The Design Space
Our redesigned piston must have the same form factor as a McMaster piston to allow for easy descope. Given the team's Fall 2017 semester experience with tie-rod pistons, we elect to continue using this style.
Desired Performance
The piston must be able to supply enough force to break the shear pins with a ?x safety factor before material deformation (this will allow for a smaller, but significant, safety factor on piston use in-flight). This safety factor was chosen using the 1974 NASA Aerospace Pressure Vessel Safety Standard [3].
As of 12/27/2017, we plan on using 180lbs of shear pins, making for a desired yield force of 720lbs. This analysis makes use of thin-walled pressure vessel theory [2], paraphrased below:
Neglecting end effects, the limiting factor will be the hoop stress in the piston bore:
\sigma_{hoop} = \frac{pR}{t}
Given Aluminum 6061 as the material
Resources:
The following resources are useful materials for learning about pressure vessel and piston theory:
[1] Jeff Hanson, Texas Tech: Intro to Thin Walled Pressure Vessels
[2] University of Colorado, Boulder: Thin-Walled Pressure Vessel Theory
[3] NASA Aerospace Pressure Vessel Safety Standard, 1974: NSS/HP-1740.1
Note that this standard was cancelled in July, 2002.