Solar Impulse lands in California after Pacific crossing | Hacker News Reader

Solar Impulse lands in California after Pacific crossing

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90 pointsfrgewut45 comments

The Solar Impulse is really cool technology. It also shows why oil use has been so prolific. the energy-to-weight ratio of jet fuel is a marvel when you consider the size of today's jet aircraft and the distances they fly with hundreds of passengers.

Indeed. I expect we'll synthesize jet fuel or a close equivalent (something that burns and reduces airframe weight) from electric or bio sources long after most other engines have moved to electric. Perhaps we'll generate electricity on board, but it will be with a dense, wasting fuel.

And we certainly aren't going to use solar to power commercial human air transport any time this millenium; it will always be better, for people, to compress energy before a flight and travel faster, than try to make the flight itself self-sustaining and travel slower. The only exception could be recreational flyers.

We've been synthesizing one liquid hydrocarbon fuel for thousands of years; ethanol is pretty similar to jet fuel in energy density and combustion. I think the possibility that in the future we'll have cheap solar/wind grid power and use that to produce ethanol, rather than charge batteries, is more likely than people think. Ethanol production isn't terribly efficient, but if your input power is sustainable it doesn't really matter.

Ethanol is about 70% of the energy density of Jet A-1: https://en.m.wikipedia.org/wiki/Energy_density

You'd want to produce methanol. Ethanol does not have sufficient energy density.

http://phys.org/news/2014-11-artificial-photosynthesis-fuel-...

It's generally accepted that a "hydrogen economy" [1] doesn't make sense, and isn't going to work. I think we can say with reasonable certainty that that's true for cars where it looks like batteries are going to reign supreme.

I'm not at all sure that that's true for large jet aircraft though. Storing liquid hydrogen is a non-trivial exercise. On the other hand the larger the tank you're storing it in, the easier the problem becomes -- and large passenger jets would by necessity be using very large tanks. These tanks would be so large that they would impact jet performance in terms of increased drag. However, even with the larger tanks, liquid hydrogen will still provide a much better energy-to-weight ratio than you can get with jet propellent. This means you need less lift to fly the airplane and you have lower energy losses to lift-induced drag. I don't know that this would get you to break-even, but it seems likely that a hydrogen powered jet can get comparable performance to a jet using conventional jet fuel.

Of course just handling liquid hydrogen is no easy task. But a large airport should benefit from considerable economies of scale. It wouldn't be like trying to provide liquid hydrogen to cars at your local gas station.

There has been at least one experimental liquid hydrogen fueled aircraft, the Tupolev Tu-155 [2].

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

[2] https://en.wikipedia.org/wiki/Tupolev_Tu-155

The problem with hydrogen is that it has abysmally low density, being a gas (8mg / L at STP, 40 g / L at 700 bar). Even liquid hydrogen has a density of only 70 g / L. Compare jet fuel, which ranges around 800 g / L. Hydrogen is a more efficient fuel per weight, but it really loses out when you compare volume. This inefficient use of volume is probably a more severe limiting factor than the weight.

> This inefficient use of volume is probably a more severe limiting factor than the weight.

I don't necessarily disagree with you. However, weight is a really, really big deal in aircraft. And the volume becomes less of an issue as aircraft get bigger, thanks to the square-cube law. Are modern day jet aircraft big enough to largely offset the costs of fueling them with liquid hydrogen? I don't know, but they're pretty damn big.

I looked up the stats for a Boeing 787 Dreamliner. The fuel capacity is about 126,000 L. Judging from Wikipedia, you'd need ~5× the amount of space to store pressured hydrogen for the same amount of energy. Wikipedia also says that the cargo capacity of the same 787 is... 172,000 L. I don't have hard numbers on the size of passenger space, but guesstimating that as the upper half of a cylindrical tube gets you in the region of 1,800,000 L. So you're looking at taking out 10-20% of the revenue space of an aircraft to fit in hydrogen fuel tanks.

Now the 787 isn't the largest craft out there, but spitballing the 747 gave similar numbers. I also don't have numbers on how much less fuel you'd need for the lighter weight (you'd reduce the weight of the aircraft by ~⅓ with equivalent amounts of fuel). If you assume it's more or less linear, you're talking about using ⅔ the fuel, which would be cutting out "only" 5-10% of revenue space or cutting your range by ⅔ and keeping revenue space the same.

