Convair B-36

Categories: U.S. bomber aircraft 1940-1949

Image:Convair B-36.jpg
A Convair B-36D in flight

The Convair (Consolidated Vultee) B-36 was a strategic bomber operated solely by the United States Air Force. Unofficially nicknamed the "Peacemaker", the B-36 is the largest mass-produced piston aircraft as well as the largest warplane of any kind. All larger and subsequently designed military aircraft have been limited to a transport role. The B-36 began its service in 1948, and the last B-36 mission was flown in 1959. A total of 384 were built, the last in 1954.

Contents

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Some History

The Cold War began in earnest with the 1948 Berlin airlift and the 1949 atmospheric test of the first Soviet atomic bomb. The historical importance of the B-36 is very much grounded in the resulting tense geopolitical climate. A credible American nuclear deterrent required bombers capable of delivering the very large and heavy first generation nuclear bombs. The B-36 was the only American bomber with both the range and payload to carry these such bombs from airfields on American soil to targets in the USSR, as storing nuclear weapons in foreign countries was (and remains) diplomatically delicate. The nuclear deterrent the B-36 afforded may have kept the Soviet Army from fighting alongside the North Korean and Chinese armies during the Korean War.

The B-36 was arguably obsolete from the outset, because it was piston-powered in a world of jet interceptors. But its jet rival, the B-47 did not become fully operational until 1953, lacked sufficient combat range to attack the Soviet heartland from North American airbases, and could not carry the huge first generation hydrogen bomb. (These limitations also applied to the other American piston bombers of the day, the B-29 and B-50.) Intercontinental ballistic missiles (ICBMs) did not become operational until the 1960s. Hence until the B-52 Stratofortress became operational in the late 1950s, the B-36, as the only truly intercontinental bomber, was the mainstay of the Strategic Air Command (SAC).

Convair touted the B-36 as a "magnesium overcast", a "long rifle", giving SAC a global reach, thanks to a combat range of at least 3,400 miles (5,500 km). When General Curtis LeMay headed SAC (1949-57) and turned it into an effective nuclear delivery force, the B-36 formed the main part of his command. Even though the B-36 was never tested in combat, SAC defended its performance. Its maximum payload was more than four times that of the B-29, even exceeding that of the B-52. Offsetting the B-36's slow speed and lack of aerial refuelling capability was its ability to fly missions as long as 50 hours. Moreover, the B-36 was believed to have an ace up its sleeve: a high cruising altitude, made possible by its huge wing area, that put it out of reach of all piston fighters.

Nevertheless, the B-36 was difficult to operate, prone in its early service years to catastrophic engine fires and other costly malfunctions. To its critics, these problems made it a "billion-dollar blunder". In particular, the United States Navy saw it as a costly bungle, diverting funding and interest from their pet project, aircraft carrier–based nuclear bombers. In 1947, the Navy attacked Congressional funding for the B-36, alleging that the aircraft failed to meet Pentagon requirements. The Navy believed that the dominance of the aircraft carrier in the Pacific during World War II proved that carrier-based airstrikes would be decisive in future wars. To this end, the Navy designed the USS United States (CVA-58), a "supercarrier" capable of launching huge fleets of aircraft — or nuclear bombers. It then pushed to have funding transferred from the B-36 to the United States. The Air Force successfully defended the B-36 project, and the United States was officially cancelled by Secretary of Defense Louis A. Johnson. Several high-level Navy officials questioned the government's decision, alleging a conflict of interest because Johnson had once served on Convair's Board of Directors. The uproar following the cancellation of the United States was nicknamed the Revolt of the Admirals.

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Design

The B-36 concept began with a proposal by Consolidated Aircraft (later Convair) to meet a 1941 United States Army Air Corps intercontinental bomber requirement. (The same design request led to the Northrop YB-35.) If Britain fell and the USA joined the war, American bombers would need to reach Europe from bases in North America. Doing so would require a combat range of at least 9200 km, the length of a Gander Newfoundland - Berlin round trip. Though this need never arose, the B-36 was still seen as playing a possible eventual role in the Pacific war, and so the project continued.

