Boeing KC-135 Stratotanker


The Boeing KC-135 Stratotanker is an American military aerial refueling tanker aircraft that was developed from the Boeing 367-80 prototype, alongside the Boeing 707 airliner. It has a narrower fuselage and is shorter than the 707. Boeing gave the aircraft the internal designation of Model 717. The KC-135 was the United States Air Force 's first jet-powered refueling tanker and replaced the KC-97 Stratofreighter. The KC-135 was initially tasked with refueling strategic bombers, but it was used extensively in the Vietnam War and later conflicts such as Operation Desert Storm to extend the range and endurance of US tactical fighters and bombers.
The KC-135 entered service with the USAF in 1957; it is one of nine military fixed-wing aircraft with over 60 years of continuous service with its original operator. The KC-135 was supplemented by the larger McDonnell Douglas KC-10 Extender. Studies have concluded that many of the aircraft could be flown until 2030, although maintenance costs have greatly increased. The KC-135 is to be partially replaced by the Boeing KC-46 Pegasus.

Development

Background

Starting in 1950, the USAF operated the world's first production aerial tanker, the Boeing KC-97 Stratofreighter, a gasoline fueled piston-engined Boeing Stratocruiser with a Boeing-developed flying boom and extra kerosene tanks feeding the boom. The Stratocruiser airliner itself was developed from the Boeing B-29 Superfortress bomber after World War II. In the KC-97, the mixed gasoline/kerosene fuel system was clearly not desirable and it was obvious that a jet-powered tanker aircraft would be the next development, having a single type of fuel for both its own engines and for passing to receiver aircraft. The 230 mph cruise speed of the slower, piston-engined KC-97 was also a serious issue, as using it as an aerial tanker forced the newer jet-powered military aircraft to slow down to mate with the tanker's boom.
Like its sibling, the commercial Boeing 707 jet airliner, the KC-135 was derived from the Boeing 367-80 jet transport "proof of concept" demonstrator, which was commonly called the "Dash-80". The KC-135 is similar in appearance to the 707, but has a narrower fuselage and is shorter than the 707. The KC-135 predates the 707, and is structurally quite different from the civilian airliner. Boeing gave the future KC-135 tanker the initial designation Model 717.
File:Boeing B-52D-70-BO is refueled by Boeing KC-135A-BN 061127-F-1234S-009.jpg|thumb|left|A KC-135A refueling a B-52D during the Cold War. Both aircraft types were operated by the Strategic Air Command.
In 1954, USAF's Strategic Air Command held a competition for a jet-powered aerial refueling tanker. Lockheed Corporation's tanker version of the proposed Lockheed L-193 airliner with rear fuselage-mounted engines was declared the winner in 1955. Since Boeing's proposal was already flying, the KC-135 could be delivered two years earlier and Air Force Secretary Harold E. Talbott ordered 250 KC-135 tankers until Lockheed's design could be manufactured. In the end, orders for the Lockheed tanker were dropped rather than supporting two tanker designs. Lockheed never produced its jet airliner, while Boeing would eventually dominate the market with a family of airliners based on the 707.
In 1954, the USAF placed an initial order for 29 KC-135As, the first of an eventual 820 of all variants of the basic C-135 family. The first aircraft flew in August 1956 and the initial production Stratotanker was delivered to Castle Air Force Base, California, in June 1957. The last KC-135 was delivered to the USAF in 1965.
File:General Curtis LeMay arrived in Buenos Aires Argentine on November 11, 1957.jpg|thumb|On 11 November 1957, Air Force Vice Chief of Staff General Curtis LeMay tested the first KC-135 on a long-haul flight from Westover Air Force Base, Massachusetts to Buenos Aires, Argentina.
Developed in the early 1950s, the basic airframe is characterized by 35-degree aft swept wings and tail, four underwing-mounted engine pods, a horizontal stabilizer mounted on the fuselage near the bottom of the vertical stabilizer with positive dihedral on the two horizontal planes and a hi-frequency radio antenna which protrudes forward from the top of the vertical fin or stabilizer. These basic features make it strongly resemble the commercial Boeing 707 and 720 aircraft, although it is a different aircraft.
Reconnaissance and command post variants of the aircraft, including the RC-135 Rivet Joint and EC-135 Looking Glass aircraft were operated by SAC from 1963 through 1992, when they were reassigned to the Air Combat Command. The USAF EC-135 Looking Glass was subsequently replaced in its role by the U.S. Navy E-6 Mercury aircraft, a new build airframe based on the Boeing 707-320B.

