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The original KC-135 requirements
The KC-135 ranks among the US Air Force (USAF)’s most iconic aircraft. First delivered to the USAF in June 1957, the KC-135 operated for almost seventy years as the USAF’s principal refueling aircraft.
The original KC-135 requirements were generated due to post-World War II cold war tension. In Europe, Russia was consolidating its domination of Eastern Europe in the late 1940s The cold war was in progress with the Berlin blockade in 1948. The Soviet Union first exploded an atomic bomb in 1949. In the Far East the Communists had taken over in China.
As told by Don Logan in his book The Boeing C-135 Series: Stratotanker, Stratolifter, and other Variants, in both the East and West the US was confronted with the need to project a strong image to oppose Soviet expansion and Chinese threats. This had to be accomplished with overseas bases no longer secure. One response to this need was to build strategic bomber aircraft – the B-36, B-47, and B-52. These aircraft could be based long distances from an actual trouble spot, and if necessary, be on the scene in a few hours.
In-flight refueling
The Convair B-36 had global range and a tremendous payload, but it was slow and vulnerable to the postwar generation of jet fighters. The Boeing B-47, first flown in 1947 and the Boeing B-52, first flown in 1952, were designed as high altitude, high speed bombers which could overfly the targets at altitudes and speeds safe from enemy defenses. These highly successful aircraft had one serious limitation, a lack of range. To remedy this lack of global range, the technique of in-flight refueling had to be refined.
Operational in-flight refueling was first accomplished during the flight of a US Army Fokker tri-motor, the “Question Mark,” during 1929. During World War Il some experimentation was accomplished, but due to the availability of bases in England and on Pacific Islands, in-flight refueling was not required by the strategy employed. It was during the post World War II years that a serious program of development of in-flight refueling was begun, using the Boeing B-29 as the tanker aircraft.
KC-97
Two basic methods were tested; the probe and drogue, and the boom and receptacle. Both methods are still in use. In the probe and drogue method, a flexible hose was trailed from the tanker with a conical basket attached. The basket was grappled by a probe on the receiver aircraft. In the boom and receptacle method, a rigid telescoping tube, the refueling boom. was trailed from the tanker. The receiver flew into position. Using control surfaces near the end of the boom, the boom was flown into contact with a receptacle on the receiver aircraft. Because of the control surfaces on the boom. the boom was known as a flying boom.
The flying boom was the most suitable method for transferring large quantities of fuel in a short time. Fuel booms developed by the Boeing Company were used on KB-29P and KB-50D/J tankers and later on the most numerous of the flying boom tankers, the Boeing KC-97 (Boeing designation of Model 367) of which 814 were built.
Boeing built the KC-97s as the prime tanker support for the Strategic Air Command’s B-47 and B-52 strategic bombers. The propeller powered KC-97, even with the auxiliary jet engines, had a serious drawback in refueling jet powered aircraft. The KC-97s flight envelope did not match that of a jet. The jets with their swept back wings were designed for high speed flight, while the propellers of the straight winged KC- 97 were not capable of operation at higher speeds.
C-135
To accomplish a refueling the KC-97 had to operate at maximum speed while the jet was near minimum speed. The situation was particularly bad when the jet receiver was near maximum weight and flying just a few knots above low speed stall buffet. Even the most skillful pilots had problems maintaining contact at night and in turbulent air. As a result of this situation, the requirement for a jet powered tanker with a compatible flight envelope was born. With a jet tanker, refueling could be accomplished safely at high altitude and high speed minimizing the loss in time and range to the receiver.
Boeing decided to begin development of an improved tanker aircraft based on the KC-97 design. After many preliminary designs, the Model 367-80 resulted in a four-engined swept-wing jet. The Dash 80, a Boeing private venture, rolled out on May 14, 1954, and took to the air on Jul. 15 of the same year.
From the Dash 80 came an Air Force order for the KC-135A Stratotanker. The first KC-135A (Model 717-100A) serial number 55-3118, arrived in 1956. The Model 707-100 developed in a subsequent parallel program, first flew in December 1957. The KC-135A is not a 707! Even the C-135A/Bs are not 707s! The airframes are different. Only the basic shape has a resemblance. The 707 has passenger windows the C-135 does not. The USAF C-137A is a military version of the 707-100. The C-137B evolved from the fan engined 707-100B after the retrofit of fan engines on the C-137As. C-137C resulted from the 707-300B fanjet.
C-135’s 34 variants
The Boeing model numbers for the C-135 family of aircraft are:
- KC-135A 717-100A, 717-146, and 717-148
- C-135A 717-157
- C-135B 717-158B
- C-135F 717-165
- KC-135B 717-166
- RC-135A 739-700
- RC-135B 739-445B
The USAF Strategic Air Command (SAC) and Military Air Transport Service (MATS) ordered the following C-135s from Boeing:
- 732 KC-135A Stratotankers (SAC)
- 17 KC-135B Stratotankers (SAC)
- 10 RC-135B (SAC)
- 15 C-135A Stratolifters (MATS)
- 30 C-135B Stratolifters (MATS)
- 4 RC-135A (MATS)
- 12 C-135F Stratotankers (French A.F.)
- 820 Total
From these 820 aircraft in 7 varieties were to come 34 variants, the largest number of variants for a given aircraft type in the US inventory. The C-135s were delivered to the Air Force between Apr. 30, 1957 and Janu. 6, 1966. The first operational mission was flown by a KC-135A of the 93rd Bombardment Wing at Castle AFB, California on Jun. 28, 1957.
The Boeing C-135 Series: Stratotanker, Stratolifter, and other Variants is published by Schiffer Publishing and is available to order here.
Photo credit: U.S. Air Force
