• Created by Unknown User (astrocrg@mit.edu), last modified by Unknown User (janssonm@mit.edu) on Nov 28, 2017 01:41

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Overview

This was a ground test of the Hermes Pathfinder rocket. It occurred on Field A at 3:45 PM on November 10, 2017. It was the first use of the Pyrotechnic Piston to separate the rocket. The piston contained 1g of 4F black powder. The tender descender contained .2g of black powder. The shear pins used were not documented but were likely 2x 6-32 nylon pins and 1x 4-40 nylon pin. The rocket successfully separated, and the tender descender released the main parachute appropriately. After de-integration it was found that the Avionics Bulkhead was seriously deformed. This prevented the planned flight on November 11, 2017 from occurring.

Failure Investigation

Black Powder Sizing Investigation

Upon further reflection, we identified the likely cause of failure. The 1 gram of black powder used in the test was sized for a worst case scenario in which the rocket would not separate until full piston extension. This meant that when the piston was not fully extended, it applied a much greater load (due to a smaller volume causing an increased pressure) to the booster section, which in turn applied an equal and opposite reaction force onto the piston rod and thus the Avionics Bulkhead by Newton's 3rd Law.

We then tried to determine the maximum force felt by the bulkhead:

Step 1: Determine pressure in the piston prior to rod extension

Flight Piston Dimensions: An estimate for piston volume prior to rod extension is 0.5599π [1.08 in for depth and (1.44/2)2π = 0.5184π in for area]
Using this black powder calculator and plugging in a tube diameter of 1.44 in, a tube length of 1.08 in, and 1 g black powder, yields a pressure of 1101.82 psi.

Step 2: Determine force at 0 extension given PSI

The pressure pushes on the piston rod, which has an approximate area of 1π in2.
F = P*A = 3500 pounds

Step 3: Evaluate Assumptions

Admittedly this math is sensitive to the volume, which we only estimated. The BP calculator also assumes an ideal gas, which isn't true.

FEA Investigation

Solidworks FEA confirmed that the deformation is consistent with a 2-bolt load application.

To perform this analysis, we performed a static load analysis on the Avionics Bulkhead modeled in CAD as of November 16, 2017. Some notable limitations of this analysis are:

  • In actuality, this would be better modeled by an impact.
  • Since significant deformation was observed (see the image below), it is clear that a nonlinear analysis would be required to accurately model the deformation. The static load analysis was performed to get a basic idea of expected deformation shape and to prove material exceedance at the expected loads.

Additional Analysis is underway.

 

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