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  1. - Top - End - #31
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by Chen View Post
    Don't think that was safety related. They had 3 other fuel powered engines on that aircraft (only 1 of the 4 was replaced by the electric motor). The point was, to make the single electric motor provide enough thrust (comparable to another fuel powered engine) they needed a generator on the aircraft to provide enough electricity to it. That generator was basically a turboshaft engine modified for the task.
    IIRC, the engine would only run on both the generator and the batteries during periods of high performance requirements, such as take-off. During cruise it would run on only one of those two, I just can't find whether it'd run on the batteries or the generator in those cases :P
    Last edited by DeTess; 2019-09-27 at 08:13 AM.
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  2. - Top - End - #32
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Firstly, the words you are looking for with regards to just driving the fan is a ducted fan. Turbo fans are basically a gas turbine driving a ducted fan, and you can replace the gas turbine with any source of mechanical energy. As for whether you would run a gas turbine on electrical heat or use an electric motor, the turbine gets around 40% efficiency, while the motor can get over 90%. There is no contest.

    Anything a turbofan can do you could do with an electrical ducted fan, including supersonic flight with an appropriate inlet and exhaust. Molten salt batteries have the power density to pull it off, briefly. If you wanted to get crazy you could even put an afterburner on it using electric heat. If your inlet is working well you can get a surprisingly high efficiency at Mach 2 if it doesn't melt (no fuel means no coolant), and the thrust you can get is absurd. As an aside, One of the easier ways to operate a scramjet is electrically, because heating the air using electricity sidesteps the difficulties involved in getting fuel to burn at that speed.

    Liquid hydrogen has not great energy density. Roughly 1/4 of standard liquid fuel. Where you might get large savings is that it is the ultimate coolant, meaning you can cool compressor stages. You can push core efficiencies up sky high by doing that, with 80% being doable. You can equal a fuel cell electric motor combination by just being clever with the engine.

    Ideally we don't want to be dropping water at that altitude anyway though. Water might seem innocuous as a pollutant, but that's because it is very common at low levels. Airliners fly at an altitude where it has a much more significant impact, and in many ways is more important than CO2 (though carbon tends to cause dust). A zero emissions airliner would be far preferable. Everything we take up we take back down, except maybe nitrogen and oxygen.

    Air breathing batteries would be pretty good, but are tricky to make work and you end up landing heavier than you take off. You might be better off just carrying the oxidiser up too, as it would simplify the design not worrying about nitrogen.

    I wouldn't want to fly in it, but you could get a molten lithium fluorine -> molten salt battery to work, and it would give you about half the total energy of a hydrocarbon (which gives roughly parity in terms of performance because motors are more efficient). Chemistry to do better than that doesn't exist; carbon and hydrogen are fantastic as fuels if you don't need to worry about oxidiser mass, and simply impossible to beat if you do. There is a reason 3/4 of the propellant mass of a rocket is oxidiser.

    One possible 'cheat' is to use hydrogen fuel cells and semi-capturing the exhaust. Instead of landing with the water, we dump it overboard with large enough droplet size and cold enough that most of it doesn't evaporate, it just falls. Hydrogen has 1/3 the energy density, but if you were able to get the fuel cells to 80% efficient you would get 2/3 the performance from 1/9th the fuel mass.

    If the concern is environmental I would be very interested to know how a methane engine would behave with regards to contrails. Methane burns cleaner, which might result in larger droplet size (there being less condensation seeds). That could significantly affect the impact of air travel far more than you might expect just by looking at the exhaust composition.

    The best short term use for electric tech is IMO achieving higher bypass ratios than are currently available. A core driving a generator doesn't need a mechanical connection to a fan to drive it. You could run 4 fans off 2 cores, avoiding the need for gigantic fans. A battery could provide more power than the engines are capable of for take off, meaning cruise can be optimised, and the locations of the additional fans is much more flexible. The power could be used for more exotic propulsion styles, (such as manipulation of the boundary layer) without being so constrained by form factor.

    As for the safety of hydrogen, we really have very little experience. It has a bad rep from the Hindenburg, but that thing had an envelope made of nitrocellulose and aluminium powder. Nitro-glycerine was created because nitrocellulose was too dangerous to use as an explosive, and nitro-glycerine is not exactly lambs milk. Aluminium powder is a major component of solid rocket fuel despite producing no gas, because it burns so effectively. The hydrogen was the least of their problems.

