Nuclear rockets are almost exclusively proposed for use as upper stage engines or for interplanetary transfers. One major reason for this is "no one wants their

Nuclear rockets are almost exclusively proposed for use as upper stage engines or for interplanetary transfers. One major reason for this is "no one wants their launch complex to glow", which is a major downside of the currently feasible nuclear rocket engines with >>1 TWR. (Orion drive, which is throwing nukes out the back and riding the shockwave, and the Nuclear Salt Water Rocket, which is like that but continuous via a standing nuclear detonation in streams of enriched Uranium salts in water) Once you're in space, ISP matters a lot more than TWR, and is the main limiting factor on how far you can get with a given mass fraction. Nuclear rockets generally have ISPs at least double that of the current best high-thrust engines, hydrolox. (hydrogen+liquid oxygen)