Boeing B-47 Stratojet

https://en.wikipedia.org/wiki/Boeing_B-47_Stratojet

The Boeing B-47 Stratojet (company Model 450) is a retired American long-range,
six-engined, turbojet-powered strategic bomber designed to fly at high subsonic
speed and at high altitude to avoid enemy interceptor aircraft. The primary
mission of the B-47 was as a nuclear bomber capable of striking targets within
the Soviet Union.

Development of the B-47 can be traced back to a requirement expressed by the
United States Army Air Forces (USAAF) in 1943 for a reconnaissance bomber that
harnessed newly-developed jet propulsion. Another key innovation adopted during
the development process was the swept wing, drawing upon captured German
research. With its engines carried in nacelles underneath the wing, the B-47
represented a major innovation in post-World War II combat jet design, and
contributed to the development of modern jet airliners. Suitably impressed, in
April 1946, the USAAF ordered two prototypes, designated "XB-47"; on 17 December
1947, the first prototype performed its maiden flight. Facing off competition
such as the North American XB-45, Convair XB-46 and Martin XB-48, a formal
contract for 10 B-47A bombers was signed on 3 September 1948. This would be soon
followed by much larger contracts.

During 1951, the B-47 entered operational service with the United States Air
Force's Strategic Air Command (SAC), quickly becoming a mainstay of its bomber
strength by the late 1950s. Over 2,000 were manufactured to meet the Air Force's
extensive demands, driven by the tensions of the Cold War. The B-47 was in
service as a strategic bomber until 1965, at which point it had largely been
supplanted by more capable aircraft, such as the Boeing B-52 Stratofortress.
However, the B-47 was also adapted to perform a number of other roles and
functions, including photographic reconnaissance, electronic intelligence, and
weather reconnaissance. While never seeing combat as a bomber, reconnaissance
RB-47s would occasionally come under fire near to or within Soviet air space.
The type remained in service as a reconnaissance aircraft until 1969; a handful
served as flying testbeds up until 1977.

Origins

The B-47 arose from an informal 1943 requirement for a jet-powered
reconnaissance bomber, drawn up by the United States Army Air Forces (USAAF) to
prompt manufacturers to start research into jet bombers. Boeing was among
several companies to respond to the request; one of its designs, the Model 424,
was basically a scaled-down version of the piston-engined B-29 Superfortress
equipped with four jet engines. In 1944, this initial concept evolved into a
formal request-for-proposal to design a new bomber with a maximum speed of 550
mph (890 km/h), a cruise speed of 450 mph (720 km/h), a range of 3,500 mi (5,600
km), and a service ceiling of 45,000 ft (13,700 m).

In December 1944, North American Aviation, the Convair Corp., Boeing and the
Glenn Martin Company submitted proposals for the new long-range jet bomber. Wind
tunnel testing had shown that the drag from the engine installation of the Model
424 was too high, so Boeing's entry was a revised design, the Model 432, with
the four engines buried in the forward fuselage. The USAAF awarded study
contracts to all four companies, requiring that North American and Convair
concentrate on four-engined designs (to become B-45 and XB-46), while Boeing and
Martin were to build six-engined aircraft (the B-47 and XB-48). The powerplant
was to be General Electric's new TG-180 turbojet engine.

Swept wings

In May 1945, the von Kármán mission of the Army Air Forces inspected the secret
German aeronautics laboratory near Braunschweig. von Kármán's team included the
chief of the technical staff at Boeing, George S. Schairer. He had heard about
the controversial swept-wing theory of R. T. Jones at Langley, but seeing German
models of swept-wing aircraft and extensive supersonic wind-tunnel data, the
concept was decisively confirmed. He wired his home office: "Stop the bomber
design" and changed the wing design. Analysis by Boeing engineer Vic Ganzer
suggested an optimum sweepback angle of about 35 degrees. Boeing's aeronautical
engineers modified the Model 432 with swept wings and tail to produce the "Model
448", which was presented to the USAAF in September 1945. It retained the four
TG-180 jet engines in its forward fuselage, with two more TG-180s in the rear
fuselage. The flush-mounted air intakes for the rear engines were inadequate
while the USAAF considered the engine installation within the fuselage to be a
fire hazard.

The engines were moved to streamlined pylon-mounted pods under the wings,
leading to the next iteration, the Model 450, which featured two TG-180s in a
twin pod mounted on a pylon about a third of the way outboard on each wing, plus
another engine at each wingtip. The Army Air Force liked this new configuration,
so Boeing's engineers refined it, moving the outer engines further inboard to
about ?3/4 of the wingspan. The thin wings provided no space for wheels to
retract into so a "bicycle landing gear" was chosen, the two main gear
assemblies arranged in a tandem configuration and outrigger struts fitted to the
inboard engine pods. As the landing gear arrangement made rotation impossible,
it was designed so that the aircraft rested on the ground at the proper angle
for takeoff.

