The Ultimate Superfortress: The B/RB-54A

Concept art of the B/RB-54A in flight
(Boeing Historical Archives)

During the Second World War Boeing worked extensively on further improvements to the B-29 Superfortress. The most important of these improved variants was the B-29D that involved swapping out the Wright R-3350 radial engines with the more powerful Pratt & Whitney R-4360 Wasp Major radial engine. In July 1945 the USAAF signed a contract for 200 B-29Ds, but with the end of the war and the rapid postwar demobilization, the B-29D contract was canceled. With the creation of an independent United States Air Force in 1947, there was a need for interim bombers pending the arrival of more advanced jet bombers. The USAF was already getting the Convair B-36 which took on the mantle of the heavy bomber, but the USAF also wanted the B-29D which would be redesignated as a medium bomber. The USAF had the B-29D redesignated as the B-50 to avoid the appearance of ordering a "wartime" bomber. Making its maiden flight on 25 June 1947, the B-50 Superfortress would eventually result in 320 examples of all variants produced.

Boeing, however, was working on an even more powerful and longer-ranged development of the B-50. Designated the B-50C, this evolution into the ultimate Superfortress was designed to extract as much speed and performance as was possible using a new version of the Pratt & Whitney R-4360 Wasp Major engine that added what was called a "variable discharge turbine" (VDT) to the engine. The standard Wasp Major used on the B-50 developed approximately 3,500 horsepower and a Wasp Major with a VDT could easily produce 4,000 horsepower, making it one of the most powerful production piston engines in the world.

The Wasp Major VDT
(from the Engine History website)

The VDT consisted of two General Electric CHM-2 turbosuperchargers that collected the hot exhaust gases from the 28 cylinders of the Wasp Major. A portion of the hot gases were diverted through an intercooler to provide turbosupercharging at high altitudes. The bulk of the hot gases went through the CHM-2 turbines and were exhausted out a variable area nozzle that resembled a set of eyelids. By adjusting the size of the nozzle, jet thrust could be achieved that had the potential to add as much as 15% to the speed of the B-50C over the production standard B-50. The Wasp Major VDT was already flying at this point on the Republic XF-12/XR-12 Rainbow long range reconnaissance aircraft. 

The scope of the changes needed to for the B/RB-54 resulted in a redesignation to B-54 with the planned reconnaissance variant being the RB-54. The jump in power output from the use of the Wasp Major VDT resulted in a redesign of the wings that resulted in a wingspan that was over 20 feet longer than that of the B-50 with a chord increase as well- an additional six feet of chord at the wing root and an additional four feet of chord at the wing tip. This provided additional fuel capacity along with external fuel tanks which were three times the capacity of the external tanks used on the B-50A on the outboard wings. The wingspan increase was so much that outrigger gears were needed under the outermost engine nacelles. Wind tunnel testing had shown that the new wing and powerful engine output also required a longer fuselage and the B/RB-54's fuselage was stretched 10 feet. Instead of the plexiglass domes used by the gunners on the B-29/B-50, low drag hemispheric sights were used. These used a fish eye hemispheric optical element that the gunner sighted through. Glenn's Computer Museum has some great pictures of the R/RB-54 hemispheric gunsight. The tail gunner also had a hemispheric gunsight but also had a radar to direct the four-gun turret as well which was mounted in fairing above the gun turret but below the hemispheric gunsight. Fairings were also present on the nose and under the forward fuselage for bombing and navigation radars. 

As a comparison, the B-29 weighed 120,000 lbs fully loaded and the B/RB-54 would weighed in at 207,000 lbs at takeoff. The Wright R-3350 engines of the B-29 developed 2,200 horsepower and the bomber had a range of approximately 3,250 miles. The B/RB-54 would have been able to push 8,000 miles of range. The mockups were completed in 1948 and the contract was signed for 43 bombers as an initial production lot. While the Secretary of the Air Force Stuart Symington and the USAF Chief of Staff General Hoyt Vandenberg were supportive of the B/RB-54 project, General Curtis LeMay, the head of the Strategic Air Command, felt that the B/RB-54 was inferior to the Convair B-36 Peacemaker particularly the B-36D that added four J47 jet engines under the outer wings. Pending the arrival of the B-52 Stratofortress, LeMay felt deterrence was better served by the B-36 which could fly faster, farther, higher, and carry a significantly larger bomb load. In the postwar atmosphere of austerity, more B-36s couldn't be accommodated in the Air Force budget and Secretary Symington offered LeMay more B-50s instead of increased numbers of B-36s. This was even more unsatisfactory to the outspoken SAC commander who then argued that if he couldn't get more B-36s, then the funding set aside for the B/RB-54 should be shifted over to get more of the Boeing B-47 Stratojet which made its first flight in December 1947. This was agreeable to all involved, even for Boeing as it meant more funding for the Stratojet program. The B/RB-54 project was cancelled with the prototype approximately 75% complete (it was converted from a B-50A) at Boeing's Seattle facilities. In addition, the addition of the outrigger gears wasn't popular with SAC as many of its bases would need widened taxiways and runways to accommodate the B/RB-54. 

