I'd do 3 equi-spaced 1/16" diameter holes. Still well under the upper end specified by Missileworks and definitely big enough to do the job in a 3" bird with that small a volume.

W

Still well under the upper end specified by Missileworks .....
W

I'll confess that I have never seen the upper end specified in Jim's documents? FWIW, the 3 x 1/16th inch holes are nearly 6X what the formula recommends -- though I think you are probably correct in saying that it would be OK.

I just have this thing for doing a single static port? Going back to the "Dance with who brung 'ya" school of thought. Having always been successfle with one, I'm prone to use just one...

This is OK, I haven't fired up the drill yet 😉

As I recall, you were standing right there with me when my first SSS bird fired on the pad not once, not twice, but three times. It had a single 1/8" port for a Perfectflight MAWD. A breeze across the hole created enough of a variable vacuum to set it off. I'm a firm believer in 3 or 4 equally spaced holes that together equal about 2x the surface area that the Missileworks formula comes up with. It's been very reliable for me so far.

Warren

Personally, for me, one port is good for rockets up to 3" in diameter, then again what Warrens says happened to him... some altimeters are "twitchy," some are not. I prefer the ones that are not. BTW, a rocket I've had packed since Oktoberfest has one vent hole, a MAWD, a min dia 38, and only two 1/16" polystyrene shear pins, ready to go on an I357--with ends loose during hibernation, and I'm feeling "twitchy" about it, especially since the flight after that will be on a J350, without a transmitter. 😯

I am also a very firm believer of multiple ports as they do phase alot of induced issues and anomalies out.

First I ask if you could print the formula here your using. Depending on what formula you are using from Jim can make a difference. When I did my L3 Jim gave me a new formula that I know at the time was not in any of his publications. It was a much more accurate formula then the previous as Jim had found some variables that could create an issue.

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 )

I believe the issues there were involved larger ebays as the old formula had an issue with that. It created ports that were to large even for larger bays. But this is what I currently use.

Warren, I don't think it was me that was there when your altimeter fired on the pad. I believe what you are saying, but I do not recall being there.

FWIW, I'm a single-port kinda guy. I keep intricate logs. I've built 36 different rockets that fly with altimeters, and 35 of them had a single static port (I'm not counting group projects). I've never had one go off on the pad.

I'm most assuredly not saying that what I do is right --- I am saying that for me, single ports have worked well on ~150 flights, and I have never had a charge fire on the pad. It is hard for me to switch gears and go to multiple ports.

I did a strange test once. I took my rocket "Stratagem" and loaded it up with everything but powder. Live batteries, ematches, and armed. I whipped it through the wind (again, it was armed) and even tossed it on the ground roughly a few times. This is a glass rocket, very tough. I bounced it off my foot (pointed away from everyone, including me). I could NOT get the altimeter to fire. I did everything short of blowing into the static port.

I don't think wind causes this sort of thing. Also note that PF altimeters have a baro sensor that is mounted in such a way as to minimize all this anyway....

This is what I told Mike K., it is both frustrating and neat that there isn't JUST ONE WAY to do things. It's great and necessary that we welcome new people into the club, with diverse opinions and experiecnes. In the end it advances us all. As JW would say, "Dance with who brung ya." Whatever works for you, use it. With each rocket, each launch, I've tried something different, some successes, some not--okay, maybe most not. I nailed my L3, but that's no guarantee that I will nail my next M and over flight. You calculate, build well, and pray like the rest of us. 😯

0.033", or a hair over 1/32".

I haven't drilled it yet, but I have an idea on how big I'd like to make it. I'm curious on what y'all would do? 1/32" seems pretty small....

John I do not have anything under 1/8". one port will be fine...you will need room for your wires to get pushed in.

So what happens if your static hole port is larger than the calculated size. I made mine larger than the calculated value to allow access to the arming switch.... so I'm trying to understand what happens if its too big. I guess along the same lines how about the vent holes for the other compartments. I've seen several recommendations as to the values and since Im potentially flying Mach 2+ I wanted the holes a little larger to accomodate the rapid change in pressures, vs slower flights. Thoughts??

So what happens if your static hole port is larger than the calculated size. I made mine larger than the calculated value to allow access to the arming switch.... so I'm trying to understand what happens if its too big. I guess along the same lines how about the vent holes for the other compartments. I've seen several recommendations as to the values and since Im potentially flying Mach 2+ I wanted the holes a little larger to accomodate the rapid change in pressures, vs slower flights. Thoughts??

I think it mostly just means that your av-bay will equalize faster with the surrounding atmosphere (good), but maybe allow more dramatic pressure swings during transonic transitions (not a problem if you're using a Mach-immune altimeter or a correctly-set Mach inhibit delay). Wilke's shot to 25k last year had barely any transonic effect show up in the recorded data, and I suspect it was because he was using a relatively small vent hole a good distance from the nose. In my supersonic flights last year, with about a 1/8" hole right near the shoulder for altimeter arming, the recorded altitude had major transonic effects, even going negative at one point for one of the flights. But the altimeter was set up to ignore the baro data for high speeds, so it wasn't an issue.

What do you have planned to go > Mach 2? Sounds like fun. I have some high-performance flights planned, myself.