Radiation (photons and neutrons) travel in
straight lines from their source but will happily
"scatter", effectively being re-emitted in random
directions (usually with lower energy) when they
collide with nuclei. Submarine reactors have to
be completely shielded on all sides since
radiation escaping to the surrounding water can
"bounce" back into the manned compartments.
That back-scatter from reactors with limited
(shadow type) shielding was a big problem with uses in airplanes.
Scattering would only be a problem in space
(vacuum, lack of nuclei) with radiation hitting
vehicle structure not covered by the shadow
shield. The shadow shield itself would be really
glowing with x-rays though. I've recently heard this referred to as "shine".
Ed Kelleher
At 02:34 PM 04/15/2018, John Schilling wrote:
The calculations for radiation protection pretty much all assumed you would be doing that, and provided only a "shadow shield" to protect a compact volume some distance away. Turns out it's not safe for humans or electronics to stand a couple hundred meters from an unshielded gigawatt nuclear reactor, and the necessary shielding is heavy even if it "only" covers a few square meters.
      John Schilling
      <mailto:john.schilling@xxxxxxxxxxxxxx>john.schilling@xxxxxxxxxxxxxx
      (661) 718-0955
On 4/15/2018 11:17 AM, John Dom wrote:
If I got this right, the reactor engine mass is in part required for radiation protection. Many science fiction writers (like Clarke in 2001 and before him) were advised to present their fantasy spaceship reactor mounted on a long stick pushing forward behind the crew module on top. The stick spacing would then sufficiently  protect without requiring radiation mass penalty. Any calc on this issue?
The same subject. In the late 50-ies or so, there was a plane based on the B36 bomber configured to fly with a fission (or was it two?) reactor aboard for propulsion. It did fly a few times before it was concluded it soon became too âhotâ for crews aboard and this could then not be remedied.
So I wonder what the Russians worked out for their recently advertised fission motor cruise missile bomber with enormous range. Of course such contraptions are not designed to fly with a crew inside.
John
Â
From: <mailto:arocket-bounce@xxxxxxxxxxxxx>arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of William Claybaugh
Sent: zondag 15 april 2018 17:42
To: <mailto:arocket@xxxxxxxxxxxxx>arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Circa 1968 video about NERVA nuclear rocket program
Â
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On Sun, Apr 15, 2018 at 9:08 AM Perry E. Metzger <<mailto:perry@xxxxxxxxxxxx>perry@xxxxxxxxxxxx> wrote:
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.Â
>
> 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.)
Â
2006 Near-Earth Object Survey and Deflection Study, Final Report. December, 2006; NASA HQ.
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Perry
--
Perry E. Metzger        <mailto:perry@xxxxxxxxxxxx>perry@xxxxxxxxxxxx