The B-36 took shape as an aircraft of immense proportions: its 162 ft (50 m) length was two-thirds longer than that of the previous superbomber, the B-29 Superfortress. The wingspan and tail height of the B-36 exceeded those of the DC-8-63 and the Antonov An-22, the largest mass-produced propeller-driven aircraft ever. The wingspan of the B-36 exceeded even that of the C-5 Galaxy. Only with the advent of the Boeing 747 and the C-5, both designed two decades later, did aircraft larger than the B-36 and capable of lifting a heavier payload, become routine.

The propulsion system of the B-36 made it a very unusual beast. All versions featured six Pratt & Whitney R-4360 radial piston engines, the most powerful and perhaps most sophisticated piston aircraft engine ever built. Each engine drove an immense three-bladed propeller, 19 ft (5.8 m) in diameter, mounted on the trailing edge of the wing. This was the rarely employed pusher configuration, favored by aerodynamic theory, because the unavoidable propeller turbulence does not interfere with wing lift.

The limited power of the R-4360 engines made for long lumbering takeoffs and a low top speed. Convair addressed these deficiencies, beginning with the B-36D, by suspending a pair of General Electric J47-19 jet engines, modified to run on aviation gasoline, from each outer wing. (The same engine powered the B-47.) Most extant B-36Bs were then retrofitted with these jet engines. The resulting enhanced power improved performance while taking off, climbing to altitude, landing, cruising at over 35,000 feet, and (hopefully) during combat.

Thus the B-36 came to have 10 engines, probably a record for a mass-produced aircraft, and was probably the most important hybrid jet-piston aircraft ever made. If all engines functioned normally at full rev during the pre-takeoff warmup, the lead flight engineer would say to the captain "six [engines] turning and four [engines] burning". Erratic reliability led to the wisecrack "two turning, two burning, two joking, and two smoking", with two engines not accounted for.

The standout characteristic of the B-36 was its enormous wings, whose span made 10 engines possible. The wing size gave the B-36 enough fuel capacity to fly great distances without in-flight refueling. The wing area made possible high cruising altitudes that enabled all versions to fly above the operating ceiling of 1940s-era fighters, jet as well as piston. All versions of the B-36 could cruise at over 40,000 feet (12,000 m). Some claim [1] that the low wing loading resulting from the large wing area made the B-36 more maneuverable at high altitude than the jet interceptors of the day, which either could not fly above 40,000 feet, or if they did, were likely to stall out. Retired fighter pilots of that era do not necessarily agree. Later reconnaissance versions are believed to have cruised above 50,000 feet (15,000 m), maybe even as high as 60,000 feet (18,000 m), although it is questionable whether such altitudes were possible while carrying a bomb payload.

The single-wheel landing gear of the XB-36 required the largest single tires ever manufactured up to that time, 9 ft (2.7 m) tall, 3 ft (1 m) thick, and weighing 1,320 lb (600 kg), with enough rubber for 60 automobile tires [2]. These tires placed so much weight per unit area on runways that the XB-36 was restricted to the Fort Worth airfield where it was manufactured, and to a mere two USAF bases. Hence the single-wheel landing gear was soon replaced by a more conventional [3] 4-wheel bogie. A [4] tanklike tracked landing gear was tried on the XB-36 but proved heavy and noisy, and so was quickly abandoned.

The B-36 initially required a crew of fifteen, with reconnaissance work adding seven more. The seven gun turrets eliminated under Featherweight I reduced the crew to nine. As in the B-29, the pressurized flight deck and crew compartment were linked to the rear compartment by a pressurized tunnel through the bomb bay. In the B-36, one rode through the tunnel on a wheeled trolley, by pulling oneself on a rope. The rear compartment led to the rear gun turret, and featured six bunks and a galley for rest and relief on long missions.

