So you have .588 in^2 as your area that you need in holes. So now you wanted 6 holes around the airframe. This would be .588 in^2 /6 or .098 in^2 for each hole. A = pi*r^2 so r = SQRT(a/pi). Therefore we get r= SQRT(.098/pi) r = .1766 in. That is the radius, so for diameter that is twice so that mean each hole would have to be .353 in. Interesting that the two formulas get two different answers.

Edward

😯 HUH!!!!!!!! Is that in Spanish???

To clarify, having too many or too few ports, too large or too small should NOT cause your altimeter(s) to fire when you power them up. If the ports are too small, that would not do it. If they were too big? That wouldn't do it, either. I often power things up after it is laid out, with one end of the ebay open. In essence, I have a "static port" the size of the ID of the coupler tubing at that point.

Mike, you had indicated earlier that you had identified the root cause of the problem. Was there a wiring or battery or other issue? Port size would just simply not translate into things going off (twice). Inquiring minds want to know 😉

Definitely not the ports, and, JW, you're right that certainly wouldn't have set things off. That issue would only have an effect during flight. Mike will say what happened when he's 100% positive why what happened did. That sentence from an English major? 🙄

Hey Bruce, those of us who studied English have to stick together. I hear ya'. Incidentally, my wife gave me one of those t-shirts that says:

ENGLISH MAJOR
(you do the math)

Now if someone needs a sonnet written about their project, Bruce and I will handle it.......

A mathematician confided
That a Mobius band is one-sided.
And you'll get quite a laugh
If you cut it in half,
For it stays in one piece when divided.

My questions on the ports and their size wasn't referring to Mikes Failure. Just seemed when he posted how many and how large they were that he might have to much and could create an issue only under flight.

Here is the formula I currently use

Bay Volume = radius * radius * length * Pi

Single Vent Diameter = 2 * SQRT ( volume * 0.0004908 / Pi )

Single Vent Radius = Single Vent Diameter / 2

Single Vent Area = ( Single Vent Radius ) * ( Single Vent Radius ) * Pi

Multi Vent Diameter = 2 * SQRT ( ( Single Vent Area / # of holes ) / Pi )

I know, guys, we're just tap dancing here until Mike reveals his findings...
if Mike's calculations on MWC's are correct, and I feel they are, 0.196 inches is 4.78 mm (0.196 * 25.4), close enough for (6) 5 mm holes.

Here's my poem, Ken, JW:

There once was a rocketeer from Nantucket,
Who...

Here is the formula I currently use
Bay Volume = radius * radius * length * Pi
Single Vent Diameter = 2 * SQRT ( volume * 0.0004908 / Pi )
Single Vent Radius = Single Vent Diameter / 2
Single Vent Area = ( Single Vent Radius ) * ( Single Vent Radius ) * Pi
Multi Vent Diameter = 2 * SQRT ( ( Single Vent Area / # of holes ) / Pi )

The diameter of each of the 6 holes, using the above formula comes out to .157" or 4 mm vs the 5 mm holes I put in. It appears the differences is in the arbitrary factors ".0004908" as in the above and "400" used by MW's. MW's formula, that I used, is the one referenced in Mark Canepa's book "High Powered Rocketry".

Not knowing how these factors were chosen, I can only assume they were derived with consideration of pressure drops and venturi effects of fluids traveling through orifices using Bernouli's equations at common rocket velocities. I can't dispute them either way.

Another on-line Static Port Calculator calculates "the required size of static ports for altimeter pressure sensors (using the 'one ¼" hole for every 100 cubic inches of bay space' rule)". My L3 avbay has a volume of 236 in^3 so, using this guidline, I would need 2.36 each .250 inch diameter holes, which equates to a total area of .1158 in^3. This is 1/3 of what MW's and the above equations suggest. Fudge factors like this scare me.

I will gladly embrace any proven science based formula. I can easily fill up my 6 holes and re-drill them if necessary.

Hi Mike,

Since I'm also in the middle of my L3 I have been running these same calculations. What happened to you on Saturday concerns me, as I also have an RRCX mini. According to my calculations, I don't see a factor of three between the MW formula and the 1/4" hole per 100in^3 approximation.

