I'm not sure there is a single rule. Basically, it's where there is a significant pressure difference between the air trapped in the rocket (at ground) and the air outside the rocket (ambient).At what altitude do I need to be concerned with venting the airframe?
I've never launched any Electronics on a Rocket, save for a Flashlight and an 808 Keychain Camera, but from what I've read here, if you are going to use an Altimeter, a Vent Hole is necessary.
I made an app for calculating this. It will even make suggestions on number of holes and standard drill bit sizes to use. https://rockets.davidward.org/Tools/Entries/2013/10/4_Static_Port_Calculator.htmlI've never launched any Electronics on a Rocket, save for a Flashlight and an 808 Keychain Camera, but from what I've read here, if you are going to use an Altimeter, a Vent Hole is necessary.
OP is asking about a vent hole not a sampling hole.
I don't think anyone mentioned it, so I'll add this: One important factor is how much "air" you have enclosed. It's that air that wants to equalize with outside ambient pressure. If you didn't have any air (say, for instance, the bay was completely flush with parachute and cords), you wouldn't need to worry at all about vents. Don't forget to include any air pockets that are connected, such as above the motor in the motor tube, if that's connected to the section in question. In addition to altitude and rate of climb, the more "airy" you make your sections, the more you have to factor in venting.
I think dward's calculator assumes worst case (completely empty bay), since I didn't see a factor for that.
I made an app for calculating this. It will even make suggestions on number of holes and standard drill bit sizes to use. https://rockets.davidward.org/Tools/Entries/2013/10/4_Static_Port_Calculator.html
Holes for an altimeter need to be larger than those used to prevent early deployment due to the development of a pressure differential between the rocket's interior and the pressure at altitude:I've never launched any Electronics on a Rocket, save for a Flashlight and an 808 Keychain Camera, but from what I've read here, if you are going to use an Altimeter, a Vent Hole is necessary.
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