The C-97 Stratofreighter, with its "double-bubble" fuselage, was basically a military version of the Boeing Model 377 Stratocruiser; though its wing, engine, and tail were acquired from the B-29 Superfortress bomber. The C-97 served a varied mission when active; in addition to both transport and tanker roles, it was used for medical evacuation and Search And Rescue (SAR).
The KC-97 Stratofreighter aerial tanker version was an instrumental factor in providing SAC with genuine intercontinental capability. It was introduced in 1950 (first flown on 15 November 1944) using the "flying boom" in-flight refueling system. Originally, the aircraft was equipped with only four Pratt & Whitney R-4360 radial engines and designated as KC-97Gs. Later, two J-47 turbojet engines were added to allow the redesignated KC-97L to safely refuel the B-47 and B-52 jet aircraft.
The KC-97 saw service well into the Vietnam Conflict era. There were 888 C-97s built between 1951 and 1956, 814 of these were modified to KC-97 tankers.
Because of its slow cruising speed and low cruising altitude, the KC-97 had difficulty being an efficient refueler to high-speed jet aircraft. To refuel a faster, jet aircraft, it performed a maneuver called "tobogganing." The refueling connection would be made high up and then the tanker and jet flew "downhill" together enabling the tanker to pick up more speed.
The KC-97L, with its jet engines mounted under each wing, gave the tanker added speed required for takeoff and flight. This setup also enabled it to refuel jet aircraft without "tobogganing." The KC-97L carried both AVGAS and jet fuel. The AVGAS was used to power its radial Piston engines while the jet fuel was carried to power its two jet engines and to be off loaded to its receivers. In the late-1950s, two KC-97Gs were converted to a KC-97Js. These aircraft incorporated YT43-P-5 turboprops and were used as flying test-beds by the USAF.
The U.S. Air Force really needed an all-jet transport/tanker and chose the new KC-135 Stratotanker, which remains in service almost 40 years later. The KC-97 is the direct predecessor of the KC-135 Stratotanker aircraft. In fact, the last KC-97 and the first KC-135 were rolled out of the Boeing assembly plant on the same day.
When acting as a transport, the C-97 could carry 68,500 pounds of cargo or up to 96 fully-equipped troops. In the tanker role, the KC-97 was capable of off-loading 15,000 gallons of fuel.
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