Pleased with the refined Model 450 design, in April 1946, the USAAF ordered two
prototypes, to be designated "XB-47". Assembly began in June 1947. The first
XB-47 was rolled out on 12 September 1947, a few days before the USAAF became a
separate service, the United States Air Force, on 18 September 1947. According
to aviation authors Bill Gunston and Peter Gilchrist, Boeing subjected the first
prototype to "one of the most comprehensive ground-test programmes ever
undertaken".


Role
Strategic bomber/Aerial reconnaissance

National origin
United States

Manufacturer
Boeing

First flight
17 December 1947

Introduction
June 1951

Retired
1969 (B-47E)
1977 (EB-47E)

Status
Withdrawn from service

Primary user
United States Air Force

Number built
2,032

Unit cost


US$1.9 million (B-47E)
($17.1 million in 2018 dollars)

The USAF Strategic Air Command operated multiple B-47 models (B-47s, EB-47s,
RB-47s and YRB-47s) from 1951 through 1965. Upon entry to service, its
performance was closer to that of contemporary fighters than SAC's extant B-36
Peacemaker bomber, setting multiple records with ease. It handled well in
flight, the controls having a fighter-like light touch. The large bubble canopy
enhanced the flying crew's vision and gave a fighter-like feel, but also caused
internal temperature variations for the crew. The three-man crew consisted of
the commander, copilot, and a navigator/bombardier or a crew chief.

It was not until 1953 that the B-47 became operational. It was sluggish on
takeoff and too fast on landings, an unpleasant combination. If landed at the
wrong angle, the B-47 would "porpoise", bouncing fore-and-aft; if the pilot did
not lift off for another go-around, instability would quickly cause it 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. The B-47 was regarded as a maintenance "hog". General
reliability was good, the only major issuing relating to the avionics, typical
given the vacuum tube technology used and the placement of equipment outside the
pressurized crew compartment. Much work was done to improve avionics
reliability, but remained problematic throughout its operational life.

Starting in 1950, several models of the B-47 included a fuel tank inerting
system, in which dry ice was sublimed into carbon dioxide vapor while the fuel
pumps operated or while the in-flight refueling system was in use; the carbon
dioxide was pumped into both the fuel tanks and the fuel system to ensure low
oxygen levels throughout. It was implemented largely to reduce the probability
of an explosion from static electricity discharges.

Initial mission profiles included the loft bombing of nuclear weapons. As the
training for this imposes repeated high stress on the aircraft, the airframe
lifetime would have been severely limited by metal fatigue, and this maneuver
was eliminated. 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, having
only three crew members to operate it. Boeing's B-52 Stratofortress, in
contrast, generally had six crewmen, five officers and one enlisted, with more
internal cabin space.

Stress and fatigue incurred in low-altitude operations led to a number of wing
failures and crashes, and an extensive refit program was begun 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, in the so-called 1958 Tybee Island B-47 crash. A B-47
based out of Homestead AFB, Florida, was engaged in a simulated combat exercise
against an F-86 fighter. As was the practice at the time, the B-47 was carrying
a single 7,600 lb (3,400 kg) Mark 15 nuclear bomb, without its core. During this
exercise, the two aircraft collided; the F-86 crashed after the pilot ejected,
while the B-47 suffered substantial damage, including loss of power in one
outboard jet engine. After three unsuccessful landing attempts at Hunter Air
Force Base, the bomber pilot had to "safe" soft drop the Mark 15 weapon off the
coast of Savannah, Georgia near Tybee Island, after which the B-47 landed
safely. An extensive nine-month search for the unarmed bomb proved futile.

During 1963, final phaseout of B-47 bomber wings began, the last bombers were
out of service with SAC by 1966. The last USAF operational aircraft, WB-47Es
assigned to the Air Weather Service, were withdrawn from use in September 1969.
Shortly before, a B-47E, Air Force Serial Number 53-2280, was used as a testbed
for a newly developed fly-by-wire system. The U.S. Navy kept specialized EB-47E
test aircraft from USAF inventory in occasional use to support the Fleet
Electronic Warfare Systems Group (FEWSG) until December 1977, being replaced by
government owned/contractor operated (GOCO) NKC-135 Stratotankers also loaned
from the USAF.

