For Questions That Don't Merit Their Own Thread

[ChrisSpace]

Would an Alderson Disk have more habitable land area than a Ringworld? And which one of the two would require more building material? Alternatively, is there any similarly vast artificial habitat idea with even more area?

[Steel]

25 minutes ago, ChrisSpace said:

Would an Alderson Disk have more habitable land area than a Ringworld? And which one of the two would require more building material? Alternatively, is there any similarly vast artificial habitat idea with even more area?

Well according to the Wikipedia page, an Alderson disk would be more massive than the Sun and require more material to build than is present in the Solar system. I'm going to guess that this means it would take more to build than a ringworld (depending on the size of the ring world, I suppose).

An Alderson Disk on it's own probably doesn't have a very large habitable zone, you would need life-support to enlarge it. There are also numerous questions about the direction of gravity at any given point of the disk as well as questions about how its would retain an atmosphere.

It's worth noting that while a ringworld is a sci-fi idea, the Alderson disk is not. It originated as a setting for a fantasy swords-and-sorcery world and so the physics of it are sketchy at best.

Edited November 12, 2017 by Steel

[kerbiloid]

On 08.11.2017 at 9:10 AM, James Kerman said:

I had read about this car but needed to see it to fully appreciate how ugly it is:

If you dislike Ford Nucleon, buy Ford Seattle-ite.

Fallouts' Corvega looks like this, and there is a lot of them, broken.

Spoiler

On 10.11.2017 at 12:33 AM, shynung said:

 

 

Martian mobile fastfood with toilet cabin looks great.

 

On 08.11.2017 at 10:01 AM, DerekL1963 said:

It was never built, and the reactor "design" never advanced beyond the most basic assumptions and bar napkin calculations.

The smallest operational (mobile) power plant that I'm aware of would be NR-1's...   And that looks to be about sixty feet long and twelve feet in diameter.

I was trying to calculate similar transport (being a student, twenty years before first time read about it), but was shocked when realized how thick would be the reactor shielding.
Since then I know, that only dump trucks can use this.

[James Kerman]

I am attempting to get an accurate distance between two points on Kerbin using KER.  I am assuming KER is only giving me the straight line distance  between the two vessels and I would like to know how to calculate the arc length but alas I am a mathematical idiot and the "for dummies" explanation does not have a "for profound dummies" section.
What I have is this:
Kerbins Circumference= 3769911m
Kerbin's radius= 600000m
chord length= 713393m

Is it possible to calculate the arc degree from the above information?  That is where I'm falling down.

[Green Baron]

Assuming that Kerbin is a sphere and not an ellipsoid and you only regard points on the surface without height you can use the formula for a Great Circle from a cord length:

https://en.wikipedia.org/wiki/Great-circle_distance

For elliptical orbits a short search revealed for example this:

https://math.stackexchange.com/questions/433094/how-to-determine-the-arc-length-of-ellipse

:-)

[James Kerman]

Thanks, @Green Baron.  You truly are a gentleman and a scholar but I'm afraid it's above my level - I have to take my shoes off to count to twenty.

You have given me the equation however so I am thankful.

[Green Baron]

[p1t1o]

What is the inverse of rotation?

This comes from a/the discussion about how bicycles stay upright and how two opposed wheels with opposite rotation still have a gyroscopic effect.

A forward vector can be cancelled out completely with a vector in the opposite direction, however with rotating objects, when you take two of them rotating in opposite directions, some things cancel out but there are still terms left over - hence two wheels rotating in opposite directions still have a gyroscopic effect.

Lets consider the wheel, all points on the rim are constantly accelerating inwards at a rate proportional to the speed of rotation. This occurs in equilibrium, a steady state, no points change distance from any other point. It stores angular momentum. Unless energy is removed, it will continue forever.

So what is the opposite of that? What arrangement of masses and velocities can directly and completely oppose the angular momentum of a wheel? Is there one? Does the question make sense in this universe?

The only thing that comes close in my mind is one of those sprinklers where water enters a spinny thing, shoots out the ends making it rotate? Its the only example I could think of with constant outwards acceleration, but even this isnt in equilibrium as the water leaves the system entirely. And it still looks quite a lot like a wheel.

 

 

Edited November 15, 2017 by p1t1o

[Oiff]

On 14/11/2017 at 4:04 AM, James Kerman said:

I am attempting to get an accurate distance between two points on Kerbin using KER.  I am assuming KER is only giving me the straight line distance  between the two vessels and I would like to know how to calculate the arc length but alas I am a mathematical idiot and the "for dummies" explanation does not have a "for profound dummies" section.
What I have is this:
Kerbins Circumference= 3769911m
Kerbin's radius= 600000m
chord length= 713393m

Is it possible to calculate the arc degree from the above information?  That is where I'm falling down.

Since you know the straight line distance you don’t need to use spherical trigonometry which you would need to use if you only had the coordinates.

You can use regular trigonometry to calculate the angle between the two positions then multiply this by 1/360th of the circumference.

2xasin(713393/2/600000)x(3769911/360)

763963.7m

[James Kerman]

19 hours ago, MattJL said:

I got a value of 763,962 m for the arc.  That sounds about right, and along the lines of what you estimated.

