[Nuclear1]

One free of the stoopid.

All in favor?

[Scotty]

I concur.

Actually, you think someone could track down most of Herra's awesome physics posts and dump them in here?  That'd be pretty damn free of stoopid, dontcha think?

[General Battuta]

I'll be in my bunk.

[Nuclear1]

Actually, you think someone could track down most of Herra's awesome physics posts and dump them in here?

This.

I hereby nominate Scotty to go hunt them all down.

All in favor?

[qazwsx]

I'll be in the engine room, count me in.

[redsniper]

[13:17] <redsniper> I think we should bring 'shiny' as a positive adjective into our everyday speech
...
[13:18] <redsniper> hot damn, that BP mod is SHINY
...
[13:18] <redsniper> ugh, these GD threads... so NOT shiny :|

[headdie]

I like the idea of a "Best of GD" thread, perhaps lock it so only mods can post here and then they just copy the good bits and link to the thread the discussion is active in

[redsniper]

We should all be friendly! We're brothers in FS fandom!

[Kolgena]

wat is GD

[Scotty]

Actually, you think someone could track down most of Herra's awesome physics posts and dump them in here?

This.

I hereby nominate Scotty to go hunt them all down.

All in favor?

I accept.

[Snail]

This thread is no longer free of the stoopid.

[headdie]

wat is GD

it is the murky swamp we call  General Discussion, lots of mud, water and **** but with a few gems hidden if you dig deep enough

[Scotty]

Megadump commencing (this is from the first five pages of Herra's recent posts):

Don't believe the myth that a matter/antimatter blast converts 100% to destructive force. You lose about half the blast energy to neutrinos. (If the neutrinos were crazy dense enough, they could cause breakdown of nearby fission weapons, hilariously enough - rendering nearby nukes inoperable.)

I don't see neutrinos involved, really.

Electron-positron annihilation produces only gamma photons of about 511 keV energy (or more if the particles had lots of momentum before their collision).

Proton-antiproton annihilation (at lower than about 2 GeV energies) produces four gamma photons and a chargeless pion, which decays into two gamma photons.

Neutron-antineutron annihilation produces two gamma photons directly.

What you lose in destructive force is the fact that gamma rays are stupidly penetrating stuff. Most of the gamma radiation passing through stuff like human tissue just goes through doing nothing, which means you need quite large intensities of gamma radiation to actually cause a harmful amount of ionizing impacts on the tissue (this level depends on the tissue, some are more sensitive to damage than others). Additionally, the intensity (of course) reduces fast as distance grows, in the inverse square of distance.

Hence, despite the massive amounts of raw energy unleashed, harnessing it into mechanical work is not quite straightforward. The gamma radiation itself doesn't cause immediate damage unless it is so dense that it transfers huge amounts of energy into nearby materials and vaporises it, but due to the penetrating properties of gamma radiation this would require quite a massive yield, and most of the energy would still escape.

The best way to actually utilize the energy of gamma burst released from the annihilation is to encase it in a non-solid fragmentation shell made of lead or other very dense element (but lead is probably the cheapest, depleted uranium tends to be a bit sparse) which would attenuate the gamma radiation significantly, heat up by a corresponding energy, and turn into expanding cloud of lead vapour and parts of the outer fragmentation shell. This could cause more damage in the vacuum.

In atmosphere, similar system could increase the amount of energy captured at the blast site. If that were not done, the annihilation would produce a mass of gamma rays which would probably heat up the air enough to cause an impressive fireball and lots of radiation sickness nearby, but if there was a shroud around the annihilation that would gather the majority of the gamma rays released, it would constrict the energy release to a much smaller area, and would likely result in pretty much larger destructive power overall.

EDIT: Typio

The gamma radiation itself doesn't cause immediate damage unless it is so dense that it transfers huge amounts of energy into nearby materials and vaporises it, but due to the penetrating properties of gamma radiation this would require quite a massive yield, and most of the energy would still escape.

Considering that the hulls are made to block cosmic radiation and withstand nuklear weapons (like the torpedoes) wouldn't that mean they aren't easily penetrated by gamma rays?
So unless they found a way to deflect radiation away from the hulls, wouldn't that mean instead of going through without causing harm, those gamma rays instead heat up the inside of those armor plates?

