, based on the B-17, first flew in September 1938. Potentially a very successful airliner, Boeing was forced to cease production at just 10 aircraft to concentrate on military contracts when World War II erupted. After the war’s end, Boeing’s attempt to break back into the civil market met with little success, its Model 377 Stratocruiser (a civil version of the Model 367 C-97 Stratofreighter) being little competition against the Lockheed Constellation and Douglas DC-6. Boeing therefore concentrated on military production while planning a long term comeback into the commercial market that would not only revolutionise airline travel, but also give Boeing a market lead that it has never lost.

Secret development

The 367-80 design can be traced back to 1950, when Boeing initiated studies to develop a successor to the piston engined C-97, to be powered by either turboprop or turbojet engines. Very soon the move towards a jet transport was made, with one early design known as the Model 367-64 incorporating a C-97 type fuselage fitted with moderately swept wings and powered by two pairs of pylon mounted Pratt & Whitney J57 turbojets. Even as the aircraft developed into a jet transport and changed shape significantly, it was decided to retain the Model 367 designation to give the impression that a new Stratofreighter variant was still being designed.

Despite its early development, Boeing’s aircraft would not be the first jet transport to fly, however it would without doubt be the greatest leap forward in design. Preliminary studies for the world’s first jet transport, the De Havilland DH104 Comet, commenced in 1943, detailed design began in September 1946 and first flight was achieved on 2 April 1949 (refer July & Sept 89 AA). The first Comet delivery was made on 2 May 1952, eighteen days before the Dash 80 was approved for final development and construction.

But while the Comet was at least moderately successful as the world’s only first generation commercial jetliner, the first two North American jet transports were another story. The world’s second jet transport, the Avro Canada C-102, was only built in prototype form, featuring a straight wing design with jet engines merely placed where piston engines should have been. The first US jet transport, the XC-123A, was perhaps even worse, being an all metal Chase G-20 assault glider fitted with two B-47 jet pods. The jet engines, intended to give mobility, were so unsuitable that piston engines were instead chosen before the aircraft eventually entered production as the Fairchild C-1238.

Thus there was little knowledge to be gained from these early designs, except perhaps in that straight or quite bulky wings with unsuitable engine mounts was not the way to go. Fortunately, Boeing also had access to extensive German wartime files which confirmed the advantages of swept wing configurations, which offered less drag and provided the same wing area but with appreciably less span. This knowledge was put to good use in Boeing’s first jet bombers, the 8-47 and B-52, and experience with these aircraft proved to be of great advantage when designing the Dash 80.

Using the experience of military jet bomber design, and having a firm fuselage basis in the C-97 Stratofreighter, Boeing’s design team proceeded to develop on paper a very modern aircraft. It designed the Model 367-80 with a modified figure eight Stratocruiser type fuselage with the same 132in width but with the pronounced centre fuselage crease smoothed out to enhance aerodynamic performance. The wing was to be an all new design, with the same 35 degree sweep as the military jets but much more rigid and with pronounced dihedral built in, a feature also of the horizontal tail. Wing flaps would be divided opposite the inboard nacelles to avoid engine exhaust interference, while two sets of ailerons would be used, small ailerons for high speed cruise and larger ones near the wingtips for takeoff and landing. Add four Pratt & Whitney JT3 turbojets (commercial variants of the J57) installed on single pods (two pairs of B-52 style twin mountings were ruled out as being unsafe in the event of a 50% power failure) and the 367-80 on paper looked a real winner.

And so in 1952 Boeing’s jet transport design was ‘frozen’. Its potential was well recognised by the company, but strong lobbying to both airlines and, as an alternative, the military, oddly enough produced little official interest. The US Air Force would not even consider purchasing the unproven ‘paper plane’.

Dash 80 developed

Despite the less than enthusiastic response of both airlines and the military, Boeing were certain that the 367-80 could be developed into a very successful aircraft. Therefore on 20 May 1952 the company’s board formally approved the construction of the prototype jet transport, budgeting an unprecedented $US16m (1952 values) of Boeing money for the project. Boeing remained well aware of its past commercial setbacks and the commercial lead held by rivals Lockheed and Douglas, so the 367-80 model number was retained to mask the fact that the jet transport was being built. Secrecy was to be maintained for as long as possible, though sketchy details of the project were revealed as early as September 1952 Boeing only ever planned to build the one Dash 80. It was strictly to be a proof of concept experimental aircraft and was never intended to enter service with either a military or civil customer, as had previously been the usual practice with most prototype aircraft. Being a one-off, the aircraft could be built with few refinements, the fuselage interior therefore being virtually bare when completed. It featured few cabin windows but two large cargo doors at each end of the 90ft (27. 45m) long main cabin, neither of which would be standard on either the military or civil derivatives.

