An analysis by John Bucknell (x-Spacex senior engineer) describes an 11 meter diameter robotic vehicle with a 6,000-megawatt nuclear thermal rocket in a NTTR arrangement. The rocket would be single stage to orbit and would be immediately be able to refly after landing and refueling much like todays airliners. Even fully reusable Spacex rockets where all stages are resused would need to be re-assembled.

He describes SSTOH missions to place a 21 meter minor and 214 meter major diameter toroidal habitat in space, capable of full terrestrial gravity simulation by spinning at 3 rpm. The habitat begins as two thin films defining the interior and exterior surfaces of the torus, which is then inflated with lunar-sourced water in a 1m thick shell and allowed to freeze.

Access to space is driven by the economics of launch vehicles. A previously published rocket propulsion cycle called the Nuclear Thermal Turbo Rocket (NTTR) is able to achieve payload fractions of more than 45% to Low Earth Orbit (LEO). This rocket is intended to be completely reusable for the launch mission as it is a Single Stage to Orbit (SSTO) vehicle, which improves economics vastly. However, providing material to LEO is not always the most economical solution for permanent space-based habitation. In-situ resource utilization (ISRU) has been proposed as a method for avoiding the Earths gravity well for space-based construction with solutions proposed using Lunar, Martian as well as other resources.

The Air enhanced nuclear thermal rocket has been described a couple at times at Nextbigfuture.

The proposed space station would be close to the size of Titanic but the space station would consist of mostly water ice

Water ice can be used as both reaction mass for propellant in liquid form and as structure in solid form. Nuclear Thermal rockets in particular are well-suited to in-space propulsion as they can add enthalpy to a variety of propellants for thrust without requiring processing plants to achieve chemically active reactants, thus saving on mission payload mass. A mission is proposed that leverages the NTTR vehicle as well as ISRU to construct an orbital habitat of Lunar water ice with a single terrestrial launch (Single Stage to Orbital Habitat SSTOH).

The lunar water ice is extracted from permanently shadowed regolith on the Lunar south pole, where the NTTR vehicle propulsively lands and places 54 tons of payload. The lunar payload is comprised of a small 30 MWth nuclear reactor and associated mechanisms able to extract sub-surface ice.

NOTE NASA will soon officially confirm that there is surface water ice at the lunar pole. The Lunar Reconnaissance Orbiter did find evidence of frost on the moon earlier in the year, but there is a NASA paper that will be released soon that will confirm surface water ice.

The NTTR vehicle fills its propellant tank with 720 tons of lunar water, and using the water as a propellant delivers 400 tons of water to the habitat in LLO before returning to the Lunar water extraction plant. The reusable NTTR vehicle makes 100 trips to inflate the 40,000-ton habitat, with approximately one trip per 24 hours. Subsequently, the lunar water extraction reactor can be transported to the habitat as a power supply and the NTTR vehicle can push the habitat to a Lagrange point.

The 40,000 ton habitat would be just short of the max cargo of a Panamax container ship. The ISS weighs 450 tons.

In such a fashion, a single vehicle of low investment can produce a 199,000m^3 habitat within 5 months of launch.

In 2015, Bucknell presented the Nuclear Thermal Turbo rocket which added air-breathing to a nuclear thermal rocket. Bucknell design would have 1664 ISP. 60% more than the best prior nuclear thermal rocket designs.

Link:

Plan for a mostly water ice space station 90 times bigger than the ISS - Next Big Future