Why it's so hard to build a jet engine

I've always wanted to ask...

Why do turbines have a static duct and micron tolerances for the blades (and creep requirements) instead of a rotating (attached to the blades) duct that can be tensioned separately, and (presumably) no creep/micron tolerances?

Your comment is really interesting, but I didn't fully understand.

What do you mean by "metals don't actually withstand temperature"? As in the raw metal would melt were it not for the cooling vanes?

'If powered down, the engine would destroy itself' - from what? Overheating?

The lower power setting on shutdown does what? Spin it at a low RPM so it doesn't decrease in temp too quickly?

Many rocket engines, especially the reusable sort, require active cooling of the throttle and combustion chamber. A portion of the fuel is split into channels which run through the combustion chamber, throat, and the nozzle. Generally it is a close loop system, so the fuel makes back to be injected into the combustion chamber.

To get max performance modern engines run hot, aka ox rich, and the regen cooling is generally not enough. So in addition to that, critical surfaces such as nozzle also get protected by injecting a thin layer of fuel. This biases combustion to be fuel heavy in localized areas which is less hot. Of course all of this happens in an extremely dynamic environment where gasses are moving at 2km/s+.

In production?

I just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?

Now I'm thinking of the Koenigsegg Dark Matter, "an 800 hp, 1250 Nm patent-pending Raxial Flux e-motor". What if it was used as a generator? It's 39Kg, although 6 phase.

Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D

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I guess nobody cares about efficiency in their model car engine, so it doesn't matter if you need to refuel every 5-10 minutes. But that would be a problem for pretty much any other use case.

Does anyone know how the efficiency per liter of engine volume compares to these small turbine engines?

How long can these engines be ran at rated power before you have to overhaul or replace?

There's millions of radial turbines in cars around the world today. They use an internal-combustion engine for their combustor, and they're called turbo-chargers.

What about for “microgrids”? If it was possible for a household (or neighborhood) to install one and run completely on corn based ethanol… that might be something better than the IC generators we have today (I understand that corn ethanol isn’t completely green).

So it makes sense for Batman, but not for my next car. Got it.

A Merlin’s lifetime run-time, even with 25+ reuse launches, is just a hair over two hours (162 first stage time times 25 times two for the static burn). That’s assuming the high reuse stages keep all the engines even.

There are likely some compromises engineers can make when the engine is only running for that amount of time with refurbishment in between each 6 minute runtime.

Well, per wikipedia: "On September 28, 2023, Capstone Green Energy declared Chapter 11 bankruptcy"

I think he's talking about aerodynamic heating. Turbines compress air, and exhaust generates more thrust than resistance, so it's sort of obvious that compressor stages can be temperature limited where the airframe that hosts it is temperature constrained or something.

I'm not sure how it has to do with electric propulsion, though - I'd think systems like NERVA is a more exciting solution in this kind of domain(jk).

You need the electric equivalent of a glider tug plane to get you up to altitude. It can then return to base taking its drained batteries with it while you continue to your destination with fully charged batteries.

On the other hand, only some fraction of the energy inherent in jetfuel is converted to work. So fuel based airplanes have to carry a lot of "extra" energy that is then just wasted as heat.

Think of it this way, if I took 1lb of air on the ground and put it into a box that box would have sides of x. As I go up x gets bigger because pressure is dropping with altitude so to get the same mass of air I need a bigger box. When you burn fuel you need a ratio of fuel to air that is determined by mass, not volume so I need to take that really big box at altitude and squeeze it down a lot to get the same density as at sea-level (and then squeeze it even more to get the right mixture in the combustion section). The thing is though, 'hot air rises' so just squeezing down to 1 atmosphere of pressure air at altitude is way hotter than the air on the ground and then you squeeze it even more to get it to the density you need for the engine and it is -really- hot. Engines are generally torque limited on the ground and TIT limited at altitude because as they go up you are power limited by TIT (turbine inlet temp, or some other temp limit related to the engine) because of this compression. Designing engines that can handle that massive heat and that massive force is really hard, but electric has the huge benefit of just needing to produce torque so it is way easier to build and can keep producing power at much higher altitudes. There are definitely challenges there, but they are likely much easier than solving both the heat and torque problems that jet engines have.

