In contrast to the B-52C, easily convertible to the reconnaissance configuration, the B-52D was equipped exclusively for long-range bombing operations. This was initially the most telling difference between the two. Like some of the B-52Bs, the preceding B-52Cs, and subsequent B-52 models, the B-52Ds could carry the newly developed thermonuclear weapons, all necessary modifications being incorporated on the production lines.
As in the case of the B-52C that it so closely resembled, the B-52D's design was initiated in December 1953.
The B-52D marked the beginning of the B-52 large-scale production. It reflected the mid-1953 decision to raise procurement and Secretary Talbott's final endorsement of a second production plant. The B-52D program also benefited from ensuing program increases, and the "D" became the second most-produced B-52 model. The aircraft were ordered under 4 separate contracts. The first, AF33(600)-28223, finalized on 31 August 1954, covered 50 aircraft; the second, AF33(600)-31267, signed on 26 October 1955, involved 51 B-52Ds and 26 B-52Es-the next model in the series. Like preceding B-52s, the new planes were to be built at the Boeing Seattle plant. The other 2 contracts, AF33(600)-26235 and AF33(600)-31155, finalized on 29 November 1954 and 31 January 1956 respectively, totaled 69 B-52Ds and 14 B-52Es-all to come from Boeing's new production facilities in Wichita, Kansas. The 4 contracts, as well as those that covered other B-52Es and subsequent B-52Fs, were of the fixed price type, with redeterminable incentives. In 1962, when production ended, 16 definitive contracts had been concluded. In addition, the B-52 program was tagged with at least 25 miscellaneous contracts for special studies, special flight tests, the procurement of mobile training units, of flight simulators, and of other related items.
The Air Force accepted the initial B-52D, a Wichita production, in June 1956, on the heels of the aircraft's first flight. The new Seattle-built B-52D, first flown on 28 September, joined the testing program immediately.
The new B-52Ds did not reach SAC before the fall of 1956. The first few went to the 42d Bomb Wing, at Loring AFB, replacing the wing's initial B-52Cs. Before the end of December, several B-52Ds had also begun to reach another SAC wing, the 93d. However, while the B-52 inventory at the time already counted almost 100 B-52s (40 B-52Bs, 32 B-52Cs, and 25 B-52Ds), combat-ready crews lagged behind, with only 16 in the 42d Wing and 26 in the 93d. But the command did quickly resolve this problem. Less than 2 years later, SAC had 402 combat-ready crews for 380 B-52s.
B-52Ds encountered the same initial problems as preceding and subsequent models. They were hampered by fuel leaks, icing of the fuel system, and malfunctions of the water injection pumps. After much frustration, the cause of the pump's failure was uncovered. It was simply due to the fact that the water pumps kept operating when the water tanks were empty. The installation of water sensors was the answer. This was done by Sky Speed teams as part of the water injection system's overall improvement program, which was completed by the spring of 1959. Other problems, however, took longer to solve.
As B-52Ds were becoming more plentiful, B-52Es and B-52Fs were also reaching SAC. Concurrently, the command's base facilities kept. deteriorating. The eagerly awaited B-52s put stresses on runways that had been designed for the lighter B-47s or the slower B-36s. SAC's problems were further compounded by the large size of the first B-52 wings, generally composed of 45 bombers and 15 or 20 tankers, all situated on 1 overcrowded base. The early and mid-fifties expansion of the bomber force compelled some of the SAC bases to support as many as 90 B-47s and 40 KC-97 tankers. In mid-1958, paving projects started at 9 of 13 bases which, the command pointed out, needed immediate attention. Paving costs alone were estimated at $25 million. Congress also approved $232 million under the fiscal year 1959 military construction program to cover projects programmed by SAC, but an additional $210 million was denied. While few of the requested alert facilities were affected, drastic cuts were made in other SAC construction projects. Strangely enough, the facilities shortage was alleviated somewhat by another problem. In the late fifties, as the Russian missile threat became more pronounced and warning time shrank, SAC bases presented increasingly attractive targets. The only immediate solution was to break up these large concentrations of aircraft and scatter them over more bases. In the B-47's case, dispersal was a long-range program. It would be accomplished primarily through the phaseout of wings in the late fifties and early sixties. Existing B-52 wings therefore were broken up into 3 equal-size units of 15 aircraft each. liwo units would normally be relocated at bases of other commands, which was not an ideal arrangement since runway deficiencies, as well as other difficulties, would be sure to materialize. In essence, after 1958 each dispersed B-52 squadron became a strategic wing, usually accompanied by an air refueling squadron of 10 to 15 aircraft. The same principle would be followed in organizing and equipping the still growing B-52 force.