Apart from possibly making those plates softer till they can cool off again (depends on the material I guess) that should also raise the ships internal temperature causing the crew some discomfort and reducing the effectiveness of the cooling systems for beams and reactors.

Also antimatter tipped warheads sound like a shaped blast to me, so in front of the impact site there would be intense energy released, probably enough to vapourise the hull there. And if that material get's into the corridors of the ship, it'd be rather catastrophic I think.

Yes, that would work - for all the ships in close proximity. Would be interesting to do the math on how much gamma radiation would be needed for hull plating to absorb enough energy to start significantly heating up, though. It'll cause radiation sickness long before that happens, I would wager, but don't take my word for it, it's just my intuition.

The problem is, gamma rays spread evenly from a point source, and their effectiveness reduces in inverse square of distance. The even distribution means that at reasonably short distance, they won't be doing much damage at all (at least immediate damage - they may cause radiation sickness, but that doesn't immediately put a warship down like compromised hull structure will.

If you instead convert most of the energy from the reaction into kinetic energy (assumign you can do this) and send shrapnel from the blast site, you get a much more "coarse" distribution of possible damage - but at the same time, the damage delivery is more efficient. The probability of getting hit by pieces of shrapnel will decrease in the inverse square of distance, but individual pieces of shrapnel don't lose their initial kinetic energy, which means the individual pieces of shrapnel retain their effectiveness regardless if distance, unlike a free gamma burst.


Or, let me put it this way.

Let's say you have a free gamma burst of a 0.1 kg anti-hydrogen warhead. Assuming a clean conversion from energy to gamma rays, you will be getting 9x10¹⁵ Joules of gamma radiation, spectrum spikes at 511 keV (electron-positron-annihilation spike) and 125, 307 and 530 MeV spikes for proton-antiproton annihilations. This is about 2.151 Megatons of TNT. This energy might sound like a lot (and it is, considering it's from 100 grams of antihydrogen), but let's break it up into what sort of effect it would actually have.

The higher the energy of photons, the more penetrating they are, which means their linear attenuation coefficient is reduced when they go through the same thickness of absorber. If you have, d metres of hull plating at attenuation coefficient α, you could relatively easily calculate the linear attenuation for each gamma spectrum spike and thus determine the percentage of absorption of pass-through gamma radiation:

I = I₀ e^-αd

Where I₀ is the intensity of radiation when it impacts the absorbing hull plate, which can be pretty trivially calculated if you know the distance from the annihilation site (and if you want, you can just deal with energies instead of power and intensity, since the time of annihilation reaction is pretty short).

I have no idea of the attenuation factors of spaceship hull material, but they'll be different for each gamma wave length. However, let us generously assume that the hull plating absorbs 80% of the gamma radiation.

The energy released in 0.1 kg antimatter warhead is, like said, about 9x10¹⁵ Joules. At 1000 metres distance, the amount of energy passing through one square metre of space is

9x10¹⁵ J / 4 π (1000 m)² = 716197244 J m^-2

Which is about 716 megajoules of energy per square metre.

For comparison, if you have a ton of water ice at 273.16 K temperature (solid form), it takes 333 megajoules to thaw it to 273.16 K temperature liquid. But if you had a cube of water ice with mass 1000 kg, at 1000 metres from the annihilation site, it would probably not melt all the way because the attenuation factor of water isn't too high; much of the gamma radiation would pass through.

So that's the practical effect of 0.1 kg annihilation warhead at one kilometre distance. If you replace the ice cube with humans, they'll heat up a bit and contract severe radiation sickness, but I doubt it would have radical effect on ship hull plating. Closer to the blast site, sure, but 1000 metres is relatively small distance even in FreeSpace.




Now, let's look at another scenario: let's say 80% of the annihilation energy is converted into kinetic energy of shrapnel pieces.

To be ambitious, let's make fragmentation shell with following configuration:

-Total mass 12566371 kg
-Mass of individual fragments on average 1 kg.

80% of 9x10¹⁵ Joules is 7.2x10¹⁵ Joules.

That as kinetic energy given to mass of 12 566 371 kg yields a velocity of 33.85 km/s.

