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Will electric motors ever propel a 747?

  1. Oct 8, 2012 #1
    I've been told that a 747 will never be able to be propelled by electric motors even with room temperature superconductivity because batteries have too low of energy density.

    So if hypothetically we were able to develop a battery with energy density as high as jet fuel, would electric motors be fine for 747s?
     
  2. jcsd
  3. Oct 8, 2012 #2

    berkeman

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    The fans in the 747's engines do not provide much of the thrust... What does?
     
  4. Oct 8, 2012 #3
    Probably not for 747s, but I imagine it's vaguely possible to make some electric prop planes. Whether they're as practical as gas-fired prop planes or as impractical as nuke-fired prop planes is unknown.
     
  5. Oct 8, 2012 #4

    Drakkith

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    First I have to ask if there is even a way to use an electric motor to propel an aircraft at 500+ MPH.
     
  6. Oct 8, 2012 #5
    I think the problem with an electrically powered jet engine is that jet engines work by sucking in air, compressing it, injecting it with jet fuel and then igniting it, like how fuel is ignited in a combustion engine

    the air-fuel mixture then shoots out the back and provides thrust for the aircraft

    at least, that is my understanding of it

    so the reason why electric jet engines would be a strange idea is that the thrust comes from the ignited air-fuel mixture exiting the back end of the jet engine at really high speeds

    sort of like a rocket engine


    so like... the whole "jet engine" concept is very much centered around the jet fuel and how it interacts with air under pressure and how it ignites
     
  7. Oct 8, 2012 #6

    AlephZero

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    There's no problem in principle with using an electric motor to drive a fan. Just so long as you realize that the engine power of a 747 at takeoff is about 100 to 200 MW. That's not so much a battery as a small gas-fired power station.

    And what do you use to drive the generators in that size of power station? Most likely a modified jet engine....
     
  8. Oct 8, 2012 #7

    rcgldr

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    Remember the jet doesn't have to carry the air it uses to combine with jet fuel. Despite that issue, there's a big difference in energy density. Jet fuel A has a specific density of about 43 mega-joules / kg, while li-po batteries have a specific density of 460 kilo-joules / kg. This means that jet fuel has 93 times the energy density of li-po batteries.

    Links to articles:

    wiki_properties_of_jet_fuel.htm

    http://www.allaboutbatteries.com/Battery-Energy.html

    There are radio control models that use electric motors with props or ducted fans or rotors for power output with li-po batteries as the power source, but flight time is less than models that use fuel instead.
     
    Last edited: Oct 8, 2012
  9. Oct 9, 2012 #8

    CWatters

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    Actually modern jets have a high bypass ratio... between 5 and 9 times.

    http://www.pprune.org/tech-log/71070-what-engine-has-higest-bypass-ratio.html

    I think any electric motor used would need to generate >20,000 horsepower to be useful on a modern jet. Say 30,000 HP to replace a jet engine on a 747.
     
  10. Oct 9, 2012 #9

    cjl

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    The fan, actually. At subsonic speeds, it's most efficient to have the majority of the power go towards spinning the fan, since you get higher thrust at the same power level if you move a larger quantity of air with a lower exhaust velocity.


    I think it's higher than that - the power going into the fan on a modern jet airliner is close to a hundred thousand horsepower or so if I remember right (more on some of the really big engines), but I'll be the first to admit that I'm going off a fairly weak memory here, so I could well be wrong.
     
  11. Oct 9, 2012 #10
  12. Oct 9, 2012 #11
    I'm a turbo machinery design engineer working with gas turbines and jet engines.

    I'm pretty sure that with the right team, I could design and successfully market an electric engine for the 747 if the electrical engineers could first do two things for me:

    Give me a battery with the same energy density as jet fuel. This battery must be able to accept a full charge in the same time it takes to fill a 747 with jet fuel, about 20-30 minutes.

    and

    Give me an electric motor with the same power to weight ratio as a turbofan engine.

    More than half of the mechanical energy developed by a jet engine goes into operating the engine. Most of the rest goes to turning the fan, which provides nearly all of the thrust. A very small amount of the thrust is provided by the jet exhaust.
     
  13. Oct 9, 2012 #12

    Drakkith

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    Seriously? I never knew that. I thought it was practically all in the jet exhaust.
     
  14. Oct 9, 2012 #13
    I've never worked out why there is a jet exhaust at the rear of the plane. What is it connected to? Where does the exhaust come from? Surely all the fuel burns inside the engines mounted on the wings.
     
  15. Oct 9, 2012 #14

    Drakkith

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    I think the jet exhaust is the hot air and burnt fuel that exits the jet engine. What are you referring to when you say jet exhaust? Some kind of exhaust port or pipe?
     
  16. Oct 9, 2012 #15
    You can more or less tell by the width/ length ratio of the entire propulsion system, how much of the energy goes into the exhaust and how much into the fan (or propeller). The wider, the more the fan uses. The longer (as in on fighter planes and supersonic aircraft), the more goes into the exhaust.

    Are you talking about the APU?

    250px-A380_APU_P1230093.jpg
     
  17. Oct 9, 2012 #16

    Drakkith

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    Sweet, thanks.
     
  18. Oct 9, 2012 #17
    If your interested in all electric aircraft propulsion you should search distributed propulsion. Core engines are used to generate power from liquid kerosene and both mechanically transfer power to a bypass fan and also use cryognically cooled wiring/motors to power an array of electrical fans.

    Any gas turbine course will tell in a performance lecture that there is an optimum jet to bypass stream ratio, due to both propulsive and thermal efficiency effects. Duct losses, mixing losses etc.. play a role here. The all electric aircraft alters this ratio due to the possible ingestion of boundary layer. Therefore, one cannot visually tell how much thrust is coming from the bypass fan in this case.

    Batteries will not be used for a very very long time, even fuel cells are ineffective.
     
    Last edited: Oct 9, 2012
  19. Oct 9, 2012 #18
    Magneto speaks well, but his turbine course is a bit obsolete. Don't hold that against him. Billions are being spent on research and development, and we learn new things all the time. The greater the bypass ratio the better for many reasons. So we make it as big as possible within physical constraints. But we are pushing thru those constraints all the time.
     
  20. Oct 9, 2012 #19
    I look forward to the time aircraft eliminate the take-off from ground level and start airborne. Then HUGE bypass ratios could be possible, of course with an FOD trade-off :wink:
     
  21. Oct 9, 2012 #20
    Where does the FOD come from in an airborne launch? Birds and hail? We can design for that. Not much different from a conventional propeller.

    Take a look at the new PW geared turbo fan. It enables a much higher NO ratio.
     
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