[AR] Re: Circa 1968 video about NERVA nuclear rocket program
- From: "Perry E. Metzger" <perry@xxxxxxxxxxxx>
- To: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- Date: Sun, 15 Apr 2018 11:07:41 -0400
On Sat, 14 Apr 2018 22:28:48 -0400 (EDT) Henry Spencer wrote:
On Sat, 14 Apr 2018, Perry E. Metzger wrote:
Specific impulse of around 900 seconds with high thrust seems
like a really useful technology, at least for off-planet use.
Unfortunately, it suffers from high dry mass, because of huge
insulated LH2 tanks and engines that are very heavy for the thrust
they produce, which hurts mass ratio badly enough to quite
significantly reduce the benefits of that appealing Isp. Isp is
about *engine* performance, but it's *vehicle* performance that
actually delivers payload, and they are not the same thing. There
are still benefits, but they aren't as large as you might think if
you look only at Isp. And they come with costs.
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.
Are there other technologies out there which give good overall
performance along with high Isp and high thrust? It seems like all the
interesting high Isp mechanisms out there (like various ion drives)
are quite low thrust. 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.)
Perry
--
Perry E. Metzger perry@xxxxxxxxxxxx
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