Ejection Charges/Canisters
Coming up on our first dual stage recovery flights this spring I started learning and preparing for altimeters, ejection charges, and everything else associated with a dual recovery.
What to use for an ejection canister has been something I've been thinking about and researching for weeks now. It seems many people have had good luck with plastic Micro Centrifuge Vials.
These are medical supplies and not the easiest thing to come by. You can find them on eBay quite regularly however. Pricing them on there I've found them to be about 15 to 16 bucks (including shipping) for anywhere from 20 to 100 vials. These are the 1.5 to 2cc vials. It so happens that 1cc in volume is basically equal to 1 gram of black powder.
While browsing a local store today I noticed something as I was walking out the door, shotgun wads. If you aren't familiar with these they are plastic cups basically that hold the shot inside of the shotgun shell, they also act as a buffering system from the blast of the gunpowder to keep from deforming the lead shot.
I began thinking rocketry of course and wondered if they'd work for ejection canisters. I decided to give it a try, at 6 bucks for 250 wads they're quite a bit cheaper than the centrifuge vials.
I wasn't quite sure by eyeballing which side I would need but guessed on 28 gauge wads. After having gotten them home I found that these would hold right at 3 grams of blackpowder, perfect for what I needed.
I have since ground tested them using both black powder and Hogden's Triple Seven powder. Both powders worked fine with my 4" BSD Horizon. The compartment pressurized for this rocket is 4"x12"
The Triple Seven only needed minor modifications. Triple Seven also is not regulated like black powder is and can be purchased and used by anybody.
Below are pictures and instructions on how I made my ejection canisters out of these shotgun wads. If you were to need larger canisters you could simply purchase 20 or 12 gauge wads, even as large as 10 gauge wads and have all the room you needed.
First here's a simple picture of what these 28 gauge shotgun wads look like.
You can see the cup at the top, the little plastic rods that come down from the cup are used to help cushion the blow from the gunpowder firing, as well as the base.
The first order of business is to cut off the rods which leaves you with the cup and the base.
The next thing I did was drill a small hole in the bottom of the cup for the ematch/igniter. Drill a hole large enough to accept the devise you are going to use.
The sides of the cup are petalled so that when leaving a shotgun barrel the wind will catch them. So the next order of business is a quick single wrap of one inch masking tape just to help the cup hold shape. You can see that the base is now going to be a nice cap when we get closer to being finished.
Primarily due to cost for testing purposes all I used were Estes igniters. Whether using those or ematches the next step is to place a 90 degree bend in the ematch and inserting the tip through the hole.
Next a couple inch piece of 1/2" masking tape was placed over the igniter/ematch and hole, and then wrapped down over the sides.
Another picture of the canister with the tape securing the ematch/igniter and then wrapped down over the sides.
Next it's time to fill the canister with black powder as shown above.
The base we cut off earlier is now placed on top of the powder and used as a cap.
Another couple inch piece of masking tape is applied to hold the cap down and in place.
And here is the completed ejection charge in it's canister. For black powder this is all that is needed, the taping again is just to keep things secure, when the canister is filled you can use pretty good force and press the tape down hard to the sides to get good adhesion.
Using Triple Seven
As was mentioned above I've successfully ground tested these canisters in my 4" BSD Horizon with Hogden's Triple Seven powder, an unregulated powder used in modern muzzle loaders that any of us can purchase at a local reloading or gun store.
This powder is also much easier to obtain than black powder, our local Wal-Mart carries this powder.
Below I will detail the couple of slight modifications that I had to make to get the Triple Seven to work. It is really not much different that what other's have already done to get Pyrodex P to work for ejection charges. I have read one report by Pratt Hobbies and the testing that they did, and performed my testing similarly.
First build your ejection canister in the same way that was detailed for black powder canisters above. This time however fill it with Triple Seven Powder instead.
Next take a 10" piece of normal everyday duct tape. Then tear the duct tape evenly into two strips, each 10" long.
Take one strip of the duct tape and wrap it completely around the sides of the canister using all of the tape. Press firmly to get good adhesion.
Take the leftover piece of duct tape and wrap it lengthwise around the canister until all of the tape is used. Again press firmly.
This is your completed assembly of a Triple Seven canister. Not knowing how long it takes to prep other ejection canisters I'm not sure how quick this process is.
I can make one in a couple minutes from scratch, however if you keep several of the canisters already partially ready in your range box, the rods cut off, hole already drilled, and the first wrap of masking tape already applied, igniter inserted and taped, it shouldn't take any longer than a minute to complete.
**NOTE**
On the Triple Seven Canisters make sure that you use the duct tape and wrap it on firmly, it needs the extra resistance to get a good burn, it's not hard we got it on our first time. And again ground test your stuff to see how much you need. 3 grams was overpowering in our test rocket, so test accordingly to see what you need.
Here are two completed canisters, Triple Seven on the left and black powder on the right. The addition of the duct tape to the Triple Seven canister makes the two types very easy to tell apart.
Again I've ground tested these and they work great for my rocket, I'll be ground testing in the next couple of days with my 4" BSD Thor to see what it takes. The payload compartment on that rocket is 4"x24" and I'll post how much of each powder is needed for it.
Update:
Both Triple Seven and 4F blackpowder worked fine in the Thor with 2.5 grams of either charge as long as they were prepped as described above.
Hope this helps....
Jason