Bonded Kevlar Shock Cord by the foot

Kevlar shock cord is incredibly strong for its weight and is heat resistant, so it makes a great shock cord in model rockets. Bonded Kevlar cord is treated to resist fraying, and is a bit stiffer and more slippery than other string, which makes it easier to work with and helps deploy your recovery device nicely. Price is by the foot. Thicker cord is stronger, ranging from 84 lbs. tensile strength, which is great for low power rockets, to 225 lbs tensile strength, which is better for mid power rockets. 

A minimum of 3 feet of shock cord is best for small rockets; larger rockets likely need more to ensure proper deployment of the recovery device. Longer shock cords also provide more shock absorbancy, though Kevlar cord stretches much less than elastic shock cords.  Many times, this will help reduce damage to body tubes from the nose cone bouncing back and hitting the body tube, but the sharper yank at deployment requires a sturdy shock cord mount. Can be combined with elastic shock cord to add shock resistance in heavier models. Just put the Kevlar closer to the motor so you benefit from its flame resistance, and add a length of elastic shock cord near the recovery device.