"Making the Bigeochintae in the original way is too inefficient."
The original Bigeochintae used a solid cast sphere. By 21st-century standards, this method had many issues. While creating a hollow spherical shell was technically excellent, it was not a method he particularly wanted to recommend in terms of production efficiency.
"Judging by the performance, mass production would clearly be better, but we need to test it."
Xiang's first move was to split the Bigeochintae sphere into two halves.
Xiang modified the design, threading one half of the sphere with a female screw and the other half with a male screw. By aligning and turning the screws on both halves, they would form a complete sphere.
Xiang also applied screws to the parts where the fuse and gunpowder were inserted. He engraved screws on the cap that covered the fuse and the cover that sealed the gunpowder hole to ensure a more secure fit.
Xiang then proceeded to the firepower tests with the prototypes.
"Let's first try without launching it from a cannon."
"Yes, Your Highness."
The test results showed that the improved Bigeochintae exploded magnificently, just as Xiang had expected.
"Remarkable!"
"Should we send them to the northern front immediately?"
Seeing the test site turned into a mess, Choe Hae-san and Lee Cheon couldn't stop their admiration. However, Xiang shook his head with a smile.
"There are still many tests to be done. We need to conduct a firing test first."
"Firing... Yes, I see. I forgot about the 'Bigeochintae' part."
In the subsequent firing test, the Bigeochintae with a 30% reduction in thickness successfully withstood the impact of firing and landing, and exploded properly. Choe Hae-san and Lee Cheon, who saw the results, were tempted to push further.
"Since the results are good, let's report them..."
"The explosion radius is too wide. There's a risk of friendly fire. Let's research a way to reduce the explosion radius while increasing its lethality."
"Your Highness's command is correct. We will consider it."
Following Xiang's orders, Choe Hae-san, Lee Cheon, the military officers, and the craftsmen began to find the optimal solution. However, it was difficult to satisfy both the contradictory conditions of enhancing lethality and reducing the danger radius simultaneously.
In the end, Xiang had to step in again.
"Bring me mud, paper, and iron balls the size of walnuts. The iron balls must be made of iron."
"...Yes, Your Highness."
When the craftsmen brought the materials, Xiang gave them instructions.
"Apply a layer of mud about half an inch thick inside the Bigeochintae shell and embed the iron balls densely. Once the clay dries, cover it with paper to ensure it stays in place."
The military officers and craftsmen, who heard Xiang's instructions, immediately exclaimed in admiration.
"Ah! What a brilliant idea!"
In the subsequent test, the modified Bigeochintae had a reduced danger radius but a significantly enhanced lethality compared to the original. Xiang, who was examining the results, looked at Choe Hae-san and Lee Cheon.
"Let's make a few more improvements. Just a little more..."
"Yes, Your Highness."
In the second round of improvements, Xiang and the craftsmen engraved grooves in a checkerboard pattern on the inside of the Bigeochintae shell and added another layer of iron balls.
After the second round of improvements and the subsequent test, Xiang, who saw the results, gave orders to Choe Hae-san and Lee Cheon.
"This seems sufficient. If we proceed further, future generations will think of us as bloodthirsty and ruthless."
"Yes, Your Highness!"
Choe Hae-san and Lee Cheon nodded vigorously in agreement. The scene created by the improved Bigeochintae was nothing short of gruesome.
"Now, let's report..."
"Let's improve the cannons that will fire these shells."
"Uh?"
Choe Hae-san and Lee Cheon looked puzzled at Xiang's words.
Xiang observed the large cannon (大碗□) used to fire the Bigeochintae.
"35 nyang (about 1.3 kg) of gunpowder in the chamber... The problem is that it's not an efficient design."
Xiang pointed out the overall design of the cannon.
"Let's change the structure!"
Xiang designed the cannon to be a traditional straight barrel, with a carriage similar to the gun carriages used in 18th to 19th-century Western cannons.
"Is it too Western? I don't think so."
Xiang tilted his head as he looked at his design. To be precise, the design Xiang created was closer to the medium-caliber infantry cannons made in Korea during the Daewongun era.
