
A Fusion Thruster For Space Travel - Bud
http://spectrum.ieee.org/aerospace/space-flight/a-fusion-thruster-for-space-travel/0
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russell
OK, I need an explanation. Carbon-12 is stable, so how come it decays into 3
helium-4 atoms. Is the carbon-12 not really an atom but some kind of
metastable thingy with too much excess energy to be stable?

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Bud
The process produces "excited carbon nuclei", not stable normal carbon-12
atoms.

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aidenn0
Hurray for talking about specific power but not specific impulse or specific
energy.

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uvdiv
It looks like they are actually talking about specific energy, and through a
series of blunders mislabeled it as specific power:

" _The specific power of the proton-triggered boron fuel would be so great
that a mere mole of it (11 grams) would yield roughly 300 megawatts of power.
(According to Chapman, using this aneutronic fusion technique with helium-3
isotopes would yield 493 MW per mole. But boron is a more attractive fuel
source because it is abundant on Earth and helium-3 is scarce.)_ "

This looks like nonsense. I think it was supposed to be megawatt-hour -- the
quote is essentially correct, after you r/megawatt/megawatt-hour/, r/MW/MWh/,
and r/specific power/specific energy/.

No idea why they are using this unit outside its native context (electricity
generation).

\-------------------------

I don't know about specific power, but the (theoretical) specific impulse is
very good. The exhaust speed of the fusion alphas is sqrt(2 * 2.9 MeV / 3.7
GeV/c^2) = 1.2e7 m/s (or 0.04 c). Since these alphas constitute all the mass
of the fuel/propellant (which are the same in this scheme), this is also the
_effective_ exhaust velocity. Or a specific impulse of 1.2e8 seconds.

This is one of the best specific impulses conceived of, and (I think) is about
the theoretical limit for a fusion-powered rocket carrying its own reaction
mass.

[http://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_...](http://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods)

For some reason I don't see it represented on this (very big) wiki page. They
do have the "fission fragment rocket", which is conceptually the same thing
applied to fission reaction products.

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ScottBurson
It endlessly annoys me how journalists confuse energy and power. It's
sometimes possible to figure out what they must really have meant, but not
always. To see this error in IEEE Spectrum -- there's just no damn excuse.

I was wondering if maybe they meant J rather than W, but you must be right --
300MJ/mol doesn't sound like much.

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uvdiv
Well, the energy per reaction of p+11B is 8.68 MeV, or 837 GJ/mol. This is
about 230 MWh/mol, so _that_ would make sense.

[http://www.nndc.bnl.gov/chart/reCenter.jsp?z=2&n=2](http://www.nndc.bnl.gov/chart/reCenter.jsp?z=2&n=2)

[http://www.google.com/#sclient=psy&hl=en&authuser=0&...](http://www.google.com/#sclient=psy&hl=en&authuser=0&source=hp&q=\(7.2890+MeV+%2B+8.6679+MeV+-+3+*+2.4249+MeV\)+*+Avogadros+number&pbx=1&oq=\(7.2890+MeV+%2B+8.6679+MeV+-+3+*+2.4249+MeV\)+*+Avogadros+number&aq=f&aqi=&aql=undefined&gs_sm=e&gs_upl=6679l9537l6l23l17l2l0l0l1l350l3306l0.2.11.1l14&bav=on.2,or.r_gc.r_pw.&fp=bd71108a3b251f45&biw=1511&bih=971)

edit: Another source for "8.68 MeV", which doesn't require math:

[http://en.wikipedia.org/wiki/Nuclear_fusion#Criteria_and_can...](http://en.wikipedia.org/wiki/Nuclear_fusion#Criteria_and_candidates_for_terrestrial_reactions)

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pavel_lishin
What's the downside to using this process to generate energy here in the well?

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jerf
Like all the other known forms of fusion that fit on a bench (and there are
several, such as the Farnsworth Fusor [1]), it doesn't produce more energy
than it consumes.

The article doesn't say that directly, I'm inferring it from the fact that if
it was net-energy positive, this would not be "hey, this might make more
efficient propulsion", but "HEY EVERYBODY, HOLY COW, FUSION IS HERE!". I am
comfortably with this inference.

I also would imagine the efficiency in question is that it has a very high
specific impulse [2], and thus requires very little mass, not anything else.
Satellites in the inner solar system generally don't have a power problem,
they metaphorically just stick a cup out the window and grab some of the power
just streaming by them nearly continuously. They have a _mass_ problem.
Converting solar power mass-efficiently to thrust is a big win.

[1]: <http://en.wikipedia.org/wiki/Fusor>

[2]: <http://en.wikipedia.org/wiki/Specific_impulse>

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DennisP
Which makes it pretty silly to talk about recovering energy from the particle
stream, instead of getting that energy from your power source directly.

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DennisP
Turns out they do actually think they can generate net power:
[http://nextbigfuture.com/2011/07/fusion-energy-without-
radio...](http://nextbigfuture.com/2011/07/fusion-energy-without-
radioactivity.html)

For a power plant they'd need a laser that doesn't exist yet, but it looks
like commercially-available lasers could do the experiment.

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vault_
> A traveling wave tube—basically an inverse klystron—captures most of the
> particles’ flux kinetic energy and efficiently converts it into electrical
> energy

Anyone else think this sounds like sci-fi technobabble?

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pjscott
It sounds like technobabble because technobabble is a crude imitation of dense
technical jargon. Sometimes reality sounds unrealistic.

