This was SpaceX's 21st launch of the year, up from a record 18 in 2017. The company has been able to cut costs and win market share by designing its rockets for reusability. This time, however, SpaceX won't attempt to land Falcon 9's first stage after the launch, the company said, citing mission requirements.

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ULA has flown eight missions in 2018, and a ninth is scheduled for no earlier than December 30 from Vandenberg Air Force Base on California's central coast.

"SpaceX has had a phenomenal year no matter how you slice it," said Luigi Peluso, an aerospace and defence consultant at AlixPartners.

"In 2019, the big race is who is going to be the first company to put humans into space and bring them back. You've got SpaceX, Boeing, Virgin Galactic and Blue Origin all vying."

SpaceX, along with Boeing, has a contract to ferry American astronauts to the International Space Station as part of what's known as the Commercial Crew program with NASA, but the timeline for the first flights has slipped repeatedly. The agency's current schedule has SpaceX's first uncrewed demonstration flight January 17 and its first flight with astronauts on board in June.

California-based SpaceX's valuation has climbed as it has racked up successful missions, making it the third-most valuable venture-backed startup in the US after Uber and Airbnb.

The Wall Street Journal reported on Tuesday that SpaceX is poised to raise another $US500 million by selling stock at a $US30.5 billion valuation, citing people familiar with the transaction.

Bloomberg