will holes for altimeter mess with ejection on LPR/MPR

I got an altimeter 2 and now need to drill holes in all my rockets. based on this https://www.vernk.com/AltimeterPortSizing.htm I think I need to drill 3 1/8" holes in my estes STM-012.

any chance they will mess with the ejection? there is a ton of volume in this rocket.

Actually AltimeterTwo uses both an accelerometer and a barometric sensor. I wouldn't expect the small holes to interfere with normal ejection, though.

The holes don't need to be that big, one 1/16" hole would be fine. An STM-012 isn't going to go mach and you're not doing any deployments with the altimeter, so you don't need to worry about a slight lag in pressure readings causing any issues.

RKeller said:

I got an altimeter 2 and now need to drill holes in all my rockets. based on this https://www.vernk.com/AltimeterPortSizing.htm I think I need to drill 3 1/8" holes in my estes STM-012.
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any chance they will mess with the ejection? there is a ton of volume in this rocket.

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I always go by the rule of 1/4" diameter vent area per 100 cu. in. of internal volume to be vented (divided between three holes) provided on the excellent web page you link to. I've never experienced anything other than very positive and energetic parachute ejection. The ejection charge causes a very rapidly rising pressure pulse which ejects the nose cone and parachute long before the pressure it creates has any time to leak out vent ports.

If you need to drill 3 holes, go with a 1/16" hole at 120 degrees apart. IMHO, a 1/8" hole is a bit much (no pun intended).

"No", I don't think those holes will affect things much. Although the further forward the holes are the better, but not too close to the end of the airframe so it has access to air flow that is more laminar.

Greg

Simple formula to determine appropriate hole size from the web page you link to:

DN = 0.02216*DT*sqrt(L/N)

Where DN is the diameter of the small static port holes for a compartment with body tube inside diameter DT, length L, and number of holes N. (All dimensions must be in inches for this equation.)

Or, just skip the holes. Depends on what your after. Once the nose cone pops, you will have the altitude. The drawback is that such may be low if ejection is late. And if you plot the data (assuming there is that ability), you won't have baro altitude until after ejection.

I have used the Altimeter 1 and one from Estes this way with no issues.


Kirk

1tree said:

Or, just skip the holes. Depends on what your after. Once the nose cone pops, you will have the altitude. The drawback is that such may be low if ejection is late. And if you plot the data (assuming there is that ability), you won't have baro altitude until after ejection.

I have used the Altimeter 1 and one from Estes this way with no issues.


Kirk

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Sure, proper vent holes aren't critical if you aren't using altimeter-based parachute deployment. I've recently needed accurate barometric apogee measurements for experiments, so I've been using that formula and drilling vents even in rockets that don't use an altimeter for parachute deployment.

Been using the altimeter one for quite awhile,never had any issues.Drill three holes 1/16" like it says..easy peasy.