The "Big Four" Modification Package began concurrently with the increasing Russian missile threat and the beginning of the B-52 dispersal program, a new difficulty came to light. Namely, there was no longer any doubt that the Soviet Union had developed formidable defenses against high altitude bombers. Of some consolation, enemy defenses were known to be far less reliable and potentially successful against low flying aircraft. Undeterred by the fact that its new B-52s had been deigned for high-altitude bombing, SAC wasted no time in planning the best way to face its new challenge. To begin with, all B-52s, except for the early B-52Bs, would have to be capable of penetrating enemy defenses at an altitude of 500 feet or lower, in any kind of weather, and without impairing the bomber's inherent high speed at high altitude. Two other necessary steps were to equip all B-52s, modified for low level, with Hound Dog missiles and Quail decoys, so far due to be carried only by the latest B-52s. SAC's fourth requirement was to add an AN/ALQ-27 electronic countermeasure (ECM) system in every modified B-52. This system, the command believed, would allow the B-52 to automatically counter ground-to-air and air-to-air missiles, airborne and ground fire-control systems, as well as the early warning and ground control interception radars of the enemy. Although the requirements outlined by SAC would involve significant modifications and the addition of complex and costly components, they were approved by Headquarters USAF in November 1959. There was an immediate exception, however. The AN/ALQ-27 production was canceled. The command had wanted 572 B-52s fitted with the new AN/ALQ-27, which promised to integrate all ECM functions into one major subsystem, but this modification alone would cost over $1 billion. The Air Staff chose instead a quick reaction capability (QRC)/ECM combination of black boxes that would cost much less. The B-52H (last of the B-52 model series) would feature this equipment from the start, and it would be retrofitted in other B-52s. However, deletion of the AN/ALQ-27 was not to be the program's only setback. Although eventually successful, the "Big Four" low-level modification-also identified as modification 1000-had to overcome numerous difficulties. First was the lack of money. In early 1960, the Air Staff constantly reiterated that a maximum effort was necessary to eliminate complexities and expensive components that promised only incremental improvements. Meanwhile, low-level modification costs had increased from $192 million in November 1959 to $241 million in March 1960. By July, the cost had risen to $265 million. In August, funds were withheld by the Air Staff pending assurance from the Oklahoma City Air Materiel Area that the work would be completed within the $265 million fund ceiling. At the same time, SAC again emphasized that basic requirements should not be compromised just to keep rising costs down. In any case, technical problems also multiplied. At first sight, the low-level modifications appeared straightforward. They called for improvement of the aircraft's bombing-navigation system, modification of the Doppler radar, and the addition of a terrain clearance radar. Low-altitude altimeters also had to be acquired, and each aircraft had to be equipped to carry its newly allocated missiles. The project was actually far more complicated than it seemed, because it covered different B-52 models. In other words, modifications had to be tailored to fit specific configurations. Airframes had to be strengthened, and they also slightly differed from model to model. As a result, low-level modification costs for each B-52C and B-52D aircraft were almost twice as much as for any other B-52. Extra structural modifications accounted for some of the additional expenditure. Another factor was upgrading of the aircraft's initial MA-6A bombing and navigation system, finally replaced in 1964 by the ASQ-48. In any case, the whole project was complex, and modifying the ASQ-38 bombing navigational system of subsequent B-52 models also proved costly. Finally, development of special terrain clearance radars proved more difficult than anticipated. Nevertheless, most low-level modifications were completed by the end of September 1963. Some ECM improvements, due to be accomplished during the aircraft's regular inspect and repair as necessary program, took longer. The ECM improvements were programmed to take place in several phases. Phase I was an emergency modification that provided the necessary minimum ECM equipment to cope with the enemy's radar and surface-to-air missile threat. Phase II was essentially an ECM retrofit that was included in the "Big Four" package. The components installed during Phase II were either equal to or nearly as sophisticated as those introduced by Phase III. The best available ECM equipment, comparing favorably to the deleted AN/ALQ-27, was fitted in Phase III and also featured in the B-52H. Except for the first 18, all B-52Hs were equipped in production for all-weather and low-level flying.
