Having finalized the ‘what, for what purpose, and how’ of the weapon’s use, Da Vinci, Kang Baek-du, and the artisans embarked on the full-scale development of the ‘Self-Propelled Incendiary Bomb.’¹ The most straightforward part was the propulsion system.

“Will the propeller even work properly underwater?”

“I bet it’ll just fizzle out with a hiss.”

While Da Vinci and a few artisans worried about this, Kang Baek-du, a naval officer, and the craftsmen who had built the rockets were brimming with confidence.

“That might be true for the gunpowder of the old days, but the stuff we have now is fine.”

“It’s not uncommon for our fire projectiles to miss their targets, plunge into the water, and still explode.”

Hearing their accounts, Da Vinci stroked his beard thoughtfully. “So, you’re saying it’s perfectly usable even after entering the water… Still, we need to establish some firm principles. The most profound lesson I’ve learned since coming to the Empire is that one must not rely on experience alone.”

The ensuing experiments yielded a clear result: as long as the gunpowder wasn’t already soaked, it could combust without issue.

“The conclusion is that we just need to waterproof it, and the best way to do that is to seal it completely, right?”

“So we just have to block it off tightly so the fire can’t get in easily.”

At the conclusion Da Vinci reached, Kang Baek-du and the artisans stared at him, dumbfounded.

“What? Is there a problem?”

“Do you know what you get when you seal it all up like that? That’s what a fire projectile is.”

“Ah…”

With that minor commotion resolved, they moved on to developing the propulsion unit. The final design, which integrated the gunpowder chamber and the combustion chamber, boasted a rather impressive appearance and performance. But from there, the problems began to cascade, one after another. The projectile, unable to withstand the powerful thrust from its rear, thrashed about wildly in all directions, failing to maintain a straight course.

“But if we reduce the propulsion, we won’t get enough speed…”

As Da Vinci and the artisans agonized over the dilemma, Hyang, who had been watching with amusement, casually dropped a piece of advice.

“Why not refine the shape of the body? Since it’s a device that moves swiftly through water, wouldn’t it be wise to reference the form of a fish?”

I can’t just present the torpedo shape I know from the start, can I? People would think I’m an alien. We need to go through a bit of trial and error.

“Ah!”

“Your Majesty, we are overcome with gratitude for enlightening our foolish minds!”

And so, Da Vinci and the artisans bought dozens of fish from the market and meticulously studied their shapes. The form that ultimately captured their attention was that of the mackerel.

“This fish is renowned for its speed. Let us model our design after it.”

“Indeed!”

Hyang, enjoying the spectacle from the side, mused to himself, A mackerel… isn’t that closer to a submarine than a torpedo?

Through a series of such trials and errors, Da Vinci’s self-propelled fire projectile gradually took on a more plausible form.

It still looks more sci-fi than reality, but… I’ll have to leave it as homework for them. If this thing is completed properly, it will establish itself as a devastating weapon for the foreseeable future.

Hyang, who was scrutinizing every report and observing the entire manufacturing process, offered his own positive assessment before adding a quiet thought. “The truly core component, the gyroscope, is a challenge worthy of the Challenge of Records. Phew. To some, this might seem like a waste of money, but we have to leave these traces behind. Only then will they keep pondering, keep striving for survival.”

He was a man who remembered the history of humanity—or, in a shorter span, the fierce technological war that raged from the late 19th to the 21st century. It was a conflict so fierce that ‘competition’ felt too soft a word; ‘war’ was the only fitting description.

“I don’t know how long this era will last, but for the Empire to survive for at least two or three hundred years, I must continuously leave behind challenges for them to tackle. No, I must engrain in them the very habit of challenging the unknown.”

This was Hyang’s resolve.

***

The development of the Self-Propelled Incendiary Bomb proceeded just as Hyang had anticipated. By refining the body’s shape and adding a long fin that ran from head to tail, they managed to minimize the projectile’s thrashing during its run.

“Does it still struggle in the water? If so, it’s unusable,” Hyang declared.

