sanity check, seperation force?

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Rex R

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do these figures sound right? I have a competitor 3(3" dia.). I plan to fly with the nose pinned (2 ea. 2-56 nylon screws), booster section unpinned. my figures suggest that I will need approximately 87 lb.s of force to shear the pins and about #50 for the booster. oddly enough my calculations show that the same amount of bp (approx. 0.8 cm^3)will work for both charges. does that sound about right? thanks,
Rex
 

Bat-mite

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I get 0.77 for the payload, which agrees with you. Booster depends on where your top CR is, for determining volume. If the MMT is 22" long, and top CR is near the top of the MMT, then you might end up with about the same volume in each tube.

Assuming the booster is friction fitted to the coupler to prevent drag separation, then that is essentially the same as having shear pins. So if the volume is the same, the charges ought to be the same.

Hopefully you will ground test with 0.8 as a starting point. Ground test withe the rocket in ready-to-fly, fully loaded configuration. But for safety's sake, stuff rags in an empty motor casing and do not ground test with a motor in it.

Good luck and let us know how it goes!
 

Rex R

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since I didn't feel like running upstairs tape measure in hand, I added 2" to the fin root length and subtracted from the length of the booster to give me a length of 30"(figured that would give me a ballpark figure. this will be my 2nd DD bird...but yes I was going to ground test, nothing quite like not having the chute deploy to ruin your day. I've got a number of unused charge well caps that I can stick on an empty casing :). thanks
Rex
 

CzTeacherMan

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3" Competitor should take 2-3g in the booster and 1.5-2.5 in the payload. My 3"Darkstar flies with 2.0/2.0 in the payload and 2.5/3.5 in the booster. Only reason for same sized charges in payload is the fact that I use 2.0 centrifuge tubes. Use larger ones in the booster...
 

Bat-mite

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Wow, really? I rarely use more than 1g in a 3" rocket. Lots of variables, though.
 

blackjack2564

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Hey Rex,

In mine [stock] I used 1.5 grams BP in both. My back up charges are 1.5 apogee and 1.75 main.
I too use 2 shearpins. If I'm using a 6 or 6xl size motor, I drop the apogee charge down to 1 gram.
This has always worked flawlessly.
 

Rex R

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so my figures were on the low side but not totally wrong, that works. thanks CJ
Rex
 

watermelonman

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Remember ground separation is not the same as air separation, and leave a healthy margin.
 

Handeman

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The think with air separation on the apogee charge, you really don't need much. If you have a 12" or 18" drogue with 1.5x diameter shroud lines and attached 12" from the upper section, you only need 48" of separation to guarantee the drogue gets into the air stream. After that the difference in drag will pull any remaining shock cord out of the rocket.

Of course sizing the apogee charge for 48" or so of separation would leave zero margin for error which wouldn't be smart. But sizing it to force the two halves apart enough to hit the ends of the cord hard is also not smart IMHO. That is just asking for a nose cone coming off and a main deploy at apogee.

For apogee charges, I've been using 1g in my 3" x 15" booster and 1.5g in the 4" x 18" booster. Those might actually be a little higher then needed, but I'm sticking with them.
 

Rex R

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
 

watermelonman

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
I use hot glue when sitting down and taking the time to do a good job.

I have been known to use duct tape or even blue tape when lazy or playa engineering. I think I even flew a few times with no seal, though the way I arrange my recovery gear and charges there is virtually no threat to my electronics.
 

timbucktoo

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woferry

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
I don't pass any wiring through, I use these and these. Go Dog House! :)
 

RocketFeller

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3" Competitor should take 2-3g in the booster and 1.5-2.5 in the payload. My 3"Darkstar flies with 2.0/2.0 in the payload and 2.5/3.5 in the booster. Only reason for same sized charges in payload is the fact that I use 2.0 centrifuge tubes. Use larger ones in the booster...
Wow, that is a lot, almost as much as we used in our 12" rocket.

do these figures sound right? I have a competitor 3(3" dia.). I plan to fly with the nose pinned (2 ea. 2-56 nylon screws), booster section unpinned. my figures suggest that I will need approximately 87 lb.s of force to shear the pins and about #50 for the booster. oddly enough my calculations show that the same amount of bp (approx. 0.8 cm^3)will work for both charges. does that sound about right? thanks,
Rex

The 87 pounds needed for the shear pins seems a bit high. I don't have a shear pin calculator, but a friend has one and we used it to calculate shear pin separation force on our big Dragonfly. Six 4-40 nylon screws needed 270 pounds of force to sheer. It seems like two 4-40 pins would need 90 pounds, about the same as your figure for 2-56 pins.

As far as BP, we ended up using only 4G of FFFF for our ejection and our airframe volume is huge - the parachute compartment is 12.625" x 36".
 

Nathan

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
Plumbers putty
 

Bat-mite

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
5-min epoxy.
 

rharshberger

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
Rubber Well-nuts
 

Rex R

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I should have mentioned that my lids are aluminum (which limits my choices somewhat). thank you all for the ideas.
Rex
 

markkoelsch

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on a somewhat related topic...what does everyone use to seal wiring pass throughs in their av-bay lids? I hesitate to use aquarium cement as that tends to corrode wiring, maybe some hot glue?
Rex
Rex, I use silicone rtv. Works great.
 

Rex R

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room temperature vulcanizing silicone rubber caulk, got it. thanks
Rex
 

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