Ground Testing Deployment

[Bret PackardParticipant]

For my L2 flight I will be using a J350 (big surprise 😉 ). In the past, I have always used motor ejection, and will be for this flight as well. My question is, can I assume the BP charge included with the motor is sufficient or do I need to ground test it? It is a 4″ airframe so the chute and harness fit in it very loosely, as long as the nose cone pops it should deploy just fine. I have never purchased black powder before and really am not entirely sure what the best way to ground test it would be if that is really necessary. Also, is there some way to know how much BP comes with the J350? I’ve looked around on AT’s website but I can’t seem to find a specific answer. Thanks.

[Ed Dawson]

Batman,

I have been in the same spot, but with a slightly larger BT. I measured the BP included with the J350 and it was approx 1.5 grams (somewhere between 1.3 and 1.6)

Sorry, I can’t remember the exact amount – I wish I wrote it down.

I ended up adding more BP to fill the well completely. I am currently looking at using a paper tube to extend the forward well for motor based ejection on a 6″ diameter tube.

[Doug GerrardParticipant]

There is an excellent site at ROL http://info-central.org/recovery_powder.shtml that discusses black powder charges and how to calculate them. My gut instinct is that the 1.5 grams with the motor will be fine. For my 6″, 24″ chute compartment and using a piston I use 3 grams.

Doug

[Chris LaPanse]

For that rocket, you should be fine. My dad uses the full charge (and no more) with his PML endeavour, and it works fine (4″ with no piston, loose fit).

[Bruce R. Schaefer]

There is an excellent site at ROL http://info-central.org/recovery_powder.shtml that discusses black powder charges and how to calculate them.

Doug,

I use this calculator, too. I’m using 6-32’s for my nose cone holding the main… the calculator goes only to 4-40’s, so I’m taking a shot for 6-32’s doubling what blasts (4) 4-40’s. Does that sound right?

Also, because I couldn’t get the materials from PR, I’m not using pistons for my L3. Gonna blast and pray (after ground testing). 🙂

[Tim Thomas]

8) Bruce, stand back from the fence!! 🙂 8)

[Doug GerrardParticipant]

I use this calculator, too. I’m using 6-32’s for my nose cone holding the main… the calculator goes only to 4-40’s, so I’m taking a shot for 6-32’s doubling what blasts (4) 4-40’s. Does that sound right?

My experience with the calculator is that it errors on the consertative side. I too need to go to 6-32 screws and although it had been said here many times before, do the ground test. I would recommend doing a ground test with the charge the calculator recommends for four 4-40’s on the four 6-32’s. My guess would be that it will shear the 6-32’s no problem. Then fly with 1.5 times that amount to make you feel better.

Doug

P.S. I tested with half the powder it recommend to shear four 4-40’s and it still sheared them. But there are too many factors to take into account so please do the ground testing.

[Bruce R. Schaefer]

I would recommend doing a ground test with the charge the calculator recommends for four 4-40’s on the four 6-32’s. My guess would be that it will shear the 6-32’s no problem. Then fly with 1.5 times that amount to make you feel better.

Thanks, Doug. That’s good advice. And, I’m using a dummy chute until I find the working amount, THEN I’ll do a last test with the correct chute.

[Warren B. MusselmanModerator]

Why 6-32’s Bruce? I have 3 x 4-40 nylon screws and they hold things together just fine and surely shear when I do a deployment test.

Warren

[Bruce R. Schaefer]

Why 6-32’s Bruce?

Why? Paranoia! My nosecone has weight in it, and like the dart, it should be loaded with inertia/momentum. I just don’t want to take any chances. 4-40’s probably would have worked just fine. That’s what I originally had and what I’m using on the booster apogee separation. Conway used 6-32’s on his nosecone, and I think that’s the way to go in my case. The ground test will show a lot.

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