Calculating recovery system loads?

I want to make sure I select the right combinations of eyebolts/anchor, shock cord, swivels, and quicklinks (or snap swivels in small rockets).

What equation(s) should I use for calculating shock loads on the recovery components?

How do those shock loads relate to "test", "breaking," and "working" strength of the various components? (E.g., 125 lb test snapswivel, or 1200 llb breaking srength on 1/4 inch tubular Kevlar, or 1750 lb test on 1/2 inch flat nylon, or "600 lb" 1.5 inch swivel, or 660 lb rated 3/16 inch quicklink.)

Are there other factors I need to consider in calculating/choosing component strengths?

Setups I want to design for:

32 ounce, 2.6" paper airframe, 42" long, dual deploy capable, 36" octagonal chute, presumably ~20 foot Kevlar shock cords in each compartment; and/or
4.5 lb, 3" fiberglass airframe, 57" long, dual deploy capable, 45" or 50" octagonal chute; probably with 25 foot Kevlar and/or nylon shock cords.

I know I can go with the, "Well, John Doe uses x, y, or z in his rocket" approach, but I would like to be a little more analytical than that.

Thanks for your input.

I'm not sure how you want to calculate the load limit of your recovery, but for dual deployment, I would use the following to calculate the max load the system might see.

If you are flying drogueless, sooner or latter you'll get a flight where the upper section will lead the fin can down and the main will open well below the fin can. The fin can will fall past an almost stopped main chute and hit the end of the two shock cords. Calculate the force the fin can, with an empty motor case, will develop when it free falls the distance of both your shock cords. This should be the maximum force you should ever see on your recovery system, excluding deployments at high speeds which are going to be failure modes, much less predictable, and probably not something you can or want to design for. The longer the shock cords, the higher this number will be. If you use a drogue, then calculate the total weight of the rocket falling the length of the upper shock cord.

Just my :2:

Thank you, Handeman. That is the kind of info I am looking for.

What equation do you use to calculate that force?

You can use this page to calculate the energy. https://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html Just plug in your values in metric and divide the N by 4.45 to get lbs.

A 3 lb fin can falling to the end of 50 ft of shock cord will hit with about 1,000 lbs of force. Double the shock cord length to 100ft and the force goes up to about 2,000 lbs.
Double the weight to 6 lbs on the 50 ft cord and the force goes up to almost 1,500 lbs and 3,000 lbs on 100 ft of cord.

Wow. That's nothing to take for granted, even with a "small" rocket.

Thanks again. Very helpful insights.