SpaceX has completed a critical test for the carbon fiber fuel tank of its Interplanetary Transport System spaceship, which is designed to transport humans to Mars. Last month, during an “Ask Me Anything” session on Reddit, SpaceX CEO Elon Musk said he believes the carbon fiber tank is the most essential part of the spaceship.

The tank is 12 meters (~39 feet) wide, the largest such vessel ever produced. According to Space.com, SpaceX tested the tank by taking it out to sea to see how it performs under pressure. SpaceX wrote on Twitter that the tank reached both of its pressure targets and that the next step will be full cryogenic testing.

In the Reddit discussion, Musk revealed the tank would be “built with [the] latest and greatest carbon fiber prepreg” – carbon fiber that’s pre-impregnated with a resin to make it tougher. “In theory, it should hold cryogenic propellant without leaking and without a sealing linker,” he said. Carbon fiber’s high tensile strength, design flexibility and low density are also major benefits.

Musk was also quick to note, though, that working with carbon fiber wouldn’t come without its challenges.

“Even though carbon fiber has [an] incredible strength-to-weight [ratio], when you want one of them put super-cold liquid oxygen and liquid methane — particularly liquid oxygen — in the tank, it’s subject to cracking and leaking and it’s a very difficult thing to make,” Musk explains. He adds that the sheer scale of it is also challenging.

However, despite potential obstacles, he did note that “early tests are promising” and that “initial tests with the cryogenic propellant … actually look quite positive.”

“We have not seen any leaks or major issues,” he added.

To learn more about the role of composites in SpaceX’s Interplanetary Transport System, click here.