Boeing B-47E "Stratojet"
Boeing B-47E "Stratojet" (AF 53-2275) at the March Field Air Museum, Riverside, California (Photos by John Shupek)
The Boeing B-47 "Stratojet" was a medium-range and medium-size jet bomber capable of flying at high subsonic speeds and primarily designed for penetrating the airspace of the Soviet Union. A major innovation in post-World War II combat jet design, it helped lead to the development of modern jet airliners. The Boeing B-47 never saw major combat use. It served the United States Air Force from 1951 through 1969 and remained a mainstay of the U.S. Air Force's Strategic Air Command (SAC) during the 1950s and early 1960s.
The Boeing B-47 arose from a 1943 U.S. Army Air Forces requirement for a jet bomber and reconnaissance aircraft that could reach Nazi Germany in the event that Great Britain fell. The next year, the requirement evolved into a formal request for a bomber with a specified speed of 500 mph (800 km/h) or more, a range of 3,500 mi (5,600 km), and a service ceiling of 40,000 ft (12,200 m). It envisioned using the General Electric TG-180 turbojet engine, then in development. The subsequent von Kármán mission inspected Nazi Germany laboratories from 1 May - July 1945. One of the members was Boeing's chief aerodynamicist, George Schairer, who became so convinced of the merits of such a design that in May, 1945 he wrote a letter to Boeing management suggesting the matter be investigated.
North American, Convair, and Boeing submitted proposals. The first Boeing proposal, the Model 424, was a modification of a conventional propeller-driven bomber design, basically a scaled-down version of the Boeing B-29 Superfortress fitted with four jet engines. Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to Martin, which had also decided to join the fray. All of the competing bombers, including the North American B-45, Convair XB-46 and Martin XB-48 would have conventional straight wings with four to six engines, and would lack the performance of the swept wing Boeing B-47.
NACA Design Tests
The National Advisory Committee for Aeronautics (NACA, the ancestor of NASA) performed wind tunnel tests on a composite model of the designs submitted by the manufacturers. The NACA wind tunnel tests showed that the Boeing model suffered from excessive drag. Boeing engineers then tried a revised design, the Model 432, with the four engines buried in the forward fuselage, but although it had some structural advantages there was little effect on drag. At this point Boeing engineers turned to the German swept-wing data. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.
Boeing modified the Model 432 design with swept wings and tail, resulting in the Model 448, which was presented to the USAAF in September 1946. The Model 448 retained the four TG-180 engines in the forward fuselage and, at the instigation of project manager George Martin, added two more TG-180s buried in the rear fuselage to provide greater range and performance. Boeing submitted the Model 448 to the USAAF, only to have it rejected immediately. The Air Force strongly disliked fitting the engines in the fuselage, since that made engine fire or disintegration catastrophic. The engines would have to be moved back out on the wings. That led straight back to the drag problem, but the engineering team came up with a clean, elegant solution, with the engines in streamlined pods attached to the wings. This innovation led to the next iteration, the Model 450, which featured two TG-180s in a single pod mounted on a pylon about a third of the way outboard on each wing, plus another engine slung from the wingtip.
The Air Force liked the new configuration, and so the Boeing team continued to refine it. One problem was landing gear. There was no space for landing gear in the thin wings, and trying to put conventional tricycle landing gear in the fuselage would have ruined the aircraft's streamlining and degraded its performance. Furthermore, the USAAF was now also insisting that the bomber be able to carry an atomic bomb. As nuclear weapons were very large at the time, that meant a long bomb bay, further limiting space for landing gear. The solution was a "bicycle" landing gear configuration, with the two main gear assemblies arranged in a tandem, not a side by side, configuration. Outrigger landing gear was to be fitted to the inboard engine pods. The concept had already been tested on a modified Martin B-26 Marauder aircraft. However, bicycle landing gear made it difficult for a pilot to "rotate" an aircraft into a nose-up position for takeoff. Again, the solution was simple: the landing gear was designed so that the nose-up position was the default. This little change would have a very pleasing effect on an aircraft that was already shaping up to be very elegant, giving the machine the appearance of being ready to leap into the air even when it was sitting still. There were some other tweaks to the design, such as a wingtip extension to improve range. This had the effect of moving the outboard engines from a wingtip position to an underwing position towards the end of the wings.