With the design and carriage of the cannon he had in mind, Xiang hung the blueprint and discussed it with Choe Hae-san, Lee Cheon, and the craftsmen.
"Mount it on a large cart with wheels. This will improve its mobility."
"Against the Jurchens in the north, improved mobility of the cannons will make things much easier, won't it?"
Choe Hae-san and Lee Cheon nodded in agreement with Xiang. Thanks to their extensive combat experience in both the south and the north, they immediately understood the advantages Xiang was talking about.
Lee Cheon, who had been observing the blueprint, concluded.
"Making carts for the cannons and carts to carry the shells and gunpowder... If even two Bigeochintae can accompany us, the Jurchens won't dare to act recklessly."
With Choe Hae-san and Lee Cheon, who were the top experts in cannons in the Joseon army, agreeing, the development of the cannons and carriages proceeded rapidly.
The new cannons being developed differed from the existing ones in that not only the exterior but also the rear part of the internal chamber was rounded.
"General Choe, do you remember the first test of the explosive shells?"
"Yes."
"At that time, the soot on the copper plate spread out in a radial pattern. This suggests that the force of the gunpowder explosion spreads out in the same way."
Choe Hae-san, who had been thinking deeply about Xiang's words, nodded.
"That seems to be the case."
"Then, wouldn't making the rear part of the chamber round make the cannon more robust? It would have the same thickness from any angle."
Choe Hae-san, who was examining the blueprint in detail, nodded.
"It seems worth trying."
After the first cannon was made, Xiang and Choe Hae-san immediately conducted a test.
They loaded twice the standard amount of gunpowder into the chambers of both the existing large cannon and the new cannon, sealed them tightly with wooden wedges, and then loaded heavy stones.
During the four consecutive firings, the new cannon performed without any issues, but the existing large cannon developed cracks at the bottom corners of its chamber.
Choe Hae-san, who confirmed the results, opened a writing board and wrote.
"The test results show that making the rear part of the cannon and the chamber curved strengthens the internal structure. It would be correct to improve other cannons or design new ones with a similar shape."
* * *
"...So, this Bigeochintae, including the shells and the design of the cannons, has all been improved by the hands of His Highness the Crown Prince."
Hearing Choe Hae-san's explanation, Sejong and the ministers looked at Xiang again.
"You... Now, even saying you are amazing makes me tired."
At Sejong's words, the ministers all nodded. They were exhausted from seeing one amazing invention after another. Indeed, the prime ministers and the minister of the Board of Revenue were already calculating the budget required for mass production, and they were feeling the pressure.
"I want to see the new cannon."
"Your command is received."
A short while later, two carts entered the demonstration ground, each pulled by two horses.
"The first cart is for the cannon carriage, and the second cart carries the Bigeochintae shells and gunpowder."
"Hmm..."
Sejong, listening to Choe Hae-san's explanation, inspected the carts. Two wooden boxes were mounted on the cart connected to the carriage.
"These boxes contain the shells and explosives to be used immediately after the cannon is set up."
The soldiers opened the lids of the boxes on the cart. As Sejong examined the boxes, he noticed small bags.
"What are these bags for?"
Sejong's question prompted Choe Hae-san to open a bag and take out its contents.
"These are the fuses to set the explosion time, the bamboo sticks to wind the fuses, 1 nyang of explosive gunpowder to ignite the main charge, and the paper tubes that contain all of these to be inserted into the Bigeochintae."
Sejong nodded at Choe Hae-san's explanation.
"I see. The explosive gunpowder doesn't ignite the main charge directly, so the explosive gunpowder is used to light it. But what about the paper tubes?"
"At first, we used bamboo, but there were issues with uniformity, so we switched to paper tubes."
"I see. That's a good idea."
"It was Your Highness's suggestion."
Choe Hae-san gave credit to Xiang, and Sejong looked at him.
"I can see your hard work."
"It's the effort of General Choe, General Lee, and the military officers and craftsmen."
