Elon Musk wants to go to Mars. He has mentioned putting a million-person colony on the planet. Such an undertaking seems unbelievably complex and costly. To do it is probably going to require more than the ambitions of a single man. For Musk is mortal just like the rest of us. He won't live forever, and this undertaking is going to take sustained effort over a long period of time.
Even if he builds a spacecraft that can move 1000 people per launch, it will take a 1000 launches just to get those people on their way. The idea is to build a lot of spacecrafts, and be able to do many launches at a very quick pace. Even so, this is one helluva project. Not to mention, this is just getting the project off the ground. That leaves a lot of other details that are yet to be worked out even after he gets his basic transportation device ready.
His Starship will be quite an innovation, provided that he is successful with it. But there may be problems ahead. Let's look at some of those.
One problem is the long distance to Mars. The trip will take months. This brings into focus several problems with keeping large numbers of people alive for an extended period of time in the most hostile environment known to man. Getting to Mars is not that big of a deal. Getting to Mars with people is a big deal. Getting to Mars with a lot of people is off the charts kind of deal. How do you provide for so many people for such a long period of time in such hostile conditions?
The Starship is designed to be reusable and is designed to carry up to 200 tons to orbit. This is impressive in itself, but it may be asking too much to try to take something that is specialized and make it into a Swiss Army knife. Getting to orbit with a reusable vehicle is a big achievement, but is it enough?
He's probably going to need another vehicle for his purposes. Maybe two more. What to do about this? The development of new vehicles is a costly and time-consuming project. Perhaps it could be streamlined. Let's speculate on what could be done...
SpaceX has already won a contract to land on the moon. Why not start there? There are advantages to working on the moon that may make it worthwhile to build something there. What if he built another vehicle on the moon itself, and launch it into space? He could make it hold thousands of people at a time. It could provide a habitat that would shield people from the elements and would enable artificial gravity. He could build a cylinder that is much more massive that anything that could be launched from the Earth.
The cylinder would be sufficiently large that it could be spun up to produce artificial gravity. It could be made of materials obtained from the surface of the moon itself. The moon has plenty of iron. His Starships are made of stainless steel, so the jump iron to stainless steel, if desired, is not that big.
Or he could use a different substance. In any case, the craft would not need to be aerodynamic, since it wouldn't be built to land on the surface, but to stay in space indefinitely. You need to build it, then launch it. Getting a massive object off the moon is going to be a lot easier than getting it off the Earth.
An object that has a 200 meter in diameter and rotates at 2 rpm will generate an artifical gravity comparable to Mars. If you could attach the rocket to it and lift it into lunar orbit, then what you need to do next is to spin it up. It could be like a torus or donut. Perhaps it could have several levels. But the main thing is that it doesn't have mass out all that much because to get to orbit requires about as much fuel as mass. This compares with the Earth which requires 50 times more fuel than useful mass to get to orbit.
Let's say you could get 25 times more people than the biggest Starship. Maybe 25k at a time. Then you would need only 4 of them to transport a hundred thousand on each trip. That would take 10 launch windows, or about 20 years to get your million people on Mars. The biggest Starship might transport a thousand people at at time. This might be better to to launch a thousand people at a time from Earth to rendezvous with this bigger craft that would house the people on the way to Mars.
Once you get to Mars, you can use a fleet of Starships to transport back from the torus to the surface. Refueil on the surface, launch and get some more on each trip. Eventually, you empty the big torus, and send it back on its way to the Moon for another load.
Is it feasible to build such a large device on the surface of the moon? Maybe not, but the thing could be launched in pieces and assembled in lunar orbit, if the launching it from the surface is too difficult. You might even build a bigger one that way.
You could build rings of steel on the moon that would be welded into place in lunar orbit. Those rings could be launched from the moon with a mass driver--- no fuel necessary. If the rings were 60 feet in diameter and x degrees of the torus in length, they would need to add up to 628 meters in length when all assembled in order to be 200 meters in diameter.
Each ring could be custom sized so as to fit together perfectly into a donut shaped torus. The living space would be inside the empty area inside the fitted rings. A sufficient number of launches from the surface of the moon could get the large mass needed into lunar orbit so that it can be assembled there. From there it could be supplied and readied for its mission.
Perhaps it couldn't carry 25k people at a time, but it would have definite advantages over the Starship. It would be a specialized craft built to do the mission of transiting large numbers of people over long distances. The Starship is better suited to get through the atmosphere and from a deep gravity well. This craft wouldn't need to do that, and could be made bigger, if desired.
There are other benefits from starting from the moon. There's plenty of oxygen there. Water can be obtained there as well. So the idea could be considered. In any case, the decision will be Musk's.
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