That underlined part is the most important thing I had missed so far; your explanation is more in-depth than what I knew, tho!
And the damned thing weighs like a... well, a huge chunk of quartz.
Hahaha! Made my day. Tsoukalos was right, after all, Chinese dragon myths were actually about alien spacecraft!
Well, by reading this thread, one important thing I've learned is that NTRs (solid-core, at least) have great Isp mostly because of the ability to use H2 for propellant rather than being confined by whatever results from a combustion (H2O at best?), not because of its inherent engineering or chamber temperature. Kinda puts some brakes on my RPG idea of using CO2 NTRs extensively on Mars.
But then, if enriched fissile material becomes more cheaply available after the next step in space colonization, perhaps using CO2 NTRs has economic benefits? The propellant is basically just atmosphere, or even better, lying around frozen at the poles.