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Compared to the moon, Mars is farther away and has a deeper gravity well.
This means that the craft has to be more substantial since the astronauts will be in there for quite a while. Also the lander needs to be more substantial since it has to escape more gravity.
The lightweight ship and small lander might not cut it.
SpaceX hasn't really explained the mars plan. They might be planning to assemble something in orbit and use a Starship as the lander.
That's the thing though: re-entry, landing, and liftoff on Mars all have substantial costs, and a longer voyage multiplies the cost of hauling all the mission's mass in and out of gravity wells.
If you need 6 months of supplies each way, a monolithic design means carrying down and then re-orbiting 6 months of supplies plus the dry mass of the other 6, plus all the fuel for the interplanetary burn back to earth, plus all the extra tankage mass for the fuel for all that stuff.
Or you could just leave all that mass in orbit like Apollo did and have a dedicated starship launcher-lander.
The moon doesn't have the long flight time issue, but it's even worse in that you're landing and launching a useless heat shield, and without an atmosphere deorbiting mass from lunar orbit is surprisingly costly. I noticed the proposed SpaceX lander for Artemis doesn't have a heat shield for that reason.
On the other hand, imagine how useful starship would be as the orbital shuttle for a dedicated interplanetary stage... If the actual starships never needed to leave low orbit, with some kept on Mars.
Maybe rather than hype, he's dreaming even bigger than I thought.
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The high-level details have been there for years. No orbital assembly, just orbital refueling. Multiple Starships as the lander(s), with cargo sent in the launch window ahead of crew so they can make sure consumables are there and refueling systems are working.
I hesitate to call this a "plan" since I don't expect it to survive contact with reality unchanged, but the changes are likely to be more along the lines of "wait to send many more cargo ships first, after some break and some get departure liftoff testing on Mars etc etc" or "redesign when Raptor turns out to be too powerful to land on unprepared Martian soil", not "orbital assembly". It would be kind of cool to see them tether two Starships together for artificial gravity in-flight, but that would be hard to combine with their "put the landing fuel tanks in between the crew quarters and the sun" plans to minimize radiation shielding weight.
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My understanding was that the Mars rocket would be assembled in LEO from parts launched in Starships, and that a tanker configuration of Starship would be used to fuel it.
Nobody has this plan. The SpaceX manned-Mars plan is that the crew/cargo configurations of Starship are the Mars rockets, that will each send ~100T to Mars after refueling in LEO from a tanker filled by several reusable tanker-configuration Starship launches. The non-SpaceX Mars probe plans are the same as they've always been, to launch <4T to Mars directly via an expendable upper stage and a separate aerobraking shell. NASA's pre-SpaceX manned-Mars proposal was generally to assemble a Mars Transfer Vehicle in LEO, from parts launched on whatever heavy-lift was politically favored at the time (I see 5 Ares V launches in the 2009 study, for example), to put 80-90T on Mars ... but the cost was always in the $100B+ range and I wouldn't call any of the studies a "plan". Looks like the latest idea was to do (relatively minimal, thanks to SLS Block 2 plus some handwaving about nuclear-electric propulsion) assembly in lunar orbit instead?
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