Science-based what-if questions

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Re: Science-based what-if questions

Postby doogly » Fri Mar 10, 2017 1:34 pm UTC

Throw a neutrino at it.
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Re: Science-based what-if questions

Postby p1t1o » Fri Mar 10, 2017 1:38 pm UTC

Passing through the centre of the sun would be like having megaton nuclear warheads detonating all over the skin of your ship, constantly (like millions of detonations per second).

There is some data to suggest that for a few tens/hundreds of nanoseconds, conditions at the epicentre of a megaton-class burst are marginally worse than those at the centre of the sun, but at the centre of the sun it is a constant, not just an extremely brief zap so the heat flux is several million times higher.

This one is really impossible without fictional materials/technology.

Sci-fi usually handles it by having a wormhole linked to outside space through which fictiony science rejects the colossal amounts of heat, or they just use a form of indestructimatter.

Pressure is also intolerable, but at these scales, pressure and temperature are almost the same concept. (That might not be technically accurate.)

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Re: Science-based what-if questions

Postby andykhang » Fri Mar 10, 2017 2:23 pm UTC

So it's time for that indestrutium we have used in that sniper rifle-shaped booster huh? Even then (and probably someway to ultilize that megaton pressure and heat, and also someway to plunge through it fast), what would happen?

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Re: Science-based what-if questions

Postby p1t1o » Fri Mar 10, 2017 2:26 pm UTC

andykhang wrote:So it's time for that indestrutium we have used in that sniper rifle-shaped booster huh? Even then (and probably someway to ultilize that megaton pressure and heat, and also someway to plunge through it fast), what would happen?


What would happen? Nothing. Its like asking "What would happen if we built and launched a submarine into the ocean?"
Either it survives or it doesnt, the sun won't notice which.

Unless your ship has a mass of significant (significant doesnt mean "large", it just means "large enough", in this context, at least as massive as Jupiter, and even that might not do much) fraction of that of the sun itself.

Adding or subtracting mass to a star will affect reaction rates and the stars lifetime and the mechanism of its eventual demise.

Having a starship large enough that this is a factor...well it wouldnt really be a "starship" but more of a mobile gas giant. And the physics behind that would be even more restrictive.

Affecting a star is difficult, much much many mass is about the only way to accomplish it. That mass might take part in, or hinder, the processes within a star, but either way, to achieve anything, you need a lot of it.
Last edited by p1t1o on Fri Mar 10, 2017 2:33 pm UTC, edited 1 time in total.

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Re: Science-based what-if questions

Postby andykhang » Fri Mar 10, 2017 2:30 pm UTC

...Yeah, guess so. That thing must at least be the size and weight of jupiter before it does anything noticable, and looking at it from inside probably blind your eye even with good protection. I does wonder about what kind of thing would you be able to see though (I originally thought this because if a weapon could pierce through the sun, almost nothing short of a white dwarf could stop it anymore)

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Re: Science-based what-if questions

Postby p1t1o » Fri Mar 10, 2017 2:37 pm UTC

andykhang wrote:...Yeah, guess so. That thing must at least be the size and weight of jupiter before it does anything noticable, and looking at it from inside probably blind your eye even with good protection. I does wonder about what kind of thing would you be able to see though (I originally thought this because if a weapon could pierce through the sun, almost nothing short of a white dwarf could stop it anymore)


I presumed that blindness would be the least of your problems, and that the technology level required to even contemplate this negates any medical intervention.

Indestructium sunglasses.

I believe the matter of the sun is largely opaque to visible light (it takes millions of years for a photon emitted at the core to escape the surface) so if you "dimmed" (Ha! understatement!) the windscreen, its not like you would be able to see very far, like if there was anoter ship inthere with you, it would be very difficult to detect.

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Re: Science-based what-if questions

Postby andykhang » Fri Mar 10, 2017 2:41 pm UTC

I guess a tourist tour inside of the sun isn't anything of interest huh? The Starpiercer would be a preeeetty good orbital bombarment weapon though (or even planet-destroying)

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Re: Science-based what-if questions

Postby p1t1o » Fri Mar 10, 2017 2:54 pm UTC

andykhang wrote:I guess a tourist tour inside of the sun isn't anything of interest huh? The Starpiercer would be a preeeetty good orbital bombarment weapon though (or even planet-destroying)


The starpiercer is capable of surviving within the core of a sun, surviving the gravity-fed plunge into it and capable of hauling itself out again.

