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The B-52 - engineered to endure.

THE B-52 ENGINEERED TO ENDURE

When the prototype for the B-52 took off from Seattle's Boeing Field on its maiden flight in 1952, little was it realized that this giant would become the most venerable and versatile military aircraft ever built. Indeed, Air Force and Boeing personnel have spent entire 20- and 30-year careers flying or supporting the B-52. But the B-52's story is far from over, and the aircraft will be flying well into the 21st century.

Several factors keep the B-52 flying and make it one of the key elements in the manned bomber leg of the United States' strategic deterrence triad. Most important, the original design was excellent. The B-52's low empty weight, low drag, substantial wing area, and large airframe meant much growth potential. In addition, the B-52 has been extensively modified and improved through the years. Some will even say that the BUFFs (BUFF stands for Big Ugly Fat Fellow, a nickname the B-52 picked up during the Vietnam War) flying today share only their tail numbers with the B-52s that originally rolled off the Boeing assembly lines. Finally, the fact that the cost to replace the B-52 would be prohibitive and funding is not available cannot be overlooked.

The basic layout of the B-52 was done over a weekend in 1948 in a Dayton, Ohio, hotel room by a small group of Boeing engineers. However, this "back of the envelope" design was reinforced by lots of experience in designing and building bombers. Boeing had built two of the most succesful bombers of World War II, the B-17 and B-29. After the war, it had a series of new heavy bomber designs on its drawing boards. Boeing had already developed the six-jet-engined B-47 strategic medium bomber. Many of its features would be incorporated into the B-52. And Boeing had recently won the contract for the XB-55, the successor to the B-47, that was soon abandoned by the U.S. Air Force in favor of the B-52.

The Monday following that weekend in Dayton, the team's brief proposal was presented to the Air Force where it was enthusiastically received. The backing of the design concept by General Curtis LeMay, the cigar-chomping head of the Strategic Air Command (SAC), played a large part in getting the B-52 development moving at full speed.

Approximately three and a half years elapsed between conception and first flight. The XB-52 rolled out of Boeing's Seattle facility in November 1951 covered with tarpaulins for security. The XB-52 and YB-52 were built as secret weapons in a highly classified assembly area. The YB-52 made its maiden flight on April 15, 1952, and the XB-52 first flew in October of that year. The X and Y designations were a budgetary ploy; the two planes were virtually the same aircraft. Boeing had already received the go-ahead to start producing the first 13 B-52As in early 1951. The first production B-52A would fly in August 1954. The first operational B-52 was delivered to the 93d Bomb Wing at Castle AFB, Calif., in June 1955, just over three years after the YB-52's inaugural flight. (Of all the models, only the XB-52 and YB-52 had the "bubble" canopy with tandem seating for pilot and copilot that was a carryover from the B-47 layout.)

Different Versions

While 13 B-52As were originally ordered, only three were actually built. The other 10 were produced as B-52Bs. The A models were used for development and test work, including a long life of research work not related to the B-52 program per se. The A models were the most expensive, at over $29 million a copy, since their unit cost included amortization of the preproduction costs. After that, the unit cost dropped dramatically.

The B-52B was the first of the stratofortresses delivered to SAC. From the outside it looked just like the A model. The main differences were its reconnaissance and bombing/navigation equipment. Of the 50 B models built, 27 were RB-52Bs; the R stood for reconnaissance. The RB-52Bs had a two-man pressurized capsule filled with electronic equipment for weather and photo reconnaissance and electronic counter-measures missions that was installed in the bomb bay.

With the B-52C, the gross weight jumped from the 420,000 pounds of the A and B models to 450,000 pounds. The C model also had huge 3000-gallon drop fuel tanks on the wings. Fuel capacity was increased from 37,550 to 41,550 gallons, as the earlier models had only 1000-gallon tanks. At the same time, the radius that the plane could fly without refueling increased from 3110 to 3305 nautical miles. Of all the models that went into operational service, the C model was produced in the smallest quantity, a mere 35.

The 170 D models were not significantly different than the B-52C. The B-52D was the first B-52 to be produced in Wichita, Kans., as well as in Seattle.

