XSBC-3 Pratt & Whitney R-1535-82 Twin Wasp Junior engine
The Pratt & Whitney R-1535, designed by Luke Hobbs, was the third of Pratt & Whitney’s two-row radial engines, following the experimental R-2270 and the R-1830. The development of the R-1535 began in May 1931, with the engine undergoing tests by November of the same year. Reflecting its early 1930s origins, the R-1535 utilized grease-lubricated valve gear, a less advanced system compared to the more reliable and efficient engine-fed oil-pressure lubrication used in later models.
The R-1535 offered power outputs ranging from 700 to 950 horsepower, with the -64 variant achieving the higher end of this range. Most engines were rated between 700 and 800 horsepower at 2650 rpm. The engine’s 1535 cubic inch displacement was achieved with 14 cylinders the size of those used in the R-985. It employed rolling-element bearings to support the crankshaft: two roller bearings at the front, one roller bearing at the rear, and a ball bearing for the center main. It featured a six-pin internal planetary reduction gear setup and a one-piece crankshaft, supported by a two-piece master rod with plain bronze bearings—a first for Pratt & Whitney.
Despite being requested by the Navy, Pratt & Whitney was skeptical about the efficiency of using 14 smaller cylinders to achieve the desired horsepower. The Navy believed that the engine’s smaller frontal area would counterbalance any disadvantages related to its displacement. As a result, Pratt & Whitney did not fully commit to the R-1535’s development. The engine’s small frontal area was best utilized by racing aircraft, such as Howard Hughes’ H-1 Special, known for its exceptional craftsmanship and performance. Pratt & Whitney’s primary focus in the mid-1930s was on the R-1830, which became one of the most produced aircraft engines ever.
An interesting fact about the R-1535 is that Pratt & Whitney pioneered the development and manufacture of two-stage superchargers, with the first experimental R-1535 engines featuring this technology, though no production models were built with it. Additionally, the R-1535 served as a basis for Pratt & Whitney’s initial experiments with sleeve valve technology.
In terms of production, Pratt & Whitney manufactured a total of 2,880 R-1535 engines at their East Hartford facility between 1931 and 1941, which was a relatively modest number compared to their later engines.