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The various jet engines currently operating on the world's
aircraft are a modern marvel of mature engineering.
They're incredibly powerful for their size and weight, and
in spite of a few recent hiccups, the reliability is pretty
phenomenal too. Of course, their days are numbered. We
all know
the future is electric, and several milestones in
the development of electric aircraft have already been
passed <link>.
The main reason for the move toward electric propulsion
are the standout numbers, electric motors top 90%
efficiency routinely. Conversely, at a 500kt cruise, the
lowly hi-bypass turbofan is languishing down in the 80%
range. Since the power required to push, say a 777,
through the air in the cruise requires ~23 megawatts per
engine any efficiency savings would be big news.
So let's replace the gas-turbine engines with electric
motors to see how good things are going to be. Using
Tesla's 285kW motor as a guide, which weighs "about 70lb"
(32kg) so the mass of motor required for a cruise engine is
only 2600kg. Now, about 6 fold more is required for take-
off, so let's say 15,000 kg for the motor alone. Then we
need a fan assembly and some shrouding, mounting
hardware, reversers, etc. Say 1500kg for a 16,500kg engine
total.
The problem with the jet, is the low efficiency creates a
lot of heat that needs to be rejected, maybe 12 megawatts
leading to 1000C+ temperatures at the exhaust. The
electric equivalent won't need to reject so much heat, BUT
it will all be inside the motor. Worse, the efficiency falls as
the temperature rises, and the max temperature of even
samarium cobalt magnets is 350C.
So, beyond cooling batteries and speed control hardware,
we have a motor heat problem. OR do we have a motor
heat opportunity, eh?
The solution: Vaporizing water is a great way of losing
excess heat, see every power station ever and sweaty
people. Water takes 2,200 ish kJ/kg of energy to vaporize,
so, 12 megawatts (12mJ/s) could be dissipated with only
5.45 kg water/second (at max thrust) and 1kg/second at
cruise. So for both engines a typical transatlantic flight
could get away with as little as 8,000kg of water. Even
better, the steam could be directed through a gas turbine
to gain extra thrust, maybe a few more % efficiency could
be gained.
So, to recap. Our electric 777 is at 32,000 kg for its two
engines and 8000kg water. All up the engine package is at
40,000kg. How does this compare? Sadly the GE90 weighs
in at 8185kg, as a complete engine with fan assembly,
shrouding and ancillaries. Hmm. Well, I hear the batteries
are really coming on...
Electric cross channel aircraft
https://newatlas.co...rbus-cri-cri/38410/
[bs0u0155, Dec 30 2019]
Boeing 777 Specs Table
http://www.modernai...7/boeing-777-specs/
[Voice, Dec 30 2019]
Efficiency comparison of gas turbine engines
https://en.wikipedi...bine_efficiency.png
[Voice, Dec 30 2019]
Energy density of various fuels
https://en.wikipedi...wiki/Energy_density
Jet fuel has 43 megajoules per kg. [Voice, Dec 30 2019]
A lithium-ion battery can have as high as 265 Wh/kg
https://www.cei.was...battery-technology/
[Voice, Dec 30 2019]
Huskys: Sensible alternative powerplants for eco-aviators
Husky_20Powered_20Aircraft
[bs0u0155, Dec 30 2019]
Boeing 777-300ER Sets Takeoff Weight Record During Testing
https://boeing.medi...cord-During-Testing
Not your garden variety Boeing 777-300 [Voice, Dec 30 2019]
is verre naase
https://www.youtube...watch?v=Ea8GyscSFaQ
True faux french [bhumphrys, Dec 31 2019]
first step of the future?