  3. - Top - End - #33
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    I think the main issue with liquid hydrogen as a fuel source is the size and weight of the tankage required for it, thanks to the very low density of the liquid. In something like an airliner you'd need that tankage to be a lot more robust than, say, the tanks on a rocket, where having one explode every few dozen flights is considered a (mostly) acceptable risk of doing business. There is also the water vapour emissions that you rightly bring up--ISTR water vapour is an even worse greenhouse gas than CO2 is, so injecting it at high altitude would actually be worse for the environment than burning regular fuel is.

    I think the main reason for creating an electrically-powered aeroplane is so you can generate the electricity on the ground using whatever clean method you can come up with rather than generating it in flight, but then we're back at the problems with battery technology. The molten lithium fluorine battery you mention is obviously horrifically dangerous and potentially very "dirty" in the event of a plane crash!

  4. - Top - End - #34
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by Chen View Post
    Yes a turbofan/shaft/prop would be replaced entirely by an electric motor and wouldn’t need a hot section at all. The problem, as stated numerous times, is providing energy to the electric motor.

    Take https://en.m.wikipedia.org/wiki/Airbus_E-Fan_X for example. They replaced ONE of their 4 turbofans with an electric motor driven fan. But they still needed a 4000 lbs battery AND a separate turbomachine (fuel powered) in the rear of the aircraft to provide the power to the single electric motor. It really shows the scale of power that would be needed for a pure electric aircraft engine.
    That Wikipedia link is kind of wonderful, I apologise for not knowing about that, it's kind of annoying that they don't have links to the engine, but that's Wikipedia for you.
    The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.

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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by Fat Rooster View Post
    Air breathing batteries would be pretty good, but are tricky to make work and you end up landing heavier than you take off. You might be better off just carrying the oxidiser up too, as it would simplify the design not worrying about nitrogen.
    This actually reminds me of another potential issue with batteries. To explain it, let me quickly back up to aircraft design 101 (almost literally. the course wasn't called that, but it definitely was what the Americans would call aircraft design 101). Aircraft have a lot of different weights associated with them during their design. Of interest here are the maximum take-off weight(MTOW) and the maximum landing weight (MLW). The MTOW is the maximum weight at which the aircraft can climb at the minimum rate set by the various aircraft certification authorities. The MLW is the weight at which a aircraft can land without inflicting permanent damage on the airframe from the shock.

    The reason why this is a problem with batteries is that the MTOW is often significantly higher than the MLW. To give an example from one of the airbus 320 variants, its MTOW would be 73500 kg but its MLW is only 64500 kg. If you wanted to convert a modern passenger aircraft to batteries, your weight budget for batteries would be significantly smaller than it had been for the fuel, because you need to bring all those batteries back down again.

    The reason MTOW is generally higher than MLW is because it's generally more weight efficient to get higher peak thrust out of your engines, then to increase the sturdiness of the airframe to increase the MLW, and with a fuel-powered aircraft the difference doesn't matter most of the time because you've burned enough fuel to land safely by the time you reach your destination (and when it does, there are ways for an aircraft to very quickly dump its fuel).

    If you where to design a battery-powered aircraft that needs to haul as many passengers as an equivalent non-battery powered aircraft you'd run into issues, where you'd either need to increase battery energy-to-weight ratio significantly beyond that of conventional fuel, or you'd need to strengthen your airframe to account for higher landing loads, which would cause a snow-ball effect of weight increases across your entire plane.
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    One option for powering an electric aircraft might be an MSR, or Medium/Small Reactor. Currently only in early development the idea is a nuclear reactor that runs on stuff like thorium, so 1) fuel byproducts don't last 10000 years, 2) can operate at regular atmospheric pressure so even in the event of a melt do we would just let it run its course and clear the site rather than worry about the whole thing exploding, and 3) would be about the size of a standard shipping container when assembled.

    Current projections put an MSR at producing few a hundred kilowatts, more than enough to operate a large aircraft, and superheat air for a jet engine.

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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by Beleriphon View Post
    One option for powering an electric aircraft might be an MSR, or Medium/Small Reactor. Currently only in early development the idea is a nuclear reactor that runs on stuff like thorium, so 1) fuel byproducts don't last 10000 years, 2) can operate at regular atmospheric pressure so even in the event of a melt do we would just let it run its course and clear the site rather than worry about the whole thing exploding, and 3) would be about the size of a standard shipping container when assembled.

