* The B-47 was refined over its history, with improved bomber versions featuring more powerful engines and improved avionics, as well as a series of photographic and signals intelligence platforms. This chapter discusses the main B-47 variants; special variants and modifications are discussed in the next chapter.
* The B-47 arose from a 1943 US Army Air Forces (USAAF) requirement for a jet bomber / reconnaissance aircraft, which evolved into a formal request the next year. The request specified a speed of 805 KPH (500 MPH) or more; a range of 5,635 kilometers (3,500 miles); a service ceiling of 12.2 kilometers (40,000 feet); and use of the General Electric TG-180 turbojet engine, then in development.
North American, Convair, and Boeing submitted proposals. The first Boeing proposal, the "Model 424", was submitted in January 1944. It was a jet implementation of a conventional straight-wing propeller-driven bomber design, basically a scaled-down version of the Boeing B-29 Superfortress, powered by four TG-180, turbojets fitted in a pod of two engines on each wing. The US National Advisory Committee for Aeronautics (NACA, the ancestor of the modern US National Aviation & Space Administration or NASA) performed wind tunnel tests on a model of the design, or more specifically a composite of the designs submitted by the manufacturers, since the three submissions were generally similar.
By this time, the war in Europe was obviously winding to a close. General "Hap" Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Karman, of the California Institute of Technology, to form up a committee of American scientists to go to Europe and examine captured German technology. The result was the "Scientific Advisory Group". One of the members was Boeing's chief aerodynamicist, George Schairer. During his visit to Germany, Schairer examined data obtained by German aircraft manufacturers on the advantages of swept wings, and became so convinced of the merits of such a configuration that in May 1945 he wrote a letter to Boeing management suggesting the matter be investigated.
* Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to the Martin company, which had also decided to join the competition.
The NACA wind tunnel tests showed that Model 424 suffered from excessive drag. Boeing engineers then went to a revised design in April 1944, the "Model 432", that retained the straight wings, but had the four engines buried in the forward fuselage. However, although the Model 432 had structural advantages, changing the engine layout didn't really reduce drag all that much. The Boeing engineers turned to the swept-wing data obtained from the Germans and promoted by Schairer. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.
Boeing then modified the Model 432 design with a swept wings and tail. resulting in the "Model 448". The Model 448 still had the four TG-180s in the forward fuselage as had the Model 432, plus two TG-180s buried in the tail. The Boeing project manager, George Martin, had decided that the company's entry into the bomber competition needed greater range and performance, and that led to six engines instead of four. Boeing submitted the Model 448 to the USAAF in October 1945, 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 under 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. Since such weapons were very big at the time that meant a long bombbay, further limiting space for landing gear.
The solution was a "bicycle" landing gear configuration, with the two main gear assemblies arranged in a tandem instead of 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, the "Middle River Stump Jumper", named after the Martin plant at Middle River, Maryland.
However, bicycle landing gear made it difficult for a pilot to "rotate" an aircraft into a nose-up position for take-off. Again, the solution was straightforward: 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.
* 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 1946. 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. However, there was a widespread disinterest in the machine through the rest of the Boeing company, it seems partly because it was so futuristic, leading many to dismiss it as a whizzy experimental aircraft that would be impractical for operational use. Pictures of the initial rollout of the first XB-47 prototype show only about a hundred people watching. The aircraft was given the name "Stratojet", but nobody ever really called it that in practice. In fact, the bomber would never receive any nickname that stuck through its entire history.
The XB-47 prototype first flew on 17 December 1947, the 44th anniversary of the first flight of the Wright Brothers, 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.BACK_TO_TOP
* The XB-47 was an attractive, "Buck Rogers" aircraft. The 35-degree swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted on a pylon in a very neat pod, and the outboard engine tacked under the wing short of the wingtip.
The airfoil sectional chord had a height-length ratio of 11. This unusual thinness was required to attain high speed, but the wing's flexibility was a concern. It could flex as much as 1.5 meters (5 feet) up or down, and major effort was expended to ensure that flight control could be maintained as the wing flexed. As it turned out, most of the worries proved unfounded. The wings were fitted with a set of flaps that extended well behind the wing, known as "Fowler flaps", to assist in take-offs.
The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bombbay, with small outrigger wheels carried on the inboard twin-jet pods.
