WASHINGTON — Two months after he unveiled a revised design for SpaceX’s next-generation launch system, Elon Musk suggested that the company is making more changes to the design.

In a series of tweets Nov. 17, Musk said that SpaceX was no longer pursuing an upgrade to its existing Falcon 9 vehicle that would make the vehicle’s second stage reusable. The company’s focus, he said, would instead be on speeding up work on SpaceX’s heavy-lift reusable launch vehicle formally known as Big Falcon Rocket, or BFR.

“Accelerating BFR instead,” Musk said. “New design is very exciting! Delightfully counter-intuitive.”

Btw, SpaceX is no longer planning to upgrade Falcon 9 second stage for reusability. Accelerating BFR instead. New design is very exciting! Delightfully counter-intuitive. — Elon Musk (@elonmusk) November 17, 2018

Asked by another Twitter user if he was referring to the company’s latest design, released in September, or other changes, Musk responded, “Radical change,” without elaboration.

The latest version of the BFR, unveiled by Musk at a Sept. 17 event at SpaceX’s headquarters in Hawthorne, California, featured a number of design tweaks to the vehicle’s “spaceship” upper stage. Those changes included using the same version of the company’s Raptor engine under development as on the lower booster stage, increasing volume of the spaceship’s cabin and changing the vehicle’s tail fin design.

At the time, Musk said those changes would be among the last for the BFR, as the company prepares to begin testing of the spaceship with “hopper” test flights as soon as late 2019 at SpaceX’s future South Texas launch site. “I feel like this is the final iteration in terms of broad architectural decisions” for BFR, he said at the September event.

Musk’s comments come 10 days after he said the company was planning to modify a Falcon 9 second stage to perform tests related to the BFR. “Falcon 9 second stage will be upgraded to be like a mini-BFR Ship,” he announced on Twitter. That design, he said, would allow SpaceX to test technologies like the vehicle’s heat shield and control surfaces during reentry from orbit that can’t otherwise be tested.

The company didn’t disclose additional details about those efforts since, including whether this would be a one-off test or a potential operational, reusable upper stage. Musk had indicated earlier in the year that the company was collecting data on how to recover the Falcon 9’s upper stage. “I’m actually quite confident that we’ll be able to achieve full reusability of the upper stage,” he said during a May press conference.

Musk, in his latest tweets, said no major changes to the Falcon 9 were now on the table. “Yes, no upgrades planned for F9,” he wrote. “Minor tweaks to improve reliability only, provided NASA & USAF are supportive.”