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Old 08-29-2017, 11:11 PM   #11
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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 08-30-2017, 12:37 AM   #12
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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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, 01:01 AM   #13
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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, 01:50 AM   #14
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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It's always thrust vs. reaction mass economy.
Not in this case. The straight antimatter rocket probably is better on both counts than the water rocket. It just is really radioactive.
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Old 08-30-2017, 07:32 AM   #15
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Not in this case. The straight antimatter rocket probably is better on both counts than the water rocket. It just is really radioactive.
There is a fundamental physics tradeoff. You can optimize to use less reaction mass per unit thrust, or less energy per unit thrust, but not both. If you optimize for less mass, you necessarily have more energy per unit thrust

Since the reaction mass, and hence its energy content, must be contained in at least some directions to get any thrust out of it, and the energy content of a system is limited by the strength of the materials resisting that energy blowing it apart (the Virial theorem), the maximum thrust to weight ratio you can physically achieve goes down as the energy content per unit thrust goes up.
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Old 08-30-2017, 08:04 AM   #16
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

Assuming that one method isn't significantly more efficient at converting fuel mass into energy, which I don't think is the case here, when one is annihilation reactions and the other is boiling water.
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Old 08-30-2017, 01:11 PM   #17
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Assuming that one method isn't significantly more efficient at converting fuel mass into energy, which I don't think is the case here, when one is annihilation reactions and the other is boiling water.
Er, both the things you proposed use annihilation reactions.

One expells the annihilation products directly, the other uses them to heat (water) reaction mass, yes. The water reaction mass will almost certainly be much lower ISP than the annihilation product reaction mass...but it's also a lot easier to confine, so it's very likely you can get much higher thrust out of it.

I mean, really, what you're talking about is right on SS23: Antimatter Thermal Rocket vs. Antimatter Pion.
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Old 08-30-2017, 01:36 PM   #18
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Since the reaction mass, and hence its energy content, must be contained in at least some directions to get any thrust out of it, and the energy content of a system is limited by the strength of the materials resisting that energy blowing it apart (the Virial theorem), the maximum thrust to weight ratio you can physically achieve goes down as the energy content per unit thrust goes up.
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.

However, there's another factor that causes higher ISp to have lower thrust: thermal limits. If you've got a physical nozzle, the reaction mass can't be so hot that it melts or corrodes the nozzle (this generally caps ISp at 1-2k for hydrogen reaction mass, much worse for anything else). Whether or not you've got a physical nozzle, you are limited by your ability to get rid of waste heat (this generally limits high ISp designs to fairly low power density; low ISp designs use reaction mass as coolant, but for high ISp the heat capacity of the reaction mass is inadequate).
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Old 08-30-2017, 03:02 PM   #19
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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
Not in this case. The straight antimatter rocket probably is better on both counts than the water rocket. It just is really radioactive.
It looks like I accidentally omitted the word, "almost" in there.
(I have a strong tendency to hyperbole, but usually try to restrain myself in text.)
But even with a total mass annihilation drive like antimatter, I think large amounts get lost by way of ejecta that can't be used for propulsion.

But as has been said, nearly any effective rocket is an even more effective weapon.
No plausible drive to gently lift and push a ship into space exists that I know of.
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Old 08-30-2017, 04:27 PM   #20
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Default Re: [Spaceships] Anti-Lithium for Drives Does this work?

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Er, both the things you proposed use annihilation reactions.

One expells the annihilation products directly, the other uses them to heat (water) reaction mass, yes. The water reaction mass will almost certainly be much lower ISP than the annihilation product reaction mass...but it's also a lot easier to confine, so it's very likely you can get much higher thrust out of it.

I mean, really, what you're talking about is right on SS23: Antimatter Thermal Rocket vs. Antimatter Pion.
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.
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