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.

The First Steps to a Turboprop Transport, Part Two

A week and a half ago I had blogged about how the USAF was getting turboprop transport experience by setting up a test squadron at Kelly AFB to operated transport aircraft that had been converted to turbine power: 

52-2693 and 52-2672 in flight together.

On 15 June 1954, the headquarters of the Military Air Transport Service (MATS) activated the 1700th Test Squadron (Turboprop) at Kelly AFB, Texas, with the task of developing maintenance procedures and techniques for the employment of turboprop transport aircraft pending the arrival of the C-130 and C-133 into the USAF service. The squadron had three flights with each flight dedicated to a single type for the testing of standard transport aircraft that had been converted to turboprop power. The first of the three flights to be activated would operate the Convair YC-131C. Two aircraft were converted from standard C-131 Samaritan transports (the USAF version of the CV-340 airliner) to use early test versions of the venerable Allison T56 turboprop.

Back in January 2010 I had written a short posting about the second of the demonstrator aircraft that were operated by the 1700th Test Squadron and operated in the second flight of the unit- the Boeing YC-97J, a Pratt & Whitney T34-powered version of the C-97 Stratofreighter. I had recently picked up Cal Taylor's voluminous tome on the Douglas C-133 Cargomaster and he devotes considerable space to the YC-97J and its operational use by the 1700th TS. The YC-97J made its first flight at Edwards AFB on 19 April 1955 and given that it used the same T34 engines as the upcoming C-133, the USAF was keenly interested in flight testing the engine in an operational environment with the YC-97J. From my previous posting about the YC-97J: 

Boeing converted two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Inflight study of the YC-97J during its Edwards flight test program.

In addition to using the same T34 engines as the C-133, the YC-97Js also used an early version of the same Curtiss turboelectric three-bladed propellers planned for the C-133. The first YC-97J completed its flight testing at Edwards and was delivered to Kelly AFB on 14 September 1955, nine months after the YC-131Cs had arrived. The second YC-97J arrived at the end of the month. After a short series of flights operating within the continental United States, the USAF authorized the aircraft to begin overwater missions with the first overwater flight being to Kindley Field in Bermuda- the aircraft covered the 1,700 mile route from San Antonio to Bermuda in 4 hours 42 minutes, the fastest time at that point by a prop-driven aircraft. On 26 January 1956, the YC-97J departed for Rhein-Main AB in West Germany staging through Dover AFB in Delaware, then Newfoundland and Scotland. Despite record breaking cold weather on the trip, the YC-97J performed flawlessly without any of the usual maintenance headaches that were commonplace for the piston-driven C-97s. On the leg between Newfoundland and Scotland, four hours were shaved off the usual flight time when using C-124s or C-118s, the run being made in only 6 hours 30 minutes. It was clear that the time savings was tremendous on long distance missions. The international aviation press covered the flight with interest. On an outbound stop in London, the YC-97J was climbing out of Heathrow at 2,500 feet per minute and London ATC asked the pilots to slow the rate of climb as the radar dish was too slow to keep up! The return flight from Frankfurt stopped in Paris, London, the Scotland (Prestwick), Newfoundland (Goose Bay) then Selfridge AFB in Michigan before returning to Kelly AFB. It was the first round-trip trans-Atlantic crossing by an American turboprop aircraft. During the mission to West Germany and back, no engine or prop maintenance was needed and the aircraft's four engines used a mere four quarts of oil for the entire trip. Needless to say, the USAF was very enthusiastic about the aircraft!

In March 1956 the two YC-97Js were put on a scheduled cargo run between Kelly AFB to Ramey AFB in Puerto Rico via Charleston AFB in South Carolina and the return routing stopped over at Brookley AFB in Alabama (now Mobile Downtown Airport). Average flying time between San Antonio and Puerto Rico was 16 hours and despite the stopovers, it was still nine hours faster than what piston-driven USAF transports took to cover the distance. But it didn't stop there- that same month the first YC-97J made the first trans-Pacific crossing by a turboprop aircraft, averaging 360 mph over the 18,000 mile round trip. The longest leg of the route to Tokyo was between Midway Island and Yokota AB outside of Tokyo- on this leg the YC-97J flew at 30,000 feet and averaged 400 mph. 

In preparation for the arrival of the Douglas C-133 Cargomaster, the first group of air crew and mechanics arrived at Kelly AFB from Dover AFB for familiarization training with the T34 engine and its Curtiss propellers. The three-week course had pilots flying an average of 38 hours on the YC-97Js to build turbine experience while the Dover mechanics worked side by side with the Kelly AFB maintenance team to keep the YC-97Js flying. The reliability of the turboprop over the piston engine was now unquestionable and in the summer of 1956, both YC-97Js would fly a total of 46 hours 35 minutes together in a single calendar day as proof of the reliability of the turboprop. The engine overhaul time (TBO) over the course of the test program with the 1700th started out at 150 hours and ended up at 1,000 hours. 

