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I found an interesting Wikipedia article, quoted in an answer here.

I'm looking into writing a story where someone has built a Death Star. The purpose of this craft is important to the story (and the world) because it's used to keep the populace of the planet in check (it's easy to get world leaders to agree to your "requests" when you can obliterate their country, easily....)

The questions are:

  • Given current technology, how many man-years would it take to build? (assume you already have enough steel, see question below)
  • How much metal is needed to build this ship?
  • Would it really take 833,000 years to get the necessary steel (as quoted in WP article, above)?
  • What would it cost? (anywhere near the White House's estimate of $850 quadrillion?)
  • Any scientific reason (ignoring the superlaser) why this ship is scientifically impossible?
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    $\begingroup$ If you need to keep the people in check on only one planet, it may be simpler to have a smaller weapon? After all, the Death Star is a weapon able to destroy planets. It's like if you were to kill a fly with a Rocket launcher in a 3m by 3m room. $\endgroup$ – Vincent Nov 2 '14 at 4:15
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    $\begingroup$ @Vincent True....my world has several planets. But even on one, who can argue with a Death Star? ;-) $\endgroup$ – Shokhet Nov 2 '14 at 4:16
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    $\begingroup$ Any what-if.xkcd readers here? :-) $\endgroup$ – Kos Nov 2 '14 at 19:24
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    $\begingroup$ The White House has an answer. $\endgroup$ – mouviciel Nov 3 '14 at 15:30
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    $\begingroup$ @JasonHutchinson See this answer as well as this one. You might be surprised. $\endgroup$ – Shokhet Nov 7 '14 at 18:56
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Update: The original version of this answer was based on an arithmetic error which underestimated the required material by factor 1000. It was subsequently rewritten.

How do you define "Death Star"? Depending on what your space station is going to be capable of, there is no upper bound to how complicated the engineering challenges can become. But to establish a lower bound for the project, let's take the most basic implementation: A hollow sphere, 120km in diameter, constructed from aluminum (like the hull of the international space station) with an average thickness of 1mm.

Material cost

A 120km diameter sphere has a surface area of (4*Pi*60²) 45216 km².

One km² covered with a 1mm layer requires 1000 m³ of material, so we need to launch a total volume of 45 million m³.

Aluminum has a density of 2.7 g/cm³ so a m³ has a mass of 2.7 metric tons.

That means the total mass of our aluminum sphere is about 120 million tons.

The world production of aluminum is about 5000 tons a month, so when the whole worlds aluminum production would be routed towards this project, we would require 2000 years to mine it all.

Launch cost

A present-day commercial heavy-lift rocket brings up to 25 tons to low-earth orbit. There are some super-heavy launch vehicles in development like the SLS Block 2 or the Long March 9 which can carry up to 130 tons. These currently only exist on paper, but there is no good reason why they couldn't be built with present day technology. So depending on which rockets you use, you need about one million rocket launches.

Launch costs could maybe be reduced by optimizing the rocket construction process or by using something other than rockets. But you said current day technology and any alternative launch systems are still in the realm of science fiction.

This does not yet include the price for launching any personnel and equipment required for the assembly. Humans working in space suits are neither very productive nor very cheap to accommodate and we are talking about a very repetitive task here, so I believe that having it built completely robotic would likely be more effective. The number of assembly robots you need depends solely on how fast you want to finish, so let's ignore this factor.

So would it be possible?

Not within our lifetime and not without unreasonable economic sacrifices. Even if we would focus the whole world economy solely on this project it would still take centuries, maybe millenia.

And this just gives us the most basic version - a paper-thin, hollow aluminum sphere which doesn't do anything. Depending on how you want to design the interior, the cost can only go up by several orders of magnitude.

