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I'm about halfway through a story I'm writing, where aliens invade, beat us up, one spaceship blows them all up, etc, etc, and everyone lives happily ever after. Except they don't.

So after they chase off the aliens, they begin construction of three hundred spaceships and an orbital defense system. How long would it take to make those?

For spaceships, there are mainly two types- spaceships and big spaceships. For spaceships, think mobile homes with coilguns and nuclear generators. For big spaceships, they'd be at least four times the size with bigger generators, more coilguns, and a pair of short range powerful lasers.

90 will be big spaceships, 210 will be regular spaceships.

The orbital defense system would be a series of platforms orbiting in a roughly grid-like pattern. every third platform in a row is a large laser. Two rows without lasers separate them from more lasers. Laser platforms are 500m wide due to the lens needed to cause damage to a target far away.

The other platforms are coilguns. They are 20-30m wide.

I haven't decided how many platforms to put into space yet, so just give a rough estimate on the time.

Technology is slightly more advanced than it currently is, mainly coilguns and spaceships. Also has alien shields from the last war.

related question that might help

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  • $\begingroup$ Do the aliens have a place to call home, or does the entire race live on these spaceships? $\endgroup$ – Ishaan Saha Jul 29 at 15:05
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    $\begingroup$ @IshaanSaha, I havent thought about that or put it in the story yet, so its still open. im thinking they do, though. $\endgroup$ – Ceramicmrno0b Jul 29 at 15:08
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    $\begingroup$ This depends primarily on the industrial capabilities of the Earth post-alien invasion, which could be pretty much anything, from "still on wartime priority production" to "literally bombed back to the stone age". $\endgroup$ – Cadence Jul 29 at 15:38
  • $\begingroup$ Did humanity manage to capture any of the alien spacecraft or support craft intact? Preferably including technicians who are willing to talk? $\endgroup$ – notovny Jul 29 at 17:10
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    $\begingroup$ You know the manufacturing triad: Good, Fast , Cheap. Pick Any Two $\endgroup$ – Carl Witthoft Jul 29 at 17:20
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This falls into my "how many angels can dance on the head of a pin? Answer: As many as wanting" category. The numbers you're looking for are small enough that the world's manufacturing processes can easily absorb construction. But there are a couple of assumptions.

Assumption #1: Earth is completely united.

Meaning the entire world's scientific and manufacturing capacity is at your disposal.

Assumption #2: The existing military-industrial complexes and corporate-industrial complexes were not substantially destroyed.

Kinda ditto Assumption #1.

Assumption #3: You didn't have these ships to begin with. You're designing, as it were, "from scratch." This would be the equivalent of designing and producing a new ship from concept to the broken bottle of champagne.

These assumptions may not be at all the reality of your story, but they produce a worst-case-estimate that allows you to scale back the construction time to suit your story.

We're also going to need to change your design a little bit. There are some truths to space flight under the conditions you're describing that will limit us.

  1. We can use real-tech nuclear reactors as found on nuclear submarines, but this means your small ships will NOT be the size of mobile homes. They'll be the size (at best) of nuclear submarines. This is actually a reasonably realistic conclusion. And here's where your life begins to really feel pain.

Taking 14 years to design and build and costing around £1bn ($1.6bn) , the British nuclear submarine Astute is one of the most technologically advanced and controversial machines in the world. Weighing in at 7,000 tonnes, measuring 100 metres (330ft) long and boasting three decks, how do you put such a craft together? (Source)

Nevertheless... rail guns for torpedoes, ion engines for propellers, use disposable fuel tanks and external engines to get the sub into orbit (not impossible, but I leave it as an exercise for the user to figure out if this is practically possible) and you have your small ship.

  1. Gratefully, government satellites take an average of only 7.5 years from design to construction which means the space-tech part of this fits within the design limitations of the submarine space ship.

So... 14 years for one small space ship.

  1. I couldn't find an easy spec for the number of submarine-capable shipyards in the world, but there are 26 countries with shipyards. I doubt there are 26 sub-capable shipyards, but let's run with the number. 210/26 * 14 = 113 years.

113 years to build 210 small ships

And it's worth noting... there are approximately the same number of astronaut-bearing space agencies in the world, so it's plausible that you could build 26 ships at a time.

  1. Your big ships are 4X the size of your small ships. It's a guess as to whether or not efficiency scales. It often doesn't when it comes to construction, but let's say it does. So, 2X the time. You're using the same shipyard resources. 90/26 * 14 * 2 = 97 years

97 Years to build your big ships

  1. That sub I mentioned is only 100 meters long. Your big platforms are 500 meters. So, 5X the size. The smaller platforms are 30 meters or 0.3X the size. You didn't mention quantities... but they would be a hair easier as they wouldn't need substantial propulsion. GUESS: say 1.5X for the big platforms and 0.15X for the small platforms, meaning each group of 26 big platforms add 26 years to the total and each group of 26 small platforms add about 2 years.

Conclusion

Whether or not it takes less time depends on just how far into the future your technology goes. It takes time to build big, complicated things.

Whether or not it takes more time depends on just how many of the construction resources (from primary manufacturing down to supporting resources, like raw mining) were destroyed. If the aliens destroyed 90% of the capacity you need to 10X the time.

I don't have any idea how quickly new shipyards could be brought online. They're BIG. But the odds are that in a world-wide panic you might cut your ship building time in half by building shipyards as fast as possible. NOTE that this would be DEVASTATING to national economies. In fact, just cranking out the ships and satellites will very likely be devastating. I don't know how fast (for example) the U.S. retooled for defense production during WWII, but compared to the total length of the war, it was a non-trivial amount of time and had massive impacts on both the economy and on society — for both good and ill. Impacts that we are still bearing today almost a century later.