However, despite being a one-off aircraft and despite the fact that Boeing was also aiming to capture the USAF jet tanker contract, the Dash 80, once it was officially made public several months prior to its rollout, widely became known as the 707 prototype. The Boeing sales team was pushing the jet airliner concept very hard to potential airline customers, so all Boeing publicity began to refer to the aircraft as the Model 707. When Jane’s All the World’s Aircraft briefly described the project in its 1953/54 edition, it wrote: “The Model 707, for some time known as Project X , will be the first American jet transport to fly in prototype form [ignoring the Chase XC-123A]. ” The Dash 80 continued to be called a Model 707 for several years by Boeing sales people and the general public (Jane’s continued to refer to the aircraft as the 707-80), though it was always known as the 367-80 to Boeing and FAA personnel. Only during the late 1960s, perhaps as a mark of respect for its unique position in aviation history, did the aircraft come to be more widely referred to as the 367-80, or more popularly the Dash 80

Flight testing

After two years of secret development and over a year of behind closed doors construction, the Boeing 367-80 was officially rolled out of the Renton factory on 14 May 1954. The Dash 80, somewhat appropriately registered N70700, took off on its first flight at 2. 14pm on 15 July, and by all accounts was an impressive sight as, with its 130ft wingspan and four deafening JT3P turbojets, it lifted into the air after using only 2,100ft of runway. The aircraft climbed steadily, test pilot ‘Tex’ Johnston later noting that “she wanted to climb like a rocket . . . we had to throttle back to keep within airspeed limits set for this first flight”. The flight lasted 1hr 24mins, with no unexpected technical problems.

The initial flight testing period then began to prove the aircraft’s worth. Results were impressive – for one thing the Dash 80 cruised at 478kt (885km/h), more than 85kt (107km/h) faster than the Comet 2 then in service. The US Air Force was now taking much notice of the new aircraft, and just three weeks after first flight it confirmed that a Dash 80 derivative was being considered for its jet tanker requirement; eight months later formal contracts were signed. However, the public and airlines alike remained wary of jet transportation, particularly in the light of the spate of Comet crashes during 1953. But at least Boeing possessed a proof of concept aircraft which could be evaluated to prove jet transportation’s safety. Douglas on the other hand had reacted to the Comet crashes by suspending design of its first jetliner, and instead chose to develop the DC-7, a turbocharged piston engined DC-6 derivative!

From its first flight, the Dash 80 commenced a series of test flights to prove the jet transport concept, the ultimate aim being to attract commercial customers so the 707 production line could be given the green light. The 367-80 was extensively tested so that every flight parameter was scrutinised, every part of the aircraft’s structure tested. All did not go well however, for instance on 5 August 1954 brake failure resulted in an unfortunate ground accident. With Tex Johnston at the controls (as he almost always was during the initial test period) the brakes failed on landing and the aircraft continued a fast roll towards the end of the runway. In an effort to slow it down, Johnston took the Dash 80 off the runway into soft ground, continuing to turn the aircraft so as to remain within the airport perimeter. Unfortunately, he ran into some hidden cement blocks and the Dash 80 finished up nose down, minus its front landing gear.

Unfortunate though this accident was, it did serve the purpose of revealing an interesting weakness in the aircraft’s hydraulic system, which was corrected very soon after. Prior to the flight that day a series of brake tests had been conducted and in the process the hydraulic brake fluid overheated, but then “cold soaked” and contracted during flight so that there was not enough fluid to actuate the brakes, just as if there had been a line rupture. On a later flight following more brake testing the brakes themselves overheated, to such an extent that at 22,000ft a small explosion was felt and an F-86 chase plane observed smoke pouring from the wheel wells. Some tyres had actually caught fire and exploded, but despite this a safe landing was executed after the flames had been blown out by lowering the gear.