The Starpiercer can do anything. Flying through a planet would be like driving through a small patch of fog.

Only a neutron star or similar would pose any consideration at all.

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Re: Science-based what-if questions

Postby andykhang » Fri Mar 10, 2017 3:50 pm UTC

And then someone overdesign it and it could pierce through neutron star as well :D. TBH, i thought it would take until a black hole would you need any amount of illegal science, in the first place.

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Re: Science-based what-if questions

Postby p1t1o » Fri Mar 10, 2017 4:10 pm UTC

andykhang wrote:And then someone overdesign it and it could pierce through neutron star as well.


Considering its made of fantasy, all you would need to do is turn the knob to "neutron star" 8-)

***

So I have a question - if you are orbiting a neutron star at say 1AU and look at it, what do you see? A dark circle? Something like a small star? A little red dot? A big red polka dot? Would gravity lensing warp the view? Is my (normal - not fantasy) ship ok?

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Re: Science-based what-if questions

Postby andykhang » Fri Mar 10, 2017 4:46 pm UTC

In what definition would you considered your ship "normal"? If it a regular space shuttle, the circuitry of your ship would be fried by x-ray and magnetic field, along with you, I think?

Edit: For your first question... probably, you won't see the star's shape as all, as it only have a diameter of 10km. Most of it energy also come from x-ray, so technically, you could see it... if it's only because the x-ray right now is burning through your retina. If that is so, you could see a white blob I guess?

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Re: Science-based what-if questions

Postby Sableagle » Fri Mar 10, 2017 8:04 pm UTC

Going back to your weapon (is this a necropost?) I think "rifle" is the wrong starting point.

Spoiler:
Check out Starburst MANPAD. That "pop-WHOOSH" effect is a two-stage rocket motor. The first stage is a very short burn to get the rocket up to speed before its rear end clears the launch tube. It then goes on a short ballistic trajectory to get to a distance far enough from the shooter not to burn him up, and ignites the second stage to get itself up to Mach 4. No, not the razor. 1600 m/s. That Mach 4. It's guided by a laser grid projected from the launcher and detected by sensors on the rear of the missile, so it can be steered in flight as long as you turn it gently enough not to yank the grid clear off it.

There's a weakness there if you're using a really LONG-range version, which is that the laser light is shining at your target and will get there long before your projectile, and laser light from your target could get back to you just as quickly. I'm not sure how big a laser array weapon would need to be to fry the shooter, but if the missile is relying on you to guide it the retaliation really only has to burn your eyes to make it miss.

If you go for a kinetic energy weapon, at low velocities KE approximates to 0.5mv2, so to make it more effective you increase the mass (go from 9x19mm pistol bullet to 12-gauge shotgun slug) or increase the velocity (go from a .45 ACP to a .30-06 Springfield). Either way, you're also increasing momentum, and momentum is conserved via that whole "equal and opposite reaction" thing, so you can launch a 16 g bullet at 250 m/s, a 10 g bullet at 400 m/s, a 4g bullet at 1000 m/s or, hypothetically, a 1 g airgun pellet at 4000 m/s for the same "kick." A 20 g bullet at 4 km/s would have 20 times the kick. That would be an issue. In energy terms, those are 501 J, 802 J, 2006 J, 8024 J and 160479 J. Faster and lighter has a higher punch-to-kick ratio, but requires higher pressure (and you need high pressure to make a 1000 mm barrel useful) which means stronger chamber and barrel walls which means more weight and has another problem:

