Heavy Nose Cone HED Shear Pin # Question?

[PBic]

I’m looking for some advise on the number of nose cone shear pins to use for a rocket with a heavy nose cone in a HED configuration. It’s a 4” rocket with a 29” NC. The nose cone alone weighs 27.37oz, and the parachute, blanket, shock cord and quick link add another 22.22oz, for a total of 49.6oz. Do you think 3 ea. 2-56 nylon shear pins are going to be sufficient when the NC/AV bay hits the end of the booster shock cord? I’m planning on a 15” drogue at apogee with 2 or 3 shear pins in the booster. I’d really appreciate input from anyone that flies HED with a heavy NC, and have gone through pretty much every HED thread here. Thanks!

 

[Nytrunner]

Lets see, 49.6 oz is 3.1 lbs
3.1 lbs times 50 G'sof opening shock is 153 lbs
Each 2-56 nylon pin is good for ~30 lbs

Highly recommend upping to 4-40 nylon screws and using 4 of them

 

[mikec]

It's always hard to predict shock loads at apogee since so much depends on the elasticity of the shock cord material (kevlar loads are much higher than nylon but I have no hard numbers). I agree that maybe 3x2-56 is not enough for you and going up to 4-40 might not be a bad idea. But obviously ground test carefully to make sure you can break those when you want them to break.

 

[Handeman]

I'm not sure what you are asking. The apogee charge separates the rocket, but the nose cone is still attached to a payload tube? You need shear pins to keep the nosecone with HED attached after the apogee charge?

If everything is in HED doesn't that separate at apogee? If it does, what do you need shear pins for?

Confused.... I've never done HED if you haven't figured that out yet.

 

[mikec]

In HED the nose cone is attached to the top of the avbay and shear pins are needed to keep it from coming loose, just as they would be for a standard DD setup. For example, https://www.madcowrocketry.com/faqs#

 

[rfjustin]

3 or 4, 4-40's would be good IMO. Be sure to ground test.

 

[PBic]

Thanks very much Nytrunner, mikec and rfjustin. Glad I asked. Is 50 G’s a typical rule of thumb in this scenario?

 

[mikec]

Is 50 G’s a typical rule of thumb in this scenario?

There's really no way to know without flying an accelerometer with enough range and speed to properly sense it. Seems a little high to me; I've always used 3x 2-56 for my Punisher 3, but I did have an apogee deploy with an all-kevlar shock cord.

 

[PBic]

I'm not sure what you are asking. The apogee charge separates the rocket, but the nose cone is still attached to a payload tube? You need shear pins to keep the nosecone with HED attached after the apogee charge?

If everything is in HED doesn't that separate at apogee? If it does, what do you need shear pins for?

Confused.... I've never done HED if you haven't figured that out yet.

Hey Handeman, this thread does a nice job of covering it also - https://www.rocketryforum.com/threads/cjs-punisher-build.120615