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B-47 Stratojet

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B-47 Stratojet
Boeing B-47E-65-BW (S/N 51-5257) during rocket-assisted take off test.
Type Strategic bomber
Manufacturer Boeing
Maiden flight 1947-12-17
Introduced June 1951
Retired 1966, B-47E
1977, EB-47E
Status 15 on static display
Primary user United States Air Force
Number built 2,042
Unit cost US$1.9 million (B-47E)[1] (1947 Dollars)

The Boeing B-47 Stratojet jet bomber was a medium range and size bomber capable of flying at high subsonic speeds and primarily designed for penetrating the Soviet Union. A major innovation in post-World War II combat jet design, it helped lead to the development of modern jet airliners. While it never saw major combat use, it was the mainstay of U.S. Air Force Strategic Air Command strategic striking power in the 1950s.

Development

The 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 the UK fell to Nazi forces, which evolved into a formal request the next year. The request specified a 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.

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 fitted with four jet engines.

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 three submissions were generally similar.)

By this time, the war in Europe was obviously winding to a close. General Henry H. "Hap" Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Kármán, 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 design 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 Martin, which had also decided to join the competition. 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 B-47.

The NACA wind tunnel tests showed that the 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.

An overall view of a Boeing B-47 Stratojet aircraft in flight.

Boeing modified the Model 432 design with a 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 such weapons were very big 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.

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 (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 the moniker was infrequently used in practice. In fact, the bomber would never receive any nickname that stuck through its entire history.[citation needed]

The XB-47 prototype first flew 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, 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.

Operational history

B-47E's on a runway.

The USAF Strategic Air Command had B-47 Stratojets (B-47s, EB-47s, RB-47s and YRB-47s) in service from 1951 through 1965.

When B-47s began to be delivered to the Air Force, most crews were excited about getting their hands on the hot new bomber. The thing was so fast that in the early days, the B-47 set records everywhere it flew without even trying. The aircraft handled well and comfortably in flight, with a fighter-like light touch to the controls. The big bubble canopy also enhanced the fighter-like feel of the big aircraft, though it could make the cockpit a "hotbox" on sunny days, while the navigator in the nose shivered.[citation needed]

However, it took the Air Force until 1953 to turn the B-47 into an operational aircraft. The big aircraft was sluggish on takeoff and too fast on landings, a very unpleasant combination of circumstances. Furthermore, if the pilot put the machine down at the wrong angle on the bicycle landing gear, 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 to its destruction. Because the wings and surfaces were flexible and bent in flight, low altitude speed restrictions were necessary to insure that basic flight control surfaces remained effective.

Improved training led to a good safety record, and few crews felt the aircraft was inherently unsafe or too demanding, but apparently there were aircrews who had little affection for or were even afraid of the B-47. Crew workload was also high, with only three crew members to keep the B-47 flying right. The B-52 Stratofortress, in contrast, generally had six crew, with much less cramped accommodations. While the original XB-52 used a fighter-style canopy, production versions used a conventional flight deck.

The B-47's reliability and serviceability were also regarded as good. The only major problem was that the electronics systems were not very reliable, unsurprising given the technology available at the time. Much work was done to improve the reliability of the electronics systems, but they remained something of a maintenance headache all through the B-47's operational life.

Several models of the 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 in flight. 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.

Strategic Air Command B-47 Stratojet bombers. The world's first swept-wing bomber.

By 1956 the US Air Force had 28 wings of 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 at forward bases in the UK, Morocco, Spain, Alaska, and Guam. B-47 bombers were often set up on "one-third" alert, with a third of the operational aircraft available sitting on the runway, loaded with fuel and nuclear weapons, crews on standby, ready to take off for no-holds-barred attack against the USSR at short notice.

Crews were also trained to perform "minimum interval take-offs (MITO)", with one bomber following the other into the air at intervals of as little as 15 seconds, to get all the bombers on the way as fast as possible. MITO could be very hazardous, as the bombers left turbulence and, with water-methanol injection, dense black smoke that blinded pilots in the following aircraft.

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 B-47 would be the backbone of SAC into 1959, when the B-52 began to take over and the B-47 wings started to be cut back. Actual B-47 production had ceased in 1957, though modifications and rebuilds continued after that.

Operational practice for 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 (425 knots) and then climbing abruptly on nearing 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 target area before the bomb fell back down and detonated.

However, stresses due to low altitude operations led to a number of 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.

