THE STRATOJET

In 1943, the USAAF became interested in developing a reconnaissance bomber powered by the recently perfected jet propulsion engines. After the war, the jet bomber design was enhanced by the use of captured German designs utilizing a thirty-five degree swept wing, which produced less drag than straight wings, giving the plane greater speed. The finished aircraft of these efforts proved to be the first long-range jet bomber in the USAF inventory, the Boeing B-47 Stratojet.

The B-47 represented a major innovation in post-World War II combat jet design, which contributed to the development of modern jetliners. The USAAF was so impressed with the B-47 design, that it ordered two prototypes in April 1946, designated XB-47 with its first flight taking place on December 17, 1947. A formal contract was signed in September 1948, authorizing the production of ten B47-A bombers. Boeing faced stiff competition from a number of aircraft firms in order to secure the B-47 contract, among them were North American, Convair and Martin. The initial contract for the ten aircraft would soon be followed by much larger contracts. The B-47's initial specifications called for a maximum speed of 550 mph, a cruise speed of 450 mph and a range of 3,500 miles. The thirty-five degree swept wings were shoulder mounted, with the inboard turbojet engines mounted in twin pods, about a third of the span, and the outboard engines mounted singly near the wingtip. This configuration reduced bending at the wing roots, saving structural weight with the mass of the engines acting as counter-flutter weights. When the B-47 first began test flights in the late 1940's the bomber ranked as the fastest aircraft type in the world. In fact, the B-47 was as fast as most jet fighters of the day. However, early jet engines did not develop good thrust at low speeds. To remedy this, especially when the plane was heavily loaded, the Stratojet was equipped with mounts for nine rocket assisted (RATO) units built into each side of the rear fuselage to provide additional takeoff power when needed. Most of the space within the upper fuselage was taken up by self-sealing fuel tanks, the wing being unsuitable for storing fuel.

Speaking of wings, their thinness provided no space for the tricycle main landing gear to retract, so a large bulge aft of the bomb bay would be necessary for lateral stability. The only way to get a bomb bay long enough for a nuclear payload was to use a bicycle landing gear configuration, with the two main gear assemblies arranged in a tandem fashion with outrigger struts fitted to the inboard engine pods. The Stratojet was so sleek that a rapid descent from cruise altitude to a landing pattern required dragging the deployed rear landing gear. The high wing loading of the aircraft required a high landing speed of about 180 mph. To overcome the high landing speed, USAF test pilots proposed using a German-developed ribbon type drag chute to shorten the landing distance of the B-47, since reverse thrusters had not yet been developed. As an additional remedy, B-47s were equipped with an anti-skid braking system-the first massed-produced aircraft to utilize it. The first prototypes of the Stratojet were fitted with General Electric J35 turbojets, the production version the TG-180 engine, which offered 3,970 lbs. of thrust. By the time the B-47 became operational in 1951, the J35 turbojets were being replaced by General Electric J47 turbojet engines, which offered up to 7,200 lbs. thrust. The B-47 was designed to carry a crew of three in a pressurized forward compartment, with the pilot and co-pilot seated in tandem underneath an elongated bubble canopy that could be pitched upward and slid back, as with a fighter aircraft. Though the bubble canopy provided a high level of visibility to the pilots, the cockpit was too high off the ground for direct access by the crew, which had to enter via a door and ladder on the underside of the nose. The Stratojet was armed with twin .50 caliber machine guns in the tail, remotely operated by the co-pilot. B-47 crews flew in relative comfort, since both heating and refrigeration systems were available to control the cockpit climate.

In May 1951, B-47s began replacing piston-powered B-29s and B-50s in the Strategic Air Command's (SAC's) medium bomber units. While the speed of the Stratojet was about 200 mph higher than that of the intercontinental range B-36 bomber, the B-47s range was less than half that of the B-36, which meant some B-47 squadrons were rotated to foward bases near the USSR on a temporary basis. The B-47's designated payload was 25,000 lbs. in the bomb bay with the aircraft capable of mid-air refueling. The forward deployments were normally of three-months duration, which were reduced to three-weeks as a result of the Reflex Action program in 1957. Until the late 1950's, the Stratojets were refueled by the Boeing KC-97, a piston-powered tanker aircraft. Since the KC-97 was considerably slower than the B-47, this meant B-47 crews had to refuel at speeds just above the stall speed of the plane. This situation was remedied when KC-135 jet tankers became available in the late 1950's. To facilitate the inflight refueling process, several models of the B-47 included a fuel tank inerting system. Beginning in 1950, the inerting system sublimed dry ice into carbon dioxide vapor while the fuel pumps operated or while the in-flight refueling system was in use. The carbon dioxide was then pumped into the fuel tanks and the rest of the fuel system, ensuring that the amount of oxygen in the fuel system was low, reducing the probability of an explosion.

By the mid 1950's another mission evolved for the Stratojet-that of a strategic reconnaissance platform. Beginning in 1952, a number of aerial overflights of Soviet territory provided valuable information about Soviet military and industrial facilities, in addition to signal intelligence and monitoring Soviet radar networks, with the Stratojet as the prime mover of these activities over a period of eight years. By 1956, the USAF had twenty-eight bombardment wings of B-47s, with five wings of RB (Reconnaissance Bomber) 47s. The Stratojets were a force in readiness, with a one-third alert concept having one-third of a wing ready to take off at a moments notice, fully loaded with crew, weapons and fuel. Crews were also trained to perform Minimum Takeoff Intervals (MITO) with one plane following the other in as little as fifteen seconds, to launch all bombers as fast as possible. The B-47 reached its zenith in the number of aircraft in SAC in 1958, with the larger Boeing B-52 Stratofortress being the dominant aircraft from 1959 on. When production ceased in 1957, some 2,042 Stratojets were built with the last aircraft phased out in 1965. Being the first long-range jet bomber, the B-47 introduced new technology and maintenance concepts during its service life-concepts which are still in use on the jetliners of today.