There are a couple of reasons it’s not feasible to replace a large percentage of a turbine engine’s weight by using carbon fiber parts.
First, let’s talk about the heaviest and thickest parts in most turbine engines; prime targets for replacement if the goal is weight reduction.
Here’s a pretty good diagram:
The thickest and heaviest parts are found in the “hot section;” specifically the “turbine section.” The thickest and heaviest parts are usually the first stage stator and first stage turbine wheel. These parts are usually some kind of super high temperature resistant Nickel-Cobalt-Steel alloy.
You have to understand how unimaginably hot some parts of a turbine engine get. The hottest parts (in the combustion chamber) can exceed ~4,000 degrees F (~2200 C). That’s hotter than molten lava, and approaches the temperatures at which some metals are welded (the exact max temp in the engine varies based on the specific engine model).
I’m not intimately familiar with the various formulations of carbon fiber, but most of the epoxys and resins used to make carbon fiber likely cannot get anywhere near these temperatures without instantly being reduced to the strength of an over-cooked ramen noodle.
Even the metal alloys used in this part of the engine cannot withstand these temperatures. These parts are so thick and heavy because we know they will slowly erode away during normal engine operations. That’s why engines are serviced on a strict schedule. After X hours of operation the engine is taken off the plane and sent in for service. Even though the engine is working perfectly when it’s removed we know the turbine section is becoming critically worn-out just because it’s been running for that much time.
Clearly, “hot section” parts are not viable candidates for replacement. So what about parts in the “cold section”? Well, that brings us to our second problem… Balance.
Here’s a zoomed in copy of the same diagram:
The main turbine shaft at the center of the image is supported in just 2 places; these are the main engine bearings. In this case, they are located near the start and end of the combustion chamber. The exact bearing support locations will vary by engine model, but the same general concept applies to most turbine engine designs. There are good reasons for this kind of design, but that would be a whole answer by itself…
With this kind of design the weight of the turbine section must be balanced by the weight of the cold section (specifically the weight of the compressor section). If the sections are even slightly unbalanced across these bearings it will cause vibrations and/or uneven wear. Eventually, this kind of problem will cause the engine to fail catastrophically.
If we start making the compressor section significantly lighter then we have to make the turbine section lighter to maintain this critical balance. However, making the turbine section lighter really isn’t an option because those extra thick and heavy parts are absolutely critical to withstanding the extreme temperatures created in the combustion chamber.
There absolutely are some limited areas where composite materials could be (and are) used on modern engine designs like the shrouds and cowling (parts of the engine that don’t spin and don’t directly impact the balance) or even the forward most fan blades.
However, you’re not going to get the kinds of “significant increases in agility and speed” you’re thinking of because the heaviest parts of the engine cannot be improved this way.
In addition, commercial aircraft designers (like Boeing and Airbus) would not create a commercial aircraft with “significant increases in agility and speed.” That’s because fuel is an airlines largest operating cost. So when airlines buy new aircraft they look for increased fuel efficiency at the current performance levels; they are not interested in aircraft with increased performance at the current fuel efficiency levels.
Author – Rajan Bhavnani (R&O Engineer)
