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 food-safe cake dowels into the cake base for stability; remove them before serving.
Add black and silver details with food coloring or edible pen.
Chill 30 minutes before serving.