On August 31, 1956 at just after 1pm local time, a new Boeing jet took to the skies for its first flight. That jet, Boeing KC-135A Stratotanker 55-3118 named ‘City of Renton’, started a story that is now 60 years old – and continuing! In 2016, the KC-135 is still the backbone of the United States Air Forces aerial refuelling capability and one of the longest serving military aircraft in the world.

A brief history of US Air Refuelling

Ever since that historic moment when man lifted from the ground in the first powered flight in 1903, aviators have been constantly trying to find ways to go faster, higher and further. Whilst the first two aspects naturally came about through technological and design improvements over the years, the final point was something that would always be constrained by a finite fuel source. To that end, with aircraft evolving at a rapid pace during these formative years, it was just 18 years on from that first flight that the first attempts to perform aerial refuelling took place – albeit not by conventional means. With barnstorming antics at the heart of the attempt, wing-walker Wesley May is reported to have walked across the wings of a Lincoln Standard J-1 biplane with a five gallon fuel can strapped to his back before climbing on to the wing of a Curtiss Jenny and emptying the contents in to its fuel tank. Although successful, the reality of the concept was that it was little more than a stunt and not a suitable solution going forwards.

Just under two years later, the United States Army Air Service performed an aerial refuelling that symbolised the fledgling start of the systems in use today. Utilising a pair of Airco DH-4 aircraft, the concept itself was straightforward – the ‘tanker’ aircraft flew in the lead whilst the ‘receiver’ formatted line astern and below. When in place, the lead aircraft slowly let out a fuel hose which the second crew member of the receiving aircraft had to ‘catch’ and, once safely in his hands, insert in the fuel tank leaving gravity to transfer the fuel. Although primitive and still fraught with danger, this successful method enabled the aircraft to stay in the air for an incredible 37 hours – setting the benchmark for others to follow.


Over the years that followed, this method continued to be used to extend the duration of flights which were now limited only by engineering reliability and human endurance. This trailing hose method certainly had its risks, most notably with the possibility of fuel leaks or spillages and, whilst the Americans had pioneered the method, other countries were certainly not standing still in developing and enhancing their own solutions. A key figure involved in this development was Sir Alan Cobham, founder of Flight Refuelling Limited – a company that still exists today as part of Cobham plc.

The first practical system developed was essentially an extension of the primitive ‘trailing hose’ technique and became known as the grappled-line looped-hose system. With the receiver aircraft trailing a steel cable, the tanker ‘fired’ a grapple line to snag the cable and reel it in where once at the tanker the refuelling hose was attached and it became the receivers turn to reel it back, attach to the refuelling point and start the fuel transfer. This was done initially using a gravity feed system before being further developed in to a pump driven one. A fairly complex solution requiring multi-crew aircraft, it may not have been the most efficient however became the first system used by the USAF when the Boeing B-29 Superfortress was modified to the KB-29M specification. This system was used during the first non-stop round-the-world flight when KB-29 aircraft refulled B-50 Superfortress ‘Lucky Lady II’ four times during the record-breaking flight.

However, as has been mentioned, this system was only practical when refuelling multi-crewed aircraft such as bombers and, even then, was considered quite complex. With single seat fighters restricted to the amount of fuel they could physically carry, it was soon clear that air-to-air refuelling would provide a massive boost to the aircraft if a suitable system was found. Sir Alan Cobham, realising the shortfalls in the existing system, had started work an alternate solution by the late forties – the Probe and Drogue system that is still in use today by many air forces around the world. Working with the Royal Air Force to develop it, the solution was similar in concept to the looped hose system however, instead of having to reel the fuel line in, the receiver flies a probe into a basket attached to the end of a hose trailed by the tanker to commence refuelling. Not only is this method used by dedicated tanker aircraft, it has also been successfully used in ‘buddy’ tanking between fighter types such as A-4 Skyhawks, Buccaneer and F-18 Hornets.

Of course, the Americans were also developing their own system to overcome the shortcomings which led to the flying boom solution that is still a prominent part of the countries air refuelling capability today. First fitted as an upgrade to the KB-29, the boom sits flush with the tanker during standard operations before being lowered and extended in to position for the aerial refuelling. Unlike the hose and drogue system, the boom has an element of control over its flight in the air with a ‘boomer’ manipulating small flying control surfaces towards the the end of it to help position with the receptacle on the receiving aircraft.

