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Comment by montenegrohugo | Hacker News Reader

I understand the advantages of an electrical motor to an ICE, but I have doubts regarding your third paragraph. The energy density of Li-Ion batteries best-case scenario is around 1 MJ/kg, whilst Kerosene is 42.8-42.6 MJ/kg. That's a 40x !!! difference in energy/weight. Of course the electric engine will have a big efficiency advantage (think 80% vs 40% for ICE), but the gap in energy density is still enormous.

The other (huge) advantages in flight safety and reduction in noise and overhead costs still apply, but I don't believe electrical planes will be as disruptive as portrayed without significant advances in battery technology.

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Apples and oranges. You also need to factor in the weight of the engines and the reenforced fuselage you need to support jet engines. It's not just the fuel that is heavy. Electrical engines are really light and compact. The resulting planes can be much lighter.

Also electical engines can be throttled up and down with much better granularity. An idling jet engine still produces a lot of trust and burns a lot of fuel even before it leaves the ground. So the efficiencies you get from better control over the output are substantial as well.

Basically the current state of the art gives you a flight time of typically 1-1.5 hours with most electrical planes. If you double or triple that capacity through better batteries you basically get planes that can do short haul. The expectation is that that is basically happening over the next decade.

> Apples and oranges. You also need to factor in the weight of the engines and the reenforced fuselage you need to support jet engines. It's not just the fuel that is heavy. Electrical engines are really light and compact. The resulting planes can be much lighter.

Electric engines are lighter than ICEs but batteries are very heavy and unlike jet fuel they don't shed their weight as they are used.

It's also a lot faster to refuel a plane than to recharge those enormous batteries. That's longer turnaround times between flights, huge(r) delays if you need to stop to refuel due to weather, etc.

Depends, it takes about 20-30 minutes to start a jet; do all the pre-flight checks, etc. Add to that boarding, cleaning, etc. and most planes would be on the ground for at least half an hour and probably closer to an hour in between flights. That's plenty of time to recharge.

That can be mitigated by using modular batteries designed to be easily replaceable during any stop. Land, open the cargo bay doors, remove battery modules, add battery modules, take-off. This sort of operation can take far more time than unloading luggage due to the advantages of using standardized containers.

The first paragraph is incorrect. The wing has to be built to withstand lift, which far exceeds the weight of the engines. The weight force of the engine typically counteracts the lifting force, which means -- if you're clever with your design -- you can build a lighter wing structure with a bigger engine. This is why the engines are typically located on the wing in the first place -- aside from being easier to access for maintenance, it reduces the weight of the aircraft.

The second problem with battery-electric aviation is that most large aircraft aren't designed to land at their take-off weight. Landing gear typically comprises some of the heaviest materials on the airframe, so landing at a lighter weight than you take off at is a big advantage. This also manifests in the range of the vehicle. Burning fuel reduces the power needed to maintain lift, which give you additional range with your remaining fuel. By comparison, range with battery-powered aircraft would be linear.

The last hurdle is just the sheer energy density required. A 747 requires 50-60MW just to stay airborne at cruise, and will produce on the order of 250MW during take-off. Even a business jet produces ~10-15MW at max power, and will cruise at around 1.5-3MW. Over the course of a 10 hour flight, the lower bound is somewhere around 10-15MWh, which would be 37-56 metric tonnes of battery mass using the upper limit of current battery energy density (265Wh/kg). Keep in mind, it's not enough just to get to your destination, you also need a reserve while you're in the holding pattern, plus an additional safety factor.

There might be interesting new markets opening up in the personal air-taxi space, but I'm skeptical. Personally, I think the new wave of short-haul rooftop-hopping electric aircraft will prove to be a black hole money-pit, and a regulatory nightmare. The aerospace industry is littered with broken dreams.

Could we rethink short haul flights somewhat? What if instead of a 400 mph aircraft, we had a 200 mph aircraft with high aspect ratio wings? In other words try to make a more efficient aircraft to compensate for the lack of energy aboard.

I suppose people want to get there in a hurry, but a flight that goes from 30 min to 1hr or 1 hr to 2hr might not be too bad.

Maybe, though you probably still want to fly above the weather, which puts a lower bound on your power requirements. To get above the weather, the energy needed is mgh plus whatever kinetic energy your aircraft still has (i.e. it's velocity); if you trade that for speed, you won't have a 200mph aircraft anymore.

Customers are definitely willing to trade time for cheaper flight (Boeing made this gamble and won in the last airframe generation), but there are certain practicalities that need to be met.

If you mean the airlines when you say customers, I would say you are correct. Planes are flying slower these days because it saves fuel and one can offer a lower fare. If you mean the people who fly, when you say customers, I would say that is incorrect. People have no idea that flights can have different lengths and the only way to search for fares is by destination, number of stops, and price. It would be great if airline ticket buyers had search parameters for leg room and flight time so that those very important parameters could be selected for. Otherwise evolution will continue to force slow flights and smaller seats.

