Port hole size help.

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Starkiller

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So I built a darkstar jr. for my first dual deploy rocket, and put a perfect flight stratologger cf in it. Now I did the math and I get a hole dia. of .024 for four holes.
My av bay is 2.00 X 7.5 does this seem to small to anyone? Plus every time I read something else or do a different calculation I get a different size. Even when I do the math and check
against the numbers in my altimeter manual some don't add up to the same in the book. I had read on one site that the smallest hole should be a .032. The .024 hole are already in my bay.
thanks for any help.
 
If you go a little large on the holes, the consequences are usually very insignificant. If you go too small, depending on how well sealed the bay is, inside pressure can lag behind actual outside pressure. This means the rocket will arch over and start down until the inside pressure catches up to the outside and it finally senses apogee. If it's only off by a little, no big deal, but if its off by a lot, you can get some significant downward velocity before it finally senses apogee. Of course the altimeter will always sense a lower apogee then actual.

I use 1/16" holes (0.0625) on most bays, unless calculations say I need more. That has worked well for me.

BTW, the only calculation I use is the area of a 0.25" diameter hole for each 100 cubic inches of volume.
 
Thanks for the info guys new to electronics. I know this is popular subject... but an important one. I was thinking of opening them up to .032 since I already have the 4 .024 holes in already.
 
I would open them to 1/16". That is still small enough that it won't be severely affected by cross winds and large enough to have plenty of flow.
 
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I have always been told not to use a even number of holes. You could get a cross wind right through the tube, And things may go boom on the pad.
 
In my scratch built I felt the holes seemed to small as well. I also found good information about having the hole diameter not be smaller than the wall thickness. So I figured wall thickness or calculated diameter whichever is larger.
 
In my scratch built I felt the holes seemed to small as well. I also found good information about having the hole diameter not be smaller than the wall thickness. So I figured wall thickness or calculated diameter whichever is larger.

That is interesting. Where did you come across the info? It makes sense to me, but that doesn't make it is correct. I would like to hear the engineering behind the wall thickness recommendation.
 
I found it odd that the RRC2+ user manual suggests much larger holes than other's, so does the rrc3 quick start guide, here is a cut and paste.


Vent Holes
Vent holes in avionics bay allow the altimeter to sample air pressure. Here are typical some 3-hole example sizes:
A. 38 mm by 6 in. 3 x 1/16” holes
B. 54 mm by 7 in. 3 x 1/8" holes
C. 3 in. by 9 in. 3 x 5/32” holes
D. 4 in. by 12 in. 3 x 3/16” holes
E. 6 in. by 18 in. 3 x 1/4"holes
Make sure when drilling holes they are clean and clear from fuzz and debris for smooth, clear airflow. Locate your vent holes
equally spaced around the avionics bay in a single circumference. If you want more information about proper venting sizes,
refer to the RRC3 User Manual section on Static Pressure Ports
 
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I found it odd that the RRC2+ user manual suggests much larger holes than other's, so does the rrc3 quick start guide, here is a cut and paste.


Vent Holes
Vent holes in avionics bay allow the altimeter to sample air pressure. Here are typical some 3-hole example sizes:
A. 38 mm by 6 in. 3 x 1/16” holes
B. 54 mm by 7 in. 3 x 1/8" holes
C. 3 in. by 9 in. 3 x 5/32” holes
D. 4 in. by 12 in. 3 x 3/16” holes
E. 6 in. by 18 in. 3 x 1/4"holes
Make sure when drilling holes they are clean and clear from fuzz and debris for smooth, clear airflow. Locate your vent holes
equally spaced around the avionics bay in a single circumference. If you want more information about proper venting sizes,
refer to the RRC3 User Manual section on Static Pressure Ports

This is the guide I use. Better a bit too big then too small, IMO.
 
At the risk of adding fuel to the fire, I'm in favor of the larger holes as Missile Works suggests. Smaller holes mean that the air pressure takes a little longer to equalize since air has mass and viscosity and it takes time to pass through the holes. This is both a good thing and a bad thing. On the positive side, it actually helps keep rapid fluctuations in air pressure from affecting the real-time baro readings, so the smaller holes act like a kind of low-pass filter. On the negative side, because of the same phenomena, the real-time baro readings may not be as accurate. With software filters, the hole size is probably not as relevant as it once was, back in the day when you had to set a "mach timer" to prevent premature deployments due to trans-sonic pressure buildup. I've tried all kinds of hole sizes while testing Eggtimers and have not seen any significant difference in deployment behavior or any evidence that the altitude readings are skewed one way or another due to hole size.