Given the amount that airlines are trying to squeeze out of the existing revenue space already, it doesn't look like a particularly appealing trade-off.

I'm not claiming it would be cost competitive with current jet fuel prices. However, if fossil fuel prices skyrocket (either because of a substantial carbon tax or simple resource exhaustion), then the economics change in hydrogen's favor.

I'm also not claiming that existing airframes could be easily retrofitted to fly on hydrogen. Not only do you need a much larger fuel volume, you can't practically store it in wing tanks. On the other hand if you took an existing design and stretched the fuselage to accommodate larger tanks, or attached large external tanks to the wings, you could get something that's economical. That's because you have an aircraft that's larger (more expensive, all other things being equal) but also lighter (cheaper, all other things being equal).

I agree hydrogen will probably never be used as a general purpose fuel, but if you have energy you can synthesize hydrocarbon fuels.

I wouldn't be so sure this would not happen this millennium as you say. Maybe not this decade, but I certainly imagine sooner or later we will have I.e. a big satellite beaming power directly to a plane (laser or microwave, you choose). Once launch costs make this plausible and it's deemed safe and secure, why not?

Bonus, you now have an orbital death ray if you decide to go Bond villain.

That would be about as much of a "bonus" as the Chernobyl disaster was a "bonus" to nuclear power. In other words, a threat.

Flight actually scales up really well. Double the volume means less than double the energy required. So, larger craft can get a better cargo to energy efficency and or more speed.

Solar impulse is large, but light and can carry two people plus stuff. Keep scaling it up, ditch the pilot, and you can have something between big slow boats and fast, but expencive jet aircraft. A full 50,000 lb 20' shipping container and 1 week China to US is probably viable. (Boats take 2 week to 1 month and unload at ports.)

Flight actually doesn't scale up that well, especially with solar.

OK, it's better than 1:1 vs cargo even for solar assuming you alter the design to fit your new scale. Beyond that, it's a question of what you want to call "Well" :)

No it doesn't scale 1:1 or any way else at least not in any specific manner.

The scaling depends on how much lift you generate, the lift is a question of speed and your the available lift surface the nominal lift produced is then divided by surface area to give you your wing loading.

Solar is pretty much the most inefficient way of powering virtually any aircraft because you needlessly increase the weight of the lifting surface and batteries weight considerably more than fuel and that weight is dead weight during the entire flight while a fueled aircraft becomes lighter.

That said the scaling between the volume and the "surface area" isn't even in question here because you are going to be hitting a wall much quicker and that's takeoff weight as mentioned before lift is a factor of the surface area that can provide lift and the speed of the aircraft (as well as air density and some other things but lets ignore it) the heavier the aircraft the more energy you need to actually get to takeoff speeds, and when you make your lift surface bigger you increase drag as well as weight.

The speed you can reach is going to be more or less limited because even a 100% efficiency solar powered engines are going to be limited to the max cell efficiency * cell surface area, the 2nd part of the equation is your wing surface and eventually there is a limit to how big your wings can be and still support themselves and most importantly not flutter. When you take all that and more into account there is absolutely no fucking way that you'll make an aircraft capable of lifting a 50,000lbs of crago, heck you won't build a solar aircraft capable of lifting 50,000 lbs of anything including itself. To put that into perspective 50,000lbs is 1/6th of the maximum structural load that a 747-8F (freighter) can carry, and about 1/4th of the nominal cargo load.

To put it into even more perspective Solar Impulse weights 3000 lbs, it's maximum takeoff weight is about 4000 lbs and it has a max speed of 80km/h with a wing span greater than that of a 747. To put it simply there is no freaking way to scale this up, there is a good reason why nature haven't developed heavy flyers the heaviest bird capable of flight weights 35 lbs, the rule of thumb is that when you double the weight you need a 4th time as much wing area and spend 4 times more calories to achieve flight and while mechanical flight isn't exactly the same it's not that different - the amount of energy you can produce more or less limits how well you can fly because at the end if you stick 2 SRB's to a cargo container you don't even need wings.