    Current projections put an MSR at producing few a hundred kilowatts, more than enough to operate a large aircraft, and superheat air for a jet engine.
    Would you be fine with a couple of these crashing into random locations every year though? And how potent would these be as a weapom of terror? I don't know enough about these to know how big of a problem this would be.
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  8. - Top - End - #38
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by Beleriphon View Post
    One option for powering an electric aircraft might be an MSR, or Medium/Small Reactor. Currently only in early development the idea is a nuclear reactor that runs on stuff like thorium, so 1) fuel byproducts don't last 10000 years, 2) can operate at regular atmospheric pressure so even in the event of a melt do we would just let it run its course and clear the site rather than worry about the whole thing exploding, and 3) would be about the size of a standard shipping container when assembled.

    Current projections put an MSR at producing few a hundred kilowatts, more than enough to operate a large aircraft, and superheat air for a jet engine.
    Thorium doesn't work like that. Waste lasts just as long, and high pressure parts are currently the only way we have of getting good heat engines. Thorium is actually a particularly bad fuel for small mobile reactors, because any reactor capable of burning thorium is capable of burning regular low enriched uranium and producing weapons grade plutonium as a by product. Those are the last reactors you want somebody to be able to fly away with and hide.

    A few hundred kilowatts is very low for what you want it to do. An airliner turbofan will be putting something like 50MW through the fan at peak power.

    If you are going with nuclear anyway you might as well use it thermally rather than running generators into motors. Run a NaK loop through an otherwise standard turbofan and it won't care that the heat isn't coming from burning fuel. Biggest issue is not being able to fully turn off your engines, because they are also part of the coolant loop of a nuclear reactor. The largest plus is that using the airflow through the engines as your primary coolant means you are able to use high power outputs. The limiting factor for nuclear reactors is how you cool them, and 500mph airflow through the engine is hard to beat. The Mach 5 ramjet design shows what is possible.

  9. - Top - End - #39
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by aspi View Post
    But I wouldn't expect you to take my word for it based on a 100 word post. That's why I linked the video, which was made by an actual engineer and is a very nice and concise discussion of the viability of battery-powered commercial air transportation.
    I find videos unhelpful in general. 3/4 of them don't start, with a pop-up covering the screen saying something like "Before you begin, agree to our you have no privacy agreement", which is a straight "no" from me (by means of returning to the previous webpage I was looking at). Another large percentage start loud and get louder. The number that are actually informative is minute.

    So if you just want to argue for argument's sake, I'm sure you can find plenty that are up for it (I'm not one of them).
    I am only interested in argument as a way to find the truth.

    But if you want an answer to your question and a proper explanation that includes some of the math behind why it's not a reasonable assumption to expect battery-powered commercial flights anytime soon, I pointed you in the right direction.
    I'm not interested in maths. Anything that can be said in mathe can be said in English, or French, or any other spoken language, maths is a jargon and I don't want to learn it. As I said batteries have changed radically in my lifetime, and it's reasonable to expect that to continue, it may well not be as fast as the rate of change in computers from 1980 to 2010, but almost nothing else in history has been that fast.
    Last edited by halfeye; 2019-09-30 at 05:01 PM.
    The end of what Son? The story? There is no end. There's just the point where the storytellers stop talking.

  10. - Top - End - #40
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    Default Re: How should we make an electric air turbine (a jet motor that runs on electricity)

    Quote Originally Posted by halfeye View Post
    I find videos unhelpful in general. 3/4 of them don't start, with a pop-up covering the screen saying something like "Before you begin, agree to our you have no privacy agreement", which is a straight "no" from me (by means of returning to the previous webpage I was looking at). Another large percentage start loud and get louder. The number that are actually informative is minute.



    I am only interested in argument as a way to find the truth.



    I'm not interested in maths. Anything that can be said in mathe can be said in English, or French, or any other spoken language, maths is a jargon and I don't want to learn it. As I said batteries have changed radically in my lifetime, and it's reasonable to expect that to continue, it may well not be as fast as the rate of change in computers from 1980 to 2010, but almost nothing else in history has been that fast.
    It is reasonable to expect continued progress, but there simply is no chemistry which beats lithium fluorine for energy per kilo, and that gives you half the performance of current engines unless you rewrite thermodynamics (Is 'half' too much maths for you), and it is wildly impractical. It's not that we haven't found it, it does not exist. Every possible combination of light elements has been considered, and the heavy ones perform strictly worse. Carbon and hydrogen cheat by using many times their own mass in oxygen from the air.

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