Supercavitation

 

A swarm of small Bahamuts appeared—though all of them were avatars.

Each Bahamut split off and began explaining different sections of the ship. Cat  Familias or Therianthropes acted as interpreters. If more detailed communication was necessary, the Bahamuts would write floating letters in the air.

Kou, Asia, and the others were led to what seemed to be the core of the spaceship.

「So this is the combat command center? You want me to sit here?」

One Bahamut persistently pointed to a specific seat. It was unmistakably the captain’s chair.

「Is it really okay for me to sit here?」

「Who else but you would sit there?」

「…Right.」

As soon as Kou sat down, the Bahamut joyfully bounced up and down.

Asia’s Aimer took the seat beside him.

「Thanks, Bahamut. You’re really good at organizing things.」

「Impressive indeed. As expected of a guardian dragon. Poor Libya, though—she can’t interact with them like we can.」

「Just until she gets a body of her own.」

A view from the real Bahamut’s perspective was displayed—it was diving through the deep sea at astonishing speed.

Nyanta and the others had also been guided here by a different Bahamut. Flack, unfamiliar with the command center, looked distinctly out of place.

「I heard from Astraea that most spaceships can withstand deep-sea pressure.」

Originally, the ship’s pressure resistance had been designed for supergravity gas giants like Jupiter, not Libya’s deep oceans.

「A slow and steady deep-sea exploration… this isn’t so bad.」

「Bahamut’s breaking the sound barrier now and then, though.」

「Underwater?」

「Underwater.」

「How’s that even possible? Moving underwater at those speeds is…」

Kou was left speechless. For someone who’d struggled to exceed the speed of sound even on solid ground, the idea of a colossal spaceship like Bahamut doing it underwater was simply baffling.

A Bahamut close to them pointed at a screen. It displayed a detailed schematic of the ship.

「This is… jet propulsion using supercavitation. Fluid dynamics at work. Cavitation is a phenomenon where vapor bubbles form in water due to pressure differentials. When a high-speed object like a torpedo or submarine moves through fluid, vapor bubbles appear behind it. If the object remains surrounded by those bubbles, much of its surface stays dry, minimizing friction with the surrounding fluid. Essentially, the fluid around the object vaporizes thanks to cavitation. The spaceship’s propulsion system makes it possible.」

「So the fact that it’s a spaceship is key here?」

「Right. To fully harness supercavitation underwater, you can’t use propellers—they interrupt the bubble layer. That’s why this is perfect for torpedoes.」

「Because you have to keep the entire body surrounded by bubbles. Propellers would just break that up.」

Kinukawa explained smoothly.

「So it’s like a rocket-propelled torpedo, barreling through the deep. We’re heading toward the trench Prometheus indicated… but I wonder if Bahamut knows what’s there?」

The Bahamut that Kou addressed responded through a conversation with Asia’s Aimer.

「Gyugyu!」

「Nyaa?」

「Gyu!」

「Nyanya!」

Kou couldn’t quite believe that something was getting across with those sounds—but he kept that thought to himself.

Even Nyanta, listening nearby, looked stunned. Clearly something surprising had come up.

「I wanna learn cat language too…」

「Humans can’t, nya…」

Nyanta delivered the harsh truth with visible regret.

「Yeah, I figured.」

「But Bahamut told us something amazing. He is taking us to the oldest structure he knows—something he’s been protecting.」

「The oldest structure?」

「It might date back to the Pioneering Era. If it’s not a ship, maybe it’s a residential colony capable of interstellar travel.」

「Something like that actually exists?」

「That era was on a whole other level. If it’s a colony, there’s a good chance it’s inert—its systems blocked by the technology seal. Humanity’s population has declined so much that there’s no need for them anymore.」

「Still, if it’s functional, it could become Libya’s vessel.」

「Without a doubt!」

Bahamut displayed their current depth.

「We’re now deeper than three thousand meters…」

「That’s the bathyal zone. You’d expect to see huge jellyfish or crustaceans. Below sixty-five hundred meters is the abyssal zone. The hadal zone begins at eighty-five hundred meters—that’s about the limit for fish survival.」

「Because of water pressure?」

「Exactly. At that depth, it’s hard for lifeforms to even synthesize proteins. At ten thousand meters, the pressure hits 100 MPa. The newly refitted Triton types might barely function down there. The MCS is fine, thought.」

「They started as escape pods, after all.」

「Right. There were Silhouette models designed for use on supergravity planets during the Pioneering Era and the Interplanetary War Era. Deep-sea usage was mainly for resource extraction. Most of that was handled by robots.」

With the decline in population, underground cities and deep-sea facilities had lost their relevance. Now, they were only used where deep-sea resources demanded it.

「An ancient structure, huh… What kind of ship could it be?」

Bahamuts moved in a straight course toward their destination—ten thousand meters beneath the sea.

At eight thousand meters, something appeared on radar.

「What… is that?」

Kou stared, dumbfounded, at the anomaly.

「Its size can’t be determined. Even Bahamut’s processors are throwing measurement errors. Its shape is unidentifiable—for now.」

Asia looked grim.

「Nyanya?」

「Gyugyu!」

Asia translated the exchange between herself and the nearby Bahamut for Kou.

「It said… this place is incredibly important. So he asked his friends to protect it in his place when he is not around.」

「Friends, huh? So there’s more than one.」

Kou felt a quiet relief knowing the kind-hearted Bahamut wasn’t alone down in the sea.

「Looks like they’re here to meet us.」

「That must be it…」

A massive disc-shaped object approached.

It was mechanical, yet its form resembled the shell of a sea turtle.

「Kou-kun. Could that be… Ga—」

「It’s a turtle. A giant turtle.」

Kou cut Kinukawa off before he could say more.

「Bahamut, what’s your friend’s name?」

「Gyugyu!」

「It’s Genbu, nya.」

Nyanta nodded in approval. A proper guardian beast.

「Makes sense. Bahamut and Genbu… and if there are more Eidolons involved too, then this really must be a place of great significance.」

「Yes… Perhaps this is the planet’s golden wheel. The pillar of the world.」

「Asia?」

There was something weighing on her mind—she grew uncharacteristically quiet.

「It’s just a feeling… Bahamut. Lead the way, please.」

「Gyugyu!」

Bahamut launched forward toward their destination.

They arrived at the heart of it all.

「A cylindrical structure with a radius over 300 kilometers… And the diameter… it’s unmeasurable.」

「We know the diameter—Libya and I. It’s a planetary ellipsoid. Earth-like planets rotate, so they become oblate spheroids. This structure is embedded in the short axis—the planet’s polar radius. It’s one of four planetary control pylons that were launched from a conical planetary administration unit over 100,000 kilometers in diameter. Just this one pylon alone is about ten thousand kilometers long.」

「…What?」

Kou stared in disbelief at Asia, who rattled off the explanation without pause. Beside her, Bahamut was looking at Asia’s Aimer clearly in distress.

Asia’s Aimer’s tears streamed without end.

 

 

AN:

Kou and the others board Bahamut and embark on their final expedition.

Their destination: the planet’s deepest depths. And at the far end of the abyss, something awaits—

This chapter features the inspiration for Bahamut’s design: a supercavitation ship called “Ghost.”

It’s a high-speed surface vessel, reminiscent of the F-117, with a catfish-like shape.

Some German supercavitation torpedoes have supposedly hit speeds of nearly 800 km/h. Maybe that’s true?

Supercavitation torpedoes are still in development around the world.