The B-29 lineage would live on, though, in the C/KC-97 Stratofreighter (the last examples being retired in 1978) and in the commercial Boeing 377 Stratocruiser. But neither would have matched the leap in performance of the B/RB-54, the "ultimate" Superfortress.

The Retromechanix page has a series of superb photos via the National Archives that show the B/RB-54A mockup in detail as well as some schematic drawings. It's well worth the time to browse them!

Source: Boeing B-29 Superfortress (Crowood Aviation Series) by Steve Pace. The Crowood Press Ltd, 2003, p166-168. Boeing B-50 (Air Force Legends Number 215 by Geoffrey Hays. Ginter Books, 2012, pp 118-121.

Flying High This Past Week: 22 February-1 March

So here's what's been getting a lot of page views this past week here at TAILS THROUGH TIME:

• The Boeing PBB Sea Ranger: The Best Flying Boat at the Worst Possible Time: Quite naturally the most recent aviation history article for this blog is going to lead things off on this installment of "Flying High This Past Week". One of the most fascinating areas of aviation history for me are those aircraft that either never went into production like the Sea Ranger or those that never left the drawing boards like the Focke Wulf Ta 183 "Huckebein". Look for more articles in the future from me that cover either unbuilt aircraft designs or aircraft that never went into production. What are often aviation dead-ends often leave a legacy for future designs. In the case of the Boeing PBB Sea Ranger, it left an unexpected legacy with Boeing that still plays a big part of the company today. 
• Tactics Over Technology: The Thach Weave: There's no argument that first six months of the Second World War in the Pacific that the Japanese Mitsubishi A6M Zero was ascendant as one of the aircraft apex predators of the skies. The Battle of Midway wasn't just a turning point in the Pacific War but it also represented a turning point in the fortunes of the Zero even though the stubby Grumman F4F Wildcat was still the main fleet fighter of the Navy and definitely not in the same class as the Zero. At Midway, the skipper of VF-3, Lt. Commander Jimmy Thach, instituted a new tactical maneuver called the "Thach Weave" that gave Wildcat pilots a fighting chance against the Zero.
• The Most Ambitious UAV Ever: Quartz/AARS: In the 1980s over $1 billion was spent on a stealthy UAV called the Advanced Aerial Reconnaissance System which was code named Quartz. It was a flying wing UAV that would penetrate Soviet airspace in times of war and search for the road and rail mobile ICBMs of the Soviet Strategic Rocket Forces. Boeing won the contract for the AARS over Lockheed but in 1992, Quartz was canceled, but it's believed the work that went into the design may bear fruit with a next-generation bomber to succeed the B-2A Spirit should the joint Lockheed/Boeing team be selected over Northrop Grumman. 
• The Ultimate Superfortress: The B/RB-54: This was the ultimate evolution in the Superfortress design lineage as a longer legged, faster, and even bigger version of the B-50 Superfortress which was itself quite an improvement over the wartime-era B-29. While it never flew let alone enter production as it came on the scene just as jet aircraft like the B-47 Stratojet were entering the picture, it did leave the drawing board. The B/RB-54 prototype was 75% complete when the program was canceled with its funding shifted to the Stratojet program. 
• One Powerful Helicopter Gunship: The ACH-74A "Guns A-Go-Go": Only four of these gunship Chinooks were built and flown into combat in Vietnam 1966-1968. To the best of my knowledge, these beast machines are the only helicopter gunships to ever go into combat with an unobstructed 360-degree field of fire.

That's all for this installment of "Flying High This Past Week". Stay tuned to TAILS THROUGH TIME- there will be a new aviation history article posted here every five days. The next one will be going up on 3 March. And don't forget last week's installment of "Flying High This Past Week"!