Vent Hole Question

[calambert]

I have a 4" diameter L1 rocket (no AV bay) I've been flying that is not vented. I have not flown it higher than
2500 ft. so I have not experienced any problems. I'm planning to fly it on 2 larger motors that
will top out at 3500 and 4K ft. I'm also adding a jolly logic altimeter. I was thinking of adding 3
evenly spaced 1/8 or 3/16" vent holes about 4" below the nosecone shoulder to relieve pressure and also
receive accurate altimeter data. Does this sound about right? My concern is to not vent so much as
to lose ejection charge pressure, but at the same time vent enough for accurate altimeter data.

TIA

 

[rharshberger]

I have a 4" diameter L1 rocket (no AV bay) I've been flying that is not vented. I have not flown it higher than
2500 ft. so I have not experienced any problems. I'm planning to fly it on 2 larger motors that
will top out at 3500 and 4K ft. I'm also adding a jolly logic altimeter. I was thinking of adding 3
evenly spaced 3/8" vent holes about 4" below the nosecone shoulder to relieve pressure and also
receive accurate altimeter data. Does this sound about right? My concern is to not vent so much as
to lose ejection charge pressure, but at the same time vent enough for accurate altimeter data.

TIA

Those are really large holes, .186 or .250" ones would do the job since you are also dealing with an altimeter. For my payload bays I use a single .125 or .186" hole to vent pressure. Its also not a good idea to expose altimeters to BP residue or the pressure spike that goes with an ejection charge firing.

 

[calambert]

Thanks Rich. I made an error in my post. Meant to say 3/16 not 3/8. Was thinking of attaching altimeter to shock cord far enough back from nosecone to be wrapped in nomex next to the chute. So you think just one 3/16" vent hole is adequate?

 

[rharshberger]

Thanks Rich. I made an error in my post. Meant to say 3/16 not 3/8. Was thinking of attaching altimeter to shock cord far enough back from nosecone to be wrapped in nomex next to the chute. So you think just one 3/16" vent hole is adequate?

If using an altimeter I would go with 3 holes sized appropriately for the payload bay in question, a single hole is fine for pressure relief type venting but I am not sure how it will affect the quality of the data from the altimeter. The altimeter will still reports max altitude just fine with either setup.