Shear pins for drogue AND main?

[neond7]

I was reading a build thread the other day, and the builder had installed shear pins on the drogue as well as the main. Is this commonly done?

I've build a few dual deploy rockets, but they haven't flown yet. I did install brass shims in the nose to help cut the shear pins, they worked great on the ground testing.

Are they also necessary on the drogue as well??

 

[mikec]

I was reading a build thread the other day, and the builder had installed shear pins on the drogue as well as the main. Is this commonly done?

Some people do it, I never have. People are worried about drag separation, but it takes unusual conditions (very draggy lower section and/or very heavy upper section) for drag sep to occur at this joint.

 

[COrocket]

Yes, they are useful for the drogue compartment as well. Assuming you are running a conventional setup, the drogue pins prevent drag separation. The main pins help prevent a main at apogee, by retaining the nosecone if the drogue event is a little more "energetic" than planned and the shock cord becomes taught.

 

[dixontj93060]

I install on both. I want my deployment sequence to be very (VERY) well-defined and deliberate.

 

[G_T]

Recently I was flying with a friend who launched a good sized N project. The drogue shear pins were left off... Well, it drag separated at fairly high speed, within a second of motor burnout. Everything was solidly built, so the only casualty was a drogue which was never recovered.

I have a fraction of the experience of many here, but I've always used shear pins at every joint. Personally I consider it a good practice. Are they always necessary? Certainly not! But whether they are necessary or not can depend on circumstances. How fast is it going to go on this launch? Was a little nose weight added this time to improve stability for a larger motor? With properly done shear pins there is no chance of a drag separation. Without shear pins being routine, one should assess their need for each flight. I prefer to stay on the side of certainty.

Gerald

 

[ClayD]

I do it both ways.
whuuups...

I shear pin for a reliable ejection force. if you take an assemble a rocket where the couplers and airframe have a failry consistant fit (how snug they are), the shear pins give an exact force for the charge to separate the two sections.
Friction fitting the couplers to hold for flight, is iffy, each flight will vary in the force required. (possibly, cause binding.)

The higher the dynamic forces in flight, the more the need for consistant retention such as shear pins, the lower the dynamics of the fligh, the more iffy, friction fit type stuff. THat might be right or wrong, but thats my rule of thumb.

 

[neond7]

If I decide to use shear pins on the drogue, do I need to add brass shims to the av-bay like I do on the nosecones? Is there a problem with the hole elongating? I have already coated the av-bay with CA and sanded it smooth. The body tubes have a 2x wrap of 6 oz fiberglass.

 

[Fdog]

When I started flying dual deploy, I used shear pins on both sections.

...Then, on the advice of an "old and crusty", I left off the pins in the Apogee union. Wasn't pretty!

Having been given a lesson with instant feedback, I now pin both sections. As dixontj93060 said:

I install on both. I want my deployment sequence to be very (VERY) well-defined and deliberate.

All the best, James

 

[neond7]

When I started flying dual deploy, I used shear pins on both sections.

...Then, on the advice of an "old and crusty", I left off the pins in the Apogee union. Wasn't pretty!

Having been given a lesson with instant feedback, I now pin both sections. As dixontj93060 said:



All the best, James

Did you use brass shims on the drogue?

 

[kandsrockets]

If I decide to use shear pins on the drogue, do I need to add brass shims to the av-bay like I do on the nosecones? Is there a problem with the hole elongating? I have already coated the av-bay with CA and sanded it smooth. The body tubes have a 2x wrap of 6 oz fiberglass.

Epoxy some fiberglass pieces to the inside of the coupler where the shear pins will go. This will keep the holes from elongating.

 

[Fdog]

Did you use brass shims on the drogue?

We have a two-headed "blessing" for living here in Reno: we are an hour and a half from the Black Rock Desert.

It's GREAT! because we get to fly at the Black Rock pretty much any time we want*.
It's terrible! because you get forced into flying mostly fiberglass.


So to answer your question, no brass - all my stuff is fiberglass, with drilled and tapped 2-56 holes for the shear pins.

All the best, James



* well, except for right now. It's a mud bog and frickin freezing to boot.

 

[Coop]

I considered leaving them off the drogue separation point--but figured it wouldn't hurt any to put them in and it might help. I could consistently replicate the separation during ground testing. I wasn't building for snot-slick airframe with an eye on an altitude record, so a little extra turbulence I could forgive myself. It worked in the air--way I figure it, that's what counts. I reinforced the area with CA soaked in... multiple (10+) ground-tests and a single flight, and the holes have held up...


Later!

--Coop

 

[Handeman]

I've only flow up to 4" diameter, although I've had +Mach flights, but I've never used shear pins on any joint. Never had a drag seperation or early deployment of the main. I just add tape to the joints until I can lift the rocket by the nose cone without any of the joints slipping. Has worked perfectly since I started DD flights 5 years ago.