Chapter Fifty: Truly Unimaginable

After a period of cooling, the eight impeller castings were finally ready. The three men made their way into the workshop, where two impellers were still undergoing sand removal, and one fully cleaned impeller had been placed neatly on the floor.

As he examined the cleaned impeller in his hand, the technical backbone who had personally accepted Zhong Jianshe’s test piece assignment grew increasingly dissatisfied. He pursed his lips, set the impeller down, and spoke up.

“There are a lot of problems here: flash lines in the bearing holes, sand wash at the thin-walled sections, and sand holes as well—a whole host of issues.”

He shook his head as he spoke. Although he had anticipated some difficulties, he hadn’t expected the flaws to emerge so quickly and all at once in the very first casting. He could hardly imagine what the remaining seven castings would be like.

Zhong Jianshe picked up the casting from the table and, sure enough, found numerous problems. After inspecting it, he handed it to Yang Hui. Yang Hui, from a distance, had already noticed the defects, but he still took the casting from Zhong Jianshe and examined it closely.

Upon closer inspection, he realized the problem wasn’t just in the flawed areas—rather, the parts without issues revealed just how well the casting had been done. The roughness was of a high grade, and the roundness of the entire outer rim was excellent.

“Jianshe, did you really set such high standards for this casting?” Yang Hui wasn’t an expert in casting, but he could see that if this casting succeeded, it would nearly reach the level of precision investment casting—a level that, at their facility, was simply unattainable for now, and even in the rest of the country, this technology hadn’t been mastered yet.

“Yes, this is a one-piece casting. For small parts like this with so many curved surfaces, it’s simply impossible to machine those curves after casting. Of course, you could have a fitter grind them by hand, but that would be overkill, so I opted to raise the casting precision instead.”

Now Yang Hui understood. The issue was simple: since the curved surfaces of the cast part couldn’t be machined afterward, the only solution was to increase the precision during casting itself. It was an approach, but clearly, their current capabilities couldn’t meet the requirements.

As Yang Hui and Zhong Jianshe were talking, the remaining seven impeller castings were cleaned and brought over. The technician called them over to inspect the pile.

“This is what the finished products look like. At present, we simply can’t achieve the tolerances specified in the drawings. I only agreed to this test piece to see whether we could make it at all, but I clearly underestimated the difficulty.”

He picked out the best of the eight impellers and handed it to Yang Hui and Zhong Jianshe.

“This one is the best—no sand wash, only a few sand holes, and the flash line here is scheduled for further turning anyway, so it’s not a big issue. You can just barely use this for testing. It’s a substandard piece, but not a scrap.”

He had to think for a long time before he could say this. After all, it had been many years since any substandard castings had left his hands, and it was only to help Yang Hui and his team meet their deadline that he allowed this one through. Otherwise, without at least one usable casting, their follow-up tests would have to be halted.

“Alright, we’ll take this casting. When we get back, we’ll find a way to relax the precision requirements. We’ll have to trouble you again in the future.” Yang Hui couldn’t say much more at this point. It was clear that the next improved casting would require the technician’s help again. According to the workers here, he was already the top technical expert in the factory, so they were sure to need his assistance next time as well.

After bidding farewell to the factory, they returned with the one substandard casting. The test, of course, couldn’t proceed as things stood; they’d have to find a way to optimize the impeller and lower the casting precision requirements.

He put the casting down and pulled out the design specifications. The simplest approach now was to see if any technical indices could be relaxed; if they found one, it might allow them to lower the overall requirements for the impeller.

First, he looked at the compression efficiency. If the efficiency could be lowered, that would be the easiest solution—the casting precision could be relaxed accordingly. But after reviewing the compression efficiency, it was clear that this index couldn’t be reduced; it was already just above the minimum air compression requirement for the combustion chamber.

Next, he considered the design lifespan: 27 hours. Zhong Jianshe had left a two-hour margin when he designed it. Now, he could simply sacrifice those two hours and change it to 25 hours, which matched the combustion chamber’s design lifespan. The extra two hours on the impeller were pointless since cast impellers are generally quite durable and unlikely to fail under normal operation.

But shaving off those two hours was far from enough, and the time margin didn’t significantly affect the required precision.

