It was the single most expensive military project of world war two costing 50% more than the Manhattan project to build the atomic bomb and is the only aircraft to drop nuclear weapons in combat.
The B-29 was the most sophisticated propeller-driven bomber of the world war 2 that brought a new wave of technology that changed not only war planes but future commercial ones too.
However, the US military was in a hurry, it needed a super bomber to replace the B-17 and fast, first to fight the Germans then almost exclusively the Japanese across the vast distances of the Pacific because it was the only bomber that could reach the Japanese mainland in large numbers and with equally large bomb loads.
Because of this rush, mistakes were made not least with the Curtiss-wright engines used to power them.
Of the 414 B-29s lost during the Japanese bombing offensive, 147 were downed by the enemy and 267 were due to engine fires, mechanical failure, takeoff crashes and other “operational losses”.
The grim joke amongst the aircrew was that Curtiss-wright would more likely kill them than the Japanese.
So what was the cause of this and how did NACA, the predecessor to NASA help fix the problems?
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During world war 2, the press and US military heaped praise on the Boeing B-29 Superfortress as the saviour that would win the war and whilst some in the navy and army might disagree, there is no doubt that it was an effective weapons system.
It’s range and load capacity brought the mass bombing raids that had been used against Nazi Germany to the once thought-safe Japanese home islands which decimated not only Japanese industry but many of their major cities and towns and in the end the atomic bombs that brought the war to rapid close.
But behind the scenes, there were long-standing problems with the engines that powered the plane that meant for the crew of the B-29’s almost every take-off was a careful balancing act of getting the heavily loaded aircraft into the air without burning up the engines, this would continue when cruising and getting home.
The story of the B-29 goes back to the mid 1930s when the United States Army Air Corps or the USAAC were looking for a heavy bomber that had superior strength, bomb load, speed and range. The result was the B-17 flying fortress, it had all these apart from range. The USAAC wanted a range of 5000 miles but the B-17 could only manage about 2000 miles or 3200km with a 6000lb or 2,700kg bomb load.
But If Britain were ever to fall to the Nazi’s then British airfields wouldn’t be available and bombers would have to fly from the US to hit Germany, too far for the B-17’s combat range and if was a war in the Pacific that would also rule out the B-17. Such was the need for the new bomber, 1600 were ordered before the prototypes were even built which laid the path for the problems ahead.
In February 1940, flying prototype orders were placed for a very heavy, very long-range bomber to replace the B-17 that could fly from the US to Germany with a 10,000lb or 4,500kg bomb load.
Although four aircraft makers were approached only two made prototypes, Boeing and Consolidated-Vultee. Boeing had been working on a pressurised heavy bomber since 1938 and immediately started adapting their design for the new requirements. Consolidated-Vultee developed what became the Consolidated B-32 Dominator as a backup in case the Boeing design could not be realised but it was nowhere near as sophisticated and the Boeing.
The B-29 as it would be known would have a pressured cabin, a first for a bomber, remote control gun turrets, Analogue gun fire computers that would allow for airspeed, lead, gravity, temperature and humidity and allowed one gunner to operate two guns remotely and carry a crew of up to 11.
It would also be bigger and fly higher and faster than any other heavy bomber including the British AVRO Lancaster, with a flight ceiling of 31,850ft and maximum speed of upto 357mph or 575 km/h.
All this made it the heaviest bomber of the war coming in at an empty weight of 74,500 lb or 33,793 kg, twice the weight of a Lancaster and it had a maximum take off weight of 135,000lb or 61,000kg with a combat overload.
To do this each B-29 was fitted with four of the most powerful aero engines which at the time was Curtiss-Wright R-3350 Duplex-Cyclone radial engine producing 2,200hp each.
Even with these engines it was going to be hard work to get fully loaded B-29’s off the ground.
The Curtiss-Wright R-3350 Cyclone was an air cooled, supercharged twin-row 18 cylinder 54.9 litre radial engine which means that it had two rows of nine cylinders spread radially around a central gearbox and had been developed in competition with the 18 cylinder Pratt & Whitney Double Wasp 44 litre engine.
At the time there was very little demand from airliners for these big engines and as such they had received less development compared to their smaller versions which would be fitted into medium bombers in volume.
However, in 1940 when three out of the super heavy four bomber designs came back, they specified the R-3350 Cyclone and it was seen as the future engine for army aviation.
The pressure was now on to make it into a reliable production engine but there were still several areas which caused problems when fitted in the B-29. The biggest was its tendency to rapidly overheat the rear cylinders because of the restricted space between the cylinder baffles which is the metalwork to guide the incoming cool air around the cylinders and the cowling around the front of the engine, this could cause mechanical failure or fire.
When the first XB-29 prototype flew on Sept 21st 1942 everything went as expected with an uneventful flight but things didn’t stay that way and over the next few months there were very few uneventful test flights.
