NTR

I do not recommend looking up "NTR" on the interweb. If you know what it means, I assume you are a mainline conservative and/or a hentai afficionado; if you don't know, I am not here to convince you to become either. For the rest of you: nuclear thermal rockets. NERVA, for my older readers. For neo-NERVA UltraSafe is planning six years to demo.

Up 'til late last year I kept confusing NTR-ry with the Orion project (itself not to be confused with the Orion brand of chemical rocket); and I didn't figure out where to assign NERVA until this week. Late last Tuesday, Winchell Chung tossed up to his site another subsection, for the next generation: to do with centrifuging. Anyway, I spent some weeknights this week reading up. I am here and now trying to, er, sus it all out.

Nuclear thermal doesn't run by the pulsed explosions of an Orion. It is more like the steady plasma-ejection of an ion drive. Chung proposes a fine way to understand it (wish I'd known before this week): a nuclear reaction where they use the coolant for propellant. Toward the end, when the rocket needs to quit propelling, they still have to run the coolant through the system, to prevent overheating and a state-change, assuming they don't simply eject the core. So it takes some planning as to what gets run, when, and how quickly.

The general notion is that if propellant is hot, it goes out the nozzle faster. Faster means more momentum, by Newton's Laws; that the craft might go forward, faster. Unlike the Orion they're not looking for some massive short-term thrust to get off to orbit. They want something that accelerates to get from high Earth orbit over to some other planet, faster: usually to Mars.

Classic NERVA systems, from the [para-]Orion era to the UltraSafe era, use a solid reactor. Those designs kept/keep it all from melting down or, in zero gravity, from melting all over the place. Problem with that remains, a temperature-limit: the melting point of the nuclear metal. NERVA constrained the temperature and, with it, UltraSafe's thrust/weight ratio. Below t/w = 1, no rocket is getting off the ground. On the plus side, UltraSafe can use diluted fuel - for Uranium they call it "HALEU" - so don't deal with the AEC, just with the FAA. Classic solid NERVA needs to hitch a lift on a rocket, to get into space.

Post-NERVA liquid solutions by contrast can get themselves off the ground... if they get clearance to use higher-grade uranium (so-called "weapons grade") and nobody minds spewing fallout all over the place. With centrifuging, at least you know where the melted uranium is: on the outer edge of the 'fuge. Although this, I believe, is a solution for microgravity, not for getting off a planet which has its own "centrifuge" in ms^-2, hence why no-one cared to post its t/w for Chung. It would compete with fusion. UPDATE 2/24: Pluto II.

As efficiency goes I generally prefer the magnetic force over brute heat. If nothing else I think a magnet might direct more of my super-speedy plasma out of the engine. As I look at all the NTR options, I also cannot help but worry about melting my engine... simultaneously, about not melting my engine, for when the neutrons start corroding it. Space Chernobyl, as the meme goes; like all the best memes, it's more true than not.

NERVA did, however, get you 49000 N of thrust against Ebrahimi's best-case 100 N and it provides its own energy-supply. We can assume similar for UltraSafe. "Efficiency" for overall speed is all well and good... for the outer planets. Inwardly we need acceleration and deceleration; we need higher thrust. Venus is an inner planet. UPDATE 2/22: NASA is now leaning NERVA for Mars too.

These solutions might end up being tested in space once we're already better-located in space. I do note that nobody is keen on "weapons grade" nuclear fuel here on Earth. That will assuredly change when Mars and/or the Belt have room to play in. Nobody here's in any position to stop a Martian from using Martian ores for Martian interests . . .

... at which point, we'll need to research this stuff too; to keep Mars from, well, subjecting us to NTR.