Putting holes in recovery bay?

I hope it's not too much of a bother that I have 2 threads side by side

Currently I have a design featuring a single bay dual deployment, featuring a nosecone w/ payload at the top, then the recovery bay containing the parachutes, tenders, and a system (undecided) to pressurize the bay, with an avionics bay contained below featuring the flight computers and altimeters. The avionics bay has its own vent holes of course, but should the parachute bay have them as well?

The rocket is planned to fly to an altitude of 15+ km, and will have enough shear pins to (in theory) hold in the pressure difference of a sealed bay at that altitude, but is it better to be safe in that regard at the cost of potentially losing pressure during actuation? And if so, what are some rules to follow in regards to putting holes there and the reasoning for any specific numbers?

Yes, you need pressure bleed holes for your chute bay(s) as well. There can be a significant pressure applied as the rocket ascends. More force as the diameter increases.

I usually just use one hole per bay. Size depends on volume, ascent rate and NC retention .

OverTheTop said:

Yes, you need pressure bleed holes for your chute bay(s) as well. There can be a significant pressure applied as the rocket ascends. More force as the diameter increases.

I usually just use one hole per bay. Size depends on volume, ascent rate and NC retention .

Click to expand...

Alright, I understand I should still keep shear pins to deal with drag separation? How would I go about sizing the pins?

Another option is removable rivets...

https://www.apogeerockets.com/Building_Supplies/Misc_Hardware/Removable_Plastic_Rivets

steven soccer said:

Alright, I understand I should still keep shear pins to deal with drag separation? How would I go about sizing the pins?

Click to expand...

I would do some calcs based on things like 4-40 screw diameters, M3 fasteners or whatever you are going to use, based on cross-sectional area of the fastener. You can look up the shear failure pressure for the material (usually Nylon), work out the shear force needed to cut the pins/screws, multiply by the number of fasteners and translate that to a pressure based on the airframe diameter. There are charge calculators that give pressure from BP charges so you can work backwards to a charge quantity. Alternatively use the required pressure to size a CO2 system.

You could also do this empirically.

Don't forget to test on the ground, whatever way you come to your answers!

Or someone who is at a computer could share the shear pin calculator spread sheet ( it has been posted on TRF many times). If not I will do it as soon as I get back to my computer.

That's taking all the fun out of it .

I don't remember seeing that calculator.

Just a quick followup question: Where do I put the holes?

If I have just one hole, do I put it at the bottom of the bay so that air vents out easier, or at the top so that actuation is more reliable? Or just in the middle for a nice balance?

If I have multiple holes, do I arrange them in a circle around the bay (and if so, where on the bay), or in a line going down it?

steven soccer said:

Just a quick followup question: Where do I put the holes?

If I have just one hole, do I put it at the bottom of the bay so that air vents out easier, or at the top so that actuation is more reliable? Or just in the middle for a nice balance?

If I have multiple holes, do I arrange them in a circle around the bay (and if so, where on the bay), or in a line going down it?

Click to expand...

I usually put mine at the bottom and usually go with one. However when I do more than one I space them evenly around the circumference of the airframe. As far as the differences in location for a vent hole, I don't think it matters. I would say just keep it away from anything that may interfere both externally and when the bay is packed.