In other early testing Boeing engineers hooked up an icing machine to one engine to determine the effects of the worst possible icing conditions on the engine pods. A grid of tubes was mounted six feet in front of the number three engine nacelle (right inboard), through which water was sprayed directly into the engine inlet duct, creating an icing condition of minus 50 deg Fahrenheit. Using the test experience, Boeing engineers developed a system to prevent ice from entering the engine in its solid state, the only ice around the engine being that which formed on the nacelle and was soon blown away by airspeed. Operation ‘Ice Machine’ showed that engine icing would not be a serious problem during high speed, high altitude flight.

Yet another test series involved high speed flutter investigation. During one flight Johnston exceeded the maximum Mach speed by . 001 (less than 2km per hour) and experienced rudder flutter causing “an appreciable vibration”, surely an understatement given the tact that the flight engineer’s panel was torn off in flight. The problem was corrected and subsequent flights proved that the solution did work very well, with no flutter or vibration experienced during a series of dives which took the Dash 80 up to at least Mach . 97 speed.

Such testing of the 367-80 proved that both the 707 and the KC-135 would indeed be successful aircraft. By the time the USAF had placed its official order for the KC-135, the 367-80had already demonstrated its tanker capability with a refuelling flying boom, though USAF interest had been confirmed even prior to this. Airline interest in the 707 steadily grew also, and as it was proved during research and both ground and flight testing that the problems experienced by the DH Comet had been overcome, orders began to flow in, the first tor the 707 being signed by Pan American on 13 October 1955.

But besides flight testing, it was an unofficial flight performance not in the text book that perhaps best showed what the new jet transport could do. Some months after flight testing began, Johnston was detailed to conduct a flypast of the Gold Cup Hydroplane Boat Races being held on Lake Washington, Seattle. On the water was Boeing president, William Allen, onboard a yacht entertaining guests from the prestigious Aircraft Industries Association. As the 367-80 roared overhead and alleyes were facing skyward, the aircraft was suddenly and most unexpectantly put into a near perfect climbing barrel roll, a sight indeed to behold. Tex Johnston, the ex 1930s barnstormer, showman and undoubted natural pilot, had initiated (albeit without prior consent) what was without doubt Boeing’s best ever publicity stunt. Some strong words were reportedly later exchanged behind closed doors, where it was agreed never to perform the act again. However news of the flight quickly spread and the Boeing jet transport age was on its way.

But still more testing to the very end

Once the military had ordered the KC-135 and airlines had opted for the new 707, the 367-80 could conceivably have been retired, its prime aims having now been achieved. However this was riot to be the case, as the aircraft was destined to continue experimental flying up until 1972, cementing the claim that it is the most tested aircraft ever.

As Dash 80 continued its test career, its characteristics changed quite noticeably several times, mainly in the nose area. Less noticeable perhaps, but considerably more important, is the series of modifications undertaken on the aircraft’s wings. Before being grounded in 1972, the Dash 80’s wings underwent considerable change, with its plantorm, thickness and aerofoil being altered by building a sleeve to fit over the inboard end of the wing structure. This increased the wing area by 39. 1m2, while new wingtips brought the span up to that of early model 707s.

Early tests involved titting the 367-80 with a special soft ground landing gear featuring twenty wheels as against the usual ten. This enabled the aircraft to be landed on the soft surface of Harper Lake, thereby testing the possibilities of a large, unsealed airstrip capable military jet transport. Back in the air, the Dash 80 at one stage also tested the feasibility of the rear fuselage engine mount concept for use on the 727, a necessity required tor the 727 design to proceed.

Some experimental flap installations reportedly had quite interesting side effects on flight characteristics. Tests were conducted with boundary layer control and a massive set of enlarged wing flaps which extended 85 degrees, but were merely bolted onto the aircraft so could not be retracted. The flaps were combined with boundary layer control, in which bleed air was forced over the surfaces, such as the flaps, to further slow flight speed capability. The obvious result was that the Dash 80’s airspeed was dramatically reduced to the point that the usual F-86 Sabre chase planes had to make way tor slower piston engined aircraft. The Dash 80 in this configuration often landed at about 78kt (144. 8km/h), while its flying speed was once reduced to just 65kt (120km/h) before stalling. The system was designed to allow an aircraft to be developed with very slow takeoff and landing speeds but with no effect on cruise speed.