Following on from the modified B-29’s, the United States continued to use aircraft developed from the same basic airframe for tanker duties in the early years. As both fighter and bomber aircraft developed, especially in terms of the use of jet engine variants of the latter, the KB-29 soon struggled to fly at the sufficient speeds needed to safely provide refuelling. Using the probe and drogue system in a triple hose configuration – a centerline hose along with one on each wingtip – the KB-50 Superfortress was the first to emerge and was fitted with a pair of General Electric turbojets to give it additional performance. Following on, and providing an upgraded flying boom platform was the KC-97 Stratofreighter – the first purpose built tanker aircraft to be produced. Despite this, at a time when the air forces bomber inventory was being upgraded to types such as the B-47 Stratojet and B-52 Stratofortress, it was still clear that the piston-engine tankers were lagging was behind and forcing the receivers to fly dangerously slow in order to take on fuel.

The search for a new tanker platform

In 1954, with this in mind, the USAF Strategic Air Command (SAC) set out to find a more modern, jet powered tanker that had a performance envelope more similar to that of the aircraft it would be refuelling. It should be of no surprise that two of the largest companies in the US at the time both vied for the honours – both Lockheed and Boeing putting aircraft forward. Of particular importance though is the gamble that Boeing had taken some years earlier in the development of a new series of jet aircraft, the prototype of which was named the Dash 80 (after the model designation 367-80). Whilst SAC had only decided to look for a new tanker in 1954, the Dash 80 had already been built and took to the skies just nine days after the announcement of the new tanker proposal. Despite Lockheed winning the competition, the need for a more capable tanker platform was immediate and led to the Boeing option being ordered as a ‘stop gap’ before the eventual decision to utilise it as the sole platform was taken. Thus, on August 31, 1956 when the first production KC-135A took to the skies, a new chapter in aerial refuelling began, one that has now lasted an incredible 60 years and provided a platform for a number of other interesting aircraft uses and roles.

Just under a year after its first flight, the first operational KC-135 was delivered to the 93rd Air Refueling Squadron based at Castle Air Force Base, California. Powered by four Pratt & Whitney J57-P-59W turbojets, the use of water injection provided nearly 14,000lbs of wet thrust along with a tremendous noise and characteristic black smoke trail caused by unburnt fuel. Along with these engines, the swept wing was also more in line with modern aircraft design making the KC-135 a big leap forward in capability. As well as that, the receiver no longer had to fly with full flaps and gear down to fly slow enough to refuel. This advancement also saw the type set a number of distance and speed records during its very first year of operation along with assisting the refuelling of other aircraft attempting similar feats.

With the KC-135 delivered at the height of the Cold War, it wasn’t long before it became involved in the very task that it was originally ordered to fulfil – the refuelling of the strategic bomber force. With the B-52s providing an airborne nuclear strike capability, Strategic Air Command set about maintaining a continuous airborne alert presence of armed aircraft. Of course, to support this effort, the role of the KC-135 was vital in providing the required fuel need to keep the bombers airborne for up to 24 hours at a time. For eight years these missions continued, mostly under the name Chrome Dome (other names were used at both the start and end of the series of missions) until they were disbanded following a B-52 crash at Thule Air Base, Greenland in January 1968.

By this time, the US was also involved in the war in Southeast Asia. Supporting both bomber and fighter forces – often en-masse – KC-135s were significantly involved in operations with over 813,000 aerial refuellings taking place during nearly 195,000 sorties over the nine year period. With this many, it’s no surprise that a number of them saved the lives of many a pilot flying low on fuel – a story that repeats itself during later conflicts the KC-135 has been involved in. Amongst these is one of the most well known stories that earned a KC-135 crew the coveted Mackay Trophy for the most meritorious flight of the year. On May 31, 1967, a routine tanker sortie led by Major John H. Casteel happened to be in the right place at the right time when a pair of US Navy Douglas A-3 Skywarriors had an immediate need for fuel. No sooner than that got underway, a pair of Vought F-8 Crusaders appeared – also in dire need of fuel. Whilst the A-3s received their fuel, the F-8s were able to use the tanking system in the Skywarrior to take on fuel for themselves leading to a three tier ‘buddy buddy’ refuelling taking place. Not complete, the tanker crew went on help out a pair of McDonnel Douglas F-4 Phantoms before also continuing on their way to its expected trade in a pair of F-104s. The importance of aerial refuelling in combat during Vietnam was more than proven in what was, essentially, the first tanker war.

