Replacing chemical combustion with electrical heating in a heat engine was an idea that popped into my head a number of years ago while thinking about the heat exchanger for Skylon's SABRE engine. I'll skip the drunken, derailed train of thought that made that particular leap, but off and on since then I've been really intrigued by it, and spent more time thinking about it than I'd care to admit!
First and foremost, I agree that the compressor is probably the most difficult part (though theoretically, if you pre-ionized the gas, you could use magnetic compression). Also, I completely agree that, given current energy densities for electric storage, this is only something that could be useful in some really niche applications.
But that being said, some of those applications are really cool! For example, one of the major challenges of VTOL aircraft is that the rotational inertia of turbines is so great that it's very, very difficult to rotate them during transition from vertical to horizontal flight. Something like this would massively decrease the rotational inertia, making it much simpler mechanically to create tiltwing aircraft.
Also, my understanding is that typically, conventional jet engines are limited primarily by the maximum temperature limit of the turbine blades. Because your compressors here would have to be powered by electricity as well (nothing else makes any sense!), there's absolutely no reason to have a turbine at all; you'd just want a plain old expansion nozzle. That means you could pump way more heat into your plasma, making your engine much more power dense. In other words, your engine could be potentially much smaller for the same thrust, which would be a big deal. Turbine blades need to be both very strong due to their rotational velocity, and extremely temperature resistant because they're literally sitting in the exhaust of a jet engine, which makes them not only really expensive, but also very, very challenging from a metallurgical perspective.
Another, potentially very interesting, application is if you have too little oxygen in your atmosphere to support combustion -- for example, on Mars. Sure, we're about to send a mini electric rotorcraft there, but the atmosphere makes it really very challenging to do that, because the classic "my rotor tips are too close to the speed of sound" problem is much, much more difficult there.
Any kind of ramjet, as you mention, is a possibility, but this would also make it a lot easier to make transition engines (like the J58 that powered the Blackbird) that start as a conventional compressor-fed jet engine and, at cruise speed, transition into a ramjet.
Regardless of application, this is such a fundamental change to the design limitations of jet engines that a lot of the usual design logic simply doesn't apply anymore. Thermodynamics are infamously complicated, which makes it really difficult to draw performance comparisons between an 80-year-old mature technology and something so radically new and different. One way this gets substantially more complicated is that in a traditional jet engine you need to be worried about combustion efficiency, flame stability, etc etc, plus you have to siphon out enough energy to run the engine's compressor, and power the rest of the aircraft (likely indirectly, through an APU!). All of those take a big efficiency hit in traditional engines, whereas this would be, nominally, much better. So my gut would be that, all other things being equal, the powerplant on an airplane with an electric jet engine would be both smaller/lighter and more efficient. But again, this is hard to reason about!
I would be ecstatic to see one of these flying around, but don't expect it to end up in a passenger aircraft any time soon or anything. For that, we need better batteries!
> There's absolutely no reason to have a turbine at all; you'd just want a plain old expansion nozzle. That means you could pump way more heat into your plasma, making your engine much more power dense.
I agree that replacing the turbine with an electric motor seems to be the only way to go with this, but the point I am trying to make here is that if you merely increase the temperature of the working fluid without changing the pressure ratio, you will get some increase in thrust, but at the cost of a worse Carnot-cycle efficiency: quite a bit of the additional energy input goes to waste in the form of a hotter exhaust, because it cannot be expanded enough to convert it to useful work.
So can we increase the pressure ratio? if it were feasible to do so with current technology, we would already be doing so, as combustion jet engines would also benefit from increasing it. When comparing plasma and combustion jet engines, we must assume that both will be operating at the highest feasible pressure ratio.
That does not automatically rule out this technology, as it may offer something in trade-off for its limited efficiency, but in the case of electric propulsion, the storage options are currently so limited that efficiency is highly valued.
