EBay Vent hole diameters

ac2fv

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After doing some research, Ive come up with vent hole sizes for 3 rockets. This is my first foray into dual deployment. Specs are below:

Loc IRIS 3.1: 3" payload diameter, ebay is 7.75" in length, 3 vent holes 5/32" in diameter
Loc Ariel: 3" payload diameter, ebay is 6" in length, 3 vent holes 5/32" in diameter
Loc Andromeda: 4" diameter payload, ebay is 5.75" in length, 3 vent holes 3/16" in diameter.

My question: Do the above vent holes look reasonable, and if the holes are larger than what they should be, what is the effect on a successful DD recovery?

Thanks!
Scott
 

Voyager1

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Handeman

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My understanding of vent hole sizes is that the recommendations are to minimize the time lag to (de-)pressurize the av-bay to ambient pressure.

If the holes are too small, the internal pressure stays higher than the external as the rocket reaches apogee and it's possible for the rocket to arch over and start coming down before the pressures equalize and starts going back up, which is what triggers apogee detection.

The only downside I've heard tell of with overly large holes is that wind can affect the av-bay pressure while on the ground/pre-launch generating false launch detects, although I've never heard of anyone saying this has happened to them so that may be more wives-tale.

When I started DD 16 years ago I used the recommended vent hole sizes and 4 holes. I still use 4 holes, but for the last decade they have been up to twice as large as recommended. I never had ground issues with wind and apogee events always seem to be accurate whether recommended sizes or double recommended sizes. YMMV
 

waltr

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The only downside I've heard tell of with overly large holes is that wind can affect the av-bay pressure while on the ground/pre-launch generating false launch detects, although I've never heard of anyone saying this has happened to them so that may be more wives-tale.

I have read that his had been a problem with early Baro based altimeter/DD systems.
Newer ones all seem to have better software to prevent this. One way is having an LDA (Launch Detect Altitude) of 100 feet or so. Any Baro pressure change due to wind gusts cause less than LDA altitude change so no launch detected.
 

cls

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The only downside I've heard tell of with overly large holes is that wind can affect the av-bay pressure while on the ground/pre-launch generating false launch detects,

From more than 10 years ago... I know of two incidents of false launch detect, and 1 or 2 early apogee detect, from too large vent holes. All using older baro altimeters, likely not having data smoothing algorithms as you suggested.

So yeah it happened.
 
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