Mmmm - you know, if it is lots of mass you need, maybe inside of launched it from Earth, you send robots to a nice metallic asteroid and install some ion engines or HUGE solar sails and then move it to under the Van Allen belts. It is the Van Allen belts that make the ISS a safer place to live than interplanetary space, right? Then hollow out that asteroid, install an environment for a crew, and trim the excess mass and polish it up. And then, we have a safely shielded spaceship for interplanetary travel! Heavy, but you are not sacrificing yourself to get to Mars. And a whole lot less delta-V than launching lead from earth. It will be mostly nickel and iron, so the hull needs to be thicker than lead but should mass about the same. And it should never touch an atmosphere - unless something goes terriblity wrong. You need landing craft for the planet you are visiting.

Lots of new engineering techniques to learn before we get there, but that's all part of the fun.

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I came for the astronomy but I do love the physics!