Jin hurriedly inspected Reiko’s body. Not a single scratch could be found. She was truly the world's most powerful automata, crafted entirely from rare materials.
"Reiko, help me fix the laboratory."
After making her stop her dogeza, Jin gave her instructions. Reiko obeyed him meekly.
"Yes, Father. Let's have Soleil and Luna help as well."
"That's a good idea."
Lately, Reiko’s assistant golems, Soleil and Luna, spent most of their time looking after Jin's house next to the laboratory.
The two had come out, startled by the noise of Reiko smashing through the laboratory wall, so Jin decided to have them help repair the exterior. It was a stroke of luck that the room she had broken into was currently vacant.
They fitted the fragments back into their original places and repaired the structure using the craft magic Fusion. In less than thirty minutes, the laboratory had been restored to its original state.
"Phew, that should do it. Soleil, Luna, thank you for your hard work."
"Yes. We shall return to the house then."
"Please call for us if there is anything else you require."
Soleil and Luna bowed and returned to Jin's house. Jin then turned his attention back to Reiko.
"Reiko, do you understand why that happened just now?"
"Yes, Father. It was because the magi reactor's output was too high. Even so, I was using less than one percent..."
"To do that with so little... I thought as much."
Jin had a faint inkling. The force generator was extremely efficient.
"When using gravity magic to make my weight near zero, I required about twenty percent output. Is this new force generator really that much more efficient?"
Jin nodded.
"Yeah. Gravity magic is, so to speak, a magic that generates gravitational acceleration. However, this force generator, as its name suggests, is generating actual force."
He explained his reasoning: while gravitational acceleration theoretically affects everything along a virtual straight line, force acts upon a single point of action. It was only natural that the efficiency would differ.
Jin couldn't explain every detail perfectly. According to Newton's second law of motion, $F=ma$, force is the product of mass and acceleration. He suspected that the nature of this acceleration was fundamentally different from gravity magic, but since he wasn't a physicist, that was as far as his explanation could go.
Nonetheless, it was enough for him to recognize the facts, grasp them, and apply them. He added that in the case of Stone Bullet, if they could eliminate more waste, they should be able to launch large stones—no, boulders—with very little magical energy.
"I understand. Next time, I will try with even less magical energy."
"Wait. It might be dangerous here, so let's go to the coast."
And so, Jin and Reiko boarded Pegasus 1 and traveled to the shore. At Tatsumi Bay, Marine 1 and Aqua 1 were in the middle of drying tengusa-modoki.
"Master, what brings you here?"
Jin gave a brief answer to Aqua 1's question.
"Oh, I've come to test Reiko's new function. It might be dangerous, so could you step back a bit?"
"Understood."
Jin also deployed a solid barrier for himself. Thinking back on it now, it sent a chill down his spine to imagine what would have happened if he had been sent flying during Reiko’s earlier runaway. That said, since the only thing Reiko had failed to control was the magnitude of the force and not its direction, such an accident shouldn't have been possible.
"Alright, go ahead."
Jin, standing safely behind his solid barrier, gave the signal through his manacam. Reiko replied through her internal manacam.
"Yes, Father. Here I go."
The moment she spoke, Reiko began to dash forward toward the sea at about fifty kilometers per hour. Or rather, since her legs weren't moving, perhaps "dash" wasn't the right word. Leaving a single line in the sand, Reiko flew forward in a straight line.
"Oh, amazing!"
Just before reaching the sea, Reiko suddenly changed direction. She turned instantly and came back in a straight line, stopping dead right in front of Jin.
"Father, I have grasped the approximate adjustment required."
"So it... seems."
Looking at her feet, which had stopped precisely at the starting point without a millimeter of error, it was clear that Reiko had mastered this new function.
"Now, I shall attempt the 3D maneuvering you mentioned, Father."
"Don't overdo it."
"Yes. Well then, here I go."
This time, Reiko’s body began to move upward.
"O-Oh!"
While Jin watched, Reiko slowly ascended to an altitude of about ten meters and hovered there.
"Father, please watch."
Having said that, Reiko suddenly shot off horizontally. Her speed was likely exceeding one hundred kilometers per hour. Heading for the sea, she flew in a straight line at first, but once she was over the water, she performed a loop. After performing ten consecutive loops, she performed an outside loop.
After one rotation, she performed a normal loop again. In other words, Reiko was demonstrating figure-eight loops. Next came steep climbs and dives, followed by horizontal flight in wide arcs. As time passed, her speed increased and her turning radius grew tighter.
Jin watched with excitement. Then, Reiko shot upward diagonally with great momentum.
"Whoa!"
A booming shockwave rang out—a sonic boom caused by breaking the sound barrier. Naturally, Reiko was no longer visible to the naked eye. A moment later, she returned in an instant and came to a halt above the sea. Moving slowly from there, Reiko landed next to Jin.
"Father, is this sufficient for the test?"
Jin deactivated his barrier and reached out to Reiko.
"Yeah, it's plenty. You're amazing, Reiko! It's a great success!"
But he let go immediately.
"Hot, hot, hot!"
Having flown around at supersonic speeds, Reiko had gathered considerable heat due to the adiabatic compression of the air.
"Are you alright, Father?"
The body temperature generation function installed in Reiko also possessed the ability to cool her body down. Thanks to that, while she would normally have been hot enough to cause severe burns, she was only slightly hot to the touch.
"Yeah, I'm fine."
Despite the heat, Jin was smiling happily.
"Reiko, are there any defects? Also, was there anything you noticed?"
"Yes, there are no defects. As for what I noticed, rotating the body is difficult."
"Ah, I see."
The current function was only for linear motion in the three-dimensional axes—X, Y, and Z. If he added a rotational motion function, she would be able to move and change direction freely within three-dimensional space.
"Alright, let's improve it right away."
Jin and Reiko boarded Pegasus 1 and returned to the laboratory.
"Rotational force... torque. In that case, the efficiency will be poor if the moment arm is short, so..."
Muttering to himself, Jin began adding the new functions. To generate rotation, it was desirable to place the point of action far from the center of gravity. Therefore, he equipped force generators for attitude control on both shoulders and both hips, making it so their directions could be changed freely.
"That should do it. Reiko, try it out."
He woke Reiko up again, and after performing a re-check, they conducted a third test.
"Yes, here I go."
This time, they stayed in front of the laboratory. As a precaution, Reiko ascended to an altitude of one hundred meters. She started with a slow rotation around her body's axis and gradually increased the rotation speed.
If she were human, her head would have been spinning and she would have fainted long ago, but Reiko remained perfectly composed. To confirm her control, she performed corkscrew climbs and descents. She even curled her body into a ball and rolled through the air.
"Oh, that’s magnificent..."
Jin had boarded Pegasus 1 and was observing from one hundred meters up.
"Hmm, at this point, I really need to start thinking seriously about air resistance..."
For an object with a shape like Reiko’s to reach supersonic speeds, the efficiency was still poor. The heat generated by the adiabatic compression of air was also a problem.
"I wonder if I can manage something by applying a breeze barrier..."
While flying freely through the air, Reiko waved her hand at Jin. Watching her, Jin began to formulate new ideas.