General > Discussion
Alien propulsion won't work IRL
TallTroll:
I'd always the aliens were using some sort of gravity drive, which was why they needed AM fuel to power it. All they are missing is reaction mass anyway. They use metallic H2, greatly accelerated by the huge power provided by their AM reactors. Simples
Sandro:
:facepalm: Guys, what are you writing? Newtonian physics does not apply to annihilation drive, since it violates the key axiom of Newtonian physics -- the law of mass conservation. You should use the relativistic energy-impulse conservation law instead.
Here's the short overview that mentions antimatter-using engines and their expected parameters: http://www.strangehorizons.com/2004/20040119/nuclear.shtml
Two short quotes:
--- Quote ---A modern liquid hydrogen/oxygen rocket, such as one of the Space Shuttle's main engines, can produce a specific impulse of 450 seconds.
...
Matter and antimatter mutually annihilate each other on contact and are converted to pure energy. This energy takes the form of gamma rays, neutrinos, antineutrinos, and/or pions. This total energy conversion makes forms of antimatter very attractive as a spacecraft fuel. Antimatter rockets are thought to be able to provide specific impulses of up to 10 million seconds.
--- End quote ---
(to convert Isp to impulse provided by 1kg of propellant, multiply by 9.81)
Jon_dArc:
--- Quote from: morse on May 30, 2012, 04:37:32 pm ---Don't quite get it. In the description of antimatter storage you say that it can store 10 grams.
--- End quote ---
The point is that unlike ordinary fuel, where you take 10 grams of whatever substance or combination of substances, cause it to release a bunch of energy, and then get 10 grams (minus some vanishingly small amount) of matter flying out the back of the craft, when you annihilate 10 grams of antimatter with 10 grams of matter you get (assuming perfect efficiency) 0 grams of matter left to fly out the back.
--- Quote ---In game mechanics it stores 1000 points, so you can easily calculate the mass of 1 point. But even if we increase that number, we'll need to increase it drastically, up to 1000 times at least, to make it look anywhere close to reality in respect to the linear momentum. But after that there will be a question: how can we miss something like this on our radars. The amount of energy released will be more than enough to boil the oceans, and should be seen by every single radar on earth surface.
--- End quote ---
So I'd realized, between my first and second edits, that the photons would have momentum and that I don't think there's anything that would make conservation of momentum not apply in this case (meaning that I think 10 grams leaving the ship at the speed of light is a reasonable rough approximation—it'd actually probably have to be half that, as the net momentum of the annihilating particles would probably be quite low meaning that for some amount of momentum's worth of photons to go bounce against the ship, propelling it forward, an equal amount of momentum's worth of photons needs to simply fly out the back without providing any thrust). The thing is, though, that all you need to do to improve the amount of momentum generated is to pitch some additional matter into the annihilation chamber so that it flies out the back of the ship as a result of the energy from the annihilation. I'm too lazy for precise calculations at the moment.
Edit: a very important caveat! I'm assuming that the annihilation produces no non-massless particles (and am also, less significantly, assuming that they only produce photons). I think this is highly likely, as I can't think of a good reason to encourage massed particle formation, but I am reminded that this is not in fact a guaranteed feature of annihilation.
--- Quote from: Starbug on May 30, 2012, 05:03:42 pm ---Though unfortunately, that IS what the UFOpedia currently says (using the explosive force):
--- Quote ---It uses direct matter-antimatter annihilation to generate thrust by injecting protons and antiprotons into the reaction chamber, then channeling this explosive force out the back of the engine.
--- End quote ---
so... :P
--- End quote ---
Well, yes, the UFOpedia currently says a lot of dumb things (witness the pistols firing rifle-caliber rounds and the assault rifle/machine gun firing teensy little ones). I guess my point is that fixing it is simply a matter of adding a reference to reaction mass, rather than fundamentally rethinking the mechanism.
--- Quote from: Sandro on May 30, 2012, 06:07:27 pm ---:facepalm: Guys, what are you writing? Newtonian physics does not apply to annihilation drive, since it violates the key axiom of Newtonian physics -- the law of mass conservation. You should use the relativistic energy-impulse conservation law instead.
--- End quote ---
This is an irrelevant objection—it's true that the conservation of momentum being followed isn't strictly Newton's third law, but momentum is still conserved. The fact that some of this momentum cannot be accounted for by multiplying velocity and rest mass is of only pedagogical importance.
~J
morse:
--- Quote from: Jon_dArc on May 30, 2012, 06:46:47 pm ---I guess my point is that fixing it is simply a matter of adding a reference to reaction mass, rather than fundamentally rethinking the mechanism.
--- End quote ---
I do not propose any fundamental rethinking. Additional mass is indeed the best choice possible. The best candidate in earth' atmosphere is of course air. That way we'll get a ramjet, just like I wrote in my first post. In the space... Well, I read somewhere in the ufopedia that liquid nitrogen is used to cool the engines, so the said liquid nitrogen is the first in line to go out. The UFO flies from space, replenish nitrogen reserve, and then flies back.
Do we have any space dogfight planned for the later stages of the campaign? If not, then we do not really need the ability to make manoeuvres in space, just to get to the mothership.
Nutter:
--- Quote from: Jon_dArc on May 30, 2012, 06:46:47 pm ---Well, yes, the UFOpedia currently says a lot of dumb things (witness the pistols firing rifle-caliber rounds and the assault rifle/machine gun firing teensy little ones).
--- End quote ---
The pistol quip, I'm presuming is an assault on the 7.62x25mm Tokarev used since before the second world war.
The 4.73x33mm, got dumped when the German reunification effed up the Bundeswehr funding the G11 rifle stopped being worthwhile.
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