Image

In the first 50 yards that bullet loses around 265 ftlbf. From 200 to 250 it loses around 210. From 950 to 1000 it loses less than 80. Adding extra velocity at the muzzle gets harder and harder as you get your bullet faster and faster, and gives you less and less extra reach. If you want to simply scale that bullet up until it could level a city block, you need ... well, now, how would I look up that figure? Let's go with a Grand Slam bomb. According to wikipedia,
Filling weight 4,144 kg
Torpex is a secondary explosive, 50% more powerful than TNT by mass.
TNT is 4184 J/g so that should be 26 GJ of explosive power or, in this case, kinetic energy. I wonder whether Bill StClair can handle that.
... 520 kg at 10 km/s, 3.25 t at 4 km/s, 52 t at 1 km/s, 325 t at 400 m/s or something else on that curve. 52 tonnes is "only" a modern main battle tank. Get one of those going at .30-06 Springfield muzzle velocity and it'll be very dangerous indeed to whatever gets in its way ... if it gets there. Air resistance is a thing, and that energy has to go somewhere. 4776682.35 times the mass of the .30-06 bullet, same velocity? In the first 50 yards that bullet presumably loses around 1.26582 * 109 ftlbf or 1.71622 * 109 J. 1.7 GJ, not even a fifteenth of a Grand Slam bomb's power, getting dumped into the air in the 50 yards immediately in front of you, no problem, right? Projectile melting may be a problem. Make it 5.2 kg of neutron star at 100 km/s (or 0.1 gigametres per second if you like SI prefixes) and your bullet's new 5.2 * 10-8 cubic millimetre volume means air resistance is less of an issue, even at that velocity. Also, you'll be shielded from the blast by the gun barrel "because DAMN." Like wise for 5.2 mg at 100000 km/s, which ... Wait. Rewind. "At low velocities KE approximates to 0.5mv2" 108 m/s ain't really a low velocity. That's 39 GJ, so slightly overkill for a mere city block. Also, that energy dump may be ... more than just hot air. Let Randall Munroe explain:
Air molecules vibrate back and forth at a few hundred miles per hour, but the ball is moving through them at 600 million miles per hour. This means that as far as the ball is concerned, they’re just hanging there, frozen.

The ideas of aerodynamics don’t apply here. Normally, air would flow around anything moving through it. But the air molecules in front of this ball don’t have time to be jostled out of the way. The ball smacks into them so hard that the atoms in the air molecules actually fuse with the atoms in the ball’s surface. Each collision releases a burst of gamma rays and scattered particles.

These gamma rays and debris expand outward in a bubble centered on the pitcher’s mound. They start to tear apart the molecules in the air, ripping the electrons from the nuclei and turning the air in the stadium into an expanding bubble of incandescent plasma.


I don't think you can throw something hard enough to do that much damage with just kinetic energy without a gun too big to be carried cross-country or even something too big to be in a country, like a Star Destroyer.
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Re: Science-based what-if questions

Postby Soupspoon » Fri Mar 10, 2017 8:11 pm UTC

From an already extant canon... If you want to piggy-back on that concept rather go your own way.

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Re: Science-based what-if questions

Postby Tub » Fri Mar 10, 2017 10:51 pm UTC

p1t1o wrote:So I have a question - if you are orbiting a neutron star at say 1AU and look at it, what do you see?

You probably won't see a neutron star. Those things are tiny.

The exact details depend on the neutron star, of course. At 1 AU, a 10km big ball is very hard to see, no matter its mass. There's a bit of gravitational lensing, but not enough to clearly see from your distance. As there's no fusion, the only light it emits is from blackbody radiation. But even the hottest neutron stars are way darker than a star, and with their small size, 1 AU is too far away. Besides, most of their radiation is x-ray and thus invisible to your eyes. Older neutron stars are colder and darker, and even harder to spot.

Unless the neutron star is accreting matter. For that case, I haven't found any numbers, but I guess what you'd see is the color of your own blood before you drop dead.

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Re: Science-based what-if questions

Postby gmalivuk » Sat Mar 11, 2017 12:35 am UTC

A million-kelvin neutron star with 10km radius would put out about 1/5 as much power as the Sun. Sure, you still wouldn't be able to see it on account of the whole x-ray thing, but I wouldn't call that way darker than a star.
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Re: Science-based what-if questions

Postby andykhang » Sat Mar 11, 2017 3:33 am UTC

@ Sableagle:...Isn't that math only applied to that particular kind of gun though? Not for something that could accelerate itself up to 1/3 the speed of light by mean of beam core AM drive (the rifle design are there to generate the magnetic field that would guide the charge pions) ? And you kinda lost me halfway there, in short?

Edit:...though,probably, I could overdesign this a bit more if I somehow manage to ultilize the gamma ray...What could you used the gamma ray for, in this particular situation?

Edit 2: Oh yeah, another question: How would you ultilize the intense heat and pressure coming from when you're inside a Sun?