The phenomenon of fatigue was yet to be fully understood by 1960, but a great deal had been learned from the B-47's structural problems. For instance, it was well established that takeoffs and landings formed one of the primary sources of fatigue damage. In this case, the B-52, with its wing fuel loads, promised to be especially vulnerable. Moreover, there were other known causes of fatigue: atmospheric gusts, maneuver loads, downwash turbulence from tankers during refueling, taxi, buffet, sonic noise, and stress corrosion. Although flying the B-52 at low level was absolutely necessary, SAC knew there would be a price to pay. The extent of the damage could not be fully predicted, but gusts at 800 feet were 200 times more frequent than at 30,000 feet. At best, it was believed that low-level maneuvers and gust loads would speed the B-52's structural deterioration by a minimum quotient of 8. Justifying the Air Staff's as well as SAC's opinion, Boeing cyclic testing of a B-52F soon showed that numerous manhours would have to be spent on every B-52F in order to alleviate stress in critical areas of the aircraft. Even though the B-52F contingent was not large, strictly mandatory modifications would total at least $15 million. Meanwhile, following the cyclic tests of a B-52G in early 1960, numerous structural fixes were ordered for the entire B-52 fleet, the B-52Bs included. These modifications, soon carried out as the
Hi-Stress Program, initially consisted of 2 phases. The Phase I High Stress fixes were scheduled when the aircraft approached 2,000 flying hours; (Phase I counted 9 fixes. The main ones consisted of strengthening the fuselage bulkhead and aileron bay area. Other important fixes were the reinforcement of boost pump access panels and wing foot splice plate.). Phase II, when it was nearing 2,500 (Phase II called for modification of the upper wing panel splice inboard of inboard engine pods, reinforcement of lower wing panel supporting inboard and outboard pods, reinforcement of upper wing surface fuel probe access doors, and strengthening of a bottom portion of the fuselage bulkhead. Some work was to be done also on the upper wing panel splice, 8 feet inboard of the outboard engine pods.). The Hi-Stress Program was not to interfere with the "Big Four" modification package; it was not allowed to fall behind schedule and was practically completed by the end of 1962. Concurrently, because of the results of the B-52F cyclic tests, an unanticipated third phase was started. The High Stress Phase III consisted of inspecting and repairing, as necessary, wing cracks in all early B-52s. Sky Speed teams and personnel of the Oklahoma and the San Antonio Air Materiel Areas again took care of most of the work. But these modifications, as thorough as they were, only marked a beginning. In the mid-sixties, the B-52 remained SAC's primary bomber and modifications were necessary to offset structural weaknesses caused by aging. An engineering change proposal (ECP 1128), approved in 1964, was scheduled for completion in June 1966. It called for various structural improvements, including replacement of the vertical fin spar and skin. It would enable most of the B-52s to resume unrestricted operations, but was expected to cost $230 million. In the early seventies, similar projects would be undertaken either to beef up or to modernize selected models of the elderly B-52s.
Less than 6 months after the B-52s became involved in the Vietnam War (B-52Fs were the first to go), the Air Force initiated a special modification program to allow the B-52Ds to carry more bombs. Referred to as Big Belly, the modification program left the outside of the aircraft intact. Modified B-52Ds could still carry twenty-four 500-pound or 750pound bombs externally, but the internal changes were significant. Reconfiguration of the B-52D bomb bay allowed the aircraft to carry 84, instead of twenty-seven 500-pound bombs, or 42, instead of twenty-seven 750 pounders, for a maximum bomb load of about 60,000 pounds-22,000 pounds more than the B-52E
B-52Ds of the 28th and 484th Bomb Wings, deployed to Guam in April 1966, immediately began to replace SAC's B-52Fs in the Vietnam conflict. All B-52Ds committed to Southeast Asia had been modified to carry more bombs than the planes they relieved. In the spring of 1967 modified B-52Ds began also to operate out of U Tapao Airfield in Thailand. From there, the aircraft would complete their mission without inflight refueling, which was necessary when operating from Guam. This saved both time and money.
Because of the war, SAC established on 15 April 1968 a Replacement Training Unit within the 93d Bomb Wing's 4017th Combat Crew Training Squadron at Castle AFB. The unit's purpose was to cross-train every B-52 crew, from the B-52E through the B-52H model, in the operation of B-52D aircraft. After 2 weeks of training, the crews were used to augment the cadre units in Southeast Asia. This spread out combat duties more equitably among the entire B-52 force and provided the crews needed to meet the increased bombing effort.
