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"For the love o’ pete, Please, PLEASE, go look at the mission profiles for the Constellation program. The ORion spacecraft is only part of the system. One thing ISS has taught us is that building spacecraft IN SPACE is not only feasible, but works rather well."

Obviously anything other than the 1950s Project Orion is going to have to be built in space. And nobody is crazy enough to think anyone would attempt to fly one of those today. Its fun to talk about, but nobody is going to chuck a few hundred A-Bombs into orbit.

However it doesn't matter if they build it in space or in a shipyard on the moon or buy it at the local Walmart. If its chemical rockets it has a limited ISP and you are looking at a mulit year mission in a giant fuel tank with very limited crew space. Use a NERVA and you get twice the ISP of the best chemical engine, or more assuming the designs for the reactor are better now than in the 1960s. Also as a bonus you get a power source that doesn't degrade as you get farther from the sun like solar. For the ISS or the moon chemical is all you need. But for Mars you need to go bigger and faster than any chemical reaction can throw you.

Plasma or Ion might work, but unless powered by a reactor most of your ship would just be solar panels. And you are still going to run into problems with small living space. And a ship dependent on solar power is harder, although not impossible, to spin up for gravity.

I think you still haven’t gone to look at the information on Constellation.

As for chem rockets to Mars, I totally agree. I am putting my money (figuratively) on VASIMR. The main guy who is working on the technology, Dr. Franklin Chang-Diaz, has a mission plan that takes two weeks to Mars for the crewed mission, and some months for the Cargo ship. Serious as a heart attack - two weeks, LEO to Mars Orbit.

Yes, it would take nukes. Someday, sombody is going to realize that as an inevitability.