Each fragment has thus velocity of 33.85 km/s and, unsurprisingly, kinetic energy of 572.9 MJ (which is 80% of the radiation of gamma rays passing through the one square meter area.

You might wonder why I picked such a random-looking value for the amount of shrapnel particles. Well, the reason is that the surface area of a sphere with radius of 1000 metres is 12 566 370.6 square metres, and when you evenly distribute 12566371 shrapnel particles on that area, you should get about one shrapnel piece per square metre, which is important in comparing the efficacy of this setup compared to the free gamma ray setup.

We've established that there will be about 12.5 million 1 kg pieces of shrapnel flying at almost 34 kilometres per second, and by average at distance of 1000 metres you will get one of them per one square metre of surface area.


Now you'll remember that the effect of gamma radiation on a block of ice was quite... lacking in destructive power. By contrast, I don't think anyone has any difficulties about what happens to a block of ice when it's impacted with one kilogram mass moving at 34 km/s.


Similarly, a projectile with 34 km/s is much more efficient against space ship hulls than an even spread of gamma radiation. Against organics, sure, you'll achieve a high mortality rate with induced radiation sickness, which may be an interesting way of acquiring ships relatively intact; gamma radiation does not irradiate materials like neutron radiation does, so in a way this would be much better way of dealing with space ship crews without causing excessive damage to the ship itself.

But, in open combat this would not work. The crew would stay operational for some while after the gamma burst. That's why as a pure weapon, the shrapnel based one would function better.



Any thoughts? Spot any calculation errors?

I see a lot of possible paths for a proton-antiproton annihilation, and most of them don't include charged pions.

Let's assume that the predominant reaction is the one that results in a pion and anti-pion, and their further decay.*

In this scenario, yes:

proton + antiproton -> pion⁺ + pion^- + gamma ray

pion^- -> muon + muon neutrino
pion⁺ -> antimuon + muon neutrino

muon -> electron + electron neutrino
antimuon -> positron + electron neutrino


So basically in this case we do have two muon neutrinos and two electron neutrinos (though since they can oscillate into each other the specific type doesn't really matter all that much).

The problem is determining how much of the energy is actually carried away by neutrinos. I'm having difficulty finding actual experimental data on how much energy bleed goes into them, but on a gut feeling I doubt it's as much as half of the energy, considering neutrinos being the lightweights of standard model.

But ok, let's accept that the total mass sets just an upper limit for the (useable) annihilation energy yield. This is, after all, strictly academic exercise in determining the effects of future space warfare weapons...

...right?


*This apparently assumes that the proton and antiproton are at rest (have very little kinetic energy at the beginning). This may be the case initially for the first reactions, but the hydrogen - antihydrogen mixture would rapidly form a high-energy plasma, though I hesitate to speculate on how much collision energy the particles may have in this situation.

I think implementing terminal velocity would be beneficial to this system.

Not only is it mathematically reasonably simple, but it's also going to work as a rudimentary atmospheric friction model.

The maths of it goes like this:

F = -G - kv^n

where

F is the resultant force vector
G is gravitational force (weight) (G=-mg)
k is friction coefficient (determined from values such as shape coefficient and air density)
v is velocity
n is the exponent for velocity, which varies depending on the speed itself, but for a rudimentary system it's more or less safe to approximate that n = 2. Basically this number increases as velocity increases, but that makes the equation needlessly complicated. Usually the friction coefficient is experimentally defined and ranges from zero to 1; for most FS2 ships I suppose it would range between 0.5 and 1.

As for solving the equation, using n = 2:

F = dp/dt = m dv/dt

m dv/dt = -mg - kv^2

dv/dt = -g - (k/m) v^2

1 / (-g - (k/m)v^2)  dv = dt  || Int(..)

v                                     t
∫ 1 / (-g - (k/m)v^2)  dv  = ∫ dt
v₀                                    t₀
...


v(t) = - (√g√m / √k) * tan ( [√g√k) / √m] t - arctan [(√k / √g√m) v₀] )

And there you go; of course, this is a simplified equation where movement is limited to one axis. If you allow movement in all three dimensions, then you have to formulate the air friction so that it's always vectorized against the velocity vector, while gravitational force always pulls "down" (in homogenous gravity field approximation, that's the same direction everywhere in the game world).

