Shear pins with motor ejection

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Nathan

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I'm building an AMW FiberMAX. It will use motor ejection and a Jolly Logic Chute Release. Normally for a rocket like this I would put two #2-56 nylon shear pins on the nose cone. But since I'm just using motor ejection I would have no way to do a deployment test to verify that the ejection charge will shear the nylon shear pins. Should I just assume that the motor ejection charge will shear the pins and hope that it doesn't come down ballistic? I plan to fly it on both 38mm and 54mm motors.
 

dhbarr

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Just plug the motor tube with a dowel, no aft closure. Yay, it's an ejection charge test!
 

fyrfytr310

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Couple layers of cardboard taped over the MMT works too.
 

fyrwrxz

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Okay- first items first-RMS or DMS? Aerotech or CTI? You can actually add a little BP to the top of CTI by building a masking tape dam around the circumference to add a little on top of the existing charge. Aerotech has a generous well you can also *overfill* a little. First you need to find what pops the top and go from there. If you haven't drilled yet, look into downsizing your shear pins. Not claiming to be a expert, but a lot of times you can go smaller for more reliability. I had 2-56 bend and not shear on my Thumper and the crater was impressive. The 'extra' exhaust coming out the back would have fine if it was still going up. And yes- I added more bp for the motor deploy-shear pins were my downfall (no pun intended!)
 

Nathan

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At least initially, motors will be CTI Pro38 and Pro54.
 

wighty44

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I've built this rocket. My NC - BT fit was loose enough that I felt using tape was not the answer. To make a long story short I used two 4-40 nylon screws to secure the NC. The MMT is 54mm and the default CTI 54mm BP charge is 2.0g. My ground tests showed 2g was way too much BP (video on my FB page). I found that 1.3g BP was just right and with the two nylon screws and 25 ft of SC the ejection force worked well.
 

dixontj93060

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Yep, fill the fore closure hole with hot glue. Pour in your BP and seat an ematch in there at the top. Cover well with tape as normal and fire the ematch. Perfect ejection test.
 

Handeman

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So why do you think you need sheer pins with a motor eject?

Sheer pins are used to hold the nosecone on against the shock of the upper section hitting the end of the shock cord during apogee deployment on a dual deploy setup. Since there is no shock like that with motor eject, some vent holes in the BT and a little friction fit should be all you need.

Is there some other shock loading happening that requires the sheer pins?
 

Nathan

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Shear pins also prevent drag separation at motor burnout.
 

Handeman

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Shear pins also prevent drag separation at motor burnout.
This is true, but drag separation usually occurs when you have a weighted nose cone and light booster with large draggy fins. The drag on the fins has to slow the booster faster then the nose cone with the frontal drag of the rocket pushing it back. Most rockets can launch with a loose nose cone and it won't drag separate because of that frontal drag.

Just my opinion, but I think sheer pins are more trouble then they are worth in most motor eject rockets.
 

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