Flying Vehicles (FV) that do not need a runway for take-off/landing have been undergoing explosive growth in numbers. It is not difficult to imagine a metropolitan area like Greater Boston to reach a million FVs someday, transporting cargo and people constantly, and performing various other tasks (photography, cleaning, maintenance, etc.), in autonomous or semi-autonomous modes. Most of these FVs will be electric, meaning they will be powered by rechargeable batteries and/or fuel cells, because electric motors are much quieter than internal combustion engines (ICE), and do not emit pollutions like soot, lead, SOx/NOx. We define these rechargeable batteries/supercapacitors and/or fuel cell powered FVs as Electric Flying Vehicles (EFVs). EFVs need to be recharged, either by high-power rating electric recharging points as found in land-based electric vehicle charging stations (e.g. DC fast charging such as CCS or CHAdeMO, or AC fast charging), and/or gaseous or liquid fuels like compressed hydrogen, methanol, ammonia, etc.
We propose below a tree-like civil service structure that can range from tens to few hundred meters tall, with a vascular-network type electrical plus fluid feedthroughs, and multi-purposed charging points that can dispense electricity and fluids to EFVs, and occasional ICE powered FVs. Liquid water may be provided for automatic cleaning of the FVs, and other service fluids. In addition to recharging and servicing, the more interior “stem space” of this tree-like civil structure (TCS) can provide spaces for longer-term parking of the FVs. We envision that tens of such TCS will be sufficient to serve a metropolitan area, instead of thousands of land-based “flat” charging stations/gas stations that are built along road networks, and many thousands of “flat” parking lots in a city. These civil structures most likely will be co-located with infrastructural nodes for the electric grid, such as substations, battery Energy Storage Systems (ESS) that store renewable electricity from wind and solar generations, and hydrogen production (water-splitting or steam reforming) stations and/or cryo-compression stations. The economic benefit of co-locating TCS with renewable energy generation/storage/conversion nodes comes from the reduced capital cost and transmission losses.
At present there are 168,000 gas stations in the US, built along a 2D road network system. There are millions of “asphalt-desert” open parking lots. The Eisenhower Highways constructed in the 1950s was a disaster to city development and ecology [ https://www.vox.com/2015/5/14/8605917/highways-interstate-cities-history ], because they partitioned the cities to inaccessible districts that are difficult to traverse (see Fig. 1). EFV infrastructure design will have deep socioeconomic and ecological consequences.
We would like to argue that a 3D tree-like structure (can either be columnar like Poplar trees, canopy like dragonblood tree, or conical like fir trees, or other tree shapes, see Fig. 2), taking advantage of the vertical take-off/landing abilities of modern autonomous FVs, would be superior to any 2D infrastructure for flying vehicles. Ride hailing service means that people no longer need to own their “personal” FVs, which is highly wasteful of mineral resources, and do not need to build individual garage space for storing them either. The energy cost of elevating FVs to 20meters to 1000 meters is not a major issue, because in order to cut down noise pollution and reduce traffic jamming, the FVs may be ordinanced to fly at significant elevation in most of their trips. Also, the gravitational energy gained in the parking process will be useful in the flight later.
Some numerical estimates are provided below. The Empire State Building is ~400 m tall, and has 280,000 m2 of floor area. We envision the tree-like structure to be mostly for automatic, human-unnecessary service/parking and thus the floor height can be halved, giving something like 500,000 m2 of floor area. Given a standard car-parking area is 17 m2, but a car weighs 1.5 tons, we believe most FVs, which will be much lighter, will need only 10 m2 or less. Thus, we estimate a structure of such scale and building cost (40million 1930 dollars) would be sufficient for the long-term storage of 50,000 FVs. Battery powered EFVs (BEFV) tend to be smaller, and they will likely get in and out of the structure more frequently since their flying duration is currently 30–40min and is unlikely to get beyond 1 hour. Thus, the BEFVs will likely inhabit the lower floors of the TCS. Because of their frequent in-and-out, an open space design (with almost no exterior or interior walls, and few pillars) that allows maximum flux is likely. Typically, it takes about 20minutes in battery fast-charging (3C rate). In comparison, the time for EFV elevation by flying and for locating vacant spot should be on the order of 1 minute. Thus, this does not add significant time overhead vis-à-vis a “flat” parking lot. Also, in terms of energy overhead, modern battery pack has gravimetric energy density on the order of 150 Wh/kg. Suppose it is required to lift an EFV total weight 5× the battery pack weight, up 200 meters, this will take 5kg × 200m × 9.8 m/s2 = 10,000 J = 3 Wh per kg of battery pack, which is 2% of the electrical energy stored in the BEFV if fully charged. Furthermore, this gravimetric energy is almost like “stored energy” in hydroelectric energy storage, and can be used to expedite subsequent flight in an “eagle’s swoop” upon dispatch. So the net energy inefficiency should be less than ~1%, which should be acceptable.
In contrast to BEFVs, hydrogen PEM fuel powered fuel cell FVs (FCFV) can have up to 4 times longer flying time and range than BEFVs and could evolve to much heavier vehicles. These could inhabit the upper floors of the TCS. The charging of hydrogen to FCFVs usually takes a few minutes in land-based fuel cell vehicle demonstrations, so elevation by flying and for locating vacant spot on the order of 1 minute still would not add too much of a time overhead. And since the EFV-total weight averaged stored energy is higher than that in BEFV, the net energy overhead should still be less than ~1% even when going up to 800 meters, which is the height of the Burj Khalifa skyscraper in Dubai, the tallest man-made structure on Earth. This shows our design of 20meters to 1000 meters tall TCS should be reasonable.
The exact height, size and shape of each TCS and their locations will need to be carefully designed with modern transportation and urban planning tools. For suburbs and smaller townships, perhaps 20–100meters tall TCSs would be enough. With flying taxis that take people in a beeline from point A to point B, even across waters, we should no longer need large 2D highways in cities, or family garages. The cities should become walkable almost everywhere.
There needs be a significant aesthetic component to TCS design, as they will be visible from afar, and a constant bustling of FVs in and out with lighting and noise may induce visual discomfort that need to be minimized. There need to be fire and accident safety considerations. It is impossible to have energy/traffic infrastructure without some negative impacts, so tradeoffs are necessary. The 3D vascular design of TCS overall should offer significant advantages over 2D “airports” and road network/gas station/parking lots/personal garages.