The question to be answered is this: for a given electricity source, will this give me anything of value over using all the power in an electric motor driving a fan? For subsonic flight, I am very skeptical that it can even come close to having anything to offer.
The J58 is often described as a hybrid turbo-ramjet, but that is hype to some extent: it is a low-bypass turbojet with an afterburner and a pressure-recovery intake, but that describes every supersonic airplane. It is the pressure-recovery inlet that makes all these engines somewhat ramjet-like, and in the J58 there is just more of it. The distinction is a matter of degree; even subsonic jets take advantage of pressure recovery.
Pressure recovery does two things: it increases the overall pressure ratio, and it slows down the inflow to the compressor to below supersonic speed. The former is only a benefit for heat engines (Carnot efficiency, again.) Therefore, I think the only reason for having such an inlet in an electric-fan jet engine is if a supersonic fan is infeasible, and they may well be. If so, then it may be the case that makes sense to use some of the available electric power to heat the compressed flow downstream of the fan, but it is not obvious to me that this would be a better use of that power than using it all in a bigger fan.
I see from here [1] that supersonic compressors, and therefore presumably fans, are feasible, though have not been very successful (maybe because pressure recovery is a better option for heat engines.)
By using electric power in a heat engine rather than in a non-thermal process, you are already committed to throwing about two-thirds of it away, so there have to be some quite compelling benefits elsewhere to make it a net win overall.
First and foremost, I agree that the compressor is probably the most difficult part (though theoretically, if you pre-ionized the gas, you could use magnetic compression). Also, I completely agree that, given current energy densities for electric storage, this is only something that could be useful in some really niche applications.
But that being said, some of those applications are really cool! For example, one of the major challenges of VTOL aircraft is that the rotational inertia of turbines is so great that it's very, very difficult to rotate them during transition from vertical to horizontal flight. Something like this would massively decrease the rotational inertia, making it much simpler mechanically to create tiltwing aircraft.
Also, my understanding is that typically, conventional jet engines are limited primarily by the maximum temperature limit of the turbine blades. Because your compressors here would have to be powered by electricity as well (nothing else makes any sense!), there's absolutely no reason to have a turbine at all; you'd just want a plain old expansion nozzle. That means you could pump way more heat into your plasma, making your engine much more power dense. In other words, your engine could be potentially much smaller for the same thrust, which would be a big deal. Turbine blades need to be both very strong due to their rotational velocity, and extremely temperature resistant because they're literally sitting in the exhaust of a jet engine, which makes them not only really expensive, but also very, very challenging from a metallurgical perspective.
Another, potentially very interesting, application is if you have too little oxygen in your atmosphere to support combustion -- for example, on Mars. Sure, we're about to send a mini electric rotorcraft there, but the atmosphere makes it really very challenging to do that, because the classic "my rotor tips are too close to the speed of sound" problem is much, much more difficult there.
Any kind of ramjet, as you mention, is a possibility, but this would also make it a lot easier to make transition engines (like the J58 that powered the Blackbird) that start as a conventional compressor-fed jet engine and, at cruise speed, transition into a ramjet.
Regardless of application, this is such a fundamental change to the design limitations of jet engines that a lot of the usual design logic simply doesn't apply anymore. Thermodynamics are infamously complicated, which makes it really difficult to draw performance comparisons between an 80-year-old mature technology and something so radically new and different. One way this gets substantially more complicated is that in a traditional jet engine you need to be worried about combustion efficiency, flame stability, etc etc, plus you have to siphon out enough energy to run the engine's compressor, and power the rest of the aircraft (likely indirectly, through an APU!). All of those take a big efficiency hit in traditional engines, whereas this would be, nominally, much better. So my gut would be that, all other things being equal, the powerplant on an airplane with an electric jet engine would be both smaller/lighter and more efficient. But again, this is hard to reason about!
I would be ecstatic to see one of these flying around, but don't expect it to end up in a passenger aircraft any time soon or anything. For that, we need better batteries!