The YC-97J departs San Diego Lindbergh Field.

In addition to its scheduled cargo flights, the YC-97Js were also flown on demonstration flights for interested groups ranging form the US Navy to other defense contractors like Pratt & Whitney and North American Aviation. On a three day demonstration in Connecticut for Pratt & Whitney, the YC-97J made 78 engine starts, 19 takeoffs and landings, 7 air starts and 15 flights without any malfunctions of the engine or propellers. By October, one of the T34 engines became the first American turboprop engine to reach 1,000 flight hours since its last overhaul. It was removed from the YC-97J with 1,001 hours and 20 minutes flight time and in that time, it only needed 44 hours of unscheduled maintenance and used a miserly 392 quarts of oil in that time frame, a fraction of what the regular C-97's piston engines would have used in 1,000 flight hours. The propellers also proved to be extremely reliable and when the first C-133 Cargomasters were delivered to Dover AFB, the engines and propellers were already rated at 1,000 hours TBO, a significant feat in that day. 

The 1700th TS's flight test program with the YC-97Js concluded on 15 November 1956, six weeks ahead of schedule. However, the aircraft were kept operational until 17 January 1957 as they were used in Operation Safe Haven to fly refugees from the 1956 Hungarian Revolution from Europe to new homes in the United States. The first YC-97J, would go on to create more aviation history, though- it was modified to become a Super Guppy transport. Aero Spacelines president Jack Conroy had already flown a piston driven Super Guppy, and aware of the pending retirement of the YC-97Js, acquired one as the turboprop engines made his conversion not only faster, but more efficient. The new turbine Super Guppy used a swing nose instead of a tail break as was the case with the original design and it was put into service with NASA in 1966, its first job transporting the second stage of the Saturn IB rocket from Huntsville, Alabama, where it was built to the Kennedy Space Center in Florida. It was subsequently retired to the Pima Air and Space Museum in Tucson, Arizona. 

Stay tuned for the final installment in this series which will look at the turboprop-powered YC-121F Super Constellation!

Source: Remembering an Unsung Giant: The Douglas C-133 Cargomaster and Its People by Cal Taylor. Firstfleet Publishers, 2005, p29-43. Photos: Smithsonian Institution, SDASM.

During the late 1940s and early 1950s many aircraft manufacturers were conducting studies on the feasibility of converting existing piston-powered transports to turboprop power. The use of turboprops was seen as a low-risk advance that combined proven airframes with higher performance engines without sacrificing fuel economy, one of the weaknesses of jet engines of the day. With the Boeing C-97/KC-97 Stratofreighter in service with the USAF at the time, Boeing had pitched to the USAF several times a turboprop-powered Stratofreighter. All were under the same Model 367 number and at one point in 1953 Boeing went as far to built a partial mockup of the proposed Model 367-41.

The USAF, however, showed little interest in Boeing's proposals but in 1955, decided to investigate further the concept of a turboprop-powered C-97/KC-97 by commissioning Boeing to convert two aircraft (52-2693 and 52-2672, both KC-97Gs) to turboprop power. Pratt & Whitney YT34 turoprop engines (which would later be used on the Douglas C-133 Cargomaster) delivering 5,700 horsepower were substituted for the four R-4360 radial engines. For a brief time the USAF considered redesignating these two Stratofreighters as C-137, but ended up assigning them the designation YC-97J (ironically the C-137 got used for the Boeing 707s used by the military, itself a development of the Model 367-80 prototype).

The conversion to turboprop power shaved nearly 5,0000 lbs off the aircraft's weight as the YT34s were much lighter but more powerful. The first flight was made on 19 April 1955 and the YC-97J demonstrated significant improvements in overall performance. The top speed was 417 mph compared to 375 mph for a regular Stratofreighter and the YC-97J took only 14 minutes to reach 20,000 feet whereas the regular Stratofreighter took 50 minutes!

Both aircraft were flown in regular transport duties as well as trials work by the USAF until 1964, but by the time both aircraft had flown, Boeing and the USAF were shifting their efforts to developing the KC-135 Stratotanker and its even greater potential than the YC-97Js.

The first YC-97J, 52-2693, upon retirement in 1964 was used to provide parts and sections for the prototype Aero Spacelines B-377SG Super Guppy.

Source: International Air Power Review, Volume 20. AIRtime Publishing, 2006. "Warplane Classic: Boeing C/KC-97 Stratofreighter" by Bill Yenne, p128-129.