From very slow speeds the Dash 80 later went to very fast speeds, at least in theory. By 1965 it had been rigged to determine flight characteristics of NASP:s supersonic transport (SSD. A long needle nose was added and the copilot’s seat was set up for use by an SST evaluation pilot, the SST test flight controls being adapted with the use of a computer to operate as if the aircraft were indeed an SST. Thus while the Dash 80 flew at its usual speed, the Dash 80’s test equipment recorded supersonic flight characteristics.

And so the tests continued from the mid 1950s to 1972. The aircraft flew with numerous experimental devices, both internal and external. It helped develop new aircraft, test new engines (it at one stage was even powered by three different engine types), conduct scientific experiments and generally fly as the ultimate aviation ‘guinea pig’. But finally in 1972 the much flown, scarred veteran of over 2,000 flights was retired. Donated to the National Air and Space Museum, it sat cocooned in the Arizona desert tor eighteen years due to a lack of exhibition space; viewed however by thousands of enthusiasts each year passing through the MASDC, at Davis Montham AFB, Tucson.

But the testing was not quite over. In 1990 the 367-80 was refurbished for its final flight back to its birth place Seattle, to go on display at Boeing’s Museum of Flight. Prior to display, the aircraft was destined to receive a final paint job to restore it back to past glory – and what better opportunity to test a new, environmentally friendly paint stripping method. Using dry ice pellets leaving no toxic chemical waste, Dash 80 was stripped bare, repainted, and placed on display (with the similarly historic, but slightly less significant prototype 747). And so the world’s most tested aircraft’s final act prior to being put to rest was to symbolically undergo one last test, the last of many.

So marked the end of the line for the historic 367-80, which the Smithsonian has declared one of the 12 most important aircraft of all time. The Dash 80 will never be forgotten, just as its contribution to aviation can never be underestimated as its many descendants ply their trade in the skies above KC-135 developments

It did not take the US Air Force long to evaluate the Boeing 367-80 and appreciate its service potential. Often mistakenly regarded as a 707 variant, the USAF KC-135 is in fact a distinct type in its own right, and without doubt one of the most successful military aircraft of all time.

The importance of air-to-air refuelling in the jet age soon became apparent to the US Air Force, particularly with the rapid growth of its jet fighter fleet during the Korean war. In the late 1940s 116 B-29 Superfortresses had been converted to KB-29Ps with Boeing’s flying boom refuelling equipment. In 1951 the first of 814 Boeing KC-97 Stratofreighters entered service, alongside 136 KB-50 Superfortress tankers. With the increasing performance parameters of jet fighters and the entry into service of jet bombers came the addition ofjet pods to these otherwise piston engined tankers, though such pods could only be an interim measure – an all jet tanker was needed.

Towards the end of 1953 the USAF called tor expressions of interest to supply the Strategic Air Command (SAC) with some 800 jet tanker/transports to support its B-47, B-52 and projected B-58 jet bomber fleet. Boeing, Convair, Douglas, Fairchild, Lockheed and Martin were invited to make formal submissions for the project in May 1954, by which time the Dash 80 was less than two months away from making its first flight, giving Boeing a substantial lead over its competitors. The Dash 80 made such an impression on US Air Force observers that just three weeks after its first flight on 15 July 1954 (and before its refuelling capability was even demonstrated) the USAF revealed that it was seriously considering the purchase of 29 Boeing jet tankers. The new aircraft (Boeing model 717) was given the KC-135A designation on 5 October 1954 when the contracts were officially formalised.

The first production KC-135A Stratotanker (serial 55-3118) was rolled out of Boeing’s Renton factory on 18 July 1956 and made its first flight on 31 August 1956. After an initial testing period deliveries commenced on 30 April 1957 and the type entered service with SAC on 18 June that year. By that date 215 KC-135As (costing about $US3. 67m each) had been ordered, and by the time of the last delivery on 12 January 1965 that figure had climbed to 732. Such had been the success of the design, that the USAF had in the meantime also ordered 17 KC-135B Stratotankers and 10 RC-135B reconnaissance variants for SAC, as well as 15 C-135A and 30 C-135B Stratolifters and 4 RC-135As for the Military Air Transport Service (MATS). All A models were powered by Pratt & Whitney J57 turbojets rated at about 13,7501b, while B models flew with four 18,000lb thrust P&W TF33-P-5 turbofans.

