spacex mars spaceship its flickr 29343909374_cbaa807fd1_oElon Musk's spaceship is designed to take people beyond Mars.SpaceX/Flickr (public domain)
Less than a month ago, Elon Musk shared his audacious plan to launch a million people to Marsand beyond, all in hopes of backing up humanity for if and when some future apocalyptic calamity dooms Earth.
Musk's 63-minute presentation showed off intricate computer renderings of rockets and spaceships, all parts of his so-called Interplanetary Transport System.
Because he had to skip over a lot of technical details, however, Musk logged on to Reddit on Sunday afternoon to let his most discerning fans squeeze information from him in an "Ask Me Anything" session.
During that Q&A session, Musk shared news of a very important test that SpaceX has planned "in the coming weeks": filling up an enormous carbon-fiber fuel tank, shown below, that will be essential to making the ITS spaceship work.
The tank, which happens to be the spaceship's core structure, has to withstand incredible pressures and stresses at blisteringly cold temperatures — otherwise it might leak or even explode.
Musk shared images of the massive structure toward the end of his September 27 talk at the International Astronomical Congress.
When user nalyd8991 asked Musk for more information about it on Sunday in the r/SpaceX subreddit, Musk said the gargantuan carbon-fiber tank "was really the big news" of his IAC talk — or at least "for those that know their stuff."

'The hardest part of the spaceship'

"This is really the hardest part of the spaceship," Musk said at the September IAC talk. "The other pieces ... we have a pretty good handle on, but this was the trickiest one. So we wanted to tackle it first."
In short, the entire spaceship will be built around such a tank — so getting it right is crucial. Here's where designs place it in the spaceship:
Carbon-fiber fuel tanks for spacecraft aren't a new concept, as Boeing and NASA began work on a huge ones 2014, but we've never seen one this enormous at roughly 40 feet wide.
Here's what it looks like from the inside:
inside tank spacex elon musk mars colonizationSpaceX/YouTube
Engineers most likely chose to make the tank out of carbon fiber because that material is lighter, shrinks less, and is stronger than the metal alloys that many cryogenic fuel tanks are made out of. So it will presumably make for safer transport to Mars while using less fuel (though at a considerably higher cost of construction).
The material isn't easy to work with, though, as Musk explained during his IAC talk:
"Even though carbon fiber has incredible strength-to-weight, 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.
"Just the sheer scale of it is also challenging, because you've gotta lay out the carbon fiber in exactly the right way on a huge mold, and you've gotta cure that mold at temperature, and then it's ... just really hard to make large carbon-fiber structures that can do all of those things and carry incredible loads."
During his Reddit AMA on Sunday, Musk dropped a few bits of new information about the tank as well:
  • "The flight tank will actually be slightly longer than the development tank shown, but the same diameter," Musk said.
  • He also said it was "built with latest and greatest carbon fiber prepreg," or 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. "Early tests are promising."
  • Finally, he teased a much grander test: "Will take it up to 2/3 of burst pressure on an ocean barge in the coming weeks."
Though Musk didn't answer any follow-up questions about this mysterious ocean test, we assume that by "barge" he means one of the robotic drone ships that have caught a handful of SpaceX's self-landing Falcon 9 rocket boosters in the recent past, such as the one shown below.
We also don't yet know how SpaceX plans to pressurize the giant black tank. It could be with plain old air, but it might be its fuel of choice for Mars: flammable (and explosive) methane gas.
Several things might suggest this: First, lugging a giant orb out into the middle of the ocean sounds like no simple effort, which implies the company is doing so out of an expectation that the device could not just merely leak but burst or explode. If no one is around and it blows up, its shrapnel (and possibly flames) can't hurt anyone. SpaceX's ocean barges, however, have proved capable of withstanding punishing explosions in the past:
Second, Musk said SpaceX performed "initial tests with the cryogenic propellant" that "actually look quite positive."
"We have not seen any leaks or major issues," he added.
If the company has already pumped methane into the spherical tank before, it stands to reason that it will try again to get even more data on its performance for incorporating into a flight version.
SpaceX representatives declined to provide Business Insider with more details on the test, including what would be pressurizing the orb or when it might happen.

NOW WATCH: Elon Musk on going to Mars — ‘The probability of death is quite high'

h