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Boeing B-47
Stratojet - History |
B-47 research and development began in 1945 with the first
prototype flight in December 1947. The Air Force wanted a
high-altitude, medium-range, subsonic bomber. At that time, four
contractors were developing bombers. Two designs were conventional
bombers in the mold of the B-29, while the more radical designs were
the Northrop flying wing and the Boeing swept wing jet. In this era
before the SAM, fighter aircraft were considered the main threat to
bombers. World War II had shown that stripped down B-29's with
near-fighter speed and a higher altitude ceiling could only be
successfully intercepted from the rear.
With the speed and maneuverability of the fighters of the late
forties, Boeing's swept-wing XB-47 won the bomber competition and
swiftly transformed the XB-46 and the XB-48 into aviation footnotes.
Six Allison J35-2 turbojet engines slung in pods beneath the
swept-back wings gave the prototype Stratojet nimble performance,
and helped to validate a design concept still widely used today.
Although uprated J47-GE-3s were soon substituted, the B-47 also
carried mountings for 18 solid-fuel booster rockets in the aft
fuselage to shorten the takeoff roll. Flight testing continued
through 1951, and B-47's began entering the inventory in 1952. ICBMs
and SLBMs did not yet exist, and the penetrating bomber was the only
nuclear strike vehicle available. A total of 2,039 B-47's were
funded and built in a serial production that lasted until 1956.
Although heavier than the heaviest World War II bomber, the B-47
was designed to be a medium-range penetrator with approximately a
3,500-nm range. This was not a problem in the early 1950's since
forward basing was available in the United Kingdom, Spain, Morocco,
Guam, and Alaska. In addition, the B-47 was equipped with an air
refueling capability and, on several occasions, 36-hour missions
were flown. Thus, when it initially entered the inventory, its range
was sufficient.
The aircraft's payload capacity was limited to 20,000 pounds
internally. Since nuclear weapons were large in the early 1950's,
the bomb bay was limited to one or two of high yield. But this lack
of payload capacity was compensated for by the large numbers of
B-47's that were purchased (at a cost of less than $2 million per
airplane) which resulted in an acceptable overall weapon delivery
capacity. The B-47 was also capable of carrying 13 500-pound or 8
1,000-pound conventional bombs.52 Although no B-47 squadron was ever
equipped with any type of missile, the B-47 was used on several
occasions as a test aircraft for missile launches. The biggest aid
to the B-47 payload was nuclear weapon technology which developed
smaller weapons.
Serial production made incorporating changes easier; the most
numerous models were the B and E series. There were many production
improvements made to include more powerful engines with water
injection, the addition of tail guns, ejection seats, increased
maximum gross weight, and bomb bay modifications for new weapons
technology. Once deployed, modifications were numerous. The most
significant was the structural revision to convert the B-47 from a
high to a low altitude penetrator due to the development of Soviet
SAMs in the mid 1950's. In May 1960, Gary Powers' U-2 was shot down
by a Soviet SAM, vividly demonstrating Soviet high altitude defense
capabilities.
Besides structural modifications, ECM and other avionics were
updated. Some B-47's were modified into reconnaissance and other
specialized aircraft. Since space was a limitation, most aircraft
modified for reconnaissance and special missions were not capable of
carrying bombs. However, the RB-47B could be converted back to a
bomber. The B-47 had the capability to be modified but was
restricted by space limitations.
The Air Force accepted a grand total of 2,041 B-47s (including
the first 2 experimental planes and the prototype of a
never-produced configuration). All other B-47s in the Air Force
operational inventory, be they weather reconnaissance aircraft
(WB-47Es), ETB-47E combat crew trainer, QB-47 drones, or others,
were acquired through post-production reconfiguration.
In December 1953 SAC had 8 B-47 Medium Bomber Wings, and a year
later the SAC inventory counted 17 fully-equipped B-47 wings. By
early 1956 a total of 22 medium bombing wings had received the B-47
while another 5 wings were undergoing conversion to the B-47. Thus,
by the end of 1956, SAC had 27 combat-ready B-47 wings, with 1204
combat-ready B-47 crews assigned. By 1956, B-47 deployment had
reached its peak with 1,306 aircraft assigned to SAC. In addition
about 250 RB-47s were in SAC at that time. In all, SAC had 30 Bomb
Wings (Medium), each with four squadrons of 15 aircraft per
squadron, along with four Strategic Reconnaissance Wings (Medium),
one Combat Crew Training Wing and four Support Squadrons/Post-Attack
Command and Control Squadrons which also flew different types of
B-47s.
