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Old 08-30-2017, 05:56 PM   #21
malloyd
 
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Originally Posted by Anthony View Post
That doesn't exactly follow. The energy content of the reaction chamber is limited, but that isn't a limitation on power, since a higher exhaust velocity generally results in lower time spent in the reaction chamber.
I'd have to think about that more. At best I think it just changes the slope of the curve - the energy content increases a lot faster (with the square of the exhaust velocity) than the residence time falls (as 1/v), but I have a sneaking suspicion there's time limit in there that's a function of the speed of sound in something between the front wall of the combustion chamber and the end of the nozzle.
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Old 08-30-2017, 06:27 PM   #22
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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I'd have to think about that more. At best I think it just changes the slope of the curve - the energy content increases a lot faster (with the square of the exhaust velocity) than the residence time falls (as 1/v).
At constant thrust the mass flow rate is inversely proportional to velocity, and the residence time is also inversely proportional to velocity, so the mass in the chamber is inversely proportional to the square of velocity. The energy per unit mass is proportional to the square of velocity, and thus the energy in the chamber is constant (it's just pressure * volume * a constant that depends on how closely the propellant resembles an ideal gas).
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Old 08-30-2017, 06:52 PM   #23
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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We seem to be circular here. If you are doing this, then you have two different rockets, rather than the OP's contention that you would take off, transit, and land, all with the same engine.
The OP's proposal was just to use a hydrogen reaction mass antimatter thermal rocket. No water, and no mode-switching. That was something else entirely... The only trick to it was the antilithium giving 6x delta-V, as per the blog post. Which makes no sense, as per the thread.

An antimatter thermal rocket isn't necessarily the best liftoff engine by the book since it's got only 0.4 G thrust per system, but you can use it and it's not particularly egregious - its backblast is just rather hot hydrogen, not anything particularly nasty.
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Old 08-30-2017, 08:22 PM   #24
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

Antilithium should not really have any benefits (antimatter-matter reactions become problematic because of kaon, muon, and pion radiation after they get beyond position-electron reactions [which are the only antimatter reactions that just produce gamma rays]), and it should be even more expensive than antihydrogen by orders of magnitude. Unless you are able to harvest it from a natural source (which would require very high levels of superscience to avoid becoming annihilated the moment you started mining), you would have to create antihydrogen and then fuse antihydrogen to antilithium. You would have to have a fusion reactor that could not only do proton-proton reactions but could do so using antiprotons. Is it possible? Yes, but it is not plausible by any conceivable level of technology.
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Old 08-30-2017, 09:16 PM   #25
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

It's hard to to say what will absolutely never be done by any human culture or descendant thereof in the infinite future, let alone what no alien species will do in the entire universe ever.
If it's even remotely barely technically possible, I bet someone/something somewhere has or will eventually do it, no matter how bizarre, inefficient, or plain suicidally dumb it seems.
But it does seem hard to imagine under what circumstances such a fuel would ever be economical.
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Old 08-30-2017, 09:25 PM   #26
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Antilithium should not really have any benefits (antimatter-matter reactions become problematic because of kaon, muon, and pion radiation after they get beyond position-electron reactions [which are the only antimatter reactions that just produce gamma rays]), and it should be even more expensive than antihydrogen by orders of magnitude. Unless you are able to harvest it from a natural source (which would require very high levels of superscience to avoid becoming annihilated the moment you started mining), you would have to create antihydrogen and then fuse antihydrogen to antilithium. You would have to have a fusion reactor that could not only do proton-proton reactions but could do so using antiprotons. Is it possible? Yes, but it is not plausible by any conceivable level of technology.
Antihelium has been done. Not in any significant quantity of course, but for TL8 not bad.

I don't think making antilithium is inconcievable for TL 11. We're talking a tech level that can build a weapons-grade x-ray laser that weighs a third of a pound, here.
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Old 08-30-2017, 10:17 PM   #27
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Antilithium should not really have any benefits (antimatter-matter reactions become problematic because of kaon, muon, and pion radiation after they get beyond position-electron reactions [which are the only antimatter reactions that just produce gamma rays]), and it should be even more expensive than antihydrogen by orders of magnitude. Unless you are able to harvest it from a natural source (which would require very high levels of superscience to avoid becoming annihilated the moment you started mining), you would have to create antihydrogen and then fuse antihydrogen to antilithium. You would have to have a fusion reactor that could not only do proton-proton reactions but could do so using antiprotons. Is it possible? Yes, but it is not plausible by any conceivable level of technology.
You could shoot lithium ions through a non-orientable wormhole.

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Old 08-30-2017, 11:30 PM   #28
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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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, 10:21 AM   #29
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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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 08-31-2017, 10:31 AM   #30
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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You could shoot lithium ions through an orientable wormhole.
The terminology is non-orientable wormhole. But yeah, I've been thinking that since I started reading this thread.

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