Re: What's a good "proton reflector"?

mechdan@xxxxxxxxx wrote:
> I have a concept for fast interstellar propulsion with current
> technology, but there's one part of the design I'm not sure
> about.
>
> Basically, the starship is a magsail propelled by high
> velocity puffs of protons from specially designed nuclear
> bombs. These bombs look like traditional Teller-Ulam
> fission-fusion bombs, except that there's a conical
> "shaped charge" inside the fusion stage. Pictures of
> the concept are here:
>
> http://members.cox.net/mechdan/ppd/index.html
>
> The conical "shaped charge" liner implodes, squeezing
> out hydrogen protons in the process. At the speeds and
> energies involved here, both the liner and the hydrogen
> are dense plasmas.
>
> My intuitive understanding is that the liner's heavy nuclei
> will act as proton reflectors instead of simply mixing with
> the protons because the thermal motion of heavy nuclei
> is an order of magnitude slower than that of the protons.
> Thus, a proton entering the field of heavy nuclei will
> randomly bounce around, probably exiting the heavy
> nuclei field after a relatively small number of "bounces".
>
> Is that right? Please correct me if I misunderstand
> what would really be going on.
>
> Also, I'm neglecting the possibility of fusion. I think I
> want an imploding liner material which has a high
> atomic weight, but not if it's something likely to absorb
> protons via fusion.
>
> So, what's a good "proton reflector" material to use?
> Is U238 a good material?

Your picture of "squirting" stuff as you have in the picture
is fairly non-numerical. That is to say, it probably won't
work that way. Materials in such situations don't behave
the way you might expect them to. I'd hate to say it
could not possibly work, but that would be the way I'd
bet unless you have experimental efforts to show it.

Just as one reason to think there might be a problem:
Consider that transferring that much energy is not going
to happen at 100% efficiency. It's going to make plasma
out of the entire thing long before it gets anywhere near
5% of c. That's going to tend to make the entire thing
start to expand, causing all kinds of losses. The steeper
the angle of the cone the worse that will be. The explosion
will tend to either go past the "puff" or the puff will puff in
all directions instead of where you want to go.

It's not just protons that will be flying around in such a
system. There will be plenty of gamma energy, and other
particles as well. Depending on what reaction you use
for the explosion, you could have significant neutrons.
I don't know, there might even be a significant portion of
the energy in hot electrons. Though I have never done
any weapons work, so that could be junk. A thermo-nuke
certainly uses lots of stuff besides hydrogen, so there's
lots of stuff besides protons flying around.

To figure out how to get a puff, if it's even possible, would
be a huge calculation. And probably a huge amount of
"tuning" would be required. The cone shaped part can't
just be something that reflects protons. It has to be such
that it catches lots of energy from the blast, *and* transfers
that to the "puff" or you don't get a puff. Your picture makes
it look like squeezing a pumpkin seed between your fingers.
But materials don't behave that way at weapon energies.

In fact, it's such a gnarly calculation that I'd not have any
confidence in it at all without at least a few experiments.

Next, your system is grossly inefficient. If only the portion
that gets puffed (assuming it *does* get puffed) winds up
being propellent, then the rest is parasitic weight. In the
system you've got here, it looks like the bulk of energy
will go into useless flashing. That means you need way
more explosions than the standard Orion.
Socks

.

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