Electric jet engines

[Linguofreak]

I've thought of a few modified turbojet/fan engines, and I just wanted to get an idea of how feasible they might be for various uses:

1) Put an electric motor on the shaft of a normal turbine engine, and plaster the wings with solar panels. Advantage: during daylight, the solar panels take up some of the load of driving the compressor and fan, if present, reducing the amount of fuel that needs to be burnt to drive them. Disadvantage: at night, the motor and panels are dead weight. Does anybody have any idea of how these would balance out in practice?

2) On a multi-spool turbofan, replace all but one spool with electric motors, and put a generator on the remaining spool (since motors and generators are heavy, you can save weight by connecting at least one fan/compressor stage directly to the turbine). If desired, you can also add solar panels as in the case above. Advantages: Mechanically simpler than a multi-spool turbofan. Disadvantages: Probably heavier.

3) If operating somewhere with an inert atmosphere (say, Venus), use solar power alone, and cut out the turbine, leaving just a compressor and fan. Where the combustion chamber would be, have a bunch of tungsten filaments (probably in some type of ceramic tubes to keep the filaments from being knocked about too much by the airstream). When operating at low speed, you just run the fan. As you start moving faster, you kick in the compressor and put current through the filaments, heating up the "combustion" chamber in order to heat the airstream and produce jet thrust. Advantages: No fuel required. Disadvantages: Solar power may not provide sufficient power to operate at speeds where jet thrust (as opposed to fan alone) is needed, and the energy density of batteries is low. A traditional jet carrying both fuel and oxidizer may do better, as might a nuclear jet. Also, the types of activities likely to be performed on worlds with inert atmospheres are unlikely to require high speeds (you're unlikely to have long-distance commercial aviation or much military activity).

What do people think?

 

[N_Molson]

If operating somewhere with an inert atmosphere (say, Venus), use solar power alone

I'd say that Venus atmosphere reflects or absorbs quite a lot of sunlight. It's like an eternal acid mist down there. :hmm:

 

[RisingFury]

The electric motors are not the problem.

The RC community has been using electric motors now for 15 years since the brushless designs hit the market. They're incredibly powerful and quite efficient.

There are two types:
- Outrunners have a stator in the middle and a rotor around it. They provide lots of torque at the expense of RPM.
- Inrunners have the rotor inside, but the stator outside. The provide lots of RPM, but limited torque.

Whichever one you pick, both can have lot of power. You can easily buy 10 kW electric motors for RC planes, though they are expensive.

The motors are also very efficient. 1 kW motors are around 85% efficient and they just go up from there, with multi kW motors being 95% or more efficient. 10 kW or more will be 99% efficient.

That's where the good news ends.


Along with the motor, you need two more things: A battery or other power supply and an electronic speed control, an ESC.

You're thinking of solar panels as a power source. They would not be able to provide enough power, but might take the edge off. The question is: Is their weight worth it. I don't know the answer to that.

Batteries are a problem as well. Our current best batteries are LiPo. They store only 1 / 30 of the energy that fuel does, but engines are inefficient at converting energy. With efficient motor and ESC, you can convert 90% of the battery power into thrust, while jet engines will at most convert 30-50%. That means a battery is effectively 1 / 10 of the energy capacity of fuel.

Better batteries are needed. There are two concepts: Lithium Sulfur, which might hit the market in a year or two. They're better than LiPo, but brand new. Still not enough.

The last concept is Lithium Oxygen. That one is promising, but very difficult. The idea is that the battery would pick up the oxygen from the air when needed and release it back when no longer needed. That'd make the battery much lighter than LiPo or LiS, because both have to carry Lithium or Sulfur to react with.

LiO batteries are promising and are what would be needed to deliver, but so far they're far off, if even possible.


The last thing you need is an ESC. It is also quite an expensive and fragile bit. It converts DC power into pulsed DC, turning on and off the coils in the motor. The ESC that can handle 10 kW is really expensive...
Electric cars already have high power ESCs, so this should not be too much of a problem.



