Will electric motors ever propel a 747?

[Ryuk1990]

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?

 

[berkeman]

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?

The fans in the 747's engines do not provide much of the thrust... What does?

 

[Angry Citizen]

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?

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.

 

[Drakkith]

First I have to ask if there is even a way to use an electric motor to propel an aircraft at 500+ MPH.

 

[SHISHKABOB]

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 engineso 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

 

[AlephZero]

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...

 

[rcgldr]

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.

 

[CWatters]

The fans in the 747's engines do not provide much of the thrust... What does?

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.

 

[cjl]

The fans in the 747's engines do not provide much of the thrust... What does?

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.

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.

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.

 

[harrylin]

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.

Nothing to imagine!
http://solarimpulse.com/en/airplane/
http://www.howitworksdaily.com/transport/how-does-the-solar-impulse-plane-work/

However, that plane weighs almost nothing.

 

[Pkruse]

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.

 

[Drakkith]

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.

Seriously? I never knew that. I thought it was practically all in the jet exhaust.

 

[mikeph]

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.

 

[Drakkith]

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.

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?

 

[Lsos]

Seriously? I never knew that. I thought it was practically all in the jet exhaust.

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.

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.

Are you talking about the APU?

 

[Drakkith]

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.

Sweet, thanks.

 

[MagnetoBLI]

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.

 

[Pkruse]

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.

 

[MagnetoBLI]

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

 

[Pkruse]

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.

 

[MagnetoBLI]

I'm mean HUGE bypass ratios that would touch the ground in conventional T&W configs; much larger than the P&W turbofan. Even distributed propulsion has limits due to wing span etc.. Design of bird damage would rise considerably though.

 

[mfb]

Seriously? I never knew that. I thought it was practically all in the jet exhaust.

That would be similar to a rocket - possible, but extremely inefficient.
It is easier (requires less power) to accelerate more air with a smaller velocity difference, at least small compared to the air speed.

 

[Pkruse]

The goal at cruise for an airliner is for the fan exhaust to be one mph more than air speed. Can't really do that, but that is the theory. If the outside of the exhaust flow approaches that, it is OK for the center of the flow to be faster. They also shape the nozzle for smoother and more gradual mixing.

 

[Ryuk1990]

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.

Don't chemical engineers typically work on batteries instead of EEs?

 

[Pkruse]

Ryuk1990: Yes and no. I have a friend who is a professor of chemistry at UCF, as is his wife. They have been working exclusively on battery development for 40 years. Certainly, these are the people who develop new technology. But to me working a real project on a real airplane, I'd expect the battery to be spec'ed out by an EE. He may very well coordinate with the professor and his wife, but as a mechanical design engineer, I would not know that.

 

[Ryuk1990]

Check this out. Elon Musk, CEO of Tesla Motors and SpaceX is planning an electrically-powered supersonic jet that also hovers! What the hell?

http://www.slashgear.com/elon-musk-is-planning-a-supersonic-electric-powered-hovering-jet-04245866/

 

[Pkruse]

Ryuk1990: Two things are true in that article beyond dispute. He is not an aeronautical engineer, and this will be much more difficult than designing a spacecraft to dock with the ISS. Everything else he says can be debated.

But he has proven himself extremely adept at identifying excellent engineering talent, and then hiring it and retaining it. He is also very good at inspiring other people to invest lots of money in his ideas. With those skills, he can accomplish anything.

 

[Enthalpy]

An electric motor is very easy to make as good as a gas turbine. It needs no superconductors. You can take an absolutely banal squirrel-cage motor. Accept slightly higher losses than usual, for instance 5% or 10%, cool it accordingly, and then it gets as compact and light as a gas turbine.

Have a look at power plants with a gas turbine: the generator is smaller than the turbine. Sure, at a power plant, the turbine is optimized for efficiency more than mass or size, but so is the generator.

Better: electric motors improve with peripheral speed and with size. If you have an impression of big and heavy electric motors, it's because they're small at rotate at 5m/s. On a plane, they would be granted 100m/s.

This one is a steam turbine, but the general impression holds for a gas turbine:
http://de.wikipedia.org/w/index.php?title=Datei:Turbinenhalle_KSP.jpg&filetimestamp=20070806180528
the generator is yellow and the turbine blue.

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A light battery is difficult to make despite so many brilliant people work hard on it. Already useable:
http://yuneeccouk.site.securepod.com/Aircraft.html
and since motors, propellers and airplanes improve with size, I suppose a commuter airliner is feasible.

But fuel cells may be ready sooner. And especially if they burn hydrogen, the mass of fuel necessary for the same trip is less than kerosene, and the volume isn't that huge, so the plane design is perfectly possible - we "only" need better fuel cells, with the propoer power and mass.

Some planes, especially drones, fly already on electric motors for tens of hours.

 

[Ryuk1990]

^Hey, which specific drones fly on electric motors?

 

[Enthalpy]

These search keywords give half a million hits:
drone "fuel cell"

http://www.tgdaily.com/sustainability-features/63000-green-drone-hydrogen-fuel-cell-powers-scaneagle
http://www.msnbc.msn.com/id/4522779...s-technology-choice-power-us-military-drones/
not necessarily the best examples. I believe to remember that at least one drone is operational, not just a prototype.

Few years ago I put some figures on an electric engine to propel a commuter, replacing the turboprop. Even without a gear, the electric motor was easy. This helped:
- Accept a good diameter (no exaggeration needed). Take many poles and a hollow rotor to save weight.
- Electronics engineers shall make the inverter with the proper frequency...
- Accept some losses, like 5% or even 10% instead of the usual <1%, since cooling is easy.
- My paper attempt was a squirrel cage. No permanent magnets needed.

I got the same mass, diameter, torque and power as a turboprop - except for the electricity source... I had put it in some forum, including the propellers distributed along the wing; was it Physforum back then?

Knowing that, I don't really understand why the recent paper design (at MIT? Read it at bbc.co.uk) wants to have superconducting alternators (at the turbine) and motors (at the distributed propellers). Copper or aluminium are good enough and available right now; an optimised design would give the proper efficiency.