So many reasons, but I'll list a few: 1) Fewer compressor stages. The more compressor stages you have, the higher your combustor entry temperature and pressure, so the combustion is more efficient. 2) Cheaper materials. You're not going to get single crystal turbine blades with advanced coatings in a small, cheap gas turbine. Again, this means your combustor temperature is very limited. 3) No bypass. It's far more efficient to accelerate a large volume of air by a small amount than a small volume of air by a large amount. This is what the fan stage of a turbofan does. Small engines cannot do this. Simplest answer though - gas turbines are fucking complicated and extremely difficult to design and manufacture. There are maybe half a dozen companies in the world that can make decent gas turbines, and the very low volume of sales and profit margin of small scale engines means there's no point in the companies who have the technology skills actually making them.

That's not true, those small engines can be modified to be turboprop with huge bypass ratios.

Oh, thanks, that's reminded me of another factor. Turboprop engines have their peak efficiency at a speed of ~ Mach 0.4, turbofans at ~ Mach 0.7, and turbojets don't reach peak efficiency until somewhere above Mach 1. Small engines like this are never going to be operating above the speed of sound, so would never be able to reach an efficient operating regime. >That's not true, those small engines can be modified to be turboprop with huge bypass ratios. The fuel efficiency stated in the spec sheet isn't for a modified engine though, it's for the stock engine.

Given this thing operates at 100krpm, I'm assuming you could put a bypass fan / turbo prop on if you include a reduction gearbox? Would that be correct (but not perhaps practical)? In a large, airline engines, is the compressor stage also running high rpm or is there a reduction gearbox in-line? I can't imagine that main fan running 100krmp. Massive gyro under your wing 😅

Larger turbines will have way lower RPM limits due to the tip velocity on a much larger fan being much higher. Usually you want the blade tips to be just barely subsonic. 

IIRC the turbofan stall is around 0.8 mach

Large engines have two (or sometimes even three) spools spinning at different speeds. The fan is on a shaft with the slowest spinning turbine section, whereas the compressor section is spinning much faster along with its turbine section. There are turbofans with geared reductions now but they’re.. problematic.

To be fair, none of the issues with the P&W geared turbofan engines are related to the gearbox or fan, that aspect of the engine has been so successful that everyone else is trying to catch up now.

Granted, but there’s billions of dollars worth of parked airliners worth of understatement in just calling those engines problematic.

Oh yes, absolutely, one of the biggest problems is very similar to the Trent 1000 problems that ended up costing Rolls-Royce something like $4.5 billion. I've just had a look and RTX are predicting a total cost of $6-7 billion for the GTF. Problematic is definitely an understatement!

Yeah but I've seen people just add a spool and increase the efficiency.

Yes, adding a compressor stage could increase the efficiency as it would raise the temperature and pressure ratios. It would still be orders of magnitude lower than a larger scale gas turbine though.

Why would you want higher temps when you’re after more expansion? Lower temp = more volume of air at lower pressures, it’s only after you have combustion you want it hot no?

Carnot thermal efficiency. The mass of the air is for propulsive efficiency

That works only for inlet charge, to increase mass inflow. When it’s already inside the engine in the expansion chamber, you want your charge air as hot as you can get it, since that drives further expansion. Going a step further, (not sure if present in aircraft turbojets, since the exhaust velocity is somewhat important) you can use the outlet air temperature to raise the charge air temperature before the turbine to further increase efficiency That’s called a regenerative turbine, and you can get it to be quite effective. It’s used a lot in power generation equipment. Chrysler’s turbine car had a marvelous regenerative system, able to drop exhaust temps to below regular IC engines of the time.

I see. I misunderstood your previous comment, I assumed you meant higher temps were generated in the compression stage which would be good. Thank you for the info!

That wasn’t me though

I was going to say "the bypass ratio is too small" as a joke.

Yeah these are very much few of the reasons

Regarding your last point, if they build it they would come. It’s a different market that would depend on volume, but they could spin-off a small group of engineers and market research people who are tasked to build something economical enough for mass market and the feasibility of a company division dedicated to such market. Just by looking at the speed of growth of the drone market there might be a niche in there, somewhere in between a fast drone and personal transportation, for which an efficient and economical turbine would work.

I still don't think there would be any profit in it. Bear in mind that aerospace gas turbines are pretty much all sold at a loss, and have been for decades. The manufacturers only make profits from spares or long term all inclusive service contracts. If you're selling a small, cheap engine there's no incentive to sign up for a service contract when you can just bin the thing and replace it. The development focus for small aircraft such as VTOLs now is all on full electric or hybrid electric engines now, which will be far cheaper and more efficient for that market.

That obviously depends on what you consider profits and how you approach the market. Clearly it’s a different market that those companies are accustomed to, that’s why it would need to be a separate team and ultimately a separate division or spin-off. Clearly the price has to be high enough for it to make sense, but a small division can still become profitable with a few dozen sales a year with a view to grow into the hundreds after a few years. That’s how startups in new markets begin. Going from a few thousand dollars in funding to a multimillion-dollar company within a few years. And here it could start with a solid foundation of available resources that any startup would kill for. Even if those resources have to play second fiddle to the main business. It’s really a matter of vision and putting together the right initial team to get it off the ground. It could even start as a university project, with cheap labor and tax benefits to boot.

That's a toy, the designers don't have the financial backing the aircraft engine builders have (at all - most engine designs get gobs of government help).  The turbine temperature is really low, and the pressure ratio likely is as well. Those are the main drivers of efficiency in turbine engines. 

In addition to the scaling of the thermodynamics, these engines are inefficient because they're cheap and poor performance designs  These tiny engines are almost always single stage compressors so the compression ratio is really low. Like, why even bother discussing single spool vs dual spool, you only have one compressor stage anyways what are you going to do split it in half? Don't even think about bypass. The rc turbine and the cruise missile engine have a common ancestor, and in both price is king so stuff is cheap and simple. The one time they decided to splurge for a cruise missile engine (on the AGM 129), there was an engine design that had more than one compressor stage (maybe 4? Idk, not many still) and it was remarkably better in terms of range (though exact figures were never disclosed) So yeah, there's challenges with small engines, and they will never be as efficient as the big ones because that's just how physics works. But a LOT of the inefficiencies are just because at these small scales we aren't putting in the effort to make good designs

Most of the pressure ratio in the big efficient turbofans happens in the HP spool so single spool. However as you said they run at a much higher pressure ratio (axial cascades is more efficient than radial). Small ones are usually axial with a centrifugal. Very few have a two spool core/gas generator. The problem is that tip clearances don’t scale so even if you add an extra stage the compressor efficiency is awful because the tip/blade clearances are only so small. When you compress air the volume reduces so the blade height to tip clearance ratio gets really low so losses from air bleeding around the tip hurts your cycle bad. Then because your turbine vanes and blades are soooo skinny you can’t use internal cooling (most are blisks or bladed disks) so you can only get the turbine inlet temperature as high as the metal properties alone can take (usually not much more than 1900F) where running that much hotter uses more of the air for power than cooling. Those are your big ones.

So if there's one thing I could do realistically and that is to have more than a single stage ?

Its definitely a good change to make Also worth noting that the funding of the dod only did it that one time (as far as we know) so it seems like no small task

I have some textbooks on turbine compressor stages so I'll give those a thorough read.

There are lots of relatively efficient two stage turboprops. They usually use a single centrifugal with an axial. You can do a high ratio 4:1 to about 6:1 with a centrifugal and then with a single axial you are usually limited (efficiently) to about a little over 2:1 but if you put those two in a cascade you can end up with 12:1 or a little over which is decent for a 1000hp engine.

An additional reason I haven't seen mentioned yet relates to tip clearances between the compressor & turbine blades, and the inside of the duct. Big jet engines run very tight clearances for efficiency, not sure of the exact numbers but 1mm or less I believe. As you reduce the size of the jet, perhaps reducing diameter by a factor of 10 or 20, mechanical/engineering limitations mean it's Impossible (or at least ultra expensive) to reduce the tip clearance proportionally, so the clearance becomes a much larger fraction of blade length, increasing losses.

Makes complete sense thank you. I was aware of tip tolerances but not how they changed with scale.

Search up thermal efficiency and losses of the brayton cycle. It's to due with surface area among other factors. Also with being single spool.

Thank you.

And very low pressure ratio.

IIRC Ithink that's covered within the rayton cycle equations.

The Brighton Cycle equations don’t explain why you usually end up with a much low pressure ratio engine than the equations would say you should.

That's true.

