This combo pack provides 3 each of the most popular Estes engines for use in most model rockets. The B6-4 is a medium impulse engine that delivers high altitude flights. It works great for launching larger, heavier rockets, too. The B4-4 model engines which is suitable for use in rockets weighing roughly between 1 and 2 ounces which is typical of most rockets being 1 to 2 feet tall. The A8-3 is a single stage engine designed for model rocket flight. It is a standard engine designed for flights in rockets weighing less than 3 ounces, including the engine.