Quantity Variable MWC Conway
Bay Radius r 2.5 in
Bay Length L 12 in
pi pi 3.141592654
Holes h 6
bay Volume V 235.619449 in^3

Single Port Diameter s_d 0.589048623 0.383718647 in

Single Vent Diameter v_d 0.383816189 0.383718647 in
Single Vent Area v_a 0.115700826 0.115642026 in^2

Multi Vent Diameter m_d 0.156692303 0.156652482 in

Total Vent Area: 0.115642026 in^2
Actual Vent Area (6xØ5mm) 0.182605438 in^2
Actual Vent Area (2.36*Ø.25) 0.115846229 in^2
1.579057763

The last number above is the ratio of your total vent area to the calculated total vent area, and it's only 50% larger. That doesn't seem like too excessive of a deviation.

Having said all that, what really troubles me is that both altimeters were in the exact same bay, under the exact same conditions yet only one of them fired prematurely - twice. This suggests rather strongly to me that, rather than there being a problem with the ports, there is something wrong with that particular altimeter.
You said the root cause of the failure had been determined. I, too am quite interested to know what the cause was, but I don't think it was the ports.

Ok first to explain the formula I have used. The formulas like the one in Mark Canepa's book have one issue as airframes increase in diameter the port also has a tendency to get exponentially larger then it should thus throwing off the real size needed and can cause some flight issues with anomalies and pressure noise. So the formula I use fixes this issue. It now makes the port holes more accurate in much larger airframes and works even in smaller airframes. If you look at the difference between the 2 formulas the formula I use came up with slightly smaller ports. Again as you increase airframe diameter and internal volume it skews the one formula. Now does that mean the one formula is inaccurate or faulty. No, for most peoples needs it is not. Mine were different needs. Anyhow regardless. The issue that Mike has is not this issue. I only asked because when he stated what his port sizes were they seemed a little large for as many of them there were to me as Im used to a different formula. Anyhow Mike is fine and this is not an issue.

Ken, I have two mini's and plan to use one on my next "big" rocket and one for J's-K's. Just kinda hang in there. Mike's av-bay environment was unique, and he will explain as soon as possible.

JW has a good thread on static ports in another area... can't remember where... hold on... oh, yeah, in Construction. 😉

Having said all that, what really troubles me is that both altimeters were in the exact same bay, under the exact same conditions yet only one of them fired prematurely - twice. This suggests rather strongly to me that, rather than there being a problem with the ports, there is something wrong with that particular altimeter.
You said the root cause of the failure had been determined. I, too am quite interested to know what the cause was, but I don't think it was the ports.

Your right as I and others have stated. It is NOT an issue with the ports. Its also not an issue of it being a defective altimeter or anything anyone should worry about or get concerned with. Mike is working with his TAP's and has spoken with the manufacture and discussed the issue and its special circumstances creating the problem... As Bruce has stated its not an issue for anyone to get worried that they have an altimeter that has issues or anything like that as this situation is quite unique and could have happened to almost any manufacture.

Conway

Plus, Conway, didn't you use a mini on your L3, if not for the certification flight, definitely right after? Ken, that's not the issue. Hang in there...

Plus, Conway, didn't you use a mini on your L3, if not for the certification flight, definitely right after? Ken, that's not the issue. Hang in there...

No, I didnt use a mini. Ive used RRC2x,2 differnt ARTS, and an RMCS, in my L3 flights. BUT I have had some experience with the mini. I helped Jim with testing on a unit at OF last year with several flights and have seen it used by several flyers now even at LDRS and not one issue yet. There is a learning curve. I highly recommend to get it down and learn it. Even set it up with the instructions every time. ( I do that with all my altimeters, saves me every time)

Also let me mention I have no issues running a Mini if I had one. Been to busy to get one yet.. But I will.

I only have two duel deploy.
One 3" and One 5.5".
Both use the RRC2X.

Used Conway's calculations on both. Both have worked perfect.

My third duel will be launched at MHM 08. I feel 100% confident
in the Missile Works RRC2X, Missile Works Pet 2 Timer, and
using Conway's calculations.