I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class.
> The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. [0]
> A similar microturbine built by the Belgian Katholieke Universiteit Leuven has a rotor diameter of 20 mm and is expected to produce about 1,000 W (1.3 hp). [0]
Efficiency is not fantastic at these scales. But, imagine trying to get that amount of power from a different kind of thermodynamic engine with the same mass-volume budget. For certain scenarios, this tradeoff would be amazing. EV charging is something that comes to mind. If the generator is only 50lbs and fits within a lunch box, you could keep it in your car just like a spare tire. I think the efficiency can be compensated for when considering the benefits of distributed generation, cost & form factor.
One of the other advantages of the smaller engines is that you can use techniques that are wildly infeasible in larger engines. For example, Capstone uses a zero-friction air bearing in their solutions:
> Key to the Capstone design is its use of air bearings, which provides maintenance and fluid-free operation for the lifetime of the turbine and reduces the system to a single moving part. This also eliminates the need for any cooling or other secondary systems. [1]
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?
The physics of gas turbine engines is one reason I am really excited about electric aviation. People don't realize that you are temp limited at altitude. They think the air is cold, but it is about getting mass through that engine so compressing that air to the density needed brings its temp way up. Electric doesn't have that issue so electric engines could go much higher which means those aircraft could become much more efficient. People focus on the problem of putting enough energy into an electric airframe, but they don't realie the potential massive efficiency gains that it can bring because of the physics of flight.
They are not temperature constrained at altitude. It's much colder up there.
They are air, oxygen really, constrained.
You are right that the electric motors themselves won't suffer from the same oxygen starvation, but as the other commenter noted, the props or impeller blades will. They need something to push, there isn't much up there.
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).
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.
Duuuuuude, TIT is the temperature after combustion, not compression. Adiabatic compression isn't even close to the main contributor to TIT--heat input from burning fuel is. Also you may be confusing turbofans and turboshafts--helicopters have torque limits (not a helo guy, but my understanding is this is a gearbox or masthead structural limit rather than an engine limit), but if your turbofan can't hit RPM limits on the ground on a cool day you should seriously consider bringing it back for maintenance instead of going flying.
There are a lot of variations. I am most familiar with turbo props so shaft hp is the limiter on the ground and TIT is the limiter at altitude. TIT, of course, gets most of the heat from burning and I did mess up my explanation a bit. Sorry about that! You may be surprised how much of that TIT comes from compression though. The main point still stands, check the temp of that compressed air at sea level vs at altitude and for the same PSI out of the compressor you get a lot more heat at altitude. Either way though, the original point remains, electric has no TIT limit and you don't have to deal with 1000c materials spinning at 100k RPM so way simpler and easier to build something that keeps delivering thrust at extreme altitudes.
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?
Theory != Practice. If that were the only variable, then yes. Electric would be great. But it's not. It's far from the only thing in play. Lift also suffers from thinner air. Pure electric (as-in battery/solid state energy storage) could have 100% efficiency (specifically in converting prop/turbine torque to thrust of moving air), and it'd still have a terrible efficiency problem with current day tech.
Electric's primary efficiency and efficacy issue is regarding the total operating weight of the aircraft compounded by how that weight does not meaningfully decrease as the battery banks are depleted as compared to consumable fuels. Weight is your biggest enemy in flight, not power nor mechanical efficiency.
Hybrid electric (be it consumable fuel through a generator or fuel cells) is much more promising, but rarely what people mean when discussing "electric propulsion" (without the hybrid qualifier), and still has issues of it's own.
if you stay subsonic. I hear the U-2 has like 1-2kt of leeway when it is at its max altitude because if it went faster it would be supersonic but any slower and it would fall out of the sky.
Can you recommend a place to learn more about this? I have been curious about this topic but have struggled to find resources online describing the basic physics of electric flight propulsion.
Electric has the virtually insurmountable problem that they have to haul the entire weight of the batteries around even if they are drained. This is a MASSIVE loss as itliners can burn off over half their weight during the flight.
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.
Very much disagree. Air to air refueling is done in a very stable manner at cruise altitude. Takeoff is a much more dynamic flight regime where things can go very wrong very quickly.