Adding 4 jet engines raised fuel consumption and reduced range. Meanwhile, heavy defensive armament fell out of favor, as new air-to-air missiles made hand-aimed guns obsolete. The remotely operated power turrets of the earlier versions were heavy, unreliable, and required significant manpower. Eliminating these turrets made it possible to carry more fuel. Hence all extant B-36s eventually underwent two rounds of weight reduction:

  • I, which removed most defensive hardware.
  • II also removed the rear compartment crew comfort features.

The last 14 B-36Js were manufactured without these items, and with no guns other than a radar-aimed tail turret. All B-36Js also came with extra fuel tanks. This "Featherweight" configuration allowed the B-36 to fly farther and to reach altitudes above 50,000 ft (15 km), features especially valuable for reconnaissance missions.

Image:B-29 and B-36.jpg
The new XB-36 alongside the first superbomber, the B-29 Superfortress.
From the Maxwell Air Force Base website (original image).
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Weaponry

The four bomb bays could carry up to 39 metric tons of bombs, almost seven times the load carried by that WWII workhorse, the B-24 Liberator. The B-36 was not designed with nuclear weaponry in mind, because such weapons were top secret during most of the period when the B-36 was engineered (1941-46), and their mode of delivery had yet to be determined. Nevertheless, the B-36 stepped into a nuclear delivery role immediately upon becoming operational. In all respects but speed, the B-36 could match what was arguably its Soviet counterpart, the Tu-95 (believed still in service). Until the B-52 came on line, the B-36 was the only means of delivering the first generation Mark-17 hydrogen bomb [5], 25 feet (7.5 m) long, 5 ft (1.5 m) in diameter, and weighing 42,000 lbs (19,000 kg), in size the largest American nuclear weapon ever. Carrying this massive weapon required merging two adjacent bomb bays.

The defensive armament consisted of six remote-controlled retractable gun turrets, and fixed tail and nose turrets. Each turret was fitted with two 20 mm cannon, for a total of 16 cannons, the greatest firepower ever carried by a bomber. The recoil of these cannons during gunnery practice could cause the pre-solid state on-board electronics to malfunction, even to the point of contributing to the crash of B-36B 44-92035 on November 22, 1950.

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Related models

In 1951, the USAF asked Convair to build a prototype all-jet variant of the B-36. Convair complied by adding swept wings and eight Pratt & Whitney XJ57-P-3 jet engines to a B-36F. The result was the B-36G, later renamed the Convair YB-60. Although the YB-60 could carry a heavier bomb load than the YB-52, it did not go forward because it was also over 100 mph (160 km/h) slower and suffered from handling problems. [6].

The B-36 was the basis for the Convair XC-99, a double-decked military cargo plane. The sole prototype built was extensively employed for nearly a decade, especially for cross-country cargo flights during the Korean War. As of March 2005, plans are afoot to move the unique XC-99 from San Antonio, where it has been retired since 1957, to the National Museum of the United States Air Force at Wright-Patterson Air Force Base near Dayton, Ohio.

A commercial airliner derived from the XC-99, the Convair Model 37 never left the drawing board. It would have been the first "jumbo" airliner.

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Production

Main article: Convair B-36 variants
VariantBuilt
XB-361
YB-361
B-36A21
B-36B62
B-36D26
RB-36D24
B-36F36
RB-36F24
B-36H83
RB-36H73
B-36J33
Total384

The first prototype XB-36 flew on August 8 1946. The speed and range of the prototype failed to meet the standards set out by the Army Air Corps in 1941. As is often the case with aircraft pushing the size envelope, the XB-36 experienced a number of problems. (For instance, the B-29 Superfortress was plagued by engine problems, and available engines were too weak to afford the Boeing XB-15 a useful top speed.) Many problems with the XB-36 stemmed from its "placeholder" engines, weaker engines used until the intended powerplant became available.

