The Boeing Model 450 B-47 Stratojet was a long-range, six-engined, jet-powered medium bomber built to fly at high subsonic speeds and at high altitudes. It was primarily designed 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 XB-47, which looked unlike any contemporary bomber, was described by some observers as a "sleek, beautiful outcome that was highly advanced". The 35-degree swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted in neat pods, and the outboard engines tacked under the wings short of the wingtips. With the exception of a change from the shoulder-mounted wing configuration to being under the fuselage, most future airliners would use a similar configuration, with the engines mounted in underwing pylons.
The wing airfoil was identified by Boeing as the BAC 145, but this was actually the NACA 64A(.225)12 mod airfoil. The wing's flexibility was a concern, as it could flex as much as 5 ft (1.5 m) up or down, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down. As it turned out, most of the worries proved unfounded. The aircraft's maximum speed was limited to 425 knots (787 km/h) to avoid control reversal, where aileron inputs by the pilot would cause the wings to twist and produce a roll in the opposite direction to that desired by the pilot. The wings were fitted with a set of Fowler flaps that extended well behind the wing, to enhance lift at slow speeds.
The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot, in tandem, in a long fighter-style bubble canopy, and a navigator in a compartment in the nose. The copilot doubled as tail gunner, and the navigator as bombardier. The bubble canopy could pitch up and slide backward, but as the cockpit was high off the ground, crew entrance was through a door and ladder on the underside of the nose.
Engines and performance
The first prototypes were fitted with General Electric J35 turbojets, the production version of the TG-180, with 3,970 lbf (17.7 kN) of thrust. Early jet engines did not develop good thrust at low speeds, so to help a heavily loaded bomber take off, the XB-47 prototype had provisions for fitting 18 solid-fuel rocket-assisted takeoff (RATO) rockets with 1,000 lbf (4.4 kN) thrust each. Fittings for nine such units were built into each side of the rear fuselage, arranged in three rows of three bottles.
The performance of the Model 450 design was projected to be so good that the bomber would be as fast as fighters then on the drawing board, and so the only defensive armament was to be a tail turret with two .50 in (12.7 mm) Browning machine guns, which would in principle be directed by an automatic fire-control system. The two XB-47s were not fitted with the tail turret as they were engineering and flight test aircraft; indeed, the prototypes had no combat equipment at all.
Fuel capacity was enormous, at 17,000 gal (64,400 l), more than triple the 5,000 gal (19,000 l) on the B-29 Superfortress. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be a very critical copilot duty. The total bombload capacity was to be 10,000 lb (4.5 metric tons). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.
Drag chutes
A related problem was that the aircraft's engines would have to be throttled down on landing approach. Since it could take as long as 20 seconds to throttle them back up to full power, the big bomber could not easily do a "touch and go" momentary landing. A small "approach chute" (drogue parachute) provided drag so that the aircraft could be flown at approach speeds with the engines throttled at ready-to-spool-up medium power. Training typically included an hour of dragging this chute around the landing pattern for multiple practice landings.
The aircraft was so aerodynamically slick that rapid descent ("penetration") from high cruise altitude to the landing pattern required dragging the deployed rear landing gear.
Unusually high wing loading (weight/wing area) required a high landing speed of 180 knots (330 km/h). To shorten the landing roll, Air Force test pilot Major Guy Townsend promoted the addition of a 32 ft (9.75 m) German-designed "ribbon" drag chute. (Jet engine thrust reversers were still a "far-future" concept.). As a consequence, the B-47 was the first mass-produced aircraft to be equipped with an anti-skid braking system.
General characteristics
- Crew: 3
- Length: 107 ft 1 in (32.65 m)
- Wingspan: 116 ft 0 in (35.37 m)
- Height: 28 ft 0 in (8.54 m)
- Wing area: 1,428 ft² (132.7 m²)
- Airfoil: NACA 64A(0.225)12 mod root and tip
- Empty weight: 79,074 lb (35,867 kg)
- Loaded weight: 133,030 lb (60,340 kg)
- Max takeoff weight: 230,000 lb (100,000 kg)
- Powerplant: 6 × General Electric J47-GE-25 turbojets, 7,200 lbf (32 kN) each
- Zero-lift drag coefficient: 0.0148 (estimated)
- Drag area: 21.13 ft² (1.96 m²)
- Aspect ratio: 9.42
Performance
- Maximum speed: 607 mph (528 kn, 977 km/h)
- Cruise speed: 557 mph (484 kn, 896 km/h)
- Combat radius: 2,013 mi (1,749 nmi, 3,240 km) with 20,000 lb (9,000 kg) bombload
- Ferry range: 4,647 mi (4,037 nmi, 6,494 km)
- Service ceiling: 33,100 ft (10,100 m)
- Rate of climb: 4,660 ft/min (23.7 m/s)
- Wing loading: 93.16 lb/ft² (454.8 kg/m²)
- Thrust/weight: 0.22
- Lift-to-drag ratio: 20.0 (estimated)
Armament
- Guns: 2× 20 mm (0.787 in) M24A1 autocannons in a remote controlled tail turret with AN/APG-39 Gun-laying radar
- Bombs: 25,000 lb (11,000 kg) of ordnance, including:
- 2 × Mk15 nuclear bombs (3.8 megaton yield each), or
- 1 × B41 nuclear bomb (25 megaton yield), or
- 1 × B53 nuclear bomb (9 megaton yield), or
- 28 × 500 lb (230 kg) conventional bombs