The thinner the air, the more efficient your flight can be, but I never saw this as a temperature problem. My understanding is that there just isn't enough oxygen. Maybe there's an issue with the amount of heating that occurs when you try to compress enough air to get enough oxygen to run your engine?

In any case, electric engines don't need oxygen.

I think they would basically be just the fan bit of a turbofan (where they replace a turbofan). A turbofan generates some of its thrust from the fast, hot exhaust, which you wouldn't have in an electric fan engine.

Not sure about electrifying engines for slower planes, that currently use turboprops. Would that be an electric prop too?

Ducted fans are by nature less efficient than propellers. This is one reason that the next big leap in engine efficiency may come from what are essentially unducted fans.

They didn’t catch on for all the reasons you state:

* High RPMs are bad in a aero engine. There are very few (no?) propellers on GA airplanes which operate above 3k RPM, so you need a reduction gearbox. That cuts into your weight savings and also reduces reliability.

* Vibration isn’t a significant concern for GA airplanes.

* Throttle response is not a significant concern in GA-sized reciprocating engines.

* Poor efficiency is a major problem because 1 lb of extra fuel is 1 lb less payload. All airplanes are limited by takeoff weight. (The 3000 HP-class radials built by Wright and P&W at the end of WWII are some of the most efficient reciprocating engines ever built)

* Maintenance is a huge concern for GA owners because labor costs $200/hr. Airplanes with 1200 TBO engines sell for a noticeable discount to airplanes with 2000 TBO engines.

>and already have significant maintenance expectations.

Exactly. It's bad enough with conservatively engineered piston engines. Adding apex seals is gonna make it a whole lot worse.

https://youtube.com/watch?v=-fDM9Eb16Do&t=176s&pp=ygUcY29saW...

I like this one

We're comparing with jet engines, and those fairly well funded aerospace companies weren't in the league to attempt that kind of complexity.

Yes.

Russia lags far behind the US in producing spaceships for some decades. There are other things necessary for the society to build and maintain companies capable of going to the stars.

In a gas turbine, the phases of the thermodynamic cycle happen simultaneously in time, but in different places inside the turbine.

While a portion of air progresses through the turbine, it passes through the phases of the cycle.

During the first phase, the air passes through the compressor section of the turbine, where it is compressed adiabatically. During the second phase, fuel is added to the air and it burns, heating the air, which expands at an approximately constant pressure. During the third phase, the exhaust gases pass through the expander section of the turbine, being expanded adiabatically.

The last phase of the cycle, which closes the thermodynamic cycle, by reaching the ambient temperature and pressure, happens in the external atmosphere, for the exhaust gases. The meaning of this phase for an open-cycle engine is that its computation provides the value of the energy lost in the exhaust gases, which reduces the achievable efficiency.

This thermodynamic cycle, which approximates what happens in a gas turbine, is named by Americans the Brayton cycle, even if the historically-correct name is the Joule cycle.

(George B. Brayton has patented an engine using this cycle in 1872, without explaining it, but James Prescott Joule had published an article analyzing in great detail this cycle, “On the Air-Engine”, already in 1851, 21 years earlier. Moreover, already in 1859, a textbook by Rankine, “A Manual of the Steam Engine and other Prime Movers”, where all the thermodynamic cycles known at that time were discussed, attributed this cycle to Joule, 13 years before the Brayton patent. Not only the work of Joule happened much earlier than that of Brayton, but the publications of Joule and Rankine have been very important in the development of the industry of thermal engines, unlike the engines produced by Brayton, which had a very limited commercial success and which had a negligible contribution to the education of the engineers working in this domain. Therefore, the use of the term "Brayton cycle" does not appear to be based on any reason, except that Brayton was American and Joule British.)

And PBS is Czech, to name one.

Safran can not make a competitive commercial jet engine on their own and Rolls Royce is a generation behind both GE and P&W, and given the state of the UK not likely to catch up. Right now there is really P&W and GE and then everyone else.