Estes 10031 F15-4 Bagged Rocket Engines with Starters and Plugs 2 Pack The Estes F15-4 engine delivers high-thrust power and long burn times, making it a reliable choice for mid-power rockets. With a 4-second delay, its optimized for heavier, single-stage rockets needing a smooth boost-to-deploy transition. Built for performance, the F15-4s extended 3.45-second thrust duration […]