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Re: Science-based what-if questions

Postby Sableagle » Sat Mar 11, 2017 11:45 am UTC

andykhang wrote:Edit:...though,probably, I could overdesign this a bit more if I somehow manage to ultilize the gamma ray...What could you used the gamma ray for, in this particular situation?
If you can make a matter-antimatter drive unit work, you don't need to use it to drive a giant bullet up to gigajoule speeds. You just need to deliver it to your target and turn it on.

What you could do with a gamma burst from your own location? Modify your indestructium. Make it gamma-ray-reflective. Make it concave parabolic. Put your bomb at the primary focus. Aim it at your target. You can move the bomb a little further away from the dish to pull secondary focus back from infinity and aim at a smaller area of your target, but your target's quite large, right? Detonate bomb. The deeper your dish, the less you'll make yourself unpopular with the neighbours, although again you have diminishing returns to scale here. Gamma radiation hits target in a very short, very intense pulse. Everybody inside the target dies.

Parabolic.gif
Parabolic.gif (2.74 KiB) Viewed 1336 times

Diminishing returns to scale: the innermost parts of the dish are doing the most, the outer parts the least.






What was that yield we needed? 6216 kg TNT, I believe. 5% of 26 gigajoules is 1.03 GJ. From wiki:
The gray (Gy), which has units of joules per kilogram (J/kg), is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.

An acute full-body equivalent single exposure dose of 1 Sv (1000 mSv) causes slight blood changes, but 2.0–3.5 Sv (2.0–3.5 Gy) causes very severe syndrome of nausea, hair loss, and hemorrhaging, and will cause death in a sizable number of cases—-about 10% to 35% without medical treatment. A dose of 5 Sv[28] (5 Gy) is considered approximately the LD50 (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. A dose higher than 5 Sv (5 Gy) brings an increasing chance of death above 50%. Above 7.5–10 Sv (7.5–10 Gy) to the entire body, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see Radiation poisoning).[citation needed] (Doses much larger than this may, however, be delivered to selected parts of the body in the course of radiation therapy.)


1 GJ, 100kg of target, 10 million Grays, 2 million times the LD50. I guess a foot of lead or gold would save someone after all. Catch a dense crowd in the open, though, and you've got enough gamma radiation there to guarantee the deaths of a million people. Maybe not much use against a military bunker, but if there's some guy on a stage in a stadium droning on about how the woman who did all the work to make their relationship work until he left her without a goodbye or an explanation should be waiting at home, making a fresh sandwich every hour, for him to come back rather than going out with her friends and having fun and occasionally talking to people he didn't personally introduce to her and you just can't stand it any more and really, really have to shut him up ... yeah, it'd do that.

The less indestructible your indestructium, the further from the blast your reflector has to be and the larger and heavier it has to be to cover the same percentage of the angles around the blast and reflect the same percentage of it towards the target.
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Re: Science-based what-if questions

Postby andykhang » Sat Mar 11, 2017 12:03 pm UTC

...You have such an interesting way to write huh? Though, like I said before, the character I was working on doesn't really care whether or not it's practical (there's always some charm to just power a bullet to 1/3 the speed of light). I was thinking more or less in the line of using the gamma ray to add more specific impulse to the bullet (thought the left-over would make for a good nuclear flamethrower...). In that case, what would be the best used?

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Space air

Postby andykhang » Sun Mar 12, 2017 12:30 pm UTC

So I just read this ,and got into "Rocket are not fighter plane" :http://www.projectrho.com/public_html/rocket/misconceptions.php. It said that rocket can't turn like plane do, since there's no air in space, so in order to turn, it have to divert it foward momentum toward the side you wanted to turn instead of letting the air do it for you. Since in a space battle, agility is the most important thing in order to let you avoid being space cheese, I guess that as some point, people are going to seriously consider the application of "Space air": Gasly (or gasly-like) Particle, either being spray everywhere (with enough consitency to avoid it being too thin) or using a natural source (probably from star nursery or a dense nebula) that, combine with "breaker" (wing design that allow for more effective turn), could add much needed agility for effective fight. Is it practical, or even possible to do it in space, despite the cold temp, cosmic wind, and the general lack of gravity? If so (and even if not so), what's the minimum requirement, and drawback for this method?

