Recovery webbing and shock cord are two very similar materials used to hold a rocket together after separation, and attach it to its chute(s). The functions of these differ slightly during a separation event.
The difference
During separation, the rocket parts will move in opposite directions until the end of the cordage is reached, and the parts are pulled back together, or the momentum from the separation is damped by the atmosphere (usually the former). Shock cord refers to cordage that is specifically designed to stretch to help dissipate some of the energy from deployment when used in conjunction with recovery webbing. Recovery webbing refers to cord that is not designed to stretch. Webbing will still stretch, however, and can cause the separated parts of the rocket to come back together and smash into each other, although this usually only occurs for very forceful deployments.
Usually, only recovery webbing is used because it's simpler to pack and easier to implement, so "webbing" and "shock cord" often refer just to recovery webbing.
Which cord do I use?
Below is a table matching types of webbing/shock cord. Each material has varying sizes and tested load limits. It's usually best practice to take the tested load of the cord and divide it by 100 to get the maximum rocket weight to fly with that cord. This is twofold: shock loads are difficult to characterize in flight (between drogue opening shock, and the shock when the cord runs out of length), and knots severely decrease the performance of the cord. The performance can probably be pushed beyond this rule of thumb, but
| Material | Kevlar | Tubular nylon | Paracord |
|---|---|---|---|
| Relative cost | Most expensive | More expensive as thickness increases | Cheaper |
| Notes | Highest strength:weight ratio - 1500lb test is commonly used for HPR. | REI climbing rope (1" tubular nylon) is standard for our competition rockets | A bit easier to damage with ejection charges, but light and cheap |
How do I attach it?
(Does this section belong elsewhere?)
Proper attachment of shock cord is key for all parts of the rocket - if it isn't attached, you will likely lose at least part of your rocket! Also, if the cord is being placed in the same section as an ejection charge, wrap it in Nomex.
Water knots can be used to create a loop at the end of the cord. They are performed as shown: (PICTURES)
Thicker cordage can also be sewn into loops, and subsequently heat-shrunk, as seen here: (PICTURES)
Cord is typically attached using quicklinks, attached to recovery hardware. It can also be tied directly to the rocket, but this is less favorable because the attachment point is either not removable, and/or the knot needs to be re-tied often during integration.