I always look at flight time since I'm flying with kids.

You're right, this is an important clarification. Airlines like smaller airframes that are easy to fill, have fewer engines, and they're okay with breaking journeys up into multiple smaller hops. Passengers don't necessarily have a choice.

I agree getting up over the weather would be a good thing. And as long as we're dreaming, may as well dream big. Maybe we can do something like they did with jato bottles on the c-130 at takeoff.

Attach some batteries with wings to the aircraft that you release after you get up to altitude. These days you can put an autopilot on the batteries and fly them back to the airport for recharging. You'd save some weight once at altitude and could get a 'free' lift up to altitude. Sort of like how a sailplane can get a tow. Maybe you could take them up to 30-40k ft at a good speed and let them trade some height for speed along their path to the destination.

Clearly there is a cost for the added complexity, but it might be worth it given the cheap computation and improving autopilot software.

That sounds absolutely terrifying. Iirc, the first rule of building rockets is, always assume it will explode - not even starting on all the stress it would put on the frame.

Rather than JATO bottles, a better option may be to have an 'electric runway', that allows you to draw ground-power while accelerating down the runway, which might save a non-trivial amount of battery capacity, and may allow you to run your motors at a higher power output than the batteries would normally allow. As an added bonus, the runway could be shorter.

Slowing down does not make an aircraft more fuel efficient. Best efficiency, as in fuel per passenger-mile, occurs with large aircraft at the very high altitude where air resistance is least. And to fly up there you need to move quickly. An a380, almost supersonic at mach 0.85, is more fuel efficient than Cessna at down low at mach 0.2. Even at lower altitudes, there is little advantage in flying slower. You just end up spending more time/energy keeping yourself up rather than moving forwards. A high-lift wing, great for maximizing time aloft, becomes a big draggy airbrake when you want to actually go somewhere.

In the extreme, the most efficient place to fly is in the most cold and dry air. That occurs at the tropopause, which is generally between 35,000 and 50,000 feet (-60*C). You don't get to play around at that altitude without a large complex aircraft.

https://www.windows2universe.org/earth/Atmosphere/tropospher...

> The first paragraph is incorrect. The wing has to be built to withstand lift, which far exceeds the weight of the engines. The weight force of the engine typically counteracts the lifting force, which means -- if you're clever with your design -- you can build a lighter wing structure with a bigger engine.

But couldn't you achieve the same effect by locating batteries in the wing? The point of the previous comment is that comparing fuel weight with battery weight is deceptive because electric engines are lighter. That doesn't mean an electric engine system is lighter weight than a conventional jet engine system. (So the structural design you're describing sounds feasible.)

Yeah you could; you would probably place the batteries in the wing where fuel is located, which potentially has benefits.

I was pointing out that the reasoning was flawed. The engine is not the limiting factor in the design of the wing, and in fact can let you reduce the mass of the wing. An electric motor and airframe might be lighter than an engine and airframe depending on the design. However, you do need to compare the battery weight with the fuel weight, because the energy has to come from somewhere, and an airframe is designed with that reduction in weight in mind. It's hard to stress just how important energy density is for an aircraft.

You're taking current batteries as the benchmark and comparing them to a 747. A 747 takes about 50K gallons of kerosene. Or roughly 150 tonnes; according to wikipedia the maximum takeoff weight for a 747 is around 500 tonnes. That's also a good explanation of why these big planes are being replaced by smaller ones; using that much fuel is stupendously expensive.

The reason why the engines and fuel are in the wings has more to do with the center of gravity. This needs to be where the wings are for the plane to be stable because of the non trivial amounts of fuel that get burned and the weight of the engines. If you'd store this in the back or front of the plane, the plane would get progressively harder to control through the flight.

With batteries, this is a non issue. You can put them wherever as long as the center of gravity is in the right place. It won't shift as you deplete the batteries. But still, the wings are probably going to be a popular place for this.

Comparing with a long haul 747 is of course a bit unfair. It is worth noting that these planes are being replaced by more efficient two engine planes that are lighter and much more efficient. But I agree it will be a while before we see cross Atlantic flights at that scale. But we're talking short haul here; 1-2 hour flights.

I didn't just compare with a 747 -- the battery numbers I ran were for the business jet. The 747 obviously looks far worse. If you want mass for a 2 hour journey in an electric business jet, divide those numbers by 5. Also the battery numbers I ran were for the upper range of current lithium-ion tech. If there are batteries on the market that have an energy density of 265Wh/kg, I haven't seen them. I'm no battery expert so I could be wrong, but from what I understand, most current batteries sit at around 60-70% of that.