Engine retrofits

All KC-135s were originally equipped with Pratt & Whitney J57-P-59W turbojet engines, which produced of thrust dry, and approximately of thrust wet. Wet thrust is achieved through the use of water injection on takeoff, as opposed to "wet thrust" when used to describe an afterburning engine. of water are injected into the engines over the course of three minutes. The water is injected into the inlet and the diffuser case in front of the combustion case. The water cools the air in the engine to increase its density; it also reduces the turbine gas temperature, which is a primary limitation on many jet engines. This allows the use of more fuel for proper combustion and creates more thrust for short periods of time, similar in concept to "War Emergency Power" in a piston-engined aircraft.
File:KC-135R-Taxi.JPG|right|thumb|A nose-on view of several re-engined KC-135R aircraft taxiing prior to takeoff. The new engines are CFM56-2 high-bypass turbofans.|alt=The front of several gray aircraft are centered in the image.
In the 1980s, the first modification program retrofitted 157 Air Force Reserve and Air National Guard tankers with the Pratt & Whitney TF33-PW-102 turbofan engines from 707 airliners retired in the late 1970s and early 1980s. The modified tanker, designated the KC-135E, was 14% more fuel-efficient than the KC-135A and could offload 20% more fuel on long-duration flights. Only the KC-135E aircraft were equipped with thrust reversers for aborted takeoffs and shorter landing roll-outs. The KC-135E fleet has since either been retrofitted as the R-model configuration or placed into long-term storage, as Congress has prevented the USAF from formally retiring them. The final KC-135E, tail number 56-3630, was delivered by the 101st Air Refueling Wing to the 309th Aerospace Maintenance and Regeneration Group at Davis–Monthan Air Force Base in September 2009.
The second modification program retrofitted 500 aircraft with new CFM International CFM56 high-bypass turbofan engines produced by General Electric and Safran. The CFM56 engine produces approximately of thrust, nearly a 100% increase compared to the original J57 engine. The modified tanker, designated KC-135R or KC-135T, can offload up to 50% more fuel, is 25% more fuel-efficient, and costs 25% less to operate than with the previous engines. It is also significantly quieter than the KC-135A, with noise levels at takeoff reduced from 126 to 99 decibels. This 27 decibel noise reduction results in a sound pressure level of about 5% of the original level.
The KC-135R's operational range is 60% greater than the KC-135E for comparable fuel offloads, providing a wider range of basing options.
Upgrading the remaining KC-135Es into KC-135Rs is no longer in consideration; this would have cost approximately US$3 billion, $24 million per aircraft. According to USAF data, the KC-135 fleet had a total operation and support cost in fiscal year 2001 of about $2.2 billion. The older E model aircraft averaged total costs of about $4.6 million per aircraft, while the R models averaged about $3.7 million per aircraft. Those costs include personnel, fuel, maintenance, modifications, and spare parts.

Avionics upgrades

In order to expand the KC-135's capabilities and improve its reliability, the aircraft has undergone a number of avionics upgrades. Among these was the Pacer-CRAG program which ran from 1999 to 2002 and modified all the aircraft in the inventory to eliminate the Navigator position from the flight crew. The fuel management system was also replaced. The program development was done by Rockwell Collins in Iowa and installation was performed by BAE Systems at the Mojave Airport in California. Block 40.6 allows the KC-135 to comply with global air-traffic management. The latest block upgrade to the KC-135, the Block 45 program, is online with the first 45 upgraded aircraft delivered by January 2017. Block 45 adds a new glass cockpit digital display, radio altimeter, digital autopilot, digital flight director and computer updates. The original, no longer procurable, analog instruments, including all engine gauges, were replaced. Rockwell Collins again supplied the major avionic modules, with modification done at Tinker AFB.

Further upgrades and derivatives

The KC-135Q variant was modified to carry JP-7 fuel necessary for the Lockheed SR-71 Blackbird by separating the JP-7 from the KC-135's own fuel supply. The tanker also had special fuel systems for moving the different fuels between different tanks. When the KC-135Q model received the CFM56 engines, it was redesignated the KC-135T model, which was capable of separating the main body tanks from the wing tanks where the KC-135 draws its engine fuel. The only external difference between a KC-135R and a KC-135T is the presence of a clear window on the underside of the empennage of the KC-135T where a remote controlled searchlight is mounted. It also has two ground refueling ports, located in each rear wheel well so ground crews can fuel both the body tanks and wing tanks separately.
Eight KC-135R aircraft are receiver-capable tankers, commonly referred to as KC-135R. All eight aircraft were with the 22d Air Refueling Wing at McConnell AFB, Kansas, in 1994. They are primarily used for force extension and Special Operations missions, and are crewed by highly qualified receiver capable crews. If not used for the receiver mission, these aircraft can be flown just like any other KC-135R.
The Multi-point Refueling Systems modification adds refueling pods to the KC-135's wings. The pods allow refueling of U.S. Navy, U.S. Marine Corps and most NATO tactical jet aircraft while keeping the tail-mounted refueling boom. The pods themselves are Flight Refueling Limited MK.32B model pods, and refuel via the probe and drogue method common to Navy/Marine Corps tactical jets, rather than the primary "flying boom" method used by Air Force fixed-wing aircraft. This allows the tanker to refuel two receivers at the same time, which increases throughput compared to the boom drogue adapter.
A number of KC-135A and KC-135B aircraft have been modified to EC-135, RC-135 and OC-135 configurations for use in several different roles.