if a jet engine catches fire at 30,000 feet you turn off the fuel. jet fuel in the tank is inherently stable. Although there is a case where a fuel tank exploded and destroyed the plane the fuel is almost inert in the tank. you can throw lit matches on it and it won’t ignite. It has to be injected at high pressure to get it atomized enough to burn or heated enough to do the same thing.
Lithium batteries have a different set of problems. If they go into thermal overload the resulting fire is not something that can be contained. The NASA site I linked to actually lists this as a reason for research into the liquid batteries I described up-thread.
There was a real world test of that theory on 9/11. According to that article the contrails reflected sunlight back and dropped the temperature during the day. It also reflected heat back at night. So it’s not a cut-and-dried issue but at least there was test data from 9/11 to compare against modeling.
that’s also at the expense of lifespan. I’m deeply involved in R/C vehicles, and I’ve seen (and backed slowly away from) more puffy, swollen LiPo packs than I care to mention.
yeah, I had a motorcycle battery do that. Scary stuff. the motorcycle sits in a detached garage so I don’t worry about it burning the house down with me in it. Sadly the battery box is too small (IMO) for a proper sized lead acid battery. I really like the power the Li battery can deliver.
The main reason that fuel-burning engines are more efficient is that they get to keep the most reactive of their reactants outside of the fuel tank, in the atmosphere as a whole. The important ingredient isn’t the hydrocarbons per se; we could replace those with something else. The important ingredient is the oxygen, but we take it for granted because it’s so easily available.
What if hydrocarbon burning planes were required to capture all the CO2 instead of emitting it to the atmosphere. Wouldn’t be quite so efficient anymore, would it?
Hence the lithium-air battery. Use ambient oxygen at the cathode instead of dense metal. Promising, but needs a great deal of development before they’re appropriate for vehicle use.
They’re *really *pushing the extremes, though. A commercial aircraft doesn’t need to zip up at 3 gees like a racing quadcopter can. Back off from that just a bit and you can have a more reliable system.
Also, I suspect the biggest problem with LiPo isn’t with using a high discharge rate but rather squeezing it to the highest possible voltage. I’ve had a couple of phone LiPos go puffy on me and they weren’t discharging at a high rate. Instead I suspect they were charging to a higher level than was safe because the phone maker decided to make the phone 0.1mm thinner by sacrificing battery volume. That’s not something one would do in a more safety critical situation.
Transcontinental and intercontinental flight will require major, unknown developments in battery tech, though. I’m glad NASA is researching this, but it also shouldn’t stop us from electrifying regional flights.
Electric aircraft can use ducted fans with no problem. Propellers are limited by the prop tip speed–you really don’t want it to be supersonic (otherwise you end up with a plane loud enough to cause seizures). Put it in a duct and it’s fine because the shockwaves can’t propagate outside.
Props may still make sense at lower speeds, and tend to be more efficient than ducted fans, so maybe we’ll see those first. Turboprops are more popular on short routes anyway.
Only if, for some reason, you require the plane to carry the carbon-capture equipment with it. Just leave that on the ground: As long as it’s still capturing as much as the plane is producing, it’s fine.
Most aviation energy is not consumed by general aviation or business aviation – it is by commercial air transport. I don’t know the exact numbers but I’d estimate over 90%, maybe over 95% is by commercial air transport, ie airline and cargo operations. Therefore any meaningful electric aviation solution must include that or at least articulate a plausible path using attainable technology to achieve electrically-powered commercial air transport.
“Short haul routes” are under 3 hr, although some authorities use 3,200 km (1,727 nm).
What would be the energy or battery requirement for airline transport operations over (say) 1,500 km? Ideally this should provide roughly similar payload and travel time performance, else all of society is plunged back to the pre-jet era. Electric-driven jets are not really possible but electric-driven propellers are. Propeller-driven airliners with near jet performance have existed: the Tu-114 (based on the Tu-95). Fortunately its engines are rated in shaft horsepower which avoids the tricky conversion of pounds thrust to horsepower. Tupolev Tu-114 - Wikipedia
The Tu-114 had 4 x 14,800 hp engines. If we assume cruise power at 77% or 45,584 hp, that is 33.9 megawatts. A short-haul flight of two hours would therefore require roughly 67.8 megawatt-hours. How much battery power would be required to fulfill that?
33.9 MW is engine output, not including gearbox and propeller efficiency. To provide this electrically we must consider battery efficiency (say 90%) and electric motor efficiency (say 90%) for overall efficiency of 81%. We won’t consider efficiency losses from motor controllers or other sources. This means the batteries must hold about 83.66 megawatt hours (not including reserves) for a two hr “short haul” flight of about 1,500 km.
