I assume they would not land the rocket with the nuclear engine but rather keep it in orbit when they arrive at Mars?

It got me thinking... what do you think about assembling a large nuclear powered space craft in Earth Orbit akin to the ISS? This could later be used as Mars cycler or something.

I assume ISS was expensive because rocket launches were so expensive, but with significantly cheaper launch costs it should be possible to assemble structures in space much cheaper?

Would be interesting to hear your thoughts Russel since you seem to think a lot about this stuff. I have just toyed with the ideas of alternatives to the SpaceX vision of flying really large rocket from planetary surface to planetary surface and back again.

One of my inspirations thinking about this has been Robert Zubrin with his Mars Direct and how he described Lunar colonization and exploration. He tends to offer smart ideas based on simpler hardware.

He has an article where he describes landing on the moon using only Falcon 9 and Falcon Heavy rockets. His idea is that you send up the spacecraft that takes you two the moon separate from the capsule you use to get into Earth orbit and land. E.g. you go on a drag capsule to LEO where you dock with a space craft sent with e.g. a Falcon Heavy launch. That takes you to the moon. On return you will dock with the dragon capsule in orbit and return to Earth.

This way of solving complex space challenges by combining smaller rockets kind of appeals to me. It made me think of combining many smaller rocket launches to get to Mars. The USNC nuclear thermal engine can supposedly be launched with a Falcon 9 into orbit. In theory then it should be possible to arrange for these engines to be coupled onto other modules.