Here’s what we learned from Elon Musk’s SpaceX Reddit AMA

SpaceX founder and CEO Elon Musk took to Reddit this evening for an Ask Me Anything session, which he noted was “meant to be supplemental” to the talk he gave at the International Astronautical Conference in Guadalajara, Mexico on September 27th.

After the talk, there were plenty of lingering questions about his Mars colonization plans: what about radiation? Where would people live? During the AMA, he followed up on several topics that he didn’t cover at the initial talk.

On rockets and propulsion

Several people asked about the rockets that SpaceX intends to use for the journey to Mars.

When asked about whether or not the Interplanetary Transport System would be able to hover over the surface of Mars while the crew searches for a landing spot, he noted that “there wouldn't be any hovering unless it encountered a problem or unexpected wind conditions,” to save on fuel.

The ITS is also designed for a maximum acceleration of 20 Gs, but “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.”

Another user asked how the Falcon 9 and the big booster would line up, to which Musk wrote “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.”

When asked about what the company had next to do with the ITS, Musk replied that their goal had originally been designing “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.”

Their next goal is to seal their tanks from oxidizing.

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 welded together on the inside.

Musk also noted that they were probably a year or two away from revealing how the ITS would be configured. We “will aim to release details of the habitation section when we have actual live mockups.”

The ITS will likely also be configurable, with the earliest missions suited more for cargo than passengers, with additional cargo packed for the ride. “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.”

Finally, Musk revealed that they’ll likely be changing the name Interplanetary Transport System to something a bit more friendly: “I'm using BFR and BFS for the rocket and spaceship, which is fine internally, but...“

Musk thinks that the Falcon 9 rockets will be able to be used indefinitely

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 6 to 8 months, so there isn't much point in ground testing Block 3 or 4 much beyond a few reflights.

Refueling on Mars

Musk did talk about refueling on Mars, but didn’t go into a whole lot of detail as to how it would work out. When asked “What equipment and procedures will be required for refueling operations on Mars?” Musk explained that they were still in the early stages of figuring out how this would work, but that SpaceX would likely 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.” Those scouting missions would be followed up by the spaceship Heart of Gold with the equipment needed to set up a fuel plant and a manned mission to complete the station.

Inhabiting the surface of Mars

One of the biggest things that hadn’t been covered during his IAC talk was details on how people would actually live on Mars once they arrived. When asked about the habitats, Musk had a couple of more details:

“Initially, glass panes with carbon fiber frames to build geodesic domes on the surface,” Musk wrote, “plus a lot of miner/tunneling droids. 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.”

Droids?

Musk has talked about robotics in the past, while discussing his support of OpenAI, but within the context of household robots. This seems to be a bigger project in and of itself, although Musk offered no details during this AMA.

Musk wants to double the number of flights each Earth-Mars orbital rendezvous

One of the revelations was Musk’s timeline. He he said he was going to “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.”

But, an Earth-Mars rendezvous isn’t always necessary

One commenter asked if in instances where mass wasn’t an issue, if the rockets would be used outside of those rendezvous windows. “It could be used for emergency purposes such as medical supplies/instruments and experts,” he noted, “or for other high priority but low mass cargo like critical replacements.”

Musk had a simple answer for this: “yes.”

That seems to be it for the AMA, which garnered over 5,000 comments. We’ll update if Musk adds any new answers.