Here is a thread where various forms of nuclear propulsion are discussed.

williatw wrote:The key is to reduce the cost to orbit by a couple orders of magnitude by some means. Once your in orbit you are half way to anywhere as Heinlein said.

An oft repeated meme. But wrong.

(Start Mass/Final Mass) = e^(dV/Ve)

Where e is Euler's number, about 2.72.

dV is delta velocity, the change in speed needed.

Ve is exhaust velocity.

With hydrolox, the above doubles every 3 km/s you add to the delta V budget.

Each 3 km/s is a square on the above chess board. Getting to LEO is about 9 km/s. Adding another 6 to that quadruples your initial mass. So if you go by initial mass, LEO is 1/4 of the way. Possibly even more as you would likely need to add stages.

If trip times are 8 months, you need more robust radiation shielding and life support more reliable and trouble free than the ISS (remember, a Mars Transfer vehicle can't enjoy supply and maintenance trips as does the I.S.S.)

williatw wrote:Reusable rockets like Musk wants may be the best near term solution.

Here we agree. But the reusable rockets to LEO must be chemical for a number of reasons.

Recovering a working stage after 8 km/s re-entry is a problem.

Presently SpaceX is working on the Grasshopper. SpaceX videos depict the two stages using reaction mass to propulsively shed mass as well as using aerobraking. Can Musk et al pull this off? They need to achieve some difficult mass fractions -- I given them less than even odds.

But if Musk's upper stage could refuel in LEO, I would give a reusable Grasshopper much better than even odds.