[mannykannot]

"The essential idea is that air is ionized to a plasma, which is rapidly heated and allowed to expand to generate thrust."

So this is just a conventional heat engine, with an electric heater. This heater may be able to get the air much hotter than other methods, but the thing about a heat engine is that you cannot get useful work out unless the working fluid can expand sufficiently. An afterburner creates more thust by heating the air to a higher temperature than could be tolerated by the turbine, but the air is by then at a relatively low pressure, and so the Carnot efficiency is very poor - most of the extra fuel's energy goes into producing a hotter (and very visible) exhaust plume.

So, for this to be a component of a jet engine, it will need a compressor comparable to, or with an even higher presure ratio, than in current jet engines, and that compressor will have to be powered somehow (IIRC to the tune of about 50,000 SHP in the biggest engines now in use.)

For the most part, it makes no sense to use electricity to power a heat engine. In guessing where this might be useful, the only scenario I have come up with is for hypersonic ramjets, where electric motors turning fans are not an alternative, and possibly especially on worlds where the atmosphere does not support combustion.

[tgflynn]

Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma. Then you might be able to reach much higher exhaust velocities than with a conventional jet engine. I don't think your thermodynamic objections would apply to that case since the energy would remain organized, it wouldn't just be heat.

To do that though you'd need to find a way to maintain overall electrical neutrality.

[jcims]

>Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma.

That's essentially what they are doing, but in an oscillating path rather than a continuous path:

In the waveguide, the charged particles in the plasma start to oscillate with the microwave field (aka: RF) while rapidly heating. The ions, atoms, and electrons collide with each other frequently, spreading the energy from the ions and electrons to the neutral atoms, heating the plasma rapidly. As a result, the researchers claim that the plasma rapidly heats to well over 1,000°C.

I don't know if you can rectify RF energy like you can with voltage. That might be one way to create a more continous path. They might also be able to add an electromagnetic field to create a net velocity out the provebial barn door as they do with ion thrusters, but I think the problem there is you just can't get enough free air path to accelerate to very high speeds...it'd be like trying to drive an F1 car at full throttle in bumper to bumper traffic. You could also pass a current through the plasma to generate lorentz forces like they do in plasma driver rail guns.

One huge advantage with this design is no moving parts and presumably extremely low manufacturing costs. So it might not be awesome for commercial airliners but it could be useful for high endurance UAVs. Pack a few dozen of them and use just the ones you need.

[tgflynn]

> I don't know if you can rectify RF energy like you can with voltage.

You can in essence, because charged particles have inertia, they don't instantly follow the EM field. That's how RF particle accelerators work. But there you're working with a beam of charged particles, not a neutral plasma.

It still sounds to me that they are essentially using the RF energy just to heat the plasma and in that case the parent's objections seem valid, though I don't know much about jet engine design.

[mannykannot]

A few months ago, there was a demonstration of a lightweight model airplane propelled by the air currents produced by a corona discharge [1], and ion rocket motors have great specific impulse, but they are both low-thrust devices. I believe you are right, that these are not heat engines, any more than an electric motor is: in all these cases, the moving part is directly accelerated in the direction we want it to go.

[1] https://cosmosmagazine.com/technology/researchers-successful...

[taneq]

> For the most part, it makes no sense to use electricity to power a heat engine.

Exactly. And modern aircraft engines generally just use the turbine to power a high bypass fan, at which point why pretend to be a heat engine when you could just spin the fan directly and save a tremendous amount of power?

[Retric]

Modern high bypass turbines provide a lot of power for the weight and extreme efficiency. Essentially, they are a gas turbine which then gets to extract extra energy from their exhaust gasses without adding a lot of weight or mechanical complexity.

There are a lot of trade offs involved, but for large 500+ MPH aircraft high bypass turbofans are simply the most cost effective option.

[taneq]

Yeah, exactly. Being able to stick the turbine in the middle of the ducted fan and then use its exhaust for extra thrust while directly driving the fan with the generated gases is the cherry on top.

If we were going to replace a high bypass turbofan with anything, though, I'd expect it to be a high power AC motor directly driving a ducted fan. Realistically for long haul air travel, though, my guess is we'll stick with turbofans and just use biodiesel or something.

[jacobush]

An idea I've had is to put a small generator instead of a shaft and gearbox, and put the AC motor powered fan somewhere else. Could improve aerodynamics, stealth properties etc.

[VBprogrammer]

A generator is necessarily on the same order of magnitude in size as the motors it is powering.

[jacobush]

Is your point that they must necessarily be heavy? Because I don't think so. Modern electric motors are very compact and powerful.

Also you'd not need gearbox and shafts. You can completely decouple the fan from the turbine. You could put the fan downwards and the turbine back for instance, or the turbine on the top to hide its heat signature somewhat.

[nbadg]

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!

[mannykannot]

> 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.

[1] https://en.wikipedia.org/wiki/Flader_J55

[himinlomax]

> So this is just a conventional heat engine, with an electric heater

The key is that it heats very fast.

[mannykannot]

While it is certainly true that any device that heats slowly would be impractical on that basis alone, I do not think rapid heating provides any relief from the laws of thermodynamic efficiency. Combustion heats very rapidly, though if this device is even faster, perhaps that would be significant in hypersonic applications? I believe one of the problems of supersonic-combustion ramjets is maintaining a stable flame.

I keep coming back to hypersonic ramjets, not because I think they are the perfect application, but because I am not sure there is any application in an otherwise conventional jet engine - i.e. one involving a rotary fan or compressor - where you would not be better off using all the available electric power in an electric motor to spin a somewhat larger fan, and not bother with heating at all.

Update: Maybe, for replacing the turbojet engines of supersonic airplanes, you need heat to reaccelerate the flow above supersonic speeds, in a convergent-divergent nozzle? But supersonic wind tunnels seem to achieve those speeds without heating the flow? I have wandered well into my area of ignorance here...