High Performance Electric Aircraft Propulsion

[Nuke]

The performance of small brushless motors have lead to a boom in their use in r/c aircraft. Then we started seeing them in sport class aircraft, solar powered research aircraft, etc. This has made me somewhat curious about the maximum limits of electric propulsion which can be achieved. Currently there are limits on the power output of aircraft power systems. It takes many megawatts (mechanical) to keep a commercial airliner flying. Suppose this problem went away. perhaps you have or or more 100MW polywells onboard. regardless of the power supply, what could you do with it as far as propulsion goes? could you achieve supersonic or hypersonic flight purely on electrical power? what about non-mechanical systems like what you see on ionic lifters, would they offer any improvement over electric ducted fans or props?

[Aghanim]

Actually I've been thinking about this since I know about polywell. The polywell proponents say that the engine could work by using extremely high voltage to create relativistic

electrons, and shooting them into a gas stream, heating them. Search for quiet electric discharge engine.

However, what I think might work is a compressor driven by superconducting motor feeding a plasma heater

[Idobox]

Arc jets heat air by using electric arcs, and can be adapted to any form of jet engine, including ram and scram.

In the upper atmosphere, plasma, Hall effect of electrostatic thrusters would work wonders, and would easily allow suborbital flights.

[YourEverydayWaffle]

The problem with brushless motors (I own a quadcopter), is the bearings often come out and cause a HUGE amount of vibration. Also, they and the ESCs (Electronic Speed Controllers) get really hot the longer you use them, and if one fails, which is fairly common, the motor will do all sorts of weird things. ESCs also can catch on fire, which is pretty bad. They aren't exactly the most reliable things for expensive projects, like big solar planes, or whatever. Other than those things though, they're great and really powerful. They sure do work with a quadcopter.

Edited April 14, 2014 by YourEverydayWaffle
Autocorrect

[Nuke]

The problem with brushless motors (I own a quadcopter), is the bearings often come out and cause a HUGE amount of vibration. Also, they and the ESCs (Electronic Speed Controllers) get really hot the longer you use them, and if one fails, which is fairly common, the motor will do all sorts of weird things. ESCs also can catch on fire, which is pretty bad. They aren't exactly the most reliable things for expensive projects, like big solar planes, or whatever. Other than those things though, they're great and really powerful. They sure do work with a quadcopter.

i also own an r/c heli. hobby grade parts work for us but are not that well designed. they are built for mass production and are made more cheaply that what you would see in actual aircraft applications. the motors and speed controllers used in r/c applications are not quite the state of the art (if they were we wouldn't be able to afford them). the primary point of failure in a brushless motor is the bearing. the smaller motors are much more sensitive to imbalance than their larger cousins. as motors get bigger you can start using fluid, air, or magnetic bearings instead of ball bearings and active balancing. hobby grade speed controllers have to be small and light while pushing a lot of power, they get hot as a result and are often designed with insufficient heat sinking. larger motor controllers will need to push even more power, so they use higher rated components, need better cooling systems, possibly active cooling. its an engineering problem that is quite workable.

[Seret]

DC motors have been improving rapidly, but for most applications where you require decent power and high reliability you're still looking at an AC induction motor.

[Nuke]

if that is the case, then comes the problem of building an inverter that can move 100MW. you will be using tubes, i dont think solid state can do that without letting out the magic blue smoke. at these power levels everything needs to be superconducting to keep components from melting.

[Aghanim]

Solid state already do that regularly:

That huge stacks are thyristors stacked togeher

Now the problem is how do you fit that inside an aircraft?

[K^2]

Any idea where I can get some of these for my super villain lair?

Anyways, the only reason why AC motor can have advantages is because of the limitations of the solid state controller for the brushless. And these are about the same as limitations for the solid state inverter. So you might as well go with brushless.

[Idobox]

Polywells running proton-boron fusion generate alpha particles with 2.9MeV of energy, which means you can easily get 1.4MV DC voltage between the core and the outer shell, a lightweight, very efficient conversion process, so you don't need a rectifier.

That being said, the best solid-state devices I know, IGBT and thyristors, can typically handle 5 to 10kV and 1 to 2kA, so you would need to stack a lot of them to deal with a MV power source.

The best option is probably vacuum tubes (limited only by practical size), thyratron (similar to vacuum tube, but filled with gas) or mechanical contacts (low frequency and arcing, but extremely good properties in steady state).

[Nuke]

Solid state already do that regularly:

http://upload.wikimedia.org/wikipedia/commons/0/04/Pole_2_Thyristor_Valve.jpg

That huge stacks are thyristors stacked togeher

Now the problem is how do you fit that inside an aircraft?

sorry, i say stupid things when its 4 am and im tired. of course solid state can do it. its just things get really big and frankensteiny when the power goes up.

Polywells running proton-boron fusion generate alpha particles with 2.9MeV of energy, which means you can easily get 1.4MV DC voltage between the core and the outer shell, a lightweight, very efficient conversion process, so you don't need a rectifier.

