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Comment by MayeulC | Hacker News Reader

What would it take to power an airliner?

Wireless power transmission would be ideal for a lot of applications, transportation (electric cars, planes, maybe trains, UAVs, boats) chief among them.

You could shave a lot of weight out of an electric plane by reducing its battery capacity to an emergency supply. This would also greatly reduce the carbon impact of flight transportation, while making a case for bootstraping a beamed power constellation and ecosystem.

I guess that's something Nikola Tesla first envisioned :)

Unfortunately, from what I know, microwaves are not really harmless, unless diffused over a large area, and you then need a large collector. An airliner could do, if big enough. I don't know either if a beam can be steered fast enough, and what would the economics behind be.

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http://large.stanford.edu/courses/2013/ph240/eller1/: “The power required to keep a Boeing 737-300 flying at a constant altitude and speed is 7.2 × 10⁶ watts.”

So, ballpark 7 MegaWatts, more during ascent, and add a bit if passengers want to watch movies, eat hot meals, drink soft drinks, etc.

On the plus side, if the airplane doesn’t have to carry fuel, carrying capacity could go up (with some redesign to make the plane strong enough to land fully loaded), so that 737 might be equivalent to a 747.

That 737-300 has about 125m² of wing area, and, guessing, about the same are for the body, so that would be 7MW/250m² = 28kW/m². The sun delivers about 1,400W/m², so that’s 20 times what the suns delivers on a cloudless day.

Now, most of the time, the ground station (or solar space array) beaming the energy wouldn’t be directly below/above the plane, so the required power/m² could go up a lot ⇒ I would make sure that any windows in the plane wouldn’t let the microwaves through.

Doable? Possibly, but on ground level, I guess it would be wise to use a lot more area for sending that amount of power up, to prevent killing birds.

I don't know if you could kill birds with this, considering that the beam would move at something like 250 m/s during regular flight. Is microwave radiation more deadly than solar radiation?

Might be a problem at the base station, but there should be multiple of those for greater fault tolerance anyway.

I'm pretty sure the beam would be traveling at 299,704,644.54 m/s. It is electromagnetic energy.

The point where the beam is aimed would be moving with 250m/s. (or about so) Therefore the hypothetical bird which crosses the beam would only get the energy for a fraction of a second.

I don't know if this reasoning is correct or not, but definitely has nothing to do with the speed of light in air. I'm not even sure why you are bringing it up.

I think he's referring to the motion of the beam to track the target.

Sun light is about 1kW/square meter.

A 737 has a wing area of 125 square meters. To deliver 7.5MW to those wings will need 60kW/square meter.

That's a far greater energy density.

As for "time in beam" of a moving beam. Presumably one end of the beam is fixed, and depending on the target's velocity the beam might not be moving much at all (oncoming plane).

The beam’s horizontal motion speed goes down linearly with height if the beam is sent from a fixed location.

So, picking 30,000 feet for flight height and 250m/s for the plane’s speed, at 900 feet the beam’s speed would be 7½ m/s. That’s quite doable for birds.

More importantly, if the beam isn’t sent from directly below the plane, and the plane flies more or less directly towards or away from the beam’s source, the speed at which it moves horizontally goes down considerably (1/cos(α), where α is the deviation from the vertical, I think), and its width when flying through it in the direction of the beam’s source at constant height will go up considerably.

The beam’s width, and, with it, intensity, will likely be larger on the ground, but my gut feeling is this won’t be enough to correct for both these factors in all cases (corrections welcome)

I started writing a complex demonstration, but the intercept theorem, (Thales' theorem) indeeds shows quite nicely that from a ground station, tracking a plane flying at (𝒽1,𝓋1) means the beam goes at 𝓋2=𝓋1*𝒽2/𝒽1. So, 5 m/s for a bird at 200m.

That said, I was mostly imagining space-based stations.

