Premature Main Chute Ejection

[renisnceman]

I flew a large rocket yesterday on a K1275, and the NC and 10' main ejected at the same time as the booster and drogue. The technical detais are as follows. Dual deploy with redundant electronics (2 Adept 22's). Rocket height is 102" and weighed 38 lbs with motor. Booster had 48" drogue and 35' of tubular nylon. Used 2-56 shear pins. Nose cone was attached to a D-bag and came down under its own chute. NC also had 3 2-56 shear pins. Ground tested ejection charges (2 gms for drogue in 8"x8" payload bay, 3.5 gms main in 8" x 13" main payload bay). 10' main was attached to payload bay with 30' tubular nylon. We heard both the main and main back up charges fire at 1200' and 900' respectively-but this was after the main already ejected. Checked the wiring on both altimeters-nothing wrong there. One seasoned flyer suggested I should use a longer shock cord on the drogue and 4-40 shear pins on the NC instead of 2-56. Also, both of the Adepts beeped out similar altitudes. Thankfully it didn't drift far, but I'd like to get this worked out before I fly a bigger motor.

[COrocket]

Sounds like good advice. Longer shock cords and larger shear pins would help. The only thing i can think of is after the drogue fired the nosecone reached the end of its cord, sheared the pins, and kept going, pulling out your main. Do the Adept 22's support an apogee delay? You could try to slightly reduce the drogue charge and use a slightly larger backup charge to keep the drogue deployment a little less "energetic". 8" diameter tubes have a lot of surface area, so the PSI to shear the pins is pretty low. Or you could try switching to a smaller drogue. There could be a pretty good deceleration with a 48" parachute, and inertia could be pulling on the main/nosecone. A smaller drogue would lessen the stresses put on the recovery system/rocket during apogee deployment. Just a few thoughts.

[ECayemberg]

Definitely bigger shear pins. 35' of TN should be sufficient.

How heavy is the nose itself? The heavier the nose, the easier it is to jar itself loose.
How tight/loose is the nose cone shoulder-to-airframe fit? The looser the fit, despite the shear pins, the easier it is to jar itself loose.
When the nose is pinned in place, can you wiggle it, or is it non-moveable? If there is enough wiggle room, the nose can often produce enough motion to easily shear the pins.

Taping the tubular nylon together in a zig-zag fashion can also help ease the deployment, slowing down the separation speed at deployment, and therefore decreasing the force when the ends reach their limits and pull the TN taught.

-Eric-

[llickteig1]

I flew a large rocket yesterday on a K1275, and the NC and 10' main ejected at the same time as the booster and drogue. The technical detais are as follows. Dual deploy with redundant electronics (2 Adept 22's). Rocket height is 102" and weighed 38 lbs with motor. Booster had 48" drogue and 35' of tubular nylon. Used 2-56 shear pins. Nose cone was attached to a D-bag and came down under its own chute. NC also had 3 2-56 shear pins. Ground tested ejection charges (2 gms for drogue in 8"x8" payload bay, 3.5 gms main in 8" x 13" main payload bay). 10' main was attached to payload bay with 30' tubular nylon. We heard both the main and main back up charges fire at 1200' and 900' respectively-but this was after the main already ejected. Checked the wiring on both altimeters-nothing wrong there. One seasoned flyer suggested I should use a longer shock cord on the drogue and 4-40 shear pins on the NC instead of 2-56. Also, both of the Adepts beeped out similar altitudes. Thankfully it didn't drift far, but I'd like to get this worked out before I fly a bigger motor.

By the sounds of things, you're set-up pretty well. 30' and 35' of recovery harness should be plenty.

The only thing I can think of that I would do (and maybe you did it and did not mention it), would to be sure you have an adequate pressure relief hole in both the upper and lower airframe sections. Internal airframe pressure at atlitude could have been high enough that the force of the ejection cause the pins to shear and pop the nosecone due to the internal pressure. If you did that, more or larger shear pins would be my next test. Be sure to ground test again to make sure your charges are large enough for the new shear pin configuration.

I also do the zig-zag harness thing Eric mentioned on large, heavy rockets. It helps keep things in order until it all gets out into open air.

--Lance.

[fyrwrxz]

I flew a large rocket yesterday on a K1275, and the NC and 10' main ejected at the same time as the booster and drogue. The technical detais are as follows. Dual deploy with redundant electronics (2 Adept 22's). Rocket height is 102" and weighed 38 lbs with motor. Booster had 48" drogue and 35' of tubular nylon. Used 2-56 shear pins. Nose cone was attached to a D-bag and came down under its own chute. NC also had 3 2-56 shear pins. Ground tested ejection charges (2 gms for drogue in 8"x8" payload bay, 3.5 gms main in 8" x 13" main payload bay). 10' main was attached to payload bay with 30' tubular nylon. We heard both the main and main back up charges fire at 1200' and 900' respectively-but this was after the main already ejected. Checked the wiring on both altimeters-nothing wrong there. One seasoned flyer suggested I should use a longer shock cord on the drogue and 4-40 shear pins on the NC instead of 2-56. Also, both of the Adepts beeped out similar altitudes. Thankfully it didn't drift far, but I'd like to get this worked out before I fly a bigger motor.

If you flew at ROC I saw the flite and it looked as if it was a pressure separation. I believe the LCO even made a comment to the effect. Remember-your chute can plug that relief hole and make it worthless. Other than that- Nice flite on a good looking bird!

[Polyalcoholic]

Another possibility not mentioned...

The Adept 22 doesn't support an apogee delay. Although unlikely, it's possible that both apogee charges fired together giving an effective 4 gram separation charge.

[renisnceman]

Thanks for everyone's response. The Adepts don't have an apogee delay. Main Adept measured 1686, back up measured 1712. I would assume this means they both didn't fire at the same time, and it would indeed be rare even for two alts of the same brand to do so. However, I think the pressure relief hole on the main BT was way too small, there was certainly some play in the NC fit and moving to 4-40's sounds like sage advice, as well as zig-zag taping the shock cords. I'll make the changes, ground test and plan on flying it again at NARAM. Thanks again.

[llickteig1]

With an 8" airframe, consider going to 4 shear pins. Also, the pins don't have to be all at the same level. Put two down 1.5" - 2" from the joint and put two 4" - 5" down. That should help reduce any side-to-side movement or "the wiggles". You can take out some of the play with masking tape, too. Your pressure holes should be just below the nosecone shoulder and the coupler so the internals can't block them as easily.

Good luck.

--Lance.