A thought occurred to me, and I wondered if it could be made practical...

This video talks about the possibility of electrical battery powered planes, and in short, while it may be practical to build small aircraft using battery power, unless batteries get an energy storage density about ten times greater than what we have today, making commercial airliners using them never will. But what if the airliners were powered by ground-based microwave transmitters? We've apparently managed to get the transmission losses to less than 20%, so how practical would it be to build relays of airplane power transmitters along land-based flight routes and use them to power fully electrical airliners?


There is practical problem: microwaves is also what radar uses

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the object(s).

To power your airplane you will need a focused beam, and this means that as soon as a radar reflective object (another plane, cloud, rain) is between your source and your airplane, the plane will receive less power, eventually even no power at all. Either because the beam is reflected or because the relative phase between the beams is disturbed, leading to a loss of focus.

Moreover, it would be difficult to supply planes flying in the same direction, since they would need to share the same emitter.

All in all I doubt that such a system could reach the reliability expected by aviation industry.

  • $\begingroup$ your last sentence covers it. ;) $\endgroup$ Feb 26 '20 at 2:41
  • $\begingroup$ Not to mention the raw power requirements. How strong does the transmitter have to be to supply adequate power to an aircraft that is, at minimum, 30000 feet up and moving anywhere from a few hundred miles per hour up to just under the speed of sound. $\endgroup$
    – Paul TIKI
    Feb 26 '20 at 4:48
  • $\begingroup$ There is one more point: if reflected beam doesn't hit plane, then what would it hit? And I (living near major aiport) really wonder would it be my home or not? $\endgroup$
    – ksbes
    Feb 26 '20 at 9:21
  • 1
    $\begingroup$ @ksbes the inverse square law will protect you extremely effectively, unless you are already dangerously close to a large jet aircraf in which case you already have more serious problems to deal with. $\endgroup$ Feb 26 '20 at 11:04
  • $\begingroup$ @StarfishPrime, I think a thousand part of airplane engine power would be "enough" for me. $\endgroup$
    – ksbes
    Feb 26 '20 at 11:48

TL;DR: flight is very energy intensive, and you'll have to build a lot of power stations and then build a vast and expensive network of directed energy weapons and persuade travellers that having you shoot at them is a good thing.

It is a little bit hard to pin down exactly how much power is required to keep an airliner in the air, but this KLM blog post suggests that a Boeing 777 produces about 23MW at cruising altitude. Takeoff is significantly more energy intensive, but lets imagine there's some other solution to that problem.

This link suggests that there are 8000-20000 aircraft in the air at any given time, though of course most of these won't be airliners. If we assume that there are, say, 5000 airliners in the air, and each needs 20MW of power, that's a nice round 100GW of received power. High energy microwave production isn't the most efficient thing in the world, but a nice gyrotron maser should manage at least a 50% efficiency, so including transmission losses of 20% that's a total required generating power of 240GW. The USA in 2017 had a total installed summer generating capacity of 1072GW, and the total world grid supply in 2014 was about 6142GW, so you can see that this is a non-trivial amount of power to handle (and that 240GW is a super-ultra optimistic total best case, by the way. It'll be much, much higher, see below).

I'm not going to try and estimate the number of ground stations you'd need, or the power switching capacity you'd need to feed them. You'll need to lay them out along flight paths so they won't be completely evenly distributed, so they can at least have their own purpose built power supply lines, which will of course need to be redundant because they are safety critical systems, but maybe someone else will consider this issue and let you know how inconvenient it will be (I'm guessing somewhere between "quite" and "very").

Next up is power density at the target.

A Boeing 777 has a wing area of about 428m2. If you cover that with your microwave rectennae, you need to be delivering ~47kW/m2. In the US, occupational safety limits for microwave radiation are ~10mW/cm2. Proposed microwave solar power sats would be limited to a peak intensity of 23mW/cm2. Your beams are ~470x more intense that the OSHA limit, and ~200x more intense than the powersat design. There are some serious risk assessements required in there, I think you'll find! This is also the best-case scenario where the plane is being zapped from directly below it (in level flight). As the beam angle with vertical increases (because the plane is flying away from the emitter) the power density will necessarily have to increase to the point where it starts looking like an energy weapon, not a power supply. More ground stations can reduce this problem, but vastly increase cost and complexity of the system.

This also assumes that all the emitted energy is absorbed by the rectenna grids, of course. in practise, you'll have pointing errors, so you'll need to be producing a much higher energy beam that fully encompasses the silhouette of the target aircraft, so you'll be illuminating the fuselage (often filled with meaty cargo) and wasting masses of power that simply misses any rectenna and flies off into space, substantially increasing the power requirements of the ground stations. I'm not going to estimate how much, because phased array pointing accuracies and diffraction shapes are fiddly to work with, but I'm going to say that it will at least double the power requirements, and will probably require much more juice than that.

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    $\begingroup$ +1 and imagine the problems when a delay on the ground means that the number of aircraft circling a given airport exceeds the number of emitters available to keep them in the air. $\endgroup$ Feb 26 '20 at 21:51
  • $\begingroup$ @KerrAvon2055 you could maybe juggle them a bit. At cruising altitude, an unpowered airliner will fly for well over 100km before making an unscheduled landing, so there's plenty of time to throw em to the next ground station. I mean, it hasn't made the idea any less appealing or practical, so you may as well do it... $\endgroup$ Feb 26 '20 at 21:57

The concept of beamed power to aircraft has been around for quite a while, and was first demonstrated by William C. Brown in 1964.

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William C Brown and a microwave powered helicopter. The large square is the antenna to receive the microwave beam

Since 1964, various other designs have been proposed, using beamed energy from lasers or microwaves, and using the energy to turn motors or directly or indirectly heat working fluids. The main sticking point is the lack of beam infrastructure (particularly large and powerful enough beams to actually power commercial aircraft). Dean Ing and Leik Myrabo wrote a book outlining the state of the art in the 1980's called "The Future of Flight". Myrabo did extensive studies on the use of laser technology to propel spacecraft, but many of his inventions and ideas also translate into atmospheric flight as well.

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Early concept for a "lightcraft"

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More advanced idea, including using the skin of the "Light craft" to manipulate the airstream using MHD fields

With present technology, the most probable means of achieving laser of microwave assisted flight would be to have the emitters in orbit and beaming "down" to the top of the aircraft. The engines would burn normal fuel until they reached cruising altitude, then establish a "handshake" with a beaming station overhead and use the laser or microwave energy to heat the air in the combustion section of the jet engine, reverting to on board fuel for landing.

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Mirror in orbit. A practical system will need hundreds of these to direct and focus laser beams

So while the idea is theoretically possible, to successfully carry it off requires a massive investment in infrastructure in space just to accomplish a fairly modest fuel saving (the most use of fuel is during take off and landing). Myrabo himself suggested that the best use for laser powered lightcraft would be to create ballistic capsules which would simply blast into the sky and fly a ballistic arc to the destination, where it would receive another input of laser power to land. The passengers would experience a "roller coaster" ride with both positive and 0 G during the flight, but also be able to fly from London to Sydney in a half hour.

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Concept of a lightcraft accelerating into a ballistic trajectory. Part of the energy is reflected back in the flight path to create an area of low pressure to reduce drag at hypersonic speed.

Using different forms of energy would seem to create the conditions for different forms of flight.


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