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Thread: Supersonic Electric Aircraft

  1. #31
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    Quote Originally Posted by swampyankee View Post
    Have you ever heard of the F-84H (video: https://youtu.be/YItexQxJS9U)?

    Deafen, nauseate, and possibly cause incontinence, but no reported deaths
    Well, you didn't LET it get to that point silly! I know they're merely Airforce ground crew but I'm sure they might have SOMEBODY who cares about them.
    Time wasted having fun is not time wasted - Lennon
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  2. #32
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    Quote Originally Posted by swampyankee View Post
    The combustors are as small as they are because they're direct contact heat exchangers; electric heating elements can provide comparable heat transfer rates to liquid metal or molten salt heat exchangers. The latter were planned for Pratt & Whitney's turbojets to be used on the (thankfully) cancelled nuclear-powered bomber.
    Simpy supplying an equivalent amount of heat is only half the issue. There is still the issue of whether heating inert air causes it to expand as much as combustion of fuel.

  3. #33
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    Quote Originally Posted by DaveC426913 View Post
    Simpy supplying an equivalent amount of heat is only half the issue. There is still the issue of whether heating inert air causes it to expand as much as combustion of fuel.
    Why would it not? Gas turbines work perfectly well with combustion reactions that result in a smaller volume of gas at STP: 2H2 + O2 = 2H2O. The combustion reaction with kerosene results in a greater number of moles of product, but that's the only effect. Combustion is only a source of heat energy for the Brayton (gas turbine) cycle; closed-cycle gas turbines have been made and proposed; see for example, this and this.

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  4. #34
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    Quote Originally Posted by swampyankee View Post
    Why would it not? Gas turbines work perfectly well with combustion reactions that result in a smaller volume of gas at STP: 2H2 + O2 = 2H2O. The combustion reaction with kerosene results in a greater number of moles of product, but that's the only effect.
    Is that not the effect we seek? Greater expansion?

    Without this increase in output, an air-heating engine would surely need to be comparably larger/heavier than its fuel-burning counterpart, to achieve the same level of thrust.

    Quote Originally Posted by swampyankee View Post
    Combustion is only a source of heat energy for the Brayton (gas turbine) cycle; closed-cycle gas turbines have been made and proposed; see for example, this and this.
    I'm afraid my layperson's knowledge of engine design doesn't help me assess these as to whether they're valid comparisons.

    I'll have to take your word that an electric heater can expand a volume of fast-moving, inert air - to the same volume, and at the same rate - as that produced by the combustion of fuel - using comparable weight and size of engine.

  5. #35
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    Quote Originally Posted by DaveC426913 View Post
    Is that not the effect we seek? Greater expansion?

    Without this increase in output, an air-heating engine would surely need to be comparably larger/heavier than its fuel-burning counterpart, to achieve the same level of thrust.


    I'm afraid my layperson's knowledge of engine design doesn't help me assess these as to whether they're valid comparisons.

    I'll have to take your word that an electric heater can expand a volume of fast-moving, inert air - to the same volume, and at the same rate - as that produced by the combustion of fuel - using comparable weight and size of engine.
    Compare the quantity of air going through the engine with the quantity of fuel consumed by it, and consider that most of the air is inert nitrogen, and not even all of the oxygen participates in the combustion reactions.. Even for those reactions that produce a volume increase, the effect is tiny.

  6. #36
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    Quote Originally Posted by BigDon View Post
    Well, you didn't LET it get to that point silly! I know they're merely Airforce ground crew but I'm sure they might have SOMEBODY who cares about them.
    Umnh, this is the same era in US history when soldiers were put near nuclear airbursts to see how they would function....

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    How do things fly? This explains it all.

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  7. #37
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    Quote Originally Posted by cjameshuff View Post
    Compare the quantity of air going through the engine with the quantity of fuel consumed by it, and consider that most of the air is inert nitrogen, and not even all of the oxygen participates in the combustion reactions.. Even for those reactions that produce a volume increase, the effect is tiny.
    What do you mean the effect is tiny? The ignition of jet fuel in a stationary container is sufficient to cause an explosion and disintegration of the container and an impressive area surrounding it. That is entirely due to the chemical release of energy - before we even get the engine moving.

  8. #38
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    Quote Originally Posted by DaveC426913 View Post
    What do you mean the effect is tiny? The ignition of jet fuel in a stationary container is sufficient to cause an explosion and disintegration of the container and an impressive area surrounding it. That is entirely due to the chemical release of energy - before we even get the engine moving.
    It's not that important in a gas turbine engine. The reaction is about Cn+((3n+2)/2)O2-->nCO2+(n+1)H2O. The combustion reaction may produce fewer moles of reactant than there were moles of product. The important effect is the heat energy added to the air; it doesn't matter from whence it came.

    Besides, there's no need to go electric > Brayton cycle > thrust. Modern supersonic aircraft, several of which super cruise, get some portion of thrust from the engine's unheated bypass air, so driving the fan stage with an electric motor would work.
    Last edited by swampyankee; 2017-Oct-09 at 09:23 AM.

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    How do things fly? This explains it all.

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  9. #39
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    Quote Originally Posted by swampyankee View Post
    Umnh, this is the same era in US history when soldiers were put near nuclear airbursts to see how they would function....
    My father was at Crossroads and was 32 miles from Castle Bravo.

    Everybody involved was proud to be there, not victims.
    Time wasted having fun is not time wasted - Lennon
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  10. #40
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    Quote Originally Posted by BigDon View Post
    My father was at Crossroads and was 32 miles from Castle Bravo.