The French air force also ordered 12 C-135F Stratotankers (similar to the KC-135A), making it the only non US service to operate any KC/C-135 variant. The -135 production line closed in February 1965 with the completion of the last RC-135B (the 820th -135 airframe, though the 14 RC-135s were given the Boeing model number 739) but the final RC-135A was not delivered o the USAF until January 1966 because of the need to install special equipment.

KC-135A

Very soon after entering service the KC-135A to prove its worth with SAC, its refuelling and transport capab11it1es being unprecedented at the time. The fuel carrying capacity of the KC-135A is considerable, with up to 21 tanks tor both refuelling and its own use being contained. Besides its refuelling capability, the KC-135A has considerable troop/cargo capacity. A 1. 83 x 2. 9m cargo door is located in the port side forward of the wing. Some 80 to 160 troops can be transported, or 37,650kg of payload. A typical crew of tour consists of pilot and copilot, navigator and boom operator. During normal flight, the aircraft utilises the centre wing tank (with six bladder cells) for its own fuel consumption. The remaining fuel capacity consists of two main tanks and an outboard reserve tank in each wing, four bladder cells in the forward lower fuselage body tank, five bladder cells in the aft body tank behind the main landing gear, and one tank in the rear of the fuselage above the cabin floor. These are normally used for refuelling purposes, though for additional range the cross feed fuel system can supply any engine with fuel from any tank, while for maximum refuelling capacity all but 1,000 US gal(3,785 litres) can be used from any tank.

The flying boom refuelling system adopted in the KC-135A was a B _ oeing designed invention first used in the earlier piston engined tankers. Two types of refuelling booms are available for use, high speed and standard speed interchangeable booms both typically constructed of two concentric tubular section capable of being extended 8. 5-14. 3m. Standard speed booms can be used at up to 330kt (612km/h) calibrated airspeed or Mach 0. 85, whichever is less. High speed booms have no speed restriction, and were typically used by specialised KC-135O tankers to refuel SR-71 Blackbirds. Both boom types are rigid pipe like structures, which are lowered from the tanker by the boom operator and with the small winglets at their outer limit are “flown” until the tip makes contact with the receiver aircraft formating behind and below. Fuel is pumped from the tanks through check valves into the air refuelling manifold, from there to the air refuelling line valve, through the refuelling pressure regulator and fuel flow transmitter, then out through the boom to the receiver aircraft.

Some tankers are equipped with a reverse flow system so that they themselves can take on fuel from receiver aircraft through the refuelling booms. Many others are also equipped with an air refuelling receiver (ARR) system, with a refuelling slipway and receptacle installed on the top centreline fuselage just aft of the cockpit, and covered when not in use. In 1960 a hose and drogue refuelling system was also designed so the KC-135A could support Tactical Air Command (TAC) fighters then fitted for probeand-drogue refuelling. The collapsible drogue was fitted to 1. 07m of hose and 1. 22m of rigid tubing, which when fitted to an adaptor formed an assembly 3. 96m long.

C-135

Whilst the Boeing KC-135A fast became the standard SAC tanker, the C-13 _ 5 variant was procured for the USAF Military Air Transport Service. However, unlike its tanker stablemate the 135 ‘:”as destined to be no more than a stop gap aircraft pendIng delivery of Lockheed C-141A Starlifters. The C-135A was powered by the same P&W J57-P-59W turbojets as the KC-135A, while the B introduced P&W T F33-P-5 turbofans with thrust reversers. The first three C-135As were actually converted KC-135As delivered on 19 May 1961 which reverted to KC-135A when the purpose built C-135s entered service. The standard C-135 featured a taller fin than the early KC-135A with the powered rudder of later tankers. Fifteen C-135As were delivered along with 17 Bs. Most served initially with either MATS’s Eastern Transport Air Force (EASTAF) flying routes from the us to Europe and t _ he Middle East, or WESTAF flying to Asia. One aircraft was assigned to the Los Alamos Scientific Laboratories for research purposes.

. The C-135s flew in the transport role until 1965, from which time the C-141 progressively became the mainstay of the USAF’s Military Airlift Command (MAC). Most c:. . fa5s were then assigned to the Air Force Systems Command (AFSC) test fleet, S _ AC reconnaissance fleet, MAC VIP fleet or the Air Weather Service fleet. Five C-135Bs were converted into VC-135Bs in 1975 for personnel transport operations, though at least three later reverted to C-1358 standard.