If you can hook accelerations into the engine directly instead of velocities, that would make things easier still, since solving the diff eq wouldn't be even necessary, you could just plug the forces divided by mass into the engine and it would apply the accelerations to objects.

This topic is relevant to my interests.


10 x mass or 10 x dimensions?

With ten times the mass, we would be 10^1/3 larger than currently (that's 2.15 times taller). A 1.80 m tall individual would, if he had a mass increase by a factor of ten, tower at 3.87 metres of altitude.

To cope with the increased body mass, Either our bone density or cross-section area would need to be enlarged by factor of ten to counter the ten times larger body mass and the following dynamic force increases (for example, either bone radius increases by factor of 10^2/3, or 4.64, or bone density or mineral strengths (yield, compressive, tensile, shear and impact stresses all need to be much stronger). Tendons and muscles would need to be either scaled up or powered up significantly to provide reliable and swift locomotion and balance. The attachment points of tendons to bones would need to either have higher torque (further from the joints) or be much much thicker and stronger. In short words we wouldn't look much like humans.

I also shudder to think of the stresses that our knees, ankles, hip joints and most importantly the spine would be subjected to.

Increase of blood pressure would be required, and subsequently a re-write of most tissues because you can't just supersize cells, you need to increase their amount, so basically you would need an overhaul of cardio-pulmonary system as well as digestive system and nervous system.

Speaking of that, your brain is now ten times more massive. But if it were just upscaled model, you wouldnt' get any benefits - except ten times larger energy consumption. Which is another issue - we would have (approximately) ten times higher basic metabolism and probably significantly higher stress metabolism. Thermal balance would also change - it would be more difficult to regulate our temperatures in hot areas, because our surface area to body mass ratio would be significantly smaller. So we would require additional cooling systems as well. In short, most of our cooling happens through skin; now our body mass is ten times the current one but our skin surface has only increased by a factor of 4.64, so that's almost half the reduction in our ability to expel a given percentage of our thermal energy. That means strenuous activity would risk thermal shock in much lower temperatures than currently. We would need about ten times as much liquid too.


And remember - this has been calculated with the (more sensible) option of having our bady mass increase to tenfold of current.

I'm not even going to go into how ridiculous it would be to think of ten times taller than current humans. 18 metres tall human would have 1000 times the mass of 1.8 metres tall, proportionally similar human.

If the original human had body mass of 80 kg, the supersized version would have body mass of 80 tons.

That's ****ing insane considering that a good-sized humpback whale is about 18 metres tall and has body mass of about 40-48 tonnes, and they're water animals. The largest present day land animal, African bull elephant, has a record weight of 11 tons, and the largest land animals ever (I'm using the Argentinosaurus here because they seem to have the most amount of evidence supporting their claim) had length of around 35 m and estimated body mass of 80-100 tons, so that's what you would be aiming at with your super human.


Incidentally, this is why I find humanoid mechas a rather humorous concept.

No wonder the Evangelion units had such short operational times without the umbilical chord... or S2 organ... But then again, compared to the hostile Angels appearing in the series, the EVA's are positively rational and realistic.

I have found this thread. It can no longer hide from me.

That said, let the nitpickery commence.

"Fast" is in fact a bit of a slippery concept when it comes to spacecraft. Speed in space is all relative to begin with; the more useful measure for a spaceship is delta v,  "change in velocity" - especially, how much you can change your velocity before you run out of gas. For any given propulsion technology, the way to get more delta v isn't a more powerful engine but a bigger fuel tank. What a powerful engine does give you is higher acceleration - so you can achieve any given delta v more quickly.

This is, of course, wrong.

The delta v of any given space ship is not as simple as the writer assumes. Calculating the movement of a variable-mass system like space ship filled to brim with propellant is complex and annoying and time-consuming, so let's simplify the problem by looking at a short duration burn, where the mass of expended propellant is significantly less than the entire mass of the ship:

m << M


The mass of the propellant is given ejection velocity v_e, and the conservation of momentum assures that the ship gains equal but opposite momentum.

Now, the momentum given to the propellant mass is

p = mv_e

while the momentum that ship gains is

p = Mv


And conservation of momentum means that

mv_e = Mv

and solving the equation for the end velocity of the ship, we get

v = mv_e / M


From here, you can see the following statements can be made:

1. Increasing propellant mass expelled during the burn while keeping ejection velocity and ship mass constant will increase the end velocity v.

