Boeing B-47 Stratojet - Variants

Variants

Section source: Baugher.

XB-47

Two prototype aircraft. Two built as Model 450-1-1 and 450-2-2 respectively,(46-065 and 46-066); powered by six Allison J-35-GE-7 turbojet engines for the first flights. The second, and subsequent aircraft were built with the specified General Electric J-47-GE-3 engines, which were retrofitted to the first XB-47.

B-47A

The first 10 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 5,200 lbf (23 kN) 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 mounting two 20mm cannons; 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. Minimum safe ejection altitude was about 500 ft (150 m). In the 1950s there were no high-tech "dummies" to test ejection seats and live people had to be used. A number of volunteers were injured in the development of the B-47s downward ejection seat. The first person to successful test the B-47s downward ejection seat was on 7 October 1953, by USAF Colonel Arthur M. Henderson who was ejected over Chocktawhatchee Bay, near Elgin AFB, Florida for safety reasons(i.e. in a later ejection, this proved wise when during one test, a volunteer dislocated his shoulder).

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 Boeing KC-97 Stratotanker production and other urgent programs.

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 10 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 aircraft were 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 lb (8,200 kg). All B-47Bs carried the tail turret with twin 20 mm (0.79 in) guns and the B-4 radar-guided FCS. The B-4 FCS proved troublesome, and 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 the senior Air Force leadership because of the limited range of the initial design. Solution of this problem was a high priority, and so an "in-flight refueling (IFR)" receptacle was fitted in the right side of the nose for "boom"-style refueling from KB-50 and KC-97 aircraft. This was the main reason for the deletion of the plexiglas nose cone for the bombardier navigator.

The B-47B was also fitted with a pair of jettisonable external tanks, carried between the inboard and outboard engine assemblies. These external drop tanks were very large, with a capacity of 1,780 gal (6,750 l).

The 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. In 1956 all the surviving B-47Bs were modified to the same standard as the B-47Es, ejection seats were added.

Between 1945 and 1956 Boeing at Wichita modified the surviving B-47Bs from line numbers 235 to 399 to the same standard as the B-47E under program High Noon, this included fitting ejections seats and enhancement of systems. The was followed by the Ebb Tide program that modified the early line numbers from 1 to 234, this included 66 aircraft from the 135 to 234 batch to the same standard as the High Noon aircraft, another 30 in the same range would have addition modification as drone directors DB-47Bs, and the 1 to 134 range which would have the same High Noon modification but without some of the non-combat changes. Following the modification programs the aircraft are sometimes referred to as the B-47B-II.

YRB-47B

The U.S. 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, 91 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 YRB-47Bs. They were capable of daylight reconnaissance only and when the RB-47Es were delivered they returned to the bomber role.

TB-47B

A total of 66 B-47Bs which were survivors of the first batch of 87 non-combat B-47Bs were re-designated TB-47B in 1953, to alleviate logistics problems due to different engines and systems. Most of them were used as trainers and some were modified for Air Training Command by Douglas at Tulsa under the Field Goal program, the simple modification added a fourth seat for an instructor and removed the tail turret. These aircraft were upgraded to the latest B-47E standard in 1956 under the Ebb Tide program and were joined by 41 more early build aircraft also designated TB-47B. These aircraft provided valuable crew training through most of the 1950s.

MB47-B

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 canceled on the appropriate date of 1 April 1953.

YDB-47B

There were various flight tests through the 1950s for using the B-47B as a launcher for the big 31 ft (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.

WB-47B

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 with the Air Weather Service of the Military Airlift Command (MAC) until the mid-1960s.

KB-47B

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 (including the YB-52 prototype) 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 did not make sense, and the notion was finally put to rest for good in 1957. Around the same time the KB-47 tanker prototypes were being tested, Boeing tested its aerial refueling equipment using its Dash 80 which evolved into the KC-135 Stratotanker, which has greater fuel capacity.

Canadair CL-52

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 canceled 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.

YB-47C / RB-47C / B-47Z / B-56

A four-engined variant of the B-47, the YB-47C, was proposed by Boeing in 1950 to be powered by four Allison J35-A-23 turbojet engines, providing 10,090 lbf (45 kN) thrust each, in place of the six GEs J47s. The J35 turbojet engine was being developed during the late 1940s, and it was provisionally rated at 9700 pounds (with afterburner) or 8500 pounds thrust without AB. Thus 4 * 8500 = 34,000 pounds using that engine, as compared to 6 * 5200 = 31,200 pounds in the production B-47. So the conversion would be lighter, simpler and more powerful. J71-A-5

A contract was signed with Boeing in January 1950, calling for rework of one B-47B aircraft. The date for first flight was projected as April 1951.