With the KC-135 proving itself so early in its career, it is no wonder that it has become relied upon during every conflict that the US has been involved in since, providing tactical air refuelling alongside transportation and medevac for 60 years. From the invasion of Panama during Operation Just Cause, operations during both Gulf Wars including support of the no-fly zone exclusion, to protecting home soil following terrorist attacks of September 11, 2001, the KC-135 has had a long a varied history.

KC-135 Modifications

As with any aircraft, improvements and modifications have played an important part in its history. In order to combat metal fatigue in various key areas, programs were launched to address weaknesses in the center wing and vertical stabilizer along with a re-skin of the wings with stronger materials. Other upgrades, such as Pacer CRAG and Global Air Traffic Management systems have focused on internal system upgrades such as to the radar and cockpit in order to bring them more in line with technological advancements.

The most noticeable external upgrades however came in the form of engine replacement programs. As has been noted, the original wet thrust engines fitted to the A variant were both noisy and smokey as well as rapidly becoming inefficient when compared against modern designs. The first upgrade saw a number of KC-135As belonging to the Air National Guard and Air Force Reserve Command upgraded with Pratt & Whitney JT3D turbofan engines – these being redesignated TF33. In sourcing these engines, the USAF looked towards the type’s civilian counterpart – the Boeing 707. Having also been developed from the Dash 80 the engine was externally almost identical while it provided a cost effective method of upgrading the machines – the USAF buying airframes from airline operators who had themselves been upgrading. With an additional 4,000lbs of thrust compared to the A model and 14% more fuel-efficient, the range and offload capability was also greatly improved.

Following that, further upgrades to the remaining KC-135A fleet (and a small number of KC-135Es later in the program) led to them receiving the CFM International CFM56 turbofan engine with the type redesignated the KC-135R. Yet another step above the JT3D, the statistics when compared to the original engines are impressive – 22,000lbs of thrust, 50% higher fuel offload, 25% more efficient, increased range and service ceilings and, possibly most impressive of all, an astounding 95% quieter.

Two other externally noticeable modifications have been made to a much smaller number of airframes within the remaining fleet. Whilst the KC-135 is normally fitted only with a boom, flights in support of the US Navy and many NATO allies which rely upon the probe and drogue system require an adapter to be fitted to the boom itself in order to provide refuelling. Furthering this capability, twenty KC-135s have been modified with wingtip mounted Multi Point Refuelling System (MPRS) Mk32 refuelling pods supplied by Flight Refuelling Limited. These provide a solution to the refuelling of both probe and drogue and boom configured aircraft during the same operational mission. The second noticeable modification is featured on just eight airframes operated by the 22nd ARW at McConnell AFB and allows the inflight refuelling of the KC-135s themselves. Given the KC-135R/T designation, these airframes are rarely seen and provide a very unique capability for the fleet.

A final key variant of the KC-135, in the tanker role at least, was that of the specialist KC-135Q. With the advent of the legendary Lockheed SR-71 Blackbird, the KC-135Q was key to ensuring its tanks were topped up for the long range and high speed missions that it undertook. Unlike other aircraft in the inventory, the SR-71 used JP-7 rather than the usual JP-4 fuel – this causing an issue for the tankers. To solve this, 56 KC-135As were converted to Q standard which saw the main fuel tanks split into two separate entities – the main body carrying the receivers JP-7 whilst the wing tanks carried the tankers own JP-4 fuel. Following the retirement of the Blackbird and the CFM56 engine upgrades, these jets became the KC-135Ts.

Of the 820 airframes built, almost all of them have served within the USAF during their life. In fact, when it comes to foreign sales, only the French Air Force purchased a number of C-135 variants for use from new. A few other air arms have acquired ‘second hand’ models with the Turkish being the first to receive seven airframes in 1995, followed by Singapore with four KC-135Rs in 1999 and, most recently whilst the USAF were relegating the airframes not selected to be upgraded to the R configuration, Chile opting a trio of KC-135Es.