    Everybody involved was proud to be there, not victims.
    Thirty-two miles was probably far enough, but the VA is investigating excess cancers among servicemen involved in US nuclear nuclear tests. See https://www.publichealth.va.gov/epid...pons-tests.asp. Also, many of the health risks, especially long-term, weren’t understood.
    Last edited by swampyankee; 2017-Oct-09 at 09:52 PM.

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  11. #41
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    Quote Originally Posted by DaveC426913 View Post
    What do you mean the effect is tiny? The ignition of jet fuel in a stationary container is sufficient to cause an explosion and disintegration of the container and an impressive area surrounding it. That is entirely due to the chemical release of energy - before we even get the engine moving.
    Ignoring the fact that jet fuel in a closed container will not explode...as already pointed out, hydrogen serves as a clear counterexample with the end product having fewer molecules (2 H2O for every 2 H2 and 1 O2) and a lower volume at a given temperature and pressure than the inputs. A hydrogen-oxygen mix will still explode nicely, because the release of energy is in the form of heating, not an increase in the number of molecules. The change in molecule count is even less significant for hydrogen in air, because most of the working fluid doesn't participate in a chemical reaction.

    For jet fuel, assuming decane, C10H22, 2 molecules of decane combine with 31 molecules of O2 to form 20 molecules of CO2 and 22 molecules of H2O...overall, an increase from 31 to 42 molecules.

    https://web.stanford.edu/~cantwell/A...ngine_Data.pdf

    The GE-90 has a mass flow rate of 576 kg/s at cruise (mach 0.85, 10.67 km altitude), plus 1 kg of fuel...assuming decane, C10H22. Simplifying the composition of air to 80:20 nitrogen-oxygen, that's an input of 3600 moles of oxygen, 16500 of nitrogen, for a total of 20100 moles.

    Combustion of 1 kg of decane burns 7 moles of fuel, consumes 108.5 moles of oxygen, and produces 147 moles of end products, for an increase overall of about 0.2%.

    Jet engines are heat engines, the work is almost exclusively done by the thermal changes resulting from combustion. Non-thermal effects are way down in the noise. There's no reason an electrically heated jet engine wouldn't work, though again, that's not how you would actually implement an electrical aircraft, electrical motors being far simpler and more efficient.

  12. #42
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    Quote Originally Posted by cjameshuff View Post
    Ignoring the fact that jet fuel in a closed container will not explode...
    I did not say it was full of jet fuel to the brim. It's partly full, the other part being air.

  13. #43
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    The supersonic for the F-104 is about 0.048; that for the Phantom is about 0.0439. is usually based on wing area. At 15,000 meters, std day conditions, temperature is 216.65 Kelvin, density is 0.19476 kg/m^3, c=295.0695 m/s

    For an F-104-sized aircraft, wing area is 18.21 m^2. Using only , drag is about 55,000 newtons at M=1.8 (531 m/s), so power required is a minimum of about 30 MW. This may be a bit of a challenge. The polytropic efficiency of a low-bypass engine's fan is probably about 85%
    Last edited by swampyankee; 2017-Oct-10 at 12:33 AM.

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



  14. #44
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    Quote Originally Posted by DaveC426913 View Post
    Is that not the effect we seek? Greater expansion?

    Without this increase in output, an air-heating engine would surely need to be comparably larger/heavier than its fuel-burning counterpart, to achieve the same level of thrust.
    Confirmed btw that the contribution of the fuel mass to the thrust from a jet engine is negligible.

  15. #45
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    You can get silly amounts of power out of lithium ion batteries. You might damage them, but look on the bright side: you probably won't have enough power to land or a vehicle that can glide effectively, so that's not that big of a problem in the grand scheme of things.

    http://www.espritmodel.com/thunder-p...ery-packs.aspx
    About 1600 kg of those would manage 30 MW. The problem's that they'd only do it for <25 seconds.

  16. #46
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    Quote Originally Posted by VQkr View Post
    Confirmed btw that the contribution of the fuel mass to the thrust from a jet engine is negligible.
    Mass wasn't at-issue; expanded volume was.

  17. #47
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    Quote Originally Posted by DaveC426913 View Post
    Mass wasn't at-issue; expanded volume was.
    At a modern engine's turbine inlet condition, CO2 and H2O are behaving pretty much like perfect gases; volume is pretty much dependent only on the number of moles of product.

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



  18. #48
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    Quote Originally Posted by swampyankee View Post
    At a modern engine's turbine inlet condition, CO2 and H2O are behaving pretty much like perfect gases; volume is pretty much dependent only on the number of moles of product.
    I don't see how that comment follows from either my comment or VQkr's.

    VQkr said that the mass of the fuel doesn't contribute much. I don't know why he said that; no one was suggesting it did. What we were talking about was volume.

  19. #49
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    Quote Originally Posted by DaveC426913 View Post
    I don't see how that comment follows from either my comment or VQkr's.

    VQkr said that the mass of the fuel doesn't contribute much. I don't know why he said that; no one was suggesting it did. What we were talking about was volume.
    Accelerating mass causes the reaction force in a jet or any other engine. If "volume" helps you think of it more clearly then yes that too...

  20. #50
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    Quote Originally Posted by DaveC426913 View Post
    I don't see how that comment follows from either my comment or VQkr's.

    VQkr said that the mass of the fuel doesn't contribute much. I don't know why he said that; no one was suggesting it did. What we were talking about was volume.
    As was I. At turbine entry, a modern engine has a pressure of about 3 to 4 megapascals and a temperature of about 1500 K. At that that tempurature and pressure, behavior of the combustion products is close to perfect, so volume at a given temperature and pressure is dependent only on the number of molecules of gas -- the number of moles -- not its mass: a mole of xenon has the same volume as a mole of hydrogen.

    In other words, talking moles is talking volume.

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



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