The only 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, 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 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 B-47. These missions became impractical upon the introduction by the Russians of the trans-sonic MiG-19. (Similar to the performance of USAF F-100)

Final phaseout of 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 test aircraft in occasional use up to 1976. The final recorded flight of a B-47 was on 17 June 1986, when a B-47E was flown from the Naval Air Weapons Station China Lake, California, to Castle Air Force Base, California, to be put in the air museum there. There are at least 15 B-47s that survive on static display, but none are still flying.

Variants and design stages

XB-47 / B-47A

The first Boeing XB-47 built (S/N 46-065), taken on Dec. 1, 1947

The XB-47 was an attractive aircraft that looked unlike any contemporary bomber. The 35-degree 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 (.86 Mach), but the wing's flexibility was a concern. It could flex as much as 5 feet (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 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.

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-cal 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 US gal (64,400 liters), compared to 5,000 US gal (19,000 L) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be 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 take off (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 9.75 m (32 foot) a German-designed "ribbon" drag chute. (Jet engine thrust reversers were still a far-future concept.)

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 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 pounds (4.5 tonnes). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.

The second XB-47 prototype first took the air on July 21, 1948, and was equipped with much more powerful General Electric J47-GE-3 turbojets with 23 kN (5,200 lbf) 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.

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.

B-47A Stratojet, the fastest known bomber in the world, flies near the Boeing Aircraft Co. production plant in Wichita, Kansas, August 11, 1950.

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 September 3, 1948. These ten aircraft were designated "B-47A", and were strictly evaluation aircraft. 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, offering the same 23 kN (5,200 lbf) thrust as the earlier J47-GE-3, and they also featured the built-in RATO 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, 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 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 500 feet (150 m).

While the XB-47s had been built by Boeing at their Seattle, Washington, plant, the B-47As and all following Boeing B-47 production were built at a government-owned factory in Wichita, Kansas, where the company had built B-29s 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 B-47As 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 B-47As 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.

B-47B

Boeing B-47B-40-BW (S/N 51-2212) of the 306th Bomb Wing (Medium) landing with drag chute.

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 of the type. The first B-47B flew on April 26 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 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 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 5,800 lbf (26 kN) thrust. Early production was retrofitted with the improved engines. They all featured the built-in RATO system used on the XB-47 and B-47A.

All featured 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 vertical tailplane with a squared-off top, rather than a rounded top as with its predecessors.

The bomb bay of the B-47B was shorter than that of the XB-47 and B-47A, since nuclear weapons had shrunk in the interim. However, the B-47B could carry a much larger bombload, of up to 18,000 pounds (8,200 kg). All B-47Bs carried the tail turret with twin 20 mm 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 directly.

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. This 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 big drop tanks were very large, with a capacity of 6,750 liters (1,780 US gal).

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, this was not a very popular measure with crews, as 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."

B-47E

Boeing B-47E-50-LM (S/N 52-3363) in flight.

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 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 27 kN (5,970 lbf) 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 32 kN (7,200 lbf). 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.

In addition, although early B-47E-Is had the 18 built-in JATO bottles, they 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.

Interestingly, 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 B-47Es was cut to 14,627 US gal (55,369 liters) 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 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 .50 cal guns (12.7 mm) in the tail turret were replaced with twin 20 mm 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 25,000 pounds (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 miles (390 km). 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 this was rarely used.

A total of 1,341 B-47Es were produced. 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most B-47Bs were rebuilt up to B-47E standards. They were given the designation of B-47B-II, though it appears that in practice they were simply called B-47Es.

RB-47E/RB-47H/ERB-47H/RB-47K

A typical reconnaissance route from Thule AB to Soviet Union flown by RB-47H crews.

The B-47E was also the basis for a number of important long-range reconnaissance variants.

Boeing-Wichita built 240 RB-47E reconnaissance variants, similar to the B-47E but with a nose stretched by 34 inches (0.86 meter), 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:

  • 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.
  • 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 70,000 liters (18,400 US gal). 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.

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 ERB-47H. A distinctive bulged radome fairing replaced the bomb bay 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 to Forbes AFB, Topeka, Kansas. 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 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 Silverking specification.

The RB-47H and ERB-47H were highly capable aircraft, but the EWO compartment was not only cramped, with sitting room only, but 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 leveloff at 10,000 ft.

Operations of the RB-47H and ERB-47H were top secret with the 10 hour missions generally flown at night and even base commanders often not knowing the details. 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.

Strategic operations of the 2,000 B-47s required 800 KC-97 Stratotankers. On a typical RB-47H reconnaissance mission covering 5,200 nm (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.

