Why do turbine (jet) engines cost so much more than piston engines?

Factual Questions
1

In some recent(ish) threads discussing general aviation planes it was noted that twin-piston engine planes have some notable drawbacks and that a single turbine (jet…could be a turboprop too which is a jet with a propeller attached) are much more reliable and powerful.

Which makes one wonder why you would ever get a twin piston in lieu of a turbine and the answer seems to be cost. The turbine engine costs a LOT more. Easily many tens of thousands more and it just keep going up.

My question is why are turbines so vastly more expensive?

My understanding is turbine engines are (comparatively) pretty simple engines. That is one of their benefits. By being simple and having few moving parts they are very reliable compared to a piston engine which has lots of moving parts (and thus more things likely to break).

Also, turbines are an old engine design. Not as old as pistons but well, well into a mature technology. A very common general aviation turbine engine is the Pratt & Whitney PT6 which is a 60 year old design. The company has certainly refined the engine over time and there are variants but the basic engine is older than I am. Also, they have sold a lot of them (51,000+) so this is not a handful of engines a year. The R&D and tooling and all that stuff has long since been paid off.

Conversely I see nothing in a piston engine that would make someone think it should clearly be cheaper because it is cheaper or easier to build than a turbine.

I am not saying these engines are “cheap” to build. They are complex machines built to incredibly exacting standards but all things are relative and I do not see why a turbine should cost more beyond “because people will pay for it”.

What am I missing?

2

I’m not a pilot myself, so I could be completely wrong, but I’ll venture a guess and say that it could be simple economies of scale. Pilots start, of course, flying piston when they get their PPL, and transitioning to jet requires lots of additional training. Most private pilots never make that step. So the market for piston engines is much bigger than that for jet engines, and that drives down per-unit costs for piston.

3

I’ll hazard a guess:

First, the blades for the turbine and compressor are expensive by themselves. This is particularly true for the turbine blades, which have to tolerate considerable heat during the normal course of operation. They use exotic manufacturing techniques and exotic metallurgy/ceramics to achieve the desired performance characteristics with acceptable weight. Here’s a short video that shows the steps for compressor blade fabrication:

Contrast with piston fabrication for a reciprocating engine: chuck up some aluminum in a lathe, turn to a piston-shape, bore a wrist pin hole, done.

Second, there are a lot of these blades in a gas turbine engine. I mean really a lot. Hundreds of them.
agentjayz services gas turbine engines and posts a lot of videos on YouTube. Here’s one where he shows the innards of the compressor section from a gas turbine engine, both the stator and the rotor:

That was just the compressor section; the turbine section is more of the same.

From what I’ve seen, each blade is selected and installed by hand. The rotor spins pretty damn fast, so mass balance is critical : each blade has a documented weight, and each blade site on the rotor wants a blade with a particular range of weight, so it’s on the assembler to get these matched up.

And of course the last piece of the puzzle is the marketplace. A gas turbine engine is more reliable and has better power-to-weight ratio, so it provides more value, which means the manufacturer can demand more money for it.

TL,DR: a gas turbine engine has a lot of very expensive parts and a lot of assembly labor, and many (but not all) customers are willing to pay for all of that.

4

Not to mention that some turbines (but not all) have super space-agey manufacturing involved like this, while piston engines are still just a matter of machining.

https://www.americanscientist.org/article/each-blade-a-single-crystal

5

It is the great precision to which parts must be manufactured and machined, and the materials they must be made of to withstand the enormous stresses and high temperatures. Piston engines can continue to run inefficiently with some problems, turbines fail rapidly, completely, and often destructively if there is any sort of problem at all.

There would be some economy of scale if there was more demand for small turbines, but they are actually no more efficient than piston engines, probably less so by a few tenths of a percent. For the high maintenance costs involved they are unlikely to replace small piston engines.

6

That is true but misleading. Piston engines max out a few 1000 RPM while gas turbines spin at a few 10,000 RPM range.

Centrifugal (or centripetal) forces generated, scale with the square of the RPM, hence the need for precision and balancing.

Also the turbine maybe simple, but the compressor is not. A turbine is useless without a compressor.

A single set of blades in a compressor does a pressure ratio of about 1.5, So you will need 5 such sets to go from 15 psi absolute (atmospheric pressure) to 90 psi absolute (75 psig). And you want these blades to be super efficient or you will just heat the air up.

7

Another complicating feature of gas turbine engines is variable-pitch stator vanes, as seen in this video. FF to about 1:20 to see how the mechanism moves:

Note that several stages of stators are variable-pitch on this engine, with dozens of moving parts on each stage, all requiring tedious hand assembly.

