Bake cake in a greased 9x13 pan at 350°F for 25–30 minutes; cool completely.
Cut cake into rocket body shape: one rounded top, tapered bottom. Reserve scraps for crumbs.
Crumb coat with 1 cup buttercream; chill 20 minutes.
Roll white fondant to 1/8" and cover cake smoothly. Trim edges.
Shape nose cone from extra cake scraps or fondant; attach with frosting.
Roll red and orange fondant into flame shapes; layer and press to look like flames.
Insert two paper towel tube supports into cake base for stability.
Add black and silver details with food coloring or edible pen.
Chill 30 minutes before serving.