It would take a LOT of mass, even just using water. You need a 1m thick layer of water to provide adequate protection in space outside of LEO. There's a good explanation here:

https://space.stackexchange.com/questions/1336/what-thickness-depth-of-water-would-be-required-to-provide-radiation-shielding-i

TL;DR - You'd need 3 Saturn V launches of just water to fill the shielding for a tiny capsule going to Mars. The example they used was "a cylinder roughly 3.5m by 20m" for the crew compartment.

If they could somehow pull the water from space (comets, asteroids) then it would become easier, but launching that much water from the surface of Earth is just not logistically feasible with our technology.

[–] [email protected] 1 points 7 months ago

The volume of shielding water needed is the difference between those two cylinders, or 22⋅π⋅2.752−20⋅π⋅1.752≈522.68−192.42=330.26m3. As one cubic meter of water weighs 1 metric ton (1,000kg), that's 330,260kg to get into space

Honestly, 330 tonnes doesn't seem too unacheivable to me. That's less than the mass of the ISS, which was built using much smaller rockets than are currently in development.