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  • $\begingroup$ Nice answer(s)! Thanks! .....I'll hold off accepting, for now, just to see what else comes in, but this is a really good answer!! $\endgroup$ – Shokhet Nov 2 '14 at 14:18
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    $\begingroup$ The 40 years is a bit too low. You forget that in order to build the launch-systems and to get the workers/robots in space you will need a lot of extra aluminum, besides the stuff for just the sphere. $\endgroup$ – Tonny Nov 2 '14 at 14:42
  • $\begingroup$ @Tonny That's true. But I also forgot to mention that you could also substitute aluminium with steel or titanium alloys in both the launchers and the station. $\endgroup$ – Philipp Nov 2 '14 at 15:05
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    $\begingroup$ Note that purchasing all the aluminum produced in 41 years would, for all intents and purposes, certainly effect an appreciable increase in the price of the commodity. Consider the fluctuations of non-precious metal prices over the last decade, during which only relatively ordinary events have affected them. I suspect that you could conservatively assume that if someone were to purchase all the aluminum produced during a 41 year period, the price of aluminum would increase by an order of magnitude. $\endgroup$ – Hal Nov 2 '14 at 15:45
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    $\begingroup$ @Hal I took the current comodity price as an indicator of what it costs to mine and refine aluminum. You certainly couldn't execute such a megaproject in a free-market economy, so I would assume that whoever performs such a project has the power to also take control of the worlds aluminum production. $\endgroup$ – Philipp Nov 2 '14 at 16:07
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If the design goal is to have weapons that can wipe out all life on a planet, and move between planets, then the main problem is just transporting planet-destroying weapons (with existing tech, probably thermonuclear warheads) to other planets, and largely depends on how far away the other planets are, and how much range, desired operating time away from base, and how many planets you want to be able to destroy before returning. And, countermeasures against existing weapons. Every requirement adds weight, which adds to fuel requirements. The most efficient in terms of cost might probably be a stealth vessel that returns to resupply after every attack, depending on the layout of targets, but that would reduce its terror power, since with existing technology, it would take a long time to return to resupply.

The part about building a metallic small moon seems to not help the stated intention much at all. In fact, it makes it infeasible in many many ways, and accomplishes very little against current technology other than making it a great big slow-moving target.

So, missing features of the design listed (compared to Grand Moff Tarkin's Death Star) above include, in ascending order of impracticality:

Omitted features that might be practical and useful in a sci-fi fututre with somewhat higher technology levels and higher-tech-opponents than currently exist:

  • No shields.
  • No TIE fighter squadrons.

Impractical features that don't materially aid planet-destroying goals, but are quite possible with current technology:

  • No pits to knock victims into.
  • No trash compactors with monsters living in them.
  • No place to land the Millennium Falcon.
  • No equatorial trenches.
  • No thermal exhaust ports.

Features which are theoretically possible with current technology, but are so impractical that they are almost impossible to do, and are crazy:

  • No small-moon-sized massive metal ball.
  • No lodging for millions of stormtroopers.

Features which there is no known way to do with current technology:

  • No ability to jump the giant metal moon into hyperspace and quickly travel to other star systems.
  • No ability to race a giant metal moon around a solar system.
  • No conversion of planet into asteroid field.
  • No artificial gravity.
  • No long-range tractor beams.
  • No giant flashy laser effects on planet-destroying weapon.

Given current technology, how many man-years would it take to build? (assume you already have enough steel, see question below)

Practical version: Depends on range specs. To make something to go nuke Mars - ~10 years?

Giant moon version: Insane amounts of man-years - you need to get the workers into space and keep them supplied, etc - use droids - you will still be overthrown (and/or die of old age) first unless you have an actual galactic empire with Star Wars technology.

How much metal is needed to build this ship?

Practical version: Probably not more than for a carrier battle group or two, probably best is light aluminium and other light materials.

Giant moon version: Using heavy metal is probably mostly silly, but if the point is you want a metal moon, then you want crazy amounts of metal. Probably your best bet is to find a way to use an asteroid belt (planet-size quantities, already in space), assuming you have one with high iron content.

Would it really take 833,000 years to get the necessary steel (as quoted in WP article, above)?

Depends on what is working on it in what conditions on what kind of planet etc., but I can imagine trying to model it and getting a result like that, yes. I think you'd be much better starting with an iron-rich asteroid belt or an existing small moon...

