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No sure what vent hole size to use? Jackson made the attached vent hole size calculator for his minimum diameter N5800 project. The spreadsheet uses pressure data from RockSim to calculate the maximum amount of force that will build up inside the area to be vented during the flights ascent.
For ordinary flights, it is easy to approximate the vent hole size based on what has worked for other flights. But, there is not always sufficient opportunity to test the vent hole size on a high mach flight. We knew we only had one shot at it. Better to do some calculating in advance.
The spreadsheet uses recursion of a formula based on an analogy to an RC filter. The RC filter and the formula were first conceptualized by David Schultz. Schultzs original work is referenced in the spreadsheet.
The spreadsheet contains both a data sheet, and an instructions page. Read the instructions page. But, the basic gist of it is: sim your rocket and export the pressure data. Then, put this data into the spreadsheet. Enter the length and diameter of the area to vent. Then, enter a vent hole size. You will have to use the sheet recursively. Continue to try various sizes until the max force that is calculated is small enough for your own comfort zone. Remember that 2-56 nylon screws fail at 25 pounds. The max force should be less than what your shear pins will hold.
We have attached an Excel spreadsheet. We also have a Numbers version, but could not upload that file. PM us if you want the numbers version.
View attachment Vent Hole.xls
For ordinary flights, it is easy to approximate the vent hole size based on what has worked for other flights. But, there is not always sufficient opportunity to test the vent hole size on a high mach flight. We knew we only had one shot at it. Better to do some calculating in advance.
The spreadsheet uses recursion of a formula based on an analogy to an RC filter. The RC filter and the formula were first conceptualized by David Schultz. Schultzs original work is referenced in the spreadsheet.
The spreadsheet contains both a data sheet, and an instructions page. Read the instructions page. But, the basic gist of it is: sim your rocket and export the pressure data. Then, put this data into the spreadsheet. Enter the length and diameter of the area to vent. Then, enter a vent hole size. You will have to use the sheet recursively. Continue to try various sizes until the max force that is calculated is small enough for your own comfort zone. Remember that 2-56 nylon screws fail at 25 pounds. The max force should be less than what your shear pins will hold.
We have attached an Excel spreadsheet. We also have a Numbers version, but could not upload that file. PM us if you want the numbers version.
View attachment Vent Hole.xls
