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The dimensions for the Hermes cup were chosen in part due to an investigation on Hermes Packing Procedures and Volumes. Here, we found that pathfinder packing volume is roughly appropriate. Thickness of the cup increased, but we used this new thickness when simulating for mass saving cuts. Because the airframe is 5.8885" ID, we want some tolerance between the OD of the cup and the ID of the airframe. On the Pathfinder vehicle, this was measured as ~0.25" in tolerance by using paper to measure the circumferences. In reality however, the Pathfinder tubes are quite out-of-round and visual measurements show a maximum tolerance around 0.15" (between diameters). Because we would like to limit frictional forces that will detract from the useful work done by the piston, we will go with the former tolerance, bringing the required outer radius of the cup to be 2.8158" and correspondingly the inner radius to be 2.69675". Due to the need to maintain roughly the same packing volume as Pathfinder, the overall length of the cup must be adjusted to 15~15"

Resources

NASA, Buckling of Thin-Walled Circular Cylinders