A Fusion Thruster For Space Travel | Hacker News Reader

A Fusion Thruster For Space Travel

spectrum.ieee.org

39 pointsBud19 comments

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?

According to Wikipedia, it's actually Nitrogen-14 and Carbon-11.

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

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

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).

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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_...

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.

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.

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.google.com/#sclient=psy&hl=en&authuser=0&...

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

http://en.wikipedia.org/wiki/Nuclear_fusion#Criteria_and_can...

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

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

Ah, this makes sense. If you've got energy and you want to turn it into thrust without losing too much mass then using it to fire alpha particles off at damn near the speed of light is a great way to do it.

Seems likely. This is basically then just more in the category of VASIMR. However, compared to VASIMR this method seems somewhat less complicated and possibly capable of a higher Isp and higher thrust to mass ratio.

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

Turns out they do actually think they can generate net power: http://nextbigfuture.com/2011/07/fusion-energy-without-radio...

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.

Polywell?

I'm thinking 'well' is referring to 'gravity well' (i.e. 'here on Earth').

Only thing I can think of that is also aneutronic pB11 fueled fusion reactor.

http://en.wikipedia.org/wiki/Polywell#Possibility_of_net_pow...

Yup. I've read too much Niven.

> 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?

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

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