A combination of delays and less-than-expected performance of the J35 led to the consideration of other engines. The Allison J71 was proposed, however problems with this engine meant that this was not feasible for the by-then redesignated B-56A. The Pratt & Whitney J57, eventually rated at 17,000 pounds thrust, was also considered, but that engine was still in development, and the Boeing B-52 Stratofortress, which was being concurrently developed (first flight was April 1952), had priority for this engine.

The B-56 was cancelled in December 1952 before conversion of the prototype was started. The donor fuselage that would have been the basis of the XB-56 protoype was then used as a ground instructional airframe.

XB-47D

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-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. The B-47 also incorporated the production model with the radar controlled rear tail turret.

Early production "B-47E-Is" also known featured J47-GE-25 turbojets with 5,970 lbf (27 kN) thrust, but they were quickly changed to J47-GE-25A engines, which featured a significant improvement in the form of water-methanol injection. This was a scheme in which a water-methanol mix was dumped into the engines at takeoff, increasing mass flow and so temporarily kicking the thrust up to 7,200 lbf (32 kN). Methanol was apparently added to the water as an anti-freezing agent. The engines left a trail of black smoke behind them when water-methanol injection was on.

Jet-Assisted Take Off or JATO modifications were performed on early 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.

The internal fuel capacity of initial production B-47Es was cut to 14,627 gal (55,369 l) as a weight-saving measure. This was considered acceptable because of the use of the big external tanks and the fact that the USAF had refined mid-air refueling to the point where it could be relied upon as a standard practice.

One welcome change in the B-47E relative to the B-47B was the return of the ejection seats, the Air Force senior leadership having reconsidered the earlier decision to delete them. In addition, the twin .50 in guns (12.7 mm) in the tail turret were replaced with twin 20 mm (0.79 in) cannon to provide more firepower, backed up by an A-5 FCS in early production and an MD-4 FCS in later production.

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 lb (11,300 kg), though the bomb bay was once again shortened because of the introduction of more compact nuclear weapons.

Another improvement was the introduction of the MA-7A BNS, a major step up from its predecessors. The MA-7A included the AN/APS-64 radar, with a range as long as 240 mi (390 km). The AN/APS-64 could be used as a long range "identification friend or foe (IFF) transponder" interrogator to allow a 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. A total of 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.

TEE TOWN B-47E

In 1955, a 100 B-47E-Is were modified to carry two removable 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, Kansas, location of Forbes AFB) and so these aircraft were known as "Tee Town B-47s". They retained their normal bombing capability.

EB-47E

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 that could not carry bombs, but did retain the tail turret.

B-47E 52-0410 and 52-0412 were converted to EB-47Es in the mid-1960s for service with U.S. Navy's Fleet Electronic Warfare Support Group (FEWSG). Considered to be on indefinite loan from USAF, these aircraft were 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 and joint exercises. These two aircraft were the last B-47s in operational service, and 52-0410 performed the very last operational flight of a B-47 on 20 December 1977, when it was flown to Pease AFB, NH and put on display at the main gate. Following the closure/realignment of Pease AFB in 1991 and its conversion to Pease International Tradeport and Pease ANGB, this aircraft was disassembled and trucked to Ellsworth AFB, SD where it donated its nose and engines to RB-47H 53-4299, which is in the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, OH.

EB-47E(TT)

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.

ETB-47E

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.

DB-47E / YDB-47E

Two B-47Es were converted to YDB-47Es to support the GAM-63 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.

JB-47E

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.

JTB-47E

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.

NB-47E

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.

QB-47E

A total of 14 RB-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.

RB-47E

The B-47E was also the basis for a number of important long-range reconnaissance variants. The only B-47s to see anything that resembled combat were these reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and often probed Soviet airspace.

Boeing-Wichita built 240 RB-47E reconnaissance variants, similar to the B-47E but with a nose stretched by 34 in (0.86 m), giving them an arguably more elegant appearance than the bomber variants of the 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 gal (70,000 liters). The navigator controlled the cameras, becoming a "navigator-photographer" instead of a "navigator-bombardier".

WB-47E

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 Air Weather Service of 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.

RB-47H/ERB-47H

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, 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 (3,000 m).

Operations of the RB-47H and ERB-47H were often classified Top Secret, with the 10-hour missions generally flown at night. When crews were asked what they were doing, they always answered that such information was classified. On inquiries on what the blunt black nose was for, they would sometimes reply that it was a bumper, used in in-flight refueling in case they nosed into the tanker. This reply was often believed.

The final RB-47H to be retired from service, 53-4296, 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 at Eglin AFB, Florida, fitted with a bomber nose.

YB-47J

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.

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.

EB-47L

Between 1961 and 1963, 36 B-47Es were modified 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 by 1965.