On July 1, 1960, a PVO Strany MiG-19 shot down an RB-47H (S/N 53-4281) reconnaissance aircraft in the international airspace over the Barents Sea with 4 of the crew killed and 2 captured by the Soviets and released in 1961. The co-pilot reported that the MiG-19 jammed (whited-out) his MD-4 FCS scope rendering the RB-47H defenseless.

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 December 29, 1967.

The final 15 RB-47Es built were fitted with additional equipment, including a "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 B-47F, B-47G, and B-47J variants, but these were all one-shot conversions of B-47Bs or B-47Es, to be discussed later. There never was a B-47I variant. The Air Force never designated a B-47I, because the "I" suffix was too easily confused with the numeral "1."

QB-47

The QB-47 was a drone variant of the B-47.

Fatal Crash at Eglin - On August 20, 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, 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, even though both vehicles were so badly damaged they were hardly recognizable anymore. The QB-47 that crashed was used for Bomarc Missile Program tests, which normally operated from Auxiliary Field Three, located approximately 15 miles from the main base.

Conversions and special modifications

Aside from production aircraft, there were quite a number of conversions and oddball special modifications in the B-47 line.

B-47B conversions and special modifications included:

  • The Air Force had considered building a specialized RB-47B reconnaissance variant to complement the B-47B bomber version, but as it turned out schedule slips and the like ensured that the RB-47E was the first production reconnaissance variant. As an interim measure before the RB-47E went into service, 24 B-47B bombers were fitted with a heated pod with eight cameras that was stowed in the forward bomb bay, and these aircraft were designated RB-47Bs. They were capable of daylight reconnaissance only.
  • A total of 66 B-47Bs were also converted into TB-47B trainers, through the simple measures of adding a fourth seat for an instructor and removing the tail turret. These aircraft provided valuable crew training through most of the 1950s.
  • With the introduction of the hydrogen bomb, the USAF contemplated the conversion of a few B-47Bs into MB-47B drones, which would essentially be huge cruise missiles carrying H-bombs. The program was known as "Brass Ring". Closer examination of the scheme showed that it was impractical, and Brass Ring was cancelled on the appropriate date of April 1 1953.
  • There were various flight tests through the 1950s for using the B-47B as a launcher for the big 31 foot ((9.5 meter) liquid-fueled AGM-63 Rascal missile, and one B-47B was modified to become a YDB-47B Rascal launcher. However, the Rascal program was politically problematic, and never became operational, though a total of 74 B-47Bs were modified into DB-74B Rascal launchers before the program got the axe.
  • In 1956, a single B-47B was converted into a WB-47B weather reconnaissance aircraft and operated by the Military Air Transportation Service (MATS), making it one of the few B-47s that wasn't operated by SAC. This aircraft remained in service until the mid-1960s.
  • In 1953, two B-47Bs were modified for testing the probe-and-drogue refueling system. The tanker was given the designation KB-47G and was known as "Maw" by flightcrews, and was fitted with a British-built tanker kit. The refueling test aircraft was given the designation YB-47F and was known as "Paw", though other aircraft were also used as refueling targets. The program was cancelled in 1954 as it turned out the KB-47G simply could not carry enough fuel to make it a useful tanker. The idea of fielding B-47 tanker conversions came up again a few years later, but the economics didn't make sense, and the notion was finally put to rest for good in 1957.
  • One of the oddest B-47B conversions was the Canadair CL-52 which was a B-47B loaned in 1956 to the Royal Canadian Air Force to test the big Orenda Iroquois turbojet for the Avro Canada CF-105 Arrow interceptor. Canadair Aircraft, the sub-contractor, attached the Iroquois engine to the right side of the rear fuselage near the tail, of all places, simply because there was no other place to put it. Flying the CL-52 was reportedly a nightmare. After the Arrow project was cancelled in early 1959, the B-47B/CL-52, with about 35 hours of engine flight tests to its credit, was returned to the US. Some sources claimed it was bent out of shape by the tests, but in any case, it was subsequently scrapped. The CL-52 was the only B-47 to be used by any foreign service.
  • The YB-47C (originally the YB-56), was proposed by Boeing in 1950. It was to be a major variant, with four big Allison J71-A-5 turbojet engines, providing 45 kN (10,090 lbf) thrust each, in place of the six GEs J47s. A B-47B was set aside to be converted into the prototype for the series. The J71 engine program suffered setbacks, and so the Air Force decided to use the Pratt & Whitney J57 turbojet instead. However, the USAF then had second thoughts about the entire program, since by that time the future was clearly the B-52, and the B-47 was essentially an interim type, and gave up on the idea in December 1952.
  • Beginning in 1951, two XB-47Ds were modified from B-47Bs as purely experimental platforms, with a big Wright YT49-W-1 turboprop engine spinning a huge four-paddle prop, replacing each of the inboard two-jet pods. Difficulties with engine development delayed first flight of the XB-47D until August 26, 1955. The aircraft's performance was comparable to that of a conventional B-47, and its reversible propellers shortened the landing roll, but the USAF did not follow up the idea.

