BMW 803

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BMW 803 at the Deutsches Museum

The BMW 803 was BMW's attempt to build a high-power aircraft engine by "coupling" two BMW 801's back-to-back driving contra-rotating propellers. The result was a 28-cylinder 4-row radial engine, but due to cooling concerns, the entire engine was liquid cooled. As with most coupled engines, the 803 never really worked right, and did not enter production.

One problem with scaling up any engine design is that eventually a point is reached where the crankshaft becomes a major engineering challenge. This was a problem that affected almost all engines of the 2,500 hp class, including their own 18-cylinder BMW 802 project. For the 803 the engineers decided to avoid this problem by simply not using a common crankshaft, and driving a set of independent contra-rotating propellers. The front engine drove the front propeller directly, while the rear engine drove a number of smaller shafts that passed between the cylinders of the front engine before being geared back together to drive the rear prop. This layout resulted in a rather large gearbox on the front of the engine, and the front engine needing an extended shaft.

With no common crankshaft all of the accessories had to be powered by one engine alone, in this case the rear engine. The supercharger alone would use up several hundred horsepower, so the rear prop ended up delivering considerably less power than the front one.

The engine weighed a staggering 2,950 kg (6,490 lb) dry, and 4,130 kg (9,086 lb) fully loaded, displacing a massive 83.5 litres. For all this weight it delivered 3,900 PS (metric hp) (2,868 kW). Although this made it the most powerful German engine design, its power-to-weight ratio was not at all impressive, at about 0.60 hp/lb, comparing rather poorly with other large designs like the Junkers Jumo 222 at 1.04 hp/lb. Specific power was likewise poor, at about 34.4 kW/l, compared to the 222's 40 kW/l.

The engine was intended to be used only on the largest of designs, notably the Focke-Wulf Fw 238 and other large bombers. FW also used it in single-seat fighter design, and it also appeared on several Blohm und Voss designs as well. None of these designs was particularly inspiring, and as the engine never matured the project was cancelled. A single example remains in the Deutsches Museum.