The final recorded flight of a B-47 was on 17 June 1986, when a B-47E was
restored to flightworthy condition for a one-time ferry flight. This aircraft
was flown from Naval Air Weapons Station China Lake, California, to Castle Air
Force Base, California, for static display at the Castle Air Museum, where it
presently resides.

The only B-47s to see anything that resembled combat were the aerial
reconnaissance variants. The first overflight of Soviet territory with a B-47B,
fitted with special radar and cameras in the bomb bay, occurred on 15 October
1952, overflying Soviet airfields in Northeastern Siberia. RB-47s operated from
almost every airfield that gave them access to the USSR and routinely probed
Soviet airspace. Occasionally, they would avoid confrontations via speed and
evasion; at least five aircraft were fired on and three were shot down. The
RB-47s fired back with their tail turrets, although it is uncertain if they
scored any kills; these were the only shots fired in anger by any B-47.

On 8 May 1954, after a top secret reconnaissance mission near the Kola
Peninsula, a 4th Air Division / 91st Strategic Reconnaissance Wing RB-47E, flown
by then-Capt (later Col) Harold "Hal" Austin, overflew the Soviet Union at high
altitude, out of reach of MiG-15s. However, unknown to USAF intelligence,
MiG-17s capable of intercepting it had been stationed nearby; the RB-47E was
chased by several MiG-17s, firing upon it with their guns over Soviet and
Finnish airspace. While taking damage, the RB-47E escaped over Sweden to its
home base at RAF Fairford, Gloucestershire; its top speed and combat radius
superiority to the fighter jets being decisive factors. The mission marked the
first time a jet aircraft equipped with modern aerial photography equipment,
K-17 and K-38 cameras, was used for USAF reconnaissance over the Soviet Union.
The incident was kept secret by all parties.

Other interceptions resulted in losses. An RB-47 flying out of Alaska was
scouting out the Kamchatka Peninsula on 17 April 1955, when it was intercepted
by Soviet MiG-15s in international airspace. The RB-47 and its crew disappeared.
Between 21 March and 10 May 1956, 16 RB-47Es and five RB-47Hs operating from
Thule performed overflights the length of Siberia 156 times under Project
HOMERUN. The Soviets filed an angry complaint with the U.S. government, which
attributed the overflights to "navigational difficulties". MiGs intercepted
RB-47s on three separate occasions in late 1958: over the Black Sea on 31
October, over the Baltic on 7 November, and over the Sea of Japan on 17
November.

On 1 July 1960, a PVO Strany MiG-19 shot down an RB-47H (AF Serial No. 53-4281)
in international airspace over the Barents Sea, killing four of the crew while
two were captured by the Soviets and released in 1961. The co-pilot reported
that the MiG-19 jammed his MD-4 FCS (that aimed the tail guns), rendering it
defenseless. The last known confrontation between MiGs and RB-47s occurred on 28
April 1965, when an ERB-47H was intercepted by two North Korean MiG-17s over the
Sea of Japan. While hit by the MiGs, it returned to Yokota Air Base in Japan
with three engines out. A few operated during the Vietnam War on missions like
relaying ELINT data but were replaced by more efficient and capable Boeing
RC-135s. The last RB-47H was retired on 29 December 1967.

Specifications (B-47E)

General characteristics
Crew: 3
Length: 107 ft 1 in (32.65 m)
Wingspan: 116 ft 0 in (35.37 m)
Height: 28 ft 0 in (8.54 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)
Zero-lift drag coefficient: 0.0148 (estimated)
Drag area: 21.13 ft² (1.96 m²)
Aspect ratio: 9.42
Powerplant: 6 × General Electric J47-GE-25 turbojets, 7,200 lbf (32 kN) each

Performance
Maximum speed: 607 mph (528 kn, 977 km/h)
Cruise speed: 557 mph (484 kn, 896 km/h)
Combat radius: 2,013 mi (1,749 nmi, 3,240 km) with 20,000 lb (9,000 kg) bombload
Ferry range: 4,647 mi (4,037 nmi, 7,478 km)
Service ceiling: 40,500 ft (12,300 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 (0.787 in) M24A1 autocannons in a remote controlled tail turret
with AN/APG-39 Gun-laying radar
Bombs: 25,000 lb (11,000 kg) of ordnance, including: 2 × Mk15 nuclear bombs (3.8
megaton yield each), or
4 × B28 nuclear bombs (1.1-1.45 megaton yield each), or
1 × B41 nuclear bomb (25 megaton yield), or
1 × B53 nuclear bomb (9 megaton yield), or
28 × 500 lb (227 kg) conventional bombs





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