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 12.7-millimeter (0.50-caliber) Browning machine guns, in principle be directed by a radar-directed automatic fire-control system. The two XB-47s were not fitted with the tail turret, since they were engineering and flight test aircraft, and in fact the prototypes weren't fitted with any combat gear at all.
Fuel capacity was an enormous 64,400 liters (17,000 US gallons), compared to 19,000 liters (5,000 US gallons) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be very critical.
The first prototypes were fitted with GE "J35" turbojets, the production version of the TG-180, with 17.66 kN (1,800 kgp / 3,970 lbf) thrust each. Early jet engines did not develop good thrust at low speeds, so to assist in take-offs in heavily loaded condition, the XB-47 prototype was to have 18 solid-fuel "jet-assisted take off (JATO)" rockets with 4.41 kN (450 kgp / 1,000 lbf) thrust each. Nine such units were built into each side of the lower rear fuselage, arranged in three rows of three bottles.
Similarly, the great weight of the aircraft and the lack of thrust reversers made for an unpleasantly high landing speed. Late in flight testing, following a suggestion by Air Force test pilot Major Guy Townsend, a 9.75-meter (32-foot) drag chute was introduced to provide landing deceleration. A conventional parachute would be torn to shreds, so a ribbon parachute, derived from German designs, was used instead. 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 do a "touch and go" and circle around for another landing attempt if something went wrong. As a result, a second, half-diameter chute was added that would be deployed on approach so that the engines could be kept throttled up until the pilot felt sure his approach was safe.
The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot 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 since 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 4.5 tonnes (10,000 pounds). Production aircraft were to be equipped with, by the standards of the time, advanced electronics for navigation, bombing, countermeasures, and turret fire control.
* The second XB-47 prototype first took the air on 21 July 1948, and was equipped with much more powerful GE J47-GE-3 turbojets with 23.54 kN (2,400 kgp / 5,200 lbf) thrust each. The J47 or "TG-190" was an improved derivative of the J35 / TG-180. 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 fixed by the addition of a "yaw damper" control system that applied rudder automatically to damp out the weaving. The prototypes also had a tendency to pitch up, and this problem was solved by tacking small vanes, or "vortex generators", onto the wings. The vortex generators set up turbulence to prevent separation of airflow from the wings.
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 Air Force (the "USAF", 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!
* 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.
The XB-47 was clearly an aircraft of enormous potential, but it was still so exotic that many USAF generals didn't take it seriously. At the end of July, USAF General K.B. Wolfe, in charge of bomber production, visited Boeing in Seattle, and Boeing president Bill Allen suggested that the general take a ride on the XB-47. Wolfe was reluctant, but Allen and others managed to talk him into it, and Guy Townsend gave Wolfe an amazing ride. In early August, Wolfe contacted Boeing and indicated that the Air Force wanted to place an order for ten more of the new Boeing jet bombers. A formal contract was signed on 3 September 1948.
These ten aircraft were "B-47As". They were strictly evaluation machines. The first was delivered in December 1950. The configuration of the B-47As was close to that of the initial XB-47 prototypes. They were fitted with J47-GE-11 turbojets, providing the same 23.54 kN (2,400 kgp / 5,200 lbf) thrust as the earlier J47-GE-3, and they also featured the built-in JATO bottles.
Four of the B-47As were fitted with the K-2 bombing and navigation system (BNS), with an HD-21D autopilot, an analog computer, AN/APS-23 radar, and a Y-4 or Y-4A bombsight. Two B-47As were fitted with the tail turret, one of them using an Emerson A-2 fire control system (FCS), another early version of the GE A-5 FCS. The eight other B-47As had no defensive armament.
The B-47As were fitted with ejection seats. The pilot and copilot ejected upward, while the navigator ejected downward. Minimum safe ejection altitude was about 150 meters (500 feet).
Although the XB-47s had been built by Boeing at the company's Seattle, Washington, plant, the B-47As and all following Boeing B-47 production were produced at a government-owned factory in Wichita, Kansas, where the company had built B-29s in the past. The switch was made since the Seattle plant was burdened with KC-97 production and other urgent tasks, and the Air Force also thought Seattle was too vulnerable to a Soviet bomber attack.
Most of the B-47As had been 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 B-47As through their paces, the Cold War was rising to full force, with a hot war flaming away in Korea. The USAF's Strategic Air Command (SAC) needed an effective nuclear deterrent to keep the Soviet Union in line; the B-47 was an excellent tool for the task, and Boeing was already working on production bombers.BACK_TO_TOP
* Following a series of preliminary contracts for production B-47s, in November 1949, even before the first flight of the B-47A, the Air Force had ordered 87 "B-47Bs", the first operational variant. The first B-47B flew on 26 April 1951. A total of 399 was built, including eight that were assembled by Lockheed and ten that were assembled by Douglas, using Boeing-built kits. The USAF was impatient to get their hands on as many B-47s as they could as quickly as possible, and so signed up Lockheed and Douglas for additional production. Lockheed-built aircraft were designated by a "-LM (Lockheed Marietta)" suffix and Douglas-built aircraft were given a "-DT (Douglas Tulsa)" suffix. Boeing production was designated by a "-BW (Boeing Wichita)" suffix, except for the Seattle-built XB-47s and B-47As, which had a "-BO" suffix.
The initial batch of 87 B-47Bs featured the same J47-GE-11 engines as the B-47As, but all subsequent production featured substantially uprated J47-GE-23 turbojets with 25.8 kN (2,630 kgp / 5,800 lbf) thrust each. Early production was retrofitted with the improved engines. They all featured the built-in JATO system used on the XB-47 and B-47A.
All were fitted with full combat systems. Early production retained the K-2 BNS installed on some of the B-47As, 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 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 B-47B had a tailfin with a squared-off top, instead of a rounded top as with its predecessors.
The bombbay of the B-47B was shorter than that of the XB-47 and B-47A, since nuclear weapons had shrunk in the meantime. However, the B-47B could carry a much larger bombload, of up to 8,165 kilograms (18,000 pounds). All B-47Bs featured the tail turret with twin 12.7-millimeter guns and the B-4 radar-guided FCS. The B-4 FCS proved troublesome, in fact so troublesome that in some 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.
In practice, the enormous fuel capacity of the 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 senior Air Force brass 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. That was the main reason for getting rid of the plexiglas nose cone.
The B-47B was also fitted with a pair of jettisonable external tanks, carried between the inboard and outboard engine assemblies. These were really big drop tanks, with a capacity of 6,750 liters (1,780 US gallons).
The B-47B suffered a considerable gain in weight compared to the 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 B-47A contributed to this decision. Whatever the case, deleting the ejection seats was not a very popular change with the aircrews, since getting out of the aircraft even at altitude was troublesome. Bob Robbins recalled that George Martin, the B-47 program manager, showed him the letter from the Air Force ordering Boeing to remove the seats. Martin told Robbins: "Bob, put that letter in a safe place. The day is going to come when the Air Force is going to regret this decision, and we want to be able to make it very clear where the decision came from to do away with the ejection seats."BACK_TO_TOP
* The designations "B-47C" and "B-47D" were applied to special variants that never went into production, described later, and so the next production version of the B-47 was the definitive "B-47E".
The first B-47E flew on 30 January 1953. Four "blocks" or "phases" of the B-47E were built, each incorporating refinements over 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" featured J47-GE-25 turbojets with 26.59 kN (2,710 kgp / 5,970 lbf) thrust each, but were quickly updated to J47-GE-25A engines, featuring 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 take-off, increasing mass flow and so temporarily kicking the thrust up to 32.03 kN (3,265 kgp / 7,200 lbf). The methanol was apparently just added to the water as anti-freeze. Water injection caused the engines to pour out black smoke, no doubt due to incomplete combustion of fuel, and produce a thunderous roar.
The introduction of water-methanol injection reduced the need for JATO. JATO was expensive and had its hazards, and it would now be reserved for emergency alerts, when bombers had to get off the runway as fast as possible. It was otherwise only done once a year or so as a training measure. Early B-47E-Is had provisions for the 18 built-in JATO bottles, but there were 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. Since JATO launches were rarely performed, the built-in JATO system was deleted, to be replaced by an external, jettisonable "split vee" or "horse collar" rack fitted under the rear fuselage. The rack carried 33 JATO bottles, in three rows of 11 bottles. The racks were expendable, being dropped onto specific range areas after take-off.
The internal fuel capacity of initial production B-47Es was cut to 55,371 liters (14,610 gallons) as a weight-saving measure. That was considered acceptable because 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 particularly welcome change in the B-47E relative to the B-47B was the return of the ejection seats. Air Force brass had reconsidered the decision to delete them and realized it didn't make sense. In addition, the twin 12.7-millimeter guns in the tail turret were replaced with twin 20-millimeter cannon to provide more punch, backed up by an A-5 FCS in early production and an MD-4 FCS in later production.
A final change in the B-47E was that most of the windows in the nose were deleted, with only one left on each side. However, many pictures of B-47Es show them with the full set of windows used on the B-47B. Whether the number of windows varied through B-47E production or whether these were B-47Bs updated to B-47E specification is unclear.
* The "B-47E-II" featured only minor changes from late production B-47E-Is. The "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 "B-47E-IV" was a much more substantial update, featuring stronger landing gear, airframe reinforcement, greater fuel capacity, and a bombload uprated to 11,340 kilograms (25,000 pounds), though the bombbay 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 of up to 385 kilometers (240 miles). The AN/APS-64 could be used as a long range "identification friend or foe (IFF) transponder" interrogator to allow a B-47E-IV to find a tanker or other B-47, or it could be used as a high-resolution ground-targeting radar. The B-47E-IV retained the optical bombsight, though it was rarely used.
BOEING B-47E-IV: _____________________ _________________ _______________________ spec metric english _____________________ _________________ _______________________ wingspan 35.4 meters 116 feet wing area 132.66 sq_meters 1,428 sq_feet length 32.6 meters 107 feet height 8.5 meters 27 feet 11 inches empty weight 35,860 kilograms 79,075 pounds max loaded weight 104,300 kilograms 230,000 pounds maximum speed 1,030 KPH 640 MPH / 555 KT service ceiling 10,100 meters 33,100 feet take-off run 3,170 meters 10,400 feet range with tanks 6,495 KM 4,035 MI / 3,510 NM _____________________ _________________ _______________________
A total of 1,341 B-47Es was produced, with 691 built by Boeing, 386 by Lockheed, and 264 by Douglas. Most B-47Bs were rebuilt up to B-47E standards under two consecutive programs, designated HIGH NOON and then EBB TIDE. They were given the designation of "B-47B-II", though it appears that in practice they were simply called B-47Es.BACK_TO_TOP
* The B-47E was also the basis for a number of major long-range reconnaissance variants.
Boeing-Wichita built 240 "RB-47E" reconnaissance variants, similar to the B-47E but with a nose stretched by 86 centimeters (34 inches), giving them an arguably more elegant appearance than the bomber variants of the B-47. The long nose was used to stow up to 11 cameras, which could include:
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 70,000 liters (18,400 US gallons). The navigator controlled the cameras, becoming a "navigator-photographer" instead of a "navigator-bombardier".
* A total of 32 "RB-47H" models was built for the electronic intelligence (ELINT) mission, along with 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.
The bombbay was replaced by a pressurized compartment, which accommodated "electronic warfare officers (EWOs)", informally known as "crows" or "ravens". There were three EWOs on board the RB-47H, but only two on the ERB-47H. A distinctive bulged fairing replaced the bombbay doors. The RB-47H / ERB-47H 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. The ELINT B-47s proved so valuable that they were put through a "Mod 44" or "Silverking" update program in 1961 to provide them with updated electronics systems. Silverking aircraft could be easily recognized by a large teardrop pod for ELINT antennas attached to a pylon, mounted under the belly and offset to the right side of the aircraft, as well as an 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 Silverking specification.
* The RB-47H and ERB-47H were highly capable aircraft, but the EWO compartment was cramped, with sitting room only, and suffered from both poor noise insulation and climate control. This made long missions very uncomfortable and tiring, and some sources say that the EWOs even had to deal with fuel leaks on occasion. Getting out in an emergency was also a problem. The EWOs had ejection seats that would cut a hole through the compartment floor when fired, but nobody regarded such a scheme as very credible, and the scuttlebutt was that test firings with dummies had often resulted in decapitations.
Operations of the RB-47H and ERB-47H were top secret, with the missions generally flown at night and even base commanders often not knowing what they were all about. 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. 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 for the weather reconnaissance mission, and were given the designation of "RB-47K". Initial deliveries were in December 1955. 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 "F", "G", and "J" B-47 variants, but these were all one-shot conversions of B-47Bs or B-47Es, discussed later. There never was a "B-47I" variant -- the Air Force didn't use the "I" suffix because it was too easily confused with "1".BACK_TO_TOP