SpaceX is swiftly moving toward achieving its ultimate goal of rapid reusability: flying a single booster twice within a 24-hour time period. It’s a goal that Elon Musk says SpaceX will achieve later this year—but in order to make good on that promise, the company must first say goodbye to its hardest-working rocket yet.

That would be the full-thrust Falcon, known to SpaceX followers as the Block 4. Each booster is capable of flying two or three times—but if SpaceX is to meet its ultimate goal of flying daily, it needs to do even better. So with 36 flights under its belt, we bid farewell to the moderately reusable Falcons of yesterday and say hello to the more capable and reusable Block 5. On June 29, at 5:42 am EDT, smoke will billow and flames will light up the predawn sky as SpaceX sends its final Block 4 Falcon into space—completing its 15th mission to resupply the International Space Station.

The booster, officially named B1045 by SpaceX, is not only the last one of its kind to fly but also the last to be commissioned. Built for NASA, the booster last ferried the TESS mission, which launched in April, before sticking its landing on the company's drone ship waiting in the Atlantic Ocean.

SpaceX readied B1045 for its next and final mission and in record time. Typically, the amount of time between booster flights has been several months; this time, it took just two. Following several weeks of checkouts and refurbishments, B1045 trekked out to Pad 40 at Cape Canaveral Air Force Station for a routine static fire—essentially a dress rehearsal. Engineers secured the rocket on the pad, loaded it with fuel, and fired the engines for several seconds to ensure that it’s ready for launch.

As with the previous two Falcon 9 missions, B1045 won’t attempt a landing, instead splashing down in the Atlantic Ocean. But its demise opens up opportunities for a new generation: the Block 5 Falcon, which first took flight in May. Elon Musk says that we will see a Block 5 launch, land, and relaunch within the same day sometime this year. A culmination of more than 10 years of development, the Block 5 is designed to re-fly with no action taken between flights—just like commercial airplanes.

The design changes are intended to help the booster hold up better to the stresses of launch: improved engines, a more durable interstage, titanium grid fins, and a new thermal protection system. According to SpaceX, each Block 5 is capable of flying 10 times or more before it needs light refurbishments, and up to 100 times before the booster is retired.

SpaceX plans to use the Block 5 in future Falcon Heavy rockets, as well as to launch astronauts to the space station. In order to achieve that goal, it has to meet NASA’s requirements, which include flying at least seven times without any design changes.

It also will have to address a pesky hardware concern. In 2016, a Falcon 9 exploded on the launchpad after helium-filled bottles that sit inside the rocket’s fuel tank—called composite overwrapped pressure vessels—malfunctioned. You can’t have that risk with human cargo on board. So SpaceX has been working to upgrade the COPVs. “The amount of testing and research that’s gone into COPV safety is gigantic,” Musk said during a prelaunch call prior to the first Block 5 launch. “This is by far the most advanced pressure vessel developed by humanity.”

As it turns out, the Block 5 that launched in May lacked the COPV upgrades and does not count toward the seven launches SpaceX needs before it can launch people. But testing and development of the new COPVs is complete, and the new hardware will be on board when SpaceX launches its first commercial crew demo flight, currently scheduled for this August. Dubbed Demo-1, that initial mission will send an unoccupied Crew Dragon to the Space Station.

If all goes well, the first crewed flight of the Dragon will launch in December. In order to meet NASA’s requirements, this means that SpaceX will need to fly six more Block 5s after Demo-1 and before December to meet that schedule. With at least a dozen more launches on the manifest for this year, anything’s possible.

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