KC-135s and the 32 C-135s, 14 RC-135 reconnaissance variants (4 As and 10 Bs) were also delivered to the SAF from the production line. The principal mission of these aircraft was photomapping and geodetic survey work. A camera compartment replaced a portion of the forward fuselage fuel tank, with cameras installed in the floor. By 1974 the four A models had their camera equipment removed and passenger seating installed, and by 1981 the last had been remodified to KC-135D tankers.

Vietnam

Whilst the refuelling capability of the KC-135A was readily appreciated, it was during the Vietnam War that the -135 came into its own, with as many as ten different variants flying in the theatre. With the rapid build up of US air power in and around Vietnam during the 1960s, the need for air-to-air refuelling also grew rapidly. In early 1964, when the US build-up was still in its early stages, there wasjust one unit of KB-50J tankers at Yokota, Japan and several KC-135As stationed at Clark and Anderson AFBs, Philippines. In 1964, however, the KB-50 was grounded and from 1964 over 30 KC-135A equipped squadrons were deployed to south east Asia to a range of bases, including Anderson and Clark (Philippines), Don Muang, Takhli and U-Tapao (Thailand) and Kadena (Japan). Those at Anderson and Clark AFBs generally supported B-52 ‘Arc Light’ bombing missions from 1965, whilst Thailand based KC-135As usually supported tactical types (F-4, F-105, F-111, etc) based in Thailand and South Vietnam. Twelve colour coded tracks were usually available to be flown by the aircraft with refuelling link-up points controlled from ground stations.

The tanker force in SE Asia was reduced from 1972 when the air war was scaled down. However late in 1972 it was quickly increased(up to at least 168 KC-135As) as the North Vietnamese launched their offensive. The peak number of tanker sorties was flown during 1972 as B-52s and tactical types mounted numerous strikes against North Vietnamese forces. The KC-135As rarely flew over North Vietnam itself, generally operating instead in its approaches at 26,000ft for 8-52 refuelling and 15,000-20,000ft for other types, well above ground fire. From a total of 194,687 refuelling sorties flown in 1972, just five aircraft were lost. Some 4,072m kg of fuel was transferred during 813,878 refuellings.

The C-135s played a lesser role in Vietnam, carrying troops to war and evacuating wounded but soon being supplanted by C-141s and C-5As as the main US transpor t aircraft. But while the transports faded from the scene, electronics and reconnaissance variants remained important weapons in the war. For instance, as the North Vietnamese anti aircraft system developed and became more accurate, the need developed to coordinate air strikes and rescue missions with ground communications. Initially radio relay services were flown by slightly modified KC-135As, but from 1969 these were replaced by specialised EC-135L electronic communications aircraft based at U-Tapao. The other significant variant to operate over Vietnam was the RC-135, which flew strategic intelligence gathering missions. No 82 Strategic Recon Sqdn flew RC-135Ms and Us from Japan alongside 6 Strategic Wing’s RC-135Ds on a range of elint(electronic intelligence) and sigint (signals intelligence) tasks from 1967 to 1974.

Variants

While the Vietnam conflict saw at least ten KC/C-135 variants in active service, these were just a small portion of the incredible number of variants which have been developed from the basic -135 airframe. Besides the initial KC-135A, six other variants left the production line(the KC-1358, RC-135NB and C-135NB for the USAF, and the C-135F for the French air force). T henumber of variants, however, has now grown to at least 40, each identifiable by prefix and suffix letter designations; there are as well a number of sub variants. Most variants stemmed from the basic KC-135A- mainly the result of either engine changes or conversions to suit new roles- though none of the original versions has escaped conversion or updating.

The first conversions stemmed from the need to have the C-135 enter service as soon as possible, hence the temporary conversion of three KC-135As to interim C-135A standard. Iron- ically, once the C-135s themselves were replaced a range of modification programs was undertaken, most subsequently becoming test aircraft. Five of the C-135Bs were converted into VC-135BVIP transports, a role in which they served for several years from 1975 before reverting to C-135Bs to serve in various specialist transport roles.

