Why it's so hard to build a jet engine
19 comments
·February 28, 2025jmward01
iancmceachern
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.
TylerE
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.
mhandley
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.
55873445216111
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.
sokka_h2otribe
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?
jmward01
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.
russdill
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.
0manrho
> 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.
zeusk
thinner the air, harder it is to generate lift as well.
Coffin corner is a real thing.
bob1029
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]
gtirloni
Aren't these engine designs patented very heavily? How were clones popping up less than a decade later?
avmich
> 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.
adiabatichottub
For anybody interested in gas turbine engineering, I recommend Gas Turbine Theory by Cohen & Rogers.
orbital-decay
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.
smitty1e
> 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.
ge96
Had to last sucking in dust
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.