USAAF Selects Boeing
The USAAF was very pleased with the refined Model 450 design, and in April 1947, 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 they were working on a breakthrough in aircraft design. On 18 September 1947, the USAAF became a separate service as the newly-established U.S. Air Force. A few months later, the XB-47 prototype flew its first flight on 17 December 1947, with test pilots Robert Robbins and Scott Osler at the controls. The aircraft flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington state, in a flight that lasted 52 minutes. There were no major problems, except that Robbins had to pull up the flaps with the emergency hydraulic system and the engine fire warning lights kept popping on, the sensor technology being very unreliable at the time. Robbins reported that the flight characteristics of the aircraft were good.
Design Stages XB-47
The Being XB-47 looked unlike any contemporary bomber, described by some observers as a "sleek, beautiful outcome that was highly advanced". The 35° swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted in very 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 under-wing pylons. The airfoil was 11 times as wide as it was thick. This unusual thinness (dry, no fuel tanks) was believed to be necessary to attain high speed (0.86 Mach), but the wing's flexibility was a concern. 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. (Wing "twist" limited tree-top speed to 425 knots (787 km/h) to avoid control reversal.) The wings were fitted with a set of Fowler flaps that extended well behind the wing, to enhance lift at slow speeds. The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bomb bay, with small outrigger wheels carried on the inboard twin-jet pods.
Performance and Engines
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, and in fact the prototypes were not fitted with any combat equipment at all. Fuel capacity was an enormous 17,000 gal (64,400 L), compared to 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 co-pilot duty. 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 assist in takeoffs in heavily loaded condition, 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.
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 was provided for "drag" so that the aircraft could be flown at approach speeds with the engines throttled at ready-to-spool-up medium power. Typical was 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 heavy wing loading (weight/wing area) required a high (180 knot) landing speed. 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 Boeing B-47 was the first mass-produced aircraft to be equipped with an anti-skid braking system.
Crew and Loads
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. Total bombload capacity was to be 10,000 lb (4.5 tonnes). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.
Second Boeing XB-47
The second XB-47 prototype first took the air on 21 July 1948, and was equipped with much more powerful General Electric J47-GE-3 turbojets with 5,200 lbf (23 kN) thrust each. The J47 or "TG-190" was a redesigned version of the TG-180/J35. The first XB-47 prototype was later retrofitted with these engines. Flight testing of the prototypes was particularly careful and methodical, since the design was so new in many ways. The prototypes initially suffered from "Dutch roll", an instability that caused the aircraft to weave in widening "S" turns. This problem was remedied by the addition of a "yaw damper" control system that applied rudder automatically to damp out the weaving motion. The prototypes also had a tendency to pitch up. This problem was solved by adding small vanes called "vortex generators" onto the wings that caused turbulence to prevent airflow separation. Boeing test pilot Rob Robbins had originally been skeptical about the XB-47, saying that before the initial flight he had "prayed to God to please help me" through the flight. The aircraft was so unusual that he simply didn't know if it would fly. Robbins soon realized that he had an extraordinary aircraft.
In early 1948, the United States Air Force (having become a separate service in 1947) sent up a chase plane from Muroc (now Edwards) Air Force Base in California to help calibrate the bomber's airspeed system. Robbins reported later: “The chase plane was a P-80 "Lockheed Shooting Star" and Chuck Yeager was flying it. Chuck's a hell of a good pilot, but he had a little bit of contempt for bombers and a little disdain for civilian test pilots. Well, we took off, climbed out, and got up somewhere within four or five points of full throttle speed. At that point, Chuck called me on the radio and said: "Bob, would you do a 180?" I thought: Hey, Chuck's smart, he just wants to stay reasonably close to Moses Lake, he doesn't have as much fuel as I do. Well, I turned around, got stabilized, and looked for Chuck. He wasn't there. Finally, I got on the radio and said: "Chuck, where are you?" He called back and rather sheepishly said: "I can't keep up with you, Bob." So Chuck Yeager had to admit to a civilian test pilot flying a bomber that he couldn't keep up! That was something!”
Yeager would test-fly the XB-47 later in its development cycle and would years later note that the aircraft was so aerodynamically clean that he had difficulty putting it down on the runway.
