An explosion on the launch pad during a test of a massive SpaceX rocket prototype was “actually not good” and the company is assessing the damage, company CEO Elon Musk said on Twitter.

The rocket, an early version of the vehicle SpaceX is calling Starship, could be seen lighting its engines as it remained anchored to its South Texas launch pad in footage captured Monday by NASA Spaceflight, a media outlet with no relation to the space agency. Then, a blaze of uncontrolled flames erupted from the engines, followed by plumes of gray smoke. After the smoke cleared, no visible signs of external damage could be seen from NASA Spaceflight’s livestream.

Musk has long described Starship as the linchpin for SpaceX’s stated goal of getting the first humans to Mars. He’s talked about the vehicle for years, and laid out SpaceX’s evolving plans for its design through several public speaking engagements over the past decade.

Last year, SpaceX also won a $3 billion NASA contract to use Starship for the space agency’s plans to return astronauts to the surface of the moon for the first time in five decades.

Musk explained what he thought went wrong during the engine test in a tweet Monday.

“Cryogenic fuel is an added challenge,” said Musk, referring to the propellant that SpaceX plans to power Starship that must be kept at extremely cold temperatures, “as it evaporates to create fuel-air explosion risk in a partially oxygen atmosphere like Earth…More later.”

Essentially, the fuels selected for the rocket have a higher risk of fiery mishap when they mix with gases already in the ambient air.

The Starship consists of two parts: a large, bullet-shaped spacecraft that’s intended to house dozens of people or large satellites, which sits atop and a towering rocket booster that’s designed to haul the spacecraft to orbit.

It was a rocket booster, which is slated to have more than 30 rocket engines and exert more thrust than any rocket humans have ever launched, involved in Monday’s test. For comparison, the Saturn V, which took the NASA Apollo missions to the moon more than 50 years ago, used five larger main engines on its first stage.

SpaceX has been preparing to launch a Starship booster and spacecraft on an orbital launch for the first time this year, following about a dozen test flights the company had put just the spacecraft portion of Starship through in the past few years. The spacecraft tests went from hopping a few feet off the ground to soaring more than 30,000 feet. A few high-altitude tests ended in explosions as the test rockets smashed back into the ground. But its latest test flight of the Starship spacecraft, in May 2021, managed to land upright.

SpaceX did not immediately return a request for comment, including specifically whether or not the latest incident would delay the company’s flight schedule.

SpaceX and Musk have repeatedly stated that they view failed rocket tests as a good thing, giving engineers the chance to quickly build, test and change a rocket design as needed rather than taking a slower approach that might guarantee fewer testing mishaps.

Last month, after more than a year of waiting, the Federal Aviation Administration, which licenses commercial rocket launches, granted SpaceX environmental approval to launch Starship to orbit out of South Texas. But the FAA is still reviewing the company’s application for an overall launch license.

“SpaceX’s application must meet FAA safety, risk and financial responsibility requirements,” the FAA said in a statement on the matter last month. “The FAA will make a license determination only after SpaceX provides all outstanding information and the agency can fully analyze it.”

The agency declined to say when a launch license decision is expected.

Meanwhile, SpaceX is already building a second Starship launch site at Kennedy Space Center in Florida, where the majority of US rocket launches take flight.

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