Ejection Charge Ground Test

Yesterday I pretty much finished all the details that were left to finish on Stinger, so he was officially ready to fly. But I knew that I had to test the ejection charges, considering I had never used anything but motor ejection. The test was a success! The ejection charge system really was a success, but I learned two very important lessons (the kind that you want to learn while ground testing, not flying!):

  1. 2 grams of black powder is a bit much.

  2. 6 feet of shock cord is a bit short.
Somehow this resulted in a severed shock cord. This was 1" wide braided nylon! Yikes, I'm glad I found this out the easy way, a failure like that would cost me my certification:


The EGE said...

2 grams seems like a bit much. AT doesn't give that much for their 29mm HPR loads, and even then I use only half.

Also, the failure may have been more nylon burning than failing. How is it attached, and where did it break?

KenKzak said...

Powder quantity depends on airframe volume, which I don't know, but 2g is quite a bit.

6 ft of ANY shock strap is short in high power. It takes distance, thus length, for seperating components to decelerate before they hit the end.
I once had a 10foot x 1/2" tubular nylon strap comedown ripped halfway through. That strap was rated at 500 pounds. I changed to a 15 footer, and been good ever since, though an upgrade to 20 feet of tubular kevlar is in the offing. Lighter, longer, stronger, and easier to pack.

DTH Rocket said...

Yes, next time I'm going to use just under one gram of bp, use a 15 ft shock cord, and probably use more ejection wadding. Come to think of it, EGE's probably right, the nylon seems to be a bit singed where it broke. So I'm hoping the combination of those three changes will ensure a safe ejection.