By mid-1948, the Air Force's bomber competition had already been through one iteration, pitting the North American XB-45 against the Convair XB-46. The North American design won that round of the competition, and as an interim measure the USAF had decided to put the North American bomber into production on a limited basis as the B-45 Tornado. The expectation was that B-45 production would be terminated if either of the remaining two designs in the competition, the Boeing XB-47 and the Martin XB-48, proved superior. In the end, Boeing president Bill Allen is said to have resolved the draw by taking USAF General K.B. Wolfe, in charge of bomber production, for a ride on the XB-47. A formal contract over ten aircraft was signed on 3 September 1948.
The USAF Strategic Air Command operated Boeing B-47 "Stratojets" (B-47s, EB-47s, RB-47s and YRB-47s) from 1951 through 1965. When Boeing B-47s began to be delivered to the Air Force, most crews were excited about getting their hands on the hot new bomber, an aircraft whose performance was closer to that of jet fighters of the period than SAC's extant B-36 Peacemaker bomber. The Boeing B-47 was so fast that in the early days the aircraft set records with ease. The aircraft handled well in flight, with a fighter-like light touch to the controls. The large bubble canopy for the pilot and co-pilot enhanced the fighter-like feel of the aircraft with improved vision, but the design would also cause variations in internal temperatures for the 3-man crew. It took the Air Force until 1953 to turn the Boeing B-47 into an operational aircraft. The aircraft was sluggish on takeoff and too fast on landings, a very unpleasant combination. If the pilot landed at the wrong angle, the aircraft would "porpoise", bouncing fore-and-aft. If the pilot didn't lift off for another go-round, instability would quickly cause the bomber to skid onto one wing and cartwheel. Because the wings and surfaces were flexible and bent in flight, low altitude speed restrictions were necessary to ensure effective flight control. Improved training led to a good safety record, and few crews felt the aircraft was unsafe or too demanding, but apparently there were some aircrews who had little affection for the Boeing B-47. Crew workload was high, with only three officer crew members to keep the Boeing B-47 flying right. The B-52 Stratofortress, in contrast, generally had six crewmen, 5 officers and 1 enlisted, with far more internal cabin space.
Training and Problems
The Boeing B-47's reliability and serviceability were regarded as good. The only major problem was poor avionics reliability, normal in this environment given the vacuum tube technology at the time, and the need to place some equipment outside the pressurized crew compartment. Much work was done to improve avionics reliability, but they remained problematic throughout the Boeing B-47's operational life. Several models of the Boeing B-47 starting in 1950 included a fuel tank inerting system, in which dry ice was sublimated 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, and thereby reducing the probability of an explosion. Ten carbon dioxide tanks and heaters were involved. The system was implemented largely to reduce risks from static electricity discharges occurring during in-flight refueling. Initial mission profiles included the loft bombing (Low Altitude Bombing System) of nuclear weapons. As the training for this imposes repeated high stress, the airframe lifetime would have been severely limited by metal fatigue, and this maneuver was eliminated.
By 1956, the U.S. Air Force had 28 wings of Boeing B-47 bombers and five wings of RB-47 reconnaissance aircraft. The bombers were the first line of America's strategic nuclear deterrent, often operating from forward bases in the UK, Morocco, Spain, Alaska, Greenland, and Guam. Boeing B-47 bombers were often set up on "one-third" alert, with a third of the operational aircraft available sitting on hardstands or an alert ramp adjacent to the runway, loaded with fuel and nuclear weapons, crews on standby, ready to attack the USSR at short notice. Crews were also trained to perform "minimum interval takeoffs (MITO)", with one bomber following the other into the air at intervals of as little as 15 seconds, to launch all bombers as fast as possible. MITO could be hazardous, as the bombers left turbulence and, with first generation turbojet engines with water injection systems, dense black smoke that blinded pilots in the following aircraft. Boeing B-47 bombers apparently performed training missions in which they penetrated Soviet airspace in numbers. The facts behind these missions remain controversial, with some claiming that Curtis LeMay ordered them without presidential knowledge or approval.
