Shear Pins on MPR Rockets?

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bronicabill

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I am currently building my second AeroTech ARCAS HV kit and, as usual (for me anyway), have converted it to dual-deployment utilizing the included coupler as my avionics can, an Apogee e-bay sled kit upgraded to my particular needs, and a MissleWorks RCC3 dual-deployment altimeter. (Technically this is my third, as I built my second one a few months ago and totally screwed up my dual-deploy conversion due to not following known working methods, so decided to start over!!!)

What I'm wondering is if I should even consider 2-56 nylon shear pins on the upper section of this rocket to retain the nose cone until main chute deployment? My first version of this rocket, built nearly 15 years ago used friction fit, but I never felt 100% comfortable with that idea. (I must add that I only flew it once as dual-deploy before "life issues" forced the sale of it as I couldn't afford to fly it anymore.)

My biggest worry is the paper body tubes instead of FG or CF, and the need to reinforce any shear pin holes in the outer body so they don't eventually deform or rip out. Seeing as how this is only a 2.6" body tube, I don't think I'd need more than 2 shear pins max. Then... there is the additional need of a stronger BP charge to ensure the pins are sheared as needed... every single time!

Any input on this would be greatly appreciated! I've been flying model rockets for 50 years now, but practically all of them have been LPR. My ONLY venture into MPR was 12-15 years ago with my first AeroTech ARCAS kit!

Thanks!!!
 

Titan II

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I use them in all rockets, regardless of size and material. Use thin CA to harden the holes. One or two is fine in the rocket you mention. I frequently use one and do not buy the binding concern some have.
 

JoePfeiffer

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I use shear pins in all of my dual-deploy rockets; I find the nose cone has a tendency to wiggle out under drogue and deploy early if I don't, and shear pins are more consistent than trying to guess how much tape to use to get a good (but not too good) fit.

It seems like everybody uses 2-56. For my small rockets (both MPR and HPR), I find two 0-80s work. How heavy is your nose cone (and laundry), and what G forces are you expecting at drogue deploy?
 

bronicabill

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<<snip>>
It seems like everybody uses 2-56. For my small rockets (both MPR and HPR), I find two 0-80s work. How heavy is your nose cone (and laundry), and what G forces are you expecting at drogue deploy?
I haven't weighed the nose cone and laundry yet, and I have zero idea how to calculate the G forces from drogue deployment.

I just want to be sure those 2-56 screws DO actually shear off as required instead of leaving my nose cone stuck in place. If 2 screws will shear okay, then I'll probably go with that. Seems like only 1 screw would cause the nose cone to twist sideways during ejection due to one side being restrained temporarily while the other is free to move. If that's not the case, then I may go with just 1 screw instead!

If anyone uses just 1 shear screw and has had no problems with it, then please let me know. Thanks!
 

mhanna

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We have a guy at our field that uses two strips of masking tape instead of shear pins on his rockets. Definitely simpler, doesn't look as nice but I've never seen it fail to hold. He's done it on 2.6" to even a 5.5" short rocket.
 

teepot

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I didn't use shear pins on my first attempt at dual deploy and everything came out at apogee. Made for another mile long hike. I won't make that mistake again.
 
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