B-47 Stratojet
| B-47 Stratojet | |
|---|---|
 |
|
| Boeing B-47E-65-BW (AF Serial No. 51-5257) during rocket-assisted takeoff test. | |
| Role | Strategic bomber |
| Manufacturer | Boeing |
| First flight | 17 December 1947 |
| Introduced | June 1951 |
| Retired | 1966, B-47E 1977, EB-47E |
| Status | 15 on static display |
| Primary user | United States Air Force |
| Number built | 2,032 |
| Unit cost | US$1.9 million (B-47E)[1] (1947 Dollars) |
The Boeing B-47 Stratojet jet bomber was a medium-range and medium-size 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. While the B-47 never saw major combat use, it remained a mainstay of the U.S. Air Force's Strategic Air Command (SAC) during the 1950s and early 1960s.
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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 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 May 1 - 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.
Preliminary designs
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.
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 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 three submissions were generally similar.)
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 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 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 (correctly as it turned out) 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
Description
The XB-47 looked unlike any contemporary bomber, described by some observers as a "sleek, beautiful outcome that was highly advanced".[2] 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 five 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 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-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 U.S. gal (64,400 liters), compared to 5,000 U.S. 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 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.
Drag chutes
A related problem was that the aircraft's engines would have to be throttled down on landing approach. Since it could take as long as 20 seconds to throttle them back up to full power, the big bomber could not easily do a "touch and go" momentary landing. A small "approach" chute 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 B-47 was the first mass-produced aircraft to be equipped with anti-lock 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 pounds (4.5 tonnes). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.
Second X-model
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 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.
X-model competitions
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.citation needed A formal contract over ten aircraft was signed on 3 September 1948.
Operational history
Early years
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 bomber was so fast that in the early days, the B-47 set records with ease. The aircraft handled well in flight, with a fighter-like light touch to the controls. The big bubble canopy enhanced the fighter-like feel of the aircraft with improved vision, but the design would cause variations in internal temperatures for the 3-man crew.
It took the Air Force until 1953 to turn the 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 B-47. Crew workload was high, with only three crew members to keep the B-47 flying right. The B-52 Stratofortress, in contrast, generally had six crewmen, with more space.
Training and problems
The 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 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. 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 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.
Prime years
By 1956, the U.S. 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 from forward bases in the UK, Morocco, Spain, Alaska, Greenland, and Guam. 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.
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 assume nuclear alert duties and the B-47 wings started to be cut back. B-47 production 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 at 425 knots 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.
Later years
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 B-47 occurred on 5 February 1958 near Savannah, Georgia. A 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 B-47 was carrying a single 7,600 lb Mark 15 thermonuclear weapon without its core. During this exercise, the F-86 collided with the B-47. The F-86 pilot ejected and the fighter crashed, while the 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. [3] [4] [5]
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.
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 B-47 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, for static display in the museum. There are at least 15 B-47s on static display, none flying.
Production numbers
The total number of B-47s built was 2,032.[6]
| 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 |
Variants
- Section source: Baugher.[7]
B-47A
The first 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 had a downward ejection seat built by Stanley Aviation.[8] Minimum safe ejection altitude was about 500 ft (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
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 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 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.
Avionics and bombing
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.
B-model modifications
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, even 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 U.S. 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.
B-47E
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.
JATO
Jet-Assisted Take Off or JATO modifications were performed on early 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 B-47Es was cut to 14,627 U.S. 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 welcome change in the B-47E relative to the B-47B was the return of the ejection seats. The Air Force senior leadership 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.
E-model modifications
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
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 m), 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 18,400 U.S. 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 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.
H-models delivered
The first RB-47H was delivered in August 1955 to Forbes AFB, Kansas. The ELINT 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 leveloff at 10,000 ft.
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.
Strategic operations
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 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 while being chased by three Soviet MiG-17 jet fighters. The Soviet fighters had 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.
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.
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 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.
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 very badly damaged. The QB-47 that crashed was used for Bomarc Missile Program tests, which normally operated from Auxiliary Field Three (Duke Field), 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
The 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 1 April 1953.
- There were various flight tests through the 1950s for using the B-47B as a launcher for the big 31 foot (9.5 m) 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 most unusual 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 new, powerful Orenda Iroquois turbojet (rated at 19,250 lbf (85.6 kN) dry, 25,000 lbf (111 kN) afterburning) 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; due to the large exterior diameter of the engine, no other location was feasible. 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 U.S. 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 26 August 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-47Es
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 20 December 1977.
- Three B-47Es were converted to the highly specialized EB-47E(TT) "Tell Two" configuration to be used for "telemetry intelligence", picking up radio signals from Soviet missile tests and space launches. The Tell Two was the precursor to the RC-135S Rivet Ball and Cobra Ball. The EB-47E(TT)s featured a "Crow capsule" in the bomb bay loaded with the appropriate gear and two ECM operators (known as Crows), and also featured odd and distinctive 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 antennas on the nose of the aircraft attracted a good deal of attention from base 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. In reality, they were specialized receiver antennas used for intercepting telemetry signals from Soviet space and missile launches.
