Jet engines are some of the most complex machines ever created. They have to be as light, efficient, safe, and reliable as possible. There's a reason that most new airliners recently have been affected by delays from engine manufacturers. This is a hard balance to engineer when on a schedule and budget.
Jet engines could certainly be cheaper to develop and buy. You can get them at relatively "affordable" prices for remote controlled aircraft. But cost certainly increases with scale, and an aircraft owner expects an engine to run for thousands of hours with minimal maintenance while burning as little fuel as possible and not hurting anyone. Each new generation of engines has been more efficient than the last, and those improvements do not come for free.
If it's just designing a jet engine, then raw materials cannot
possibly be a major factor even if it's something like titanium or
It's not just the raw materials, but the processing involved. Modern engines push materials to their limits and beyond. Advanced manufacturing technologies have to be developed.
Let's say you have a new material or process you want to use. It can easily take at least hundreds of thousands of dollars just to develop one, and a new engine could include many of these. Even for a cheap raw material, the amount of labor required to create test articles, set up tests, run them, and document the results grows very quickly. You want to be sure you understand how the new material or process will work before moving forward with it. If things go wrong, you create big problems for your customers (aircraft manufacturers, and their customers).
How many prototypes could you possibly need? I mean I hope it's not
all trial and error.
"Trial and error" is sometimes also called "science" which is what you need to develop new technologies. Obviously as testing progresses and the risks increase you'd like the "error" part to keep decreasing. But the trial part is very important for understanding how things will actually work (or not). This means not just full scale prototypes (which will go through several design iterations, even through airplane certification) but also subsystems and components. And you need to do enough tests to have statistical confidence that the results can be reliably reproduced.
The other thing is computer software, which I thought would make
things easier and cheaper to design.
This is certainly true and these technologies have decreased the amount of physical testing that has to be done. But either way it's going to cost you money.
With products like jet engines, better tools does not generally mean "how cheap can we make this process" but "how much more performance can we get for the same money."
So what makes it so expensive? Is there some super costly
Yes. People like to fly on planes with engines that keep working and don't explode. This means rigorous regulations and certification. For the FAA, 14 CFR Part 33 covers the certification requirements for jet engines, to try to make failure events as rare as possible. Here are just some of the tests required by regulations:
- Full operating range
- System and component tests
- Rotor lock
- Full teardown
- Blade containment/rotor unbalance
- Rain, hail, and bird ingestion
Some of these tests are going to be destructive, either by design or by accident. Some of them are going to take a lot of time and effort. Just the paperwork involved with understanding all these requirements and documenting to the regulators that you've met them could easily take a good chunk of your 100 people.
Maybe someone can explain the general process of jet engine design in
the first place because I'm sure that would be helpful. They way I
imagine it, you just go through stage by stage and try to get each
blade shape and diameter right.
It sounds like you have the basic idea. But engineering is about the devil in the details.
First, modern engines could have 20 or more stages, attached to 2 or 3 separate spools. The engineers have to decide the optimum number of stages and spools for the engine design. This means analyzing many different configurations, complexity tends to increase exponentially, as each stage affects the rest of the system.
Yes, the process is relatively simple if you're given static conditions to analyze. Of course it's important to optimize fuel consumption at cruise. But the engine still has to operate across a huge range of conditions. Then there are the dynamic conditions of acceleration and deceleration. The engine has to start and be stable in both crosswinds and tailwinds. It has to be able to start on the ground or in the air after getting extremely cold. Weird things can happen as things expand and contract with temperature.
If you're looking at simple analysis of how pressure and temperature change through a jet engine, there's probably a lot of hand waving about a stage called the "combustor" where you magically get an increase in temperature. The process of burning the fuel in the extreme conditions of a jet engine is extremely complex. The air rushing in the front has to be compressed, then slow down enough to not extinguish the flame. The flame has to be contained in the combustor section throughout operation, and not overheat the turbine stages behind it.
Higher temperatures and pressures provide better efficiency but materials are pushed to their limits. New superalloys and manufacturing techniques have to be perfected to create materials able to withstand extreme temperatures while spinning at thousands of RPM. They have to put small holes and passages into the blades to force out cooling air that covers the surface of the blade so it doesn't directly contact the extremely hot air in the turbine.
Then you also have mechanical energy being extracted by a generator, and pneumatic energy being extracted for the aircraft's bleed air system. The engine has to be able to cope with varying demands of these systems.
There's also the problem of various spools rotating and thousands of RPM and not causing too much friction heat or prematurely wearing out. Engineers need to understand the temperatures, aerodynamics, and rotational stress on each part, through the whole operating range of the engine, and how it affects the rest of the engine.
And it's not just enough to get something that works. Someone will always be asking the question, "How can we make this more efficient?" Modern engines are pulling many different tricks to squeeze out every bit of efficiency that they can. Air is bled off and vanes can be adjusted to make the engine stable in all operating conditions. New concepts and technologies are developed. Modern turbofans have the problem of a low pressure turbine in the back that needs to spin as fast as possible to be efficient connected to a fan in the front that needs to spin much slower to be efficient. For the Pratt & Whitney example you give, their solution was a gearbox to allow the two to turn at different speeds. This was a very difficult challenge that took them decades to finally get into an end product.
All of this complexity has to be managed by software that monitors an array of sensors throughout the engine and continually adjusts the many parameters to maintain stable and efficient operation. This software has to run on computers that will operate across a huge range of temperatures and under constant vibration.
You also have to keep in mind how all of these thousands of parts will be manufactured and then assembled, and then maintained through the life of the engine. You need people planning to ensure that a mechanic will have access to the right components with the tools they need, and what processes have to be followed to assemble and disassemble the various parts.
Then there are also collateral effects like noise and pollution. There will be engineers tasked with understanding how these are generated and how they can be reduced to acceptable levels with as little cost as possible.
This is just an overview of the many areas involved in designing a jet engine. There are certainly more, and each detail here could easily require a specialized team working on it.