Crossover Shipgirls: Building a Dimensional Empire from Scratch

Shua—

Setsuna descended once more onto the Starbase.

”Eh? Commander? Back again? Is the Underworld renovation going smoothly?”

Observer floated over on her rigging.

”Or are you planning to build a Heaven or something next? Hmm... In the future, everything above and below in this universe will be built by our own hands.”

”No, to be precise, built by the Bulins.”

As she spoke, she rubbed the head of a colorful Bulin beside her.

Foreman Bulin was leading a group of gold and purple Bulins, carrying tools and hammering away at a massive construction framework.

”There’s no issue with the Underworld. Ereshkigal is very satisfied with her miniature universe. A range of two light-years is more than enough for her to use for a long time.”

Setsuna waved his hand.

”I do need to use a few more miniature universes, though. They’re for modifying the Eternal Snowfall.”

”Huh?!”

Observer froze for a moment, then waved her tentacles to summon Compiler, Purifier, and several other Siren girls.

”Commander, you mean...”

”Exactly what I said. I plan to mount a miniature universe onto the Eternal Snowfall.”

...

An hour later.

Weng!!—

The main body of the Eternal Snowfall—the towering Juggernaut—was manifested and suspended within the Starbase dock.

No matter how many times one saw it, an ordinary person standing before a planet-class warship like this could not help but feel their mind tremble.

”Aaa...”

Tiamat tilted her head up, a hint of confusion in her crimson eyes.

”This is... the child’s ability?”

”Yes. It is me, and I am it. We are one and the same.”

Setsuna nodded.

”Child... amazing...”

When she had visited the Underworld, Tiamat had shown little reaction. To her, Ereshkigal was merely a tiny being.

But in the face of this ultimate technological creation, she felt genuine awe.

Kh—

Setsuna gathered the Siren girls stationed at the Starbase.

”Purely in terms of volume, I believe the Eternal Snowfall’s modifications have reached their limit.”

”Making it bigger would only make it look more intimidating. It wouldn’t serve much purpose. After high-dimensional expansion, its size in three-dimensional space is already more than sufficient.”

”There are only two paths left for further upgrades: expand the internal space, or raise its dimensional level.”

From cruiser to battleship, from battleship to Titan, and then to a planet-class Juggernaut, the weapons mounted had grown increasingly numerous and powerful.

But this kind of material-stacking upgrade had its limits. It never stepped beyond three-dimensional spacefaring technology.

The Starsea Empire possessed enough resources to construct a star-sized battleship, but the meaning of doing so was minimal. It would instead become exceedingly inconvenient to maneuver.

”I agree.”

Empress raised her hand.

”Even if you build a catapult larger than a spaceship, it still can’t compare to a light spear, right?”

”So it’s time to put that old plan into practice—install a miniature universe onto the Eternal Snowfall.”

Setsuna said.

Ever since Miniature Universe No.1 was created, the Research Division had proposed a host of bizarre ideas.

Now that the technology was mature, it could finally be implemented.

”Hmm...”

A group of Siren girls circled the Eternal Snowfall, examining it from all angles.

”A miniature universe is essentially an entrance to an alternate spacetime. Assigning it a fixed coordinate isn’t difficult.”

”Originally, as a shelter, it operated independently. Nothing from the outside could interfere with the miniature universe’s internal environment, and likewise, it could hardly affect the greater universe.”

”But if we attach it to another world, energy exchange between the two can be achieved—just like the Underworld.”

Compiler pushed a holographic interface in front of them, speaking in her calm voice.

”Can you put that in simpler terms? Something I can understand?”

Gudako was already dizzy.

”It means it’s theoretically feasible. After installing the miniature universe onto the Eternal Snowfall, it will lose its independence and can no longer serve as a shelter, but it can either draw energy from the starship or supply energy to it.”

Purifier explained succinctly.

She led Setsuna back into the previous laboratory.

Weng—

Rows of miniature universe entrances rose from beneath the ground once more.

”Commander, what size do you want?”

Observer casually made a lewd joke.

”No idea. Don’t need it. Haven’t used it.”

”Hey???”

Several shipgirls nearby suddenly flushed red.

”Eh? Aren’t there normal people here too?”

Purifier blinked.

Setsuna gave her a meaningful look.

”Times have changed.”

”???”

”Ahem...”

”I mean, how large of a miniature universe are you planning to mount onto the Eternal Snowfall?”

Observer coughed twice and pointed at the row of pale white ”doors.”

”The Underworld’s scale is considered relatively small among planetary-system-class miniature universes. Ereshkigal’s current ability can’t control anything larger.”

”Commander, what about you?”

”As big as possible. Install as many as we can.”

Setsuna replied without hesitation.

”...I knew it. As expected of the Commander.”

The little girl selected a row of miniature universes.

”These are the largest worlds we’ve created so far. Their volumes range from twice the size of the Solar System to ten times the Solar System.”

”On a cosmic scale, they’re insignificant. But for a starship, that’s already enormous, right? If it’s not enough, we can expand them later.”

