07 April 2015

The Development of the Boeing Flying Boom

World War II USAAF tests with B-24 tankers and B-17 receivers
Though the United States had explored using air refueling to extend the reach of strategic bombing missions during the Second World War, nothing operational had come of the work by the time the war ended in 1945. In the immediate post war years, the newly independent United States Air Force and its nuclear deterrent arm, the Strategic Air Command, had both the weapons and the aircraft to carry out nuclear strikes, but what was lacking given the technology and geopolitical climate of the day was overseas bases that would allow SAC's bombers to reach the Soviet Union. As it was, the Boeing B-29 Superfortresses that were the main strike force of SAC lacked the range to hit Soviet targets nonstop from bases in the United States. While the US government placed priority on securing overseas bases for SAC, the USAF made inquiries to the leading experts of air refueling of the day, Flight Refueling Limited in Great Britain. It was Flight Refueling that consulted with the US Army Air Forces during the Second World War and assisted with several trials using B-24 Liberators as tankers and B-17 Flying Fortresses as receivers. Several sets of air refueling equipment were procured from Flight Limited and installed on a very limited basis on several B-29 Superfortresses to get crews trained on the procedure. However, the USAF was dissatisfied with the system as it took time to rendezvous and get into the proper position, change positions, and then transfer fuel. Using Flight Refueling's method, the receiver trailed a hauling line with a weight and hook at the end. The tanker approached from the side and below and deployed a contact line that crossed over the hauling line of the receiver and engaged the hook. The tanker then moved above the receiver, pulling in the hauling line with the contact line. The refueling hose was then attached to the hauling line and it was then pulled down to the receiver which had a refueling receptacle in the tail gunner's position and refueling commenced. The lines and refueling hoses used created tremendous drag that imposed air speed restrictions that may have been acceptable for a piston-engined bomber but wholly impractical for a future jet-powered bomber. 

The USAF contacted Boeing in November 1947 if they would be willing to look at the air-refueling problem within the purview of the company's ongoing research programs. In the following month, the Preliminary Design Group and the Experimental Manufacturing Division at Boeing formally signed a contract with the Air Force to work on improving air refueling. Boeing's first step was to determine what formation can two Superfortresses operate most closely for an extended period of time safely to conduct air refueling. Boeing's engineers figured the refueling solution would be easier the closer the aircraft could fly to each other and not have to do the position changes that the Flight Refueling method entailed. To this end, in May 1948, the USAF ran a series of tests out of Wright-Patterson AFB in Dayton, Ohio, using B-29s flown in every possible formation and relationship to each other. Escorting aircraft photographed the formations from every angle and Boeing's team would then analyze the photographs to determine their three-dimensional relationship to each other. For every possible formation, the flight crews involved were also queried on things like workload and visibility in maintaining the formation. As a result of these test flights, it was determined the optimum position that provided a relatively low workload with good visibility was to put the aircraft in trail formation with the trailing aircraft vertically displaced 25 feet and longitudinally displaced 10 feet. This gave the flight crew in the trailing aircraft the best view of the lead aircraft with the closest possible distance. Pilots in the trailing aircraft found that if they flew less than 25 feet vertically displaced below the lead aircraft, they got buffeting from the wake of the lead aircraft which gave the formation an inherent safety feature. 

After determining the most optimal close formation, the next step for the Boeing team was to figure out the best fuel transfer method. Five different refueling systems were explored. The first three systems were probe-and-drogue applications with the tanker trailing a hose with a drogue at the end with the receiver flying a probe into the drogue to make the connection. Though this method is used today by the US Navy and US Marine Corps as well as a large number of air arms like the RAF, the Boeing team felt that the hose movement could be unpredictable in rough air and required too much maneuvering by the trailing aircraft to make hose contact. Such maneuvering might be fine for a smaller tactical aircraft, but Boeing was less than thrilled about the prospect of a large receiver aircraft having to maneuver frequently before contact so close to the tanker. 

