Shear pins - when to use?

billdz said:

Should the shear pins be used if the flight is single deploy?

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Generally No. Shear pins are used with DD to hold a nose cone on a payload when they hit the end of the shock cord after a vigorous apogee ejection. As long as you can pick up the rocket by the nose cone and give it a slight shake without the nose cone coming off, you should be able to fly almost anything single deploy without a problem. If you have a heavily weighted nose cone and large draggy fins you might consider shear pins on a single deploy but I would consider the need to be a rare occurrence.

I have a LOC Iris and a K&S Arctic Wolf. Both rockets use heavy cardboard tubes and are about 3 inches in diameter. I tried shear pins once on the Iris and I did not get a nose cone separation on that flight. The shear pins did not shear, but sort of bent over and held the nose on. In hindsight I did not ground test enough. I also had damage where the shear pins ripped out the cardboard during ground testing.

I now use a different method to keep the nose on that I found on this forum. I fold over the shock cord a few inches and wrap it in tape. I flew the Iris this Saturday and had 16 taped loops in the shock cord. After the successful flight I saw that 12 of the taped loops had snapped, leaving 4 intact. The taped loops act as shock absorbers as each one absorbs some of the energy and they keep the shock cord from snapping hard when fully extended. I have used this method 8 times between the two rockets without a problem.

Thanks for the replies, lots of different ideas.

What about air pressure change holes to prevent drag separation? I've read that they should be drilled. I'm guessing they also impact the shear pin calculation, since some of the pressure will vent through the holes, which means less force on the NC. Or are the holes too small for this to be significant?

Vent holes are not for drag separation. Shear pins are. Vent holes are to keep the NC from popping off due to pressure differential between atmosphere & payload. I use one 1/8” hole on all my rockets, both payload & booster. Some will say if you have shear pins, you don’t need vent hole.
I drill my vent holes during build but I don’t think they will have much impact if any on force required to shear the shear pins.

billdz said:

Thanks for the replies, lots of different ideas.

What about air pressure change holes to prevent drag separation? I've read that they should be drilled. I'm guessing they also impact the shear pin calculation, since some of the pressure will vent through the holes, which means less force on the NC. Or are the holes too small for this to be significant?

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Calculations only give you a starting point for your testing. No calculator that I know of is designed to take in every possible detail that makes no two rockets exactly alike. Calculate, then test and adjust.

Zeus-cat said:

I now use a different method to keep the nose on that I found on this forum. I fold over the shock cord a few inches and wrap it in tape. I flew the Iris this Saturday and had 16 taped loops in the shock cord. After the successful flight I saw that 12 of the taped loops had snapped, leaving 4 intact. The taped loops act as shock absorbers as each one absorbs some of the energy and they keep the shock cord from snapping hard when fully extended. I have used this method 8 times between the two rockets without a problem.

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I love z-folding and its benefits, but I'm totally blanking on how it keeps the nose on.

Now I'm curious

Nytrunner said:

I love z-folding and its benefits, but I'm totally blanking on how it keeps the nose on.

Now I'm curious

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I think he's assuming that the amount of inertia on the nose, while perhaps significant enough to get premature ejection, might not be significant enough to break all the tape. If so, this is unsound reasoning. Yes to Z-folds lessening cord shock, not to using it as a NC retention method.

Nytrunner said:

I love z-folding and its benefits, but I'm totally blanking on how it keeps the nose on.

Now I'm curious

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Bat-mite said:

I think he's assuming that the amount of inertia on the nose, while perhaps significant enough to get premature ejection, might not be significant enough to break all the tape. If so, this is unsound reasoning. Yes to Z-folds lessening cord shock, not to using it as a NC retention method.

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Although he didn't state it, I believe the z-folds are on the apogee shock cord. It reduces or eliminates the shock when the payload hits the end of the cord so there is little inertia to make the nose cone come off or the main chute to push it off. With the reduced shock, a few wraps of tape on the nose cone shoulder will provide sufficient friction to keep the nose cone on during apogee deployment.

Yes, the Z folds are on the apogee cord. I assumed that would be obvious as they would contribute nothing if on the nose cord. I do make sure the nose has a good friction fit.

I use shear pins on all HPR flights. I z-fold all harnesses.