RC community has long known the idea of an electric jet. We call them electric ducted fans or EDF:

It's just an inrunner with a fan.

It's the poor man's jet turbine, which will offset you several thousand dollars...

 

[boogabooga]

When you talk about "multi spool" turbofans, are you thinking on a commercial gas turbine engine scale?

Power you get from a wing area worth of solar panels:
might meet the electricity needs of your house if you are lucky.

Power you get from a modern high-bypass turbofan engine:
might pull your house off of its foundation and distribute the debris for several hundred meters.

Different scale.

It would be like augmenting your car's engine with a wind-up music box.

 

[Linguofreak]

When you talk about "multi spool" turbofans, are you thinking on a commercial gas turbine engine scale?

Yeah, but for the multi-spool replacement idea specifically I wasn't thinking so much of augmentation with solar panels as an integral part of the design, in that case it's mostly just a scheme to have the fan and different compressor stages working at their own optimum speeds without needing each to be driven by its own turbine on a separate spool.

 

[Urwumpe]

Well, I would maybe use electricity rather for augmenting classic jet engines by letting electrics do all the stuff that currently requires to tap off power from the shaft, waste it in a constant speed drive and then let it to stuff in the most inefficient way.

But maybe that's just me.

For something like Venus, I would rather think about other concepts. For example the Meredith effect - if you use the heat generated inside the probe for heating the air in a Ramjet like arrangement, you can theoretically get more thrust than drag. The question is, how much energy is needed and how effective could it be compared to a propeller?

 

[Artlav]

First, solar panels are a no-go. A wingful of them can generate kilowatts, jet engines produce megawatts. It's like trying to augment a fusion reactor with human power - off by 3 orders of magnitude.

Second, a "jet engine" on a commercial plane is a high-bypass design, effectively a big ducted fan spun by a little turbine engine.
Instead of trying to hybrid it, just replacing the turbine engine core and driving the fan with electric motor would be a much more practical solution.

Realistically, it's not the motor that is a problem, but storing enough electric energy for the flight. There are no batteries even remotely close to the power and energy density of kerosene.

Sure, you can build a heavy, slow going plane that would be full-electric, but you'd get about the performance of a zeppelin for ten times the price and a tenth of the cool factor.

 

[RisingFury]

Power you get from a wing area worth of solar panels:
might meet the electricity needs of your house if you are lucky.

Ok, so that's the instant dismissal of the idea, but turns out things are more complicated.

Yes, planes would only receive a few percent of energy needed for the trip during the trip, assuming daylight. But if the panels and engines are light enough, airlines will adopt them because they'd save... a few percent of fuel. But it gets more interesting than that.

Planes don't just fly. They spend a lot of time waiting on the ground. Loading, unloading, fueling,... panels on planes would generate power during such time.

Obviously not enough energy would be generated, so batteries would still need to be charged from the grid, but if the panels are light enough, they might be economically viable.


The real issue are batteries. We need batteries at least 10 times better at storing energy per unit of weight and ones that can be reliably charged and discharged over many cycles.

 

[MaverickSawyer]

Nah, the added complexity of the solar panel wings would cost more than the savings. And on the ground, they use grid power to keep the plane energized when waiting for more than, oh, 30 minutes or so. Fast turns, they just keep the APU running. Saves time, which is also money saved.

 

[RisingFury]

Nah, the added complexity of the solar panel wings would cost more than the savings. And on the ground, they use grid power to keep the plane energized when waiting for more than, oh, 30 minutes or so. Fast turns, they just keep the APU running. Saves time, which is also money saved.

For small, regional planes the turnaround times are fast, but the jumbos typically fly an average of once per day.


The first electric planes are starting to take off. This is an electric you'll soon be able to buy:

Pipistrel Alpha Electro

http://www.pipistrel.si/plane/alpha-electro/overview

Yes, it's a small 2-seat ultralight.

