Shear pins for nose cones.

Space Oddity · May 26, 2015

What is the point of using shear pins to retain nose cones and body tube sections before ejection?

I can't find an answer on the web apart from on the Apogee advice site. I think they are saying that the purpose of the pins (rivets) is to retain the cone if there is a big pressure differential between the inside of the tube and ambient. That seems to be a logical answer but is it correct? If so, that is a very big pressure difference. I'm sure there must be another reason for using them?

Again, if that is the reason, I'm supposing that acceleration is a factor. At what point should shear pins be used? I can't think of any instance where they would be applied to low, medium, or "low level" high power rockets.

Would appreciate some basic advice on the reason for using them.

SO.

jeff2space · May 26, 2015

Shear pins are used on nose cone in dual deploy rockets. You don't want nose cone with main chute to deploy with Apogee charge or when rocket is falling until Main charge ignites.

ckjohnson · May 26, 2015

Shear pins are used for a couple of reasons, but primarily one. In a dual deploy rocket, the main parachute is typically in the forward section and ejects out with the nosecone. When the drogue section fires, it can cause the forward section to hit the end of the shock cord. The weight of the main parachute and support equipment pushing behind the nose cone can cause your nosecone to deploy at apogee. You use shear pins to keep the nose cone on, but still allow deployment with the ejection charge when the main charge fires by shearing the pins.
You can counter the pressure changes with vent holes, so your drogue section will have vent holes. You don't need shear pins on the drogue because it doesn't get the laundry pushing on it.

Unless you are going very high very quickly, vent holes in your drogue section and shear pins in your main section works great.

I hope that clears it up some.

jpoehlman · May 26, 2015

As for the pressure differential issues, I had a 3" LOC without a vent hole. Successful flights on I-140, but when I flew on an I-357 it had an early separation event at speed. Shock cord shredded at both ends and floated away with chute. Upper and lower sections came in at terminal velocity very near flight line!

I learned my lesson an pressure differential vs acceleration. The I-357 had a lot of kick off the pad! I should have pay more attention to the guy that asked about a vent on the way to the RSO!

Currently planning my first dual deploy. I will definitely use shear pins on the nose cone, and screws for the electronics bay to upper body section. As well as a vent hole on the lower section.

Space Oddity · May 27, 2015

Thanks for your clear answers. I guess I won't have to worry too much at level 1. Duel deployment is the next challenge.

SO.

Nathan · May 27, 2015

Shear pins are also used to prevent drag separation. Your rocket decelerates very quickly at motor burnout due to aerodynamic drag and this force can cause premature separation right after motor burnout. I'm not sure of the exact reason for this. It is either because there is more drag on the lower section than the top section, or because the top section is heavier and has more inertia so that it tends to decelerate more slowly from drag than the lower section. Maybe someone can clarify this.

Here is a video of my 4" diameter 10 lb rocket (without shear pins) drag separating at Red Glare this spring. This is with a CTI J381 Skidmark.

[video=youtube;2KDEqTTXIUk]https://www.youtube.com/watch?v=2KDEqTTXIUk[/video]

molten_dragon · May 27, 2015

Sort of a related question. Do you use vent holes and shear pins, or one or the other? Is there a certain altitude at which you'd want both?

I'm working on the rocket I intend to get my level 2 with, and if I launch it later using a larger motor, open rocket shows an altitude of roughly 10,000 feet. Assuming no vent holes, that's a pressure differential of roughly 4.6 PSI, which on a 3 inch rocket comes out to about 30 pounds of force.

I found estimates of the shear strength of #2-56 nylon screws between 30 and 45 pounds, so with 3 of them I should be more than covered and not need vent holes, right?

blackjack2564 · May 27, 2015

"so with 3 of them I should be more than covered and not need vent holes, right?"

Wrong.....you need vent holes to relieve the internal pressure, on the fin can & payload. You're tempting fate with-out them.

It's only a rocket.....what can go wrong? Lol

molten_dragon · May 28, 2015

blackjack2564 said:
Wrong.....you need vent holes to relieve the internal pressure, on the fin can & payload. You're tempting fate with-out them.

I'm not following the logic here. How is the pressure difference going to harm the fin can? I thought the whole purpose of vent holes was to equalize pressure in the body tube to prevent premature separation.

The only payload I'm launching is the dual deploy altimeter, which is vented obviously.

rharshberger · May 28, 2015

molten_dragon said:
I'm not following the logic here. How is the pressure difference going to harm the fin can? I thought the whole purpose of vent holes was to equalize pressure in the body tube to prevent premature separation.

The only payload I'm launching is the dual deploy altimeter, which is vented obviously.

The fin can needs vent holes to prevent pre-mature separation, just like the main parachute bay, pressure combined with drag could cause damage to fin can IF the velocity is to high at the time of the separation.

blackjack2564 · May 28, 2015

Not damage per se.....but the pressure build up [think ears popping when traveling up & down in mountains.] when traveling fast, under thrust, has to go somewhere.

That "somewhere" usually results in pushing the payload off the fin-can at or near motor burn out, resulting in a shred---zipper--or rocket tearing itself apart. A simple vent hole [1/8in. for rockets up to 3in diameter, it must also be sized like altimeter vents, but not so critical] allows pressure to bleed off in a timely fashion, preventing damage.

Just one of those things learned, when progressing up the high power ladder.

Edit: Look at the video in post #6... caused by pressure build up and forcing the parts apart when flying under full head of steam. Even if that was not the cause.....it's what happens or looks like, when vent is forgotten.

To Nathan.........
No one commented on the video.....did you have a pressure vent?

molten_dragon · May 28, 2015

blackjack2564 said:
Not damage per se.....but the pressure build up [think ears popping when traveling up & down in mountains.] when traveling fast, under thrust, has to go somewhere.

That "somewhere" usually results in pushing the payload off the fin-can at or near motor burn out, resulting in a shred---zipper--or rocket tearing itself apart. A simple vent hole [1/8in. for rockets up to 3in diameter, it must also be sized like altimeter vents, but not so critical] allows pressure to bleed off in a timely fashion, preventing damage.

Just one of those things learned, when progressing up the high power ladder.

Edit: Look at the video in post #6... caused by pressure build up and forcing the parts apart when flying under full head of steam. Even if that was not the cause.....it's what happens or looks like, when vent is forgotten.

To Nathan.........
No one commented on the video.....did you have a pressure vent?

Okay, I get the issue now, I wasn't clear enough.

There are 3 breaks in the rocket. Where the fin can connects to the av bay, where the av bay connects to the payload section, and where the payload section connects to the nose cone. It's designed to separate where the fin can connects to the av bay for the drogue, and where the nose cone connects to the payload section for the main chute. Both those places have 3 shear pins. The connection from the av bay to the payload section uses 3 steel screws to prevent it from separating there.

Shear pins for nose cones.

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