I think 3 holes is the optimum number. Having only one hole can theoretically cause pad deployments if you have "just the right" gust of wind, and two is almost as bad because you get a crossflow effect. With three holes you don't get the crossflow, and sudden pressure rises due to wind gusts will be equalized by the other two holes.
 
I find the recommendations interesting. Using the recommendation of a 0.25" diameter hole (0.049 sq in) area for each 100 cubic inches of volume would say the recommended 3 - 1/16" holes are almost twice as much as needed for the 38mm 6" bay, but the 3 1/4" holes in the 6" x 18" bay would be about 2/3 of the recommendation based on 0.25" holes per 100 cubic inches since 3 * 3 * pi x 18 = 509 cubic inches and would recommend 5 1/4" holes.

I don't mind a list of recommended sizes, but I would like to know how and why the sizes were selected to be on the list of recommended sizes. The list sizes are not linear in respect to the av-bay volume, which is OK, but I would like to know why and how those sizes were arrived at.
 
Just to let everyone know I did all lot of looking around and calculations and came up with a lot of different scenarios. But I decided to go with the 4 1/16 holes. I know three might have been better but I already had four holes in, and my sled and components wont let air blow straight through. But I did find some very useful info. and a port calculator that's downloadable on Off We Go Rocketry's web site. Gary who runs the business is in our Tripoli club in MN. and what is nice about this one is he explains the why some equations aren't as accurate. His spreadsheet came up with 4 .047 holes so I feel pretty confident on the .0625
holes. I suggest you guys check it out I don't know why it never came up in my searches. Thanks again for the info guys. I will let you know how it works.
 
I found it odd that the RRC2+ user manual suggests much larger holes than other's, so does the rrc3 quick start guide, here is a cut and paste.


Vent Holes
Vent holes in avionics bay allow the altimeter to sample air pressure. Here are typical some 3-hole example sizes:
A. 38 mm by 6 in. 3 x 1/16” holes
B. 54 mm by 7 in. 3 x 1/8" holes
C. 3 in. by 9 in. 3 x 5/32” holes
D. 4 in. by 12 in. 3 x 3/16” holes
E. 6 in. by 18 in. 3 x 1/4"holes
Make sure when drilling holes they are clean and clear from fuzz and debris for smooth, clear airflow. Locate your vent holes
equally spaced around the avionics bay in a single circumference. If you want more information about proper venting sizes,
refer to the RRC3 User Manual section on Static Pressure Ports

The list sizes are not linear in respect to the av-bay volume, which is OK, but I would like to know why and how those sizes were arrived at.


Well......................... I wrote the quick-start guide & the "other Jim" Amos wrote the manual.
How & Why...

I did almost 2 years of testing comprised of 38 fights of the RRC3 & 22 of the RRC2+.
Add into that over 200 more flights of all those exact size av-bays, which are the sizes that Wildman uses in his kits, & I use in scratch built rockets, many Rocketry Warehouse sizes were the same or close ay the time. Plus the fact other bays may be a bit longer or shorter. This would be a basic guide that will work for those not wanting to take the time and figure it out for themselves.

The vents holes were sized for each different bay after 5 yrs of testing. All this was done with the designer of MW altimeters [Jim Amos] on board. Things that were considered were:
Thickness of airframes.
Possibility of wires sleds, tape,rods, etc. partially blocking one or more holes constricting airflow. Internal volumes.
The partial blocking airflow if using "twist & tape" which runs the switch wire through one or two vents. [depending on use of 1 or 2 altimeters] The fact that on small bays with tiny holes many will make one hole larger so a "switch' wire can pass through. It's hard to try and figure in, every crazy thing flier's might come up with, and still make this guide cover most of it. It's never as simple as one size fits all...lol

The speed of flights depending on size, weight, and propellent used, can make evacuation of internal air fast or slow.
Most common size of bits found in "everyday" packages sold & used. Yes you can get any size if ordered or go to specialty tool shops, but most of us just have the common packages found at HD or Lowes etc.