- 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. Initially assigned to the Military Air Transport Service (MATS), they became part of the Military Airlift Command (MAC) when that organization was established. 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 U.S. 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. It is now on display at the Air Force Armament Museum, Eglin AFB, Florida, fitted with a bomber nose.
- 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.
Operators
- Royal Canadian Air Force
Survivors
A large number of surviving airframes exist in museum collections worldwide.
Specifications (B-47E)
Data from Quest for Performance[9]
General characteristics
- Crew: 3
- Length: 107 ft 1 in (32.6 m)
- Wingspan: 116 ft 0 in (35.4 m)
- Height: 28 ft 0 in (8.5 m)
- Wing area: 1,428 ft² (132.7 m²)
- Airfoil: NACA 64A(0.225)12 mod root and tip
- Empty weight: 79,074 lb (35,867 kg)
- Loaded weight: 133,030 lb (60,340 kg)
- Max takeoff weight: 230,000 lb (100,000 kg)
- Powerplant: 6× General Electric J47-GE-25 turbojets, 7,200 lbf (32 kN) each
- Zero-lift drag coefficient: 0.0148 (estimated)
- Drag area: 21.13 ft² (1.96 m²)
- Aspect ratio: 9.42
Performance
- Maximum speed: 607 mph (527 kn, 977 km/h)
- Cruise speed: 557 mph (484 kn, 896 km/h)
- Combat radius: 1,749 NM (2,013 mi, 3,240 km) with 20,000 lb (9,000 kg) bombload
- Ferry range: 4,037 NM (4,647 mi, 6,494 km)
- Service ceiling 33,100 ft (10,100 m)
- Rate of climb: 4,660 ft/min (23.7 m/s)
- Wing loading: 93.16 lb/ft² (454.8 kg/m²)
- Thrust/weight: 0.22
- Lift-to-drag ratio: 20.0 (estimated)
Armament
- Guns: 2× 20 mm M24A1 autocannons
- Bombs: 25,000 lb (11,000 kg) of ordnance, including:
- 2× nuclear bombs, or
- 28× 500 lb (230 kg) conventional bombs
Popular culture
The B-47 is featured prominently in the 1955 film "Strategic Air Command" starring Jimmy Stewart. The film features good aerial footage of both the B-47 and the B-36.
The 1 July 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 Soviet Union.
See also
- Strategic Air Command
Related development
Comparable aircraft
- B-45 Tornado
- Convair XB-46
- Martin XB-48
- English Electric Canberra
- Avro Vulcan
- P4M Mercator
- Tupolev Tu-16
Related lists
- List of bomber aircraft
- List of military aircraft of the United States
References
Notes
- ↑ Knaack 1988, p. 142.
- ↑ "Air Power Classics." Retrieved: 23 February 2008.
- ↑ B-47 "Broken Arrow" 1958 Savannah, GA
- ↑ Broken Arrows
- ↑ Aerospaceweb.org | Ask Us - Broken Arrow Nuclear Weapon Accidents
- ↑ Boeing B-47 History Boeing official site, 19 November 2007.
- ↑ Baugher's Encyclopeda B-47 Index of Variants
- ↑ Stanley Ejection Seat Website
- ↑ Loftin, L.K. Jr. "Quest for Performance: The Evolution of Modern Aircraft." NASA SP-468. Retrieved: 22 April 2006.
Bibliography
The initial version of this article was based on a public domain article from Greg Goebel's Vectorsite.
- Bowers, Peter M. "The Boeing B-47" Aircraft in Profile, Volume 4. Windsor, Berkshire, UK: Profile Publications Ltd., 2nd revised and enlarged edition, 1970. ISBN 0-85383-013-4.
- Boyne, Walter J. "The Long Reach Of The Stratojet." Air Force Magazine Vol. 66, issue 71, December 1997.
- Dennison, Robert C. "Stratojet!" Air Combat, July /August 1997. (Dennison is a retired USAF officer and B-47 pilot who works with the B-47 Stratojet Association.)
- Gunston, Bill. Bombers of the West. London: Ian Allan Ltd., 1973, p. 126–153. ISBN 0-7110-0456-0.
- Jones, Lloyd S. U.S. Bombers, B-1 1928 to B-1 1980s. Fallbrook, CA: Aero Publishers, 1962, second edition 1974. ISBN 0-8168-9126-5.
- Knaack, Marcelle Size. Post-World War II Bombers, 1945-1973. Washington, DC: Office of Air Force History, 1988. ISBN 0-16-002260-6. Post-World War II Bombers, 1945-1973 (PDF).
- NASA History on Jet Bombers: Primary article source
- Tegler, Jan. B-47 Stratojet: Boeing's Brilliant Bomber. New York: McGraw-Hill, 2000. ISBN 0-07135-567-7.
External links
- Description of the July 1, 1960, incident at USAF Museum website
- B-47 Crashes into Wright Peak (Adirondacks)
- Museum of Flight - Seattle, Washington - WB-47E - (51-7066)
- RB-47E USSR Overflights by the 91st SRW- PDF on The Cold War Museum website
- Listing of Air Force Aircraft Serial Numbers, including B-47 aircraft
- B-47 Stratojet Association
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