The final B-47E was delivered on 18 February 1957 to the 100th
Bomb Wing at Pease AFB, New Hampshire. This was the 29th and last
SAC bomb wing to be equipped with B-47s. The beginning of the
phase-out of the B-47E coincided with the delivery of the last
example in 1957. In 1960 there were still almost 1,100 B-47s. This
dropped to about 400 in 1964. SAC's last two B-47s went to storage
on February 11, 1966. A few RB-47s were retained until 1967. In
March 1961 President Kennedy had requested funding to support an
increase in the number of SAC aircraft on 15-minute ground alert
from one-third to one-half the total force. At this time the B-47
phase-out was accelerated to provide the aircrews needed to support
the higher alert rate of B-47 and B-52 bomber forces [which was
attained by July 1961].
In the strategic bombing role for which the B-47 was designed,
weapons delivery at the target was originally intended to take place
from high altitudes. By the mid-1950's, however, the increasing
effectiveness of methods for detecting aircraft at high attitudes,
as well as the growing capability of surface-to-air missiles and
fighter aircraft, required the development of new methods of weapons
delivery. As a means of avoiding detection by radar, penetration of
enemy airspace was to take place at high speed and at an altitude of
only a few hundred feet. At the target, the aircraft was to execute
an Immelmann turn with weapons delivery taking place in near
vertical flight. (An Immelmann turn consists of a half loop followed
by a half roll from inverted to normal flight attitude at the top of
the loop. A change of 180° in direction coupled with a gain in
altitude are accomplished during the maneuver.) This method of
weapons delivery was known as LABS (low altitude bombing system) and
was intended to provide the aircraft a means for escaping
destruction from the blast effects of its own weapon.
Constant practice of the LABS technique subjected the B-47 fleet
to the severe gust-load environment of high-speed low-altitude
flight, as well as the maneuver loads associated with weapons
delivery. The aircraft was not designed for this type of service. As
a consequence structural fatigue problems were encountered, and
several aircraft were lost as a result of structural failure. At one
point, the entire B-47 flee was grounded for inspection and
incorporation of necessary design modifications. Both the structural
fatigue problem and the much greater capability of the Boeing B-52,
which began entering the inventory in 1955, played a part in the
retirement of the B-47 from first-line service. Its life with the
Strategic Air Command began in 1951 and ended 15 years later in
1966.
The phase out of the B-47 medium bomber coincided with the rapid
build up of ICBM and SLBM deployment by the US. The B-47 had shown
flexibility in adapting to a low level mission profile that was
required by the introduction of SAMs. But modifications to a large
fleet (especially structural modifications) cost vast sums of money.
Moreover, forward basing of strategic nuclear forces was becoming
unpopular with US allies, and there was not enough tanker support to
make up the range difference for CONUS basing of all the B-47's. The
B-58, planned as a replacement for the B-47, started entering the
inventory in 1960. Also, the B-52, designed as an outgrowth of the
B-47, was proving to be a very capable strategic bomber. Thus, the
combination of mission profile changes, which limited B-47's
usefulness and the emergence of a replacement medium-range bomber
and a truly long-range strategic bomber, led to the retirement of
the B-47's after 14 years of service.
In summary, the B-47 was a technological innovation in bomber
aircraft design with swept wings, jet engines, the ability to be air
refueled, and an operational envelope equal to the fighter aircraft
of the early 1950's. The Stratojet was also one of the more
beautiful airplanes to grace America's skies. However, it was a
medium-range bomber that had limitations on space, payload, and
range. When deployed, the B-47 was adequate to perform its designed
mission. While it was able to adapt to changes in the threat at
significant cost, it was not able to keep up with a heightened
threat and greater range requirements, thus making other available
aircraft that did the job better more viable. |
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