B-47E conversions and special modifications included:

  • In 1955, a number of B-47E-Is were fitted with external pods, one mounted on either side of the bomb bay, with each pod containing four AN/ALT-6B jammers. The pods were known as "Tee Town pods" (for Topeka, KS, location of Forbes AFB) and so these aircraft were known as "Tee Town B-47s". They retained their normal bombing capability.
  • The Tee Town B-47s then led to a specialized ECM conversion of the B-47E, which was given the designation EB-47E. The initial EB-47 conversion featured a set of 16 jammers in a removable cradle stored in the bomb bay, plus radar warning receivers and chaff dispensers. These were known as "Phase IV" or "Blue Cradle" EB-47Es. The more advanced "Phase V" EB-47E featured a pressurized module that was stowed in the bomb bay, with 13 jammers under control of two Crows. While the Phase IV jammer system was "broadband", blanketing a wide range of frequencies in hopes of jamming radars operating somewhere within that range, the Phase V jammer system could be selectively tuned to specific radar frequencies by the crows, permitting much higher jammer power on the frequencies that did the most good. A radar jammer tends to announce its presence and location by the radio signals it emits, and EB-47E crews were perfectly aware that they were unlikely to return from an operational mission into the USSR. If they could cover for B-47 bombers, however, the sacrifice would be worth it. About 40 B-47Es were converted to EB-47Es. Of course they couldn't carry bombs, but they did retain the tail turret.
  • Two more B-47Es were converted to EB-47Es in the mid-1960s for U.S. Navy service, on indefinite loan from the USAF. They were very much unlike the USAF EB-47Es, with some of their ECM gear fitted into pods carried on the external fuel tank pylons. They were used for tests of naval ECM systems and as "electronic aggressors" in naval exercises. These two aircraft were the last B-47s in service, and one performed the very last operational flight of a B-47 on December 20, 1977.
  • Three B-47Es were converted to the highly specialized EB-47E(TT) configuration to be used for "telemetry intelligence", picking up radio signals from Soviet missile tests and space launches. The EB-47E(TT)s featured a "Crow capsule" in the bomb bay loaded with the appropriate gear and two Crows, and also featured odd and distinctive "armrest" antennas just below each side of the cockpit. All three of these aircraft were operated out of Turkey, and stayed in service until 1967. The armrest antennas attracted a good deal of attention from nosy personnel, and crews made up imaginative stories about them, for example claiming they were part of a "return to fighter (RTF)" defensive system that would cause Soviet air-to-air missiles to loop back and shoot down their own launch fighters.
  • As with the B-47B, a few B-47Es were converted to trainers, with a fourth seat for an instructor, and given the designation ETB-47E. These aircraft were used to replace TB-47Bs that had got too long in the tooth, and served into the early 1960s.
  • Similarly, two B-47Es were converted to YDB-47Es to support the Rascal stand-off missile program, and two more B-47Es were converted to DB-47Es in preparation for the operational introduction of the missile before the program was axed. These two DB-47Es were later used as drone controller aircraft.
  • Following the single WB-47B weather reconnaissance conversion, in the early 1960s 34 B-47Es were converted by Lockheed into WB-47Es for weather reconnaissance. These aircraft were stripped of combat gear, including the tail turret. They were fitted with cameras in the nose to take pictures of cloud formations, and carried a special meteorological instrument pod in the bomb bay. The last WB-47E was retired on 31 October 1969, and was the last B-47 in operational USAF service.
  • In 1963, the Air Force modified 35 B-47Es to carry a communications relay system. These aircraft were given the new designation of EB-47L, and were used to support US flying command post aircraft in case of a nuclear attack on the US. The EB-47Ls only remained in service for a few years, as improved communications technologies quickly made them redundant.
  • A total of 14 B-47Es were converted to QB-47E target drones in 1959 and 1960. These aircraft were radio-controlled, and included such interesting features as self-destruct charges and arresting gear to assist in landings. They also carried pods mounted on the external tank pylons to help in scoring weapons tests. Apparently most of the missiles fired on them were directed for a near-miss, but the QB-47Es were nonetheless eventually whittled down to two survivors that were retired in the early 1970s.
  • A single B-47E was modified to test the MA-2 BNS for the B-52, and given the designation YB-47J. Other B-47Es were also apparently used in the MA-2 tests, but not given a special designation.
  • Several B-47Es were assigned to other specialized test duties and given the blanket designation of JB-47E. One was used in the late 1960s to test "fly by wire" control system concepts.
  • Two B-47Es were also used for secret flight experiments in the early 1960s and given the designation JTB-47E, and a third, even more mysterious modified B-47E was given the designation JRB-47E. They appear to have been test platforms for ECM systems.
  • The final RB-47H to be retired from service was later pulled out of the "boneyard" and used for tests of avionics for the General Dynamics FB-111. This RB-47H was fitted with an F-111-style nose and flew into the early 1970s. It was not given any special designation.
  • In 1963, the USAF demonstrated three B-47Es to Australian crews, as a possible stopgap before Australia acquired the General Dynamics F-111. Nothing came of this exercise.
  • Finally, a B-47E was loaned to the U.S. Navy to help test the GE TF34-2 turbofan for the Lockheed S-3 Viking carrier-based antisubmarine warfare aircraft. This B-47E was given the designation NB-47E and performed test flights from 1969 through 1975.

