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


Puppet_Sock wrote:
>mechdan@xxxxxxxxx wrote:

>>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.

>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.

I am not a physicist. That's why I have questions about
exactly what will happen and what would be the best
material to use.

>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 part, I'm not worried about. Even in the worst case
scenario of fusion contribiting NO PRESSURE WHATSOEVER,
the inner liner can be made out of U238 and implode
itself to 5%c via fission. In fission-fusion-fission
bombs, there are so many neutrons generated by the fusion
stage that the final U238 fission stage is very high
yield (maybe 80% or more). This results in fission fragments
ablating away at roughly .037c. Assuming 80% yield, a
mass ratio of 5.4 gives an velocity of .05c.

>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.

The inner liner should hopefully absorb/deflect most of
the particles, so they contribute to the implosion
momentum rather than contaminating the "particle puff".

Any high energy photons which penetrate past the
U238 will sail right on through the hydrogen propellant,
unfortunately. The ideal thing would be if all photons
magically heated up the hydrogen propellant puff directly.
As it is, the U238 liner will block practically all photons
which might have otherwise contributed directly to hydrogen
propellant heating.

>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.

The electrons are certainly a concern, and I don't
comprehend the role they would play. My guess is that
they're basically helpful. Compression heating will
send them bouncing around at near-C velocities, so I
think they'll be "pulling along" the protons rather
than the other way around.

>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.

Surely some people out there are familiar with how
appropriate materials behave at weapon energies.
I've been mentally modelling the behavior by mostly
ignoring the electrons and thinking in terms of
"nuclei billiards" and behavior of gas particles.

U238 nuclei are so heavy compared to protons that
a proton entering a field of U238 nuclei ends up
bouncing around with erratic brownian motion having
relatively little effect on the positions/velocities
of the U238 nuclei. It's a random drunkard walk,
and probabilities dictate that the proton will
most likely leave the U238 field on the same side
as it entered after not many bounces. Thus, the
proton is "reflected". The bulk effect of all of
the proton collisions is some amount of mixing and
fast temperature equalization. The U238 nuclei end
up with thermal velocities about 1/15 of the proton
thermal velocities.

However, I am assuming the protons are deflected by
the U238 nuclei rather than absorbed (fusion of U238
into Np239). This depends on the fusion cross section,
of course, which I simply don't know. Even if I read
what the fusion cross section was in a physics
reference, I don't actually understand what the
numbers mean.

Also, I may be making a grossly critical mistake
in ignoring the electrons. I know they must have
some effect, but I simply lack the understanding of
plasma physics to analyze the effects.

>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.

Yes. I estimate mass ratios of maybe 1000:1 between
the "parasitic weight" and the puff mass.

>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.

No, what it means is that there is a lot of overhead in
the devices so they need to be made bigger/heavier.
This has a linear effect on the mass budget of the mission.
For example, if overhead reduces efficiency by 80%,
then that overhead magnifies the mass budget fivefold.
If a mass ratio of 1000:1 is required without that
overhead, the overhead increases the necessary mass ratio
to 5000:1.

This is actually extremely good, compared to Orion style
rocket propulsion. The big advantage a "runway" drive over
a rocket is that you don't need to accelerate a fuel
tank. Any overhead only has a linear effect on the mass
budget, as opposed to an exponential effect.

For example, with a perfect 100% efficient Orion style
drive, an exhaust velocity of .037c may be possible.
With a mass ratio of 1000:1, this is good enough to
get up to .25c. However, what if each pulse unit has
an 80% overhead? That implies a 1000:1 mass ratio is
only good enough to reach .05c. To get up to .25c,
you'd need a mass ratio of 1,000,000,000,000,000:1.
Needless to say, that mass ratio is not practical!

Orion style rocket propulsion is already a marginal
concept for interstellar propulsion assuming the best
of conditions. It doesn't take much overhead to take
it completely out of contention for plausible interstellar
missions. In contrast, the practicality of particle
puff propulsion is relatively insensitive to overhead
realities.

Isaac Kuo

.

Relevant Pages

  • Re: Whats a good "proton reflector"?
    ... What if the particle puff ... I like this magnetic acceleration method because there's ... of using "unobtanium" to reflect protons, ... >>the fuel supply is not accelerated, a poor mass ratio only has ...
    (sci.physics)
  • Whats a good "proton reflector"?
    ... but there's one part of the design I'm not sure ... The conical "shaped charge" liner implodes, ... out hydrogen protons in the process. ... My intuitive understanding is that the liner's heavy nuclei ...
    (sci.physics)