A non-shifting static margin is certainly a benefit for electric aircraft, but we already successfully design aircraft that do have a shifting static margin. The interesting question is whether or not a constant mass would let you design airframes that would be impossible with fuel and ICE.

1-2 hours looks possible using current and near-future technology, but has anyone actually done a cost analysis on flying electric aircraft that are always at "full fuel" weight versus standard aircraft doing the same journey with half a tank? I can't imagine it's completely cut and dry, because the aerospace industry has had its eye on various forms of electric propulsion for decades.

Also, what's the turnaround on these things? Airlines want to make money, so they want to minimize the amount of time the aircraft spends on the ground. That's going to be a major hurdle, since even charging 2MWh over the course of an hour or so requires 2MW of power going into the vehicle. That's not an ungodly amount, but it's still non-trivial.

Most domestic flights in Norway last less than an hour, like Bergen - Oslo and Stavanger to Oslo.

Jet enegines have a combustion efficiency of 100% at low altitude and 98% at cruising altitudes. High bypass jet engines will operate 90%+ propulsive efficiency when you reach cruising speed and altitude.

This can include torbo props.

https://en.m.wikipedia.org/wiki/Propulsive_efficiency

Fuel has also a very big advantage over batteries and that is that you shed off weight when you use it while an empty battery will weight as much as a full one so you’ll end up carrying much more dead weight for the entire flight.

Fuel cells might be a possibility but I don’t think airplanes with hydrogen tanks are a good idea.

Out of my depth here. Why are tanks of Kerosene safer than tanks of hydrogen? Pressure?

Hydrogen is gaseous at normal temperatures. Both Kerosene and hydrogen are flammable, but being gaseous means that the resulting exothermic reaction will happen much faster and thus the energy will be released much more violently (think:explosion).

Additionally, as you said, storing hydrogen densely enough means pressured containers, or extreme cooling (like for rockets).

Hydrogen leaks out of literally any container it's stored in, if you fill up a pure lead bottle(a 25kg bottle only holds 1 litre of hydrogen) it will be empty after 2-3 weeks of doing absolutely nothing - hydrogen molecules are so small they just permeate through any safe to handle material. Obviously planes couldn't use tanks made out of lead because of weight, so whatever material they used would leak even faster - and as hydrogen forms an explosive mixture with air, that's not a good thing.

How do consumer-level hydrogen fuel-cell vehicles solve this issue? Does this mean you can't store them in a garage, because the leaking hydrogen could cause an explosion?

They do not - you're not supposed to keep them in enclosed spaces because of this exact issue.

Can't you just vent to outside of the plane? It's not like they are usually in enclosed spaces.

Hangars?

How does the metal cylinder of anaerobic gas mixture in my lab that has a percent of hydrogen in it keep it in indefinitely then?

Don't know - I only ever handled industrial hydrogen containers and I know they couldn't keep hydrogen in for more than couple weeks. I suspect that for it to be useful it needs to be stored as liquid, which means huge pressures = much higher leakage rate. If it's kept at gas it's probably still leaking,just not at a rate that would matter for a while.

Other posters already mentioned the storage factor, so I'll mention the safety factor:

Hydrogen is flammable, but kerosene is not; kerosene is instead combustible. Here's the technical difference between the two[0], but in practical terms it means that you can drop a lit match or cigarette into a tank of kerosene and nothing will happen. You can even heat a tank of kerosene directly with a propane torch without igniting it[1].

[0] https://en.wikipedia.org/wiki/Combustibility_and_flammabilit...

[1] https://youtu.be/7nL10C7FSbE?t=105

Liquid hydrogen is really, really, bad as a fuel.

The energy density per mass is very high, but it's also the least dense gas, so the volumetric energy density is absolute crap, even at high pressure. A bucket of kerosene contains a lot more hydrogen atoms than a bucket of liquid hydrogen, and unlike the hydrogen, it's safe to keep at room temperature, at normal pressure, and it's neither corrosive, flammable, or explosive.

I would say kerosene is flammable...

Flammable has a very specific definition - it must be easily ignited at room temperature. Kerosene is not easily ignited at room temperature (e.g. you couldn't light a puddle of it with a match), so it is not considered "flammable". It does burn at room temperature though, so it is considered "combustible"

You are confusing combustion efficiency with thermal efficiency. Having a combustion efficiency of 100% does not mean that 100% of the energy stored in the propellant is transformed into kinetic energy (the thing the plane is interested in).

The ICE engine with highest thermal efficiency reaches just 50%: The Wärtsilä-Sulzer RTA96-C turbocharged 2-stroke Diesel [1]

[1]https://en.wikipedia.org/wiki/Thermal_efficiency

That's quite an engine - I just looked up a picture of it https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg

The latest F1 tech has started to exceed this using their jet ignition system.

https://www.autosport.com/f1/news/131772/mercedes-engine-hit...

https://www.enginelabs.com/news/f1-tech-what-is-turbulent-je...

