This is part of it: “Falcon Heavy is one of those things that at first it sounded easy,” Musk said. “We’ll just take two first stages and use them as strap-on

This is part of it: “Falcon Heavy is one of those things that at first it sounded easy,” Musk said. “We’ll just take two first stages and use them as strap-on boosters. And like, actually no, this is crazy hard, and required a redesign of the center core, and a ton of additional hardware. It was actually shockingly difficult to go from a single core to a triple-core vehicle.” http://spacenews.com/spacex-to-launch-falcon-heavy-with-two-flight-proven-boosters-this-year/ http://spacenews.com/spacex-to-launch-falcon-heavy-with-two-...