Lydia's Deal Elastic Cord 1/8 Inch (3MM) x 50 Yards White, Stretch Elastic String for Sewing, Crafts, Jewelry, Beading, Mask Ear Loop, Strong & Soft Elastic Roll

Did it just snap or did it get scorched/burned? If it just snapped upgrade your shock cord to something thicker and longer (something basic like this is plenty for that kind of rocket https://www.amazon.com/Elastic-String-Sewing-Crafts-Loop-White/dp/B0B9BRFW4M). If it shows any black burn marks add more wadding next time.