A second aircraft, the YB-36, flew on December 4 1947. It featured a redesigned high-visibility "bubble" canopy, which was later adopted for production. Altogether, the YB-36 was much closer to the production aircraft. Additionally, the engines used on the YB-36 were a good deal more powerful and more efficient. The YB-36 was actually beaten to the air by the first production model: a single B-36A was built with enough equipment to fly to Wright Field, where its airframe was subjected to a battery of physical tests.

The first of 21 B-36As were delivered in 1948. They were admitted interim airframes, intended for crew training and later conversion. No defensive armament was fitted as none was ready. Once later models were available, all B-36As were converted to RB-36E reconnaissance models. The first B-36 variant meant for normal operation was the B-36B, delivered beginning in November 1948. This aircraft met all the 1941 requirements, but had serious problems with engine reliability, and with the availability of armaments and spare parts. Later models featured more powerful variants of the R-4360 engine, improved radar, and redesigned crew compartments.

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Reconnaissance Capability

More than a third of all B-36 models were reconnaissance models, designated RB-36. The RB-36 performed a number of rarely acknowledged reconnaissance missions, and is suspected of having carried out numerous penetrations of Chinese (and possibly Soviet) airspace. The RB-36 was well-suited for such reconnaissance missions. Its high cruising altitude (at least 50,000 feet (15,000 m)) made it difficult to intercept, and its fuel capacity enabled missions perhaps 50 hours long. RB-36s were distinguishable by the bright aluminum of the camera compartment (contrasting with the dull magnesium of the rest of the fuselage), and by a series of radar domes under the aft fuselage, varying in number and placement.

Before the development of the Lockheed U-2, the RB-36 was the mainstay of American photo reconnaissance over hostile territory. It was the only aircraft having sufficient range to fly into Asia from bases in the USA, and enough size to carry the huge high-resolution cameras of the day. The first of the four bomb bays was permanently converted to a pressurized camera compartment. The aft bomb bay was usually loaded with extra fuel tanks. The cameras carried by RB-36 could produce very high resolution photographs: pictures of a golf course taken from 40,000 ft (12,000 m) show recognizable golf balls.

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Operational history

Unlike the B-52, which saw much action in the [[Vietnam War][ and the two Gulf Wars, the B-36 had zero combat experience. The closest a B-36 came to directly shaping history was the following "Broken Arrow" incidents. On February 13 1950, the crew of B-36B 44-92075 jettisoned the unarmed atomic bomb it was carrying before it crashed in the wilds of British Columbia. This was the first loss of an American nuclear weapon. On May 22 1957, a B-36 accidentally dropped a Mark 17 hydrogen bomb on a deserted area while coming in for a landing at Kirtland AFB in Albuquerque NM. As the bomb was unarmed, only the conventional explosives in the trigger detonated. See list of military nuclear accidents [7] [8]. These incidents were classified for decades.

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Maintenance

The B-36 needed a great deal of maintenance between flights; although in an emergency an aircraft could be "turned around" in a few hours for a ferry flight, it took much longer to get the airplane ready for an operational mission. In January 1951 a B-36 amassed 200 hours of flight time (8.3 standard 24-hour missions), an apparent record.

The B-36 was too large to fit in most hangars. Moreover, even an aircraft with the range of the B-36 needed to be stationed as close to the enemy as possible, and this meant the northern USA, Alaska, and the Arctic. As a result, most "normal" maintenance, e.g., changing the 56 spark plugs on each of its six engines (always at risk of fouling by the leaded fuel of the day), replacing the dozens of bomb bay light bulbs shattered after a gunnery mission, was performed outside in 100-degree summers and 60-below winters. Special shelters were built so that the maintenance crews could enjoy a modicum of protection while working on the engines. Often, ground crews risked slipping and falling from ice-covered wings, or being blown off by a propeller running in reverse pitch. Some procedures even required a mechanic to sit astride a running engine, a 19-foot diameter propeller at his feet, his hand near the 34-inch diameter cooling fan.