“But, Your Majesty, this is the best we can achieve.”

“Go back to the beginning and reconsider everything.”

“Yes, Your Majesty…”

At Hyang’s command, Da Vinci and the artisans reviewed every aspect of the design from scratch. As a result, the body of the Self-Propelled Incendiary Bomb became slimmer and longer, the warhead grew more pointed, and the distinctive long fin was more firmly established.

“It feels like we’re almost there now, doesn’t it?”

“Right?”

“Alright! Let’s push just a little harder!”

“Let’s do this!”

Thinking the worst was behind them, the artisans and Da Vinci encouraged one another as they tackled the next task. This, however, proved to be one of the most formidable challenges of all: maintaining depth.

“I said it should travel underwater, but it only needs to run at a depth that makes it difficult to spot with the naked eye from the surface.”

“Difficult to spot with the naked eye, you say?”

“That’s right. A depth where it’s only visible when it’s right upon you. That would be ideal.”

Following Hyang’s directive, Da Vinci and the artisans took to the sea, conducting experiments on the Han River, the West Sea, and the East Sea.

How deep does it need to be to become invisible to the naked eye?

After dozens of repeated measurements, the results were in. From a higher vantage point, one could see deeper into the water. However, from the deck of a Challenger-class warship, the projectile became difficult to find if it was submerged just half a jang³ (about 1.5 meters) below the surface. The optimal depth, they concluded, was one jang (about 3 meters) below the surface.

“We’ve found the right depth, but how do we maintain it?”

After a period of intense deliberation, the answer Da Vinci and the artisans devised was to ‘fill it with water.’ They designed the body of the Self-Propelled Incendiary Bomb with empty, partitioned compartments. On the outer shell of each compartment, they installed a plug. Just before launch, the crew would open the plugs corresponding to the desired depth. As the projectile traveled, water would fill the empty spaces, theoretically allowing it to settle at the correct depth.

“This seems promising!”

“I think this is the answer!”

However, when they built and tested a prototype, the results were far from ideal. The depth-control compartments filled with water far too slowly.

“Why?”

“Why is this happening?”

Confounded by the unexpected outcome, Da Vinci and the artisans wracked their brains for a solution, but an answer remained elusive. They began to consider submitting the problem to the Challenge of Records.

“Should we register this in the Challenge of Records?”

As they seriously debated this, the most senior artisan among the onlookers chimed in. “Add another plug. You have things trying to get in and things trying to get out using the same single hole. Do you really think that traffic will flow smoothly?”

“Ah!”

“What in the world did you learn when you were building steam engines? Tsk, tsk, tsk.” The old craftsman clicked his tongue and turned to his colleagues, continuing his complaint. “Don’t they teach this stuff in the middle schools or the engineering academy these days? It’s the most basic of principles.”

Ultimately, this issue was reported all the way up to Woo.

“He has a point.”

Agreeing with the sentiment, Woo immediately ordered an investigation and discovered a problem with the educational curriculum. The engineering courses were teaching outdated techniques, forcing graduates to relearn too many things once they entered the field.

“This is a major problem.”

Woo reported the matter to Hyeon, which led to a massive overhaul of the national education curriculum and its content.

***

After overcoming countless more trials and errors, the Self-Propelled Incendiary Bomb finally took shape. In the Navy’s official evaluation of the finished product, it received a partial pass. Its speed and destructive power were excellent. The problem, however, was its range, which was disappointingly short—barely half that of the standard cannons mounted on a Challenger-class warship. A vessel would inevitably suffer damage from enemy cannon fire while trying to close the distance to use the new weapon. Furthermore, the sheer space required for the launch apparatus and the inherent danger of storing the projectiles made it difficult to integrate them alongside other cannons on a standard warship.

“Its speed and power are truly impressive, Your Majesty, but its other aspects are too lacking. If not for those flaws, it would be a truly fine weapon,” the Minister of Defense and the naval admirals concluded.