In 1967, eight former MAC C-135As were converted by Douglas into EC-135Ns. These were notable in that they featured a large, bulbous nose radome housing a 2. 13m diameter parabolic dish antenna for the tracking of Apollo/Saturn spacecraft. Four were also fitted with the Airborne Lightweight Optical Tracking System (A-LOTS) for use during tracking missions. Flying in pairs, the EC-135Ns operated over the Atlantic and Pacific Oceans and the Gulf of Mexico during manned space launches, their use being vital to permit continuous voice communications between the spacecraft and ground stations. By 1980 all eight aircraft had been demodified and redesignated C-135Ns to support Air Force Systems Command test missions.

Three C-135As were modified to NC-135A standard for NASA use, initially for atomic test diagnostics but more often used for comet photography or to conduct solar eclipse or cosmic ray studies. In 1969 at least one NC-135A was reassigned to the US Atomic Energy Commission to collect fine atmospheric particles to analyse not only US but foreign nuclear tests. By 1977 all three were grounded, and at least one was eventually scrapped. Other test aircraft with the AFSC include the JKC/NKC-135As, 14 of which have at one time or another been used in such roles as testing countermeasures equipment, airborne laser, icing, satellite communications, missile vulnerability, weightlessness and various technical equipment programs. Two NKC-135As were loaned to the Naval Electronics Systems Command in 1975 for electronic warfare research. They are the only two aircraft of the KC/C-135 family to serve in USN colours.

Two very important -135 families include aircraft modified for electronic and reconnaissance duties. At least 40 aircraft have at some time been modified to EC-135 standard (EC-135A/C/G/ H/J/K/UP). The electronics aircraft have been equipped with a variety of equipment for use in several roles. The six EC-135As were for instance modified to serve as radio relay link aircraft in support of SAC’s Post Attack Command Control System. The 14 Cs were redesignated as airborne command posts, with additional HF probe antennae on each wingtip, two wire antennae extending from the vertical fin to mid and forward fuselage points, a trailing wire antenna housed in a lower fuselage fairing and blade antennae installed along the belly and upper fuselage centre line. Five Gswere conver ted from KC-135As to serve SAC as Airborne Launch Control Centre and radio relay link aircraft, with similar antennae to the C but without the trailing wire. Five Hs were converted with similar blade and wingtip antennae, as well as a saddle shaped radar over the fuselage centre line aft of the overwing exits, and a trailing wire antenna pod located just forward of the right main landing gear wheel well. Three Ks, converted KC-135As (including the very first KC-135A) were pro-duced in the mid1960s to act as airborne command posts for the Tactical Air Command. Most EC-135s have retained refuelling booms and ARR equipment, and several remain in service in their specialist role.

Reconnaissance variants form the other important group of non tanker -135 variants. These include ten RC-135Bs delivered in 1964-65 (later redesignated RC-135C). Large Side Looking Radar (SLAR) cheeks are a notable feature of the RC-135B/Cs, all of which were subsequently converted to RC-135U orV standards. Other RC-135 reconnaissance variants included the D (thimble nose and long, tubular antenna fairings on the fuselage side forward fo the wings) which were reconverted back to KC-135A standard in 1975-79. One branch of this family is that used for weather reconnaissance, at least ten having been converted from C-135B to WC-135B and based worldwide for weather research duties, equipped with U-1 foils on the side of the fuselage to trap air particles.

Post Vietnam operations

The importance of the non tanker operational variants has been shown on many occasions, having long been used to gather intelligence from ‘unfriendly’ nations. In fact at least five -135s (one KC-135A, 2 RC-135Ss, E and one T) have been lost on what are believed to have been operational reconnaissance missions, four being lost (officially in accidents) off the east coast of the Soviet Union. In addition, one RC-135U returned safely from a mission off Libya in 1980 after having been fired upon.

Libya in fact produces a prime example of how both the tanker and various specialist -135 variants are key USAF offensive weapons. Operation ‘El Dorado Canyon’, the April 1986 US bombing raid on Tripoli, relied heavily on several -135 variants. Prior to the actual raid several RC-135Ws and an RC-135V were flown to Hellenkion in Greece to fly intelligence gathering missions off Libya. The KC-135 then became of key importance because both France and Spain denied the US overflight rights for the raid. With a small number of KC-10As, KC-135As and a few Qs were used to refuel the attacking F-111 force at least four times en route Libya and at least twice on the return flight. Without these aircraft, the F-111 raid would not have been possible. Besides the tankers and reconnaissance variants, an EC-135E is believed to have acted as a tactical command centre for the raid, while the final act was carried out by a C-135C which flew photographic evidence of the raid back to Washington.