The battery pack on an 85 KWhr Tesla Model S weighs about 540 kg (1,200 lb). We would need about 984 of these which would weigh about 1.18 million pounds. The Tu-114 had a payload of about 55,000 lb, but 130,000 lb of fuel for a total payload of 185,000 lb.
So very roughly, we’d need an improvement in battery energy/weight ratio of about 10x to make this feasible. To recharge 10 of these after landing would require an on-site dedicated two gigawatt power plant, and the assumption each aircraft can sustain an approx 100-200 megawatt charge rate.
The maximum possible improvement for battery energy density improvement using currently-known physics is about 2x for lithium ion, about 3x for lithium sulfur, and about 4x for lithium oxygen. Lithium sulfur and lithium oxygen are mostly research items, not commercial products.
This doesn’t mean the above improvements will be achieved, but those are limits beyond which improvement is unlikely using those chemistries: https://youtu.be/AdPqWv-eVIc
If you combine, say, 2x battery improvement, 2x energy efficiency improvement from lower speed, better aerodynamics, lighter materials, then it should be possible to eventually have (essentially) a battery-powered 72-passenger ATR-72 with an approx. 500 mi. range. This could serve routes like Hawaii inter-island service, etc. ATR 72 - Wikipedia
There is currently no realistic envisioned technology that could produce a battery-powered 737-class plane, much less a larger plane. It appears that all airliners from “short haul” category on up must continue to burn liquid fuels, either hydrocarbon or hydrogen.
This fascinating chart shows the dilemma: Li-ion batteries are at the lower-left of the energy volume/density curve, whereas hydrocarbons or hydrogen is at the right or center: Wikizero - File:Energy density.svg
Electric airplanes as anything other than a novelty or experimental craft are not going to happen until we have a breakthrough in battery technology, and until we solve about a million certification issues. It’s going to be extremely hard to get one of these certified for civilian private use, let alone for commercial transportation.
For example, these quadcopter-based ‘personal flying craft’ that you’re seeing in development are death traps. An engine failure on any engine will result in a crash. They fly too low to use ballistic parachutes. The range is ridiculous - half an hour to an hour aloft is not nearly enough for a serious flying machine. In fact, if I’m flying a plane and I discover that I’m half an hour to dry tanks, I’m looking for a place to land NOW.
IFR flight requires that you carry enough fuel to make it to your destination, plus be able to divert to an alternate, plus be able to land with a 30 minute reserve. No electric plane can meet that requirement, and none will be able to until we have much better batteries.
Lithium-ion batteries aren’t even approved for airplane transport, other than personal-sized batteries for laptops and such, which must be in your carry-on. There has already been a fatal crash of a jet when the lithium-ion batteries in cargo caught fire. I cannot see the FAA certifying an airplane carrying tens of thousands of pounds of the things and actively using them for power. Certainly not without huge amounts of certification trials and testing and explosion-proof or fireproof designs.
I could go on. We need to have certification trials for any electric motors used, new procedures would have to be developed for monitoring and testing them regularly, TBO’s established, yada yada. The aircraft itself will need type certification, which is extremely hard to get. One of the reasons you don’t see new aircraft designs very much is because the certification costs are outrageous. That’s why 2019 Cessna 172’s look nearly identical to versions made 40 years ago.
All-new aircraft designs are extremely difficult to certify. Cessna wound up with the Cirrus SR22 because Cirrus nearly went bankrupt trying to certify it. And those costs mean that these small aircraft, much simpler than a modern car, can cost a million bucks. Hell, just trying to get a conventional Porsche engine certified for private aviation just about broke Porsche, and they abandoned the effort.
Here we are talking about certifying entirely new airplane concepts with entirely new engines and power systems that have never been used in aviation before. There would also need to be extensive software, and that too has to be certified. Ask Boeing how that’s working out for them. Now imagine trying to stand up a complete new flight control system for a radical aircraft with a new power system. Good luck.
Most of these planes are gimmicks anyway. If you gave me a plane with a half-hour range until it plummets out of the sky, I wouldn’t leave the traffic pattern in it. It’s entire range wouldn’t even meet the requirements for VFR reserves for private pilots.
These things would also have to fly at lower altitudes, as props aren’t going to fly in the jet levels. That means they’ll be less efficient and the passengers will get to stay in the weather the whole flight. These planes also wouldn’t have the endurance to be able to climb to a reasonable altitude and then descend again.