By the mid-1950s, improvements in Soviet air defenses including interceptors, radars, and surface-to-air missiles had reached the point where the B-52 could not penetrate safely at high altitudes. As a result, the B-52E was the first of the series equipped with equipment for penetration at "on-the-deck" altitudes as well as having an improved bombing navigation system. The 100 E models were produced at both Seattle and Wichita.

The main improvement on the B-52F was the J57-P-43W engines, which produced 13,570 pounds of thrust, as compared to the 12,100-pound thrust engines used on the B through E models. The B-52F also benefited from more than 1000 minor structural changes. The 89 B-52Fs that were produced were the last of the B-52s to be built at both Seattle and Wichita.

The B-52G was essentially a new aircraft, even though externally the only readily apparent changes were a shorter vertical tail and lack of ailerons. While gross weight climbed to 488,000 pounds, the empty weight actually went down. The end result was more capacity for fuel and mission equipment. The B-52G had an unrefueled radius of 3785 nautical miles. Much of the weight savings came with the replacement of the bladder fuel cells with integral fuel tanks, the so-called wet wing. Reduction of the tail height and elimination of the ailerons saved much more weight. The tail gunner was moved from his lonely position in the tail of the aircraft up to the front with the rest of the crew. The tail gunner operated the four turret-mounted 50-caliber machine guns using radar and closed-circuit TV. The biggest change, however, was that the B-52G became a missile platform as well as a bomb dropper.

Even more changes would come on the B-52H, the last of the series, There were new engines, as the TF33 turbofan replaced the turbojets previously used. Power shot up by 30 percent and fuel consumption decreased by an equal amount. The B-52H has a radius of 4510 nautical miles. The 50-caliber guns in the aircraft's defensive fire control system were replaced by a six-barrel, 20-mm Gatling gun capable of firing 4000 rounds per minute resulting in a significant increase in firepower. Boeing produced 102 B-52H models.

A New Life

The modifications performed to keep the B-52 not only flying, but also capable of performing ever more complex missions while being able to survive constantly more sophisticated enemy threats were rather impressive. The B-52 was originally designed with a life of 5000 flying hours. This was quite appropriate since prior to the B-52, bombers became obsolete long before they wore out. Through some extensive modifications, an ongoing structural integrity program, and many more minor modifications, B-52s have been able to accumulate many times the 5000 hours originally planned.

Early on, it was discovered that the wings on the B-52G and H did not have a long structural life because these models used 7178 aluminum in the lower wing to reduce weight. It was necessary to redesign the wing using more durable and damage-tolerant 2024 aluminum. Replacing the wing structure in all the G and H aircraft was a major operation that extended from 1959 through 1964.

In the Pacer Plank program starting in 1972, 80 of the best B-52Ds in the fleet were chosen for intensive modification based on number of total flying hours, number of hours flown at low altitudes and during refueling, and battle damage sustained. These aircraft were essentially taken apart, with over 8000 parts removed. Major portions of the wings and fuselage were rebuilt using redesigned components. The wings were given new skins, structural components, and leading- and trailing-edge assemblies. The center portion of the fuselage was also reskinned.

As part of other modification programs, major portions of the fuselage, tail empennage, and other critical parts of the structure were strengthened or replaced. One modification resulted in fairings called strakelets on the leading edge of each wingroot of all B-52Gs that carry Air Launched Cruise Missiles (ALCM). This modification allows Soviet satellites to easily keep count of B-52s equipped with ALCMs as part of the SALT II agreement. Since all B-52Hs carry cruise missiles and are easily distinguished from the G model, the strakelets are not needed.

The average age of the 262 existing B-52s is 28 years. One of the key ways the Air Force is fighting the afflictions of old age - structural failure and corrosion - is through its Aircraft Structural Integrity Program (ASIP) managed by the Air Force Logistics Command at Wright-Patterson AFB, Ohio. While ASIP has been in existence for three decades, it is continually being updated as new structural analysis and inspection techniques and equipment are developed.