In the same month, coincidently, the role of more scientific variants was also displayed following the Chernobyl nuclear power plant explosion on 26 April 1986. The Soviets maintained their usual pre Glasnost silence and it was not until two days after the explosion that a Swedish aircraft collected fallout samples and the West was alerted. Following the Swedish discovery, an RC-135U was despatched from Mildenhall, UK to collect more samples. An EC-135H monitored Soviet communications and effectively confirmed the accident, while two WC-135B weather recon variants collected air samples from the easterly airstream. The -135s were therefore an important tool in forcing Soviet authorities to admit the extent of the accident.

In the Gulf War, several -135 variants – tanker/transports, reconnaissance and ECM variants, flying command posts – played a key part in the air campaign, thus adding yet another chapter to the -135’s history. The aircraft flew troops and supplies into the Gulf, provided the main in-flight refuelling capability for all US and several allied types en route to the Gulf region and to a lesser extent during the campaign, and conducted and reconnaissance operations vital to the war effort. French air force C-135FRs were also based in Saudi Arabia, flying mainly refuelling missions for French tactical aircraft.

Reengaging program

By the time of the Libyan raid and the Chernobyl accident most of the specialised -135 variants had been re-engined, a major update program having been introduced to replace the aircraft’s aging and relatively inefficient J57 turbojets with JT3D-2B turbofans. Eighteen special purpose -135s were the first to be re-engined, these being redelivered to the USAF between January and July 1981.

The JT3O-2B was widely used by 707-100B/320B/320Cs and 720Bs which were being retired in large numbers by the early 1980s, so the USAF embarked on a plan to acquire these aircraft with a view to transferring their engines to the KC-135A fleet of the Air National Guard (ANG) and the Air Force Reserve (AFR). The USAF acquired whole fleets of retiring 707s for their engines, flying the airliners to Arizona where they were stripped of their engines, pylons, tailplanes and other useful components. Follow up reengining contracts covered five more special purpose -135s and all 134 ANG and AFR KC-135s. The reengined aircraft (redesignated KC-135E) also received five rotor wheel brakes and a Mk 11/111 anti skid system, and were redelivered to the USAF between 1982 and 1988.

The operational difference made by the reengining program is shown by the fact that the JT3D-2B offers a 30% thrust increase over the J57 powered KC-135A, with 14% fuel savings, 85% less noise and 90% less emission. The KC-135E can also offload 37% more fuel over a 1,500nm radius at normal takeoff weight, and 82% more over a 2,500nm radius.

The other significant reengineering program involves replacing the J57 engines of SAC KC-135As with CFM56-2B-1 turbofans, producing the new designation KC-135R. Also being modified in these aircraft are electrical, hydraulic and performance, fuel management and flight control systems. The main landing gear is strengthened and dual APUs are also installed for quick starting. The R’s first flight was made on 4 August 1982,and modifications continue at the rate of about 4 aircraft per month with over 220 having been completed at the time of writing. The KC-135R is able to offload 65% more fuel over a 1,500nm radius and 150% more fuel at a 2,500nm radius. Increased thrust enables it to take off 762m before a similarly loaded KC-135A. There is 98% less noise during takeoff, and MTOW has been increased about 10,000kg. Eleven similar French air force C-135Fs have also been re-engined with the CFM56-2B-1, and have hence been redesignated C-135FR.

With the aircraft re-engined and the average airframe having used up just 10,000 hours of an expected 30,000 hour life, the KC-135 seems destined to fly on for many years yet. Certainly current plans have the aircraft in USAF service until at least 2020. Further updates may also be undertaken to increase the aircraft’s usefulness and its service life. Boeing for instance has proposed a two crew cockpit for the aircraft, which would introduce all modern flight deck equipment as used in current generation jetliners. Boeing claims that each cockpit update would cost $US1m, though with reduced maintenance costs and one third fewer crew members needed for the fleet, each upgrade would pay for itself in just three to five years.

Thus the KC/C-135 family has still a long way to go. It is by far the most successful tanker project ever and, if all goes to plan, by the time it is retired it will have been the USAF refuelling fleet’s backbone for well over 60 years – and a long way from the first USAF interest in the Dash 80 project.