What would it cost? (anywhere near the White House's estimate of $850 quadrillion?)

It would cost letting go of the idea that money is meaningful, and imposing dictatorial rule.

Any scientific reason (ignoring the superlaser) why this ship is scientifically impossible?

Practical version - nope.

The big metal moon version:

  • It's too heavy to move around.
  • No hyperdrive.
  • No artificial gravity.
  • No long-range tractor beam technology.
  • Mad impracticality.
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Customise a moon

Current answers have required that the aluminium be extracted from the planet and launched into space. There is an alternative.

A metallic moon, or a moon with a metallic core could be landed on and hollowed out. Solar powered mass ejectors could remove extraneous rock from the planet surface. Mass ejectors could be constructed in situ, you would only then need to land an automated robot factory in the surface, capable of processing the metal into tunnelling robots, rail guns, ammunition, security droids, and additional factories.

Assuming the moon was once molten, heavy metals such as uranium should be found near the core. There should be plenty of material for bomb making, power generation and propulsion.

A massive iron skin, a mile or more thick should provide effective shielding from any nuclear attack.

Extra material could be processed into an armada of deadly spacecraft.

Weaponry

Weaponry could be nuclear, or simply impact based. A chunk of uranium the size of a city block hurled at a planet with speed would pierce the crust of that planet like a bullet causing massive devastation, earthquakes, volcanoes, etc. It would be a shock and awe weapon, impossible to counter.

A vacuum arc weapon might also be viable provided sufficient voltage could be generated. Imagine a 5000km electrical spark stretching from one planet to another.

Potential issues

You will have issues with generating and holding an atmosphere, as you would have an absolutely enormous volume to fill. It would be likely that only certain parts of the station would be pressurised with the rest given over to shipyards, hanger space, power generation, engines and extremely big guns.

You would also have issues with gravity (or lack thereof), perhaps you might have rotating habitation modules within the hollow core.

All docked ships would need to be securely attached. Accelerating the station would cause unattached ships to smash into the walls.

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As an alternative answer with a much more pessimistic estimation I would like make a rough estimate for a fully functional station by taking the International Space Station for comparison and scaling it up linearly.

The international space station has a pressurized volume of $837\text{ m}^3$. Let's be generous and round this up to $1000\text{ m}^³$ to account for the non-pressurized parts.

A $120\text{ km}$ sphere has a volume of about $$\left(\frac{3}{4} \times \pi \times 60^3\right) 500 \text{ km}^3$$ or $500 \text{ trillion m}^2$, so we would need 500 billion International Space Stations to fill it up.

The total lifetime cost of the ISS project is estimated to be about $100 billion shared between the participating nations.

When you want a death star filled to the brim with expensive equipment just like 500 billion international space stations, you would end up with a cost of US$ 50 Sextillion (50.000.000.000.000.000.000.000). The total economic power of the whole humanity (gross world product) is currently estimated at 85 trillion dollar per year, so when the whole world economy would be concentrated solely on this project, it would take 600 million years to complete.

When we only spend one tenth of our economical resources (we still need to survive), we might get finished in 6 billion years. Coincidentally, this also happens to be the time it will take until our sun burns out, turns into a red giant and destroys Earth, so we better start building now.

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    $\begingroup$ Scaling it up linearly however makes no sense at all whatsoever. The more you do a certain thing, the cheaper it becomes. With the ISS a lot of things had to figured out for the first time and then they have been done 2 or 3 times, with such a death star project the research vs work ratio would be of an entirely different order. $\endgroup$ – David Mulder Nov 2 '14 at 15:53
  • $\begingroup$ @DavidMulder: things don't necessarily get cheaper with a larger order size, try rhodium/platinum/mathematicians $\endgroup$ – Mark K Cowan Nov 2 '14 at 17:53
  • $\begingroup$ @MarkKCowan: In the eye of scarcity it's not a rock solid rule no, but overall if you look at the price of the final products they do get cheaper. If the scarcity of one resource drives the price up too much, somehow another solution is found using a different resource. If tons and tons need to be brought up into space, somehow another solution will be found in time (who knows, maybe a space lift isn't that uneconomical after all). $\endgroup$ – David Mulder Nov 2 '14 at 18:16
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The "Death Star" as you describe it has already been built:

The purpose of this craft is important to the story (and the world), because it's used to keep the populace of the planet in check (it's easy to get world leaders to agree to your "requests" when you can obliterate there country, easily....)

