[WizardAustralis]

Water does work, it is also astoundingly heavy. That is the major issue. Elegant but impractical.

It simply blows out any useful payload volume and costings because of the amount of water required. Last estimate I have seen is they would need 1,500 tons of water to make Starship radiation resistant (80% decrease in exposure). Even at $300 per Kg, about 1/8th the cost of current launches, that is still approaching a half billion dollars per ship with a significant lost of internal space.

Technical viability and economic viability are two very different things as well. This is going to be the thing that I feel will eventually limit our travel outside of the earth system.

I suspect we will see rampant use of Starship in orbit, the occasional run to the moon and maybe once or twice to Mars but beyond that. It will be just another technological lead that ends up in civilizations recycling bin.

[skykooler]

One possibility is the use of a cycler - in other words, putting only a couple Starships with massive amounts of water onboard in an orbit that loops back and forth between the Earth and Mars. Passengers would board it for the transit, then transfer back into an unshielded starship that flies in formation for the journey. This way, the shielding mass only needs to be launched once, and can be reused for an arbitrary number of flights.

[WizardAustralis]

That is a pretty smart idea actually.