This combo pack provides 3 each of the most popular Estes engines for use in most model rockets. The A8-3 is used for first flights and small flying fields as you get used to how your rocket will perform and where it will land in your flying field. The B6-4's will almost double your altitude. The C6-5's will provide almost double the altitude that the B6-4's can provide. Refer to the instructions or packaging that came with your model rocket for the engines you should be using. Estes model rocket engines are the standard in the model rocket industry for easy to use and reliable propulsion for your model rockets.