RogerBW's Blog

Convair B-36 Peacemaker 08 March 2014

The Peacemaker was the world's first intercontinental bomber, and the largest mass-produced piston-engined aircraft ever built.

Its genesis was in 1941: it seemed likely that Britain would fall to Nazi occupation at some point, and if that happened the Americans wanted to be able to bomb Europe from bases in North America. The initial competition asked for a 275mph cruising speed, a 45,000-foot ceiling, and a 12,000-mile range. All the aircraft manufacturers reckoned this was impossible, so the constraints were reduced a bit.

The military didn't help development at first, since the B-36 project demanded materials for which they had more immediate use (e.g. for B-24 Liberators). Later in the war, with an invasion of Japan looking likely, the B-36 was given higher priority. Even so, the first aircraft were not delivered until 1948, and that after the project had nearly been cancelled in 1947 to pay for USS United States, which would have been the first supercarrier and was also intended for long-range nuclear bombardment.

The most obvious characteristic of the B-36 was its size. The Liberator had been considered a "big" heavy bomber by pre-war standards; the B-36 was twice as long, twice as wide, and three times as high. It could stay aloft for nearly two days, going 10,000 miles. It rapidly became referred to as the "aluminum overcast".

This size caused its own problems. The original design of the landing gear had such high ground pressure that the B-36 could only have operated from three airfields in the USA. Vibration, metal fatigue and fuel leakage bedevilled the aircraft throughout its life. But the wings were seven feet thick at the root, allowing internal crawlspaces for in-flight access to the engines.

Those engines were Pratt & Whitney Wasp Majors, also used in the B-50 Superfortress, the Hughes H-4 prototype, and the Boeing 377. Unusually, though, they were mounted in a pusher configuration, the twenty-foot propellers hanging off the back of the wing. This made airflow over the wing less turbulent and thus more efficient, helpful for extending range, but had the downside of allowing the carburetor to ice over (the engine design assumed the carb would benefit from warm air flowing over the main engine body). In-flight fires were a continuing problem.

To try to make the B-36 a bit less of a target, an upgrade added four J47s (the same early turbojet used on the F-86 Sabre), two under each wing. They were too fuel-hungry for constant use, but helped with takeoffs and (in theory) attack runs. The ten engines were referred to as "six turning and four burning" – or, considering reliability problems, "two turning, two burning, two choking, two smoking, and two not accounted for".

Although it had been designed with no knowledge of the Manhattan Project, the B-36 was the only aircraft big enough to carry the twenty-ton Mk. 17 hydrogen bomb, so for much of its service life it was considered primarily a nuclear bomber. It was the aircraft with which Curtis LeMay attempted to instill professionalism into Strategic Air Command.

The B-36 was very much a "defended bomber" in the standard WWII mode, and in its original role would have had to act entirely without fighter support. Its design configuration included eight turrets (each with paired 20mm cannon): reasonable in 1941. When it became apparent that air-to-air missiles had rendered gun turrets irrelevant, they were mostly removed to save weight, always a problem for the B-36: every time some weight could be clawed back, it seemed the Air Force had something new for the thing to carry.

By all accounts the B-36 wasn't particularly fun to fly; apart from constant vibration, it was just too slow and unresponsive to be a pleasant experience. In spite of its size, crew accommodation was cramped. The actual wartime mission would have been to fly from bases in the USA, drop bombs on the USSR, and then land in Europe, North Africa or the Middle East; the plane was certainly capable of it.

Because of its size and strategic role, the B-36 was used for a variety of experiments, including parasite fighters (about which I'll write another time) and nuclear propulsion. The latter was really yet another attempt to extend range: an aircraft which could remain on station for weeks at a time would be much more of a threat than one which could only stay up for a day or two. To this end, a B-36 was fitted with a 1MW reactor and appropriate shielding, and taken for several flights with the reactor critical, to evaluate shielding and the effects of radiation on aircraft systems. The project produced disappointing results, and the advance of high-speed aircraft and ballistic missiles rendered the nuclear-powered bomber obsolete before it was built.

While the B-36 never dropped a bomb in anger, the photographic reconnaissance variant saw very heavy use over the USSR. The crew was expanded to 22 to include operators for an array of bulky high-resolution cameras, and the plane even carried an on-board darkroom for developing film in flight.

However, as a bomber, the B-36's days were always numbered. The appearance of the MiG-15 over Korea in 1950 made the piston-engined bomber obsolete: the MiG could fly faster and higher than American fighter escorts and cut apart slow bombers with impunity. The advent of air-to-air missiles only made things worse. What's worse, the B-36 couldn't be equipped for aerial refuelling, and the airframes were getting close to the end of their design life. When the Korean War ended in 1953, President Eisenhower decided to put a lot of money behind upgrading the US Air Force, and the B-36 was rapidly replaced by the B-52 and B-58; the last one was out of service by 1960.

  1. Posted by John Dallman at 01:14pm on 08 March 2014

    It's a good example of trying to build something too advanced for the available technology: not impossible, but it often comes out huge, expensive, buggy and with poor performance.

    As usual for aircraft, the engines are the basic problem. The Wasp Major became a very good implementation of a concept that had been pushed too far, but the early models weren't reliable enough. The turbines that would have made the job practical weren't available at the time.

  2. Posted by RogerBW at 01:25pm on 08 March 2014

    The basic, yes, but I would argue not the only problem. Those huge thick wings were always going to lead to something ungainly no matter how much power was put behind it, and that's a materials tech problem.

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