https://mikeshouts....n-powered-aircraft/
making ions field(verb) the air in new ways [wjt, Jan 12 2020]
[link]
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// I hear the batteries are really coming on... // |
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Ah yes, clearly the Voices In Your Head have been particularly loud and insistent today ... |
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Electrochemical batteries can't ever deliver the order-of-magnitude improvement in energy density required. Nuclear isomer systems might, but are an emergent technology. |
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// Water takes 2,200 ish kJ/kg of energy to vaporize,... the steam could be directed through a gas turbine // |
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... except you don't want to be running your motors at anywhere near 100C, let alone the 150 - 200 C you need to drive a total-loss steam turbine. For the outer jacket to be at a temperature to boil water (even at 1 bar) the winding temperature is going to be hot enough to degrade the insulation and raise the winding resistance - the last thing you want, because it will send the motor into a thermal spiral and melt it. |
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Oh, and you can refuel a big aircraft with JP-1 in less than an hour - try recharging a battery that quickly. |
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Batteries could be swapped out. |
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What if the aircraft has to divert ? |
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Batteries will be airframe-specific (certification requirements); JP-1 is universal. So if there's no fully-charged battery on hand, all you can do is wait. |
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The 300 has a dry weight of 160,500 kg and a maximum takeoff weight of 299,370 kg, leaving 138,870 kg for cargo, pilots, and passengers. So if the problems [8th] mentions can be addressed that leaves about 95,000 kg for batteries and passengers. Leaving aside takeoff and landing (by begging the question that efficiency would be about the same) we can note the approximate figure of 70% efficiency, quod linky. |
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A 777-300 can hold 171,176kg of fuel, or 7,360,568 megajoules. 1 watt-hour is worth 3600 joules. To hold 7,360,568 megajoules you need a battery capacity of 2 billion watt hours. I would go on, but I've screwed up somewhere, as per my calculations you would need 7,812,500 kg of batteries. Someone check my math please. |
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I suspect it will be a long while before batteries are viable
for long commercial flights. Before then, we've got the
option of bio-
derived fuels or even (despite the storage problems)
electrolytically-generated hydrogen. And that's assuming
that we continue to move away from fossil fuels that you
can just pump out of a hole in the ground. |
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Hydrogen is several-fold more energy-dense than
conventional liquid fuels, which must go a long way to
offsetting the problems of containment. As far as I can tell,
regular fuel tanks are never designed or intended to survive
a crash; if you're going to spill your fuel upon impact, you
might as well have it disperse upwards. |
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[
placeholder for 8th to go into tedious detail
] |
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// the storage problems // |
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What "storage problems" ? The Shuttle main engine system worked perfectly well ... |
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Oddly enough, big passenger jets are one of the few non-
space situations where it might be economical to store
hydrogen as a cryo liquid rather than under huge pressure.
This is even more true when you realize that wasteful boil-
off will only happen while the plane is sitting on the ground;
once it's in the air, the constant removal of hydrogen to
feed the engines will keep the tanks nice and cold. |
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Boil-off whilst on the ground could either be burnt off, or
pulled off through a duct alongside the airbridge.
Personally, I'd like to see Concorde with a proud flare
emerging from the tip of its nose. |
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//a long while before batteries are viable for long
commercial flights.// |
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I'd already got there, what with energy densities etc. I
was having a think about how electric motors handle heat
badly, and how surprising it was that thermal difficulties
are a fairly insurmountable barrier to the electric motor. |
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I started solving that with some water cooling, then found
out that turbofan are 80% efficient, rather than my ~50%
ballpark for most IC engines. |
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So now I'm in a place where batteries are an order of
magnitude off in terms of energy density, electric motors
are capped in power output density, and electric motors
are no more, possibly less efficient at getting electricity
into thrust. They're definately less efficient when you
take grid/charging losses into account. |
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You can't make an electric jet liner, and it would be
dumb if you could. |
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It is, however, possible to make a nuclear thermal turbine - open or closed cycle - and a nuclear thermal ramjet (open cycle) too ... |
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And if you can get high enough so you can spark it off, you can use a deuteron ram. |
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<Outrageous faux-french accent> |
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That sounded more like an outrageous faux-Italian accent. |
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//Someone check my math please// |
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Working it a slightly different way, jet A1 is about
45x10^6 J/kg. Good Li-Ion is about 1x10^6 J/kg. So 45
fold. 777 can take 181,000 litres at 0.8kg/l is ~144,000 kg
(variable w/temp) so about 6.5 million kg of Li-Ion
batteries. So within the sort of wiggle room you'd expect,
same sort of numbers. The 777 may have some issues at
those kind of weights. |
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[Voice] looking at the energy density wiki, I see fat
metabolism is in the same region as gasoline for energy
density. That puts my husky powered aircraft significantly
ahead of electric alternatives. |
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It certainly puts reindeer power ahead of batteries... no eco-friendly sled for Santa for a while yet ... |
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Minimum of one order of magnitude required; realistically, two. |
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Wouldn't just a lightening bolt pulsed jet be more efficient than a hefty motor if the vast numbers of electrons were available? The question is, is there a better way with unlimited amounts of electrons and the control thereof? |
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An alternative would be to spool out two very-high-voltage
wires from the wingtips as you go, powering the plane from
the ground. |
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You're looking at maybe a cubic metre of copper (or
aluminium) wire, which shouldn't be prohibitive. Of course,
you'd have to ensure your flight path didn't cross anyone
else's. |
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Could operate like a trolleybus, with contact arms that reach out to slide along a network of high-altitude overhead wires. |
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May we draw to your attention the problems encountered by high speed rail using overhead electrical supplies ? |
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Extremely high wear rates of both the catenary conductor and the pantograph pickup are normal, because of the very high contact pressures needed to prevent bounce and arcing. |
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Now scale that to a situation where the vehicle lacks a rgid support and is travelling at 500 - 600 KIAS... |
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The answer, clearly, is to have the cables loop around
motorized spools on the wingtips. They can therefore be
used for traction. |
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Sheer genius. May we please reserve some tickets for the inaugural run of the MaxCo Mt. Everest gondola lift ? |
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No reservation is necessary - we already booked you in for the
second test run, which will happen as soon as we finish
picking the pieces of the test dummy out of the gondola walls.