Xiang gave credit to the military officers and craftsmen.
Sejong's eyes stopped at the rear part of the cannon. The part that caught his attention was the section connected to the cannon's base. There was a column with threads engraved on it, and next to it, a handle similar to a winch. Beside that, a semicircular plate with the characters 'Yuan (違), Jin (近), Chaojin (超近)' was attached.
"Prince, what is this used for?"
"Yes, this is to adjust the angle of the cannon to control the range."
"Control the range?"
"Yes. It works like this."
Xiang turned the handle similar to a winch, and the cannon's muzzle moved up and down. Sejong, who saw this, turned his head.
"By adjusting the angle of the muzzle, you can control the distance?"
At Sejong's question, Choe Hae-san immediately answered.
"Yes, Your Majesty."
"How?"
"It works on the same principle as shooting an arrow. When aiming at a distant target, you raise the bow and shoot in a parabolic trajectory, don't you?"
"Ah!"
Sejong nodded and exclaimed in admiration at Choe Hae-san's explanation. Choe Hae-san pointed to the semicircular range indicator and continued.
"By aligning the muzzle with these graduations, you can control the distance from 300 to 500 paces."
Sejong, who was examining the graduations in detail, looked at Choe Hae-san and Lee Cheon.
"Many efforts must have been made to determine these graduations."
"Yes, Your Majesty."
Sejong's words were met with nods from Choe Hae-san and Lee Cheon.
To accurately measure the distance changes under various conditions, Choe Hae-san, Lee Cheon, and the craftsmen fired hundreds of Bigeochintae. To reduce unnecessary gunpowder consumption, they filled the Bigeochintae with mud and gravel. In the end, they had to remake nearly a hundred Bigeochintae that broke due to metal fatigue. That wasn't all.
A significant number of cattle in the nearby farms in District 51 couldn't withstand the stress from the loud explosions and died, leading to considerable compensation. The meat from the slaughtered cattle was served to the craftsmen and soldiers.
"Being able to control the distance like this will certainly show better effects in battle. Can this be applied to other cannons?"
Sejong's question was answered by Xiang.
"The officials at the research institute are currently studying it."
"However?"
Sejong, hearing the answer, looked for Jeong Choe among his ministers.
"Minister Jeong, I trust you."
Jeong Choe, hearing Sejong's words, bowed and replied.
"I accept your command."
Jeong Choe's voice was filled with moisture.
'Even now, I can't go home...'
Thanks to Xiang's previous order, the officials at the research institute were already holding books filled with firing results and were engaged in a 'war of numbers.'
They were investigating the changes in range under various conditions, creating tables, and formulating equations to develop distance adjustment mechanisms for each cannon.
As a result, the officials at the research institute could only go home on their designated days off—nine days of work followed by one day off—due to the intense workload. At least, Xiang strictly adhered to this rule.
However, there were times when the officials voluntarily worked overtime. This period coincided with the time when they were making meju (fermented soybean blocks).
* * *
Joseon was known as the 'land of jang' (fermented soybean paste) due to its high consumption of doenjang (soybean paste) and ganjang (soy sauce). Every household had numerous jangdoks (earthenware jars), and the smell of doenjang permeated the residential areas of Hanyang. The smell of jang was familiar to Joseon people, but the smell of meju being made was a different story.
Especially in households with many family members, a large amount of meju had to be made, making the smell even more intense in the homes of high-ranking officials who had many servants and guests.
The palace also made a large amount of meju, but due to its vast space, the smell wasn't as strong. For this reason, officials would delay their departure from the palace as much as possible during the meju-making period.
* * *
In any case, thanks to the orders of Sejong and Xiang, Joseon became the first country in the world to create and apply firing tables for cannons.
After thoroughly inspecting the cannons and the equipment for firing the Bigeochintae, Sejong straightened his back.
"Hmm, I've seen everything, but before checking the accounts, there's one last thing I need to confirm. The armor worn by the military officers and soldiers here is different from what I'm used to seeing. Please explain it."