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Force for separation? Tools?

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tildenm

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Howdy,
Tragic case today postponing my level 2 of the dreaded zipper. It happened because the main chute bay was a bit too slippery and deployed the main at burnout. :mad: Yes, I know I should have tested the ejection, but not sure that would have seen it because it was too loose, not to tight.
I thought maybe if I use a spring scale and pull the model apart; using the max estimated drag as a bottom threshold for stages. Any thoughts?
Tilden
 

Chrisn

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Not sure if I have interpreted your post correctly or not.

Im guessing this is a standard duel deploy rocket?

So the nose cone has fallen off at apogee and pulled out your main parachute creating a zipper? Why don't you try using shear pins to hold it on? Either 1.5mm polystyrene rod or nylon screws depending on what you have available or the size of your rocket, More can be explained once I have an understanding of what has happened.
 

CCR

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A good rule of thumb for a friction fit is to hold the assembled rocket verticaly by the nose cone, wiggle it a little bit and see if it stays together. It should come apart if you then jerk it upward with some force. The shear pins are a great idea though and I prefer the use of styrene rod over the nylon screws because I have had the screws pull out without breaking and wedging between the coupler and body tube. Thin styrene, 1/16", always snaps cleanly. I have been using 3 pins on a 2" rocket lately with good success. I would recomend not using less than three pins to avoid the couple cocking in the tube.

CCR
 

MarkM

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I agree with CCR. Use 1/16" styrene rod for shears pins; easily available at most any hobby shop. I've used three "pins" of the styrene on everything from 3" rockets to my 6" L3. Cleanly shears every time.

I'm not a big fan of the nylon screws that many use for shear pins. It takes more force to shear these than the styrene and the styrene is easier and cheaper to acquire; one package will last a long time.
 
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Handeman

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Howdy,
Tragic case today postponing my level 2 of the dreaded zipper. It happened because the main chute bay was a bit too slippery and deployed the main at burnout. :mad: Yes, I know I should have tested the ejection, but not sure that would have seen it because it was too loose, not to tight.
I thought maybe if I use a spring scale and pull the model apart; using the max estimated drag as a bottom threshold for stages. Any thoughts?
Tilden
If the main deployed at burnout, I don't think it was faulty ejection that caused it, I think it was drag separation. The use of shear pins or something to increase the friction fit of the nose cone is definitely needed. The shear pin advice given here can help.

The other question I have is, do you have a vent hole in the payload sections to prevent pressure build up as the rocket gains altitude? That can also cause deployment and the combination of payload air pressure and fin can drag can combine to cause unwanted deployment at burnout.
 

blackjack2564

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How long is your drogue shock cord? An overzealous ejection charge can cause a short shock cord to snap tight and the Nose cone will fly off pulling out the main at the top. If your rocket is 5ft long the shock cord should be 20-25ft long to be safe [4-5 times length of rocket for drogue side], especially if deployment happens on the down side where the rocket will be moving at a good clip.
 

Diosces

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I have found that two 2-56 Nylon machine screws (McMaster Carr) work nicely as shear pins for 2.6" and 4" diameter rockets. Very slightly undersize the hole so the screw can be threaded on. Mind the length of the screw so it does not create a snag point for recovery lines..

Styrene is ok but iffy in that there's a chance of it falling out. I tried both and the nylon screws just give a more positive fit and confidence imho.

Whatever you use for shear pins make sure you GROUNDTEST with ejection charges.

Also keep in mind Handeman's comment regarding vent holes, this could have been part of your trouble.
 

shoitsma_86

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I tried using 2-56 nylon screws as shear pins on my EZI-65 in the nosecone, but I couldn't find the right amount of charge to use. I used up to 2.5grams (by volume) of pyrodex in the 17" payload section to push the nosecone off but it didn't shear the pins (i used 3 pins by the way). I was afraid to use any more powder for fear of blowing up the payload section. Maybe 3 pins is too much, but I spaced the holes 1/3 circumference around since I planned on using 3. I don't know if i could use 2 with that configuration or not.
 