One of the Google guys has a company that is supposedly building an electric sport plane. Or rather, it started out as a potential plane until the reality of battery energy density and the inability to certify such a plane was discovered. Then they tried turning it into a sport vehicle for use on the water, with software limits preventing it from rising more than 15 feet and 25 mph. But even with those severe restrictions they can’t get the thing safe enough, so they’ve scrapped it and are working on something else. Aviation is hard.
You’re going to continue to see wacky experimental planes, and the companies that make them will be looking for funding so they will promise all sorts of bullshit to keep the bucks rolling in. Save your money until you see one of these things fly 500 miles with several people in it, and a proper track to reasonable certification can be shown.
I believe modern ones are lithium iron phosphate (LiFePO4) with integrated battery management electronics. LiFe cells are known to be less temperamental than LiIon or LiPo cells.
Not if an engine is out. My guess is that a quadcopter that can be certified for land use will have to be at least a hexacopter, or maybe an octocopter, before the loss of an engine is survivable anywhere but on the ground. That will increase weight and complexity and maintenance costs and all the rest. But quadcopter-based designs are not passively stable either, and have other issues that may prevent them from being anything other than a novelty. For example, wait until you see how much crap on the ground is kicked up when one of these things tries to land. And some of that crap is going to go up and back through the blades and wear them down. A helicopter rotor is way higher off the ground than quadcopter blades.
We’ll also need to develop the ability to inspect these electric motors and be able to tell when they are near failure. And you would need redundant electrical systems. Hell, light aircraft still use magnetos because electronic ignition is a point of failure and hard to certify. And we use dual magnetos for redundancy. Perhaps an electic plane would need multiple battery packs with the abiliy to shut down and isolate a failed one while still powering the plane with the others. But that too would need extensive testing and certification.
If you want to register on of these as experimental, you can certainly do so. Maybe someone will offer a kit. But don’t expect to see one certified for commercial use any time soon. I’d say that even if we had all the requisite technologies available right now (better batteries, etc), it’d still be a decade or more before you’d see a plane certified to carry passengers commercially.
Consider the Beech Starship, a plane that used standard engines, control systems, etc. But it used composite construction and a pusher configuration with a canard. Designed in 1979, the first proof-of-concept plane flew in 1983, but the final certified production plane didn’t leave the line until 1989, ten years after the design had been done. And most of that airplane used off-the-shelf certified components.
Current batteries are not nearly capable of reasonable duration, even for regional flights.
A Boeing 737, a common regional jet, burns on average 750 gallons per hour. There are about 39 kWh of energy in a gallon of jet fuel. The best lithium ion batteries have an energy density of around 250 Wh per kilo, or about 159 kilos of batteries to replace a gallon of jet fuel So one hour of flying a 737, all else being equal, would require 119,000 lbs of batteries, while the fuel weight would only be 5100.
The lithium ion batteries for one hour of 737 flight would weigh more than the maximum takeoff weight of an entire fully fueled and loaded 737. And one hour of fuel would only give you a 15 minute range with a reserve. That gives an idea of how much efficiency you would have to gain to create a viable commercial electric aircraft.
Until and if batteries have much, much more energy density, these planes aren’t going to be in the air long enough to be able to climb to and descend from the flight levels. A 737 uses as much fuel to climb to altitude as it does in an hour of cruising. If our plane only has an hour of endurance, it isn’t climbing far.
Right, I was just addressing the limited range, not the robustness against failure. An airplane needs a half-hour reserve of fuel, because if you can’t land at your intended destination, then you might need to fly a half hour to find some other suitable place to land. That’s not an issue when your suitable landing spots consist of “how about right there?”.
But yes, having four independent mechanical systems, the failure of any one of which is catastrophic, is poor engineering, especially for something that’s going to carry humans.
You’ve definitely demonstrated that retrofitting an electric system onto a 50s-era Soviet craft is a non-starter. I would dispute the relevance, though.
Agreed, more or less. These are the kinds of electric planes I expect to see in the first round (aside from air taxis, etc.). They require some advancements, but no “magic”.
Size works in favor of electric planes. Electric planes will live or die on their efficiency, and larger planes are more efficient. If you can build a 72-passenger electric plane, you can build a 500 passenger plane.
What is difficult is range and speed. There isn’t much demand for an A380 with a 500-mile range (or a 9000-mile range, for that matter). And the lower speeds we can expect matter more for long routes. So initially, I think we’ll see replacements for the kind of smaller turboprops you mentioned, where an extra 15% flight time is not a big deal. It’s not because large passenger counts are impossible, but just because the smaller craft work better in that market.
If nothing else, since many of the technologies are a step change, keeping the R&D budget down by starting with smaller craft will probably be necessary.