Analyzing the graphs produced by RRC3's and"other" brands of altimeters that were flown side by side for comparison.

After all this Jim agreed the sizes chosen were close enough to his "formula" sizes that the chart would be the perfect compromise.

The fact of the matter is to get perfect port size holes, you must design them around a specific rocket going to a specific altitude range & know the humidity, temp, etc. with a specific volume in the bay, etc
Bottom line we tried to come up with something simple...reliable and works in a variety of situations & it does.

I'm sure there may be some inconsistencies in my explanations, but I am NOT a designer nor claim to be. I just tried to explain things in laymen's terms. For more exacting theory on how it was done , just call Missleworks and ask, but this is the gist of it.:grin:
 
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And if you contact the "other Jim" Amos from MissleWorks he will return your email, just use the contact email from the MissleWorks website.
 
And if you contact the "other Jim" Amos from MissleWorks he will return your email, just use the contact email from the MissleWorks website.
+1! Jim is a great guy to work with and obviously he has great products. So glad he recovered from the floods in his area a while back.
+2 to CJ for all his work in the field on this subject-much respect, brother!
 
Well......................... I wrote the quick-start guide & the "other Jim" Amos wrote the manual.
How & Why...
[............]
I'm sure there may be some inconsistencies in my explanations, but I am NOT a designer nor claim to be. I just tried to explain things in laymen's terms. For more exacting theory on how it was done , just call Missleworks and ask, but this is the gist of it.:grin:

Jim, thanks for taking the time to explain things. I think coming up with sizes based on successful testing and flight experience is as or more valid than any calculated formula. I was wondering if the non-linear vs. volume might have had something to do with wall thickness vs hole size vs flow disruptions or the pressure difference delay times, or some other heavy duty physics. Guess it's kind of like an experience marksmen and Kentucky windage can put you right on target just as well as all kinds of fancy calculation.
 
Well......................... I wrote the quick-start guide & the "other Jim" Amos wrote the manual.
How & Why...

I did almost 2 years of testing comprised of 38 fights of the RRC3 & 22 of the RRC2+.
Add into that over 200 more flights of all those exact size av-bays, which are the sizes that Wildman uses in his kits, & I use in scratch built rockets, many Rocketry Warehouse sizes were the same or close ay the time. Plus the fact other bays may be a bit longer or shorter. This would be a basic guide that will work for those not wanting to take the time and figure it out for themselves.

The vents holes were sized for each different bay after 5 yrs of testing. All this was done with the designer of MW altimeters [Jim Amos] on board. Things that were considered were:
Thickness of airframes.
Possibility of wires sleds, tape,rods, etc. partially blocking one or more holes constricting airflow. Internal volumes.
The partial blocking airflow if using "twist & tape" which runs the switch wire through one or two vents. [depending on use of 1 or 2 altimeters] The fact that on small bays with tiny holes many will make one hole larger so a "switch' wire can pass through. It's hard to try and figure in, every crazy thing flier's might come up with, and still make this guide cover most of it. It's never as simple as one size fits all...lol

The speed of flights depending on size, weight, and propellent used, can make evacuation of internal air fast or slow.
Most common size of bits found in "everyday" packages sold & used. Yes you can get any size if ordered or go to specialty tool shops, but most of us just have the common packages found at HD or Lowes etc.

Analyzing the graphs produced by RRC3's and"other" brands of altimeters that were flown side by side for comparison.

After all this Jim agreed the sizes chosen were close enough to his "formula" sizes that the chart would be the perfect compromise.

The fact of the matter is to get perfect port size holes, you must design them around a specific rocket going to a specific altitude range & know the humidity, temp, etc. with a specific volume in the bay, etc
Bottom line we tried to come up with something simple...reliable and works in a variety of situations & it does.

I'm sure there may be some inconsistencies in my explanations, but I am NOT a designer nor claim to be. I just tried to explain things in laymen's terms. For more exacting theory on how it was done , just call Missleworks and ask, but this is the gist of it.:grin:

Thanks for the info Jim! makes perfect sense.
 
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