Let me present to you the LGM 118 Peacekeeper.

LGM 118

You really do not need to have a spherical launcher in orbit, nor does it have to be an directed-energy based weapon. Nuclear weapons have been, and currently are being, used exactly for the purpose you describe. Using nuclear weapons even gives opportunity for some interesting plot twists.

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    $\begingroup$ +1, because you're right.....but I still want my Death Star :P $\endgroup$ – Shokhet Nov 2 '14 at 16:09
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    $\begingroup$ I don't think this properly addresses the question. It's like someone asking how to build a car and you point out that a bicycle is perfectly capable of the same thing. $\endgroup$ – HDE 226868 Nov 2 '14 at 16:18
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    $\begingroup$ The problem with ICBMs is that they are hidden in silos. That makes them an abstract and invisible threat which is easy to forget about, unless you use one once in a while. When you want to instill permanent fear in your population, you need something visible to them which constantly reminds them of your power to end their puny existence whenever you feel that way. $\endgroup$ – Philipp Nov 2 '14 at 18:45
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    $\begingroup$ @Shokhet you could justify the Death Star by ascribing it the ability to destroy ICBMs in flight. By analogy, as Little Boy was to Japan, the Death Star will be to the world. A tier of sophistication above all other means of warfare and influence.That said, coincidentally, the Pentagon named the first system they considered for destroying ICBMs in flight 'Star Wars'. $\endgroup$ – Hal Nov 2 '14 at 20:21
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    $\begingroup$ @Hal Interesting idea. That could render nukes entirely irrelevant, if it's played correctly. $\endgroup$ – Shokhet Nov 5 '14 at 22:10
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How about a smoke and mirrors solution?

Let's say that you own the world's only civilian space tourism business and you decide to take over the planet. Being patient in your quest, and having read the other answers to this question, you decide to fake the whole deathstar thing. After all, do you really need to own a whole deathstar, or is it enough that people just think that you own a deathstar?

Concluding that perception is 9/10ths of reality, you start by investing a billion or so in creating a space station. Competitors are already gearing up to compete with you in the orbital tourism business, so you raise the stakes, adding a "party island" to your tour. Your passengers not only get to orbit the planet a few times before returning home, (which is the current business which your competitors are targeting), now they can also hang out in your gravity free space station for a few hours midway through the flight. Your competitor's don't stand a chance! and as a side-effect, you now have a place to start staging your planetary take over. The best part is that it's all paid for by your wealthy passengers, who are literally paying you to take over the world.

So, now you have a space station and several shuttles visiting it every day. What do you do next? You start ferrying Mylar canvas up in bundles and tethering them to the outside of your station. You will need a lot of it, so keep that up for a few years, while the rest of the plan comes together.

Now all you need is some paid-off astronomers and one nuclear bomb.

On the night of the "deathstar's arrival", you go to the station and wait until both the sun and moon are below the earth's horizon. Then, under cover of earth-shadow, you spread out the Mylar into space around you, forming an enormous flat disk. With thin aluminum tent poles, you hold the fabric flat so that its broad face reflects down towards earth. You and your crew have got to work quickly because there is a lot of Mylar and tent poles to set up, but by morning you own an enormous pizza pan, floating ominously above the planet, visible for all the puny humans, striking anxiety and fear.

Suddenly your paid-off astronomers start screaming to the press, describing how they had watched the enormous artificial moon approach for deep space. Then New York is vaporized as a breathy baritone voice asks for the planet's surrender...

The next day, you're addressing the world court, accepting their surrender and becoming the Earth's first Planetary-Emperor. With total control of the world's economy and its labor force, you begin the 40-year project of building an actual deathstar.

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Asteroid belt extraction, mining and modification

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Kinetic Bombardment

I think a few points are important.

