Here is the math you need to get what you want.

Bay Volume = radius * radius * length * Pi

Single Vent Diameter = 2 * SQRT ( volume * 0.0004908 / Pi )

Single Vent Radius = Single Vent Diameter / 2

Single Vent Area = ( Single Vent Radius ) * ( Single Vent Radius ) * Pi

Multi Vent Diameter = 2 * SQRT ( ( Single Vent Area / # of holes ) / Pi )

Speaking for myself, I have often found the prescribed hole sizes to be too small -- largely because I tend to stuff wires back through instead of using switches.

Many, many of the rockets I have flown have had static ports that were "too big" according to the instructions. I've never had a problem.

Make sure your ports are in "clean air", as far away as possible from seams or rivets or transitions.

I'm assuming Conway's numbers above come from a MW manual? That is where I usually go.

Finally, again -- speaking purely for myself, I like a singular hole for the port. I'm sure three holes are fine. I've just always drilled one, basically because of the "twist and shove" technique I use...

Ive always used 3 equally spaced ports. less chance for anomaly or issues from pressure induced stuff. Holes to large cause to early of deploy. Holes to small cause late deploy Mine have always been right at the very end of apogee. The formula above is NOT from a MW sheet or instructions. It is Jim Amos's NEW formula for calculations as past formulas had some issues with expedientle sizing and cause anomalies. Had some long conversations with Jim on this and the current math problem I gave is his answer to fix all issues.

Single Vent Diameter = 2 * SQRT ( volume * 0.0004908 / Pi )

Conway, where does the 0.0004908 come from?

Doug

Not sure Doug. Jim Amos is the Master mind behind the math. I got it from him when I was doing my L3 stuff and talked with him about the formula he had previously(and others formulas that have been used and same issue) and he said to throw that out as it along with many others had a flaw in them that becomes apparent when scaling up as the size changes in a exponential fashion that creates an inaccurate port size. BUT Im sure jim knows the answer. Ive never thought to question those numbers as Jim is the genius and knows way more then I. so I have no answer for ya.

Hello Everyone,

I really appreciate all the help with the updated formula. That made a big difference !

Instead of three ports at 11/32, I will need three ports at .21 inches.

I may tweak a little towards .25 inch each, but not to much.

Thanks again 🙂

An interesting question. And being a math/physics geek, I always wonder on what principles these prescriptions arise from.

The simplistic view I have is air is rushing outward and too small a hole will cause a phase lag between actual pressure and measured. In the other direction, assuming Mach effects are not present, head scratching as to why too large a hole might lead to early deploy.

Ideally, all one wants is an unencumbered look at external pressure, so if true--big holes early deploy, it would seem Bernoulli effects and or turbulence are responsible. (Anyone know for sure the reason behind Con's observation?)

I have always had MW deploy "late" and others have seconded, but figured it was a function of software and philosophy. The key is that it does DEPLOY. I personally agree with it and if your bird can't survive the rigors of a second or two late deployment, and in need of adaptive digital filtering, then time to rethink your design IMHO.
J