As a planet-class warship, after excluding its armed wings and onboard weapons, the Eternal Snowfall’s usable free space was roughly equivalent to half a moon.

At ten times the scale of the Solar System—taking the Oort Cloud as the boundary—that meant a diameter exceeding twenty light-years.

”Uh...”

Purifier did a quick calculation. That was enough to fit fifteen billion Earths side by side.

”...Installing that many miniature universes inside a starship is terrifying just thinking about it.”

”Commander, what do you plan to use these worlds for?”

”There’s plenty we can do.”

Setsuna snorted softly.

”First, the Eternal Snowfall’s residential zone can be relocated into a small world. The spacetime within a miniature universe can be freely adjusted. I won’t even need to use time magic anymore.”

”The starship assembly plant can also be expanded further. We can shove Unicron into storage, then mass-produce a batch of planet-class warships and stockpile them. If necessary, we deploy them.”

”And for a more ambitious goal... if the miniature universe is large enough, we could even simulate a real cosmic environment inside and construct megastructures.”

”For example—a ship-mounted Aetherophasic Engine.”

Hiss!!—

Even the Siren girls were startled.

Setting aside the idea of using miniature universes as docks to store planet-class warships for combat deployment.

The concept of a ship-mounted Aetherophasic Engine was utterly explosive.

They knew the theory was feasible—carve out an independent space within a miniature universe, large enough to construct a galactic wonder.

The Aetherophasic Engine was a genuine universe-ending weapon. Not only could it drain an entire cosmos, it also provided an immense energy supply.

The ultimate technologies of two different worlds fused together into something absurd beyond imagination.

”A seemingly ordinary planet-class warship, yet within it are countless worlds—each region a separate realm...”

Gudako scratched her head.

”Have you considered renaming it an Immortal Ark instead?”

”Different paths, same destination—merge worlds into one ship, forge one ship into a world.”

Setsuna said calmly.

...

”Ahem. The Eternal Snowfall’s firepower enhancement Plus Version upgrade officially begins.”

Inside the Starbase.

Under the command of the Siren girls, Setsuna led the team as a group of colorful Bulins carried the ”entrances” of miniature universes aboard the Eternal Snowfall.

From the outside, it looked as if they were simply transporting a number of ”doors.” Yet behind each door lay a universe with a diameter ranging from as small as 4.5 light-years to several dozen light-years.

All the shipgirls gathered around to watch this modification. Compared to making the starship larger or installing more weapons, this was an entirely new design philosophy.

If everything went smoothly, it might even be applied to other shipgirls’ hulls in the future.

”Miniature universes that have reached planetary-system-class scale and meet the Commander’s requirements—there are currently sixteen in total. That’s all we have in stock for now.”

Observer tapped the frame used to secure the miniature universe entrance.

”If we need more later, we can deploy additional Bulins to produce them. Or... we could expand them on-site as well.”

”I really want to try creating a galaxy-class miniature universe. Something the size of the Milky Way... no, the Andromeda Galaxy.”

Hiss—

Belfast was startled.

The Andromeda Galaxy had a diameter exceeding 220,000 light-years. Based on past experience, creating a miniature universe of that scale would require at least five times the resources.

In other words, they would have to burn through multiple galaxies larger than the Milky Way.

”I can grant you construction permission for a galaxy-class miniature universe. We’ll simply transfer manpower and materials from other universes.”

Setsuna answered casually.

”As for a cosmic-structure-class one... I’ll probably have to think of another approach. Relying on Bulins to hammer it out bit by bit isn’t realistic.”

”Let’s finish the planetary-system-class miniature universes first.”

Shua—

He projected the structural diagram of the Eternal Snowfall.

”The first batch to be installed will be sixteen miniature universes. Based on different functions, they’ll be mounted in different sections of the Eternal Snowfall.”

”Hmm... we could call them delves? Though I’d say they’re more advanced than what that term usually implies.”

Purifier felt a strong sense of déjà vu from this kind of spatial compression technology.

Choosing where to install each miniature universe was a technical challenge.

Compiler, Observer, Akashi, and the other engineering-focused shipgirls quickly gathered around the starship blueprint, pointing and discussing.

A cluster of colorful Bulin heads squeezed in from the side.

When designing the specific construction plan, they even consulted the Bulins for their opinions.

”First, one miniature universe needs to be installed in the bridge section.”

”This will become the Eternal Snowfall’s future residential zone. It doesn’t need to be too large—two to three times the size of the Solar System will suffice.”

”That’s hundreds of billions of times larger than the original bridge. The Commander can fit as many people inside as he wants. Mm... he could even pack up the entire capital and relocate it.”

Considering that a significant portion of the Starsea Empire’s population lived within the Solar System, and that the space inside a miniature universe far exceeded the Solar System’s scale—

Even moving the entire capital inside would feel perfectly natural.

—They just wouldn’t be able to explore the greater universe anymore.

”...That’s so weird.”

Taihou twirled a strand of hair around her finger and tilted her head.