The Boeing B-47 would be the backbone of SAC into 1959, when the B-52 began to assume nuclear alert duties and the number of Boeing B-47 bomber wings started to be reduced. Boeing B-47 production ceased in 1957, though modifications and rebuilds continued after that. Operational practice for Boeing B-47 bomber operations during this time went from high altitude bombing to low altitude strike, which was judged more likely to penetrate Soviet defenses. Bomber crews were trained in "pop-up" attacks, coming in at low level at 425 knots (787 km/h) and then climbing abruptly near the target before releasing a nuclear weapon, and the similar "toss bombing" procedure, in which the aircraft released the weapon while climbing, and then rolled away to depart the area before bomb detonation.
Stress and fatigue incurred in low altitude operations led to a number of wing failures and crashes and an extensive refit program was initiated in 1958 to strengthen the wing mountings. The program was known as "Milk Bottle", named after the big connecting pins that were replaced in the wing roots. One of the more notable mishaps involving a Boeing B-47 occurred on 5 February 1958 near Savannah, Georgia. A Boeing B-47 based out of Homestead AFB, Florida was engaged in a simulated combat exercise with an F-86 Sabre, the bomber simulating an attacking aircraft and the fighter a defender. As was the practice at the time, the Boeing B-47 was carrying a single 7,600 lb (3,400 kg) Mark 15 thermonuclear weapon without its core. During this exercise, the F-86 collided with the Boeing B-47. The F-86 pilot ejected and the fighter crashed, while the Boeing B-47 suffered substantial damage, including loss of power on one of its outboard jet engines. The bomber pilot had to "safe" soft drop the Mark 15 weapon off the coast of Savannah, Georgia near Tybee Island after three unsuccessful landing attempts at Hunter Air Force Base. The bomb was successfully jettisoned and the aircraft landed safely. An extensive nine-month search was mounted for the unarmed bomb, but proved futile. The only Boeing B-47s to see anything that resembled combat were the reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and they often probed Soviet airspace, and on occasion, Boeing B-47 pilots were caught in situations from which mostly speed and evasion in retreat saved them. At least five of these aircraft were fired on, and three of these were shot down. The Boeing B-47s fired back with their tail turrets, though it is uncertain if they scored any kills, but in any case these were the only shots fired in anger by any Boeing B-47. These missions became impractical upon the introduction by the Russians of the trans-sonic MiG-19.
Final phase out of Boeing B-47 bomber wings began in 1963, and the last bombers were out of service by 1965. The very last USAF operational aircraft was grounded in 1969. The U.S. Navy kept specialized Boeing B-47 test aircraft in occasional use up to 1976. The final recorded flight of a Boeing B-47 was on 17 June 1986, when a Boeing B-47E was flown from the Naval Air Weapons Station China Lake, California, to Castle Air Force Base, California, for static display at the Castle Air Museum. There are at least 15 Boeing B-47s on static display, none flying.
Strategic operations of the 2,000 Boeing B-47s required 800 KC-97 Stratotankers. On a typical RB-47H reconnaissance mission covering 5,984 mi (9,360 km), the aircraft would fly from Thule, Greenland to the Kara Sea to Murmansk and then return only to find Thule weathered-in, forcing the flight from the air-refueling/decision point near the northeast shore of Greenland to one of three equidistant alternates: Goose Bay, Labrador, London, or Fairbanks, Alaska. Five KC-97s at Thule were required to support this scenario. Two ground spares and one air spare insured two 20,000 lb (9,090 kg) fuel transfers at a distance of over 600 mi (965 km) from Thule. Tankers returned to Thule to refuel and again repeat the flight to intercept the returning RB-47H six hours later for another air refueling.
The first overflight of Soviet territory with an RB-47 took place on 15 October 1952, when an RB-47B flying out of Alaska overflew Soviet airfields in Eastern Siberia. The Soviets scrambled MiG-15s to intercept, but the fast RB-47B got away unharmed.
On 8 May 1954, after a top secret reconnaissance mission in the Kola Peninsula, a 4th Air Division 91 Strategic Reconnaissance Wing RB-47E reconnaissance aircraft, with Hal Austin at the controls, flew west from the Soviet Union. The RB-47E was flying at high altitude, out of reach of MiG-15s, but unknown to USAF intelligence some MiG-17s had been stationed in the area and they were able to intercept the intruder, so the plane was being chased by three Soviet MiG-17 jet fighters. The Soviet fighters tried to destroy the RB-47E with their guns in Soviet and Finnish airspace, but the damaged RB-47E managed to escape over Sweden back to RAF Fairford station in Gloucestershire, England, where it had taken off, thanks to its remarkable top speed and combat radius superior to the Soviet fighter jets. It was the first mission in which a jet airplane equipped with modern photography equipment was used by American military reconnaissance. The incident was kept secret by all parties.
