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sunderll

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Hi all, I'd like to get an idea of how much XXXX to use for my 7.5" rocket. I'd like to find out what everyone thinks the correct amount for a 10.5" long chamber and of course 7.5" diameter. My calculations come up with 2.5 grams, but that's pretty potent in the last few launches. I'm backing down to 2 grams next launch. The coupling fit is about average in resistance. Thanks for any replies!!
 

rharshberger

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Hi all, I'd like to get an idea of how much XXXX to use for my 7.5" rocket. I'd like to find out what everyone thinks the correct amount for a 10.5" long chamber and of course 7.5" diameter. My calculations come up with 2.5 grams, but that's pretty potent in the last few launches. I'm backing down to 2 grams next launch. The coupling fit is about average in resistance. Thanks for any replies!!
Have you ground tested?
 

timbucktoo

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Bat-mite

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Have you ground tested?
You really need to test on the ground. You want good separation that gets the chute completely out of the payload bay, but is not so forceful as to put shock on the harness or the attachment points.

Start with a calculator, then work your way up and down from there.

What I do is get a couple of pieces of wire (spent igniters have tons of wire available). Wire them directly to your charge leads inside the coupler, then pull the ends out through a static port. Attach your controller to the wires and let her rip.

No two rockets are exactly the same, and thus the calculated charge size is a starting point. The size of your chute, blanket, length of harness, pressure vent hole(s), etc., all factor into how much pressure is needed to get good separation. A loose chute will require more pressure than a tight chute, unless it is too tight, in which case even more pressure may be needed to dislodge it.

Make sure to stuff the MMT with something when you test the drogue charge, or else you lose a ton of pressure out the back. Rocket should be in flight-ready condition for every test.
 

cwbullet

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I use calculators as a point of reference, but I always ground test. If the calculator says 2 grams, I start testing at 1.5 grams and work my way up to result I want at 0.25 to 0.5 gram increases.
 

RocketFeller

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I agree with the ground testing. We initially thought we would need twelve grams for a 12.625" x 30" compartment. In the end we only needed four grams.
 
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