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shock load / force experienced at separation calculation ?

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ether

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aiming to build a release mechanism that would release a latch to allow a drouge chute to pull out a main. Want to test it on the ground by dropping weights from a pre-defined height to simulate the max shock load it would experience at separation.

The nose cone weight is 2lbs and the main airframe with motor case would be 15lbs, assuming a 25ft kevlar cord holding them together. recognize it would depend on how fast the NC is going before the cord ends. Wondering if anyone has done this type of calculation and could share the shock load i should design around. I would test it by dropping weights from a set height to replicate what it would experience.

Thanks
 

manixFan

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I think VCP (an old venerable rocket program) will calculate velocity of the nose cone based on the ejection charge. It’s made more difficult in that gravity also comes into play. But there are so many factors at play. For example, I had an issue lately caused by excessive weather cocking that ripped a fin off the rocket.

Ground testing using the parts involved with actual change would be great, but doing a drop test should be very useful. I’ve often thought about doing the same to test the ‘snap’ strength of Kevlar.

Good luck,

Tony
 

Bat-mite

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Are you using a deployment bag? If so, then there is very little shock on the pilot chute. All it has to do is keep the bag upright while the main spills out. Sounds like you have something else in mind?
 

ether

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Yes deployment bag, basically creating something to do what a tender decender does but without pyro
 

Steve Shannon

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I’ve seen accelerometer data that shows 30 gees of acceleration as a typical acceleration during deployment. An off- normal deployment could easily be 10 x that. Plan accordingly.
 

manixFan

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I’ve seen accelerometer data that shows 30 gees of acceleration as a typical acceleration during deployment. An off- normal deployment could easily be 10 x that. Plan accordingly.
Great point. I used to (ok, maybe still do a bit) subscribe to the ‘blow it up or blow it out theory’ and have maxed out the accelerometers on 50g chips. I think I’ve seen as high as 70, but I’d have to check. In fact I’ve snapped 200# Kevlar with a 2 lb rocket. So the forces can be far more than one might expect.


Tony
 

ether

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thanks very helpful, good thing is if it fails the result would be the main parachute being deployed at apogee - long walk but not a lawn dart. thinking making it robust enough, but also not going overboard with the charges
 

Kelly

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I’ve seen accelerometer data that shows 30 gees of acceleration as a typical acceleration during deployment.
Is this acceleration due to the ejection charge going off? Or, of the snap at the end of the tether when the component comes to a stop? Or, the when the chute deploys? Or something else?
 
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