BT vent hole

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Mar 23, 2009
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I'm building an ARG Nike-Scram, 1.72" diameter, 31" tall 29mm MMT kit. Instructions call for a 1/8 in vent hole placed in the BT, 3" under the NC/payload connection. They say nothing more about it, and I may have read about that somewhere in the past, but can't recall what it is for. :confused2:
it's there for two reasons:
firstly, the pressure "up there", is less than "down here" so you need a vent hole to allow the pressure inside the BT to match outside, or the nose cone might pop off, esp. if it is loose.
secondly, as its a payload section there's a good chance that that's where you will put an altimeter, which could work by measuring air pressure, so if there's no hole, it will not work, as it will not measure a pressure change.
There's a few posts as to size of the vent hole.
Thanks Gil...
I fully understand small vent(s) in the payload area itself where, indeed, I will place my altimeter. The hole they are calling for is in the recovery (chute) area just below where the payload bay sits. I never thought of a premature separation of the entire payload/nosecone section due to pressure change. On E motors, flights as low as 2000 ft AGL, probably not an issue... but it is a lightweight model and with G motors would probably hit close to a mile in short order. In that scenario, your explanation makes good sense.

It was hard for me to imagine bleeding out ejection gases when (normally) you would want all the push you could get. I considered for a moment that it might be that, because motor retention is by friction fit, there might be some benefit of venting some of the ejection pressure, to reduce likelihood of popping a motor out.
I would say the hole is there to equalize the pressure while assending to prevent an early seperation. I also think you are right in your assumption that E powered rockets to 1,000 - 2,000 feet don't really need it. Getting a G flight to 5,000+ feet may need it, but I think you will have other design issues to contend with if you want to get a G motor that high.
Attached is a spreadsheet that shows the internal pressurization load that is attemption to push the nose cone off a non-vented compartment versus altitude and airframe diameter.

For your rocket it's roughly 1 pound load for every 1,000' of altitude so it's really not that high for a 1.7" rocket, but having a small vent never hurts.

It's essential for a 10" diameter rocket even at 1000'.


View attachment Internal Pressurization Load.xls