2. Increasing the ejection velocity of propellant while keeping other variables constant will increase the end velocity.

3. Reducing ship's mass and keeping engine parametres constant will increase the end velocity.


By now you should see the flaw in the reasoning in the quoted paragraph. "More powerful engine" does not only mean that it can dump bigger propellant mass per dt out into space, it can also mean that it can do it at higher ejection velocity, which is in fact the more important term for the efficiency of the engine.

Basically, the higher the ejection velocity, the less propellant you need for a given change of momentum (and velocity). Keeping your ship as light as possible will also help immensely, but as far as propulsion technology goes (as long as it's based on conservation of momentum), increasing the amount of propellant is much less efficient than increasing the ejection velocity.

EDIT: Of course, it could be that when the writer says "any given propulsion technology", he means to say that the ejection velocity is fixed or in the same order of magnitude; however I disagree with this sort of classification, because especially with electromagnetic engines you can increase the ejection velocity pretty much linearly, unlike with chemical rockets where you are limited by materials and the properties of the expanding gas and chemical energy available from reactions.

@Herra: that is a bit of a nitpicking, if you checked more of the articles then it will be abundant that the writers are *very* much aware that specific impulse (the speed at which propellant leaves the engine, divided by its mass) can have massive effects.

That's why most realistic designs use electric (ion, field-effect, etc.) engines since they have a massive isp.

You also made a not entirely correct assertion, as merely lowering the craft's mass (without changing any engine parameters) would include carrying less fuel, which we all know won't result in more delta-v. However if we amend, that lowering the mass of ship components beside fuel, then yes - it would produce more delta-v. However that's the very same thing as increasing the ship's fuel-to-mass ratio.

Yep, I meant the dry mass of the ship. But propellant-to-ship mass ratio and total delta v doesn't translate directly to combat efficiency - increasing propellant/ship mass ratio by increasing the amount of propellant onboard will increase delta v but reduce acceleration (assuming the engine stays the same), while increasing propellant/mass ratio by reducing dry mass increases both acceleration and delta v.

What the entry refers to is sticking a "bigger" engine on a craft. It will give more thrust, but isp for a given technology is set or at least won't scale that much with size... as a result you won't gain any (significant) delta-v, only the ability to accelerate, ergo reach your delta-v faster.

So to sum it up:
Delta-v is the result of your technology specific isp and your fuel fraction.
Acceleration is the result of your technology specific isp and the size of your engine.

Yeah, I amended my post with an edit, because I frankly disagree with the assumption that specific thruster technology would always have more or less same ejection velocity.

With chemical rocketry, this assumption holds true because the gas expansion velocity is limited. However, with most other technologies, the ejection velocity can be scaled up by increasing reactor output power to engine.

Though I suppose the maximum factor limiting the ejection velocity of an ion drive engine would be vacuum's permittivity - that would largely define how strong an electric field you can use to accelerate the propellant without creating arcs of electricity through the engine itself. Using a nuclear reactor to heat up gas, accelerating it by expansion would be limited by the maximum reactor temperature which of course is limited by materials' thermal durability.

So maybe there are natural maximum performance limits for each specific propulsion technology. But the assumption that all the engine systems would be developed to the maximum is quite interesting.

I do agree with what the emphasis of the text was - that increasing propellant flow doesn't increase delta v, only acceleration (and propellant lasts shorter duration of time, obviously) and that if ejection velocity stays constant, the only way to increase delta v is to increase the ship's propellant/mass ratio.

I just felt that it was necessary to specify that the "thrust" of the engine can be increased by either increasing mass flow or ejection velocity. To be absolutely accurate, you can't even measure meaningful "power" for a thruster, so the term "more powerful" is a bit badly worded in either case...

With single core, it's easy enough - 3.0 GHz = 3.0 GHz (non-geeky equation), but with all these new ones that only say they have about 2.2GHz, is that per core or the total processing power? Does that mean a new quad core 2.0 GHz CPU will have 8.0 GHz total?

No. The clock frequencies announce the processor's cycle time - N cycles in a second. With 2.0 GHz clock frequency, the processing unit has two billion cycles in a second (2,000,000,000 Hz).

