The KC-97: Boeing’s Big Tanker Set the Standard

The KC-97 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 offloading 15,000 gallons of fuel when not burdened with cargo.

Variants and Refinements

Three EC-97Gs did Electronic Intelligence (ELINT) work for the CIA. Twenty-two HC-97Gs were used by 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 maintaining directional stability was challenging.

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.

KC-97 Phased Out

When Boeing’s replacement for the KC-97, the KC-135A Stratotanker, entered service in 1957, Strategic Air Command turned their backs on their KC-97s. But Tactical Air Command (TAC) was plenty happy to have them. When refueling tactical aircraft like the North American F-100 equipped with a refueling probe, the boom was equipped with the required drogue.

Happy to have KC-97s, too, were Air Force Reserve and Air National Guard (ANG) units. KC-97s served across the country and around the world with more than 100 SAC, TAC, Reserve, and ANG Air Refueling Squadrons (ARS) as well as Israel and Spain. In 1978, after 26 years of service, the last ARS KC-97Ls were retired by the Texas Air National Guard and the Utah Air National Guard.

Moving On From the KC-97

We’re fortunate to have more than twenty KC-97s on display in museums and at Air Force Bases today. There are actually two KC-97s still flying- though you might not recognize either of them right off. The first began life as KC-97 52-0828, and that airframe donated its wings, tail, empennage, cockpit, and landing gear to NASA 941- the sole remaining Super Guppy.

The other now-airworthy KC-97G (52-2718) was converted back to the non-tanker C-97G configuration when restored to flight in 2017 by Berlin Airlift Historical Foundation (BAHF) after spending quite a few years in the desert at the Boneyard.

Stratofreighter Trivia

On 9 May 1957, KC-97F USAF SN 51-0258 suffered a double engine failure while en route from Sidi Slimane Air Base in Morocco to Lajes in the Azores. The Stratofreighter successfully ditched in the Atlantic roughly 340 miles southeast of the Azores.

The crew of seven all survived the ditching, and 258 did too, floating for ten days after the crew was rescued. The Iowa-class battleship USS Wisconsin (BB-64) was on her way home from the Med when she was called upon to sink 258, which she did with naval gunfire. When it landed in one piece in the open ocean, Boeing’s 377 Stratoliner could also float for quite a while.

On 14 December 1959, KC-97G USAF SN 53-0231 of the 384th Air Refueling Squadron based at Westover AFB in Massachusetts, collided with a Strategic Air Command B-52 during a refueling mission. The collision left 231 without her left horizontal stabilizer and elevator, the upper quarter of her vertical stabilizer, and rudder- but with the refueling boom skewered through the aft fuselage!

The KC-97G crew made a no-flap, electrical power off night recovery at Dow AFB in Bangor, Maine. The B-52 recovered safely at Westover- minus two of her crew, who had ejected after the collision and were subsequently rescued safely on the ground. Miraculously, there were no injuries.

More KC-97 Trivia

On 30 March 1960, KC-97F USAF SN 51-0363 was forced to ditch in heavy seas after losing an engine while en route from Harmon AFB in Newfoundland to MacDill AFB in Tampa, Florida. 363 ditched in the Atlantic Ocean roughly 43 miles from the east coast of Florida, off Cape Canaveral, with the loss of two of the 14 souls on board the aircraft. Slightly more than 55 years later, the wreck of 363 was discovered on the bottom by divers 365 feet down in 2015.

KC-97s starred (in supporting roles) in a pair of SAC shipping-out films during the 1950s. The first was the 1955 Paramount feature ‘Strategic Air Command’ starring Jimmy Stewart and June Allyson- and a few KC-97Gs hauling support equipment and refueling B-47s.

The second film was the 1957 Warner Brothers movie ‘Bombers B-52’ starring Natalie Wood, Karl Malden, and Efrem Zimbalist Jr.- and KC-97Ls refueling B-52s.

The last KC-97G built by Boeing rolled out of the factory doors and was pushed aside to make room for the first production example of its replacement- Boeing’s own KC-135A Stratotanker.