Radial (star-shaped) engine.

I'm back～

"Look at this, Onee-san!"

"It's our masterpiece, Master!"

The engine blueprint they showed me had five pistons arranged in a star shape around the axis—a radial engine.

Amazing. I only taught them the basics, yet these two developed a radial engine on their own. Engines like this were used in reciprocating aircraft back on Earth.

"With just one piston starting at its maximum or minimum position, it's unclear which direction it would rotate initially. We figured that by arranging multiple pistons and intentionally offsetting them, we could solve that problem."

"And that's when I suggested the star design, and it turned into this shape!"

Looking at the diagram, it seems like it could actually work if made properly.

Engines like this won't work if the precision is off, but since I'll be processing it with Spatial Magic, that shouldn't be an issue.

In other words, this is a perfect engine blueprint.

"By having them pull each other, the pistons only need to move in one direction instead of both… Theoretically, it can run on a single Goblin's magic stone. Though we're not sure how fast it would spin."

Plus, since it's a basic piston-only Magic Tool, the fuel efficiency should be quite good. Rotation-based Magic Tools require a lot of energy to increase their output.

"… You two look like you're itching to try making the real thing."

"Yes! Please, Karina-oneesan!"

Alright, alright. Karina-chan has always been weak to requests from cute kids.

Let's make a model right away. Here we go—Spatial Magic crafting! Whoosh!

"It's just a wooden model, but here you go. I printed the magic circle circuits on it too."

"Oh, Onee-san works fast!"

"As expected of Master!"

"Then, let's set a Goblin's magic stone into it."

A single Magic Tool connecting five pistons. I placed a tiny magic stone about the size of a pinky nail—a so-called scrap magic stone—and yet the shaft started spinning smoothly.

"… It really moved! It's rotating!"

"We did it, Dea-sama! It's our victory!"

I don't know what they were fighting against, but Dea-kun and Aishia high-fived in celebration. Can Onee-san join too? Yay～

That said, the rotation was surprisingly powerful. I tried gripping the rod, but it took quite a bit of force and bracing to stop it. Getting this much speed and torque from one scrap magic stone… Wait, isn't this revolutionary?

"Ah… I heard a crack just now. Did a piston break?"

"Hmm. The thin wooden parts can't handle the output. Impressive."

It was Treant wood, too. Now I really want to make it with a stronger material.

I remade it with stone. Natural stone should be sturdier than wood. I'd prefer metal for its elasticity and other properties, but this will do for now.

I ran it the same way and tried stopping it by gripping the rod—again, it took considerable force to stop. No breakage this time.

"At worst, it could peel the skin off your hand. Amazing power."

"True. We can call this a great success."

What's more, it looked like the power would increase linearly if we chained two or three of these star-shaped units together. With gears, we could achieve some seriously high rotation speeds.

… Wait, we were just supposed to make a dummy Magic Tool, so why did we end up seriously developing a real engine!?

"By the way, we also came up with a plausible magic circle to install inside the cart. One designed to amplify rotation. With eight amplification links, it should fool most people. Though it might not pass scrutiny from professionals…"

"And here! This part incorporates wind and fire motifs—that was my suggestion!"

Wait, not just the engine body, but even a camouflage magic circle for inside the cart!

I was out for less than half a day, and they completed the work perfectly!!

Kuh, this is all because Dea-kun and the others turned out to be more talented than I expected! And so cute!

I must give these two a reward…!

"Alright, then I'll make you two a special treat… Today, it's a pancake party!"

"Wow, Onee-san can make sweets too? I'm looking forward to it!"

"Thank you, Master!"

Incidentally, after a few adjustments, the radial engine model ended up being repurposed as a mixer and proved very useful for stirring the pancake batter.

… The box had the pancake mix ingredients listed on it, so I could probably make more myself using that as a reference. Though I'd need to figure out how to get more baking powder.