Static Port size question

sunderll · Aug 12, 2016

Hi all; I have a question about static hole sizing. I've generally launched 2" to 5" rockets and the formulas for the static port diameter for four holes always seemed logical and correct. I'm prepping a 7.5" rocket (12" long bay) and when I ran the calculations from various sources and programs the results were all over the place; anywhere from .290" to .500". That's a huge variable. The ports will be about 3.5 diameters back from the nose cone and the rocket will be average speed about 10 Gs max on boost. I'd be interesting in hearing from anyone who has a remotely similar rocket what size ports they used successfully. .500" seems massive and concerns me for turbulent sampling. THANKS FOR ANY HELP!!!

mpitfield · Aug 12, 2016

sunderll said:
Hi all; I have a question about static hole sizing. I've generally launched 2" to 5" rockets and the formulas for the static port diameter for four holes always seemed logical and correct. I'm prepping a 7.5" rocket (12" long bay) and when I ran the calculations from various sources and programs the results were all over the place; anywhere from .290" to .500". That's a huge variable. The ports will be about 3.5 diameters back from the nose cone and the rocket will be average speed about 10 Gs max on boost. I'd be interesting in hearing from anyone who has a remotely similar rocket what size ports they used successfully. .500" seems massive and concerns me for turbulent sampling. THANKS FOR ANY HELP!!!

The sampling port calculator I use says .28. So your .29 would be in-line with that. This is based on a four hole scenario with roughly 500ish cubic inches of volume in your AV bay. What I normally do is look for the next up-sized drill bit, in this case 5/16 or 9/32.

Bat-mite · Aug 12, 2016

I feel your pain. Which calculator to trust?

I kind of like Joel Schiff's, from the Apogee Peak of Flight newsletter.. dN = 0.022dR * sqrt (L/N) where dN is the diameter of one hole based on N, N is the number of holes, D is the AV bay diameter, and L is the AV bay length.

Using your numbers, and assuming three holes, we get

dN = 0.022 * 7.5 * sqrt (12/3) = 0.165 * sqrt (4) = 0.165 * 2 = 0.33

5/16 is 0.3125; 3/8 is 0.375; 11/32 is 0.34375

So I would probably just go with 3/8 to be sure. Remember that the calc is for the minimum hole size, so no harm in going bigger.

Quickly, if you choose to go with four ports, then you need 0.165 * sqrt (3) = 0.286, so go with 5/16.

sunderll · Aug 12, 2016

THANK YOU BOTH!!! This is comforting and very helpful. NO matter how much I tried to rationalize four .500" holes, it just seemed excessive. Knowing you had success with the holes I felt were proper is good news. On the small rockets a failure isn't so significant, but when the bigger ones have issues it's so much worse. Thank you guys again, I appreciate the help!

bobkrech · Aug 12, 2016

The math for payload or parachute compartment venting is discussed here. https://home.earthlink.net/~david.schultz/rnd/pressurelag/parachute.html

A single 7/16" diameter hold would have a 0.88 second time constant. (4) 7/32" diameter holes equally spaced around the airframe has the same area and has a common mode time constant of 0.88 seconds but offers a 3.5 second differential gust time constant.

Static Port size question

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