P.S. And that's before we getting into finding a material light enough to be viable for solar flight while still being able to bare the load of 50,000 lbs without collapsing, but even if you'll take perfect carbon nano tubes or any other super material you still won't be getting off the ground.

If you want solar cargo air transport blimps are probably the way to go...

Try and make a solar powered fixed wing aircraft fly at 43 mph while being 1 foot wide and reserving 25% of it's weight for cargo and keeping it in the air for 36 hours. You have 45 kW / (200 ^ 2) = 1.1 watts peak solar output to work with.

The issue with overly large wings is mechanical stress let's try and get around that.

<This is a dumb design> As a thought experiment, take 3 Solar impulse aircraft fly them next to each other with a simple non weight baring cable to share power between them. The outer two can clearly provide more power than they need and supply that extra power to the center one. Now the center one has extra power for 'free'. So, it can have a higher angle of attack and more weight.

As to scaling birds, a Quetzalcoatlus had a wing span of 9 to 12 meters. That's huge for a land animal. https://en.wikipedia.org/wiki/Quetzalcoatlus Birds are limited by biology not so much aerodynamic issues.

Birds are definitely limited by aerodynamics, biology just says well screw this it ain't worth it because life forms are limited by the amount of energy available to them. If we take the heaviest flying bird today it has a wing span of about 3M and a total wing surface of slightly under 1 m/sq the Quetzalcoatlus you've mentioned has a wing span of about 15m and a total wing surface of about 35 m/sq, back of the envelope math means that about a mass increase of about 12 times over the Kori (heaviest bird capable of flight) results in about 30-35 times the increase in lift surface (and we are still unclear if the damn thing was even capable of powered flight or not).

And again you are missing the point Solar Impulse can lift about 1000 lbs of w/e if you want to scale it up either increase the thrust by 50 or increase your lift surface by 50 to get to that 50,000 lbs cargo you want. If we ignore load baring and drag than 3 Solar Impulse aircraft combined could lift 3000 lbs that's it.

We can argue about this till the sun doesn't shine it won't change the fact that solar flight will never be a viable option for cargo transport. But it can replace other things, things that for example are well in range of the 1000 or lbs of payload that any reasonable designed solar aircraft could carry - surveillance, communications, weather and atmospheric monitoring equipment etc. They could potentially replace satellites, and increase the endurance of both military and civilian drones considerably but no one is looking at them to carry cargo because it's simply silly.

Also if you want to move 50,000 lbs today by air it usually won't take less than 6 days, the only thing you can really ship overnight are small things that end up going on commercial flights not cargo flights. And while it's also true that a container ship can take 21-30 days from say China to the US about 14-15 of those days are spent in local transit and loading, the sailing it self a cargo ship can do within 10 days of good weather with the average being probably closer to 12 or so days due to weather and fuel economy.

The problem with shipping by boat is the variability. Your just in time supply chain needs to assume a ~6 weeks or longer in case of a strike etc. Variability goes down a little when you have more boats going to more ports.

As a final point on energy larger birds can do generally have a slower metabolism. They simply need less energy per lb to maintain flight and can travel longer distances on smaller per pound energy budgets.

Not that I think anything could convince you, but look into glide ratios and optimum glide speeds. If A and B have 15:1 ratios and B glides at 2x the speed that's more efficient.

No matter how many tricks you play eventually it comes to 2 things.

Amount of thrust (limited by solar power) * lift surface limited by materials and physics, it all boils down to math. http://www.ajdesigner.com/phpwinglift/wing_lift_equation_for...

So I really don't see any way of a solar power aircraft which uses aerodynamic lift getting of the ground even in the most perfect unattainable conditions.

A they already built a solar aircraft and B they say it can scale to 50x the size and carry 50 people. IMO, that would be more useful for lifting cargo vs people.

Second wing area and lift is related to velocity (not power) right before you stall outside of that it's a looser connection. Velocity is indirectly related to power in steady level flight based on drag. Drag in level flight is a function of weight, shape, altitude, and velocity.

Economicly I don't think this is a good idea. Physically it seems possible with cheaper and more efficemt solar cells it might make this more reasonable up to a point. Especially if they can boost altitude and thus speed.

Yes they've built an airplane with the wing span of a 747 and a lower lift than a Cessna. Please show me where they stated that they can scale it 50x the size, that would mean it will have a wing span of 10000 feet, that's 10 Eiffel towers....

