General > Discussion
Alien propulsion won't work IRL
Jon_dArc:
--- Quote from: Starbug on May 30, 2012, 03:48:20 pm ---Yes, Newton's 3rd law, 'The action and reaction forces between two bodies are equal, opposite'.
Chemical engines work by throwing out lots of mass (all that Hydrogen + Oxygen) at a high velocity (thats what all the combustion is for), giving lots of momentum (Mass x Velocity). The space shuttle, for instance, throws out lots of fuel with lots of momentum out the back, and it goes forward.
However, Newton's 3rd law ALSO applies to EXPLOSIONS. Thats how a gun works.
When you fire a gun, a bullet comes out one end, and the barrel recoils in the other. But the barrel and the bullet are not pushing on each other!! There's an explosion inbetween, which pushes on both. The barrel contains the explosion, so most of the force goes into pushing the bullet forwards and the barrel back. If the barrel wasn't strong enough, it would also explode outward, but that's not too fun.
--- End quote ---
That's simplified to the point of being wrong. Newton's Third does not "apply to explosions". The explosion causes the gas and particulate matter in the propellant to expand rapidly—matter at now-high velocity, which smacks into the bullet in a perfectly ordinary application of the law. This distinction becomes significant in your next paragraph:
--- Quote ---The antimatter engine, as described in the UFOpedia, is effectively using the explosive force to push the rocket along. Not the craziest idea we've had. (see 0.23 - 1.18). (It doesn't use little bomblets like this, but the principle is the same.)
--- End quote ---
The key difference between an explosion, as such, and a matter/antimatter annihilation is that while both produce (or release, if we're being precise) energy, the former does so with a negligible change in the total mass of the reactants. At perfect efficiency, a matter/antimatter annihilation leaves you unable to propel anything, because although you've released huge amounts of energy there's no longer any mass to push against any other mass. You can work with photon momentum, which I assume is what morse was talking about, but that's really not going to get you very far.
So you can use annihilation in place of other chemical or nuclear alternatives to produce energy, but if you're going to do that you need to carry along reaction mass. In atmosphere, you can grab reaction mass from all around you, but in space the few atoms per cubic meter simply aren't going to cut it.
Edit: actually, reviewing morse's post, it looks like there's an assumption that the mass of the original matter+antimatter is still around somehow, which means his objection is wrong. Your objection is still wrong, though ;)
Edit^2: or he could be using sloppy phrasing for calculating photon momentum via conservation of momentum, in which case I don't think he'd be wrong but the answer would be "reaction mass".
~J
morse:
--- Quote from: Jon_dArc on May 30, 2012, 04:08:11 pm ---Edit: actually, reviewing morse's post, it looks like there's an assumption that the mass of the original matter+antimatter is still around somehow, which means his objection is wrong. Your objection is still wrong, though ;)
--- End quote ---
Don't quite get it. In the description of antimatter storage you say that it can store 10 grams. 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.
Nutter:
--- Quote from: Starbug on May 30, 2012, 03:48:20 pm ---That sounds like a good idea to me. Using the explosive force of anti-matter as the propulsion did strike me as *glorified rocket engine* as well, it didn't quite sit right.
Also if you are using the antimatter as the propellent, how off-earth would you use it to go FTL?? Antimatter reactor and plasma drives seems to make more sense to me. (With FTL as some separate system, that does whatever it does using the power from the reactor)
Since the aliens can get plasma weapons working in our atmosphere fine, plasma engines sound plausible. The engines as they are look like they could be plasma engines or whatever. Antimatter reactor could definitely get lots of power out of them. No need to say how they work, just that for the aliens, they do.
--- End quote ---
Problem is, all I've found on plasma drives is mostly versions of ion engines which we already covered don't work that well on the ground.
Though, if you pinned the thing as a giant friggin plasma beam (or something) pointing backwards and not really all that in touch with our current tech, it should sound okay.
And it also allows the application of Newt's third law in the form of the (hopefully) famous Kzinti Lesson because, let's face it; it's awesome.
Starbug:
--- Quote from: Jon_dArc on May 30, 2012, 04:08:11 pm ---The key difference between an explosion, as such, and a matter/antimatter annihilation is that while both produce (or release, if we're being precise) energy, the former does so with a negligible change in the total mass of the reactants. At perfect efficiency, a matter/antimatter annihilation leaves you unable to propel anything, because although you've released huge amounts of energy there's no longer any mass to push against any other mass.
--- End quote ---
Ah, thats, quite true. I did NOT think things all the way through. :P
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
Crystan:
--- Quote from: TrashMan on May 30, 2012, 02:06:50 pm ---Also, if you want ot make the setting even mroe believalbe, set the date a bit lower. 70 years in the future, adn yet it doesnt' look like it and doesn't feel like it.
2030 or 2040 work FAR better than 2084, given the look and deisgn and "feel".
--- End quote ---
The discission to set the date to 2084 was made after the most maps and props were made afaik. The old vehicles and stuff is present just because it wasnt planed to let the game play this far in future at the time they were created. Make us new future civ vehicles and other props and you get the future feeling... ;)
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