Altimeters and pressure holes

[Jimbooregon]

Hi all,
If one doesn't have a separate payload bay for the altimeter and makes 1/16th inch holes for pressure equalizing is the escaping gas from the ejection charge minimal and not effect pushing out the chute?
Thanks for your response.
Cheers
James
Ayr Scotland

 

[Kane]

The pressure ports should not effect the ejection charge's ability to pressurize the airframe. That said, I rarely have more than one or two pressure ports. I'd be more concerned though about what's housing your altimeter if it's not in an avionics bay. Is this just a data recording altimeter or is it controlling the deployment charges?

 

[Alan R]

Hi all,
If one doesn't have a separate payload bay for the altimeter and makes 1/16th inch holes for pressure equalizing is the escaping gas from the ejection charge minimal and not effect pushing out the chute?
Thanks for your response.
Cheers
James
Ayr Scotland

I just put a couple of holes in all my airframes for altimeter. I'm just recording altitude and flight data not doing any dual deploy. Never had a problem.
Also, bigger air frames are going to need bigger holes. If you've got 3" or 4" body you might want to make those holes at least 1/8" - 1/4".
Somewhere around here there was actually a formula for pressure equalization by tube size.

 

[Jimbooregon]

The pressure ports should not effect the ejection charge's ability to pressurize the airframe. That said, I rarely have more than one or two pressure ports. I'd be more concerned though about what's housing your altimeter if it's not in an avionics bay. Is this just a data recording altimeter or is it controlling the deployment charges?

Just altitude and data.

 

[Jimbooregon]

I just put a couple of holes in all my airframes for altimeter. I'm just recording altitude and flight data not doing any dual deploy. Never had a problem.
Also, bigger air frames are going to need bigger holes. If you've got 3" or 4" body you might want to make those holes at least 1/8" - 1/4".
Somewhere around here there was actually a formula for pressure equalization by tube size.

Thanks. Helps.

 

[Ronz Rocketz]

This helps. Makes nice clear holes as long as you don't push too hard.

https://www.amazon.com/gp/product/B001DIG6D8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[OzHybrid]

There are 2 types of holes you need to make. Holes for your altimeter to be able to read the pressure correctly and a single small hole in any volume that will be sealed to prevent the pressure inside from pushing say the nosecone off as the altitude increases and the pressure externally drops.
There is plenty of information on sizing holes for altimeter bays. Make sure your barometer sensor cannot see direct sunlight coming in one of the holes and onto the barometer sensor element.

https://www.apogeerockets.com/education/downloads/Newsletter68.pdf
For volume pressure relief holes, pin holes are fine for small tubes, BT50 up to 6.4mm holes( as you're in a metric part of the world) for larger volumes.
Smaller nosecones would to some extent be protected from separation from the natural air pressure leakage of a reasonable-fit nosecone. That is, not sealed on with an O ring.....
Bear in mind that it also takes time for the rocket to reach an altitude. So the volume is constantly venting through the hole.
Hope this helps

 

[BEC]

I fly altimeters in almost everything. Many of these models have no separate bays and yes ejection gases leak out. But I've never had a failure to deploy that I can attribute to this.

Sizing the holes can be simple or complicated. Mainly they need to big enough that the pressure where the altimeter is tracks what's going on outside closely enough for you to get the data you want accounting for how quickly the rocket changes altitude. This is discussed at some length here: https://www.apogeerockets.com/education/downloads/Newsletter543.pdf.

None of the images in that article show streaks of soot below the holes from the ejection charge going out the static ports, but I have models that show them.