One RB-47 flying out of Alaska was scouting out the Kamchatka Peninsula on 17 April 1955, when it was bounced by Soviet MiG-15s in international airspace. The RB-47 and its crew disappeared. Between 21 March and 10 May 1956, 16 RB-47Es and 5 RB-47Hs operating from Thule, Greenland, performed overflights the length of Siberia 156 times under Project HOME RUN. The Soviets filed an angry complaint with the US government, which attributed the overflights to "navigational difficulties". MiGs did bounce RB-47s on three separate occasions in the fall of 1958, with one incident over the Black Sea on 31 October, the second over the Baltic on 7 November, and the third over the Sea of Japan on 17 November. In all three cases, the RB-47s got away without serious injury.
On 1 July 1960, a PVO Strany MiG-19 shot down an RB-47H (AF Serial No. 53-4281) reconnaissance aircraft in the international airspace over the Barents Sea with four of the crew killed and two captured by the Soviets but released in 1961. The co-pilot reported that the MiG-19 jammed (whited-out) his MD-4 FCS scope rendering the RB-47H defenseless. It seems the Soviets were annoyed over recent overflights and wanted to send the Americans a message that such provocations would have consequences.
The last known confrontation between MiGs and RB-47s took place on 27 April 1965, when an ERB-47H was jumped by North Korean MiG-17s over the Sea of Japan. The MiGs gave the ERB-47H a working over, but it managed to make it back to Yokota Air Base in Japan with two engines out.
While a few of these aircraft performed special duties during the Vietnam War, such as relaying ELINT data from drones, they were eventually replaced by much more comfortable and capable Boeing RC-135 platforms. The last RB-47H was retired on 29 December 1967.
The final 15 RB-47Es built were fitted with additional equipment, including the AN/APD "side looking airborne radar (SLAR)" system, and gear to sample the air for fallout from nuclear tests. The Air Force judged them different enough on delivery, beginning in December 1955, to give them a new designation of RB-47K.
The RB-47Ks were generally used for weather reconnaissance missions, carrying a load of eight "dropsonde" weather sensors that were released at various checkpoints along the aircraft's flight path. Data radioed back from the dropsondes was logged using equipment operated by the navigator. The RB-47Ks stayed in service until 1963.
Incidentally, there were Boeing B-47F, Boeing B-47G, and Boeing B-47J variants, but these were all one-shot conversions of Boeing B-47Bs or Boeing B-47Es, to be discussed later. There never was a Boeing B-47I variant. The Air Force never designated a Boeing B-47I, because the "I" suffix was too easily confused with the numeral "1."
Boeing B-47 "Stratojet" Variants
Boeing B-47A "Stratojet"
The first 10 aircraft were designated "B-47A", and were strictly evaluation aircraft. The first was delivered in December 1950. The configuration of the Boeing B-47s was close to that of the initial XB-47 prototypes. They were fitted with J47-GE-11 turbojets, offering the same 5,200 lbf (23 kN) thrust as the earlier J47-GE-3, and they also featured the built-in RATO bottles.
Four of the Boeing B-47s were fitted with the K-2 bombing and navigation system (BNS), with an HD-21D autopilot, an analog computer, APS-23 radar, and a Y-4 or Y-4A bombsight. Two were fitted with the tail turret, one of them using an Emerson A-2 fire control system (FCS), another an early version of the General Electric A-5 FCS. The eight other Boeing B-47s had no defensive armament.
The Boeing B-47s were fitted with ejection seats. The pilot and copilot ejected upward, while the navigator had a downward ejection seat built by Stanley Aviation. Minimum safe ejection altitude was about 500 ft (150 m). While the XB-47's had been built by Boeing at their Seattle, Washington, plant, the Boeing B-47s and all following Boeing B-47 production were built at a government-owned factory in Wichita, Kansas, where the company had built B-29's in the past. The switch was made as the Seattle plant was burdened with KC-97 "Stratotanker" production and other urgent tasks.
