Alien propulsion won't work IRL

[Jon_dArc]

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.

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:

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.)

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]

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

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]

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.

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]

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.

Ah, thats, quite true. I did NOT think things all the way through. 

Though unfortunately, that IS what the UFOpedia currently says (using the explosive force):

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.

so...

[Crystan]

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".

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...

[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:

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.

(to convert Isp to impulse provided by 1kg of propellant, multiply by 9.81)

[Jon_dArc]

Don't quite get it. In the description of antimatter storage you say that it can store 10 grams.

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.

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.

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.

Though unfortunately, that IS what the UFOpedia currently says (using the explosive force):

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.

so...

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.

: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.

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]

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.

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]

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).

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.

[Jon_dArc]

The pistol quip, I'm presuming is an assault on the 7.62x25mm Tokarev used since before the second world war.

Well, serves me right for not actually going back and rereading the description. Though rereading does once again point out how much time is spent extolling the armor-penetrating virtues of the starting human weapons which perform extremely badly against armor.

The 4.73x33mm, got dumped when the German reunification effed some of the stuff up and the G11 rifle stopped being worthwhile.

This time it serves me right for not going and looking up the round (and I'm a G11 fan too, for shame). And someone's gone and made base damage and damage type the same for the AR and MG, meaning that sharing the same ammo isn't dumb anymore.

As substitutes, permit me to submit the bit where the UFOpedia claims that Small Hangars are a base entry point (different kind of dumb, granted), or the plasma blade being an "anti-armour weapon […] except designed to kill armoured infantry and small vehicles rather than tanks"—that is to say, a non-anti-armour weapon.

~J

[Nutter]

Hey, armour-piercing bullets aren't used on tanks, either. Point of the name is that it's supposed to defeat armour. Doesn't necesairly mean armour in the tank sense. Though, I suspect a current tank wouldn't be able to resist that stuff. It's a shaped charge without all the bits that make using it fun. Namely, the rocket strapped to its back. Maybe R&D could fix that?
Honestly, the entire point of the blade is rather silly. Sticky bombs on tanks were bad enough during the forties. Trying those stunts on infantry...well, you're not getting out alive. But the aliens have reserves. We don't.

And I think the small hangars might've been planned as an entry point at some point in developement. Seems to me quite a lot of the text remained unmodified as developement changes happened.
Though, I think there might be some rethinking going on regarding bases as well and that's why nobody bothers with changing that. At least, I hope. Not a fan of the current stuff, to be honest.

And yes, their performance against armour is rather poor untill it's time for some friendly fire. I think it's more of an issue of the friggin aliens you spend most your time shooting at just having annoyingly resilient shit.

[TrashMan]

Ya know, I think I might actually open a new thread with re-done descriptions of items and text.....to make everything better. Everyone is free to contribute 

[headdie]

Something to chew on.  I knocked together a little comparison of the rounds in blender and came out with this.

4.7   UFO:AI      UFO:AI Assault Rifle and Machine Gun
5.56x45 NATO   NATO standard Assault Rifle Round and also used by M249/FN MINIMI light Machinegun
12.7×99mm NATO   Used in the M82 Barret Anti Material Rifle
20x102mm      Cannon round used in M61 Vulcan, Anzio 20mm. Vulcan Anti Material Rifle and likely candidate for UFO:AI Sniper

[Jon_dArc]

Hey, armour-piercing bullets aren't used on tanks, either. Point of the name is that it's supposed to defeat armour. Doesn't necesairly mean armour in the tank sense.

Given the reference to the Panzerfaust (which I omitted in my post) in the text, I think it's clearly implying armour in the same sense as "armoured column"—that is to say, tanks, APCs, self-propelled artillery, etc.

~J