Type of engine to employ
This is a 1922 patent-style technical document from NASA Technical Reports Server, a record whose title suggests guidance on choosing an engine type. Its pictures are mechanical line drawings: Fig. 1 shows a framed structure holding a cluster of cylindrical units with a central shaft and gears, and Fig. 2 shows gear trains with shafts and circular sectional views. A designer could take from them the clean, schematic way engineering drawings label parts with letters and show gears in plan and section.