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Re: Space air

Postby Soupspoon » Sun Mar 12, 2017 2:36 pm UTC

Watch the (original, at least) Battlestar Galactica series, and see the Vipers and Cylon Fighters zoom around like atmospheric fighter planes (within the limits of matt-replacement technology, and generally cinematographically keeping useful framing shots so as not to confuse the audience). That is how it would not work in space, unless you were deliberately trying to do it that way at a vast expense of propellant (visible afterburners for 'thrust' and invisible retros or gyros for the pitch, roll and yaw and maybe 'lift' in the vicinity of an airless planet/planetoid of some kind). It makes sense to us, used to flight through air, but it really does not make sense in space flight (even discounting orbital peculiarities). It also makes no sense that they have to "power cruise" even in straight and uninterupted flight (presumably constant velocity, relative to... well, at least the background stars) such as when rendezvousing or on a scouting mission or its return leg.

Now watch Babylon 5. The Starfuries, and most Human ships, work with full Newtonian physics in mind. Exotic "Gravity Drives" were used in elder-race ships, including Minbari (hence also the White Stars), as in-canon explanation for their ability to "swoop", although they also "reaction thrust" as easily, or emulated it as so. But to those without exotic drive systems, an all-axis (thrust and rotate!) system really is the best way of traversing airless space with any degree of control. Want to avoid debris flying towards you? Thrust 'up'wards (or downwards, or to the side) with your small thrusters, or spin yourself with them then punch in the big thruster to your rear for a more satisfying kick.

(Galactica's Vipers would pull back on the stick, while powering into the debris cloud, and hope to pass 'over' it, rather than just "not be where the debris would be". But then the debris cloud was likely that created by their destruction of a Cylon, which they had been chasing at nearly matching speeds, whose debris cloud would have been on average moving still at the same rate/direction as the original vessel at the moment of destruction... unless the Viper really was continually accelerating, still, it wouldn't be in any danger at all of 'threading the needle' of the cloud of destruction.)

So, the big question is why would you want to fly like a fighter plane? It might be mentally easier to handle, from a planet-bound person not expert in spaceflight and maybe only an armchair pilot in the atmospheric sense, but you'd be missing out on all the possibilities open to you in a non-aerodynamically constrained flight situation.

Then, it's the how. To get aerodynamic 'purchase', you need 'air' of some kind that is a decent proportion the density of our own (regularly flyable) atmosphere, and that gets thin enough at high altitude to be more prone to accidentally end up in spins whilst surfaces don't 'grip' enough and/or other thrust inintentionally acts far too much in an offcentered way. See the X-series of high altitude test planes, Felix Baumgartner's freefall and the principle behind the SpaceShipOnemTwo "shuttlecock" design. Thus to get something like an atmosphere to fly through you need an atmosphere to fly through. From where? Your choice, but if it's in situ then you must have prepared the ground (or chosen it, specifically, and pre-existing nebulous gas is probably too... nebulous... for our purposes), and if we're spraying it ourselves at the time of 'use' then that brings several other awkward questions into play (like "why aren't we just using this stuff as regular rocket exhaust gas?).

And then you're also adding friction to the equation (towards some 'stationary' frame of reference, at least near the centre of the dispersing cloud if it's otherwise unconstrained) which is going to act against you as much as any substantial pseudo-aerodynamic pitching/etc acts in your combative favour. And you certainly wouldn't be able to 'drift', except by essentially becoming as one with (parts of) the cloud by momentum exchange, at which point you're not really getting any benefit from using flaperons instead of monopropellants.


I'd advise against this whole idea of making your rockets (back) into aircraft, unless you have a very compelling reason (and method) for doing so. Which is not to say you might not have something like that, in mind, so perhaps if you shared your initial thinking a little more..?

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Re: Space air

Postby andykhang » Sun Mar 12, 2017 4:11 pm UTC

Well, first of the "Space air" application that I could thought of is the ability to "break" without using up your fuel. Especially in midspace supply post, where you don't have the grace of air and instead have to used your own engine (you could tried asteroid, but I don't think you would like to have a hole in your spaceship). Second is military reason to build "safe zone" against ballistic projectile and missiles (especially if you used nanomachines), and especially useful against light-based weaponry (not so much for light-based sensor though). It's also could be used to stall the enemies that's moving as great speed, since trying to do that, even in our regular atmosphere is suicidal. Another is that it could also be used to effectively collect more heat energy in some method, but that probably wouldn't be as useful.