We're quite optimistic, but would
nevertheless be grateful if you could return the Intercalary's
absinthe spoon before the test. |
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// Extremely high wear rates of both the catenary
conductor and the pantograph pickup are normal, because
of the very high contact pressures needed to prevent bounce
and arcing. Now scale that // |
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Nice idea; however, an arc can be literally "blown out" by a high velocity gas stream (vide the design of air-blast HV circuit breakers ) and the high temperature will cause erosion and oxidation of the conductors, leading to embrittlement and failure. |
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A carbon (graphite) contact pickup can be fed out (as in an arc lamp, like a MIG welder) but maintaining the arc spacing will still need some physical contact between the catenary and the pickup. |
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//then found out that turbofan are 80% efficient// |
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That’s not right. Somewhere around 45% is closer
to the mark. |
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Okay, let's split the difference and call it 7 million kilograms. The new 777ER has a dry weight of 138,100 kg and can have a takeoff weight of 307,722 kg. So fill one with batteries instead of fuel. Assume 1/23rd the range for the lower energy capacity and divide that by 1.5 again because you won't be taking off at maximum possible takeoff weight. The 777ER has a range of 9100 freedom units. You can haul 400 passengers 263 miles, suitable for some short-hop routes. But the airplane isn't suitable for most short-hop airports. If you string a power cable along the runway you might be able to extend that to 280 or 290 miles. |
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You missed a slight detail in your sums. Civil airliners are expected to have a minimum of 60 minutes fuel reserve; landing with significantly less than that amount will result in Questions Being Asked. |
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The reasons are obvious and perfectly rational. You are in the circuit at a single-runway airfield when there is an incident and the field is obliged to close. The actual reason is immaterial; it could be nothing more than a burst tyre on an arriving service meaning the runway has to be cleared and checked. That takes a while - after all, it needs to be done right. |
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So you may need to go around and hold for a while, or possibly divert. There is usually somewhere within 30 - 45 mins flying where you can land, but remember that all the other inbound traffic now has the same problem so you are not alone. |
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So your theoretical flying time - to meet regulatory requirements - needs to be (taxi and takeoff time) + (flying time) + (mandatory reserve) + (a bit more for luck). In reality, pilots get fairly intense about landing when they get even slightly close to "reserve" fuel levels. This is because they have no wish to make an unplanned descent with empty tanks. |
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On the plus side, that greatly reduces the risk of a post-crash fire. |
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So before starting your calculations, you need to including a minimum two hour flight capability before considering the actual journey. |
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Electric car out of juice ? Sit in it and curse until assistance arrives. |
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Electric boat run down ? Well, you can paddle or row, if it's small enough - otherwise, you're looking for a tow. |
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Electric aircraft, airborne with a flat battery ? Ah. Not good, particularly if it's a die-by-wire control system. Say "Goodnight", John-boy ... |
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<Delves in jar of ointment with forceps, extracts fly/> |
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Knew it was in here somewhere... |
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Now, there may be some outside the aviation industry who are oblivious to previous issues with Lithium-chemistry cells on aircraft, either as cargo or auxiliary energy storage - but within it, there's a very high level of awareness. The fact is that such cells burn ferociously if damaged, and resultant fires have caused at least one catastrophic crash. |
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Intuitively, an electric aircraft seems less vulnerable to fire than one using JP-1. But is that correct ? Jet kerosene isn't particularly volatile - and an electric system needs hundreds, if not thousands of cells to power it, each of which must be "made without flaw" and good for the design life. A single point failure can ignite a whole block of cells, with disastrous consequences. Currently, it isn't possible to build cells that are both light and fire-resistant - the biggest problem is the organic electrolyte. |
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Then there are the cables - and junctions - between the energy storage and the propulsion system, and the electronics needed to throttle the power delivery. |
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So anything put into service needs to be at least no worse than current (ha ha) technology, bearing in mind that unlike ships and ground vehicles you do not have the option to run or swim away. |
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Fire suppression can be installed in the battery compartment(s) - but that adds dead weight, and although it may contain or extinguish the fire, it doesn't compensate for the loss of capacity, or help with structural damage or combustion products which may invade the passenger compartment. With a gas turbine, the most likely place for a fire is the engine nacelle - not inside the fuselage - and the slipstream will drag the fire back away from the wing. Of course there are wheel-well fires, and electrical fires behind panelling in the cabin, but just looking at the propulsion system, an inboard fire is a low probably event. |
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Hmm, promote it up as a weight loss regime, use liposuction on the passengers to get a very calorific fuel... |
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Since the USA appears to have pretty much cornered the planet's supply of outrageously obese humans, that immediately puts them ahead in the renewable aviation fuel stakes.... |
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// have the cables loop around motorized spools on
the wingtips. They can therefore be used for traction
// |
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Put the motors on the ground and pull the cables with
the plane attached. Fuel problem sorted. |
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"You realise that this means the end of the horse drawn zeppelin !" |
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