Was the B-47 Stratojet just too good for its own time, or did its revolutionary engineering compel an entire generation of pilots and designers to reconsider what was possible and survivable in jet-warfare?
The history of the B-47 starts not in Seattle or Wichita, but in shattered German laboratories after World War II. As Allied forces raided Nazi research facilities, they found a treasure of swept-wing aerodynamics information research led by Adolf Busemann and others, who understood that tilting a wing backward would stall the formation of shock waves and decrease the crippling wave drag that bedeviled high-speed flight. Boeing’s designers took advantage of these observations to design the B-47’s now-familiar 35-degree swept wings, the template for virtually every subsequent jetliner and bomber. As aerodynamicists later demonstrated, the sweep angle decreases the normal component of the Mach number, essentially allowing the aircraft to travel faster before hitting the drag-inducing shock waves that characterize the transonic regime.
But swept wings had their downsides. In flight at high speeds, the B-47 was a wonder, flying at Mach 0.74 and leaving everything in the air short of a fighter behind. In low-speed flight, though, the sheer purity of the design proved to be its undoing. Pilots would be “so aerodynamically clean that generating the drag necessary to slow down and land safely required the use of drogue parachutes,” as Harrison Kass noted. The approach chute, a 16-foot ribbon parachute borrowed from German procedure, was not only a novelty it was a necessity. Used at final approach, it enabled pilots to maintain high engine power (allowing for an immediate response if a go-around were required) while providing drag to manage the descent rate. According to one pilot, as quoted in Smithsonian Air & Space, Boeing overcame the problem by installing an approach chute… thrusting the throttles forward would provide almost instant power because the engines were being made to work at the higher rpm necessary to overcome the drag of the parachute.
The six-engine configuration of the B-47 was also revolutionary. With three twin pods hanging underneath those thin wings, the Stratojet was equipped with General Electric J35 turbojets each generating around 3,970 pounds of thrust in early versions, subsequently increased to 7,200 pounds when water-alcohol injection was added. But jet engines of the period were notoriously underpowered at low speeds, particularly when loaded with fuel and bombs. The cure? Rocket-Assisted Take-Off (RATO) packs, a technology based on the frenzied rocket testings of World War II. Nine solid-fuel rockets, each producing 1,000 pounds of thrust, could be installed close to the rear fuselage. As explained in Coast Guard Aviation History, some of these early RATO systems, created by Aerojet and others, were nothing more than steel tubes charged with potassium-perchlorate propellant, burning for 14 seconds to reduce takeoff roll by almost half. The show was unforgettable: “On takeoff it was a rip roaring static howling stinking toxic mess,” one said, as the rocket exhaust carved trenches in the runway.
But for all its might, the B-47 required a new type of discipline from its pilots. The plane’s “coffin corner,” the small band at high altitude where the difference between stall and maximum Mach was five knots, allowed very little margin for error. “At higher altitudes, the wing didn’t generate much lift, creating a “coffin corner” flight envelope in which only five knots separated the stall speed and the maximum Mach speed,” Kass noted. Operating within this hair-thin band demanded constant attention, particularly as the bomber’s 17,000-gallon fuel load consumed itself and changed the center of gravity. Training was unremitting, and errors easily tolerated in older, sluggish bombers proved disastrous in the unforgiving environment of the B-47. As Walter J. Boyne related, “The B-47’s cutting-edge design pushed the boundaries of both aerodynamics and pilot experience”. In case after case, there was some minor but fatal human error.
Technologically, the B-47’s impact is deep. Its swept-wing, podded-engine configuration became the model for the jetliners that would shrink the world in the postwar decades. The difficulties it revealed spanwise flow, tip stall, structural flutter propelled material, manufacturing, and computer-aided analysis developments. Swept-wing designs today are dependent on composites, flow fences, and advanced control systems to hold at bay the very forces that bedeviled Stratojet pilots.
And yet, despite all its technical successes, the B-47 never delivered a nuclear bomb in combat. Its only nuclear release, a training accident off Tybee Island in 1958, is a grim aside. But in the history of flight, the Stratojet stands as a testament to the risk-taking and danger of testing the limits of flight.