Most of the Boeing B-47s were phased out of service by early 1952, though one did perform flight tests for NACA for a few more years. While the Air Force put the Boeing B-47s through their paces, the Cold War was rising to full force, with a hot war intensifying in Korea. The USAF's Strategic Air Command (SAC) needed an effective nuclear deterrent to keep the Soviet Union in line, and the "Stratojet" was an excellent tool for the task, and Boeing was already working on production bombers.
Boeing B-47B "Stratojet"
Following a series of preliminary contracts for production Boeing B-47s, in November 1949, even before the first flight of the Boeing B-47A, the Air Force had ordered 87 Boeing B-47Bs, the first operational variant of the type. The first Boeing B-47B flew on 26 April 1951. A total of 399 were built, including eight that were assembled by Lockheed and ten that were assembled by Douglas, using Boeing-built parts.
The USAF was impatient to get their hands on as many Boeing B-47s as they could as quickly as possible, and signed up Lockheed and Douglas for the additional production. Lockheed-built aircraft were designated by a "-LM (Lockheed Marietta)" suffix and Douglas-built aircraft given a "-DT (Douglas Tulsa)" suffix. Boeing production was designated by a "-BW (Boeing Wichita)" suffix, except for the Seattle-built XB-47s and Boeing B-47s, which had a "-BO" suffix.
The initial batch of 87 Boeing B-47Bs featured the same J47-GE-11 engines as the Boeing B-47s, but all subsequent production featured substantially uprated J47-GE-23 turbojets with 5,800 lbf (26 kN) thrust. Early production aircraft were retrofitted with the improved engines. They all featured the built-in RATO system used on the XB-47 and Boeing B-47A.
Avionics and Bombing
All featured full combat systems. Early production retained the K-2 BNS installed on some of the Boeing B-47s, but most production featured the K-4A BNS, which featured an AN/APS-54 warning radar and an AN/APT-5 electronic countermeasures (ECM) system.
The K-4A used a periscopic bombsight fitted into the tip of the nose of the aircraft, with the transparent plexiglas nose cone of the XB-47 and Boeing B-47A replaced by a metal nose cone. There were four small windows on the left side of the nose and two on the right. Another visible change from the earlier models was that the Boeing B-47B had a vertical tailplane with a squared-off top, rather than a rounded top as with its predecessors.
Boeing B-47B Model Modifications
The bomb bay of the Boeing B-47B was shorter than that of the XB-47 and Boeing B-47A, since nuclear weapons had shrunk in the interim. However, the Boeing B-47B could carry a much larger bombload, of up to 18,000 lb (8,200 kg). All Boeing B-47Bs carried the tail turret with twin 20 mm (0.79 in) guns and the B-4 radar-guided FCS. The B-4 FCS proved troublesome, in fact so troublesome that in some Boeing B-47Bs it was replaced with an N-6 optical sight. The copilot could swivel his seat around to face backward and sight the guns directly.
In practice, even the enormous fuel capacity of the Boeing B-47 was still not enough to give it the range the Air Force wanted, and in fact there had been substantial prejudice against the type among the senior Air Force leadership because of the limited range of the initial design. Solution of this problem was a high priority, and so an "in-flight refueling (IFR)" receptacle was fitted in the right side of the nose for "boom"-style refueling from KB-50 and KC-97 aircraft. This was the main reason for the deletion of the Plexiglas nose cone for the bombardier navigator.
The Boeing B-47B was also fitted with a pair of jettisonable external tanks, carried between the inboard and outboard engine assemblies. These external drop tanks were very large, with a capacity of 1,780 gal (6,750 L).
The Boeing B-47B suffered a considerable gain in weight compared to the Boeing B-47A, and so as a weight-reduction measure the ejection seats were deleted, and a windbreak panel was fitted to the aircraft's main door to make escapes easier. Some sources also claim that a fatal ejection-seat accident in a Boeing B-47A contributed to this decision. Whatever the case, this was not a very popular measure with crews, as getting out of the aircraft even at altitude was troublesome.
