Vent hole question…

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JasonB

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Instead of putting the hole in the body tube could I put them in the motor mount rings and venting out the back of the rocket?
 

heada

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Yes but I wouldn't. You have "odd" pressures at the aft end of the rocket. Vastly different at motor ignition vs motor burn vs motor burn out vs coasting to apogee.
 

Bat-mite

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I always put mine just below (booster) or above (payload) the AV bay.
 

rocket_troy

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Instead of putting the hole in the body tube could I put them in the motor mount rings and venting out the back of the rocket?
If you were planning on going really fast, really quickly, then you can certainly consider venting out the base for internal pressurization equalization - there are some theoretical advantages to that. I would still vent out the side for barometric measuring though for the reasons Aaron mentioned.

TP
 

JasonB

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This was just for pressure equalization and not altimeter readings… doing a few launches this weekend, starting at about 3900 asl … launching approz 6400ft agl … using a jolly logic chute release, did not really want to drill a hole in the body tube. That is why i was thinking about going through the motor mount rings from the bottom.
 

Steve Shannon

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At the base of the rocket you are apt to have lower than ambient atmospheric pressure, and like Aaron said it could be in a turbulent area so it might wildly fluctuate.
That low pressure can help keep your rocket together, but the ejection charge must overcome the additional delta P. Of course, as velocity drops, the delta P probably becomes inconsequential.
As others have said, I wouldn’t complicate things by doing it.
 
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rdrown

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This was just for pressure equalization and not altimeter readings… doing a few launches this weekend, starting at about 3900 asl … launching approz 6400ft agl … using a jolly logic chute release, did not really want to drill a hole in the body tube. That is why i was thinking about going through the motor mount rings from the bottom.
The JLCR still needs accurate barometric readings before it is ejected. I would still use body tube vent holes.
 

Alby

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If you are doing dual deployment, would you (asking everybody on this thread) put in (2) two vent holes?
One hole above the AV bay to keep the upper section from popping off and the other hole below the AV bay
to keep the bottom/drogue section from coming off?
 

Titan II

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If you are doing dual deployment, would you (asking everybody on this thread) put in (2) two vent holes?
One hole above the AV bay to keep the upper section from popping off and the other hole below the AV bay
to keep the bottom/drogue section from coming off?
Yes
 

heada

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Every "bay" should be vented. Both parachute bays only need a small hole. I use 1/8" up to 4" diameter rockets. 3/16" up to 7.5" Your av-bay needs sensing holes more than vent holes but same idea. 3x 1/4" up to 4" and 3x 3/8" up to 7.5"
 

Alby

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Every "bay" should be vented. Both parachute bays only need a small hole. I use 1/8" up to 4" diameter rockets. 3/16" up to 7.5" Your av-bay needs sensing holes more than vent holes but same idea. 3x 1/4" up to 4" and 3x 3/8" up to 7.5"

This is good info since I'm building a 4" diameter rocket right now.
So 1/8" for Upper/Lower Parachute bays and 3x 1/4" for the AV Bay.

Thanks,
 

Titan II

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Recommend you do whatever is recommended in the altimeter manual for the av bay static pressure ports.
 

Alby

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Recommend you do whatever is recommended in the altimeter manual for the av bay static pressure ports.
I have a TeleMetrum and I don't see anything about AV Bay Static Pressure Port sizes in the manual.
This is all I really see on the topic:

"The barometric sensor sampling port must be able to “breathe”, both by not being covered by foam or tape or other materials that might directly block the hole on the top of the sensor, and also by having a suitable static vent to outside air."


Source: https://altusmetrum.org/AltOS/doc/altusmetrum.html
 

heada

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Recommend you do whatever is recommended in the altimeter manual for the av bay static pressure ports.
Indeed! I didn't mean to imply my method was the only, best or even correct way. Just that it is how I do it. You should always follow the manufacturer's directions.
 

Titan II

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I have a TeleMetrum and I don't see anything about AV Bay Static Pressure Port sizes in the manual.
This is all I really see on the topic:
"The barometric sensor sampling port must be able to “breathe”, both by not being covered by foam or tape or other materials that might directly block the hole on the top of the sensor, and also by having a suitable static vent to outside air."
Source: https://altusmetrum.org/AltOS/doc/altusmetrum.html
Understood, and surprised they do not mention it.
 

Conway Stevens

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There is a calculation thats recommended for static ports for your altimeters. Its based on the internal volume of the av bay. You need to know the length and the diameter of the inside to calculate the volume. I usually like to use 3 ports for my altimeters. These are from the Missleworks instructions. The barro sensor needs these ports. Most every one of the altimeters using a barro sensor will utilize the same port sizing as Missleworks does. The math involved is pretty universal across manufacturers.
 

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