Diosces

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I tried using 2-56 nylon screws as shear pins on my EZI-65 in the nosecone, but I couldn't find the right amount of charge to use. I used up to 2.5grams (by volume) of pyrodex in the 17" payload section to push the nosecone off but it didn't shear the pins (i used 3 pins by the way). I was afraid to use any more powder for fear of blowing up the payload section. Maybe 3 pins is too much, but I spaced the holes 1/3 circumference around since I planned on using 3. I don't know if i could use 2 with that configuration or not.
Two pins are plenty. I omitted the recommendation to glue a thin strip of brass (or other sharp metal)on the male end of the connection. Strip located at the shear pin hole to cleanly slice the pin (like a guillotine) when ejection charge fires. Ejection shear GUILLOTINE strip
Without the strip the cardboard gets all frayed and ovals out and then its hard to get even a large charge to properly shear the screw, prolly what happened in Shoit's case
 
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ben_ullman

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I tried using 2-56 nylon screws as shear pins on my EZI-65 in the nosecone, but I couldn't find the right amount of charge to use. I used up to 2.5grams (by volume) of pyrodex in the 17" payload section to push the nosecone off but it didn't shear the pins (i used 3 pins by the way). I was afraid to use any more powder for fear of blowing up the payload section. Maybe 3 pins is too much, but I spaced the holes 1/3 circumference around since I planned on using 3. I don't know if i could use 2 with that configuration or not.
Youd be suprised how much it takes to "blow up" a tube. My dad used 5 grams by weight in his L2. 3 #2-56 in a 3" x 18" payload.

Ben
 

mattvd

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Using the 1/16th styrene, any tips on how long to make the "pins"? Flush to the inside and outside of the airframe/coupler . . . or a bit longer?

Thanks.
 

CCR

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I would aim for flush on both sides. Never worried too much about exact length but I guess the laundry could hang on a pin long on the inside just never had it happen.

Scrath that about hanging up the chute. The pins inside the coupler. Duh.
 
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Chrisn

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Using the 1/16th styrene, any tips on how long to make the "pins"? Flush to the inside and outside of the airframe/coupler . . . or a bit longer?

Thanks.
the thickness of the coupler and airframe is the length the pins should be, make them about 1mm (about 0.04inches) longer to insure they go all the way through.
 

m85476585

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I make them longer than they need to be, and I flatten one end with a hot soldering iron so they won't fall in. They might still fall out, but not if the hole is a snug fit. Two should be plenty for a L2 rocket.

Also, you might not want to use shear pins for the drogue section. I had one come in ballistic when I did that despite a successful ground test, and as I was picking up the pieces a much more experienced flier told me to just friction fit the drogue section and make it tight enough so it will just support the weight of the rocket if you pick it up by the top. Of course, with more BP and less shear pins (I think I used four!), it probably would have been fine, and maybe more reliable than tape, which can easily get stuck.
 

Chrisn

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I make them longer than they need to be, and I flatten one end with a hot soldering iron so they won't fall in. They might still fall out, but not if the hole is a snug fit. Two should be plenty for a L2 rocket.
Yes, make sure the holes are drill true, CA the holes (make sure the coupler is separated or you will glue it to your airframe. Instead of a soldering iron (or if you don't have a butane one and want to do your prep at the field) get a lighter and heat up knife or piece of metal, and mushroom one end of the styrene rod on it- and presto.
 

cjl

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I tend to go rather heavily on the other end from the problem that most people have. My L2 had 6 (yes, 6) #4-40 nylon screws as shear pins on the nose, requiring upwards of 200 pounds of separation force (a good estimate for #4-40 nylon screws is about 40lb per screw, IIRC). I also used 4g of black powder (in a 4" by 24" tube) though, so it came apart quite effectively :)

If your model is single deploy though, your strategy sounds like it should work fine. I would probably add a bit of extra margin in too, just to be safe.
 
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