Re: Fission-fragment rocket propulsion

On Tue, 14 Aug 2007 05:49:27 -0700, "Carey Sublette"
<careysub@xxxxxxxxxxxxx> wrote:


"IsaacKuo" <mechdan@xxxxxxxxx> wrote in message
news:1187069073.432750.253630@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Aug 13, 7:56 pm, John Schilling <schil...@xxxxxxxxxxxxx> wrote:
On Mon, 06 Aug 2007 21:18:39 -0400, Nyrath the nearly wise

<nyr...@xxxxxxxxxxxxxxxxxxxxxx> wrote:
I was bemused at this Wikipedia article about fission-fragment
rocket propulsion.
http://en.wikipedia.org/wiki/Fission-fragment_rocket
The article mentions that specific impulses of
100,000 seconds "are possible."
...
Also "specific impulse" is misleading, arguably just plain
wrong. The exhaust velocity may indeed be 1,000,000 meters
per second or so, but the exhaust represents only a small
fraction of the fuel. Most of the uranium is in the boring
old reactor part of the system, serving as a neutron source
but otherwise just dissipating heat without producing thrust.

I can understand your objection if we're talking about
an old fission fragment rocket design involving a
neutron source and a rocket "chamber" lined with
fissile material. However, neither of the fission fragment
rockets described on that Wikipedia page are of
this design. In both of them, the "reactor" uses the
fission fragment fuel itself. No auxiliary neutron
source is required.

Of the two designs described on this page, only the second concept (the
nanoparticle reactor) escapes John Schilling's criticism.

The first concept describes a system that irradiates fuel deposited on
bundles of rods. Most of the fissionable material in the this concept would
be in "a reactor core where additional surrounding fuel made the bundles go
critical", exactly what John critiques. The bit about the bundles "going
critical" is, BTW, an incorrect description of what is happening - the
reactor is critical, it produces a sustained neutron flux that fissions the
propulsion fuel on the rod surface.

Right. And I was assuming that the "reactor" listed for the second
concept, also included a fixed monolithic fuel blanket. I've since
tracked down the original reference, and they claim otherwise. But
they only give one paragraph on reactor design, and I'm skeptical.


The nanoparticle reactor works by making the fuel particles so small that
fission fragments escape from the interior and thus all of the fuel
potentially becomes working fluid. The description is engaging in wishful
thinking when it declares that this design (which relies on a magnetic
bottle to suspend and contain an unmoderated nanoparticle fuel mass)

Nit: They are depending on a fully-moderated, thermal-neutron design,
courtesy of a very good moderator/reflector blanket.

as one that "decreases complexity". Magnetic bottles are easy to sketch
on vu-graphs but notoriously difficult to engineer, especially in a
flight-ready package.

Understatement. The moderator/reflector blanket, with associated cooling
system, is another tricky part, and it doesn't help that they both want to
be in the same place.


The nanoparticle system would probably work better with plutonium, with its
smaller critical mass and the fact that it is usable with fast and
intermediate spectrum reactors.

They can't afford to go fast; they're trying to achieve criticality with
eleven kilograms of U-235 spread uniformly through a one-by-five meter
cylinder. Even with full thermal-neutron fission cross sections that's
going to be tough; BOTE suggests their moderator/reflector has to be at
least 60% efficient. If they tried the same trick with fast neutrons,
they'd need to wrap the thing with a 99.7% efficient neutron reflector.
Maybe 99.5% with fast neutrons and plutonium.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
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.

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