I've asked this before myself and after making a couple rockets like this (not enough to have great fact based evidence, just anecdotal opinion) I go with holes marginally bigger than the engine ID but smaller than the OD. For a 13 mm engine with a 10mm ID, two or three holes with surface area equal to 11-12mm.
@BABAR your mindsim vents look about the same as what I would've done and scaled in line similar to the original Twister.
I am trying to think of the alternative reasons (aside from a piston) for venting.
first I think of is for gap staging. I believe there was an actual sounding rocket that had the booster coupled by struts of girders to the sustainer. The actual bird presumably had electronic staging of the booster (I have no inside knowledge of how two stage sounding rockets actually worked.). More pertinent, there WAS I believe a two stage model rocket built on this design, and it was gap staged, and the gap (which I think was only an inch or two) was COMPLETELY open, with just 4 or 5 struts holding the two together. The key was that the struts held the sustainer and its motor directly in front of the booster motor. For practical purposes it was an infinite size vent.
My point here is that for staging vent placement rules are like Real Estate, Location, Location, Location. They can’t be too big. Obviously they CAN be too small. For 18 mm I usually use two standard notebook paper size hole punch holes, I put them on opposite sides on the theoretical chance that I put it on one side, it might, during staging, produce an asymmetric fore throwing the rocket off vertical. Probably just a rabbit’s foot. The LOCATION must be either just AT the tubing below the sustainer motor (for minimum diameter staging where the sustainer motor “nests” in the booster), or for non minimum diameter with a chimney or stuff “guide” tube, leave a 1/4” (or so) gap between the end of the extended motor mount and the sustainer motor nozzle, you can literally put the vents just about anywhere except the motor mount itself, you can even use “open” booster centering rings (holes or gaps in ALL the centering rings) and vent the gas BACKWARD put through the tail. This is both an aerodynamic as well as cosmetic solution, as there are no unsightly holes in your streamlined outer surface and
@neil_w. Doesn’t have to come up with a decal that says “Caution: Plasma Vent.”
second one I think of is cluster motors where you afraid if all the clusters ejections fire at once, you will overpressurize the tube and rupture the tube. I am curious if anyone has ever HAD this happen, I don’t fly m any clusters of this type, all except one have been staging clusters and this is not an issue. Here again, I don’t think you can make the hole TOO big, with exception for structural soundness and cosmetic and possibly aerodynamic compromise. Again I think an element solution is big gap (say 1/2” or larger) forward of the tube, some sort of flame resistant substance on the bulkhead directly in front (maybe JB Weld?), and vent the gases out the back between motor mounts.
third I can think of is outboard motors that are not ejected. These are likely best dealt with by plugged motors. (Ideally PURCHASED as plugged motors, I don’t want to start going down the “user plugged” motors rabbit hole, except to note that user plugged motors are at best strongly frowned upon if not outright refused at sanctioned NAR launches. I failed the Tripoli I.Q. Test, so I can’t speak for them
YMMV). If I WAS using retained outboard black powder motors, I’d definitely go with zero delay motors, no dedicated ejection charge (but still potentially significant force.). If not minimum diameter I would try to duct out the rear as for the clusters above. If minimum diameter, not sure, probably at least a total surface area of holes the size of the internal bore of the forward end of the motor, and LOTS of internal surface coating (again, maybe JB Weld?) as the inside would get repeated fried, assuming you have more than one successful flight!
the fourth and last case I can think of (there are likely more) is motor eject gliders, like I think the Hawk and Falcon. These out a hole up front presumably for PARTIAL venting, as the motor is intended to eject. For these, for 18 mm I always put at least two holes (standard hole punch size) one on each side, so there is no lateral force to break the boom. My suspicion however is that most broken booms on these are due to the motor ejection, which especially with Estes “Shotgun Blast” is a really big force. Because of the pylon, THIS force is off axis to the boom, and I suspect it forces the nose forward and downward, snapping the boom between the base of the pylon and the tail assembly. Just a guess.
interestingly I think
@Rktman quoted Rob Edwards as saying that the nose cone on his models INTENTIONALLY partially covered the hole in his motor eject gliders. Given his incredible series of easy to build and successful gliders, Mr. Edwards certainly should know.
custom r pack. I bought I think a 12 pack 10 years ago for around $10. Really like the nosecone. Salvage an extra body tube and the engine mount will work perfectly. We’ll see how it goes.
