Would a dog leash make for a good mid-power shock cord?
You are asking a complicated question.
A short answer is that a nylon dog leash is
unlikely to be a good solution for a mid power rocket.
Calculating Shock Cord Size and Lengths
A complete answer is a course of instruction in
fluid dynamics and
materials science.
A terse, insufficient, and disastrously imprecise internet discussion forum hand-wave might go something like this:
The "shock" that the cord is meant to absorb is not force, it is
rapidly changing force. Think of your dog running to the end of a slack leash while you stand still -- that
jerk on your arm that wrenches joints and snaps tendons is what the separated components of the rocket will experience when the shock cord is fully extended.
An elastic cord will reduce the shock loading (reduce the rate of change of the forces exerted to slow down the separated parts of the rocket after ejection). An elastic with some degree of
hysteresis (dampening) will also help to dissipate the energy of the ejection.
Making the shock cord longer gives fluid drag a larger distance over which to do work, dissipating energy. Since shock scales with mass, for rockets with large mass (the heavy rockets that can
only be launched on high power motors) it may be impractical to pack a long-enough shock cord for drag-dissipated shock reduction. Likewise, elastic cords may become friable* under large shock loads, so are liable to fail for heavy rockets. For heavy rockets, then, it is sometimes desirable to use an inelastic shock cord and harden the points of attachment against shock failure.
For high power applications, nylon webbing (tubular nylon is favorite) offers some advantages over round cords (notably, you can more easily make a couple of tape-wrapped z-folds to help "absorb" the shock -- and dissipate energy -- with flat straps than your can with round cords). It is also a little easier to manage when packing the recovery harness.
For a rocket that is light enough to go on a G motor, depending upon LOTS of other design factors, nylon webbing may not be necessary or even desirable.
*Friable seems like the wrong word. Maybe "brittle"? Not sure how to describe rupture under tensile shock.