1- How would that be done in secrecy? If other countries get a hold of what you are doing they may try to prevent the construction to ever be finished.

2- The cost for mining here on earth and then moving all the materials to space is outrageous and downright inneficient.

Solution:

Start a modest asteroid belt mining program, and start to invest on it heavily as the years go by.

Focus on autonomous machines to do the work on site, and start moving a big chunk of rock towards a lagrange point.

You need to build a self sufficient habitat on the inside of the asteroid to prevent retaliation and to have a place to run to when things go to hell on earth.

Try to use an asteroid with tungesten on it. You dont need a death ray, just melt it in big heavy rods and use them as missiles to hit your target. Thats kinetic bombardment.

From wikipedia:

A kinetic bombardment is the hypothetical act of attacking a planetary surface with an inert projectile, where the destructive force comes from the kinetic energy of the projectile impacting at very high velocities. The concept is often encountered in science fiction and originated during the Cold War. The typical depiction of the tactic is of a satellite containing a magazine of tungsten rods and a directional thrust system. When a strike is ordered, the satellite would brake one of the rods out of its orbit and into a geostationary position while directly over the target. The rod would then begin to fall towards the earth, picking up immense speed until it reached terminal velocity shortly before impact. The rods would often be shaped so as to increase the terminal velocity. In science fiction, the tactic is often depicted as being launched from a spaceship, instead of a satellite. Kinetic bombardment has the advantage of being able to deliver the projectiles from a very high angle at a very high speed, making them extremely difficult to defend against. In addition, projectiles would not require explosive warheads, and—in the simplest designs—would consist entirely of solid metal rods, giving rise to the common nickname "Rods from God". Disadvantages include the technical difficulties of ensuring accuracy and the prohibitively high cost of positioning ammunition in orbit.

Extra

All the rocks excavated to give room for the habitat and other facilites can be repositioned in front of your asteroid for an extra shield or just stay there untill you decide to hurl them at one target in a shotgun style attack (hits multiple areas, no total obliteration of target, leaves something to conquer).

Cost? What cost?

If you have the cash or manage to get the money from investors, the project pays itself. Many asteroids have rare minerals and elements in much bigger concentration than here on earth. Aside from the initial cash to get things started (it wouldnt be cheap) the extraction of rare materials could at some point pay for everything, and maybe even spill some profit.

As some minerals/elements start to get scarce on earth (or just too cost prohibitive to find/reach) you could be the only source of some materials in the near future.

Also from wikipedia on asteroid mining:

In 1997 it was speculated that a relatively small metallic asteroid with a diameter of 1.6 km (0.99 mi) contains more than $20 trillion USD worth of industrial and precious metals.[6][47] A comparatively small M-type asteroid with a mean diameter of 1 kilometer (0.62 mi) could contain more than two billion metric tons of iron–nickel ore,[48] or two to three times the annual production of 2004.[49] The asteroid 16 Psyche is believed to contain 1.7×1019 kg of nickel–iron, which could supply the world production requirement for several million years. A small portion of the extracted material would also be precious metals.

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I think that limiting yourself to "today's tech" is too limiting. If someone really needed to build a Death Star they would have to plan ahead and build the tech to build it.

I would go for self-replicating robots of some kind. I think that with current technology and a lot of research we could build a robot space probe and factory that would go out and find asteroids, set up mining and processing, and build more mining robots and more robot factories.

At some point the robots and factories are directed to start building and assembling Death Star components instead of more robots.

This assumes that all of the necessary minerals can be found in the asteroids which may not be actually true. We haven't really looked very hard yet.

There are too many variables to estimate the time needed, but with an exponential growth curve in robot population, the time is a LOT less than 833,000 years. A lot depends on how fast the robots can reproduce and how fast they "die" from hardware failure.

The cost would be the initial development and ongoing monitoring and programming updates. The cost of the actual building would be zero because the robots would be doing it like a giant 3D printer and providing the material too. Unless you start using economics and calculate the opportunity cost of building a Death Star instead of all the really useful things your space industry could be doing.

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