By December 28th 1942 the had been 16 engine changes, 22 carburetor changes and 19 exhaust system revisions. The average test flight time was just 1 hour 10 minutes with most of the time spent fighting problems and getting back to base. Further issues put the #1 prototype out of action for 7 months
On the 30th December 1942, the maiden flight of #2 prototype took place. Within 6 minutes #4 engine overheated and caught fire, even though the top test pilot Eddie Allen cut the fuel and used the fire extinguishers the fire continued. The flight was terminated, and the plane only just made it back with serious damage to nacelle and wing and the crew suffered from the heavy smoke entering the cabin from the bomb bay.
On inspection, it was found that a fire had also just started in #1 engine and #3 was close to failure. All three failed engines had less than three hours total ground and flight time.
But worse was to come on the 18th of February 1943 when Eddie and his crew took #2 prototype up to get climb and level flight performance and engine cooling data.
Just after take off, #1 engine caught fire and the plane radioed that they were coming back to the field but wing was on fire and smoke was in the cabin. Three of the forward crew jumped from the plane but at only 250 ft which was too low for the parachutes to open.
Three miles from Boeing field, the plane crashed into the Frey meat packing factory killing all on board plus 20 people on the ground. Burns on the bodies of the 3 crew that bailed showed the fire had spread into the cabin.
As time went by more problems with engine fires continued and the blame game went back and forth between Boeing blaming Curtiss-Wright for the poor engine design and Curtiss-Wright blaming Boeing for the poor airflow due to the design of the nacelles and cowls.
As the war in the Pacific increased, the pressure to create a workable super bomber meant that even after the prototyping stage and production had started there were so many changes to the early planes that as they rolled out of the factory they went straight into facilities that would start modifying and rebuilding them.
As part of making the engines lighter, they used a lot of magnesium but if an engine fire got out of hand the magnesium could start to self-combust reaching up to 3000 Celsius which could burn through the ineffective firewall which the crew called “Tin Pans” and then the aluminium wing spar just behind the nacelle in seconds. The high temperatures also helped destroy the evidence of the root cause of the fires.
On engines which did survive it was found that the exhaust valves were failing and being ingested into the engine and it became common to replace the cylinder heads after just hours of operation.
The pressure to get B-29’s in to combat meant that overheating continued to occur once in operation in the Pacific. The overheating issues were compounded by the temperatures on the runways in the tropics which meant that planes had only short window of time to get up in the air to cool the engines, any more than 20 minutes idling on the ground and they were too hot for take-off.
The climb to altitude was also an ordeal and put the crew of the often combat-weight overloaded B-29’s in a tricky position having to keep an eye on the cylinder head temperatures while balancing the engine cowl flaps that diverted more cooling air for the engines. Too little and they could overheat, too much and it would increase the drag like an airbrake and in some cases when the flaps stuck open it could make the plane almost unflyable.
Flying at altitude over Japan they encountered the then unknow Jet stream creating a 200 mph headwinds which slowed the progress over the target to a crawl making them an easy target for the enemy and putting more pressure on the already over loaded engines.
The try and get to the root cause of the overheating issues, in June 1944 a B-29 was sent to NACA, the predecessor to NASA at the Aircraft Engine Research Laboratory (now NASA Glenn) in Cleveland, Ohio.
After ground and test flights, wind tunnel testing and pulling the engines apart, they found that the cylinder heads didn’t allow for enough heat dissipation which caused the exhaust valves to fail. They designed a new longer cylinder head that would disperse the heat better and changed the metallurgy of the inlet valves.
The exhaust area was also not getting enough airflow so they added baffles to direct more air and changes the cowl flaps to work more effectively without causing more drag.
The inlet manifold was found not to be distributing fuel evenly between cylinders which could cause lean burning in some to backfire into the magnesium supercharger impeller igniting fuel in the casing and in the worse cases starting a magnesium fire. A new fuel injection system directly into the cylinder heads and impeller a was developed to reduce the potential for induction system fires.
The various modifications were done to two engines which were fitted to the left wing of the B-29 and in tests they proved that with proper fuel mixture and cowl flap settings the engines attained its maximum range on each flight and that performance increased by 38% during periods of maximum cooling which was NACA estimated could add 10,000 ft of altitude or 35,000lb of payload.
Problems still occurred but were significantly reduced though the average lifetime for an R-3350 was 265 hours before replacement.
With the modifications and a change of tactics from high altitude daytime bombing to low altitude night time raids by the new man in charge General Curtis Lemay, the stress on the engines and the failure rate dropped and the B-29 became a more reliable bomber, though it wouldn’t be around that long as on the 28th May 1946 the last B-29 was produced at Boeing’s Renton factory.
The production run had lasted just over 3 and half years from the first prototype the last one off the line, one of the shortest for any military aircraft.
While the bomber was vindicated in the end with many successful variants and special operations after the war and many of the surviving crew looked on the B-29 with great fondness, regarding it as the Cadillac of aircraft, the journey from the first prototype to the last one made was one of the most tortuous of any major US military aircraft project.
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