A friend of mine who happens to be a RABID electric vehicle fan was on a holy tear on facebook about why there are no “electric jets” and since he was able to find videos of 1/6 scale models of electrically powered ducted fan f-16’s that it should be easily “scaled up” to make electric jet fighters.
I already pointed out many of the materials used in models are nowhere near strong enough for the stresses of supersonic flight and or dogfighting, not to mention potential survival of battle damage. You can’t just make everything 6x bigger of balsa and plastic , slap in a few Nissan leaf batteries and and everything works.
He seems to be under the impression that jet engines generate significant amounts of thrust from pulling air not just pushing. I was under the impression that the vast majority of the thrust is generated from the addition of boatloads of heat expanding the already highly compressed air. Is there some meaningful fracton of thrust generated by intake vacuum and I am missing something here. I am under the impression even bypass air although helpful is not a huge fraction of overall thrust. Friend seems to be of the impression fuel is just used to keep the engine turning and that jet engines are wasting tons of energy just blowing heat out the back when according to his POV electric motors could turn the engines to generate more than sufficient thrust.
I full well know that the turbine blades in the back DO make the engine turn and thus keep compressing air to feed to the combustion area.
So could some kind of electric ducted fan jet fighter be made, on the not too distant horizon? Or is this just an electric vehicle wet dream?
I did my due dilligence and did some poking around on the latest in electric aircraft and saw quite a few propeller type situations, but nothing that looked like high speed combat aircraft.
This is probably the closest you are going to get with a modern electric ducted fan plane. It’s an experimental technology demonstrator, and with these specs it’s not even in the same ballpark as a modern jet fighter.
Take-off speed is 100 km/h (54 knots); cruising speed is 160 km/h (86 knots); maximum speed is 220 km/h (119 knots). Typical endurance is between 45 minutes and 1 hour.
Your friend could use a good primer on how jet engines work, along with a bit of tutoring about the square-cube law.
To a degree, your friend is right. Big high-bypass turbofans like a Boeing 777 uses get most of their thrust by accelerating the bypass air, not from the actual turbine exhaust. It’s essentially a ducted fan powered by a little jet engine.
I suspect the issue is more one of power density. Jet fuel has an energy density of 35 MJ/L, while the highest battery one is somewhere in the 4 range.
What this means in practice is that if you stuffed batteries in all the places that a jet plane would have fuel, you’d still only have 1/9th the energy to deal with. Probably enough to take off, and fly around a bit, but almost certainly not enough to be economical or anything but a curiosity.
As already said, high-bypass turbofan jet engines get the majority of their thrust from the fan and not from the exhaust; exact percentages vary by engine design. It’s essentially a fan powered by a turbine engine. It’s not quite correct to think of thrust being generated by “intake vacuum”, however; it’s really generated by the momentum of the expelled air + exhaust gases. This is why reverse thrusters still work on turbofans: they deflect both the exhaust and the bypass air forward.
The problem with an electric-ducted-fan engine design is simply the problem of where the electricity is going to come from and the poor energy density of current batteries. A hydrogen fuel cell might work well, however.
Pound for pound a hydrocarbon fuel has a lot more energy in it than a battery. Try taking off with 50,000 pounds of batteries and see how far you can fly even if you can get off the ground.
Now a case might be made for a fuel, maybe hydrogen, being converted into electrical energy. Then that energy would be used to turn an electrical motor which would turn a propeller or even a ducted fan. Such an aircraft could be more aerodynamic and it would be easier to put the motor in non-traditional places such as on the wing tips or top of the vertical stabilizer. Still I don’t see it happening anytime soon.
The limitation is not in the motors. The limitation is in the batteries. The energy density of kerosine is far, far greater than the energy density of even the best battery pack. Those electrically powered ducted-fan F-16s your friend loves probably have an actual working flight time of maybe fifteen minutes before their batteries are dead. If someone figures out how to make batteries with comparable energy density (and ease of recharge and longevity, etc) as hydrocarbon fuel for a reasonable cost, we will see electric powered airplanes become commonplace.
I thought the OP was asking about just how far can you downscale a jet engine, not how far up you can “upscale” a typical battery powered deducted fan design. What IS the smallest working true jet engine in the world? Just how small can you make them?
A few years back I have done a few calculations on this subject. Current electric motors that could plausibly replace the core of a turbofan engine are about 5x heavier than the turbine core. And even if they were equal, you would still need battery energy density about 1.6x higher than that of kerosene (since fuel makes up over a third of take-off weight, the plane gets significantly lighter over time - yet batteries cannot be ditched as they are gradually drained).
Hydrogen is not too promising either. For one, it is very bulky and bulk means drag. You also need to convert it into electricity, which means a very heavy fuel cell. You would be much better off burning it in a good old turbine engine.
Were you factoring in superconducting motors, which need ~200 times lighter wiring? (but additional mass for coolant and insulation)
With that said, even with superconducting motors, the energy density problem means that you end up with a jet engine somewhere else on the aircraft that generates electricity. There’s at least one airliner concept design that uses electric jets powered by a kerosene fueled generator but it’s a huge amount of complexity for marginal fuel savings.
This article might interest you. There may be a future for electric propulsion as a way of increasing aerodynamic efficiency, by distributing thrust sources, but weight will always be a limiting factor. It’s hard to beat the energy density of a liquid hydrocarbon.
He’s right in one regard: the high temperature of jet exhaust represents wasted energy. If the engine were 100% efficient, the exhaust would be at room temperature, and all of the fuel’s chemical energy would be manfested as kinetic energy in the exhaust plume. But Carnot says this is impossible.
An electric thruster could theoretically be 100% efficient - but first, you’d have to generate the electricity to charge the battery. If you’re using a heat engine to generate that electricity (e.g. power plant operating on coal, natural gas, or nuclear), then Carnot would like to have a word with you.
Room temperature? My background is not in science so I may be completely off but I thought that to be 100% efficient, one necessary condition would be for the exhaust to be at 0 kelvin. Is this not so? What’s special about 20 Celsius?
My unwritten assumption (sorry) was that the intake air was also at room temperature - the idea being that if intake air and the exhaust plume are at the same temperature, then we have converted all of the fuel’s chemical energy into kinetic energy of the exhaust stream, which is in fact the purpose of the jet engine.
This motor clocks at 1.1 kW/kg at continuous power. Turbines are about 10!
Superconducting motors can do even better, but once you add the cooling equipment and insulation, the numbers are not looking good.