Hyeon turned to Hyang. “Do you perhaps have a good solution in mind?”

“Hmm…” Hyang feigned a moment of contemplation before sharing his thoughts with Hyeon. “What do you think about building a dedicated vessel for it?”

“A dedicated vessel, you say? But warships are not exactly cheap, are they?”

At Hyeon’s point, the Minister of Defense and all the naval admirals nodded in agreement. Hyang let out a soft click of his tongue.

“Tsk.”

The sound instantly put Hyeon and everyone else on edge. Hyang continued, “Do you recall what I mentioned before? The new engine that combines the multi-bladed turbines. Imagine mounting that powerful new engine on a small warship. A small hull with great power—would its speed not be incomparable? Its small size would also make it a difficult target for enemy cannons, and it would be too far for small arms to be effective. We could operate such vessels equipped with two or four of these projectiles. Of course, we wouldn’t use just one or two, but deploy them in groups of at least ten.”

“But, Your Majesty, such a small boat would have difficulty operating on the open ocean,” one of the admirals pointed out.

Hyang clicked his tongue once more. “Tsk! Do I look like I was suggesting it for open-ocean operations? I am talking about a weapon for the coastal defense of the Empire! Think of our mainland, Daeseoldo, and Seonamdo!”

“Ah!”

At Hyang’s clarification, the faces of Hyeon and the others lit up. The Empire’s military strategy was fundamentally defensive, but its defense was heavily focused on intercepting threats far from its shores. Consequently, a significant number of officials had raised concerns, asking, ‘What if an enemy manages to enter the Empire’s coastal waters?’ They argued that a contingency plan was needed.

‘It’s not just enemies from afar that are a problem. Remember the Ming and Japan!’

It was a valid point, and the Ministry of Defense and the Navy had been devising countermeasures. However, true to the Empire’s doctrine that ‘artillery is the beginning and the end,’ the most promising solution they had come up with was a gunboat that resembled a monitor ship.

This is not my style at all… Hyang had been dissatisfied with that direction. Was this my subconscious reason for supporting the Self-Propelled Incendiary Bomb’s development?

In any case, the application Hyang proposed was the perfect solution to the Navy’s nagging problem. The Imperial Navy embraced his suggestion and began production of the ‘High-Speed Gunboat.’ An interesting footnote was the number of Self-Propelled Incendiary Bombs it carried. Faced with Hyang’s suggestion of ‘two or four,’ the Navy chose two. Their reasoning was that the pronunciation of ‘four’ was the same as the word for ‘death’ (死), and equipping more than four would require a larger hull. Two was optimal. These high-speed gunboats would later prove their worth time and time again in crises to come.

***

Not long after the official adoption of the Self-Propelled Incendiary Bomb, a strange report reached Hyang.

Leonardo da Vinci had entered a period of seclusion. It was not a retreat for conceiving new ideas, but something closer to self-imposed exile.

“Hmm?”

After reviewing the document detailing Da Vinci’s unusual behavior, Hyang immediately summoned him. “Is something troubling you? Or are you unwell?”

To Hyang’s question, Da Vinci replied with a weary face. “No, Your Majesty. I simply feel… empty. I find myself completely at a loss for what I want to do.”

Hyang nodded in understanding. Burnout? It’s understandable. But I need to squeeze a bit more out of him…

After a moment of thought, an idea struck him. “Do you remember, long ago in Italy, when you drew my portrait and were scolded for it?”

“Yes, Your Majesty.”

“In that case…”

Fifteen days later, a bizarre rumor began to circulate within the Medical Academy.

“There’s a madman in the dissection room!”

“He’s drawing everything! Every last detail of the dissected cadavers!”

The affair could have become a scandal, but the words ‘by Imperial order’ quickly silenced all whispers. Shortly thereafter, the senior physicians of the Medical Academy, having seen the ‘madman’s’ results, rushed to find Hyeon.

“We must print all of that painter Da Vinci’s drawings and compile them into a medical text!”

“They are an unparalleled treasure for the field of surgery!”