We recently were given a copy of the 1975 book Some Unusual Engines by LJK Seltright. This is a long out of print and somewhat hard to find book, with used copies going for $125.00 or more on Amazon. Several tank engines are featured in this book, including the well known A57 Chrysler Multibank and the Mitsubishi 10 ZF ten cylinder engine. One tank engine mentioned in the book that we were not familiar with at all was the Orion engine designed by General Motors. This unusual engine never made it past the prototype stage. Like the Soviet 5TD of the T-64 and the British Leyland L-60 of the Chieftain tank, the Orion utilized an opposed piston, two stroke design. However, it was a much more unusual design than either of those two engines. Rather than having the cylinders arraigned in a row, the Orion engine features six cylinders in two rows of three each on top of each other. Even more unusual was that engine was combined with a turbine and the turbine actually generated the shaft power. Below is the description from the book as well as an illustration:
We did our best to find out any other information on this rather interesting engine. The only thing we found was a brief description in the document Engine Transmission Power Packs for Tactical Vehicles 1967. Interestingly, this report gives more detail on the Orion project, as well as the name “Rigel” for the 600HP version of the engine intended for tanks. This document also gives a date for the project, noting that the program was cancelled in 1955. Oddly, it credits the engine concept to General Electric, rather than General Motors. Below are the pages from the report pertaining to the Orion program. Unfortunately, the PDF these images came from is not of very high image quality.