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SpaceX Interplanetary Transport System liftoff
An artist’s conception shows SpaceX’s Interplanetary Transport System lifting off with a refueling tanker sitting beside it. (Credit: SpaceX)

In the weeks ahead, SpaceX plans to pressure-test a prototype carbon fiber tank on an oceangoing barge, to gauge how well the technology will stand up to the oomph that’d be required for trips to Mars.

The test is one of the near-term steps that SpaceX founder Elon Musk laid out today during an “Ask Me Anything” session on Reddit’s SpaceX discussion forum, focusing on his long-term plan to transport a million settlers to Mars.

Musk signed on to the AMA session to follow up into some of the geeky questions raised by last month’s big reveal about SpaceX’s Interplanetary Transport System. Which, by the way, Musk is not happy with as a name.

“ITS just isn’t working,” he wrote. “I’m using BFR and BFS for the rocket and the spaceship, which is fine internally, but…”

The acronyms that Musk mentioned refer to “Big [Fracking] Rocket” and “Big [Fracking] Spaceship,” two of the key pieces in Musk’s architecture for Mars trips.


The rocket would be by far the biggest booster every built, and each spaceship would be designed to carry 100 settlers to Mars. About 1,000 of the spaceships would be built over the course of several decades, with the aim of setting up a mass migration every 26 months.

Musk has intentionally left a lot of details to be filled in – for example, what settlers will do when they get to Mars – but today he touched upon the type of habitat he thought was best-suited for a Mars settlement.

“Initially, glass panes with carbon fiber frames to build geodesic domes on the surface, plus a lot of miner/tunneling droids,” he said. “With the latter, you can build out a huge amount of pressurized space for industrial operations and leave the glass domes for green living space.”

Musk says the project is his life’s work, motivated by the view that humanity must become a multiplanet species in order to weather any sort of global catastrophe on our home planet. He’s toying with the idea of calling the first spaceship the Heart of Gold, in honor of the spacecraft that was endowed with an Infinite Improbability Drive in Douglas Adams’ “Hitchhiker’s Guide to the Galaxy.”

SpaceX carbon fiber tank
SpaceX has built a large carbon fiber tank as a prototype for the future spaceships that Elon Musk plans to send to Mars. (Credit: SpaceX)

So just how improbable, or probable, is Musk’s plan? If there’s any audience that can assess the possibilities in depth, it’d be Reddit’s SpaceX community. So Musk went into full geek mode during a Q&A that ran for more than an hour. Here’s are a few of the Redditors’ questions (in some cases, shortened for brevity’s sake) and Musk’s answers:

Question: Tell us more about the design, construction and role of the carbon fiber tank you showed during last month’s presentation:

Elon Musk: “Yeah, for those that know their stuff, that was really the big news :)

“The flight tank will actually be slightly longer than the development tank shown, but the same diameter.

“That was built with latest and greatest carbon fiber prepreg. In theory, it should hold cryogenic propellant without leaking and without a sealing linker. Early tests are promising.

“Will take it up to 2/3 of burst pressure on an ocean barge in the coming weeks.”

Q: What SpaceX technology or material still requires the most development for ITS to be a success?

A: “It used to be developing a new metal alloy that is extremely resistant to oxidation for the hot oxygen-rich turbopump, which is operating at insane pressure to feed a 300 bar main chamber. Anything that can burn, will burn. We seem to have that under control, as the Raptor turbopump didn’t show erosion in the test firings, but there is still room for optimization.

“Biggest question right now is sealing the carbon fiber tanks against cryo propellant with hot autogenous pressurization. The oxygen tank also has an oxidation risk problem as it is pressurized with pure, hot oxygen. Will almost certainly need to apply an inert layer of some kind. Hopefully, something that can be sprayed. If need be, will use thin sheets of invar [a nickel-iron alloy] welded together on the inside.”

Q: How differently will the spaceship be configured internally for a smaller crew, such as the first one, as opposed to the full 100-person crew? 

A:  “Probably just pack the pressurized space with cargo. Early missions will be heavily weighted towards cargo. First crewed mission would have about a dozen people, as the goal will be to build out and troubleshoot the propellant plant and Mars Base Alpha power system.”

Q: What equipment and procedures will be required for refueling operations on Mars?

A. “We are still far from figuring this out in detail, but the current plan is:

“1. Send Dragon scouting missions, initially just to make sure we know how to land without adding a crater and then to figure out the best way to get water for the CH4/O2 Sabatier Reaction. [This chemical reaction would convert Martian water and carbon dioxide into methane fuel and oxygen.]

“2. Heart of Gold spaceship flies to Mars loaded only with equipment to build the propellant plant.

“3. First crewed mission with equipment to build rudimentary base and complete the propellant plant.

“4. Try to double the number of flights with each Earth-Mars orbital rendezvous, which is every 26 months, until the city can grow by itself.”

Q: In the event of an emergency, could the spaceship return to Earth without having to wait 26 months?

A: “Yes.”

Q: How many G’s could the spaceship and the booster take?

A: “The spaceship would be limited to around 5 G’s nominal, but able to take peak loads 2 to 3 times higher without breaking up. Booster would be nominal of 20 [G’s] and maybe 30 to 40 without breaking up.”

Q: How does the landing profile for the ITS booster differ from that of the Falcon 9? What changes are you planning for the updated “final” version of the Falcon 9, and what’s the plan for reusing Falcon 9 first-stage boosters?

A: “The big booster will have an easier time of things than Falcon, as the mass ratio of the stages is lower and it will have lower density. Net result is that it won’t come in quite as hot and fast as Falcon, so Falcon should be a bounding case on the big booster.

“Final Falcon 9 has a lot of minor refinements that collectively are important, but uprated thrust and improved legs are the most significant.

“Actually, I think the F9 boosters could be used almost indefinitely, so long as there is scheduled maintenance and careful inspections. Falcon 9 Block 5 — the final version in the series — is the one that has the most performance and is designed for easy reuse, so it just makes sense to focus on that long term and retire the earlier versions. Block 5 starts production in about 3 months and initial flight is in six to eight months, so there isn’t much point in ground testing Block 3 or 4 much beyond a few reflights.”

Musk didn’t specifically address the challenges that SpaceX has been facing in the wake of last month’s launch pad explosion in Florida, which led to the loss of Falcon 9 rocket and its satellite payload. SpaceX’s launches have been suspended pending the outcome of an investigation.

Although the subject didn’t come up this time around, Musk cited a quote from Winston Churchill during an earlier Reddit session that may well apply to SpaceX’s current situation as well: “If you’re going through hell, keep going.”

Today’s Reddit session attracted more than 5,000 comments, and got into topics ranging from the planned specific impulse for the BFR to the purpose of some spherical tanks that show up in the design for the BFS. To get the complete geek-out, scan the complete transcript or just Musk’s comments.

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