Jeepers creepers...3 grams? Of BP?! On a 54mm airframe? Holy kah-blammo Batman. The 29/180 loads come with 1.5 grams and that is more than enough to push out a chute on my single deploy 30"x4" airframe. Heck, I have dual deploy bird with a main section about the same size (30"x4") that I use 2 grams on and it's enough to bust two shear pins AND push out the laundry.
-Dave
well, its not like your charging a bullet.! where too much BP will explode the case and all mayhem breaks loose exploding recievers and bolts screaming at you...
all you are doing is presurizing a volume of air.... to create "X" expansion.... causing a force equal to the force required for separation.
so.. lets take my sons darkstar (i use that cause people on here are familiar with that rocket all my others are scratch built.)
for the apogee charge... the charge is placed on the avbay at the top of the drouge complartment, where (2) 2-56 nylon screws act as shear pins..
it only takes a finite amount of force to shear these two pins, and over come the friction on the coupler that is present in flight(not much if the pins did thier job). This force again is finite and can be "proven" with testing.
Now.... that said the amount of BP is irrelevant to the force to cause separation.. (only that it is enough) - extra, is only vented out of the end of the airframe... just like a bullet no longer accelorates out of the barrel of a gun, neither would the main section of the rocket AFTER separation.
THAT is not saying, the extra expansion created during the shearpins shearing and the coupler being free of the airframe doesnt allow acceloration.. because it does. (back to the arguement on a longer drouge harness to shake off energy of exuberant charges, not saying the kevlar would break... but 120g's is rough on anything)... but, the extra BP doesnt cause problems with the "exploding the rocket" as some claim...if the rocket has a failure from the BP, its from somehting else failing before the force of the shear pins being sheared. (ever pulled a parachute out of a nose cone!!) or someone putting a charge that more resembles a bomb than a "flame front generator" which a charge should be to cause air expansion, not BP detonation.... You could use 10 grams in a 54mm darkstar... you might get some "flash" out of the end of the airframe... but wouldnt "BLOW UP"...if you did it correctly..
wheras - we allow the bp to generate a flame front causing expansion... not detonation...
(how to make a charge like that is another thread)
Likewise- if you have no air the only way the flame front can travel is through detonation!!! so if you are preping a rocket, and push out all the airspace. 2grams will exlode your rocket...
goto a cold air ejection....
my 4" birds all get 4 grams....