That being said, the best solid-state devices I know, IGBT and thyristors, can typically handle 5 to 10kV and 1 to 2kA, so you would need to stack a lot of them to deal with a MV power source.

The best option is probably vacuum tubes (limited only by practical size), thyratron (similar to vacuum tube, but filled with gas) or mechanical contacts (low frequency and arcing, but extremely good properties in steady state).

you dont need rectification (ac->dc), but you do need to use an inverter (dc->ac) if you want to use an ac induction motor. dc brushless motors just switch each of 3 or more dc coils on and off based on the phase of the motor. usually using a hall sensor or back emf to detect phase. you still need high power dc switching there. speed controllers for a multi-megawatt motor will get pretty big.

[K^2]

Polywells running proton-boron fusion generate alpha particles with 2.9MeV of energy, which means you can easily get 1.4MV DC voltage between the core and the outer shell, a lightweight, very efficient conversion process, so you don't need a rectifier.

That being said, the best solid-state devices I know, IGBT and thyristors, can typically handle 5 to 10kV and 1 to 2kA, so you would need to stack a lot of them to deal with a MV power source.

The best option is probably vacuum tubes (limited only by practical size), thyratron (similar to vacuum tube, but filled with gas) or mechanical contacts (low frequency and arcing, but extremely good properties in steady state).

If you are going to use a polywell with direct conversion as your energy source, you don't want to use it to run an electric motor. It'd be like using nukes to do urban demolition. You probably could, but why would you want to?

You have a source with considerable current and a huge voltage output. You should be building an arcjet. Dry air is what, about 30kV/cm? That makes a 1MV+ source absolutely perfect for an airliner jet engine. And you can use the arcjet principle to build anything from a high bypass turbofan, to a scram jet, to a pure rocket engine. None of these are going to be super efficient, but you're running on nuclear energy. And it can still be cleaner than a combustion jet engine, despite the efficiency drop.

You don't need to worry about throttling, either. Just change reaction rate. With direct conversion, current flow is going to change almost instantly. You'll have just as good of response as with conventional jet turbines, if not better.

[Seret]

if that is the case, then comes the problem of building an inverter that can move 100MW.

Not a problem, the power electronics used to control motors was developed by the power industry. Whether you go for AC or DC is largely going to be a choice based on what power source you're using (a solar aircraft will naturally use DC, for example), power required and how mature you want your motor technology to be.

MV isn't a practical voltage to be using on an aircraft, you'd need to step that down use it and that would cost a lot of weight.

Edited April 16, 2014 by Seret

[Nuke]

im starting to get sold on atmospheric arcjets from ground to hypersonic as k2 suggests.

would it be possible to build an electric ssto? what kind of engines would we use between hypersonic and orbit. ion engines would not offer enough thrust, mpd thrusters probibly wouldnt cut it (but might work as an oms), hydrazine or lox/lh2 arcjets maybe (if they scale up well). 100MW is a lot, but is it enough to go to space?

[K^2]

100MW should get you about 20kN of thrust from an arcjet rocket, with maybe 800-1000s of I_SP. (Some rough figures from the top of my head.) That's with ammonia, I think, but hydrazine shouldn't be significantly worse.

Anyways, with air-breathing arcjet to get you started, this should be enough for about 2T total mass, and about 7km/s to go. So your dry mass is going to be something like 400kg/450kg. That needs to include engines. Doesn't leave for much. How heavy is that 100MW reactor you're thinking of?

[Aghanim]

Why do you want to use hydrazine while a less exotic propellant like argon or hydrogen works?

Here is the problem: Although quiet electric discharge: http://www.askmar.com/Fusion_files/The%20QED%20Engine.pdf and arcjet could directly utilize the >1 MV output of a

polywell, how about electrode erosion? QED probably doesn't have that, no idea for the electron gun though, but arcjet will have some serious erosion which will limit its service life

That is why I prefer contactless way of injecting electricity into the propellant stream like helicon plasma or magnetic field oscillating amplified thruster: http://www.unoosa.org/pdf/pres/stsc2007/tech-21.pdf

[K^2]

Not looking good on the jets. Cyrus Vision SF50 has a 140kg turbofan that consumes 3MW of heat from fuel to produce 5.3kN of thrust. Assuming linear scaling, this doesn't look good for an SSTO. There is simply no way to get enough thrust from air-breathing stage at anything like reasonable efficiency without bringing too much weight for the rocket stage to carry.

So if you want to go orbital, or even suborbital, using arcjets, you'll have to go with two stages. Naturally, first stage can fly back to the point of origin as a conventional airplane. So this could still be a viable idea. I just need to know how heavy these reactors are, because I'm not familiar with polywell tech, and I'm not seeing anything useful on Wiki.

Why do you want to use hydrazine while a less exotic propellant like argon or hydrogen works?

