Skipjack wrote:A little bit of radioactivity in the atmosphere is completely irrelevant. Besides, the hydrogen reaction mass barely gets radioactive anyway. Also want to point out that subsequent projects to NERVA demonstrated much higher T/W than 1, high enough to SSTO.
If we're talking about a solid-core engine, then yes, the radioactivity is trivial. If we're talking about a leaky solid core engine (which Tom was), we're probably fine unless we're talking about a sizable percentage of the core disintegrating, or a lot of flights with the cores disintegrating. On the other hand, a fission fragment rocket wouldn't be a good thing to use in the magnetosphere ever--nor would Orion itself.
Remember, all of this stuff is likely to enter in a localized region at the poles. What happens after that exceeds my meteorological competence. I can't think of a regulatory regime that would allow more than a few tens of terabecquerels of fission products per year to be deposited into the atmosphere, and the odds of anybody approving a fission-based SSTO seem vanishingly small if for no other reason than you couldn't get it to pass a safety review.
That said, I'd think that using even a dusty nuke would be relatively uncontroversial (except to the arithmetically challenged) as long as the exhaust trajectory didn't intersect the magnetosphere. I think we're stuck with chemical launchers until we can get some kind of fusion SSTO going.