The wings roots were thick enough, 7 ft (2.3 m), to enable a flight engineer to access the engines and landing gear by crawling through the wings. This was possible only at altitudes not requiring pressurization.

The piston engines also had a prodigious appetite for lubricating oil, each engine requiring a 100 gallon (380 L) tank. A former ground crewman has written: "[I don't recall] an oil change interval as I think the oil consumption factor handled that." It was not unusual for a mission to end because one or more engines ran out of oil. Though the B-36 could continue flying with as many as three engines inoperative, the extra stress on the remaining engines put them at risk of failing.

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Crash history

Though the B-36 had a better than average overall safety record, ten crashed between 1949 and 1954 (three 36Bs, three 36Ds, and four 36H's). Goleta Air and Space Museum maintains a web site [9], with photographs and lengthy excerpts from the official crash reports. Like all large aircraft powered by piston engines, the B-36 was prone to engine fires, a problem exacerbated by the pusher configuration. When a crash occurred for any reason, the magnesium-rich airframe burned readily.

When thinking about engine fires, keep in mind that:

  • The ambient temperatures typical of the B-36's cruising altitude and of the high latitude bases where it was often stationed, were usually far below freezing.
  • Radial aircraft engines like the R-4360 are air-cooled. Hence the large volume of air flowing past the cylinders during flight underwent substantial warming.

The design of the R-4360 engine tacitly assumed that it would be mounted in the following conventional tractor configuration:

carburetor -- 28 cylinders -- air intake -- propeller

with air flowing from right to left. The carburetor is bathed in air warmed by engine cooling, and so is unlikely to ice up. The B-36 employed the following pusher configuration:

propeller -- 28 cylinders -- carburetor -- air intake

Because the carburetor is now in front of the engine, it cannot benefit from engine heat. Hence when the intake air was very cold and humid, ice gradually obstructed the carburetor air intake, increasing the richness of the air/fuel mixture, until the unburned fuel in the exhaust caught fire. The eventual solution was to add electric heating to the carburetors. Engine fires of this nature led to the first loss of an American nuclear weapon, decribed above.

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Crew experience

Training missions consisted of a 40 hour mission, followed by some time on the ground for refueling and other maintenance, then a 24 hour mission. Many missions were flown "around the flag pole"; they began and ended at the same base.

The B-36 was not a sprightly aircraft: Lieutenant General James Edmundson likened it to "...sitting on your front porch and flying your house around." Despite its immense exterior size, the pressurized crew compartments were relatively cramped when occupied for 24 hours by a crew of fifteen in full flight kit. Although the rear compartment included bunks for off-duty flight crewmen, most preferred to sleep in their seats. Convair rigged a couple of bunks at the top of the 12-foot-diameter radio compartment behind the flight deck.

War missions would have been essentially one-way: taking off from forward bases in Alaska or Greenland, overflying the USSR, and landing in Europe or the Middle East. Surviving crewmembers' recollections reveal that while crews were confident of their ability to complete their mission if called upon to do so, they were less confident of surviving the weapon delivery itself. These concerns were substantiated by the 1954 Operation Castle tests, in which B-36s flew near atmospheric detonations in the 15 megaton range, at distances believed typical of wartime delivery situations. The aircraft experienced heavy heat and blast damage.

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Experiments

B-36s were employed in a variety of aeronautical experiments. The most bizarre was the NB-36H flying nuclear reactor testbed, a forerunner to the Convair X-6 project. An operational nuclear reactor was fitted in the aft bomb bay, behind an added four ton lead shield. The crew was encased in a lead and rubber capsule with a tiny, 1 foot (30 cm)–thick leaded glass windshield. The reactor was operational but did no useful work. Its purpose was to investigate the effect of radiation on aircraft systems, to determine whether a nuclear powered airplane was feasible. The NB-36H first flew in 1955, and was scrapped in 1957 when the nuclear aircraft program was abandoned.