Boeing B-47E "Stratojet"
The designations Boeing B-47C and Boeing B-47D were applied to special variants that never went into production (described later), and so the next production version of the Boeing B-47 was the definitive Boeing B-47E. The first Boeing B-47E flew on 30 January 1953. Four "blocks" or "phases" of the Boeing B-47E were built, each incorporating refinements on the previous block, and also sometimes featuring production changes within a block. Older blocks were generally brought up to the specifications of later blocks as they were introduced. Early production "B-47E-Is" also known featured J47-GE-25 turbojets with 5,970 lbf (27 kN) thrust, but they were quickly changed to J47-GE-25A engines, which featured a significant improvement in the form of water-methanol injection. This was a scheme in which a water-methanol mix was dumped into the engines at takeoff, increasing mass flow and so temporarily kicking the thrust up to 7,200 lbf (32 kN). Methanol was apparently added to the water as an anti-freezing agent. The engines left a trail of black smoke behind them when water-methanol injection was on.
Jet-Assisted Take Off or JATO modifications were performed on early Boeing B-47E-Is. They had the 18 built-in JATO bottles, and were quickly exchanged for an external, jettisonable "split V" or "horse collar" rack fitted under the rear fuselage. The rack carried 33 JATO bottles, in three rows of 11 bottles. The built-in JATO system was eliminated because of worries about having the JATO bottles so close to full fuel tanks, and in any case once the rocket bottles were exhausted they were just dead weight. The racks were expendable, and were dropped over specific range areas after takeoff. B-47s rarely used JATO for takeoffs, as it was expensive and slightly more hazardous than a non-assisted takeoff. Apparently it was reserved for emergency alerts, when bombers had to get off the runway as fast as possible, and was otherwise only done once a year or so as a training measure. Water-methanol injection was a big help on takeoffs when JATO wasn't used. The internal fuel capacity of initial production Boeing B-47Es was cut to 14,627 gal (55,369 L) as a weight-saving measure. This was considered acceptable because of the use of the big external tanks and the fact that the USAF had refined mid-air refueling to the point where it could be relied upon as a standard practice.
One welcome change in the Boeing B-47E relative to the Boeing B-47B was the return of the ejection seats, the Air Force senior leadership having reconsidered the earlier decision to delete them. In addition, the twin .50 in guns (12.7 mm) in the tail turret were replaced with twin 20 mm (0.79 in) cannon to provide more firepower, backed up by an A-5 FCS in early production and an MD-4 FCS in later production.
Boeing B-47E Model Modifications
A final change in the Boeing B-47E was that most of the windows in the nose were deleted, with only one left on each side. However, many pictures of Boeing B-47Es show them with the full set of windows used on the Boeing B-47B. Whether the number of windows varied through Boeing B-47E production, or whether these were Boeing B-47Bs updated to Boeing B-47E specification, is unclear. The Boeing B-47E-II featured only minor changes from late production Boeing B-47E-Is. The Boeing B-47E-III featured an ECM suite, consisting of a radar jammer in a bulge under the fuselage plus a chaff dispenser, as well as improved electrical alterNATOrs. The Boeing B-47E-IV was a much more substantial update, featuring stronger landing gear, airframe reinforcement, greater fuel capacity, and a bombload uprated to 25,000 lb (11,300 kg), though the bomb bay was once again shortened because of the introduction of more compact nuclear weapons. Another improvement was the introduction of the MA-7A BNS, a major step up from its predecessors. The MA-7A included the AN/APS-64 radar, with a range as long as 240 mi (390 km). The AN/APS-64 could be used as a long range "identification friend or foe (IFF) transponder" interrogator to allow a Boeing B-47E-IV to find a tanker or other Boeing B-47, or it could be used as a high-resolution ground-targeting radar. The Boeing B-47E-IV retained the optical bombsight, though this was rarely used. A total of 1,341 Boeing B-47Es were produced. 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most Boeing B-47Bs were rebuilt up to Boeing B-47E standards. They were given the designation of Boeing B-47B-II, though it appears that in practice they were simply called Boeing B-47Es.
