For Questions That Don't Merit Their Own Thread

[peadar1987]

9 hours ago, shynung said:

What are the biggest hurdles in designing a working fusion power reactor today, in ELI5 terms?

As I understand it, there are a couple, and they depend on the type of reactor used (however, fusion isn't really my thing, so others will probably have some stuff to add). For all of them the main overarching problem is that you have to put in more energy to heat up the fuel and keep it contained than is released by the fusion of the atoms.

Inertial Confinement Fusion

Or "shooting a pellet with a laser". Essentially you have a pellet of fuel, you fire an immensely powerful laser into it, and the resulting heat causes explosive ablation of the pellet, generating the pressures and temperatures necessary for fusion. This is a relatively mature technology, as it's used for gaining theoretical data about thermonuclear weapons.

Problem is with generating a useful amount of power. Big lasers can't fire that often. They're inefficient, and so produce a lot of heat. That heat distorts the optics and requires a long cooldown period. Current designs can fire once every day or two, compared to once every couple of seconds, which is what you'd want for power generation. There is also a lot of room for improvement with the efficiency of the fusion process. Ideally you want to hit the pellet at the same time with multiple lasers, so the shock waves all hit the centre of the pellet at the same moment, which maximises the temperature and reduces the power requirements for fusion. However, that's extremely hard to achieve, as any manufacturing defects in the pellet, or tiny variations in the timing of the lasers will throw this off.

Tokamaks

Or "magnetic smooshing". This is where plasma is fed into a doughnut shaped chamber and compressed by magnets. However, turns out it's very hard to keep a doughnut shaped ring of plasma stable for any extended period of time. The control issues are crazy, and it's very hard to stop bits of plasma escaping and damaging the reactor, especially when run for longer time scales.

There are a few other types of technology, such as Polywells, but I'm not familiar enough with them to say what the issues are with making them produce useful amounts of net power.

[ChrisSpace]

How much does Sealand weigh? As in, the entire platform.

[Kryten]

42 minutes ago, ChrisSpace said:

How much does Sealand weigh? As in, the entire platform.

~4,500 tons.

[ChrisSpace]

23 hours ago, Kryten said:

~4,500 tons.

And do you have a source for that? I mean, that number's a lot higher than what I imagined.

[Kryten]

http://harwich-society.co.uk/old/info_rough_towers.htm

Buildings tend to be heavier than is intuitive anyway, and this one's an armoured fort anchored with a giant concrete pontoon.

[Flamingo]

I stood in the bathroom yesterday when this question suddenly hit me, and i have not stopped thinking about since. I have no idea if it in any way makes sense, but here it is: I have a big 100 kg cube in space and nothing effects it, then i make it move with 10 km/h towards a not moving small 50 kg cube. When it hits the small cube would the small cube move with 20 Km/h? Would the big cube stop completely? Would they both move at the same speed? Or would something different happen?

[Green Baron]

 

Assuming balls, an elastic collision, linear movement (no deformation, no energy conversion, conservation of momentum), then the ball with m/2 (50kg) would gain 4/3rds of the speed (13.333..m/s) and the ball with m (100kg) would carry on with 1/3rd = 3,33... m/s. Total speed difference stays 10m/s.

http://hyperphysics.phy-astr.gsu.edu/hbase/colsta.html

2D elastic collsions involve angles. http://farside.ph.utexas.edu/teaching/301/lectures/node77.html

If the collision is inelastic (deformation) or heat conversion gomes into play then the math gows over my head. The same with 3D-collisions, uncentered hit, spin ...

 

Edited December 25, 2016 by Green Baron

[Flamingo]

@Green Baron

Thank you!

The math gives perfect sense, and you answered it perfectly. Finally i can stop thinking about it.

Edited December 26, 2016 by Flamingo

[NSEP]

How on planet Turnovia do they make these maps? I wa t to make maps like this too.

[K^2]

On 12/25/2016 at 7:48 AM, Green Baron said:

If the collision is inelastic (deformation) or heat conversion gomes into play then the math gows over my head. The same with 3D-collisions, uncentered hit, spin ...

Inelastic collision works exactly the same. It's just that your E₁ < E₀. Figuring out the exact loss will depend a lot on material properties, but typically, an approximation where a given fraction is lost for a given material is sufficient. From there, you set up a system of equations where momentum is conserved and energy is constrained by the above and solve it exactly the same as in the elastic case.

Non-spherical bodies are slightly more complex, because you have to take normals at impact point into account, and that can generate angular momentum transfer in addition to impulse transfer. Which means you have to take moment of inertia tensor into account. As a result, the system of equations grows quite a bit larger to account for six degrees of freedom for each body. But again, you'd go on to solve the problem in exactly the same form. Total momentum and total angular momentum are conserved, and energy is either conserved or you lose some fraction to heat. Set up system of equations, solve it, get your results. Caveat is that point of impact computation and your system of equations are likely to end up being non-linear, which means you'll need a computer to solve it. Which isn't unexpected.

