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Another AV Bay Vent Hole Question

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Lowpuller

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On my Dual Deploy Twitch I plan to run redundant altimeters and want to use the ones I currently own so I will be using my new Eggfinder TRS and my RRC2+.

My AV Bay is approximately 3"x 9", actual inside dimensions are 2 7/8" x 8 3/4".

Eggfinder calls say use 3,1/4" holes

Missile Works says use 3, 5/32" holes

I am also using 2 Missile Works PCB screw switches and will need two access holes.

Based on all of the above, I plan to use 3, 1/4 holes and use two of them for PCB screw switch access, that's a total of 3 1/4" holes.

Ideas, thought, suggestions please.......
 

Tonimus

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3 equally spaced 1/4" holes meets all your requirements.
 

rms

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Yep 3 1/4 holes will work fine as would 3 5/32 holes, either or, up to you
 

Bat-mite

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After you drill, line up your AV bay such that the rivets/screws/shear pins, etc., are not directly above the static ports. Off set them so that you get a clear airflow into the bay. Make sure you drill them cleanly with no "rough edges." I agree with all that 3 @ 1/4" is fine.
 

Lowpuller

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I have a problem. I drilled the rivet holes in my AV Bay to connect the payload tube to the AV Bay, then I built the sled with electronics. Big mistake, in hind sight, as the rivets now interfere with the electronics.

I have considered abandoning these as rivet holes and using them for my vent holes. This will allow me to relocate the rivets to a clear section on the AV Bay. However I need at least 2, 1/4" vent holes in the switch band for deployment charge screw switch access.

All this means I would wind up with 5, 1/4" vent holes in my 3" x 9" AV Bay that is crammed full of electronics including an Eggfinder and an RRC2+.

*****Will having this many vent holes or this much open area into the AV Bay be a problem?*****

I have pictures posted on the Dual Deploy Twitch Build Thread.
 

tfish

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use epoxy to fill the holes you don't need! Or fill all the holes and drill new holes where you want them.

Tony
 

Lowpuller

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tfish,

Thanks for the advice but that's not my question.

Here is what I am trying to get help answering.


*****Will having this many vent holes or this much open area into the AV Bay be a problem?*****
 

OverTheTop

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I am slightly nervous with that many at that size in the bay, but I have no data or experience to back up my feelings. Feel free to disregard this post!

You could cover the excess ones with round vinyl stickers. They are surprisingly resilient to Mach+. I went to Mach 1.8 with some fairly fine writing on a rocket and most of it survived. A 1/2" or 3/4" diameter circle would probably cover it.
 

tfish

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sorry about that.

simple answer is yes.

You would be going against what the manufacture of the eggfinder TRS recommends. Go back and read page 41 in the instruction manual. I would think that 5 - 1/4" vents would fit into this category. "If you make the ports too large, then crossflow air currents can disturb the airflow and your readings will be very “noisy” during the boost and coast phases when the rocket is moving at high speed."

Tony
 

markkoelsch

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Rivets are too bulky for me. I used them before seeing Pem nuts broaching nuts. Take a little more work to install, but work great.

In that size bay the number and size you have is excessive. Fill with some epoxy, sand smooth, and try again. Other option is to buy new components.
 

Lowpuller

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Thank you for the suggestion but to properly install PEM nuts the material has to deform. They were not designed for use in wood or fiberglass even if glued in place. They are also flat and you would be placing them on a curved surface creating even less contact with the "knurled" surface, this problem is exasperated the smaller the diameter of the tube. I know they are used commonly in rocketry and can be purchased from Missile Works but I don't believe they are correct for this application.

https://youtu.be/VmTZ0WD7ctc
 

markkoelsch

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You can argue if they work or not if you want. I am here to tell that I have used them down to 38 mm airframes with nothing but success.

You do not need a press- just the right size hole, a screw, and a plate to pull against. Again, it depends on proper selection and technique.

Before arguing further I suggest you get a few broaching nuts designed for use in fiberglass circuit boards, some scrap tubing, and try it.
 
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Graduator

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As Mark said, there are different types of broaching nuts for different materials and insertion methods. There are specific ones for fiberglass, with course splines such as these:
https://www.mcmaster.com/#captive-nuts/=14ybll1
And if you're that worried about the curved surface, just cut out a small patch of flat G10, epoxy it inside the tube as a backer plate and Bob's your uncle.
This method also works great for cardboard, phenolic and Quantum tube.
 

Lowpuller

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Mark,

I have a 3" body tube, and coupler, with a 1/4" hole already in both, what size PEM nut would you recommend?

Do you use a rubber washer under the screw/bolt?

Do you epoxy the PEM nut in place?

Where do you get you PEM nuts?

I really do appreciate the advice not wanting to be argumentative I view this as a teachable moment. I researched PEMs when I found them on the MW site.

Thank you.
 

markkoelsch

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https://www.mcmaster.com/#95117a444/=14ye90j

You would want the 8x32 as it is setup for the 1/4" hole. You may need to fill the body tube and redrill the holes though.

I press them into place. I have done this in a couple of ways. One involved using a screw through a piece of metal and pulling the nut into the tube.

I also did it with a pair of pliers one time.

I do put a fillet of JB Weld around it.
 
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