Shear Pins

[slipstick]

I had a situation where my ground tests of my 3″ diameter dual deploy rocket sheared both sets of the 3ea 2-56 pins on my drogue and main sections, but in the flight the main chute section failed to separate even though the charges were 20% larger. Fortunately the rocket was well built, the drogue sufficiently large enough and the snow was soft, so there was no damage.

FYI, the drogue was in a 3″ dia x 30″ long section which I placed (2) 3 grams of charge in. The main was in a 3″ dia x 10″ long section which I place (2) 1.2 Grams in. Two altimeters were used for redundancy. They were both set at 1000 feet for the mains. (I know, I know, I should have made one go off at 800 feet and put a kick-ass charge in it just in case).

I’ve done a post mortem analysis of the flight and have concluded that prior to the flight, I had tightened all the the 2-56 screws to full stop on final assembly before traveling, whilst the ground test had the heads of the screws above the airframe surface. Note: Both the coupler and airframe were tapped at the same time after I drilled the minor diameter holes.

This was obviosly a rookie mistake, as I inadvertantly added the clamping force of the coupler and the airframe to the shear force of the screws. The lowly charge I had tested with was not enough to overcome the additional amount of resistance between the tube assembly. I have not seen anyone address this issue before, but this situation would not have occured if I had used straight .062″ dia styrene rods. Its only a problem when using screws, as the clamping force is dependant on the amount of torque, whereas in straight shear you can get the same results almost every time.

Clamping forces could also have been avoided by following up with drilling a tight clearance hole matching the major diameter of the fastener on the inner or outer tube. I suggest drilling the slip fit clearance hole in the inside tube (coupler) to make it easier to remove the sheared screw shanks later.

In essence, leave the head of the screws sticking up into the airstream. 😳

[SCOTT EVANS]

Mike
Good to know I can learn from your mistakes. 😉 On my first pinned rocket (in the next couple of months) I plan on styrene rods now.
(ok so I already have the rods but its still good to know)

At least you know why you had your failure. Even though I used moter ejection, I had 2 matches hooked to the altimeter and neither lit. hmmm

scotte

[Warren B. MusselmanModerator]

I do NOT tap the airframe holes – only the nosecone or coupler. In addition, I usually put some talcum powder on the coupler and nose shoulder to lube things a little bit. Finally, since I generally fly altitude birds, I slice off the heads of the 2-56 or 4-40 shear pin screws. Learned all this the hard way including a flight much like yours Mike. I had my 4″ carbon bird come in from over 8K on a drogue with the main hung up due to insufficient charge. Again, like yours – threaded airframe, threaded nose and the compression of the screws increased the friction to the point that the charge barely had enough to blow off the nose. When I had ground tested, I didn’t tighten the screws all the way. When I flew – because I had pre-prepped the bird at home, I tightened the screws all the way thereby introducing a variable I had not accounted for.

Mike, I had assumed you certified successfully last Saturday. My sympathies on not cert’ing. I’ve been there – 4 times before I was successful. Just to be completely clear – if the bird flew with the intent that it dual deploy, it must successfully dual deploy with both chutes fully deployed at the respective, planned altitudes (apogee and whatever the altimeters were set for on the main) or else it is a DQ. That is clear in both Tripoli and NAR rules. The mere fact that the bird returned intact is not a clean certification flight.

Warren

[Bruce R. Schaefer]

Mike, I thought you had certed L2 Saturday. Warren’s right, if your main didn’t come out, then that should DQ you. Happened to me my first time, too. Landed flat with no damage, but because the chute stuck, I couldn’t get my L2 that day. Real bummer. However, whether you certed or not is among you and the two who signed your paperwork. It is none of my business. I wasn’t there or involved. Maybe you used a 3′ drogue, and that could count as a main. It’s just a standard rule that if you choose to use dual deploy, it must work. None of my business though.
I tapped my last bird, but was advised to NOT tap the holes to hold the shear pins. Reason being, if the head is sheared off in landing, you have to drill them out, which is what happened to me. AND the added force needed to overcome the compression of a well-tightened shear pin.

[Conway StevensParticipant]

I do NOT tap the airframe holes – only the nosecone or coupler. In addition, I usually put some talcum powder on the coupler and nose shoulder to lube things a little bit. Finally, since I generally fly altitude birds, I slice off the heads of the 2-56 or 4-40 shear pin screws. Learned all this the hard way including a flight much like yours Mike. I had my 4″ carbon bird come in from over 8K on a drogue with the main hung up due to insufficient charge. Again, like yours – threaded airframe, threaded nose and the compression of the screws increased the friction to the point that the charge barely had enough to blow off the nose. When I had ground tested, I didn’t tighten the screws all the way. When I flew – because I had pre-prepped the bird at home, I tightened the screws all the way thereby introducing a variable I had not accounted for.

I had assumed you certified L2 last Saturday. My sympathies on not cert’ing. Been there – 4 times before I was successful.

Warren

I to do NOT tap my shear pin holes either. Just a strait drilled hole in the correct size. It should be tight enough that your pin is good and tight and wont come out easy, but you can push the pin in via hand or with the help from a tool. I ALWAYS ground test as I would fly. So if I plan to have the pin in all the way and tight I make sure I have a charge that gets the job done and then add just a hair more. If im going higher Alt. I bump the charge up a full gram. (now again My airframes dont have many issues with larger charges beings they are composites like CWFG,ect and deal with the bigger charges better) When I do high Altitude I also cut the head off and flush to the body.

One thing I like about not tapping the holes for the shear pins is the removal afterwards. just push them out with a pick.

[Bruce R. Schaefer]

Now I remember where I heard to NOT tap…

[Conway StevensParticipant]

Now I remember where I heard to NOT tap…

😀

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