Got bored doing repetitive calculations to arrive at "Free Volume" for use in ejection charge calculations. I ALSO needed the Free Volume for calculating Vent Port Sizing for the booster as well. The latter case is what got me started writing.
Soooo.... I threw a little spreadsheet together. Pretty sure the math is correct. The screen shot shows approx. numbers for my WM Punisher 3.
If you want this DRAFT spreadsheet, it is attached below.
Feel free to ask questions here. I have not written (nor am I likely to) any documentation for this yet.
See below for additional comments:
BANG! You have the volume to use on one of the ejection charge calculators like BP Estimator or Altimeter Vent Port Sizing.
Only BP Estimator doesn't have a volume field & Altimeter Vent Port Sizing's volume field is not directly editable, so you have to back up from the "Total Free Volume" number to get the equivalent length of Body Tube to achieve the calculated volume.
See below:
I have a Total Free Volume of 154.2729 cu in. That came from V=π*r^2*h Solve for "h".
π*r^2*h =154.2729 ---> h = 154.2729/π*r^2 ---> h = 154.2729/π*(1.5)2 --->h = 21.8252 in.
EDIT: I just decided to add the math for the above to the spreadsheet.
Take the "21.8252" and input it as the "Body Tube Length" on BP Estimator or other. The calculator will then figure the powder charge for the calculated "free volume".
In my case, I bumped the pressure to 11 psi to force "Number of #2 shear pins" to THREE (3).
The calculator gives me an estimated 0.86 grams of BP to use.
I will back that off by at least 10% and work up to the minimum "reliable" charge weight, and will also evaluate the calculator's "accuracy"/usefulness.
Hope some of you will find this useful.
Be well!
Soooo.... I threw a little spreadsheet together. Pretty sure the math is correct. The screen shot shows approx. numbers for my WM Punisher 3.
If you want this DRAFT spreadsheet, it is attached below.
Feel free to ask questions here. I have not written (nor am I likely to) any documentation for this yet.
See below for additional comments:
- Any units as long as you are consistent (all in, mm, cm).
- Input the actual measurements from your airframe components.
- Set a value for Qty on each volume type. ZERO/blank will zero out that volume type.
- Input calculated volume(s) (if desired) into one or more of the User Input Volume lines and set the "Qty" field to non-zero. Note that I have not entered any... yet. I am leaving these off for now to artificially increase free volume and add a margin to the BP required number. Once I know the REAL world charge weight, I will come back and add the rest of the actual subtractive volumes to analyze the "accuracy" of the calculator.
BANG! You have the volume to use on one of the ejection charge calculators like BP Estimator or Altimeter Vent Port Sizing.
Only BP Estimator doesn't have a volume field & Altimeter Vent Port Sizing's volume field is not directly editable, so you have to back up from the "Total Free Volume" number to get the equivalent length of Body Tube to achieve the calculated volume.
π*r^2*h =154.2729 ---> h = 154.2729/π*r^2 ---> h = 154.2729/π*(1.5)2 --->
EDIT: I just decided to add the math for the above to the spreadsheet.
Take the "21.8252" and input it as the "Body Tube Length" on BP Estimator or other. The calculator will then figure the powder charge for the calculated "free volume".
In my case, I bumped the pressure to 11 psi to force "Number of #2 shear pins" to THREE (3).
The calculator gives me an estimated 0.86 grams of BP to use.
I will back that off by at least 10% and work up to the minimum "reliable" charge weight, and will also evaluate the calculator's "accuracy"/usefulness.
Hope some of you will find this useful.
Be well!
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Perhaps I might look at the code from other calcs/sheets and see how they are getting to their "destinations". It WOULD be nice to have the volume calc stuff on the front end automagically feed through to the charge weight calcs/recommendations, et al. 