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Re: Space air

Postby gmalivuk » Sun Mar 12, 2017 5:55 pm UTC

Soupspoon wrote:Watch the (original, at least) Battlestar Galactica series, and see the Vipers and Cylon Fighters zoom around like atmospheric fighter planes
As I recall, it does need to be the original, because in the new series the Vipers move accurately in vacuum, which is one of the things that pleased me while watching it.
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Re: Science-based what-if questions

Postby andykhang » Sun Mar 12, 2017 6:31 pm UTC

...Also, turn out that in order for AM beam drive to work, the nozzle need to be at least 21 meter, which is not-sniper-rifle size(more like stationary cannon size), but the speed could still be compensate by ...violently smashing the charge pion against the bullet through a narrow tube of magnetic belt after it's reflected (we assume it's reflect by the indestrutium) (also there's still the obscene amount of radiation pressure...). So...how much speed are we talking about?

Edit: Also, you could even overdesign it abit more if you place Scramjet design onto the bullet. Not only do you mitigate drag in-atmosphere, you would also have an external source of fuel and remass (fusion fuel, infact, since the speed and pressure alone is enough for it, and even allow you to bring more antimatter than matter in the initial blast), which could accelerate the speed. Thing is, which is better:

1.Letting the desgin as is and hoping the radiation pressure the level of a neutron star's core would be enough along with the accel, risking the pressure would be too much to bare for the internal engine of the bullet to work efficiently or

2.Before shooting, close the muzzle and fill the barrel with cold hydrogen (or tritium, if you're feeling fancy), and just before shooting, release the muzzle and letting it add more additional kick to the engine, with possibly reduced radiation pressure? (also another possible inefficient, since the speed isn't going to top unless travel in a significant distance of the barrel

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Re: Space air

Postby Soupspoon » Sun Mar 12, 2017 6:43 pm UTC

To brake (without breaking, unless you make your air too thick and the thing you're slowing down to weak), you're also impeding acceleration. Selective atmosphere-only 'airlock shields' could give you a zone of air of arbitrary viscosity to aerobrake into that doesn't intrude into neighbouring bits of space where that's not what you want. (If it is what you want, then build youself a gas giant in the vicinity of your arbitrary point in space, or thicken up the atmosphere of the planet... But then you lose the ability to float/orbit 'in space', and certainly impede your transit away from any stable point you have.) That's a futuretech, though, that's not just "get enough stuff together, somehow" megaproject. More on futuretech stuff later.

Might I suggest that for a 'midspace stop-off' you instead go for something like a mass-driver (first in reverse, to capture incoming loads, then 'normally' to send them onwards, and (if possible) used in careful balance amongst various loads to equalise the momentum transfers at each stage) either with good aiming skills or a large lateral forgiveness for incoming craft.

(Your midspace supply post... Lagrange-point? Or is it just 'adrift' between stars/galaxies/whatever... Is there some benefit to having an 'interstellar truck-stop', half-way from everywhere, rather than one (or several, spaced out) craft on a trans-Lunar/Mars Recycler style of loop... Not that a "free braking" mechanism could not be useful in hitching a ride with one of those (you'd need to transfer to the transit orbit, anyway, to casually dock with it), but then at least you have economies (and safety margins) of scale on your side, for the duration, whilst the truck-stop seems to have no purpose other than independence of location. (Or, I suppose, a hand-waved plot point. As long as you're prepared to wave your hands a lot...)

Thick enough for ballistic protection (given we already know how to get at least the terminal end of an anti-tank round through solid steel, after a trip through an atmosphere) is going to be interesting. The enemy could always orchestrate a Chicxulub-like asteroid if they weren't fans of subtlety, and if there were any Jovian Blimp-Creatures there to witness it, Shoemaker-Levy 9 probably shook any of them up who were in the vicinity. As a substitute for easily imagineered Force Fields (with switch-on-and-off-ability, already much precedented in fiction), cloaking within an atmosphere of ones own creation needs a bit of anti-grav applied to the disbelief module. (Not impossible, just a stretch that you need to handle or gloss over.) Ditto any form smog vs. "photonic shielding".

