Minimum Engagement into Nose Cone?

[Chris in Idaho]

This is a fiberglass 54mm airframe, and I will be using three shear pins. I will be trying head end dual deploy, so the less the coupler/Av bay extends into the nose cone, the more space there is for parachute and shock cord.

It seems to me with the nose cone solidly shouldered against the switch band and three shear pins holding it straight and concentric, there wouldn't be any need for extra length of the coupler slid into the nose cone. Am I missing anything?

I'm considering 3/4" of engagement, giving 3/8" on either side of the shear pin holes. Sound like it will work?

 

[James Keller]

The general rule I've heard and adhered to is 1 diameter. So on a 54mm diameter rocket you would have 54mm of engagement. That being said its a general rule and if your nosecone is a snug fit I wouldn't have an issue with that. Just make sure it doesn't wobble. And always ground test! Goodluck

 

[pathtouch]

I’ve done 1.75” on a 4” airframe. This was about the max I could push it in anyways. That’d be like 7/8” on a 54mm tube so go for it. I also went (3) 2-56 shear pins.

Was fine on a K695 and K550 so far.

 

[kjhambrick]

No argument on securing nose cones with short shoulders using shear pins here ...

But @pathtouch, where did you get that beautiful airframe and coupler material ???

I want some !

-- kjh

 

[Banzai88]

Conventional is 1 caliber, HED is whatever you can get without binding.

Shear pins as necessary.

No argument on securing nose cones with short shoulders using shear pins here ...

But @pathtouch, where did you get that beautiful airframe and coupler material ???

I want some !

-- kjh

Probably this

http://www.compositewarehouse.com/index.php?route=product/product&product_id=280

 

[kramer714]

my 3" fiber-goblin only engages 1.5 inches. I use (2) McMaster https://www.mcmaster.com/90136A530/ works fine,

If you need more room, go for a compact av bay, less than the length of the coupler. The avbay for the fiber-goblin is only 1-1/2 inch long leaving more room for HEDD.. The 'top of the av bay' is at the top of the switch band.

 

[pathtouch]

No argument on securing nose cones with short shoulders using shear pins here ...

But @pathtouch, where did you get that beautiful airframe and coupler material ???

I want some !

-- kjh

Believe it or not from Composite Warehouse. This was one of their Pink Ladies. I’ve gotta say, sounds like their business practices are all over the map but their components are top notch…. if you get them.

 

[OzHybrid]

There are competing requirements. The longer the shoulder engagement is in the body tube, the longer the force from the ejection charge has to provide thrust to the nosecone before it separates and the higher the velocity of the nosecone will be on separation for a given ejection charge size. The higher velocity of the nosecone will tend to allow the chutes to separate from it better-ish.
There is an engineering principle called "stick/slip" that states that for an object to slide, the length of that object should be greater than the distance between the slides. So for a piston ejection, the piston LENGTH should be at least the diameter. That's not completely true for a nosecone shoulder as the mass of the nosecone forward of the shoulder which is moving will tend to unstick it.

But simply test what you'll do, and do what you test.

 

[lakeroadster]

It truly depends on the fit. A nice snug fit... 1 times the diameter... sure. A sloppy fit... 2 times the diameter... sure.

 

[Back_at_it]

I know other will disagree but I've never had any issues with shorter shoulders even without shear pins. If space is an issue or weight, I've ben know to cut down the shoulders. On a couple of recent 4 inch builds I've cut the shoulder down to as little as two inches. Keep in mind that I do ensure that my cone is still tight in the body tube.

 

[3stoogesrocketry]

My 5 inch HED rocket uses 1.125 of engagement into the nose cone. The only determination on how short your coupler can be is how accurate are your tube/nose mating surfaces. If they are off 20 thousandths , your nose will wanna torque over. Shear pins are a must with HED in my opinion.

 

[Titan II]

On a few occasions I have done something similar to what kramer714 did as shown in post #6 where I recessed the upper bulkhed in the av bay coupler. I take a small section of coupler, about 1/4", take a very small section out, and glue it inside the av bay coupler. That acts as a stop for the upper bulkhead.