Above is the result of spherical trigonometry and below is regular Trigonometry.  Both you guys deserve a gold star  but silver is the only option I've got.

4 hours ago, Oiff said:

Since you know the straight line distance you don’t need to use spherical trigonometry which you would need to use if you only had the coordinates.

You can use regular trigonometry to calculate the angle between the two positions then multiply this by 1/360th of the circumference.

2xasin(713393/2/600000)x(3769911/360)

763963.7m

 

[Oiff]

6 hours ago, p1t1o said:

What is the inverse of rotation?

This comes from a/the discussion about how bicycles stay upright and how two opposed wheels with opposite rotation still have a gyroscopic effect.

A forward vector can be cancelled out completely with a vector in the opposite direction, however with rotating objects, when you take two of them rotating in opposite directions, some things cancel out but there are still terms left over - hence two wheels rotating in opposite directions still have a gyroscopic effect.

Lets consider the wheel, all points on the rim are constantly accelerating inwards at a rate proportional to the speed of rotation. This occurs in equilibrium, a steady state, no points change distance from any other point. It stores angular momentum. Unless energy is removed, it will continue forever.

So what is the opposite of that? What arrangement of masses and velocities can directly and completely oppose the angular momentum of a wheel? Is there one? Does the question make sense in this universe?

The only thing that comes close in my mind is one of those sprinklers where water enters a spinny thing, shoots out the ends making it rotate? Its the only example I could think of with constant outwards acceleration, but even this isnt in equilibrium as the water leaves the system entirely. And it still looks quite a lot like a wheel.

How about a vortex?

It feeds its angular momentum with changes in altitude that result in coriolis effect.  The changes in altitude are normally eithercaused by heating or gravity.

These system can self perpetuate with an example being hurricanes, one of the most destructive things on the planet.

 

On a related note I think bycicles stay up because the steering allows for counter steer that counteracts the CoG shifting off centre not by gyroscopic precession.  Gyroscopic effects can be helpful at really high speed but your average cyclist won’t notice then.  This is also how motor bikes are driven fast, watch a race bike going around a corner and you will see the front wheel going the opposite way to the direction of turn.  This isn’t to drift like a car would but to stop the bike from falling over.

[Aghanim]

On 11/8/2017 at 1:38 PM, shynung said:

How small can a functional nuclear power generator be built, in order to use them as a vehicle's power source? We know that nuclear-powered ships and submarines exist, but can we scale the reactors down even more, while keeping a usable power-to-weight ratio?

Theoretically, a dense plasma focus based fusion reactor can scale down really well, as their ion density is constant with the size of the device, and by using direct conversion, no bulky steam generators are necessary

Quote

Thus, a comprehensive analysis also including results from other groups is presented. In particular, all the devices, from the largest to the smallest, maintain the same value of ion density, magnetic field, plasma sheath velocity, Alfvén speed and the quantity of energy per particle. Therefore, fusion reactions are even possible to obtain in ultraminiature devices (driven by generators of 0.1 J for example), as they are in the larger devices (driven by generators of 1 MJ).

http://iopscience.iop.org/article/10.1088/0963-0252/19/5/055017/meta

Imagine the usefulness of a very small fusion reactor that fits on a desktop PC tower...

The problem is that the focus fusion guys haven't successfully create a self sustaining fusion reaction on their multi megajoule device, and this:

Quote

However, the stability of the plasma pinch highly depends on the size and energy of the device.

 

[DerekL1963]

14 minutes ago, Aghanim said:

Imagine the usefulness of a very small fusion reactor that fits on a desktop PC tower...

Not very, because the neutron shielding you'll require will make the fusion volume very small indeed.  It'll probably drive it's own cooling fan though.

[Aghanim]

45 minutes ago, DerekL1963 said:

Not very, because the neutron shielding you'll require will make the fusion volume very small indeed.  It'll probably drive it's own cooling fan though.

You don't use 'dirty' neutronic fuels such as D-T for this type of reactor, you use 'clean' aneutronic fuels such as helium3-helium3, or proton-boron, which is what Focus Fusion are currently investigating. D-T fuel energy output is in form of useless neutrons that cannot be used for direct conversion to electricity, unless if you are trying to make neutrons.

Of course, there is that pesky bremsstrahlung x-rays generated from fusion reaction, which can be probably absorbed by several cm thick lead shield...

[NSEP]

Did the Soviets ever had or used SRB's in/on their rockets? It almost seems like the Soviets never had SRB's.

[wumpus]

7 hours ago, NSEP said:

Did the Soviets ever had or used SRB's in/on their rockets? It almost seems like the Soviets never had SRB's.

I doubt it.  The Germans used alcohol (a mix of ethyl alcohol and water, typically 75% alcohol/25% water, but 50%/50% also works) and liquid oxygen (this is "close" to the ISP of kerolox, and burns at about 1000K lower).  Later ICBMs and rockets (both US and USSR) used hyrdrolox, but this had the issue of requiring fueling before launch.  It was quite possible that an ICBM site could be destroyed before launch during a nuclear war.  The US went with solid rockets for ICBMs and thus had the technology for boosters, while the USSR preferred hypergolics and even use this tech with the Proton.

