When does a vent hole become necessary?

At what altitude do I need to be concerned with venting the airframe?

I'm putting small (3/32) holes (one or two) in the airframe above where the chute will be - on anything expected to go over 1500 feet. It's probably not necessary at such a low altitude, but I'd rather err on the side of caution. Besides - that way I can always stuff a bigger motor in there and have one less issue to worry about.

Some times it is not so much what altitude but how fast you get there. Most rockets are not totally sealed and will bleed off some pressure on the way up. So the faster they get to altitude the less time to bleed pressure. A low speed rocket with the same leak down could reach a higher altitude than a higher speed rocket would before the pressure separated the parts.
That said I always put a 1/16"to 1/8" hole in the recovery bay of rockets larger than 2.5" (I have yet to build one smaller.) I don't know that I have ever had one separate due to internal pressure so I plan to continue the technique.

VonMises said:

At what altitude do I need to be concerned with venting the airframe?

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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).

Taking a quick look at a chart from Wikipedia, it appears to me that there is 10% difference below between MSL and 1Km altitude. So I'd say a vent hole doesn't hurt, but I doubt it would cause your rocket to open under 1Km AGL (3280'). Halving that, just to be safe, takes you back to scsager's suggestion.

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.

David Schultz put together a superb spreadsheet to analyze this. You may be able to Google it, otherwise I will try to post a reference tomorrow when I'm on my regular computer.

TopRamen said:

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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OP is asking about a vent hole not a sampling hole.

I put them on all of my rockets because I cant think of any drawbacks of doing so and it takes only a couple of seconds to drill them out.

TopRamen said:

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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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

dixontj93060 said:

OP is asking about a vent hole not a sampling hole.

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Yeah, my bad. My Comment was not really relevant, as I was not referring to a Vent used to stabilize internal and external Pressures to prevent premature Recovery System Deployment.

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.

John Beans said:

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.

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That's right. It's really just for AV bays and assumes that they will be filled with a sled and electronics, not parachutes or anything that will make it less airy.

Thanks everybody.

I really don't like drilling holes in my airframes is it's not necessary. They just don't look right. I'm trying to come up with a way to vent (only if necessary) without making a noticeable hole.

For some reason, my airframes tend to be full no matter how big they are. I tend toward slower velocities, and like to stay at or below 3,000 ft so I can witness all of the events.

I'm glad you asked this question. I'm re-visiting this question again as I finished up another project. My recent line of thinking was, "Why drill holes in the booster section when it leaks gas past the motor case?" Nice to see comments here regarding air volume and how fast the rocket gets to it's expected apogee. Most of my rockets have very full booster sections, taken up by recovery gear and a long motor case. Since I'm an altitude junky, I've experimented with making my vent holes smaller and smaller. I'm now, successfully, down to a #60 size drill bit for rockets that fly fast to 8,300'. I should note that these are all 2.6" dia. rockets with full recovery bays. I probably won't be able to get away with this when flying a 3" dia. rocket to beyond 10k at above mach 1.5.

dward said:

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

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Pretty cool!

I had some fun and asked how big for 1/2 hole. It gave me an answer!

TopRamen said:

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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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:

Static Pressure Port Hole Size

https://www.vernk.com/AltimeterPortSizing.htm

Smaller holes can be used to prevent early deployment due to an internal/external pressure differential since the goal isn't to maintain a precise tracking of internal and external pressures as it is with an altimeter, but only just large enough to prevent the pressure differential building up enough to push the nosecone out of the airframe.