BOEING KC-135R STRATOTANKER
Where once a trip to Europe was considered an adventure, today a trip to the Middle East elicits a travel- weary reaction. 'I don't want to go to Saudi Arabia again; I've been there five times, [and] I've got enough souvenirs,' one KC-135 pilot said.
It is a hard fact that much of the 'Global Reach' of the U.S. Air Force depends on a fleet of aerial refueling tankers that are, in many cases, now older than their crews. The first KC-135 made its maiden flight on August 21, 1956, and the aircraft entered service in January 1957. A total of 798 KC-135 Stratotankers were built between 1956 and 1966. Their original, highly critical mission was to refuel the SAC fleet of B-52 nuclear bombers on their way to Doomsday (and back). Many of these planes spent decades standing alert duty, enjoying the fanatically meticulous maintenance that SAC enforced. Because the aircraft spent so little time under the stresses of flying, the -135 fleet is in surprisingly good condition. In fact, the fleet average on flight hours per airframes is something less than fifteen thousand hours, which is amazing when you consider that most of them were built in the early 1960s, and an equivalent commercial Boeing 707 might have over 120,000 flight hours! Now equipped with new engines, new wing skins, strengthened landing gear, and modernized avionics, the 552 remaining KC-135s will continue to give many years of good service. They will have to, because there is currently nothing on the drawing boards, or in the Air Force budget, to replace them.
Throughout the history of aerial warfare, the single most limiting factor has been the fuel capacity — and thus the range — of the aircraft flying the combat missions. The lack of a long-range escort fighter cost the Germans the Battle of Britain in 1940. Conversely, the P-51 Mustang, with its 'seven-league boots,' was the deciding factor in the success of the 8th Air Force in their operations over Germany. Thus, the idea of extending an aircraft's range by aerial refueling is such a simple idea, it is surprising that it took so long to catch on.
The first known attempt to do so occurred in 1921, when an aviator named Wesley May climbed from one flying biplane to another with a 5-gallon /18.9-liter can of gas strapped to his back. Later, daring young officers like Major Henry H. 'Hap' Arnold and Major Carl A. Spaatz (both became generals in World War II) experimented with simple hose and gravity-feed or pump arrangements for passing fuel from one aircraft to another. At the time, this was regarded more as a stunt to set flight endurance records than a realistic operational option; but it was a start on the road to the airborne tankers of today. World War II passed without any known use of aerial refueling by any of the combatants, though it would be the last major conflict where the technique would not be used.
After World War II, two different technologies for aerial refueling were developed in the United States. The first of these, the probe-and-drogue method, required the tanker aircraft to reel out a hose with a cone-shaped receptacle (the drogue), that could then be 'speared' by a fixed or extensible probe on the receiving aircraft. This method is preferred by the U.S. Navy, the Royal Air Force, and a number of NATO countries. The other method, Boeing's Flying Boom, required a trained boom operator with nerves of steel to guide a telescoping boom with twin steering fins into a locking receptacle on a receiving aircraft, which meanwhile is trying to maintain precise formation in the tanker's turbulent wake. This technique appealed to the USAF, who felt that the actual hookup between the aircraft should be controlled by a professional who did this odd task for a living. Tanker 'boomers,' as they are known, are usually sergeants who double as the aircraft's crew chief.
The first operational tankers, the KB-29, the KB-50, and the KC-97, were derived from the Boeing B-29 bomber. Their shortcomings were obvious. Because of their four piston engines, the early tankers simply could not keep up with the new generation of jet fighters and bombers that were quickly becoming the primary customers for these aerial gas stations. The solution, clearly, was going to be a jet tanker capable of integrating itself with the new jet combat force of the USAF.
