Monday 5pm ET Update: The Falcon 9 rocket launched on schedule, reached orbit without a hitch, and deployed the Dragon spacecraft en route to the International Space Station. This was SpaceX's seventh successful launch of 2018.

Original post: In 2017, SpaceX launched 18 Falcon 9 rockets and recovered all 15 of the first stages it attempted to land. But this year, SpaceX has landed just one of the Falcon 9 boosters it launched—its first flight of the year on January 8. Since then, the rocket company has not sought to recover any of the four Falcon 9 rockets it has launched.

On Monday, at 4:30pm ET (20:30 UTC) SpaceX will attempt another Falcon 9 launch: a mission to deliver about three tons of supplies to the International Space Station for NASA. Although this booster will have plenty of fuel left over to attempt a land-based return along the Florida coast, it too will splash into the ocean.

This will make five expendable Falcon 9 launches in a row for SpaceX, which prides itself on developing a reusable rocket. So what gives? The answer is that SpaceX is working through its inventory of recovered Falcon 9 rockets.

Since the Falcon 9's first launch in 2010, the company has developed four major versions of the booster, each more powerful than the one before it. SpaceX began to successfully land its rockets (both on land and at sea) with the third and fourth version of the Falcon 9, known as Block 3 and Block 4. However, after flying those rockets, its engineers determined that these versions could probably only fly twice in a safe and economical manner.

Block 5

Therefore, SpaceX has been hard at work designing, developing, and testing the fifth version of its Falcon 9 rocket, the Block 5. This variant, optimized for multiple flights with hardened grid fins and myriad other new features, could take flight as early as April 24, with the launch of the Bangabandhu-1 satellite for Bangladesh. Once SpaceX starts flying (and hopefully reusing multiple times) the Block 5 version of its workhorse rocket, it intends to fly only that version.

So, in recent months, SpaceX has steadily been re-flying its used Block 3 and 4 boosters before dropping them into the ocean. To be sure, on those flights the company has gathered data during those "water" landings, generally bringing them in hot and fast to optimize their algorithms for future landings on drone ships and coastal sites. But SpaceX has expended them all the same. It will do so again today.

The webcast for Monday's launch should open about 15 to 20 minutes before liftoff, and weather conditions are 80-percent favorable. Unlike with Friday's launch, there should be no "NOAA restrictions" that prevent viewing of the launch all the way through Dragon deploy.

Listing image by Trevor Mahlmann / Special to Ars