One point to remember! That 14 year number is concept-to-water. Theoretically that's the time for the first ship. It gets faster with each succeeding ship as kinks in the process get ironed out, efficiencies fall into place, etc. On the other hand, there's also the very high possibility that the Mark I ship gets replaced by the Mark II ship... etc... meaning that it's not all a bed of roses. Remember, this is your worst-case number. You modify it to your needs.

But, all things being equal: about 200 years.

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    $\begingroup$ If we use nuclear submarines as a benchmark, USSR and USA each built close to 200 nuclear subs over 40 years of the cold war without a special focus/resource priority on submarine building, so the estimate that the world would take 113 years for a comparable task is a few times too high. $\endgroup$ – Peteris Jul 29 at 23:07
  • $\begingroup$ @Peteris I used a modern-day estimate for design and construction on purpose: to accommodate the technology requirements of the OP. Your point is useful, as per my answer's "point to remember" that the first design-to-water cycle is longer than the rest. Also per my answer, the number is a worst-case starting point from which the OP may work backwards to meet his story's requirements, not a definitive minimalist answer. Keep in mind that I'm using subs as a reference point. Ion engines, rail guns, life support - all stuff that will slow construction. I don't think I'm wrong by much. $\endgroup$ – JBH Jul 30 at 15:12
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The time it takes to build these things really depend on too many factors. How badly civilization was damaged, what their current technology is, and how determined they are all play a big part. So really, we can't tell you these things. But we can give you some stats to compare to.

  • Your "big spaceships" sound comparable to a nuclear powered aircraft carrier in complexity. The Nimitz class carriers took 3-5 years each, consuming the resources of a major shipyard. And, obviously, they did not have to be launched into space.

  • At the moment, we as a species, are making about 100 launches per year. As an example of one of our heavy lift rockets, the Ariane 5 can lift 16,000kg into LEO. A mobile home is about 15,000kg, so we can lift one of your smaller space ships on each launch, if we try hard enough.

  • The heaviest lift rocket we have is the Saturn V, capable of delivering 140,000kg to LEO. We launched 13 of them total. You will certainly need something in the Super-heavy launch vehicle class, and probably something on the high end of that. This is an interesting class because, other than the retired Saturn V, most of them are concepts. The Long March 9 is expected to lift 140,000kg, the Yenisi (Souyz-5 Super-Heavy) is theorized to hit 160,000kg, the SLS is expected to clear 130,000kg in Block-II, and the Starship is expected to lift 150,000kg.

    • What is common about all of these lifters? They are all in development (other than the retired Saturn V). Thus, your timeline should include development costs which are measured in decades. One to two decades seems to be a development cycle for these beasts. If your Earth has not already developed the SHLV craft needed to get these big vehicles to orbit, add that development time.

Also, you may want to spend some time in Kerbal Space Program looking at your orbital defense system. We can't actually create a grid that stays in orbit, due to the way orbits work. They are always (nearly) in-plane with the center of the earth. If you look at real constellations, like the planned Starlink constellation they tend to be a bunch of criss-crossing paths.

Definitely think through what your humans think the next attack force will look like. That will help you spec your orbital defense system. If the aliens attack with 4 or 5 "planetkiller" nukes, you really just need a small number of powerful lasers such that you always have a clear line of sight in all directions. If you're worried about a swarm of thousands of smaller craft, you may need a larger number of individual lasers/coilguns. Look at the enemy's armaments. If they were really good at taking out capital ships in the first war, you probably want lots of little platforms. If well-armored capital ships were the only thing that saved us in the first war, you want fewer platforms with more armaments on each one.

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Between 10 and 20 Years

You have two historical examples you can look to : the Apollo missions (which launched vehicles roughly the weight of your "small" ships) and SpaceX's heavy lift vehicle (which is working on a such a vehicle).

In the Apollo case, we had to invent the technology of sufficiently heavy rockets to carry the assembled-on-Earth load into orbit. It took almost 7 years. Once it was done, about 12 vehicles were produced in the next 5 years.

In SpaceX's case, although their design is kind-of a 21st Century update of Apollo designs, much of the undocumented knowledge of how to do it (sometimes called "tribal knowledge") had been lost by the half century since the last time we'd done anything like this. They had to re-invent most of these things. It has taken them close to 20 years, so far, and they aren't quite there yet; but they are testing heavy lift vehicles now.

Mass Production

In Apollo's case, once the technology was understood building additional instances was much quicker. From the first successful flight proving the hardware in February '67, twelve missions were launched by '72.

SpaceX is being forward-thinking and trying to cut costs up front in their design of re-landing 1st and recoverable secondary stages.

Put that together, and make it a national (or worldwide) priority, and I think between one and two dozen ships per year is not out of the question. That would put 200 ships in orbit in about another 9 years.

Parallel Endeavors

While SpaceX is working on their heavy lift vehicle, they are launching satellite parts of their Starlink network. There's nothing preventing multiple organizations working on individual parts of this re-arming effort.

Your large ships (which might be liftable by the same vehicles; you're only trying to get to orbit, not the moon!) may be being built by another division or organization and going up simultaneously.

And another group or division is putting defense satellites in orbit.

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From a few decades to a century

Today, there are a handful of countries capable of manufacturing crewed spacecraft. The United States has the largest production capability. Even with more advanced technology than we have now, you'd be facing a massive challenge to build anything near the capability that you're considering. SpaceX is one of the biggest rocket producers, and even their production numbers wouldn't make a dent in the demand for the four types of objects you need. If we assume that the Earth's manufacturing supply chains haven't been heavily damaged by the aliens and that we can devote all human effort to this initiative, you'd still need at least a few decades to build everything.

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