[AR] Re: Circa 1968 video about NERVA nuclear rocket program
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 15 Apr 2018 16:39:03 -0400 (EDT)
On Sun, 15 Apr 2018, Perry E. Metzger wrote:
Unfortunately, it suffers from high dry mass...
Ah, what a pity. I had assumed that things like NERVA based tugs might
work well, too, given the nice Isp and reasonably long reactor lifetime,
but it's true, it profits you little to have an excellent Isp and high
thrust if you're tied to a huge engine mass to do it.
The idea is still of interest, just not quite as wonderful as it might
seem at first glance. Some of the final design concepts did in fact
envision fully-reusable nuclear tugs -- for example, they could boost a
Mars expedition into Mars transfer trajectory, and then separate, do a big
retrofire burn, and return to a LEO base, never getting very far from
Earth themselves.
Are there other technologies out there which give good overall
performance along with high Isp and high thrust?
Unfortunately, as noted in my post earlier today, the combination of high
Isp and high thrust inherently means *very* high power. The energy has to
come from somewhere.
Power beaming from base stations is of some interest, at least when close
to home. Making it work over solar-system distances is hard, at least in
the near term.
If you want a self-contained vehicle, right now, packing that much energy
into a reasonable mass means nuclear fission. In fact, something somewhat
better than the NERVA engines (in both Isp and T/W) would really be
preferable, and there are plausible concepts (e.g. gas-core nuclear and
nuclear salt-water), although they aren't nearly as fully developed.
Fusion is hard to do, especially with reasonable dry mass, and the more
practical fusion reactions all produce a lot of neutrons.
Longer term, antimatter is a possibility, although making it is formidably
expensive, and it too comes with radiation problems. And if we can find a
way to make small black holes (they don't seem to be common enough in
nature that we could plausibly find them ready-made), there are things
that could be done with them.
... What's an engineer to do if he's impatient and wants to move an
asteroid or comet to L4 to disassemble into raw materials? (I mean this
last bit slightly facetiously, but only slightly.)
For moving *big* masses around, nuclear-bomb propulsion has some appeal.
It's hard to make it work well for spaceships of reasonable size -- such
systems inherently want to be huge -- but for moving asteroids around on a
short time scale (including deflecting a threatening one on short notice),
it's the one halfway practical near-term approach. Although you might
want to put your disassembly facility somewhat farther out than lunar L4,
because big nuclear explosions in space can have unpleasant side effects
at considerable ranges, and the Moon's orbit may not be far enough away.
Earth's L2 point, maybe.
Henry
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