Perspective

Variant Built
XB-47 2
B-47A 10
B-47B 399
B-47E 1,341
RB-47E 240
RB-47H 32
ERB-47H 3
RB-47K 15
Total 2,042

The total number of B-47s built by Boeing's count was 2,042.

The B-47 undeniably fulfilled its design objectives. Perhaps as importantly, it led directly to the Boeing B-52, which:

  • still survives as an important US military asset, and
  • in turn provided much inspiration for the design of

The B-47 is very elegant even by 21st-century standards, but considering what most of its contemporaries looked like, it must have appeared revolutionary when it was introduced. Most bomber variants were painted white on the underside to reflect the flash of a nuclear explosion, and this color scheme helped give the aircraft a clean, elegant appearance.

The B-47 was succeeded by the B-58 Hustler supersonic bomber, which in turn was replaced by the FB-111. The XB-52 used the fighter style canopy, and tandem landing gear, but subsequent large bombers have used side-by-side flight decks.

Survivors

RB-47H on display in the Cold War Gallery at the National Museum of the United States Air Force
  • An EB-47 S/N 52-0412 is on outdoor display in the Linear Air Park at Dyess AFB, Tx.

Specifications (B-47E)

Orthographically projected diagram of B-47E Stratojet.

Data from Quest for Performance[3]

General characteristics

Performance

Armament

  • Guns: 2× 20 mm M24A1 cannons
  • Bombs: 25,000 lb (11,000 kg) of ordnance, including:

Operators


Media

  • The July 1, 1960 shoot down of an RB-47H (s/n 53-4281) by a MiG-19 over the Barents Sea was retold by the two surviving crewmembers, Capt. John R. McKone (navigator) and Capt. Freeman B. Olmstead (co-pilot), in the biographical book The Little Toy Dog by William L. White. The title refers to a small plastic toy "Snoopy" that Capt. McKone carried with him, and kept during their seven months in Lubyanka prison in the former Soviet Union.

References

  1. ↑ Knaack, Marcelle Size (1988). Post-World War II bombers, 1945-1973. Office of Air Force History. ISBN 0-16-002260-6. 
  2. ↑ National Musuem of the USAF Fact Sheet
  3. ↑ Loftin, LK, Jr. Quest for performance: The evolution of modern aircraft. NASA SP-468. Retrieved on 2006-04-22.
  • Tegler, Jan (2000). B-47 Stratojet: Boeing's Brilliant Bomber. McGraw-Hill. 
  • Bowers, Peter M. (1966). The Boeing B-47. Profile Publications. 
  • Dennison, Robert C. (July /August 1997). "Stratojet!".  Dennison is a retired USAF officer and B-47 pilot who works with the B-47 Stratojet Association.
  • Boyne, Walter J. (December 1997). "The Long Reach Of The Stratojet" 66 (71). 

The initial version of this article was based on a public domain article from Greg Goebel's Vectorsite.

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