Elon Musk held an AMA or "Ask Me Anything" session on Reddit, where he answered questions strictly related to SpaceX, and not his other companies. The session came almost a month after his presentation at the International Astronautical Conference in Guadalajara, Mexico on September 27th, where he gave details about SpaceX's plans for the colonization of Mars.
Many users wanted to know about the rockets that SpaceX will use to get first people to Mars and Interplanetary Transport System. Musk pointed out that the ITS is 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.
Musk also believes that the Falcon 9 rockets will 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.
Musk also explained his plan about sending the first people to the Red Planet.
We are still far from figuring this out in detail, but the current plan is:
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.
Heart of Gold spaceship flies to Mars loaded only with equipment to build the propellant plant.
First crewed mission with equipment to build rudimentary base and complete the propellant plant.
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.
He also talked about the first habitats that will be a home to the first people on Mars.
Initially, glass panes with carbon fiber frames to build geodesic domes on the surface, 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.
Musk pointed out that the first crewed mission to Mars will have about a dozen people "as the goal will be to build out and troubleshoot the propellant plant and Mars Base Alpha power system."
Asked about the tight cluster of 42 engines of the ITS Booster, Musk referred to one of his favorite books The Hitchhiker's Guide to the Galaxy where the answer to life, universe and everything is 42. Musk said "it had to be 42 for important scientific and fictional reasons!"
Musk plans to send the first crewed mission to Mars in late 2024, that would arrive at the Red Planet in 2025.