The problem was that until someone developed a jet transport with sufficient payload capacity, this idea was going to stay just that — an idea. Fortunately, in the early 1950s there was an international race to produce the first commercially viable jet transport, and the USAF was able to pick their new tanker from the winners. The British Comet was first into service, but an unforeseen problem with metal fatigue around the window frames led to the loss of several aircraft in flight through explosive decompression. In the United States, Boeing's long experience with designing pressurized high-altitude aircraft like the B-29 paid off in the design of a tremendously strong airframe that would become the basis for both the military C-135 transport and the 707 commercial passenger liner. In 1954, soon after the first flight of Boeing's first jet transport prototype, the Model 367-80, the Air Force ordered a fleet of Boeing tankers to support the bomber force of the then-Strategic Air Command.
Boeing's project number for what became known as the Stratotanker was Model 717. It differed from the basic 707 airliner in having smaller overall dimensions, a somewhat narrower fuselage, no cabin windows, and, of course, an extendible, finned refueling boom and tiny compartment for the boom operator under the tail. The tankers, constructed more simply, to military standards, than their commercial cousins, actually went into service before the Boeing 707 completed final commercial certification.
Now, you should understand that if the whole fuselage barrel section were filled with fuel, the plane would be too heavy to take off. Thus, the fuel carried is actually contained in a relatively small volume, leaving the inside of the cabin available for other uses. All of the tanker-related equipment is below the main deck, leaving seating space for passengers or an equivalent volume of cargo — up to 83,000 lb./37,650 kg.
Over the years, more than two dozen variants of this versatile airframe have been built, including a bewildering collection of 'deep black' intelligence collection platforms, going by such names as Rivet Joint and Cobra Ball. A small number of all-cargo versions were built as C-135 Stratolifters. The -135 has also enjoyed some modest success in the export market; a dozen C-135F models were sold to France in 1964 to support that country's tiny but potent nuclear strike force of Mirage IV bombers. Canada and Israel also purchased tanker/cargo aircraft from the 707/KC-135 family, and continue to operate them today.
If you take a walk around one of the big tankers, the first thing that strikes you is how much it looks like an old Boeing 707, but with fewer windows. This absence of viewports is one of the reasons for the Stratotankers' longevity, since each hole that you put in a pressurized airframe is just another place for structural fatigue cracks to start.
The entire KC-135 fleet was originally equipped with the noisy and fuel-guzzling Pratt & Whitney J-57 engine, which also put out a lot of smoke on takeoff. Fortunately, most of the aircraft remaining in service have been re-engined with more efficient and powerful General Electric/SNECMA CFM- 56 turbofans, creating the KC- 135R variant. The engine change reduces noise by 85 % and pollutant emissions by 90 %, and the increased power allows for a much shorter takeoff run. The biggest benefit, though, is the vastly improved fuel efficiency, which allows a KC-135R to offload up to 50 % more fuel than one of the earlier J-57-equipped birds. While most aircraft in the USAF fleet are equipped to take fuel from tankers, most KC-135s are not themselves equipped with flight- refueling receptacles. The few that are so equipped are known as KC-135RTs, and are highly coveted assets by the new Air Mobility Command (AMC), which controls most of their operations, maintenance, and use. Thus, unlike the small fleet of McDonnell Douglas KC-10 Extenders (the newest USAF tanker, based on the commercial DC-10), most KC-135s can only be refueled on the ground. This makes for an interesting set of decisions for the operators of the -135. Unlike the Extender, they can either off-load fuel or deploy to an overseas area, but not both at the same time.
You get into the KC-135 through an entry hatch in the bottom of the fuselage, on the left side of the nose. It requires a bit of a climb to get up the ladder and into the cockpit, something like the climb into the conning tower of a submarine. Once there, the first thing you will probably notice is that by the standards of current commercial airliner cockpits, the -135 is decidedly ancient. The four-person flight crew usually includes three officers (aircraft commander, pilot, and navigator/radar operator) and one enlisted airman (the crew chief/ boom operator), each with a seat in the tight little workspace up front. Very little of the modern computer age is evident, other than a digital flight management system and the throttle controls for the four CFM-56 engines. The fit of communications and navigation gear enables the tanker to maintain station precisely and talk to its customers, though the navigation gear is also a bit dated. Until the planned installation of a GPS receiver later in the 1990s, the navigator
