This article is about the 1950s U.S. Air Force bomber. For other meanings, see B47 (disambiguation)
|Boeing B-47E during a test of the rocket-assisted take-off system.
||Boeing Aircraft Company
||17 December 1947
||U.S. Air Force
||US$1.9 million (B-47E) equivalent to $19.5 million in current value
The Boeing Model 450 B-47 Stratojet was a long-range, six-engined, jet-powered medium bomber designed to fly at high subsonic speeds and at high altitudes to avoid enemy interception. The B-47's mission was primarily to drop nuclear bombs on the Soviet Union. With its engines carried in pods under the swept wing, the B-47 was a major innovation in post-World War II combat jet design, and helped lead to modern jet airliners.
The B-47 entered service with the United States Air Force's Strategic Air Command (SAC) in 1951. It never saw combat as a bomber, but was a mainstay of SAC's bomber strength during the 1950s and early 1960s, and remained in use as a bomber until 1965. It was also adapted to a number of other missions, including photo reconnaissance, electronic intelligence and weather reconnaissance, remaining in service as a reconnaissance platform until 1969 and as a testbed until 1977.
The B-47 arose from an informal 1943 requirement for a jet-powered reconnaissance bomber, drawn up by the U.S. Army Air Forces (USAAF) to prompt manufacturers to start research into jet bombers. Boeing was among several companies that responded to this request; its initial design, the Model 424, was basically a scaled-down version of the piston-engined Boeing B-29 Superfortress equipped with four jet engines. The next year, this concept evolved into a formal request-for-proposal to design a new bomber with a maximum speed of 550 mph (800 km/h), a cruise speed of 450 mph (725 km/h), a range of 3,500 mi (5,600 km) and a service ceiling of 45,000 ft (13,700 m).
In December 1944, North American Aviation, the Convair Corp., Boeing and the Glenn Martin Company submitted proposals for the new long-range jet bomber. Wind tunnel testing had shown that the drag from the engine installation of the Model 424 was too high, so Boeing engineers then tried a revised design, the Model 432, with the four engines buried in the forward fuselage. The USAAF awarded study contracts to all four companies, requiring that North American and Convair concentrate on four-engined designs (to become B-45 and XB-46), while Boeing and Martin were to build six-engined aircraft (the B-47 and XB-48). The powerplant was to be General Electric's new TG-180 turbojet engine.
In mid-1945, with the defeat of Germany achieved, the von Kármán mission of the Army Air Forces inspected German aeronautics laboratories from the beginning of May through the end of July 1945, in search of German developments that might help the United States. One of the Boeing engineers on the mission, George S. Schairer, studied German reports on the effects of wing sweepback on the performance of aircraft as they approached the speed of sound, and realizing the possible implications for the new bomber, sent word back to Boeing to stop work on the straight-winged design and switch to swept wings.
Analysis work by Boeing engineer Vic Ganzer suggested an optimum sweepback angle of about 35 degrees. Boeing's aeronautical engineers modified their Model 432 design to include swept wings and tail, resulting in the "Model 448", which was presented to the USAAF in September 1945. The Model 448 retained its four TG-180 jet engines in its forward fuselage, with two more TG-180s in the rear fuselage. The flush-mounted air intakes for the rear engines were inadequate, while the USAAF disliked the installation of engines within the fuselage, considering it a fire hazard.
The engines were moved out to streamlined pods pylon mounted under the wings, leading to the next iteration, the Model 450, which featured two TG-180s in a twin pod mounted on a pylon about a third of the way outboard on each wing, plus another engine at each wingtip. The Army Air Force liked this new configuration, and so Boeing's team of engineers continued to refine it, with the outer engines being moved further inboard, to about 3/4 of the wingspan. The thin wings provided no room into which wheels could be retracted, so a "bicycle landing gear" was chosen, with the two main gear assemblies arranged in a tandem configuration and outrigger struts fitted to the inboard engine pods. As the landing gear arrangement made rotation (i.e., lifting the nose during take-off) impossible, the landing gear was designed so that the aircraft rested on the ground at the proper angle for take-off.
USAAF selects Boeing
The USAAF was very pleased with the refined Model 450 design, and in April 1946, the service ordered two prototypes, to be designated "XB-47". Assembly began in June 1947. People involved with the project were very excited, since they believed (correctly as it turned out) they were working on a breakthrough in aircraft design.
The first XB-47 was rolled out on 12 September 1947, a few days before the USAAF became a separate service, the U.S. Air Force on 18 September 1947. The XB-47 prototype flew its first flight on 17 December 1947 (the anniversary of the Wright Brothers' first four flights on 17 December 1903), with the test pilots Robert Robbins and Scott Osler at the controls of the aircraft. It flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington state, in a flight that lasted just 27 minutes, with no major problems. Robbins had to pull up the flaps with the emergency hydraulic system, and the "engine fire" warning indicators were falsely lit. Robbins reported that the flight characteristics of the aircraft were good.
During early tests of the XB-47 prototype, the canopy came off at high speed, killing pilot Scott Osler. The copilot safely landed the aircraft. This resulted in a canopy redesign, and the hiring of pilot Tex Johnston as chief test pilot.