Although designed and built in vast numbers as a tanker, a smaller number of aircraft were built without the refuelling configuration in place and designated C-135. This small number of airframes fulfil a wide range of operational needs that are often highly specialised.

Numerous NKC-135’s have been used over the years in a test and evaluation roles. From airborne flying laboratories supporting the nuclear weapon Limited Test Ban Treaty to simple testing of new equipment for the RC-135 fleet, the versatility of the platform is clear. Possibly the best known of the NKC variants are the now retired Big Crow aircraft. Designed as an electronic warfare asset, the distinctive additions to the standard KC-135 made it stand out. It also served as a target aircraft for the YAL-1 Airborne laser tests that were fired from a highly modified Boeing 747.

The most often seen (certainly in the UK at least) variants are those that operate under various reconnaissance roles – the RC-135. With a number of upgrades over the years, they continue to support operations around the world today as the Cobra Ball (RC-135S), Combat Sent (RC-135U) and Rivet Joint (RC-135V/W) alongside TC-135 trainers. The RC-135W has also seen some export success after the RAF purchased three aircraft to replace the retired Nimrod R1 with L-3 Communications providing the conversions needed to three ex-USAF KC-135R tankers.

Whilst the KC-135 was supporting Cold War operations by keeping alert tankers in the air, another variant was also filling another role. EC-135’s were used as Command and Control posts in the air, staying airborne around the clock during Operation Looking Glass. With senior command staff onboard, the intention was that any response to a nuclear attack on the United States could be coordinated by an aircraft already in the air. The EC-135J also became the first Nightwatch platform, which is now flown by the E-4 where they provided a constantly available command platform for the president. Other EC-135’s were also used to support the US Space Program including the Apollo missions and themselves had very distinctive ‘droopy nose’ antenna’s protruding from the front.

Other specialised variants also exist, or have existed. For example, NASA employed two aircraft perform parabolic flights in order to train personnel in reduced gravity flight – the famous Vomit Comet or Weightless Wonder as these aircraft are more often known. As part of the Open Skies Treaty, which permits unarmed surveillance flights over participating countries, a pair of OC-135’s are operated whilst Constant Phoenix WC-135s provide an airborne atmospheric analysis capability to detect nuclear detonations. Then there is the Speckled Trout mission that over the years has been carried out by a number of aircraft, most recently a KC-135R, performing VIP transport duties alongside its testing role that has included refuelling testing with the KC-135s planned successor as part of the 412th Flight Test Squadron.

The Future

With the KC-135 first flying 60 years ago, and even the youngest aircraft in the fleet now being over a half decade old, the type is one of very few to be flying after so many years service. Whilst some sources suggest that, airframe hours wise, it could continue on well in to its 80s, there is already a program underway to replace it with a more modern alternative.

Although originally intended as a replacement for the retiring KC-135E airframes, the Boeing KC-46 program was shrouded in controversy with corruption charges causing the initial contract to be abandoned and failures in the bidding processes for subsequent contract, known as KC-X, led to it being withdrawn and re-ran once more. Despite this, the Boeing KC-46A (based on the Boeing 767 airframe) was finally selected and confirmed as the new platform in early 2011.

Since then, the test phase of the new aircraft has been taking place alongside the planning process for its entry in to operational service. Due to delays, the first aircraft are scheduled to be delivered to McConnell AFB in autumn 2017 – the base becoming the first active duty recipient. Altus AFB has been chosen as the training location and on the day before the 60th anniversary of the KC-135 first flight, saw the reactivation of the 56th Air Refueling Squadron in order to carry out the training.

Whilst no formal retirement date has been announced for the KC-135, the arrival of the KC-46 at bases will start to see the gradual withdrawal of the type. The KC-46 may be a more modern and capable aircraft than the venerable Stratotanker, but it is highly unlikely to have the character and distinguished career of its predecessor.

For 60 years, and still with more to come, the KC-135 has been at the heart of the United States air refuelling capability and been essential to the success of operations around the globe during times of both peace and war. Whilst fighters and bombers may be the focus of many a mission, none of this can be achieved without the support of the KC-135 Stratotanker. After all, Nobody Kicks Ass Without Tanker Gas!