Kinda crazy but might actually work for continental flights over cooperative areas. Parachute the empty batteries down with some minimal steering mechanism to land them at regularly spaced depots, then ship them back to airports fully charged.
Turnaround time for planes is short enough that you’d need to do a battery-swap rather than a battery-charge anyway.
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?
A very good article, but I was disappointed to see the misunderstanding about the de Havilland Comet failures repeated
> fatigue failures around its rectangular windows caused two crashes, resulting in it being withdrawn from service
While the accident investigation reports refer to "windows", which really doesn't help matters, the failure point was the ADF antenna mounting cutout. The passenger windows had rounded corners and did not fail in service.
The Comet was not withdrawn from service, they re-engineered and launched the Comet 4 (with oval windows, but that choice was to reduce manufacturing costs) in 1958, but the Boeing 707 was introduced that year and the DC-8 in 1959, ending the Comet's status as the only in-service jet airliner it held between 1952 and the grounding of the Comet 1 in 1954. The Comet 4 continued to fly in revenue service until at least the mid 1970s with lower-tier airlines.
The decision to bury the engines in the wings was one of the deciding factors for airlines - engines in nacelles are easier and cheaper to service and swap if required. Re-engining the Comet 4 to new more efficient turbofan engines the DC-8 and Boeing 707 introduced in 1960 and 1961 respectively required a new wing, but a podded engine was much easier to swap on to an existing airframe and this was done for many of the Boeing and Douglas aircraft.
The last Comet-derived aircraft - the Hawker Siddeley Nimrod - flew until 2011 in the RAF. They did look at upgrading them with new wings and avionics, but the plan was scrapped when they discovered that in the grand tradition of British engineering every fuselage was built slightly differently and they couldn't make replacement parts to a standard plan.
As i am sure the OP and GP know pprune has much of this, and concord related stories from a cohort of engineers and pilots who worked on these aircraft.
They did have a "best of" collection at one point, not sure now. Also a lot of flight test stories, ATC stories.
> Developing a new commercial aircraft is another example in this category, as is building a cheap, reusable rocket.
Cheap rockets can be vastly simpler than turbojet engines. Reusability (I'm talking about reusability of an orbital rocket, suborbital reusable rockets can be rather simple, as e.g. Armadillo Aerospace and Masten Space achievements show) adds a lot to the order, but increasing the size the square-cube law improves things to an extent.
One important point is missing from this: building a cheap and good engine is not enough, there are more companies and industries that can do this than it seems. But you also need the maintenance and logistics network, with a ton of professionals trained for your engine type in particular. And for that you need to penetrate the market that is already captured. This is what stopping the most.
What's beautiful to me is that that combustion turbines have the simplest possible thermodynamic cycle in theory (a steady input flow of X fluid/sec at pressure P, and a steady output flow of Y>X fluid/sec at pressure P), yet it turns out to be one of the most complex cycles to harness in practice!
> Building the understanding required to push jet engine capabilities forward takes time, effort, and expense.
This occurs in a broader cultural context. A society that dreams, enjoys science fiction, rewards hard study of advanced topics and so forth, can produce the work force to staff companies capable of going to the stars.
I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class.
> The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. [0]
> A similar microturbine built by the Belgian Katholieke Universiteit Leuven has a rotor diameter of 20 mm and is expected to produce about 1,000 W (1.3 hp). [0]
Efficiency is not fantastic at these scales. But, imagine trying to get that amount of power from a different kind of thermodynamic engine with the same mass-volume budget. For certain scenarios, this tradeoff would be amazing. EV charging is something that comes to mind. If the generator is only 50lbs and fits within a lunch box, you could keep it in your car just like a spare tire. I think the efficiency can be compensated for when considering the benefits of distributed generation, cost & form factor.
One of the other advantages of the smaller engines is that you can use techniques that are wildly infeasible in larger engines. For example, Capstone uses a zero-friction air bearing in their solutions:
> Key to the Capstone design is its use of air bearings, which provides maintenance and fluid-free operation for the lifetime of the turbine and reduces the system to a single moving part. This also eliminates the need for any cooling or other secondary systems. [1]
[0] https://en.wikipedia.org/wiki/Microturbine
[1] https://en.wikipedia.org/wiki/Capstone_Green_Energy
Tiny nitro RC engines can produce 1+ horsepower in engines that weight 1/2 lb.
How long can these engines be ran at rated power before you have to overhaul or replace?
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?
Funny, if you mouse over the graph of transistor costs, they become free in 2005! Cool!
Transistor manufacturers are like, "hey free transistors" then they bill the entire cost as shipping fees like the average aliexpress store.
The physics of gas turbine engines is one reason I am really excited about electric aviation. People don't realize that you are temp limited at altitude. They think the air is cold, but it is about getting mass through that engine so compressing that air to the density needed brings its temp way up. Electric doesn't have that issue so electric engines could go much higher which means those aircraft could become much more efficient. People focus on the problem of putting enough energy into an electric airframe, but they don't realie the potential massive efficiency gains that it can bring because of the physics of flight.
They are not temperature constrained at altitude. It's much colder up there.
They are air, oxygen really, constrained.
You are right that the electric motors themselves won't suffer from the same oxygen starvation, but as the other commenter noted, the props or impeller blades will. They need something to push, there isn't much up there.
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).
I am not clear about your description.
Electric propellor planes have similar problems at high altitude that you're pushing thin air.
What are the efficiency gains you're thinking about?
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.
Duuuuuude, TIT is the temperature after combustion, not compression. Adiabatic compression isn't even close to the main contributor to TIT--heat input from burning fuel is. Also you may be confusing turbofans and turboshafts--helicopters have torque limits (not a helo guy, but my understanding is this is a gearbox or masthead structural limit rather than an engine limit), but if your turbofan can't hit RPM limits on the ground on a cool day you should seriously consider bringing it back for maintenance instead of going flying.
There are a lot of variations. I am most familiar with turbo props so shaft hp is the limiter on the ground and TIT is the limiter at altitude. TIT, of course, gets most of the heat from burning and I did mess up my explanation a bit. Sorry about that! You may be surprised how much of that TIT comes from compression though. The main point still stands, check the temp of that compressed air at sea level vs at altitude and for the same PSI out of the compressor you get a lot more heat at altitude. Either way though, the original point remains, electric has no TIT limit and you don't have to deal with 1000c materials spinning at 100k RPM so way simpler and easier to build something that keeps delivering thrust at extreme altitudes.
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.
> can be
Theory != Practice. If that were the only variable, then yes. Electric would be great. But it's not. It's far from the only thing in play. Lift also suffers from thinner air. Pure electric (as-in battery/solid state energy storage) could have 100% efficiency (specifically in converting prop/turbine torque to thrust of moving air), and it'd still have a terrible efficiency problem with current day tech.
Electric's primary efficiency and efficacy issue is regarding the total operating weight of the aircraft compounded by how that weight does not meaningfully decrease as the battery banks are depleted as compared to consumable fuels. Weight is your biggest enemy in flight, not power nor mechanical efficiency.
Hybrid electric (be it consumable fuel through a generator or fuel cells) is much more promising, but rarely what people mean when discussing "electric propulsion" (without the hybrid qualifier), and still has issues of it's own.
thinner the air, harder it is to generate lift as well.
Coffin corner is a real thing.
if you stay subsonic. I hear the U-2 has like 1-2kt of leeway when it is at its max altitude because if it went faster it would be supersonic but any slower and it would fall out of the sky.
To the point where, if you turned too hard, you could stall one wing tip while Mach buffeting the other.
Obligatory plug for the excellent book Skunk Works by its former director, Ben Rich.
Unless you're prepared to go supersonic. Not easy to do with electric propulsion though.
Scimitar props are pretty tame, though, or what part do you mean is hard?
Can you recommend a place to learn more about this? I have been curious about this topic but have struggled to find resources online describing the basic physics of electric flight propulsion.
Electric has the virtually insurmountable problem that they have to haul the entire weight of the batteries around even if they are drained. This is a MASSIVE loss as itliners can burn off over half their weight during the flight.
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.
If that sort of complexity were viable for commercial aviation, we’d be air-to-air refueling airliners.
Air-to-air is way more difficult than just a tug, though.
If that’s the case, why doesn’t the Air Force tug up its fighters? It’d be a huge advantage.
Very much disagree. Air to air refueling is done in a very stable manner at cruise altitude. Takeoff is a much more dynamic flight regime where things can go very wrong very quickly.
Drop tanks, well, drop batteries, to get rid of the excessive mass.
Kinda crazy but might actually work for continental flights over cooperative areas. Parachute the empty batteries down with some minimal steering mechanism to land them at regularly spaced depots, then ship them back to airports fully charged.
Turnaround time for planes is short enough that you’d need to do a battery-swap rather than a battery-charge anyway.
Would electrics be ducted jet engines but with a motor instead of a gas turbine?
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?
For anybody interested in gas turbine engineering, I recommend Gas Turbine Theory by Cohen & Rogers.
https://archive.org/details/gasturbinetheory0000sara
A very good article, but I was disappointed to see the misunderstanding about the de Havilland Comet failures repeated
> fatigue failures around its rectangular windows caused two crashes, resulting in it being withdrawn from service
While the accident investigation reports refer to "windows", which really doesn't help matters, the failure point was the ADF antenna mounting cutout. The passenger windows had rounded corners and did not fail in service.
The Comet was not withdrawn from service, they re-engineered and launched the Comet 4 (with oval windows, but that choice was to reduce manufacturing costs) in 1958, but the Boeing 707 was introduced that year and the DC-8 in 1959, ending the Comet's status as the only in-service jet airliner it held between 1952 and the grounding of the Comet 1 in 1954. The Comet 4 continued to fly in revenue service until at least the mid 1970s with lower-tier airlines.
The decision to bury the engines in the wings was one of the deciding factors for airlines - engines in nacelles are easier and cheaper to service and swap if required. Re-engining the Comet 4 to new more efficient turbofan engines the DC-8 and Boeing 707 introduced in 1960 and 1961 respectively required a new wing, but a podded engine was much easier to swap on to an existing airframe and this was done for many of the Boeing and Douglas aircraft.
The last Comet-derived aircraft - the Hawker Siddeley Nimrod - flew until 2011 in the RAF. They did look at upgrading them with new wings and avionics, but the plan was scrapped when they discovered that in the grand tradition of British engineering every fuselage was built slightly differently and they couldn't make replacement parts to a standard plan.
Anyway that's my rant in to the void today :)
As i am sure the OP and GP know pprune has much of this, and concord related stories from a cohort of engineers and pilots who worked on these aircraft.
They did have a "best of" collection at one point, not sure now. Also a lot of flight test stories, ATC stories.
> Developing a new commercial aircraft is another example in this category, as is building a cheap, reusable rocket.
Cheap rockets can be vastly simpler than turbojet engines. Reusability (I'm talking about reusability of an orbital rocket, suborbital reusable rockets can be rather simple, as e.g. Armadillo Aerospace and Masten Space achievements show) adds a lot to the order, but increasing the size the square-cube law improves things to an extent.
Related:
See Thru Jet Engine [video] - https://news.ycombinator.com/item?id=32145297 - July 2022 (70 comments)
One important point is missing from this: building a cheap and good engine is not enough, there are more companies and industries that can do this than it seems. But you also need the maintenance and logistics network, with a ton of professionals trained for your engine type in particular. And for that you need to penetrate the market that is already captured. This is what stopping the most.
Aren't these engine designs patented very heavily? How were clones popping up less than a decade later?
What's beautiful to me is that that combustion turbines have the simplest possible thermodynamic cycle in theory (a steady input flow of X fluid/sec at pressure P, and a steady output flow of Y>X fluid/sec at pressure P), yet it turns out to be one of the most complex cycles to harness in practice!
> Building the understanding required to push jet engine capabilities forward takes time, effort, and expense.
This occurs in a broader cultural context. A society that dreams, enjoys science fiction, rewards hard study of advanced topics and so forth, can produce the work force to staff companies capable of going to the stars.
Let us encourage that.
Had to last sucking in dust