Hydrazine is way cheaper than argon, is liquid at room temperature, and has a high density. No need for huge cryogenic tanks, which saves a ton of space. Something less toxic might be a good idea, though. Methane should work extremely well in an arcjet. It would require a bit of pressurization, but it isn't bad otherwise.

how about electrode erosion?

Actually, if it's tuned right, anode shouldn't be hit with more than 2-3kK, so you could minimize erosion during normal operation. You'd probably have to swap them out pretty frequently anyways, but it doesn't seem like a deal breaker.

Edited April 16, 2014 by K^2

[Aghanim]

Have you consider turbine-compressor loss? Driving the compressor with electric motor will eliminate the need of a turbine, and the gas could flow freely, increasing thrust. Without turbine blade to melt, increasing the arcjet temperature works. Don't know if that will increase the efficiency or not

And where do you find the 3 MW / 5.4 kN figure? I'm already searched it in google but no result

Edited April 16, 2014 by Aghanim

[K^2]

Specific fuel consumption of Williams FJ33 is cited as 13.8g/(kN s) @ 5.3kN. And specific energy of Jet A is 43.02kJ/g. Which gives me a bit over 3MW total.

And the problem here is the mass of the engine. Adding motor isn't going to help that. I'd rather take a ~50% loss on compressor, because we have plenty of excess energy.

[Idobox]

100MW should get you about 20kN of thrust from an arcjet rocket, with maybe 800-1000s of I_SP. (Some rough figures from the top of my head.) That's with ammonia, I think, but hydrazine shouldn't be significantly worse.

Anyways, with air-breathing arcjet to get you started, this should be enough for about 2T total mass, and about 7km/s to go. So your dry mass is going to be something like 400kg/450kg. That needs to include engines. Doesn't leave for much. How heavy is that 100MW reactor you're thinking of?

2T for 20kN is far too heavy, you won't be able to do a gravity turn before burning a lot of fuel.

Also, why 7km/s, you certainly can get more than 2 or 3km/s with an airbreathing arcjet.

Here is the problem: Although quiet electric discharge: http://www.askmar.com/Fusion_files/T...D%20Engine.pdf and arcjet could directly utilize the >1 MV output of a

polywell, how about electrode erosion? QED probably doesn't have that, no idea for the electron gun though, but arcjet will have some serious erosion which will limit its service life

As said, electrode erosion is not such a big deal, also arcjet is pretty much the only way to get decent thrust out of an electric engine, and a mhpd thruster works as an arcjet at atmospheric pressures.

An EHD thruster might also be an option with this kind of power supply.

[Aghanim]

EHD thruster is inefficient, arcjet is better, MPD is arcjet with some electromagnetic acceleration added

[K^2]

That's kind of why 7km/s. You need to get quite a bit of vertical velocity to get the thing high enough for TWR of 1 being sufficient to push the thing to orbit. So your first stage eats most of the atmo/gravity losses, but you'll need most of the orbital velocity to be gained by second stage due to gravity turn ending up being done so high up.

Of course, I'm not sure you need such low TWR. Again, it depends on how heavy the reactor is. Also, I was trying to save as much of the weight as possible for the air-breathers. Which isn't necessary with a 2-stage design.

[Nuke]

100MW should get you about 20kN of thrust from an arcjet rocket, with maybe 800-1000s of I_SP. (Some rough figures from the top of my head.) That's with ammonia, I think, but hydrazine shouldn't be significantly worse.

Anyways, with air-breathing arcjet to get you started, this should be enough for about 2T total mass, and about 7km/s to go. So your dry mass is going to be something like 400kg/450kg. That needs to include engines. Doesn't leave for much. How heavy is that 100MW reactor you're thinking of?

i dont have any numbers for the reactor (it hasnt been built yet, nor do they currently have funding to do so, but they have enough to finish with wb8 tests) there are some pics of the reactor here which might give you some idea about its size and mass:

http://www.emc2fusion.org/

its not a hulking tokamak but its still pretty big. id bet most of the mass will be due to the vacuum chamber. a pollywell is mostly just some coils, electron guns, ion guns, and a few faraday cages, and some cryogenic hardware. im using the 100MW number from their demo reactor, but i have a feeling we could get them up to higher power levels if the technology has a chance to mature (proves to be viable).

there are other reactor concepts that could be smaller than this. if dpf fusion works, you get a reactor thats about a quarter the size.

Edited April 17, 2014 by Nuke

[K^2]

Yeah, that's definitely going to be too heavy to power the ship, unfortunately. A slower flying, purely lower atmo cargo plane, though, is still entirely within realm of possibility.

But who knows. First reactors also were way heavier than they needed to be. Maybe this is something that can be miniaturized, or have its power output boosted by an order of magnitude.

[Nuke]

for purely space application you can just stick the polywell outside and forget about the chamber. so i imagine having polywell driven mpd ships flying all over the place.