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Old 08-29-2017, 04:28 PM   #5
Curmudgeon
 
Join Date: Sep 2011
Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

Quote:
Originally Posted by ADAXL View Post
I recently stumbled over this article:

http://hardmaths.blogspot.co.at/2017...n-options.html

Go to the bottom of the article to “Option: Anti-Lithium (TL11)”. This is an idea for a stronger version of antimatter drives such as the antimatter thermal rocket. An antimatter thermal rocket gets much better delta-V per tank (10.8 mps) with anti-lithium.

This kind of drive combines good acceleration with good delta-V. A spacecraft could lift off from a planet (especially if it is winged and has a drive with the air-ram option that gives some free delta-V) and fly right to another planet. The old science fiction staple, the interplanetary spaceship that can land on a planet, appears possible this way.

So I wonder how “hard” the science behind drives with anti-lithium is. Of course, the idea of using anti-lithium (or just a bigger load of anti-hydrogen) seems straightforward enough, but I wonder what stress this puts on the engine and how much radiation it produces.

Can anybody with a background in science say if this makes sense? Thanks in advance!
Yes and no, at least as I follow his argument. With the caveat that most of my knowledge of antimatter was gathered in passing in the 1970s, you should take what I say with a fairly large grain of salt, both because my understanding is oversimplified and science has moved on since, but here goes...

Atoms of ordinary matter are mostly made up of protons, electrons and neutrons (which can decay into an electron and a proton). Antimatter atoms are composed of particles of the same mass but opposite charge: anti-protons, anti-electrons and anti-neutrons (which decay into an anti-electron and an anti-proton).

Anti-particles will annihilate the corresponding particle resulting in close to total conversion of the mass into energy. We can handwave the annihilation of electrons as having insignificant mass (for game purposes), but since neutrons are about as massive as protons, we do have to account for their energy.

Under Avogadro's Law, two identical volumes at the same temperature and pressure will contain the same number of particles of gas, regardless of the densities involved, i.e. a cylinder of helium will have exactly as many individual molecules of helium as the same cylinder filled with oxygen would have, despite oxygen being denser than helium. It thus follows that, in gaseous form, the relative energy density of two forms of matter for fuel purposes in an antimatter powerplant is directly proportional to the number of particles composing the nuclei of that matter. (In other words, what isotope you're using makes a difference.)

For hydrogen/antihydrogen, the most common isotope is 1 (i.e. one proton and no neutrons), giving us our base energy. Isotopes 2 (one proton and one neutron) and 3 (one proton and two neutrons) would give two and three times the energy for the same volume, respectively.

At TL10, under this scheme, you should be able to harness helium/antihelium as an antimatter fuel. Isotope 3 (two protons and one neutron) would produce about the same energy as isotope 3 of hydrogen. (Actually helium-3 gives a bit less energy than hydrogen-3 because protons are slightly less massive than neutrons.) Isotope 4 (two protons and two neutrons) gives four times the energy.

And at TL11, we have lithium/antilithium. Isotope 6 (three protons and three neutrons) gives six times the energy and is evidently what the author was thinking of. Isotope 7 (three protons and four neutrons) would give seven times the energy.

So far, all well and good. Now, I've only counted stable isotopes for the elements mentioned. There exist several more isotopes but they have short half-lives (under 8 milliseconds, IIRC) and so probably aren't suitable for use as fuel.

I've also discounted relative abundance and that is probably an issue as it may indicate difficulties in producing the anti-matter counterparts. Hydrogen is the most abundant element in the universe (75%) and isotope 1 is the most abundant isotope at 98%. Helium is the second most abundant element at 23% and 99.9998% of all helium is isotope 4. Lithium though is 25th out of the 32 most abundant elements in the solar system and the most common isotope is 7 (95% of all lithium), so a lithium/antilithium antimatter fuel doesn't strike me as particularly useful, even given superscience. Given that it doesn't quite double the energy you'd get using helium-4, the value of doing so falls off fairly rapidly. If you take 37.5% more energy to produce antilithium-7 than you do to produce antihelium-4, you're breaking even with anti-helium-4 for energy costs and your only benefit is being able to fit more fuel in the same space. Beyond 37.5%, you're paying a premium for that extra space. There's also the question of lifting the extra mass of the fuel. Lithium-7 is going to weigh seven times as much as the same gaseous volume of hydrogen-1.

So, the superscience is more or less sound in principle, but the wrong isotope may have been used and it does have its drawbacks.

Last edited by Curmudgeon; 08-29-2017 at 04:37 PM. Reason: corrected abundances
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