R-38 Pneumatic Bourke Rotary Engine

The R-38 is the third rotary engine developed after the [R-66](https://www.thingiverse.com/thing:3066355) and [R-52](https://www.thingiverse.com/thing:3668776). The R-38 measures (38 x 38 x 48) mm³, making it the smallest rotary engine in this series so far. It is also the fastest and better suited to the airflow of 4 mm pneumatic tubes than the larger models. The crankshaft is all-metal, the crank radius is 3 mm so the piston stroke is 6 mm. The piston piston surface area is (16 x 12) mm² = 1.92 cm² which yields a piston force of 38 N at 0.2 MPa (2 bar) pressure. Combined, this gives a theoretical torque of 110 Nmm (at 0.2 MPa). Filament sponsored by Fillamentum. All PLA, in following colors: Everybody's Magenta (rotor), Pearl Night Blue (housing), Gold Happens (bearing cage).