As a speed-limiter, that's what our atmosphere does (but we can still get through it, and/or not burn up in the attempt) but I'm reminded of some possibly classic SF series that employed (again) a 'field'-based approach. Some sort of a Null Zone is created where nothing (including light) can go quicker than, perhaps it was, tens of metres a second. It effectively denied entry to conventional missiles and kinetic masses (they rather splatted, from the slowdown) and people inside (no idea how they explained away nerve impulses and generally living, for the people involved) then operated, and fought with melee/low-velocity ramged weaponry, "blind" within the zone... But I can't remember what SF series this was from... Just the impression that it was a sort of Ender's Game thing ('80s publication at the latest, probably earlier).


Which makes me sort of look like I'm saying that you shouldn't try the aerobraking thing at all. That's not my aim, you just need to work harder to try to createba hard-SF reason to use (and to be able to use) Space Gas than you would to be able to conjure up something softer-SF with arbitrary projected fields and barriers...

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Re: Space air

Postby Eebster the Great » Sun Mar 12, 2017 10:32 pm UTC

It does matter how fast you are travelling. If you are moving fast enough relative to the interstellar medium, drag can be significant. Needless to say, in interstellar space, "fast enough" is pretty fast. The densest regions you have any reasonable probability of flying through (assuming you are not in or near a star or planet) are dense molecular cores, which in extreme cases can have densities up to 106 molecules/cm3. This is roughly
10-14 times the molar density of Earth's thermosphere. However, molecular clouds are very cold and comprised mostly of hydrogen, so their pressures and mass densities will be even lower still. They are thousands of times less dense even than the atmosphere at the orbit of the international space station.

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Re: Space air

Postby andykhang » Mon Mar 13, 2017 9:19 am UTC

Hm... the only hard-SF reason for using it, that I could thought of, is to directly pumping it into an atmosphere of a inhabitated planet to trigger global warming (or terrawarming, as I said), and force the inhabitant to surrender while the one using it could used to adjust the temperature and pressure to the livable level of their species. Placing these cloud in L-point work too, and, depend on range, you don't need too much of it to be effective (though the economy of scale would limit that to military operation and country (or even planet)-funded expedition and transportation, at least until better tech)

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Load bearing

Postby andykhang » Tue Mar 14, 2017 8:48 am UTC

How would you effectively divert an immediate physical force coming from outside to the ground without breaking the load bearing material?

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Re: Load bearing

Postby Zamfir » Tue Mar 14, 2017 9:16 am UTC

More details needed?

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Re: Load bearing

Postby speising » Tue Mar 14, 2017 9:16 am UTC

You have to channel the qi.

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Re: Load bearing

Postby andykhang » Tue Mar 14, 2017 9:48 am UTC

Well, suppose there's a big stick stand in a ground, and a steel ball coming as it with speed. What would you need to do for that stick to not break and instead divert the incoming force to the ground (even changing what type of stick count too, cus I want to know the requirement)

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Re: Load bearing

Postby p1t1o » Tue Mar 14, 2017 10:39 am UTC

The question does not appear to make sense.

You still want the bearing to hit the (top?) stick?

But have the force be "transmitted" to the ground without breaking the stick? (Assuming that the bearing has the energy to break the stick)

Use a stronger stick?

????

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Re: Load bearing

Postby HES » Tue Mar 14, 2017 10:42 am UTC

The same way we build bridge parapets, anti-terror bollards, railway buffers, impact resistant barriers, etc.

Make the stick stronger.

If you know which way the ball is coming from, put a strut behind it.

If there's only one ball, put something expendable in front of it.
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Re: Load bearing

Postby andykhang » Tue Mar 14, 2017 11:02 am UTC

So make it structurely rigid? I thought hard mean brittle, most of the time.

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Re: Load bearing

Postby p1t1o » Tue Mar 14, 2017 11:54 am UTC

Brittle doesnt mean weak though.

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Re: Load bearing

Postby HES » Tue Mar 14, 2017 1:15 pm UTC

andykhang wrote:So make it structurely rigid?
It depends what you want to achieve.
HES wrote:bridge parapets, anti-terror bollards, railway buffers, impact resistant barriers
I deliberately left regular crash barriers and standard bollards off the list, as those work differently.

Have several sticks, spaced at intervals. Tie them together. The first will fail, the tie then spreads the impact to the others. I guess this doesn't count as "the stick doesn't break", though.

Have a deformable stick. The ball hits it, it flexes over, the ball carries on, the stick springs back into place. Probably doesn't count as "divert the force to the ground", though.
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Re: Load bearing

Postby doogly » Tue Mar 14, 2017 3:32 pm UTC

speising wrote:You have to channel the qi.

