Optimal placement of barometric altimeter vent holes

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Aug 13, 2017
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I recently bought a JL Altimeter3, I have a question about the vent hole placement. I know they should be about two calibers below the nose cone but what is the ideal placement around the body? Three 1/16" holes around the body; on 3 fin rockets should I be putting these holes in line with the fins or in line with the area that would be between the fins? what about 4 fin rockets? does it make any difference? either performance of the rocket, rocket stability or performance of the altimeter?
The normal sensibilities say you should use three or more holes to keep the pressure as even as possible during flight. It should not matter whether they are in line with the fins or interdigitated.

Having said that, my normal configuration is two vent holes in my 3D printed NC less than a half a caliber below the NC shoulder :). Electronics is TeleMega and it has not shown any aberrant behaviour. YMMV.
I have gone with the one caliber rule on rockets that approach the subsonic <.8 range. In one case I had one flight closer to .9, so technically entering the transonic range. All of those flights were nominal.

I have not used a nosecone AV Bay above that, just more of a traditional design. Although I have some HED designs in the works that push well into the 2.5-3.3 Mach range, so at some point I will have to dig into this deeper.

As far s the number of holes, well that question has been asked and answered many times. The responses vary from a single hole, to many, but most commonly three as a min. Over the top surmised it well IMO.

I tend to line my holes up with the fins for no other reason than I like the look, which is highly subjective. This includes using three holes for a three fin rocket and four holes for a four fin rocket.

At the end of the day I do believe that this is one of those, "if you get it mostly right" then it will work, at least for most flights. I have personally never had a failure in this area with velocities up to Mach 2.4. However I have experienced inaccurate altimeter data with a mid power rocker that I retrofit a JL Altimeter Two into roughly five years ago. In that case I drilled three holes all on the same side. At that time I knew nothing about this and had zero experience.

I just fixed those holes on that rocket this summer.
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I have heard people say not to put them in line with your shear pins so that the heads of the screws don't interfere with the airflow into the AV bay.