Help with High Altitude Ejection Charges

I just finished building my 75mm minimum diameter rocket... which should go 25k-30k. The last snag that I have run into is the ejection charges. I'm not real sure what I need to do to make sure my charges work at altitude. I tried searching the forum without finding the direct answer that I was looking for. I have heard about surgical tubing, using pieces of plumbing, etc... I know the powder needs to be contained well so that it will burn fully. I was thinking about using an igniter tube from an AT load and capping the ends in epoxy? I just want to make sure whatever it is that I do is going to work!

Thanks!

Taylor- have you done any ground testing yet? How much (and what kind) charge do you need? Goex 4f is the preferred powder as substitutes need more volume. I use centrifuge tubes by drilling a small hole in the bottom and running my e-match thru and sealing with epoxy, hot glue,etc. pour in your measured amount and if there is any room left over, tamp some dogbarf in and snap the lid shut and tape it. I'd be glad to send you a handful if they are big enough.

Taylor, I have had great success using the surgical tubing method that Tony (tfish) came up with.

I use hot glue instead of zipties, it is a faster assembly. With these I've been to 25,000' without issue, and IIRC they've been to 100,000' and saved the day when other charges failed.






If you must use Pyrodex, I have found that using vinyl aquarium tubing is an adequate constraint to produce complete combustion of the powder. This however I would limit to fiberglass airframes because it produces a significantly sharper initial pressure curve (more of a "bang"). I have not tested it inside a pressure chamber, but, the aquarium tubing does not deform when pressurized to 25 psi - hence, the powder would still have complete containment even in a vacuum.




All the best, James

Thanks guys. I am using Goex 4f black powder. The surgical tubing looks simple enough... What size do you use? 1/4" ID seems like it would work.
I'm just getting ready to ground test - but it shouldn't take much powder since I'm pretty much just pressurizing the nosecone plus a couple inches of tubing haha

a ball park recommendation on tubing size and charge sizing for them.

for charges up to 2 grams 5/16" ID McMaster 5234K48

for charges 3-4 grams 3/8" ID McMaster 5234K99

for bigger or shorter charges 1/2" ID 5234K54

ground test before flying.

Tony

Okay Tony, that's pretty much what I needed to know. Always a good idea to ground test - it's not worth the risk in any way not to.

Thanks!