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The piston must be able to supply enough force at its operating pressure to break the shear pins with a 2x safety factor, which is the safety-critical guideline for parachute components presented by NASA (Section 3.3.1. [5] According to standards established by the Aerospace Corporation, there must also be a minimum 1.5 design burst factor. [45). The same source (Section 3.3.1.6) dictates a design burst pressure factor of 2 times the maximum design pressure. [5]
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:
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Another requirement of the piston is that it cannot break the shear pins prematurely due to an internal build-up of pressure caused by the altitude difference. Between 4,245 ft (the altitude of Truth or Consequences, NM) and 152,945 ft ASL (a simulated upper bound on performance as of January 4, 2018), the pressure difference is approximately (given by the 1976 Standard Atmospheric Calculator using no temperature offset) -86600 Pa. Thus, the following graph of maximum piston area based on the amount of shear pins used can be estimated:
Terminology
Factor of Safety
Resources:
The following resources are useful materials for learning about pressure vessel and piston theory:
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