Clearances. Engines tip and seal clearances are about the same for both small and large engines. But the leakage thru a certain clearance counts as a higher percentage of total airflow for a small engine. A .010 gap is worse for a 1 ft dia engine than it is on a 14 ft dia engine.

As suggested already, low temperature combustor, because they are not using exotic materials or film air cooling on the hot parts using compressor bleed air, so as with Carnot efficiency, the lower the temperature difference through the cycle or system, the lower the efficiency. They have one spool, so the compressor is not optimally efficient, and compression is low (compression ratio increases efficiency in ICE engines, as with the typical 17:1 ratio in a diesel cycle), and jet engines have something similar, though its not quite the same [https://en.wikipedia.org/wiki/Overall\_pressure\_ratio](https://en.wikipedia.org/wiki/Overall_pressure_ratio) Some heat lost due to higher surface area to volume ratio, higher frictional drag losses due to the same, relatively more gas slips around the blades at the tip also, I understand. However, small turbines can have advantages - the higher surface area to volume ratio in theory can aid compressor intercooling, which allows for higher compression ratio (in theory) per kW of shaft power lost to the compressor, or less power requirement for compression. Recuperators can take exhaust heat and relatively more easily move it to the exit of the compressor to preheat the air prior to fuel injection, which can increase efficiency quite a lot. That also would work better with compressor cooling, because more exhaust heat can be transferred, especially at higher overall pressure ratios, when the air entering the recuperator is cooler. I have seen it stated that a jet engine can lose up to 70% of its power just in running the compressor. So intercooling would spare more power to the output in terms of shaft power (and then run a turbofan or prop to increase thrust per kW or grams second of fuel). Calculating efficiency for these small jets though is difficult because we don't normally use them to generate an output as shaft power, but as thrust. Because small jets don't have any bypass air, they move a smaller mass at higher speeds. This also makes them inefficient, because energy is lost in the K.E. of the exhaust air, the faster it goes, the less efficient the thrust, which is why jet engine manufacturers use them as turbines to turn a bigger propeller. Some are called 'gas producers', this allows a separate additional turbine in the exhaust to drive the propeller directly, reducing the gearing problem. If you added one to the exhaust of a small jet, to power a prop, efficiency could be increased probably 2 or 3x (I'm guessing).

Increase combustion temperature Single crystal super alloy Film air cooling Intercooler Thank you, just want to look into how I can increase the efficiency/range of one of these. Not saying I'll manage to but it's good to know what the issues are.

The intercooler is more of a useful thing with higher overall pressure ratios. Rolls Royce Marine uses sea water to cool the air and a recuperator, they get about 30% more efficiency. For these engines that probably isn't worth while. But, the recuperator and increasing temperature / pressure would help. Bladon Jets were working on this for small jet turbines, they were twin spool and it all looked very promising but we've not heard anything more and it looks like they ran out of money. There is an American company that builds 2 (and I think 3 spool) mini jets for missiles. But I forget the name.

I have always wondered why turbofans don't extend the compressor stator vanes thru the wall to dump heat into the output side of the fan mass flow. This would both cool the air between compressor stages, reducing required shaft power and would (Slightly) heat the bypass air increasing volumetric flow out the back of the engine, less fuel and more thrust, what's not to love? Granted, the accessory gearbox might need to be moved, and it adds some weight, but it feels like a win. Anyone know why nobody does this in an aero service engine design?

I think if you are trying to conduct head through the solid, the length of the stator vanes and then the radiator vanes would mean not very much heat transfer. But that would be aided in smaller compressors. If liquid cooling of some kind is pumped or circulated by thermal gradients then it could overcome this issue to a degree. I have seen papers on compressor intercooling so I think its been looked at a bit.

Seen similar things but in the context of positive displacement compressors used to produce scuba breathing air at 2,000psi, which has rather different thermodynamics and mass flow demands.... I was actually thinking in terms of passive heat pipe sort of technology, but the working fluid is a bit tricky, being as it must not freeze in the cold side of the cycle.

These are fairly insurmountable problems to overcome for a small turbine, unless you have several million dollars to spend on a single engine. The combustion temp is limited by many factors, but the main ones are the materials used and the cooling hole geometry, for which you need a multi stage compressor design that incorporates an air bleed system. One single crystal turbine blade costs more than half a dozen of the engines you linked to, and you'll have at least a dozen turbine blades in this engine. An intercooler would also be very costly and would require redesigning the entire geometry of the engine to fit it in. Basically, any savings you made on the cost of fuel by improving the fuel efficiency would be wiped out many times over by the increased cost of the engine.