ASIP is a two-pronged attack on the problem that includes advanced predictive models combined with inspection of individual aircraft components. The end result are reports prepared by the Aircraft Management Information System at the Oklahoma Air Logistics Center located at Tinker AFB, Okla., that summarize the damage that the structure of a particular aircraft might have sustained. From this information, logistics managers can tell operational users when individual aircraft are approaching critical stress limits or when certain flight regimes could endanger the aircraft and its crew. Maintenance procedures can also be prescribed to extend the total number of flying hours.

The ASIP has included extensive inspection for cracks and corrosion in the B-52. This has been augmented by both destructive and nondestructive testing that included loading wings until they collapsed and subjecting almost all B-52Ds to a 100 percent design limit proof load test.

Information from ASIP also plays an important role in determining when modifications are required. The information also helps project how much longer key structures will last under certain flying conditions. For example, ASIP information was used when the B-52 got new wings and strengthening of its structure.

One key element of ASIP is the Individual Aircraft Tracking Program that monitors the detailed flight histories of all USAF aircraft. Much of this information is obtained through instrumentation installed on selected aircraft.

Another ingredient comprises the Stress Spectrum Surveys aimed at locating hidden flaws in critical structural components. Inspection techniques run the full gambit of nondestructive methods including eddy current, ultrasonic, X-rays, and N-ray techniques. Another technique uses liquefied rubber impregnated with metal filings that, when applied to magnetized parts, are attracted to flaws and cracks that cannot be seen with a naked eye. Black light and fluorescent dyes are also used to detect problem areas.

In the fleet damage monitoring program, special measuring equipment was installed in aircraft to keep track of structural integrity. At one time, the USAF even had a "lead the fleet" program where selected B-52s were flown much more often than the rest of the fleet so future problems could be found before they affected the rest of the B-52 force. In addition to strengthening the aircraft to survive stress, the USAF has reduced stress by flight planning that includes avoiding turbulence, reducing airspeed, and maneuvering if severe turbulence is encountered. Sometimes the solution has been to reduce flying hours, speeds, and the load carried.

Nowhere has the need to modernize been more apparent than in the B-52's electronic systems. Modernization programs have updated "black boxes" and displays in an attempt to keep up with rapid advances in electronic technology as well as to maintain the ability to survive in an increasingly hostile threat environment. The B-52's original systems used vacuum tubes and analog computer technology. Vacuum tube electronics are almost impossible to maintain as the supply sources for their components have virtually disappeared. They are also prone to failure due to in-flight vibration and jolting.

The first electronic improvements came with the expansion of the B-52's mission to low-level penetration and the need to fly at an altitude of 500 feet or less. A key element of this modification was the Advanced Capability Radar (ACR) for terrain avoidance that was added beginning in the early 1960s. In the early through mid-1970s, the G and H models received the Electrooptical Viewing System (EVS) that consisted of Low Light TV (LLTV) and Forward-Looking Infrared Radar (FLIR) that, when used with the ACR, meant much safer and effective low-level capability.

A Stability Augmentation System was added in the G and H models in the early 1970s to improve level flight characteristics, reduce structural loads, and improve controllability in turbulence. Other electronic improvements have included many upgrades to Electronic Countermeasure (ECM) and Defensive Avionics systems. The ability to communicate with the AFSATCOM communications satellite was added starting in the late 1970s. In the early 1980s, a Trail Warning System was added on the B-52G and H to provide radar warning of threat aircraft and missiles as well as automatically triggering chaff and flare dispensing to confuse enemy radar.

In 1980, a 10-year $1.5 billion program was begun to replace the analog computer-based bomb navigation system with a modern digital, solid-state Offensive Avionics System (OAS) on the G and H models. The OAS not only reduces life cycle costs, but offers greatly improved performance while maintaining compatibility with other systems on the B-52. A similar though less ambitious improvement was also carried out on the D models.

Heavy Artillery

Through the years, the B-52's mission and thus the weapons it carries have changed. These changes have required more modifications beyond those required to merely keep the B-52 flying.