When a single B-52, set aside for static testing, was subjected to final destruction back in February 1955, its wings accepted 97 percent of the ultimate up-bending load before failing-an entirely satisfactory outcome for the configuration tested. However, since that time, the B-52 had flown many hours and far more years than expected. Furthermore, many of the hours accumulated by the 10-year-old bomber had been flown at low-level, which put a great deal of extra stress on an aircraft structure, originally intended for high-altitude bombing. Therefore, the structural modifications, approved in the mid-sixties as a result of engineering change proposal 1243, came as no surprise. Started in December 1966, this modification program ensured selected B-52s of an additional 2,000 hours of service life. All Big Belly B-52Ds, reconfigured with high-density bomb bays, were automatically earmarked for the work. The others were chosen according to a very straightforward formula. Namely, B-52C, D, or F models qualified if they were nearing their flying maximum of unrestricted "E" hours and had not been tabbed for upcoming phaseout. The "E" hour was an equivalent used to indicate the fatigue damage accrued in the wing structure of all B-52C, B-52D, and B-52F bombers. The modification program was completed during the second half of 1968, at a cost of approximately $16 million, after replacing fatigued structural parts in the most critical wing areas of the involved planes.
Because they had already been fitted to carry heavier bombloads, a number of B-52Ds were earmarked for another round of modifications- The changes this time would allow the aircraft to carry extra aerial mines. As requested by Deputy Secretary of Defense David Packard in December 1968, the project had been thoroughly reviewed, the Air Force concluding that the suggested modification of later B-52 models would be less efficient and more costly-$6.9 million instead of $6.3 million. Although the Air Force's selection was approved by the Office of the Secretary of Defense in mid-1969, the B-52D special modifications were only completed in the fall of 1971. It also took time to finalize logistics agreements with the Navy for procurement, modification, storage, and delivery of mines. Not too soon, it seemed, for President Richard M. Nixon ordered the mining of North Vietnam's harbors and river inlets on 8 May 1972.
The Vietnam conflict cost SAC 22 B-52Ds. Surface-to-air missiles and other ground defenses accounted for 12 of the losses. Ten B-52Ds were lost in operational accidents of one kind or another.
The B-52D production ended in late 1957, the last 6 productions being accepted by the Air Force in November. Air Force accepted 101 B-52Ds from Seattle; 69 from Wichita. Only 1 B-52D was accepted in FY 56 (June 1956); 92 in FY 57 (between July 1956 and June 1957); and 77 in FY 58 (all in calendar year 1957). Cost per aircraft was: $6.58 million: Airframe, $4,654,494; engines (installed), $1,291,415; electronics, $68,613; ordnance, $17,928; armament (and others), $548,353.53. Another price decrease, almost $700,000 below the B-52C's cost.
In accordance with Secretary McNamara's mid-sixties decision to cut down the strategic bomber force by mid-1971, SAC inactivated 3 squadrons of B-52D and B-52E aircraft during the early part of 1967. This action, however, did not spell the immediate retirement of the aircraft that had been attached to the inactivated units. Badly needed elsewhere, the Big Belly B-52Ds were immediately used to bolster the resources of the B-52D wings committed to Southeast Asia. The B-52Ds actually outlived 2 subsequent B-52 models. In 1973, a partial retirement of the B-52D fleet was planned. Based on the age and condition of their airframe, 45 B-52Ds were earmarked for phaseout by September 1974.
In mid-1973, SAC forces still counted about 130 B-52Ds. Some of these aircraft were on their way out-45 by the fall of 1974 and a few others soon afterward. But 80 B-52Ds were expected to see unrestricted service into the 1980s. The Air Force was negotiating a contract with Boeing for the Wichita fabrication of kits and the reworking of wings that would be installed on the 80 B-52Ds, during the aircraft's regular depot maintenance. The cost of extending the B-52D's operational life seemed high, over $200 million for 80 planes, but the Air Force believed it had no alternative. As explained by Secretary of Defense Elliot L. Richardson to the Senate Armed Services Committee, without the hi-density B-52Ds, the Strategic Air Command's conventional bombing capability would be at the expense of its other missions. As approved by the Office of the Secretary of Defense on 30 November 1972, the modification, identified as engineering change proposal (ECP) 1581, promised to be extensive. It included redesign and replacement of the lower wing skin, to make it similar to the B-520 wing, and in the process Boeing was to use a more fatigue resistant alloy. The wing center panel was also to be redesigned and replaced. Finally, ECP 1581 called for new upper longerons and some new fuselage side skins. Also, the pressure bulkhead in the B-52D nose would be changed. Already delayed for lack of money, ECP 1581 had been programmed to take at least 2 years.
Two B-52Ds of the 28th Bomb Wing, Ellsworth AFB, South Dakota, established world speed records over 2 different routes. One B-52D flew at 560.705 miles per hour for 10,000 kilometers in a closed circuit without payloads; the other, at 597.675 miles per hour for 5,000 kilometers, also in a closed circuit without payloads.
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