In the above case, I talked about ground-based stations at airports. Unfortunately, those would require high angular velocities, but would already be in bird-restricted areas (airports use all sorts of tricks, including drones, sounds, people chasing them, etc, to scare away birds). You could also conceivably drastically lower the beam energy output by using multiple beams that concentrate at a single point.

Now, a fun calculation: what would be the tipping point between birds saved by not emitting carbon dioxide and birds killed by the towers, if there is one?

One last point: I'd hazard that the bird is likely to change direction if it starts running into a beam and it has time to (ie, it is not killed "instantly"). This drastically lowers the dangerous altitude range, as the bird is less likely to fly in the same path as the beam (how likely was it in the first place? One would need to go faster than the other to catch up, but not too much to actually dispense the lethal dose? I haven't done the math, but I'd say it's quite unlikely).

Thanks for the figures. I'd guess that most of that power is used to combat drag, which can be lowered by going slower. The rest is probably used to combat gravity, especially during ascent, which should be improved by lowering the amount of fuel.

A small battery, and/or a ground station at the airport could provide a bit of extra power during ascent, and the battery recharged during flight. I guess the extra power shouldn't amount to a lot more than 10kW...

Depending on the wavelength, it should be fairly easy to have the plane act as a Faraday cage to shield its passengers. I guess the geometry could be slightly tweaked as well to make a larger surface area, possibly using trailing metallic wires to improve the energy collection area at a minimal cost (I get that cylindrical wires are the worst aerodynamic shape, but it should be less of a problem behind the wing).

If the wavelength is low enough, a coarse mesh could collect power quite efficiently, but you'd have to worry a lot more about radio interference.

Drag and lift are related. Wings work by literally displacing lots of air towards the ground.

If you go slower, you need bigger wings (also no one really wants longer, slower flights in cramped airplane cabins).

Unless it's cargo. Cheaper than "normal air" / faster than sea shipping is a reasonable niche.

I don't know if it's a big enough niche. There are quite a lot of costs to air cargo aside from marginal fuel efficiency and somehow developing novel cargo-specific aircraft has to translate into both increased profits for Boeing/Airbus and decreased costs to shippers. Keeping in mind that these slower craft would be unsuitable for some of the most profitable uses of air freight today. I'm not sure it makes sense.

I guess that's part of the reason we still see propeller planes around (often with the military).

Answering to a cousin comment, buses and trains can't compete when you need to cross water :)

Well, there are boats. Passenger sailing as a means of travel is not especially popular these days.

If slower meant cheaper tickets I can guarantee that people would want to use it.

The lower cost, slower mode niche would have to compete with buses and trains, with the added penalty of TSA security theater. I'm not saying you're wrong, but I'm still not sure the financials work out.

I love posts like this that remind me that I'm on HN and that HN can solve world problems based on assumptions.

Take a scenario. Give it best-case figures. See if it works. If it doesn't, then it's impossible. If it misses the mark by just a bit, it's a remote possibility. If it works, try the worst case figures and the average case ones. Of course, you can also start with the worst case ones and show that it works.

I like to approach problems this way. Plus, one can have dreams, can't they?

Once you've shown it can theoretically work, it's time to consider the side effects: best/worst. With the above, someone now controls a multi-gigawat orbiting power source.

Upsides: well, it can power useful stuff just about anywhere on (two third of) earth when there's nothing else to do. That would be really useful to compensate renewables, or other grid fluctuations. One could even imagine making airplanes slower of faster depending on demand.

Downsides: well, somebody really does control that :)

https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Examples_of... says the jumbojet, at cruising altitude, has 17 times as much lift as drag.

So it seems drag isn’t taking most of that power.

Also, “I guess the extra power (for ascent) shouldn't amount to a lot more than 10kW” seems unlikely to me. It’s too low compared to the 7MW used at cruise height.