[Wjolcz]

Can someone explain the spectral diagram of the stars? These letters are obviously not in the right order, so I'd assume each of them has a certain meaning?

EDIT: How I understand this from Wikipedia: as new types of stars were being discovered they also started measuring their sizes, densities and things like that and came to conclusion that, for example, the B-type stars are the previous stage of evolution of the A-type stars, right? So instead of reclassifying them they decided to just move the letter B before A and keep the class of these stars the same?

Basically, it was easier to modify the scale/diagram/whatev instead of recataloging them from the scrarch?

Edited December 27, 2016 by Veeltch

[Green Baron]

1 hour ago, Veeltch said:

Can someone explain the spectral diagram of the stars?

Emancipate yourself from Wikipedia ! :-)

It's easy, and i hope this is not too simple an explanation, if it is then i apologize:

A star radiates a continuous spectrum. Parts of the spectrum are absorbed in its atmosphere, depending on the elements present. These parts correspond to dark lines in the spectrum, that's an absorption spectrum. It tells us about the elements present in a star's atmosphere, the star's atmosphere composition.

On the other hand, if a gas is heated to a point where its elements start to radiate (excited electrons fall back into their original state and send out a corresponding photon), the corresponding peaks in the emission spectrum tell us about the composition of the gas.

 

The Hertzsprung-Russell-Diagram and the categorization of stars is the compilation of our knowledge of the development stars. It maps the surface temperature (colour) of a star to its brightness (moreorless). There are different classification schemes. I am not sure, but i don't think that something as heavy as a B-type (Rigel) is a predecessor of a bright A-type (Sirius comes to mind ...),

... but this all is just a hobby of mine :-)

 

Edit: because the visible matter in the universe is mainly hydrogen and helium + some impurities do not be surprised if you stumble upon testimonies like "there is hydrogen, helium and metallicity". For astronomers, everything heavier then helium is a metal, they are not as sophisticated as the chemists ...

Edited December 27, 2016 by Green Baron

[monstah]

12 hours ago, Veeltch said:

These letters are obviously not in the right order, so I'd assume each of them has a certain meaning?

One older classification system used all the letters from A to N, in a sort of continual sequence of the visible characteristics that were perceived at the time, plus letters O, P, Q for some odd-balls. That's before it was known how they related to temperature and particular chemistry of the star.

Said system was later revised, temperature was taken to be the major influence in the spectrum appearance, so most letters were grouped into O, B, A, F, G, K, M and reordered by temperature, and some suffixes and sub-classifications were added to account for other particularities.

[Wjolcz]

1 hour ago, monstah said:

One older classification system used all the letters from A to N, in a sort of continual sequence of the visible characteristics that were perceived at the time, plus letters O, P, Q for some odd-balls. That's before it was known how they related to temperature and particular chemistry of the star.

Said system was later revised, temperature was taken to be the major influence in the spectrum appearance, so most letters were grouped into O, B, A, F, G, K, M and reordered by temperature, and some suffixes and sub-classifications were added to account for other particularities.

Oh, ok. That's what I was suspecting, since no spectral colour starts with B or O.

[TheDestroyer111]

What is the chemical composition of RP-1? Wikipedia doesn't give any source on its article about it, and I can't seem to find any serious source about that on the internet...

//EDIT: Wow, thank you very much for the quick answer!

Edited January 5, 2017 by TheDestroyer111

[SuperFastJellyfish]

From here:

PROPERTIES OF ROCKET PROPELLANTS
Compound	Chemical Formula	Molecular Weight	Density	Melting Point	Boiling Point
Liquid Oxygen	O2	32.00	1.14 g/ml	-218.8oC	-183.0oC
Liquid Fluorine	F2	38.00	1.50 g/ml	-219.6oC	-188.1oC
Nitrogen Tetroxide	N2O4	92.01	1.45 g/ml	-9.3oC	21.15oC
Nitric Acid	HNO3	63.01	1.55 g/ml	-41.6oC	83oC
Hydrogen Peroxide	H2O2	34.02	1.44 g/ml	-0.4oC	150.2oC
Nitrous Oxide	N2O	44.01	1.22 g/ml	-90.8oC	-88.5oC
Chlorine Pentafluoride	ClF5	130.45	1.9 g/ml	-103oC	-13.1oC
Ammonium Perchlorate	NH4ClO4	117.49	1.95 g/ml	240oC	N/A
Liquid Hydrogen	H2	2.016	0.071 g/ml	-259.3oC	-252.9oC
Liquid Methane	CH4	16.04	0.423 g/ml	-182.5oC	-161.6oC
Ethyl Alcohol	C2H5OH	46.07	0.789 g/ml	-114.1oC	78.2oC
n-Dodecane (Kerosene)	C12H26	170.34	0.749 g/ml	-9.6oC	216.3oC
RP-1	CnH1.953n	≈175	0.820 g/ml	N/A	177-274oC
Hydrazine	N2H4	32.05	1.004 g/ml	1.4oC	113.5oC
Methyl Hydrazine	CH3NHNH2	46.07	0.866 g/ml	-52.4oC	87.5oC
Dimethyl Hydrazine	(CH3)2NNH2	60.10	0.791 g/ml	-58oC	63.9oC
Aluminum	Al	26.98	2.70 g/ml	660.4oC	2467oC
Polybutadiene	(C4H6)n	≈3000	≈0.93 g/ml	N/A	N/A

 

Quote

NOTES:

Chemically, kerosene is a mixture of hydrocarbons; the chemical composition depends on its source, but it usually consists of about ten different hydrocarbons, each containing from 10 to 16 carbon atoms per molecule; the constituents include n-dodecane, alkyl benzenes, and naphthalene and its derivatives. Kerosene is usually represented by the single compound n-dodecane.

RP-1 is a special type of kerosene covered by Military Specification MIL-R-25576. In Russia, similar specifications were developed under specifications T-1 and RG-1.

Nitrogen tetroxide and nitric acid are hypergolic with hydrazine, MMH and UDMH. Oxygen is not hypergolic with any commonly used fuel.

Ammonium perchlorate decomposes, rather than melts, at a temperature of about 240 oC.

 

Here is the Milspec mentioned.

(I bolded the relevant parts)

Edited January 5, 2017 by SuperFastJellyfish

[magnemoe]

On 25.12.2016 at 4:32 PM, NSEP said:

How on planet Turnovia do they make these maps? I wa t to make maps like this too.

First you turn the map upside down, then you need to look at climate zones wind pattern and rain shadows.

An upside down world get winds inverted. US get more desert since wind comes in to California and rain is stopped by rocky Mountains 

[WinkAllKerb'']

still is there any way to cut the border with the highest land ratio possible instead of water ? (( )) ( ) 

Edited January 6, 2017 by WinkAllKerb''
(why, no idea just curious to sea what it look like from a fish pov instead of walker)

[TheEpicSquared]

Not sure if this is the right place, but is the Cheerson CX-32A a good quadcopter?

Edited January 6, 2017 by TheEpicSquared

[WinkAllKerb'']

eyes and touch referential no  ?

@gooddog15 (mode on the late, + forum software weirdness with page shown )

Edited January 6, 2017 by WinkAllKerb''

[Findthepin1]

So my mother made soup the other day and it got cold and she put it in the microwave to warm it up and put oregano seeds in it and when it was out of the microwave the oregano seeds had all clumped into a hexagon in the centre by themselves. What causes this?

Edited January 13, 2017 by Findthepin1

[DerekL1963]

5 hours ago, Findthepin1 said:

So my mother made soup the other day and it got cold and she put it in the microwave to warm it up and put oregano seeds in it and when it was out of the microwave the oregano seeds had all clumped into a hexagon in the centre by themselves. What causes this?

My guess, convection flow.

[p1t1o]

 

9 hours ago, Findthepin1 said:

So my mother made soup the other day and it got cold and she put it in the microwave to warm it up and put oregano seeds in it and when it was out of the microwave the oregano seeds had all clumped into a hexagon in the centre by themselves. What causes this?

Let me guess - it was one of those microwaves with a rotating table, no?

It would appear from recent research into the Hexagons at Saturn's poles that rotating fluids of different densities can occasionally form geometric shapes naturally, give it a google

[Findthepin1]

5 hours ago, p1t1o said:

Let me guess - it was one of those microwaves with a rotating table, no?

Yup

5 hours ago, p1t1o said:

It would appear from recent research into the Hexagons at Saturn's poles that rotating fluids of different densities can occasionally form geometric shapes naturally, give it a google

Thanks

[p1t1o]

4 minutes ago, Findthepin1 said:

Yup

Thanks

No worries!

But now that I re-think it, whilst the above is still correct and possible, there may be a simpler solution that doesnt require brand new science. Particles floating on a fluid, depending on the surface properties of the particles and the surface tension of the fluid, can tend to agglomerate together, and if you pack spherical(ish) together, they will try and stack up in a least-area conformation, and (at least with objects of similar size) you can get hexagonal symmetry quite easily: