How I designed a practical electric plane for a NASA competition | Hacker News Reader

How I designed a practical electric plane for a NASA competition

spectrum.ieee.org

119 pointssohkamyung63 comments

I work at NASA primarily supporting our aeronautics programs, and I'm excited about all-electric and hybrid-electric civil aircraft conceptual design analysis and optimization (for both fixed-wing and rotary-wing). It's one of the big "what-if" areas that is being looked at by NASA aeronautics in the next decade, and an area that has a surprising diversity of intuitive reactions within the industry at large.

Meaning, there is a camp if industry experts that feels that it is a dead end, and another that sees it as a practical way forward. My opinion is that there are gains to be made there, but practical systems will have to be designed and optimized in a far more integrated manner than how most civil aviation systems have been in the past (integrating, for instance, the design optimization of the propulsion, airframe, and mission profiles together at mid to high fidelity early in the conceptual design process).

And as far as I can tell, organizations struggle with doing multilevel and multidisciplinary optimization.

(I meant to upvote this, but somehow hit down. Could someone please fix that? It's actually a really good point relating to the development of new types of systems)

Fixed. I imagine a few other people might have as well, so you probably more than made up for it. ;)

> feels that it is a dead end, and another that sees it as a practical way forward

Until the Wh/kg ratio of batteries improves significantly, it will be a huge problem to have electric planes. Look at the energy contents of 1 litre of Jet-A or avgas compared to the same weight in batteries. Hydrogen + fuel cell systems to achieve the same energy density require very costly extremely high pressure tanks and fueling systems, and that is before you get into the argument of whether hydrogen is a terrible use of energy.

If your hydrogen supply is coming from an electrolysis process an IMMENSE amount of kWh has been consumed to fill your hydrogen tank, which should be considered for the total environmental cost. So unless your electrolysis plant is hooked up to a massive PV array or a 4GW nuclear power plant, the economics are questionable.

for comparison:

lithium ion batteries 0.36–0.875 MJ/kg

Jet-A: 42.80 MJ/kg

What efficiency is the engine converting those 42.80 MJ/kg into power at? I know it's still far above batteries, but what's the actual level batteries have to reach to be competitive in the power to weight ratio? Even that's not the full picture though, as for a system like this you would presumably include the engine weight for an engine that could produce comparable output, but that makes it more complicated.

>"Hydrogen + fuel cell systems to achieve the same energy density require very costly extremely high pressure tanks and fueling systems, and that is before you get into the argument of whether hydrogen is a terrible use of energy."

The main problem here is you are comparing it to a fuel source which took million of years to form naturally. Think of the energy expenditure behind that!

Battery technology will definitely need to improve before any system is practical enough to build and add to a fleet. I think there are power system assumptions being made regarding mid to far term viability of metal air batteries. Currently, they are expected in operation to be much closer to the 40 MJ/kg figure than existing battery technology:

https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...

That said, what becomes really interesting (and tricky) is that these types of batteries actually get heavier as they discharge, which throws off intuition for optimal trajectory analysis. We're used to thinking in terms of planes getting lighter as they consume their fuel...

A very enlightening read and quite excellent in breaking down the approach and technologies researched and employed in the concept! I'm an amateur in the arena[1], albeit with some first-hand flight experience, but study this kind of development every chance I can get. As an academic concept, this is superb and the aerodynamic advantage of the prop/tail design deserves high praise - that's some innovative thinking right there!

From an implementation standpoint I'm curious to how practical using a fuel cell will be from an aviation safety and regulatory standpoint. From what I've read regarding their implementation in automobiles, the testing is rigorous, so that's a plus! The author does point out that the plane needs to be accepted by the target markets, and honestly, I think the plane could be built and working by 2020 but there'd be a total of six mechanics qualified in the world to work on the thing. It might be an exaggeration, but give it a little consideration.

We're talking a significant overhaul - an advancement, sure! - to the existing notion of flight. Hey I'm cool with it if it works! I've got no love for burning tons of fuel in the upper atmosphere. Yet also I've been quite skeptical of all the excitement around electric battery tech - e.g. graphene, super-capacitors - until it starts getting used and tested in real world applications. Advancements are underway, sure; time will tell if they live up to the hype though. See also: Moeller Air Car. There's something to be said for the punch that Jet A still packs - the new FlyBoard air debuted recently and I'm not sure an electric version is possible yet, or within the near future.

[1] I participated in the LocalMotors LITECAR challenge and still am sore that an "exoskeleton" and "body panels made out of this lighter metal" entries were deemed more revolutionary than an organic composite sedan powered by microturbines and electric motors because what was said was the purpose of the contest did not match what was chosen in the long run. Grr.

There's something to be said for the punch that Jet A still packs

Personally I suspect that the US Navy nuclear-powered CO2-to-JetA system, or something like it, will be the long term solution when fossil JetA becomes prohibitively expensive. Electric prop planes are feasible but electric jets really aren't.

Very cool thought and I agree that using high energy density compounds is a hard habit to break. I'm not sure it's one that will need to be broken, as you point out. If there are "synthetic" ways to create such fuel, I think plenty of R&D is underway and we'll hear about them as they develop. I'm pretty sure the initial excitement has worn off but I'm hoping that aglae-based fuels can eventually be competitive as well.

When I read this article and also when I see quadcopter endurance tests (just over two hours) I come to the same conclusion as you do. Electric jets aren't happening.

So it is so strange and fascinating that Elon Must seems to think that it could be done.[1] Like: if we take todays lightest and most aerodynamic sail plane, add batteries and an electric motor, and it's still worse than a cessna [2] - how exactly does he reach the conclusion that electric jets, _vertically starting and landing_ jets, are feasible? I really wish for something like the hyperloop design-docs to explain the thinking behind it :)

[1] e.g. http://bgr.com/2016/02/07/elon-musk-electric-jet-great-idea/ - “Well I have been thinking about the vertical takeoff and landing of an electric jet a bit more,” Musk said. “And I think I have something that might be close. I’m quite tempted to do something about it.”

[2] MEA: https://www.iei.liu.se/machine/applications_re?l=en

> There's something to be said for the punch that Jet A still packs

I commonly hear "jet fuel" expected to be some exceptionally-potent fuel, but the truth is that it's very similar to regular old Diesel with aviation-specific additives.

Today, phrasing your statement relative to something like trucking, where hybrids are widely deployed, does not provide nearly as effective an argument.

That doesn't make it untrue, however. Fossil fuels have provided a naturally-occurring fuel source with a potency we are struggling to match. Everyone knows we've got to get away from them, though.

I have practical market experience on building a two passenger electrical aircraft and trying to market it, what I found is that "range anxiety" is a real issue for pilots and the market today for electrical planes is really limited to trainers, so we decided to modify our aircraft to an hybrid configuration.

As a private pilot: yes, you absolutely do not want to be landing on empty. Range is the same thing as endurance, and endurance means you have options when things don't go your way. Endurance is already a concern even on conventional motors - your average Cessna 150 only has a useful payload of 344 lbs with full fuel tanks. If you want to put two people in it, you're not taking off with full tanks, especially once you account for possibly thinner air above sea level or due to warm temperatures.

You need to keep a minimum (!) 30 minute reserve of fuel for VFR. The 150 has a 4-hour max endurance with full tanks. Usually you get less, because that doesn't include full-throttle time for takeoffs/climb. So if you take out half the gas (buying yourself another 76 lbs of payload) you've actually removed about 3/4 of the useful range of the aircraft. The same problem applies to battery-driven electric aircraft - extra weight kills endurance, and compensating for this causes a death spiral.

Twin-engine aircraft are really the way to fix this, but they're more expensive to get certified for, more expensive to run, more expensive to insure, etc. I'm a big fan of the Diamond DA42, twin diesels are cool as hell and it's totally nuts how slowly they sip fuel.

The other thing not mentioned in the article is that with batteries, as you drain them the output voltage also drops. That translates into reduced torque/power, and possibly an aircraft with different handling characteristics in stall situations. Conventional engines will deliver 100% power on-demand right up until they're out of gas.

The voltage droop isn't as bad as you'd think, I don't get limited power until I'm below 10% in our Tesla 85D. Even "full throttle" for most electric motors isn't near the max C rating(because amps = heat = power loss).

The rest of the stuff thought, yeah that's an issue and I feel like one of the areas where Fuel Cells might actually be applicable(you don't have the long-term leakage/storage/fueling issues as with cars).

An aircraft can't afford to be hauling around 2-3x as much battery as is actually necessary to fly, as is common in electric cars. I can assure you than when an electric airplane is at the limit of its range, its performance characteristics will change in a negative way.

Electric planes are already at a 100x disadvantage in terms of Specific Energy - liquid fuels (JP-1 jet fuel or gasoline) contain about 46 MJ/kg while Li-ion batteries contain about 0.36-0.875 MJ/kg. They cannot afford to make that a 300x disadvantage. It doesn't work in terms of lift.

As someone else posted in a thread, electric power already takes you from a 900 mile range to a 150 mile range. 50 miles is not acceptable because you're talking <30 minutes of endurance including takeoff, and you have zero safety margin if something should go wrong.

This article has me thinking about hybrid electric aircraft (is anyone working on that?). A small internal combustion engine could generate electricity to charge the batteries to run electric motors. With this set up, you could still get the efficiency advantage of the alternate propeller placement. The engine could be much smaller and lighter than it is conventionally because the additional power needed for take-off and climb thrust could be temporarily drawn from batteries. Further, in the event of engine failure, you would still have battery power available to the propellers for a short time in order to make an emergency landing.

It's definitely being worked on. Google "hybrid electric aircraft" and you'll find some discussion of the efforts.

It's less advantageous than it is for a car. For one, the difference between maximum power and cruise power is far lower in a typical light airplane than it is in a typical car. A Prius might max out at 130hp but cruise at 25hp. A light airplane often cruises at around 75% of maximum power. Another problem is that the hybrid powertrain adds significant weight, which hurts efficiency much more in airplanes than it does in cars. I don't think these kill the advantages entirely, but it's probably why it's not as far along in airplanes as it is in cars.

A light airplane often cruises at around 75% of maximum power.

Good point . . . I was probably thinking of larger, heavier, faster aircraft where cruise power is significantly lower than max power. As an extreme, I seem to recall that the Concorde cruised at about 20% power.

Obviously I don't know anything about these sorts of things, but 20% power may have been all the engines were capable of producing with the much thinner air at cruising altitude.

I guess that depends on whether by power it's generally meant "the max power the engines are capable of under ideal circumstances" or "max capable under current circumstances". I think the latter is generally what is assumed when talking about operating (since it's hard to know the former). For example, in a car, you would probably assume that meant the gas pedal half depressed (and even that would be grossly wrong to the actual meaning of the latter interpretation, given gearing and the RPM power curve).

Jet engines actually perform better at higher altitudes, bypass and propeller notwithstanding.

No, they are more efficient at higher altitudes, mainly due to the colder intake air, but they have lower performance (as in maximum available thrust), due to the air density being lower and there being less available to burn the fuel.

Right, the first thing to always remember when you're doing a hybrid is that you're stating up-front, "I want to have two engines in this machine -- that's a weight and space cost that I want to pay up-front." As mentioned by the article, with airplanes when you increase weight everything has to get bigger to accommodate it. That doesn't make it impossible, but does make it a bit tricky.

Another interesting question: the fuel cell design in the original post is probably not reversible, is it? It would be really cool if given 10-20 more years of work we then used variable-pitch propellers with reversible fuel cells allowing for a plane which, during descent, could regenerate most of the energy used to make its ascent. It might also make pilots less worried about limited range, because they'd know that in a pinch they could get some more energy to the battery.

Flight takes a lot of energy. Gliding let's you fly for longer at the cost of speed or altitude. You can also trade altitude for speed, but there is little waste in a normal descent.

> allowing for a plane which, during descent, could regenerate most of the energy used to make its ascent

The Pipistrel aircraft do almost exactly this. They claim about 10-15% energy recovery rate for thier regenerative systems. No need for variable pitch either; they simply windmill the main prop.

Max vs. cruise is a huge difference for VTOL though, which you need to replace cars for daily commute.

It's interesting that I read 'Google "hybrid electric aircraft"' without your quotes for a second and assumed Google was working on a hybrid electric aircraft, rather than you suggest people search for it with Google. Google is diversifying enough that just saying "google $term" without quotes (as you provided) can be ambiguous. Perhaps this is the long-term curse of google as a verb.

That's pretty funny, and I can totally see why you might read it that way. I guess technically now it would be an Alphabet hybrid electric aircraft? Not that I'm at all careful about saying Alphabet instead of Google.

Me neither, but I think this incident is illustrative as to why they changed it. "Google" has multiple meanings now (a site, a service, an action, and a company), and they probably want to distinguish their corporate element from those others.

I think one potential advantage is that turbofan bypass ratios are topping out due to the dimensions of the fans needed. A hybrid system would allow you have multiple fans per engine and also reroute power in case an engine failed.

I think turbofans also have issues with the difference between the rotational speed that is optimum for the turbine and that for the fans. Hybrid design would allow one to decouple the two.

I am working on a two passenger hybrid electric aircraft we decided for parallel operation.

I've looked at it before, in the context of R/C aircraft (my hobby). The problem is the same one everyone else has named: airplanes need to maximize power-to-weight ratio and hybrid-electric systems are heavier for a given power output.

My conclusion was that a battery-based system was a bad approach. Instead you would want to use a hybrid motor/generator approach - you can use a glow motor to spin an electric motor, and if you couple the electric motor to another identical motor they will spin lockstep. It's like a diesel-electric transmission on a ship or locomotive. And the funny thing is glow engines are actually a kind of diesel engine too.

https://youtu.be/6kgzrXFSDwA?t=13

This would be most advantageous for multi-engine setups because you would be able to use more energy-dense liquid fuels instead of batteries, use a single larger/more efficient motor as the prime mover, and avoid the complexity of multi-engine operation. With gas engines, you need to tune each engine to perform similarly, have a throttle servo with identical/aligned travel for each, independent fuel supplies (or at least header tanks), etc. Whereas electric motors are dead simple, but the problem is the batteries are hard to lift. So instead of batteries you have a prime mover - and on top of that you get liquid fuels too.

Ideally you'd probably want a single larger generator-motor attached to multiple small drive motors, but they would need to be chosen so that the electrical characteristics of the motors were appropriate for each other throughout the RPM range.

The downside is that it probably doesn't scale well to full-size aircraft - many small aircraft are relatively underpowered even with gas engines. And in both cases, having multiple independent power sources enhances safety, as this creates a single point-of-failure.

Pipistrel is trying: http://www.panthera-aircraft.com/technical-data

Fuel cells are energy inefficient. The future of electric air craft is elon musk's vtol super sonic design.

https://medium.com/@andrewt3000/what-do-we-know-about-elon-m...

Why would people would consider electric for airplanes? You want maximum energy density for weight. Jet fuel is pretty damn good when scored like that. We can make synthetic jet fuel from green electricity - if the process uses carbon from the atmosphere it's carbon neutral. Granted it would be expensive, but that's neither here nor there until fossil fuels surpass that price point.

I suspect well-meaning people like the idea of an electric airplane because it's greener, but FYI we live in a market economy. We will continue to extract and burn fossil fuels until it is uneconomic to do so. No matter how many people shout, no matter how much damage it does to the Earth, even if it kills us all. Markets don't care. If you have a problem with that, as I do, the only practical solution is to make fossil fuels not a cost effective alternative. Electric airplanes could be part of a solution - but only if they can compete on cost. I personally like the idea of taxing fossil fuels to factor in the negative externalities - but given the political makeup of the world as a bunch of competing nation-states, it seems unlikely. Game theory tells us there is too much incentive to be the nation state that doesn't support the agreement. Instead what I think will happen is that we'll innovate our way out of the mess by making green technologies more cost effective than fossil fuels.

"Fortunately, electric propulsion offers some flexibility that the engineers at Cirrus did not enjoy. Unlike combustion engines, electric motors are compact and efficient. These small, light motors can be placed in many more locations on the aircraft than would be practical for a combustion engine. If applied strategically, this tactic can distribute the power production across more or larger propellers. And the greater the area swept by propellers, the more efficient and quieter they become."

"The decrease in energy consumption and the elimination of the gasoline engine (and all the routine maintenance it requires) will likely reduce operating costs. What’s more, the reduction in noise level, from 92 decibels to 76 dB, should improve cabin comfort considerably. And the very high reliability of electric motors should give both pilots and passengers greater peace of mind."

Read the article. The motivations are made clear

Why would people would consider electric for airplanes?

Assuming one could build the infrastructure, beamed power for airplanes could be a huge win, given the hurdle of initial cost could be overcome so economies of scale could come into play. Much of the mass taken up by fuel could then be devoted to payload. The initial hurdles may well be too large to overcome, however, particularly for passenger service.

Interesting read. I don't have high hopes for hydrogen cars as batteries seems to solve the range problem adequately for most scenarios, and it's easier to build charging stations than hydrogen filling infrastructure. However, for planes it seems that the advantage of low weight is a necessity to electrification. I wonder if the future will be (mostly) electric road transport and hydrogen air transport.

Agreed on the perspective regarding hydrogen cars - I never quite figured out the thought process behind trying to make an experimental platform of that type into a mass-consumer product. I mean, I guess it makes environmental sense but the R&D path seemed so long and trying that EVs & hybrids really had a lot of growing up time in parallel.

For me I was always more interested in a turbine system for cars that would use an advanced biofuel (ex: eventual algae-based fuel) which would power electric systems (wheel motors, some batteries, KERS) and sort of convert they existing infrastructure into a more modern one - tanks, pumps, etc.

Even from an environmental perspective, hydrogen cars make little sense. Currently, hydrogen is a petroleum product. It's typically made from natural gas, and this process emits plenty of CO2 (plus all the pollution inherent in oil extraction). You can make hydrogen cleanly by using renewable electricity to power electrolysis, but nobody is doing that currently and the efficiency is terrible. You'd need three times as much generating capacity to power a fleet of hydrogen vehicles as you would to power an equivalent fleet of EVs.

I really can't figure out why Toyota is pushing them so hard.

>> "Submitted to a NASA competition for students, the design for this electric aircraft had to meet certain requirements, the most important of which was that it could be manufactured within five years."

Is this plane going to be in production?

A reasonable question! From my experience with general and professional aviators, the answer is a tentative "No" for quite a while. It would likely start out as an "Experimental" certification by the FAA[1]. Following that, I'm not sure what the course would be to official production for purchase (though Experimental planes can be purchasd on the free market to my knowledge).

[1] https://www.faa.gov/aircraft/air_cert/airworthiness_certific...

The easiest option is to sell it as a KIT where the buyer assembles it and requests certification as an experimental Light Sport Aircraft.

http://www.kitplanes.com/issues/29_3/builder_spotlight/Under...

I love to see it go to production.

Hydrogen fuelling infrastructure for Fuel cell + safety issues of carry two tanks of Hydrogen in a small plane need to be solved before production.

One Hindenburg like accident would kill the idea forever.

This is a fantastic read. But I'm now really curious what things look like at the big end of the spectrum: does the solution scale (or do similar benefits emerge) for 747-scale electric aircraft?

It all depends on the parameters that you are after.

Big without speed has already been done; see Facebook's 747 sized solar powered aircraft http://www.theverge.com/2015/7/30/9074925/facebook-aquila-so...

The issue is the amount of energy that you need to reach the speed of your average jet.

Just to put it in perspective jet fuel has a theoretical specific energy somewhere over 10,000 Wh/kg, compared to all of the options the author was exploring being well below 1000 Wh/kg

I am not an aerospace engineer, so take my comments with a grain of salt.

This will not really scale to the commercial airliner market. The author got most of his improvements out of propeller efficiency, and propellers are very efficient. However, they don't scale to jet speeds. I am not at all certain that an efficient electric powered jet engine could be produced (I suspect the physics just won't support it). If you are willing to take a step back and use hydrogen, I am very willing to believe that an efficient hydrogen powered jet engine could be built, but that would be completely new technology and I would bet it would take decades before it were available even if someone were to start working on it immediately. Besides, it is not clear to me what the advantage of using hydrogen over jet fuel would be. By weight, the energy density is higher, but by volume it is much lower.

Hydrogen has problems too. There is a chapter in the Ben Rich book "Skunk Works" on trying to build a hydrogen powered plane and the supporting infrastructure before they did the A12 & SR-71.

no, because:

lithium ion batteries 0.36–0.875 MJ/kg

Jet-A: 42.80 MJ/kg

quick googling says a 747-8 has a 239,000 liter fuel capacity, so the energy stored in full tanks is 10,229,200MJ

I don't know of any battery or hydrogen fuel cell type technology that can approach anywhere NEAR ten million megajoules stored in the same weight.

Having the electric motors at the top of the V tail planes indeed enables a short landing gear. But they increase the tail plane loads, requiring a heavier structure there.

I honestly cannot wait until these plane designs become more available .. electric flight would be my preferred way of getting around.

So I'm curious -- could a non-fuel cell electric generator (gas, turbine, etc.) make this design perform even better?

I was under he impression that fuel cells were very expensive compared to batteries. Is that not the case?

The contest required a cost analysis, but had no cost requirement.

http://aero.larc.nasa.gov/files/2014/09/2020-E-GA-design-con...

Yet the author thinks the cost is comparable, so there is that.

I was able to arrive at a design that, if mass-produced, could indeed compete with the Cirrus SR22. It would weigh and cost about the same, and its range would be very similar: about 920 miles. The plane’s cruising speed would be somewhat less—173 as opposed to 212 mph. But that seems a reasonable bargain, given that the electric aircraft would consume about a quarter of the energy per flight.

why not store many small batteries in the wings ?

As the article states a few times, weight is the main issue.

what part of lithium-ion batteries is heavy ? Lithium is light

The rest of them. Lithium is only the electrolyte. They also need cathode and anode materials.

Specific energy of gasoline: 46.4 MJ/kg

Specific energy of high end Li-ion batteries: 1.3 MJ/kg

it amazes me that we've achieved so many hard things (going to the moon, to Mars, to Saturn, making computers) and yet batteries are so primitive that we can't even beat gasoline

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