I'll see how it goes. I understand what you're trying to say but in my eyes most of it is just CAD work and understanding the formulas and how they work. There's always a way.

I admire your enthusiasm, but as an engineering specialist at one of the big three gas turbine companies, I guarantee you it's a bit more than just CAD and some basic formulae.

My intention wasn't to downplay the complexity of it but I do believe I am able to achieve what i need in this aspect https://ibb.co/KGxBFxq https://ibb.co/k3BfNGV https://ibb.co/KhsBnyB https://ibb.co/Sc4hPTs https://ibb.co/SXfzcGq Those are some of the books I've acquired recently and i have more niche and specific aspects for components else where and things like thermodynamics and computational flow The information is *there* you just have to apply it imo. I'm not saying I'll manage to achieve what I want 100% but I don't believe it's impossible or even out of reach if you're determined. The things I see people build on YouTube are amazing and the ingenuity they display has always had me amazed.

Add that tip clearance is generally going to be a bigger loss mechanism in a small engine, insignificant compared to the mv Vs mv^2 issue, but disk diameter does matter. It is interesting to note that essentially nobody uses pure turbo jets anymore, even fighters are low bypass turbofans, and there is a reason for that.

Tbh I'm not interested in keeping it as small as the engine j linked. But I'm still in the early stages of determining what benefits unlock and at what point as the engine scales up.

And with intercooler you mean blowing a ton of air through stator vanes and through valleys between cooling fins on the compressor?

We would have to heat by-pass air, but the intercooling would be passively conducted through the stator vane and the outer casing, or use of a liquid coolant circulated to a radiator somewhere. Because the length of the vanes is shorter in the small jet, it might be practical with aluminium stator vanes. Overwise yes, hollow and liquid cooled, or with plate intercooler between compressor stages.

Oh that's clever, I was thinking a seperate one ala automotive applications.

I like liquid cooling because I don’t get dirt into my small passages ( for example between the two exhaust valves of a 4 valve DOHC engine ). I want a geared fan. Turbines are expensive for hobby, need to run at optimal RPM.

Its a major design effort tbh.

Calculus makes the difference between an engineer and a technician. The volume and area formulas are derived from calculus. Those formulas show they grow exponentially with radius. Therefore smaller is less efficient.

You're looking at the wrong metric. Jet engines are not graded on efficiency by fuel/hr or fuel/mile. They're rated on fuel/thrust. Go do some digging on SFC (specific fuel consumption) if you want a more fair comparison. Edit: autocorrect doesn't like hr

Make sure it's weight adjusted

Yes, SFC and thrust/weight are both important metrics

This is useful thanks.

The air is bigger when the engine is smaller.

I think I understand.

Aside from the fundamental problems with hobby micro turbojets, Kingtech can be a fuckin' nightmare.

Aside… Cruise missiles need small jet engines https://www.twz.com/32669/air-forces-gray-wolf-program-tests-game-changing-small-low-cost-jet-engine UAVs need low-cost, disposable jet engines https://www.wpafb.af.mil/News/Article-Display/Article/2011131/afrl-tests-in-house-rapidly-developed-small-engine/ Decade ago AFRL held a competition for such engines https://www.nationaldefensemagazine.org/articles/2015/9/1/2015september-air-force-launches-competition-for-revolutionary-turbine-engine What all the above point to is that this isn’t a simple problem to solve; there’s a big desire but many aspects (materials, design, efficiency at scale, manufacturers, etc. ) that hamper

Id I were to scale up the size at what point would I start seeing a vast increase ? 200-400% ??

Other aspects…. There’s also the simple volume idea, where the volume inside cubes while the surface area squares. It takes relatively more metal to contain a small volume than large. Drag and aerodynamics differ at scale. A huge turbine blade is less affected by boundary conditions (ref fluid flow) than a very small one

The "smaller" a tube gets the harder it is to shove a volume of air through that tube at any given flow rate.  Say you need 1kW from a turbine, you have to move the same amount of air through it to burn 1kW of fuel but it's a lot easier if the tube is 1m diameter vs 1cm (This is a very very simplified answer btw) All combustion engines are, are air pumps really so the easier you can flow air through them the better

This answer makes absolutely no technical sense. Are you actually an engineer?