Vent holes...When/where are they needed

[Rocketry nut]

I have flown many rockets as high as 5000ft without vent holes. I understand the function of them, but when are they needed? Even my higher flying and faster HPR rockets flying over 3000ft have never used one. What's the biggest reason for needing one? Flights over X amount of feet, rocket diameter, speed, etc?

Also where is the best place to put one. Say in a DD rocket in the payload bay, would it be placed near the cone or further down?

So far I haven't had any problems, but as I continue to push my projects higher I want to know when I need one to prevent premature ejection.
Thanks in advance to any advice here.

 

[ben]

I don't have all that much experience with them but do make sure its not placed near the charge. Then you will vent the charge and no get a deployment. Part of it depends on how tight things are and how much air is in the deployment section. Also shear pins can help with this as friction fit can be pushed off a little easier than snapping off shear pins. I have then on my 7.5" Sandhawk that will go about 900mph and 12k feet.

Ben

 

[MarkM]

As a general rule you should always have vent holes! I'm surprised you haven't had a premature deployment. I've seen first time fliers forget the vent hole and they got premature deployment.

I don't know of a rule of under what conditions you should use vent holes since I've always used them. Although, the smaller the diameter and higher it goes, the risk of premature deployment increases without a vent hole. IMO, it's senseless not use a vent hole. It takes three seconds to drill one. Just use them, be done with it, and have security; there are enough other things you need to think out as you continue to advance in this hobby. This is not one of them.

I place my holes closer to the nose cone, but still a few inches away. There's really no hard/fast rule for where, but you do want them a distance away from any charge so the gases have time to pressurize the tube to pop the recovery system. Don't forget in a dual deploy rocket, you should use vent holes in both the main parachute bay AND the drogue. For the drogue your vent hole should be closer the fins since your deployment charge (from altimeter) is coming above.

 

[brianc]

My drogue vents double as hybrid N2O vents - I put in three that align
with the SRS motor lengths I own. The unused two equalize the pressure.

The payload vent is good at ground level; making it easier to push
the NC on.

 

[rocketlaw]

What is a good rule for sizing the vent holes. Any formulas anyone care to share?

 

[boris katan]

In a previous thread on this topic, a number of flyers stated they use one 1/8" hole for each chute / drogue compartment for 3" or 4" inch rockets. This is what I have done and it has worked for me.

Hole location should be close to the separation point, but not blocked by anything solid. For example, if the NC base slides 4" into a 4" tube, I would drill a 1/8" hole 5" from that end of the tube.

I would not worry about where your ejection charge is relative to the hole, as the charge is explosive and creates pressure much faster than a small hole can vent it.

I have a couple of zipper less dual deploy rockets. Having a small vent hole near the ejection charge has not been a problem.

 

[Necromancer]

yep - you need a vent hole.
i had plenty a flight which premature popped its nosecone before i was convinced of this!