The Stratofreighter Was the Proving Ground for the KC-135 Stratotanker
It is 1951. The first generation of America’s Air Force, Navy, and Marine Corps tactical jet aircraft have begun service with their respective services. Aerial refueling is a new method of extending both the mission and ferry range of these tactical aircraft and making them (almost) instantly deployable to distant bases on other continents. Boeing’s KB-29 and KB-50, both modified heavy bomber aircraft, comprise the first generation of aerial refueling aircraft. These initial tankers lack the necessary “give” capacity to provide enough fuel for more than a few tactical aircraft per tanker sortie.
Finding the Right Fit
Airlifter technology is also evolving. The Curtiss C-46 Commando and the Douglas C-47 Skytrain were the backbone of airlift during and after World War II. The Douglas C-54 Skymaster, and to a lesser extent the Lockheed C-69 Constellation, also carried the World War II war effort abroad and continued to do so while next-generation airlifters like the Boeing C-97 Stratofreighter, Northrop C-119 Flying Boxcar and the C-123 Provider were being developed. These next-gen airlifters were the most likely aircraft to execute the aerial refueling mission as it was no longer acceptable to modify the nation’s bombers into tankers.
Making the Right Fit Work
The requirements for internal tankage capacity, ability for increased capacity as required, and a fuselage configuration allowing refueling boom installation made the C-97A the logical choice for the second generation of refueling aircraft. The C-97A, which entered service in 1947, was itself based on the B-50 bomber, though early service test airframes utilized components from the B-29. The cargo hold was essentially added to the lower fuselage, wings, engines, and empennage of the B-50 bomber. In essence all that was necessary to turn the C-97 into the KC-97 was to add the refueling boom, sufficient tankage, and the requisite plumbing to connect it all.
Old Shaky Takes Over
When the Douglas C-124 Globemaster II entered service in 1950 and began taking over primary long-distance airlifting duties, the Air Force was able to modify or new-build 811 of the 888 C-97s built by Boeing as KC-97s. Some of the KC-97s were later modified back to cargo configuration. The KC-97s carried fuel for passing gas to refueling aircraft in palletized tanks located in the cargo bay or in the lower bay below the main deck. The refueling boom and operator’s station were located aft in what became, and still is, the standard placement for boom refueling equipment. The KC-97 created the original standard configuration for aerial refueling.
Service Entry and Improvements
When the KC-97 entered service in 1951, support for Strategic Air Command’s fleet of B-47 strategic bombers was its first and most critically important mission. More powerful R-4360-59B engines were added to the 159 KC-97F variants. The KC-97G was the definitive dual-role tanker/cargo variant. When acting as a transport, the KC-97G and other dual-role KC-97 variants could carry up to 68,500 pounds of cargo or up to 96 fully-equipped troops. KC-97s in most configurations were capable of off-loading 15,000 gallons of fuel when not burdened with cargo.
Variants and Refinements
Three EC-97Gs did Electronic Intelligence (ELINT) work for the CIA. 22 HC-97Gs worked for the Air Force Aerospace Rescue and Recovery Service (ARRS). Even with those more powerful engines (the most powerful piston engines available) the pedestrian performance of the KC-97 called out for more. When refueling B-47s and B-52s sidled up to the boom they were flying at such low speeds that directional stability was difficult to maintain. The unintended sideways oscillations this sometimes caused would get the hearts of both tanker and tankee crews pumping. The initial solution was to have the tanker plug the boom into the receptacle on the receiving aircraft. Then both the KC-97 and the receiver aircraft would fly together in a shallow dive attitude to maintain enough airspeed to avoid the receiver aircraft stalling while plugged to the boom. This downhill ride was (naturally if wryly) referred to as “tobogganing.”
Four Turning and Two Burning
Tobogganing sounds like fun but not when tons upon tons of volatile fuel are flying in close formation on the edge of a stall. So the engineers got together and hatched the KC-97L. This variant was equipped with a pair of underwing pylons mounting General Electric J47 turbojet engines removed from retired KB-50J tankers. Tankers so configured toted separate fuel systems for their reciprocating engines (aviation gasoline or avgas) and jet fuel for passage to other aircraft and for their own J47 jets (jet fuel). The added thrust was usually enough to avoid the tobogganing, but still not ideal for heavy and fast jet bombers or tactical aircraft. Two YC-97Js were equipped with Pratt & Whitney YT34-P-5 turboprop engines as a potential solution but the modification was not adopted. Until later.