Combined cycle gas turbines are actually higher efficiency than this. The current state-of-the-art is somewhere around 63% thermal efficiency.

True, but those are usually not on a moving vehicle and have the luxury of being as heavy and big as desired. Engines on ships instead are limited by weight and size, engines on cars/trucks a little bit more, engines on planes much more and at the edge of technology we have the engines on rockets which have to be as light as humanly possible whilst still providing high thrust-specific fuel consumption

The propulsive efficiency as defined in the Wikipedia article is the product of the cycle efficiency (the efficiency turning heat into mechanical power) and the mechanical efficiency (the efficiency turning the mechanical power generated by the engine into movement of the airplane). The mechanical efficiency can be close to 100%, but the cycle efficiency is only 40% or so.

You could use fuel cells that work with more convenient fuels.

Would Li-air batteries be advantageous in this regard if they get them working?

Agree. I think the big breakthrough is going to be with figuring out some sort of replacement for the li-ion battery in terms of eletrical powered aircraft. Batteries are just way too heavy for how much energy they hold. There has to be a better way. I think we need some out of the box thinking for this one.

I know solar energy is advancing, what about if we put thin-film solar cells on the exterior of the body of the aircraft? That might allow greater distance travel without much additional weight. There is already solar powered aircraft that traveled around the globe called the solar one. It is experimental but still a good concept to look at.

Put kerosene in a tank to drive a generator to provide electricity when taking off, and as backup to generate DC for charging batteries when necessary.

Now instead of kerosene burning in a jet engine, "we are all fucked if it doesnt work", its burning in a simple compact, "fuck it if it doesnt work", generator.

Boeing-backed startup Zunum is doing exactly that and is planning on delivering their first 12 seat aircraft to JetSuite in 2022.

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

A turbofan engine is pretty much a turbine "generator" mated to a propeller. Turbine generators are preferred over piston-based generators where weight matters. Airplanes often have a separate turbine generator (called an APU) for on-board electricity.

That's a good approach, at least on paper.

There are a few nice things about this. One is, it decouples your power generation from your thrust generation. If the generator fails, you still have a perfectly usable engine for some time. You can declare an emergency and land at the closest airport (easy to do if you are GA in the US). Same goes for all fuel starvation scenarios, fuel contamination, fuel pump malfunctions (for aircraft that use those). If you are using a turbine as a generator, it can probably burn anything you'd classify as fuel. Electric engines will probably get away with longer periods between overhauls and are more reliable.

This will not be cheap though. It also introduces a high voltage, high power electrical bus where there was none before. And it is a more complex approach, with more things to go wrong, even if it may save in maintenance. For instance, take your average Cessna: you can lose your entire plane electrical system and your engine won't care. It has (redundant) magnetos which are coupled to the engine itself and will spark as long as the engine is rotating. Fuel is gravity-fed for the most part. So as long as your have fuel in your tanks (and throttle and mixture controls are in the right positions) the engine will run. Some of your critical instruments (like the artificial horizon) are mechanically powered (vacuum pump) so you can still use them without electric power.

If you lose your electrical system in a hybrid (or electric plane), or if you have to turn it off (fire?), this is now a engine-out situation. You are also going to lose your instruments.

< what about if we put thin-film solar cells on the exterior of the body of the aircraft

Not much of a gain, unless we are talking about ridiculously large wings (think glider-sized and above).

In other words combining energy density, efficiency difference and lighter construction enabled by electric engines, the effective energy density gap is about 1 order of magnitude.

In terms of engineering development over medium term, that is not very much, so I'll bet this will happen.

I doubt that you can make batteries with a higher engergy density, otherwise they are quite explosive.

How do you make them much lighter? Lithium is a pretty light material.

Petrol is so energy dense because you are storing that energy in chemical bonds. Electrical energy is not as "dense". I think we are close to "optimal" batteries.

E.g. you change the other part of the battery. "Air breathing" LiO batteries could reach energy densities in the ballpark of other chemical fuels. I think lab demo cells are around 5MJ/kg nowadays. Traditional chemical fuels have high energy density because we don't include the oxidizer, since it's readily available in the air. Any time you stuff a lot of potential energy into a small space with a low threshold for release you're making something dangerous...

hydrogen seems a good fit here. the extraction process can be sustained from renewable or low emission sources, refueling danger risk can be mitigated since these planes are not meant for general populace access.

how much safety is of a concern depends on application. for fully autonomous cargo planes you'd already reduce it significantly and can be improved further by routing them around and not above densely populated areas.

they'd be highly situational, but for those routes they could be approved for they'd be great.

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