I don't think Russia has ever done significant work with solids (the Apollo program used hypergolics in a few places, and I think the shuttle RCS had some, but they typically don't carry much of that type of fuel).

Just checking for another thread: looks like the Titan [2] missile used a solid lower stage and a hypergolic upper stage.  I guess I shouldn't post about ICBMs...

Edited November 20, 2017 by wumpus
note about titan

[Grand Ship Builder]

ICBM = Cool if they're not actually launching.

[Racescort666]

1 hour ago, wumpus said:

I doubt it.  The Germans used alcohol (a mix of ethyl alcohol and water, typically 75% alcohol/25% water, but 50%/50% also works) and liquid oxygen (this is "close" to the ISP of kerolox, and burns at about 1000K lower).  Later ICBMs and rockets (both US and USSR) used hyrdrolox, but this had the issue of requiring fueling before launch.  It was quite possible that an ICBM site could be destroyed before launch during a nuclear war.  The US went with solid rockets for ICBMs and thus had the technology for boosters, while the USSR preferred hypergolics and even use this tech with the Proton.

I don't think Russia has ever done significant work with solids (the Apollo program used hypergolics in a few places, and I think the shuttle RCS had some, but they typically don't carry much of that type of fuel).

Just checking for another thread: looks like the Titan [2] missile used a solid lower stage and a hypergolic upper stage.  I guess I shouldn't post about ICBMs...

One of the drivers for solid rockets was the US Navy wasn't real excited about hypergolic fuels on submarines. They wanted something that was storable and fairly safe to store. The Soviet Navy appears to not have this same requirement because a cursory search reveals that most of their submarine launched ballistic missiles are all liquid fueled. The US spent a lot of time and money getting usable SRMs to a point where the Navy was comfortable with them and as a byproduct, they also made it into orbital launch vehicles as well.

Titan has optional SRBs for the Titan IIIC, IIID, IIIE, 34D, and IV and only the SRBs fire at liftoff with the main engine firing after liftoff.

[ARS]

Does a rocket with engine attached on the upper stage is is more stable than the one with engine placed on the lower stage? (The rocket center of mass is below the engine instead above the engine, so instead pushing the mass, it pulls it,)

 

[peadar1987]

28 minutes ago, ARS said:

Does a rocket with engine attached on the upper stage is is more stable than the one with engine placed on the lower stage? (The rocket center of mass is below the engine instead above the engine, so instead pushing the mass, it pulls it,)

 

https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

It's a little counterintuitive, but no. Although in KSP, engines are really heavy compared to their real-life counterparts, so putting them near the nose of the rocket shifts the CoM further up, which does increase stability.

[Aghanim]

How do you do fractional distillation in zero g?

[peadar1987]

4 hours ago, Aghanim said:

How do you do fractional distillation in zero g?

Spin gravity, most likely. Either a centrifuge or place the distillation system on the end of a tether. You don't need much gravity for the process to work, just enough to give a bit of buoyancy (and a lot of patience if you need very pure product)

[ChrisSpace]

What would be the best defence against the nightmare fuel in the new short film "slaughterbots"? Here's my current opinion:

Projectile weapons: useless

Lasers: useless unless the beam width can be expanded and the laser power can be increased significantly

EMPs: Could work

Nukes: Too hard to acquire, but would certainly work

[wumpus]

12 hours ago, Aghanim said:

How do you do fractional distillation in zero g?

I would assume a centrifuge.  I'd be curious if it is possible to have multiple "arms" spinning and have a negative pressure at the center (unequal heating?) to force equalibrium into different areas.

Don't be surprised if the real answer is really, really weird.  It might involve surface tension and carefully chosen substances as surfaces for liquids to creep along.  Zero-g can get pretty weird at certain scales.

[magnemoe]

9 hours ago, ChrisSpace said:

What would be the best defence against the nightmare fuel in the new short film "slaughterbots"? Here's my current opinion:

Projectile weapons: useless

Lasers: useless unless the beam width can be expanded and the laser power can be increased significantly

EMPs: Could work

Nukes: Too hard to acquire, but would certainly work

Emp should work but would hit other stuff too unless beamed. 
Nets would help, staying indoors, in an car or other places they can not get would work, problem is that they can defeat windows who are not bullet proof or laminated. 
Any sort of body armor including sturdy clothes and helmets with face cover. If nothing else an bucket over you head.
The slaughterbots used an very small shaped charge or an explosively shaped penetrator. 
Range and penetration is minimal. An somewhat larger one would carry an larger warhead to defeat armor and even fire at range, 

Note that this is an military weapon, making an so miniaturized shaped charge or not to talk about penetrator and integrate it is hard to do in small scale but fairly easy to do with mass production. 
An terrorist would rater use one or a few larger drones with an easier to get explosive. 

Now the question is why military would want to use them. See two settings one is hard to reach and dangerous places like underground, second would be to reduce collateral damage they would work very well in hostage or human shield situations. 
Else you could just empty some B52 over target.