8

Surely a part of the engine but that’s not the gas turbine but the air compressor.

9

“gas turbine” refers the overall type of engine. It’s the conceptual equivalent to “internal combustion engine”.

A gas turbine engine consist of several major subsystems. One of which is called the “turbine” (no “gas”). Another subsystem is called the “compressor” (no “air”). These subsystems are also called sections, as in “compressor section” or “turbine section”.

You’re correct that @Machine_Elf’s picture is part of a compressor which in turn is part of a gas turbine.

The overlapping use of “turbine” to refer to the nature of the engine and to refer to a key part of it is akin to how “internal combustion engines” are also often called “piston engines”, despite the fact a piston is just one component of the larger whole.

10

You are correct that the video only shows the compressor section. It’s visible as the rightmost portion of the entire T-58 turboshaft engine in this photo.

Coincidentally, the sign on that engine highlights the massive performance advantage (and therefore value to the customer) that a gas turbine engine provides. The T-58 engine weighs just 250 pounds, but delivers 1000 shaft horsepower. For comparison, I looked up the weight of the piston engine from a Bugatti Veyron, which makes about a 1000 horsepower; it (the engine) weighs about 1000 pounds. There’s probably a lot of room for weight reduction if you’re trying to build a 1000-hosrepower piston engine for a plane, but no way you’re getting down to anywhere near 250 pounds. For reference, the Lycoming O-540 (an actual aircraft engine) weights 438 pounds and only makes 300 horsepower. You’d have to put three of them on your plane to get the same power as the T-58, and means you’d be lugging around an extra 1064 pounds.

Note also that the T-58 is 65-year-old design; I’d guess there are modern gas turbine engines out there with an even better power-to-weight ratio.

11

Back to the OP:

Gas turbines are conceptually simple. And the early inefficient unreliable research devices of early WWII were pretty simple; certainly simpler than the highly developed V-12s, V-16s, or 18-, 21-, & 27-cylinder radial ICEs of the time.

But the devil is in the details. Nowadays turbine engines are some of the most highly stressed machinery mankind knows how to build outside of rocket engines. Insane temps, forces, and tolerances are the base price of admission to play the game. Times massive reliability and high thermodynamic efficiency. That means rather exotic materials and exotic fabrication methods.

All that costs serious coin. The original PT-6 is indeed 60 years old. There’s not very many parts in common between the original and the latest. There’s an obvious family relationship that goes beyond just the paperwork. But the innards are all new.

Switching from cost alone to your implied larger question of why aren’t GTs ruling the roost for smallish low speed airplanes …

A problem with GTs in general is they’re most efficient running at full speed = RPM = power output. They become vastly less efficient at lower RPM. For moving a load like a car or a train, it takes a lot more power to accelerate than it does to cruise. So a GT engine strong enough to do the accelerating from a standstill would be loafing inefficiently once you got the vehicle to cruise speed.

In the case of jet aircraft, we solve that by climbing high. Essentially you leave the power at full bore and use the excess thrust to accelerate first to a good speed then take the excess and use it to climb a hill just steep enough to hold a speed that’s efficient for the airplane while all the excess power created at the optimally efficient engine setting is “used up” to climb. As you climb the atmosphere gets thinner, engine power drops off, and drag declines. The drag decline means the airplane can can go faster with less power. Eventually you get so high that the reduced engine power is just enough to let the airplane fly at the aerodynamically efficient speed. You’re still running the engine flat out or nearly so. It’s a virtuous circle that means you’re always staying in the efficient part of the engine’s (and airplane’s) envelope.

Ideally you cruise that way until just the right distance from landing, then pull the power to idle and coast all the way to touchdown. Cashing in all the potential energy you stored in the climb. Even better if you could turn the engine(s) off completely until needed.

If for whatever reason your aircraft can’t climb high (e.g. unpressurized cabin, short-range mission, inherently low-altitude mission, etc.), you can’t wring nearly the gate-to-gate efficiency out of a GT engine.

Turboprops are a little different story because the prop gets into the mix on the engine side and Mach effects aren’t as significant on the airframe side, but it’s still the case that being able to climb enough to run down the engine’s flat-out power production to your desired cruise value is key to extracting efficiency.

That mission profile just doesn’t work with lightplanes. Which is why we now see things like turbo diesels or two-strokes as a way to get away from much of the clattering reciprocating machinery inherent in a conventional ICE without buying into the exotic materials & tolerances of GT, nor their difficult-to-use efficiency curve.

12

I got interested and poked around online to understand more about how special and exciting jet engine performance is. Here are some operating parameters, mostly from airliners. Jet engines begin with a compressor that might compress more than a ton of air per second by a factor of 40. The blades rotate at 1000 mph. Then a combustion chamber operates at 2000 C, 700 degrees hotter than the melting temperature of some of the metal components. Its effective exhaust velocity may be over 257,000 miles per hour. The engine develops up to 115,000 pounds of thrust, averaging more than 100,000 hours between in-flight shutdowns. Apparently some miniature jet engines run as fast as 100,000 rpm and small helicopter engine compressors around 50,000.

Conceptual simplicity is great, but just imagine the material science required to pull this off.

13

What they are omitting, perhaps deliberately, is the emissions. Higher temperatures also means higher NOx (Nitrogen Oxide) numbers.

There’s been a lot of effort on power generation gas turbines to reduce their NOx (Nitrogen Oxides). NOx in the single PPM digits has become the norm and the technology is pretty nifty to achieve that.

Airplane engines has somehow enjoyed exceptions in the past, but they are catching on.

14

To be fair you could do a similar comparison between the piston engines in the Bugatti Veyron and the engine in a Russian Lada Riva.

The tolerances and materials needed and technology used in the Veyron are light years beyond what is used in a Riva (with a price light years more expensive to match).

Certainly as manufacturers seek to improve their engines they continuously push the boundaries of what is possible at the time. Single-crystal turbine blades are a neat technology but they are a relatively new development and certainly turbines were built without that for ages.

Hell, you can build a jet engine in your garage with hand fabricated parts (done in your garage). I totally get that such an engine is in no way a fair comparison to modern turbines we see on jets today. I merely point out that you can do jet engines without access to hyper-space-age-tech. Same as you can do with piston engines.

15

I enjoy occasionally watching the YouTube channel Missionary Bush Pilot. He flies around Papua New Guinea flying into remote small runways helping to supply very rural communities that are almost unreachable by other means.

His company flies a fleet of Kodiak airplanes (think Cessna Caravan and you’d be 99% on the money). Almost all of his flying is at low altitude (below 10,000 feet) but the plane is a turboprop. Wouldn’t an ICE be better in this case?

Which is to say…I am certain the designers had good reasons for building the plane as they did and the buyers felt it was the best choice for their mission profile so I’m wondering where the tradeoff is here. Maybe having a more reliable engine when flying into remote strips regularly is worth some inefficiency. Or something else. Just curious what the thinking is.

16

Have you got a source? Some of these claims are suspect. I’m not aware of anything on earth that moves at 257,000 MPH. Nor am I aware of any engine that develops 115,000 pounds of thrust; the biggest ones I know of are on the A-380 and can make 84,000 pounds.

They do however spin pretty fast. The smaller, the faster, as centrifugal loading scales with radius. Miniature jet engines are on the same size scale as automotive turbochargers, which do indeed hit speeds in the 100,000-RPM range.

The problem is due to two factors: elevated temperatures combined with excess oxygen. Gas turbine engines have both; they must burn lean to keep turbine temperatures within reason. Good engineering helps, but chemistry imposes limits. Stationary gas turbine engines (for power generation) can be fitted with exhaust aftertreatment to reduce tailpipe NOx (same as what’s done to a car or truck), but that’s just not feasible on an aircraft.

A piston engine might have better efficiency as LSLGuy pointed out, but (speculating) maybe they wanted the turboprop for its other advantages, e.g. power and reliability?

17

Here’s my fun fact about jet engines:

the blades are all loose. Seriously, they’re floppy. And it’s deliberate.

18

That’s old technology and hardly used nowadays. Technologies like that fall in the post formation treatment category and I’ve seen maybe a handful of them in the last 20 years.

More popular are the pre formation control technologies where the fuel is burnt in stages (DLE) or water/steam injection. DLE has been steadily gaining popularity since the 2000s.

19

GE9X is supposed to be rated for 110,000 lbf thrust. It hit 134,000 in a test run, setting a Guinness record, per the wiki. It’ll push the 777X around.

The power of these engines is just silly. As is their reliability, to anyone comparing them with, say, the maintenance schedule on a run of the mill Lycoming.

20

The 257,000 mph comes from the table here for a General Electric CF6; I converted m/s into mph:

A thrust of 127,900 is generated by a GE90 here:

Not sure where I found the other one but this is even higher.