”Oh, and the flow of time inside the miniature universe is independent from the greater universe. So Commander, you can party to your heart’s content inside—if your body can handle it.”

Observer scratched her head again.

”The difference in time flow between the two universes can be set to something extreme—hundreds, thousands of times faster, or even more.”

”You could spend years or even over a decade inside, while only a day or two passes outside.”

As she spoke, she poked Setsuna with her tentacle rigging.

”Still, you’d better take it easy. If one day you drag the entire port district in there and go wild for several years straight, only to get drained dry and die inside, that’d be troublesome.”

”Reviving you would be a hassle...”

”Huh???”

Suddenly, she had a flash of inspiration.

”Oh right. Since you’re always hosting silver parties in there, why not just name this miniature universe the Silver Garden?”

”???”

”Where did you plagiarize that name from? Careful, Homura might beat you up.”

Setsuna felt the suggestion was ridiculously bold.

”...”

Kanae, Iris, and several other ordinary girls nearby fell into thought. After a moment, they suddenly clapped their hands.

”Ah! If that’s the case, couldn’t we cultivate inside it?”

”If we train for over a decade inside the miniature universe, only an instant passes outside. But our strength would increase for real.”

”It’s just like the blessed lands in cultivation novels... That’s amazing!!!!”

Since ordinary people were comparatively weak within the Starsea Empire, they paid special attention to ways of growing stronger.

Undoubtedly, the miniature universe’s virtually side-effect-free, near-infinite time distortion made it perfect for focused cultivation.

Even if mortal bodies could never truly compare to higher beings, spending ten, twenty, or even more years training would lead to explosive growth.

With super-evolution surgeries, genetic ascension, and their connections to the goddess of the Underworld, they didn’t need to worry about lifespan at all.

”Ohhhh!!”

Akame, Kurome, and the others also suddenly understood.

”Now that’s the right mindset.”

Setsuna rubbed Kanae’s head and then lightly knocked Observer on the head.

”They’re thinking about training. Unlike you, whose mind is full of parties.”

”???”

The little girl puffed out her cheeks and thumped Setsuna a few times.

”You have the nerve to say that??!”

”I was just stating your true thoughts, okay?!”

”...Which is why you shouldn’t say the truth out loud.”

After some playful chaos, they began planning the next phase of modifications.

”The Eternal Snowfall’s armory can also be stored in a separate miniature universe.”

”We can shove Unicron in there. When necessary, just throw him out to beat people up...”

”Additionally, the Commander can use the Authority of Reason in coordination with the Bulins to produce more planet-class or even larger starships and stockpile them inside for emergencies.”

After being fully corrupted, Unicron had become a combat machine completely under Setsuna’s control.

Normally, the massive entity hovered in Earth’s orbit. Due to its sheer size, maneuvering it wasn’t very convenient.

With a miniature universe, the portability issue would be resolved instantly.

”So we could airdrop dozens of battle moons from the Eternal Snowfall onto our enemies? ...”

Imagining dozens or even hundreds of planet-class warships being deployed from a Juggernaut only slightly larger than the Moon—the scene was simply too beautiful.

”That’s the idea.”

Observer nodded smugly.

”And the process wouldn’t even take long.”

”Thanks to time distortion, as long as we set up automated factories inside the miniature universe and supply sufficient resources—”

”Even if automated factories are far less efficient than Bulins, letting them work for decades or centuries inside would still be enough to assemble starships. Outside, only a short time would pass.”

”There’s plenty of time inside the miniature universe. We can just live normally in the greater universe.”

”...”

”Advanced. Truly advanced.”

Everyone sighed sincerely.

As expected of the ultimate technology once used by god-tier civilizations for refuge. Combined with advanced technologies from other worlds, it produced all kinds of incredible effects.

”Lastly... the most important matter.”

Observer gestured for the Bulins to bring up a miniature universe entrance engraved with the number ”20.”

”This is the largest miniature universe in our current inventory. Its spatial diameter is twenty light-years—enough to fit ten Solar Systems side by side.”

”If the Commander wants to attempt constructing high-tier megastructures inside, we’ll use this miniature universe as the foundation.”

She paused.

”I was thinking—besides a ship-mounted Aetherophasic Engine, we could try building something else first. For example, placing a Dyson Sphere inside? Building one in the Solar System would be absurd. But putting it in a miniature universe to power the Eternal Snowfall seems viable...”

”A Matter Decompressor could provide mineral resources. A Sentry Array could replace radar and sensors for reconnaissance. An Arc Furnace could refine alloys...”

”This way, the Eternal Snowfall could gather the power of all Galactic Wonders into itself.”

”...”

The grand vision left everyone dumbfounded.

Using miniature universes to house Galactic Wonders within a single starship—this was perhaps the strongest starship that Stellaris-era technology could produce.

It would itself be a complete world, possessing near-infinite productivity and energy, along with the power to destroy cosmic structures.

”...”

Setsuna snapped his fingers.

”Go ahead and do it.”

—

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