The imaginative fourth proposed system involved a gun-turret like assembly on the tanker's forward dorsal fuselage. The tanker would take the trailing position and the turret would deploy a rigid boom up and forward to engage a receptacle on the underside of the tail of the receiver. The boom would be maneuvered like a gun turret by an operator aboard the tanker and when not in use, the boom would slew 180 degrees and stow atop the dorsal fuselage of the tanker. While imaginative, it was soon realized the aerodynamic loads on the boom would be significant. But what if their positions were reversed? What if the tanker lowered the boom aft and down to the receiver who had a receptacle on the top of the fuselage? This way the operator did all the work from the tanker and the receiver flight crew could focus on holding the prescribed position in trail behind the tanker. Flight test personnel with experience with the flight refueling systems of the day were consulted and all agreed that a boom lowered from the lower aft fuselage of the tanker to the top of the fuselage of the receiver would be the most ideal. A rigid boom would allow fuel transfer rates much higher than a hose system and small aerodynamic surfaces would be used on the end of the boom to maneuver it to the receptacle of the receiver- which is how Boeing came to call it the "flying boom". 

B-50 "Lucky Lady II" taking on fuel from a KB-29 hose tanker
While Boeing's engineers in Seattle worked on the flying boom concept, the Air Force's first secretary after its creation, Stuart Symington, had testified before the Senate Armed Services Committee in March 1948 that the latest air refueling systems would allow the new Boeing B-50 Superfortress to reach any part of the Soviet Union, but the reality of it was that all the USAF had were the first Flight Refueling hose units on a handful of B-29s and that the flying boom was still a paper project. Quite literally on the following day, the USAF instructed Boeing's Wichita division to get as many hose units onto KB-29 tankers as possible and get the new B-50s up to speed as receivers with an interim system until the flying boom was operational. The first operational installation was ready in less than 30 days and by the end of 1948. On 26 February 1949, the B-50 Superfortress "Lucky Lady II" took off from Carswell AFB in Fort Worth, Texas, and flew around the world nonstop in 94 hours, taking fuel from hose-equipped KB-29s four times during the record-breaking flight. 

The flying boom equipped KB-29P Superfortress tanker
Despite this very public success, Boeing continued to develop the flying boom and interestingly, had funded the development internally without outside USAF funds. Two dry booms were built for KB-29s as proof of concept. Though not able to transfer fuel (hence the term "dry booms"), the dry booms were actually installed on KB-29s in June 1948, a full seven months before the circumnavigation flight of "Lucky Lady II". Dry receptacles for the purposes of flight test were installed on a B-47 Stratojet and an F-86 Sabre. Flight tests using the dry boom were conducted through the summer of 1948 out of Seattle, Wichita, and Wright-Patterson AFB in Ohio. The tests were successful and the USAF requested Boeing transfer the flying boom work to Curtiss Aircraft. As the company had an absence of work postwar, the USAF wanted to keep Curtiss in business, but quite obviously, Boeing wasn't happy with that request, particularly since development had so far involved company funds without any USAF funding. By April 1949, Boeing was already constructing wet booms (flying booms able to transfer fuel) and was resisting USAF pressure to transfer the program to Curtiss Aircraft. Boeing won the dispute with the USAF by insisting its flying boom work was proprietary and would have commercial applications in refueling jet airliners. Since no USAF funds had been used in development so far, the USAF found it didn't exactly have financial clout to compel Boeing to transfer the program to Curtiss. Up to this point, the flying boom program was classified and the USAF had insinuated that the program's classified status meant that it couldn't be used for commercial applications. But the flight test program had already been publicly revealed by the USAF itself in an October 1949 press release! Boeing did finally get its contract for the flying boom. From 1950 to 1951, the Boeing Renton plant converted over 100 B-29 Superfortresses into KB-29P flying boom tankers with the first tanker delivered to SAC in March 1950. A fixed cradle structure supported the flying boom when it was raised. A hemispheric plexiglass dome replaced the tail turret and laying in a prone position, the boom operated "flew" the boom to the receiver. Boeing had always considered the KB-29P an interim tanker and soon enough was working on a tanker version of the C-97 Stratofreighter- not only did a tanker version of the C-97 offer more fuel carrying capability, it could also carry cargo when not being used for air refueling, offering mission flexibility for the USAF. The first flying boom-equipped C-97 was flight tested by Boeing in September 1950 and so impressed the USAF that all remaining orders for the C-97 were to be completed as KC-97s. In fact, the first KC-97 was delivered to the USAF only eight months after the KC-97 contract was signed with the first units operational in July 1951. Boeing then suggested a turboprop-powered KC-97 to the USAF, but the military was ambivalent to the idea, but by that point, Boeing was already working on a new breed of transport that would eclipse even the turboprop powered KC-97. But I'm pretty sure you know how that story ends! 

Source: Passing Gas: The History of Inflight Refueling by Vernon B. Byrd. Byrd Publishing, 1994, pp 123-136. Photos: National Museum of the United States Air Force.

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