Other experiments involved providing the B-36 with its own fighter defense in the form of parasite aircraft carried partly or wholly in a bomb bay. One parasite aircraft was the tiny McDonald XF-85 Goblin. The concept was tested successfully using a B-29 carrier, but docking proved difficult even for experienced test pilots. Moreover the XF-85 was seen as no match for Soviet aircraft in any case, and so the project was cancelled.

The Tom-Tom project had F-84s dock to the wingtips of the B-36. The hope was that the increased aspect ratio of the combined aircraft would result in a greater range. This failed to take into account the B-36's powerful wingtip vortices. The project was cancelled when an F-84 flipped over onto the wing of the B-36, resulting in the loss of both aircraft and the death of both crews.

More successful was the FICON project, involving a modified B-36, a GRB-36D "mothership", carrying a F-84 Thunderjet fighter modified for reconnaissance, the RF-84K, in a bomb bay. The B-36 would ferry the RF-84K to the vicinity of the objective, whereupon the the RF-84K would disconnect and begin its mission. Ten GRB-36Ds and 25 RF-84K were built and saw active service until 1959, when they were quietly withdrawn as newer reconnaissance aircraft were introduced.

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Obsolescence

The operational life of the B-36 ended because:

  • Long range jet-powered bombers became feasible;
  • The speed and operating ceiling of jet interceptors steadily rose;
  • Radar-guided surface-to air missiles capable of reaching 20,000 meters emerged;
  • The airframe, especially the wings, proved vulnerable to metal fatigue;
  • Wing flexing led to fuel leakage, a common problem.

The B-36 was gradually decommissioned as the B-52 entered service. Only four aircraft survive, one B-36H and three B-36Js, all museum exhibits. The B-36H is on display at the Castle Air Force Base museum in Atwater, California. One B36J is on display at the National Museum of the United States Air Force at Wright-Patterson Air Force Base near Dayton, Ohio, another at the SAC Museum (formerly Offutt Air Force Base) near Ashland, Nebraska. The final B-36J built has been restored, and is in storage in its home town of Fort Worth. Officials and volunteers at the B-36 Peacemaker Museum had planned to exhibit it at Meacham Field in Fort Worth. In the summer of 2005, the Air Force Museum opted to move this aircraft to the Pima Air & Space Museum, Tucson, Arizona for permanent display.

Absent major funding and a substantial dedicated crew, it is highly unlikely that a surviving B-36 will ever fly again. It was a challenge to keep aloft even when operational [10].

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Specifications (B-36J)

General characteristics<h3>
  • Crew: 15 (9 in Featherweight configuration)
  • Length: 162 ft 1 in (49.40 m)
  • Wingspan: 230 ft 0 in (70.10 m)
  • Height: 46 ft 9 in (14.25 m)
  • Wing area: 4,772 ft² (443.3 m²)
  • Empty weight: 171,035 lb (77,580 kg)
  • Loaded weight: 266,100 lb (120,700 kg)
  • Maximum gross takeoff weight: 410,000 lb (190,000 kg)
  • Powerplant:
<h3>Performance<h3>
  • Maximum speed: 411 mph (jets on) (661 km/h)
  • Cruise speed: 230 mph (jets off) (370 km/h)
  • Range:
    • Combat radius: 6,800 mi (11,000 km)
    • Ferry range: 10,000 mi (16,000 km)
  • Service ceiling: 48,000 ft (15,000 m)
  • Maximum rate of climb: 1,920 ft/min (9.75 m/s)
  • Wing loading: 55.76 lb/ft² (272.3 kg/m²)
  • Thrust/weight: 0.078:1
  • Power/mass: 0.086 hp/lb (120 W/kg)

Armament<h3>
  • Guns: 16 (2 in Featherweight configuration) 20 mm M24A1 cannon
  • Bombs: 86,000 lb (39,000 kg) of bombs


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Media

Image:FilmRoll-small.png
B-36 construction and features.ogg (info)
Video clip of the construction and features of the B-36 bomber.
Problems seeing the videos? Media help.


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References

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