Also, if my math is correct, 1kWh lifts one kilogram about 360,000 meters, or 36,000kg (less than the weight of an empty 737) by 10 meters. So, 10kW would lift the plane by only 100 meters in an hour.

7MW in cruise, but keep in mind the power margins that are required for a transport category aircraft.

Beam steering can be done at pretty high speed, essentially limited by the weight of the mirror. And the further the mirror is the faster the effective linear speed of the 'spot'.

But the safety question is more serious. The beam is dangerous to birds, unshielded aircraft, and persons on the ground. I don't think it's great giving the US military even more power to start a fire anywhere on earth with zero warning.

Couldn't phased arrays ensure that everywhere (within region) but the target gets only a small amount of power?

In theory, yes, but practice no. The antenna required to collimate the beam from LEO, down to a person sized spot on earth, would be massive.

Single antenna apertures are limited to about 90 dB of gain, and I doubt that is near enough, but I’d have to run some calculations.

> I don't think it's great giving the US military even more power to start a fire anywhere on earth with zero warning.

They already have hydrogen bombs :-)))

Fairly hard to deploy that quietly.

True, but they also have drones and a million other ways they can kill people relatively unnoticed. And if the US Gov. really wants to, there are always Black Ops.

This is a bit like complaining about a knife in a gun shop :-)

>start a fire anywhere on earth with zero warning.

Are lasers like this detectable? If not, seems like a good way of creating targeted brain tumors. Disrupting soft infrastructure nodes etc.

Think of it this way, it can be uses safely on high altitude and trans-oceanic flights which means less fossil fuel usage,less fuel-weight to carry and more flight capacity (even lower carbon footprint).

It's not that microwaves are intrinsically harmful, it's the intensity of the beam. For example microwave ovens operate around 2.4Ghz, same as Bluetooth/WiFi. I'm not sure what power delivery operates at.

If you get hit by a megawatt beam, you're going to get burned regardless of the wavelength (unless either it's short enough to pass through you, or somehow you reflect 100% of it).

As another comment points out, you can beam steer quite fast. You only care about angular rate, and an airliner at cruise altitude moves fairly slowly (eg you can follow it with your eyes easily).

This has nothing to do with aircraft. This is about beaming power to low-orbit satellites. The problem with low-orbit radar satellites (the spy satellites tracking ships) is that they need lots of power. In the past this was done with small nuclear reactors but today that is unacceptable and virtually all satellites use solar panels. But sarge solar panels means higher drag and shorter lifetimes in low orbit. Being able to 'beam' power from a higher orbit means one could power more aerodynamic low orbit satellites.

Ah, my bad. That said, beamed power could be applied to anything, and the post I wrote above is one of the better use cases, in my opinion.

This has nothing to do with aircraft.

Actually, yes, the article explicitly mentions the possibility of powering unmanned aerial vehicles.

>> the article explicitly mentions

Ya. The public report on the super-secret space plane touts that it is studying far-off green energy solutions when those same technologies have much more immediate military effects. It's like the USAF saying that it is studying lasers in order to build better CD players. Everyone takes such statements with a grain of salt. Nobody thinks that we are getting the full story behind these experiments and we chuckle a bit at the poor junior officer instructed to "be creative" with descriptions in public statements.

I didn't say anything about green energy, and while the grandparent comment did, I don't think it was implied that it was the motivation of the experiment. Powering military drones for indefinite dwell time is a very plausible use case among many listed in the article.

It mentions "long-endurance unmanned aircraft", which I assume use orders of magnitude less power than most aircraft.

Umm, TFA is on the X-37B spaceplane, but it specifically speaks a lot about potentially using the power for aircraft and drones. And the potential for indefinite drone flight duration.

You're not wrong about low orbit sats in general, but it's only one of many potential military (and/or civilian) uses. And it's not the use the article focuses on.

I’d imagine a safety feature on the airplane to be a battery or even capacitors to help smooth out any drops in transmission. At least the batteries could be small

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