The first major change in mission came with the B-52's nine-year involvement in Southeast Asia that began in 1965. At first, B-52Fs able to carry "iron bombs" were used. However, they were limited in both the number of aircraft available and the bomb load they could carry (51 750-pound bombs). As a result, the B-52D was fitted to carry a much larger bomb load in the "Big Belly" modification program. After modification, the B-52D could carry 84 500-pound or 42 750-pound bombs internally and another 24 500- or 750-pound bombs externally from pylon racks on the wings. The modified B-52D still had the capability to carry nuclear bombs and was given the ability to carry the GBU-15 "smart bomb" that uses an onboard TV camera to guide it to its target.

The Quail, used from 1960 to 1978, was the first missile to be carried by the B-52. Up to four of these missiles could be launched from a B-52 to confuse enemy defenses. Even though it was only 13 feet long, it looked like a B-52 on a radar screen and was programmed to fly at the same speed and altitude, performing the same low-level maneuvers as a penetrating B-52.

A much more potent missile was the Hound Dog carried by B-52s between 1961 and 1976. This supersonic air-to-surface missile had a 700-mile range, allowing it to be used as a standoff nuclear weapon. A B-52G or H could carry two Hound Dogs beneath its wings. An interesting feature was the thrust from the missile's turbojet engine could be used to help the B-52 take off with the Hound Dog's fuel tanks replenished from the B-52.

The B-52 can carry as many as 20 nuclear-tipped SRAM (Short Range Attack Missiles), six on each wing pylon and another eight in the bomb bay. B-52s started being equipped with SRAMs in 1972. The SRAM has a range of 30 to 100 miles depending on the altitude and flies at hypersonic speeds virtually undetected because of its very small radar cross section.

The most complex weapon carried by the B-52 is the Air Launched Cruise Missile (ALCM). The ALCM is a small, unmanned subsonic aircraft. After launch, it is propelled by a small turbofan engine to ranges of up to 1500 miles using a combination of inertial guidance and terrain contour matching to guide it to its target. All the B-52Hs are currently being modified to carry 12 ALCMs externally and another 8 internally. In addition, 98 B-52Gs are already carrying 12 ALCMs externally. Thirty B-52Gs have also been modified to carry eight Harpoon antiship missiles in support of the Navy. B-52s are also capable of performing sea surveillance, minelaying, and antisurface warfare operations.

To the Future

Though the B-1 and the B-2 stealth bomber will take over some of the B-52's tanks, the B-52's days are far from over. At their peak in 1962, there were 639 B-52s on active duty. Today, the USAF still has some 167 B-52Gs and 95 B-52Hs in service with the Strategic Air Command.

Besides its strategic nuclear-deterrent role using SRAMs and ALCMs, the B-52 still represents the Air Force's most capable conventional bomb carrier. Indeed, the USAF is now thinking of converting the B-52G fleet for conventional missions. In a proposed $3 billion program, the bomber would get upgraded electrical, computer, and electronic countermeasure equipment, plus a navigation system that uses the Global Position Satellite system. These B-52s would be equipped with standoff munitions as well as HARM and Tacit Rainbow antiradar missiles for destroying enemy air defense systems. B-52G crews have already started extensive training in conventional tactics.

Other studies have looked at refitting the B-52s with more modern turbofan engines such as those used on jumbo airliners or using B-52s in an electronic warfare role. Though none of these ideas are currently funded, and may not be because of the tight defense budgets, it is clear that the well-engineered BUFFs will be flying for years to come.

PHOTO : First look. The prototype XB-52, shrouded in tarpaulins for security, rolled out of Boeing's Seattle plant on November 29, 1951.

PHOTO : In flight. This B-52B is equipped with a canopy at the rear for the tail gunner. For the G and H models, the tail gunner was moved forward with the crew.

PHOTO : Heavy payload. Six Air Launched Cruise Missiles can be carried on each of the B-52's wings; the model H can carry eight more inside the aircraft.

PHOTO : Electronic eyes. The twin pods under the nose of the G and H models are for the Forward-Looking Infrared Radar and Low-Light TV sensors that are part of the Electrooptical Viewing System.
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Title Annotation:strategic bomber; includes related article
Author:Siuru, Bill
Publication:Mechanical Engineering-CIME
Date:Jan 1, 1990
Words:3151
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