The article says electric aircraft holding 50 people in 50 years. Granted, not pure solar aircraft but electric.

50x the power takes 50x the surface area and 7.1x the length / width. Or less than that with a lifting body approach. Or significantly less than that if it's daytime only.

I've seen 2 things there 1. electric and 2. 50 people nothing to do with 50x time size. The Solar Impulse can lift up about 5 people as it is (if there was room), scaling it to 50 might might be possible but cargo, and at the weight you were talking about no sorry not going to happen.

Solar Impulse is a marvel of engineering, and the fact that during on of the legs of the flight a single person flew the damn thing for 6 days straight is an attestment to human ingenuity and courage but there is a very very big difference between flying 50 people using electric aircraft (again electric not specifically solar) in 50 years to flying 50,000 of lbs on solar power.

We already have electric aircraft capable of 1.5-2h of flight scaling those up on electric power alone without solar might actually be easier as you do not inherit the weight of the solar panels. Efficient fuel cells and ultra light batteries / nano super caps could potentially make electric flight possible within 50 years but again relying on solar to transport cargo long haul no sorry not happening.

Li-ion battery's are 0.265 kWh per kg. Over a 2 hour flight the lightest thin film solar cells produce 12kWh. Battery's might be improving but not 50x as good. See 6w per gram: http://news.mit.edu/2016/ultrathin-flexible-solar-cells-0226

You would probably want a thicker coating to protect the cell, but in 50 years solar can add very minimal weight.

I'd like to propose that both you and Retric are more-or-less right, if not for exactly the right reasons. Your post gets down to the real issue right at the end: the actual limiting factor for a craft like solar impulse is the strength of the wings, and the best way to get around that is to use a lifting-body rigid airship.

That's why i said blimps are the way to go. Lifting body also has it's limit, at the end solar just can't provide enough thrust for heavy lifting, if the lifting it self is done through other means such as buoyancy then it's not an issue as solar can provide enough thrust for actually going forward as well as power the compressors to control the buoyancy of the craft.

Anything that will be dependent on aerodynamic lift alone literally won't be getting of the ground with any significant payload.

Flight scales badly to slow speeds. Let me demonstrate with some very rough calculations:

Imagine three aircraft flying the same distance.

The first is a large airplane flying at 20 m/s. Solar impulse speed. The second is a large airplane flying at 200 m/s. Basic propeller plane. The third is a small airplane flying at 200 m/s.

    Slow plane:
    speed: 1x
    area: 1x
    lift: 1x
    payload: 1x
    drag: 1x
    thrust: 1x
    power: 1x
    distance: 1x
    energy: 1x
    
    Big fast plane:
    speed: 10x
    area: 1x
    lift: 100x
    payload: 100x
    drag: 100x
    thrust: 100x
    power: 1000x
    duration: 0.1x
    distance: 1x
    energy: 100x
    
    Small fast plane:
    speed: 10x
    area: 0.01x
    lift: 1x
    payload: 1x
    drag: 1x
    thrust: 1x
    power: 10x
    duration: 0.1x
    distance: 1x
    energy: 1x

You can see that you use the same energy per payload no matter how fast you go, but your plane can be a lot smaller if you go fast.

(This didn't take into account altitude. That means you can go even faster for the same energy expenditure.)

Things are far more compex than you are suggesting. The tradeoffs for spead work out largely because you can increase altitude which is very useful.

I accept that Solar does not work to supplement the onboard generator used for electrical power on jets due to weight and night time flights. At the same time solar is ~1% the fuel cost for that amount of power for 20 years. They say they can scale this up 50x and carry 50 people. IMO, it would be better to build cargo only craft in that size range as this is far to slow outside of sight seeing in VFR.

Not to mention wind. That said if you can say double the solar power and add significant altitude and speed that's much more viable. I am assuming this is significantly worse than the best possible solar aircraft we could build in say 30 years.

Well, 1/100 the cargo and 1/10 the speed means you need 1000 solar planes for every kerosene plane for the same route revenue. If we assume they cost the same to buy since they are the same size. And that doesn't take into account night flying etc...

Much better to have the solar panels on land and do synthetic kerosene for planes.

Ships go half the speed of Solar Impulse, but they have a lot more payload per dollar invested.

I'm a bit skeptical of scaling up Solar Impulse. The plane only has room for one person. If the plane could care another 250 pounds for another person and gear, I'm sure they would have done it. And Solar Impulse has a 200 foot wingspan--the same as a 747.

Progress takes time and money. Imagine they needed to take an 8 month break and raise 20M to fix a battery heating issue. Each component of a complex solution, like a solar plane, makes slow but steady progress: lighter/stronger materials, lighter/more durable batteries, etc. Software will also play a huge role automating and making resource efficiency better. The fact is that step one is cleared: flight around the world on solar power :)

I am surprised that no one has been developing (or I haven't heard of) fully autonomous airplane pilot systems. Something like that tied to a fleet of solar planes like this for shipping stuff could be a real game changer for certain types of cargo.

Flying actually seems like a much "cleaner" environment for the AI to navigate then roads do, to be honest, and the dependence on instrumentation and not visual cues as much is of course a natural fit.

There are several autopilots able to follow a route untethered with just waypoints. Apm (ardu pilot mega) and pixhawk come to mind. From what I've read it is still highly recommendable by the RC community to have telemetry links and if range allows, manual control. These modules are used mostly as flying aides but some companies are building RC planes with autopilot capabilities and market them towards the mapping and surveying industries, charging a hefty premium for it.

>surprised that no one has been developing (or I haven't heard of) fully autonomous airplane pilot systems

Facebook seems to be http://www.komando.com/happening-now/319972/facebook-is-buil...

Pilot here. I agree with other posts about energy density being an important factor here. I don't expect to see all-solar aircraft anytime soon. The best way to get aviation to become carbon neutral will be via biofuels, which several airlines are already experimenting with.

That said, I can see a couple of areas where solar will contribute.

While it will be very difficult to generate all of the power needed for an airliner via solar, there's no reason not to cover the aircraft in high efficiency (triple junction) solar cells. A large airliner will have a usable sky facing surface area of 1,000 square meters or so (rough estimate). Enough to generate a few hundred kilowatts of power. If the aircraft has hybrid gas/electric engines, that power can reduce the fuel burn a bit (even a small reduction is a big deal).

I can also see where a serial hybrid design could be workable (e.g. lots of fuel cells generating electric power that in turn drives electric fans). This would be attractive, especially in smaller planes, due to the potential to reduce the risk of mechanical failure.

Impulse propulsion flown by Piccard? Checks out

"He predicted that, 50 years from now, electric aeroplanes would be "transporting up to 50 people"."

Shame the mega storms that are swirling the planet at that point will probably rip the planes out of the sky.

50 years too late if you ask me.

What megastorms? Did I miss anything?

GP might be referring to that http://www.scientificamerican.com/article/megastorms-could-d...

Megastorms Could Drown Massive Portions of California

Huge flows of vapor in the atmosphere, dubbed "atmospheric rivers," have unleashed massive floods every 200 years, and climate change could bring more of them

To busy to find links, but the gist is that Greenland Ice Sheet melting disrupts the conveyor currents that transport heat energy between the equator and the poles. Coupled w/ greater warming, the higher temperature differential would support stronger and more frequent cyclonic storms (supported by models, current trends, and archeological evidence).

"The total number of hurricanes (particularly after being adjusted for improvements in observation methods) and the number reaching the United States do not indicate a clear overall trend since 1878"

https://www3.epa.gov/climatechange/science/indicators/weathe...

"Trends in tropical cyclone activity in the Australian region (south of the equator; 90–160°E) show that the total number of cyclones appears to have decreased to the mid 1980s, and remained nearly stable since. The number of severe tropical cyclones (minimum central pressure less than 970 hPa) is dominated by variability with periods of lower and higher frequencies of occurrence."

http://www.bom.gov.au/cyclone/climatology/trends.shtml

"Similarly, the trend in intense tropical cyclones (minimum central pressure below 970 hPa) is not significantly different from zero."

https://www.ipcc.ch/ipccreports/tar/wg1/091.htm

Or did you mean current trends in something else?

Yep. All the falsifiable predictions have turned out to be, well, false.

I hope it would be earlier, 25-30 years maximum

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