But since that first glimpse in 2011, the Falcon Heavy has been in development hell. An in-flight explosion of a Falcon 9 in 2015—followed by a fuel tank explosion in 2016 that destroyed the rocket, a customer satellite, and the launch pad—set the company back months, if not years. SpaceX bounced back in 2017, completing 18 missions, including 14 booster recoveries and 5 reflights of a previously-flown rocket. But no Falcon Heavy just yet.

SpaceX had hoped to cap 2017 with the debut launch of its heavy-lift vehicle. The company would need the Falcon Heavy to make good on a promised mission: An ambitious attempt to launch two private citizens to lunar orbit in late 2018. At first, SpaceX sold it as a glorious return to the moon for the first time since the final Apollo mission in 1972. Months later, Musk would joke that those two individuals are “brave” for flying on the mission—aboard a still-untested Crew Dragon spacecraft.

SpaceX originally had another mission planned for Falcon Heavy in 2018, too: Red Dragon. A variant of the Crew Dragon, these mission concepts would launch to Mars on supply drops, capitalizing on the Hohmann transfer window that opens every 26 months, shortening the trip between Mars and Earth. SpaceX sold Falcon Heavy as a regularly-scheduled train, establishing a cargo route between the two planets. But even with interest from NASA, last summer Musk announced that SpaceX had abandoned the concept, diverting resources into what the company describes as multipurpose interplanetary spacecraft.

Despite these big plans, though, it was only in March of last year that the massive rocket began to take shape at SpaceX headquarters in Hawthorne, California. First, it was just a full-sized mockup of the rocket’s center booster, without engines. SpaceX shipped the massive piece of hardware to its facility in McGregor, Texas for structural load tests—the ones that would show if the vehicle could actually handle the weight it was designed to lift.

Then SpaceX geared up to build the real center booster on the factory line back in Hawthorne—the very same that will fire up in next week’s demonstration flight. As for the Falcon Heavy’s side-boosters, SpaceX decided that it would use previously-flown Falcon 9 rockets.

That choice was cockier than it seems. SpaceX had chosen to use reusable rockets as early as September 2016—before it had ever successfully launched a used booster. (The company accomplished that feat for the first time last March.) And since then, the company has only grown more confident in its recovered fleet of rockets. WIRED learned from sources with knowledge of the manifest that in 2018, the company intends to fly 50 percent of its 30 planned missions on recycled rockets.

SpaceX will continue developing its reusable hardware, aiming to recover a booster and relaunch it within 24 hours. To do that, the company is leaning on the next and final version of the Falcon 9, the Block 5. The upgraded rocket will have more thrust, permanent landing legs, and a new layer of thermal protection coating—features that will end up in the Falcon Heavy, too. Eventually.

For now, SpaceX will content itself with testing small reusability-enhancing elements. Like two sets of new titanium cast fins—which, if the Falcon Heavy flight goes according to plan, will navigate two side boosters back to ground at Cape Canaveral. Flown just once before, the grid fins will be standard on the Block 5 booster; they’re meant to improve flight control and survive the blazing hot atmospheric reentries better than previous aluminum fins.