Shock Cord is the type of cord used to connect parachutes and other parts of the recovery system to the rest of the rocket. It is usually made of Kevlar that has been braided together and is sold by the foot or meter and measured in load pounds, often represented with the # symbol (i.e. 1000# Shock Cord). Only Kevlar shock cord should be used for rocketry recovery systems. Nylon and other shock cords will melt or snap under the loads of deployment. 

Selecting Shock Cord

Once you have calculated the snatch force of your recovery system, you can start selecting shock cord. The pound rating of shock cord refers to its maximum loading with lashings, no knots, and ideal eyebolts. Since we are using knots and non-ideal eyebolts, we need to add a factor of safety. In the section below we go over tying shock cord and how much of the effectiveness of the shock cord strength is lost. The factor of safety for your shock cord can be defined as 1.25 times 1 over the of the loss of cord effectiveness of the weakest knot. So, a cord with a Palomar Knot on either end that needs to withstand a 1,000 pound shock force should be rated for at least (1000*1.25*(1/0.85)) 1,470 pounds of shock load. If a shock cord of that rating is too large for the application, smaller sizes can be used if they are destructively tested multiple times and still show a factor of safety. 

A common rule of thumb in hobby-level rocketry is to use a shock cord rated to 100 times the weight of the rocket, and the length of the shock cord between sections must be at least 2 to 5 times the length of the rocket. In theory, a lighter shock cord can be used if it is longer, since it gives more of an elastic dampening of the separation forces. 

Tying Shock Cord

Knots reduce the strength of rope or cord to as little as 5%, so the type of knot we use in recovery systems is very important. There are many different knots preferred by different people for different reasons, but a good knot to use for recovery systems is the Palomar Knot. It is a common climbing and fishing knot, that in destructive testing was found to maintain 85 to 89% of the cords original strength. A new development in the knot industry is the Fish-N-Fool knot, which was found in testing to get 100% or sometimes more than 100% of the effectiveness of the original cord. Since this does not make sense the public opinion is still out on the knot. It is still an interesting knot that you can use. If you do use it, use it with the Palomar's reported knot effectiveness.