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Oct 12, 2009
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I am looking for some information. Any one know what the G-force is when the parachute charge goes off on 1/2A to D engines?

My son is trying to write a program that will determine what strength shock cord is need with what engines.

I know this is trivial, but it is a college assignment.

Thanks In Advance
Andrew From Texas


Cyborg Rocketeer
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Jan 5, 2009
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Alliance, Ohio
There are several forces at play in the seconds surrounding that moment.

Are you asking for the force of the ejection charges for the various motor sizes? This probably isn't a significant force on the shock cord (so long as it doesn't burn) so much as a significant force on the airframe.

The forces on the shock cord arise from a) the weight/mass of the rocket, b) the size of the recovery device, and c) the speed of the rocket at deployment.

I've had rockets with an attachment point that was weak, but worked fine for several launches because the chute opened almost perfectly at apogee. The next launch was not so perfect and the rocket core sampled while the nose cone drifted away on a nice big parachute. The stress on the recovery system can vary a lot depending on these factors.

G-force is probably not the best measure for any of these forces but your son could still write such a program if he makes some assumptions (such as, the weight of the rocket is the maximum that can be lifted by a motor of a given size, the deployment speed is a maximum of x, etc.).


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May 9, 2009
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Are you asking for the force of the ejection charges for the various motor sizes? This probably isn't a significant force on the shock cord ...
What might be useful to talk about is the amount of pressure on the inside of the airframe, which translates to the force on the base of the nose cone and its acceleration away from the airframe. Then the nose cone is decelerated by the shock cord acting as a spring.

The pressure is a function of the volume of the pressurized part of the airframe, which in turn is a function of the size of the ejection charge. As a rule of thumb 0.006*V grams of BP will pressurize a volume of V cubic inches to 15 psi, and I would guess a typical A-C Estes motor has something like 0.5 grams of BP ejection charge (anybody have a better number?)

I'd have to say this problem isn't all that trivial; there's a lot of physics going on even in this oversimplified situation.

El Cheapo

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Jan 20, 2009
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Wow...that's ambitious. Biggest variables off the top of my head other than finding an average charge for each type of motor is BT length & diameter.