Then, to Yang Hui’s exasperation, another specification caught his eye. Zhong Jianshe, in his thoroughness, had even considered the impeller’s resistance to foreign object ingestion—in particular, bird strikes. To ensure the engine could return safely after ingesting a foreign object, he’d upgraded both the impeller’s strength and hardness by more than one grade.

“My dear Zhong Jianshe, you even worried about bird strikes? This model plane is so small—what are the odds of hitting a bird?” Yang Hui couldn’t help but remark as he watched Zhong Jianshe still calculating the impact of the two lost hours on manufacturing precision.

“The odds of a bird strike... Ah, I was overthinking it. How likely is a model plane to hit a bird? It’s all because I read a paper on foreign object ingestion and bird strikes in engines a few days ago—my mind got muddled.” Zhong Jianshe immediately realized his mistake. He had simply overthought things and added unnecessary calculations.

“Team leader, give me a moment. I’ll recalculate to see, without considering foreign object ingestion or bird strikes, just how much we can lower the impeller’s required strength and hardness.” At this, Zhong Jianshe recalled an impeller design he’d previously abandoned and grew visibly excited.

As for the half-finished calculations about impeller lifespan’s effect on precision, he dropped them at once; after all, strength and hardness were the real factors affecting manufacturing precision.

“I’ve got it! Haha! Team leader, did you know—without the foreign object requirement, lowering the impeller’s strength and hardness actually matches another impeller I designed!”

As he spoke, Zhong Jianshe rummaged through his heap of scrap paper until he finally unearthed a battered and crumpled sheet. Carefully, he unfolded it.

What met their eyes was a remarkably simple impeller design, but among the various data on the side, one set of figures stood out: the compression efficiency. Even Yang Hui could hardly believe it. This simple impeller’s air efficiency was about 1.4 times higher than that of the previous, more complex impeller—a difference that could make all the difference.

“Team leader, this is the impeller! Look at the design—I’ve calculated it, and the compression efficiency is very high. But the structural strength is low; under high-speed rotation, any external force would immediately cause catastrophic failure. That’s why, though I was reluctant, I abandoned this impeller at the time.”

Zhong Jianshe now resembled a child showing off a treasure, eagerly presenting the design he’d once discarded, and now awaiting Yang Hui’s praise.

Yang Hui, however, had no time for compliments. He was delighted with this obviously over-performing impeller design and was already plotting how to use it. Such a high compression efficiency couldn’t be wasted. In the future, they could convert this turbojet into a pseudo-turbofan. How? That would take some explaining.

At this point, it was necessary to mention a famous engine: the General Electric F404 turbofan. Yes, it’s widely advertised as a turbofan, but careful observers have noticed its bypass ratio is only 0.34. In fact, the original version, fitted on the YF-17 fighter, had a bypass ratio of just 0.2—the outer bypass airflow was solely for cooling the engine casing and didn’t participate in combustion. Yet GE still called it a turbofan. In reality, it would be more accurate to call it a “leaky turbojet.”

Take, for example, Pratt & Whitney’s PW1120: they still call theirs a turbojet. In fact, the PW1120 is a turbojet with continuous bleed from the low-pressure compressor. See the pattern? Study it carefully: continuous bleed from the low-pressure compressor. Realize what that means? It’s the same principle as the F404—a leaky turbojet.

So now it became clear how to utilize this ultra-high compression ratio impeller: by installing a bleed valve on the casing behind the centrifugal impeller, this miniature turbojet could be rebranded, like the F404, as a turbofan—a leaky turbojet. The thought of a model aircraft using a turbofan engine was exhilarating. In future generations, hardly anyone would attempt this; the technical requirements were simply too high. Yet, by happy accident, they now had such a powerful impeller in hand—truly a blessing in disguise.

However, even though this impeller could be used to make a leaky turbojet (or, as GE would say, a turbofan), Yang Hui decided not to do so, for the usual reasons: there was neither time to redesign nor the need to move beyond technical reserves.

So he focused on disguising the impeller’s true capabilities—by further simplifying the design to make manufacturing even easier. Although it was already a simple impeller, simpler would be even better.

“Jianshe, based on the requirements of this combustion chamber, bring the compression efficiency down, and make the impeller even simpler...”