After a three-year delay, SpaceX plans to fly its Falcon Heavy launch vehicle for the first time next spring, followed quickly by three additional flights of the 28-engine rocket by the end of 2016.

Lee Rosen, SpaceX’s vice president of mission and launch operations, laid out the ambitious schedule for the heavy-lift rocket during an appearance at the American Institute of Aeronautics and Astronautics’ Space 2015 conference in Pasadena earlier this week.

The first flight in April or May will be a demonstration mission of the new launch vehicle, which features three modified Falcon 9 cores with 27 engines as its first stage. Falcon Heavy is designed to lift more 53 metric tons (58.4 tons) into low Earth orbit.

SpaceX also has Falcon Heavy launches scheduled for the U.S. Air Force, Inmarsat and ViaSat in 2016. The Air Force mission will carry the Space Test Program 2 mission, which will test a number of satellites and technologies, and The Planetary Society’s LightSail-B spacecraft.

Falcon Heavy was announced in 2011 with an expected launch date of early 2013 from Vandenberg Air Force Base in California. Launch dates have repeatedly slipped and initial flights were moved to NASA’s Kennedy Space Center in Florida.

SpaceX is completing modifications to Launch Complex 39A, which formally hosted space shuttle and Apollo flights. The company also has built a new launch vehicle processing facility near the pad.

SpaceX also will use Launch Complex 39A for flights of its crewed Dragon spacecraft to the International Space Station.