A tangentially related thought: In looking over the years at the need
for ~2000s Isp, ~ 1km/s/day delta V inner-system space transport to
supplant chemical rockets, I came up with a rule-of-thumb: That to do
this with nuclear-electric ships, the nuclear electrical generation part
of such a system needs sustained (weeks) power output on the rough order
of one kilowatt per kilogram of generating plant. Else we're back in
the mousefart thrust trap.
My impression is that the current nuclear-electric SOTA might approach
100 kg per kw sustained output. Anyone have better numbers than that,
or thoughts on alternative nuclear approaches?
Henry (V)
On 8/23/2019 10:14 AM, Henry Spencer wrote:
On Fri, 23 Aug 2019, William Claybaugh wrote:
The de facto Western ban on testing in the atmosphere means testing will
have to be off-planet.
And for something like NSWR, I'm not sure I can reasonably object to that. For a solid-core design that can be expected to pretty much contain all fission products, I could see testing on the ground using exhaust scrubbers, *perhaps* moving to open-air testing after containment was well verified. But for advanced systems like NSWR, where there's inherently a lot of radioactive garbage in the exhaust and so even one unscrubbed run would be bad, I'd worry about scrubber effectiveness and scrubber failure modes.
Moreover, just a little bit off-planet isn't enough -- you want to test (and operate) outside Earth's magnetosphere.
(Gosh, could this be something that the Lunar Gateway is actually good for? :-) )
This favors "known" designs that can be relied upon to produce a usable vehicle on the first try.
They may be favored, but if they can't meet the mission specs -- and the "known" designs realistically have at best modest advantages over chemical rockets -- then they're not even in the running. The objective is to be useful, not just usable. A modest gain over chemical isn't worth the very high development costs and nasty political hassles.
If reuse is a part of the plan, then some sort of refueling facility is implied, all in a nuclear safe orbit (1000 km, generally). Getting a new payload on a hot vehicle is left as a problem for the reader.
What probably makes the most sense is a concept that was seen in the late 60s: reusable nuclear tugs. They're not part of your mission-specific vehicle; they dock to your ready-to-go vehicle, boost it from LEO to (say) Mars trajectory, and then turn around and come back (immediately, using brute-force high-delta-V retrofire, not an economy orbit) to return to their base. So your payload never spends much time in their vicinity.
If you're going to have to fiddle around for a while during/after docking but before departure, you can fit substantial shielding around the tug docking interface, and have it moved aside five minutes before departure. (Launching it will cost something, but it doesn't *go* anywhere, so that cost only has to be paid once and then it's available for all future departures. Think of it as the equivalent of airport de-icing trucks.)
I also observe that the whole notion of "nuclear safe" orbits implicitly assumes that a failed nuclear-propelled vehicle would be abandoned and left to come down at random. This makes no sense -- quite apart from the side effects of doing so, it's too expensive to just abandon.
(Well, except perhaps in a spectacular one-shot program with no continuation or follow-on, which is expected to crash and burn once it meets its primary objective, and therefore isn't interested in building support infrastructure -- e.g. tow trucks for disabled vehicles -- for future use... and that doesn't make much sense either. Been there, done that, know better now.)
Henry
