Anything one wants to ask about traveling to Mars could be asked to a number of people who have studied the problem intently for decades, which Musk has not. That's why he's capable of making simple errors such as those we discussed. There are many truly brilliant people holed up in NASA and universities; you have never heard their names and likely never will.

Radiation protection is best handled by shortening the trip; that is accomplished by ion engines, which can reduce a 8 month trip to 40 days. Using such an engine, acceleration is low, like 1/1000 of a G. Acceleration force on the airframe such as you mentioned, capable of going to space from earth and capable of descending to Mars, is perhaps 6G. That is a difference of 6000 and you can directly note a difference of 6000 in the weight of structural support in the spacecraft.

For a ship headed to Mars, that 5999/6000 savings in structure translates directly to more payload, humans, supplies, etc. This is the primary reason we separate a "spacecraft" from a "launch vehicle." Dead weight and fractions of dead weight to useful payload.

Such ion engines require electrical power, most likely nuclear.

A good reference book, available free online is Dr. Robert Zubrin "The Case For Mars."

http://www.ifa.hawaii.edu/%7Emeech/a281/handouts/mars_case.pdf