Brian Wang reports here: Advanced Nuclear Thermal Propulsion Design for 90 Days Trip to Mars
However that simple headline covers a multitude of assumptions that I wish to unpack a bit. Nuclear Thermal Rockets have been studied since the 1940s and achieved a solid engineering basis in the 1960s. For example, the high exhaust velocity achieved by the NERVA program, reaching 8,500 m/s, is about twice that of H2/O2 rockets from the 1960s (~ 4,200 m/s) and most designs for NTRs since then have been improvements on NERVA fuel elements rather than the basic NTR design itself. Solid-Core Reactors are the kind we have real experimental experience with – all other systems, like Gas-Core, Plasma-Core or Liquid Core, are paper-designs systems. And a Solid-Core is limited by the need to keep the Core solid – thus the temperature is kept under ~3,000 K. Advanced materials might push that to ~3,500 K, but that’s as high as it can go.
From an ISRU perspective Solid-Core NTRs of any design are kind of pointless. Sourcing the favoured propellant, liquid H2, requires throwing away 88% of the mass of its most common source – water, which seems grossly inefficient. Is there a source that already has plenty of free H2 floating around? The atmospheres of the Gas Giants are too deep down their gravity wells to source it, except maybe Uranus. There is some 660 billion tonnes of gaseous H2 available on Titan, which has a escape velocity of ~2.6 km/s. With a background temperature of 94 K, chilling hydrogen down to liquid form at 20 K isn’t as energy intensive (by a large factor) as cracking it out of water anywhere else, then chilling it down.
Boosting it up from the surface of Titan could use an NTR, but a 1:8 mix of acetylene-hydrogen makes for a decent propellant for free, given the dunes of Titan are probably mostly acetylene. No oxidiser required and it gets about 260 seconds Isp at Titan’s surface pressire and up to 420 seconds in vacuum.