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Shock cord design
#1
This is a project I am been toying with for some time now and I am determined to prototype it this flying season.

We make shock cords ridiculously long and use threaded rods and quick links strong enough to tow a truck with our rocket recovery systems to deal with the dreaded SNAP and JERK if we every have a deployment event that uses all the shock cord length.  How inelegant.

If we can effectively dissapate the ejection charge created kinetic energy prior to the length of the shock cord, then will have a more reliabile system with less stress on the recovery attachment points.

So in the next few posts I will describe possible concepts that will absorb energy as the shock cord extends.
John Derimiggio
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#2
A remidy to this problem would be a god send. 
Jim Goggins NAR  L3
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#3
We are all ears John! 8)
Greg Young - L3
NAR #42065
TRA #00234
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#4
Ok, work and home got into the way of this blog.

part 1.  How much energy do we have to bleed off?

If we use shear pins (a good practice) we usually want to size the ejection charge so there is 100-120 pounds of force on the section to overcome about 50# of shear pin strength.  If you use too much shear pin then you have to generate more force which makes the energy dissapation task harder.

For a 10kg rocket and a 6" shoulder, this force will generate roughly 300 Joules of energy.  Now some of that will be consumed in breaking the pins so lets assume we have 200J of kinetic energy to bleed off.
John Derimiggio
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#5
Part 2. Methods to absorb this energy.

The 3 methods I will discuss are:
A. Masking tape sections on the shock cord ( popular method in rocketry)
B. Spring - damper
C. Sorbothane absorber in parallel with the shock cord.

A. Masking tape method.  In this method fold over sections of the shock cord with the fold overs secured with wrap(s) of masking tape.  As the shock cord extends the masking tape is sequentially ripped which consumes the ejection energy.  This is the easiest method but the disadvantage is trial and error is required in determining how much tape to use and how many sections are needed.  Who has time for that?

B. Spring and damper.  In this method you simply terminate one end of the shock cord with a spring.  When the rocket reaches the maximum separation the spring compresses and applied force gradually instead of a sudden snap.  The remain ejection energy goes into compressing the spring and you dissapate the rebound energy with foam.  This is the easiest to size but I have sized springs that will store 200J and they are beefy.  I have one and will try one but I estimate it weighs a pound.  You then have to construct something to house this apparatus and the end product in inelegantly heavy.

C. Sorbothane parallel damper. While researching materials to use for the damper with the spring method I looked to Sorbothane.  Its a specially formulated urethane designed to absorb energy for many industrial and consumer applications.  It is a very "gummy" rubber with very little rebound energy.  If you drop a ball or BB on a piece of it the ball will not bounce.  All the energy gets absorbed.  (The company that invented it and manufactures it is right in my backyard in Kent OH).

In my next post I will post a picture of the concept using this material.  You will then see how simple it is and you can make one with stuff you can buy at your local Home Depot.  (Except for the Sorbothane, but you can buy that from McMaster Carr rocketeer suppy company.

Here is info on Sorbothane
http://www.sorbothane.com/
John Derimiggio
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#6
I assume your sorbothane arrangement will have in parallel a length of shock cord that backs up the foam?
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#7
The shock cord connection is never broken.

                ^
              /    \
              /      \
            /        \            Shock cord goes through turnbuckle like hardware (through loops and each end
            /          \          In the triangular section is the sorbothane which gets compressed against the turnbuckle as
          /            \          the shock cord gets tensioned absorbing the energy.
_____/            \_______
0================0
John Derimiggio
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