Which is to say, increase elasticity.
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Re: Load bearing

Postby morriswalters » Tue Mar 14, 2017 6:41 pm UTC

Ask gun range owners. I'm trying to understand the models in the link. :roll: Another is to make the object being struck massive with respect to the impacting body. A tree will stop a bullet, a fairly massive stick as it were. I guess you have to decide exactly which mechanism you want to use. Or something.

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Re: Load bearing

Postby Sableagle » Tue Mar 14, 2017 8:11 pm UTC

andykhang wrote:So make it structurely rigid? I thought hard mean brittle, most of the time.

It does, but there's more to it than that.

https://www.youtube.com/watch?v=oMYkEMhPsO8

Curious piece of physics: temperature is the average speed of movement of the atoms or molecules within the material relative to each other. Above a certain speed relative to the others around it, one of those atoms or molecules behaves as a liquid, not a solid. If your bullet is going faster than that speed for the plate it behaves as if it's hitting liquid. If it's going faster than that speed for itself if behaves as if it is liquid.

This can be very pretty: https://www.youtube.com/watch?v=QfDoQwIAaXg&t=3s

Melting point and hardness seem to be related, so you get really hard bullets that DGAF if you put some steel plates in the way and lead bullets that'll splash against it and barely leave a mark. https://youtu.be/dYVT9R4GX2k?t=31

You also see different modes of failure of armour against different attacks. In the AR500 torture test, the (slow-moving, soft and heavy) shotgun slug bent the plate in the middle and smashed up the wooden frame and the (faster, harder, lighter) 7.62x51mm AP punched neatly through without bending it at all and even bullets that don't go through will eventually go through if they keep at it long enough.

On the subject of stopping an incoming "complicated weapon" like an implosion-initiated plutonium sphere nuke with an artificial atmosphere: why the atmosphere? At the kind of speeds where hitting unexpected atmosphere will cause it to burn up, meeting a wax slug coming the other way at a piffling 387 m/s would be devastating. So, for that matter, would an off-cut piece of 6mm wall thickness I-section steel girder from a construction project, a rusty spanner set or a magnet.

If you want to catch a ball with a stick without the stick breaking ... you need to make the stick stronger, really. Add more sticks. Add lots of sticks in a pretty pattern, which is far stronger than a single stick of the same total mass.

You could also cushion the impact with a sacrifical honeycomb, which is how bicycle helmets work (and why they only save you once).

A suggestion for braking, manouevring, docking and a host of other things: repulsion beams. You're writing this, so you get to decide whether it's okay to have them fixed in little clusters and just aim them at the universe in general or necessary to swivel them and aim them at massive objects nearby due to that whole lightspeed thing (having to wait 4 to 24 minutes* for your beam to even reach Mars before you start to slow down would make docking in Earth orbit trickier). A space station could have lots of them facing inwards and either spread their effects over an entire ship or aim them at reflectors mounted on sturdy parts of the ship to push ships around inside its own docking bay. Pilots would then only have to get into the bay with low enough relative velocity for the station to handle the rest for them.

Reflector, by the way, is just three concentric and perpendicular discs. Put three mirrors inside a box and look into them and you'll always see yourself looking back. Shine a light into the box and your lamp will be illuminated. Swipe a laser pointer across it and you'll always get the dot right back on the pointer. The reflector is like that, but facing eight ways.

* or 8 to 48 minutes, if the beam has to get back to you as well.
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Planet's spin

Postby andykhang » Thu Mar 16, 2017 5:53 am UTC

2 Question today: Just how would you stop a planet from spining in it own axis and what it would be like for a planet to have 3 axis of spin?

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Re: Planet's spin

Postby LaserGuy » Thu Mar 16, 2017 8:18 am UTC

Spinning can't be stopped unless you can transfer angular momentum somewhere else--another planet or moon, usually. The Earth's rotation is actually very slightly decreasing over time due to the tidal interaction with the Moon, which is pushing the Moon's orbit out away from us. Typically planets don't end up stopping spinning entirely... I think the stable equilibrium is usually tidal locking (the same face always pointing at each other). Mercury, for example, is tidally locked to the Sun, so it has a permanent light and dark side.

Spherical objects can only have one axis of rotation. All you can do its change the direction that the axis of rotation points.


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