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Old 08-29-2017, 02:09 PM   #1
ADAXL
 
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Default [Spaceships] Anti-Lithium for Drives – Does this work?

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!
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Old 08-29-2017, 02:21 PM   #2
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

It's pretty much nonsense. Antilithium has the same energy content per unit mass as antiprotons, and would have the same performance. There are two potential benefits, but both are probably already incorporated in regular TL improvements:
  • It's possible the annihilation products would be easier to deal with. I wouldn't bet on this, though.
  • Anti-lithium is probably easier to store than anti-hydrogen.
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Old 08-29-2017, 02:27 PM   #3
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

Any scheme for antimatter rockets for lift is immediately suspicious, if you care at all about the place you are leaving.
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Old 08-29-2017, 10:11 PM   #4
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

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Originally Posted by sir_pudding View Post
Any scheme for antimatter rockets for lift is immediately suspicious, if you care at all about the place you are leaving.
There are exceptions. In principle, for example, you could have a rocket that worked by using a working fluid (probably water) as reaction mass, energized by combining it with a tiny amount of antimatter (very tiny amounts, for reasonably-massed ships).

The result would be a rocket that produced very little radiation but could in theory give bountiful thrust. In practice, we're a long way from being able to build such a thing, we don't know how to store tiny amounts of seetee in reasonably-massed/volume storage systems, and we'd have to make the antimatter at high expense.

But it's possible in principle, and if we could built it it wouldn't be devastating to its launch area. You'd probably want to launch it in open areas, like a patch of desert, but it wouldn't be a WMD for the launch site.
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Old 08-29-2017, 11:37 PM   #5
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

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Originally Posted by Johnny1A.2 View Post
There are exceptions. In principle, for example, you could have a rocket that worked by using a working fluid (probably water) as reaction mass, energized by combining it with a tiny amount of antimatter (very tiny amounts, for reasonably-massed ships).
Sure, but such systems would actually be poor rockets compared to straight antimatter rockets with gamma exhaust. Which means that you aren't going to use the same engine for lift that you use for deep space burns, or at least your engine has two modes, one with significantly poorer ISP.
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Old 08-30-2017, 12:01 AM   #6
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

It's always thrust vs. reaction mass economy.
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Old 08-30-2017, 10:30 PM   #7
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

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Originally Posted by sir_pudding View Post
Sure, but such systems would actually be poor rockets compared to straight antimatter rockets with gamma exhaust. Which means that you aren't going to use the same engine for lift that you use for deep space burns, or at least your engine has two modes, one with significantly poorer ISP.
No, a photon drive would not be better for taking off and landing on planets, or maneuvering in atmosphere, or most of such activities. The amount of fuel for a gamma-photon rocket is way higher than for seetee-activated reaction mass. For the latter, water is cheap, plentiful, and easy to handle, and an inifinitesimal amount of seetee will turn a few tons of water into a super-hot plasma that makes a dandy reaction mass. You get plenty of thrust for relatively little fuel and very cheap reaction mass.

Nor do you have to use water, you could use any fluid or gas your drive could handle. Water would just be convenient.

(Modulo our current inability to build such a thing, of course. But we can't build a useful gamma-photon drive, either.)

If you pump out enough thrust with a gamma-photon drive to get from Earth's surface to LEO or escape, then with even a tiny spacecraft you need substantial amounts of antimatter to do it, and yes, your drive is also a WMD on a big scale. The energy-to-thrust ratio for photon drives is rotten.

A photon-drive surface-to-orbit/escape vessel is theoretically possible, in the physics sense, but it uses a genuine death ray as a means of propulsion. I foresee legal and diplomatic issues.

A photon drive is preferable if you're hoping to use antimatter to achieve relativistic velocities, yes. But at intra-solar and planetary velocities the tradeoff is different.
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Old 08-31-2017, 09:21 AM   #8
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

Quote:
Originally Posted by sir_pudding View Post
Sure, but such systems would actually be poor rockets compared to straight antimatter rockets with gamma exhaust. Which means that you aren't going to use the same engine for lift that you use for deep space burns, or at least your engine has two modes, one with significantly poorer ISP.
I've never seen a proposal for an antimatter rocket with gamma exhaust. The thing in Spaceships uses the short-lived charged mesons that are the first breakdown phase of matter/antimatter annihilation and magnetically sends those rearward.

The gamma rays resulting from some of the first-stage annihilations and then the meson breakdown are not used for thrust at all. This is what makes the ^Total Conversion Drive even more fuel efficient.

I'm afraid that Sir Pudding has jumped in his head to a TC drive that inconveniently emits all the converted energy as gamma rays while retaining a chemical rocket-like thrust-to-weight ratio. Not a hard science thing at all.
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Old 08-29-2017, 02:27 PM   #9
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Default Re: [Spaceships] Anti-Lithium for Drives – Does this work?

In terms of basic energy yield, I'm fairly sure anti-Lithium shouldn't have any more than anti-hydrogen per unit mass. It's possible that more of that energy can be used (the particle physics there is way outside my knowledge).

It would probably take less volume and containment apparatus to store, but Spaceships ignores both of those things.

I've also got nothing on why it's characterized as 6x delta-V, which ought to require 36x as much energy.
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Old 08-29-2017, 04:28 PM   #10
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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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