The fact that there are multiple cores does not magically increase the cycle rate.


However, cycle rate is integral in defining a single core's calculating power; essentially, it means it can do N processes in a second (this is grossly simplified since there are other factors that amount to the calculating power), and the calculating power is measured in FLoating point Operations Per Second (FLOPS), usually with modern computers this ranges in gigaflops which should not become as a surprise to an acute reader at this point.

This means that if each core has calculating power of 2 GFLOPS, a dual core processor has the parallel processing power of 2x2GFLOPS, quad-core has 4x2GFLOPS etc. This, of course, requires that the program itself supports parallel processing which requires that the program uses so-called "multiple threads"; one thread per virtual processor.

Yes, to complicate the matters, some processors have virtualization that enables the operating system to see one physical core as two discrete virtual processors, which can be very beneficial in some ways and problematic in others.


For older applications such as FS2_Open, which don't support multi-threaded processing, the performance of the CPU depends on a single core's performance which often can be more or less directly expressed in main clock frequency.

However, there are other things that affect the processing power in different scenarios; mainly, the speed and size of the processor's memory cache and the bandwidth to system random access memory. These form the so-called von Neumann bottleneck. This means that in many cases, computing power is not limited by the raw processing power but the ability to transfer data to and from between the system RAM and the processor. Not only this, but the processor's are required to cache their own instructions in addition to the data passing through. The latency of memory operations is what causes most significant delays in computing when the amount of data flow goes over certain threshold. If for example the processor need to use an instruction that isn't stored in the instruction cache, it needs to fetch the instruction there and the processing is stopped for the thread that requires that instruction, for the duration of fetching the instruction to the cache. Same with data - when the thread receives data from memory location to calculate (a word, in modern processors it is of length 64 bits), it processes it, spits out the result, and this result is stored back to a memory location, and the processing is halted for the thread until new data can be accessed from the memory.

The data cache speeds up the process by increasing the chunks of data that can be stored "closer" to the processor (computatively speaking) instead of moving each word one by one through the system bus between the RAM and the CPU.

Here is a diagram of a reasonably modern CPU's cache structure (AMD K8 core):




Now, messing up with process priority will not magically increase the processing power of the core. It will, however, elevate the process to higher priority which prevents threads from other processes from being processed on the appropriate core. Setting the priority to real-time does just that - it gives all of the core to the single processes demands, all the time until the priority is dropped or process terminated.

This might work on a multi-core processor when you set affinity to a single core, or the application natively only uses a single core. In this case, it leaves other core free to do important stuff like keep the operating system responsive and working, things like that. If, however, you set a multi-threaded process to real time priority, it can easily hog all of the CPU, prevent the operating system processes from working and crash your computer.

In single core systems this will happen every time if you set a "greedy" process to real-time priority. Unless the OS is protected from stupid user errors like this.

In other words: don't do it, it's misguided and silly and will only grant you a placebo effect of renders going faster. If other processes are messing up with renders, you should set their priority lower, not the rendering process priority to higher. Or you can set rendering processes affinity to a core that is not used by other processes as much. Either method works.

Incidentally, this is also why so many people experience such huge frame rate drops with FRAPS. Sure, it does decrease performance by having to dump each frame into HDD in a video file, but if you set FRAPS affinity to a different core than the game you play, it will work out much smoother... when the same core attempts to run both FS2_Open and FRAPs for example, it creates conflict of interests for both processes which slows down both process threads; when they operate in discrete cores, they are free to use more processing power simultaneously.

Bask in the glory of Herra Tohtori.

[Herra Tohtori]

You should figure out something new to discuss rather than reposting some old ramblings.

[iamzack]

this thread is gay

[Scotty]

You should figure out something new to discuss rather than reposting some old ramblings.

New contributions are always welcome.

[redsniper]

Let's talk about portals. I know we've had some interesting IRC convos about portals.

[Snail]

this thread is gay

Awesome!

[Herra Tohtori]

Let's talk about portals. I know we've had some interesting IRC convos about portals.

Let's disregard the laws of physics. What would happen according to laws of physics, in this situation?

[redsniper]

er... they would be disregard-  oh hell...