Boeing RB-47E/RB-47H/ERB-47H/RB-47K "Stratojets"
The Boeing B-47E was also the basis for a number of important long-range reconnaissance variants. The only Boeing B-47s to see anything that resembled combat were these reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and often probed Soviet airspace. Boeing-Wichita built 240 RB-47E reconnaissance variants, similar to the Boeing B-47E but with a nose stretched by 34 in (0.86 m), giving them an arguably more elegant appearance than the bomber variants of the Boeing B-47. The long nose was used to stow up to 11 cameras, which could include: An O-15 radar camera for low-altitude work; A forward oblique camera for low-altitude work; A K-17 trimetrogon (three-angle) camera for panoramic shots; and K-36 telescopic cameras. The RB-47E could carry photoflash flares for night reconnaissance. Although the RB-47E could be refueled in flight, its fuel capacity was increased, to a total of 18,400 gal (70,000 liters). The navigator controlled the cameras, becoming a "navigator-photographer" instead of a "navigator-bombardier". A total of 32 RB-47H models were built for the electronic intelligence (ELINT) mission, as well as three more specialized "ERB-47Hs". These aircraft featured distinctive blunt, rounded nose and sported blisters and pods for intelligence-gathering antennas and gear. They were designed to probe adversary defenses and then collect data on radar and defense communications signals.
Bomb bay Modifications
The bomb bay was replaced by a pressurized compartment, which accommodated "electronic warfare officers (EWOs)", also known as "Crows" or "Ravens" (both being black birds, it was a reference to "black ops" meaning classified operations). There were three Crows on board the RB-47H, but only two on the Boeing ERB-47H Stratojet. A distinctive bulged radome fairing replaced the bomb bay doors. The Boeing RB-47H / ERB-47H Stratojets retained the tail turret, and were also fitted with jammers and chaff dispensers. The only easily recognizable difference in appearance between the RB-47H and ERB-47H was that the ERB-47H had a small but distinctive antenna fairing under the rounded nose.
The first RB-47H was delivered in August 1955 to Forbes AFB, Kansas. The ELINT Boeing B-47s proved so valuable that they were put through a "Mod 44" or "Silver King" update program in 1961 to provide them with updated electronics systems. Silver King aircraft could be easily recognized by a large teardrop pod for ELINT antennas attached to a pylon, mounted under the belly and offset to one side of the aircraft, as well as a pylon-style antenna attached under each wing beyond the outboard engine. It is unclear if all RB-47Hs and ERB-47Hs were updated to the Silver King specification.
The RB-47H and ERB-47H were highly capable aircraft, but the EWO compartment was not only cramped with sitting room only, but also had both poor noise insulation and climate control. This made 12 hour missions very uncomfortable and tiring, and some sources say that the Crows even had to deal with fuel leaks on occasion. Successful ejection downward (cutting through the belly radome) was impossible on-or-near the ground. Crows sat bobsled-like on the pilot compartment access floor for takeoff and landing; having to crawl encumbered with Arctic clothing with parachute to-from their compartment along an unpressurized maintenance shelf during temporary level off at 10,000 ft (3,000 m). Operations of the RB-47H and ERB-47H were often classified Top Secret, with the 10 hour missions generally flown at night. When crews were asked what they were doing, they always answered that such information was classified. On inquiries on what the blunt black nose was for, they would sometimes reply that it was a bumper, used in in-flight refueling in case they nosed into the tanker. This reply was often believed.
Boeing QB-47 "Stratojet"
The QB-47 was a drone variant of the Boeing B-47E "Stratojet". Fourteen were converted. On 20 August 1963, a QB-47 veered off course on its landing approach at Eglin Air Force Base and crash landed on a stretch of road that ran parallel to the runway. Two cars were crushed by the crash landing, killing two occupants, Robert W. Glass and Dr. Robert Bundy, and injuring a third, Dorothy Phillips. Mr. Glass and Dr. Bundy both worked for the Minnesota Honeywell Corporation at the time, a firm which had just completed flight tests on an inertia guidance sub-system for the X-20 Dyna-Soar project at the base. Mrs. Phillips was the wife of Master Sergeant James Phillips, a crew chief at the base. Mrs. Phillips was treated for moderate injuries and released later that day. Both vehicles were destroyed by fire. The QB-47 that crashed was used for Bomarc Missile Program tests, which normally operated from Eglin AFB Auxiliary Field Number Three (Duke Field), approximately 15 miles (24 km) north of the main base.
Conversions and Special Modifications
Aside from production aircraft, there were quite a number of conversions and oddball special modifications in the Boeing B-47 line.
The Boeing B-47B conversions and special modifications included:
Boeing B-47E Stratojet conversions and special modifications included:
Specifications (Boeing B-47E "Stratojet")
Copyright © 1998-2016 Skytamer Images, Whittier, California