But this is where I remind you of the relationship between power and velocity:

P = k * v^3

Decrease speed and you'll decrease power by A LOT. The first commercial electrics we'll see will be short haul regional routes.

 

[Donamy]

Use the passengers as batteries, like the Matrix.

 

[Linguofreak]

First, solar panels are a no-go. A wingful of them can generate kilowatts, jet engines produce megawatts. It's like trying to augment a fusion reactor with human power - off by 3 orders of magnitude.

Yeah, I ran the numbers on the actual power delivered and felt really stupid: the wing area of a 747 won't even deliver the full throttle power of a P-51.

 

[Urwumpe]

Use the passengers as batteries, like the Matrix.

If you focus on managers as customers, you need less power for being afloat, because even a lead battery would allow neutral bouyancy. :dry:

 

[boogabooga]

First, solar panels are a no-go. A wingful of them can generate kilowatts, jet engines produce megawatts. It's like trying to augment a fusion reactor with human power - off by 3 orders of magnitude.

...

Sure, you can build a heavy, slow going plane that would be full-electric, but you'd get about the performance of a zeppelin for ten times the price and a tenth of the cool factor.

It's been done (there is a thread here somewhere):

https://en.wikipedia.org/wiki/Solar_Impulse

Looks like it can put out about 40HP total and puts along at 40 mph. So forget P-51, even with twice the wing area, would still be less powerful than your car.

I don't think people understand how much energy is contained in 10kg of a petroleum fuel.

 

[perseus]

NASA's X-57 Electric Research Plane
With 14 electric motors turning propellers and all of them integrated into a uniquely-designed wing, NASA will test new propulsion technology using an experimental airplane now designated the X-57 and nicknamed “Maxwell.

pecifications
(based on the proposed Mod. IV configuration)

Goal – Five-fold reduction in energy use at high speed cruise compared to traditional propulsion.

Objectives

Mod II – Demonstrate 3.0x lower energy use at high speed compared to original P2006T.
Mod III – Demonstrate 1.2x lower energy use at high speed compared to Mod II.
Aircraft Weight – Approximately 3,000 pounds.

Cruise Altitude – 9,000 feet.

Cruise Speed – Approximately 172 mph.

Critical Takeoff Speed – 58 knots (67 mph).

Batteries

Lithium Ion.
790 pounds.
140 kilowatts continuous, 300 kilowatts max.
461 volts nominal.
69.1 kilowatt hours (47 kilowatt hours usable)

Cruise Motors and Propellers (2)

60 kilowatt Joby.
Air-cooled, direct drive, permanent magnet 6-phase out-runner.
5-feet in diameter.
14-inch outer diameter.
57 pounds.

High-Lift Motors and Propellers (12)

5-blade, folding propeller.
10 kilowatts.
Air-cooled, in-runner.
1.9 feet in diameter.
Approximately 22 pounds.

https://www.nasa.gov/image-feature/nasas-x-57-electric-research-plane

Lilium

Lilium enables you to travel 5 times faster than a car by introducing the world’s first all-electric vertical take-off and landing jet: an air taxi for up to 5 people. You won’t have to own one, you will simply pay per ride and call it with a push of a button.

https://lilium.com/

 

[Ripley]

Mmmm...VTOL in an electrical sounds to me like a lot of wasted energy.

 

[Urwumpe]

Mmmm...VTOL in an electrical sounds to me like a lot of wasted energy.

Maybe, but it reduces infrastructure needs. A VTOL needs not more airport than a helicopter.

 

[jedidia]

You won’t have to own one, you will simply pay per ride and call it with a push of a button.

Now I can't stop thinking about all the things that could happen if I board one of these with my kids... :blink:

 

[Urwumpe]

Now I can't stop thinking about all the things that could happen if I board one of these with my kids... :blink:

Your kids travel alone without you? :lol:

 

[jedidia]

Me and my kids crash and burn because I couldn't prevent them sufficiently from jumping around and otherwise aggressively disassemble the darn thing in flight seems more like it :lol: