Large painting stencils are easy to make with a printer, painters tape, and spray adhesive.

Lay long strips of tape down the rocket, including well beyond the area's you want to paint to combat overspray.

Trim pages to minimize paper overlap, and gently tape them together using the guidelines for alignment.

Lay the paper template onto the rocket and adjust till properly aligned. Use a sharpie to make alignment marks on all sides of the paper. Take the paper and spray the back with spray adhesive following instructions. Stick the paper to the tape according to the previous markings.

Use a scalpel and flexible straight edge to cut out the letters.

Paint, peel, and enjoy your sharp pain lines.
Notes:
If you degrease or otherwise prepare rocket surface for painting, do it before applying the tape and use gloves when cutting out the letters to prevent contamination.
Make sure your tape is pressed down prior to painting.