Has Joby Cracked The Power Problem To Make Electric Air Taxis Work?

As a college freshman in 1992 interested in building aircraft capable of taking off and landing vertically, JoeBen Bevirt landed in the right place: the lab of flying car pioneer Paul Moller at the University of California, Davis.

But despite deep respect for Moller, Bevirt soon concluded that it wasn’t going to work. The rotary gas-powered engines Moller was developing for his Skycar were intolerably loud and research Bevirt did on batteries led him to believe it would be decades before they would contain enough energy at a low enough weight to make a quieter electric vertical takeoff and landing (VTOL) aircraft possible. He turned to robotics instead.

A few decades of battery improvements later, aviation fuel still contains 14 times more power by weight. But Bevirt says his Joby Aviation has made a breakthrough that he believes will soon enable urbanites to take their commutes into the skies in clean, quiet electric aircraft that can take off and land like a helicopter and fly efficiently on wing like an airplane. He says Joby has been able to design a five-seat air taxi that can travel 150 miles powered by lithium-ion battery cells that are on the road today in electric cars. The automotive cells he’s using pack “almost” 300 watt-hours per kilogram in specific energy, Bevirt says, well short of battery chemistries under development promising 400 Wh/kg and up that some other electric VTOL developers appear to be waiting for to make their aircraft viable.

Those are big claims that Joby, which has been incredibly secretive about what it’s been up to for the last five years, has yet to provide public proof of.

“If Joby’s asserting that we’ve solved the problem and can do 150 miles with five passengers on automotive batteries, then essentially everyone making new batteries for EVTOL can quit,” says Venkat Viswanathan, a battery researcher at Carnegie Mellon University who’s involved in one such effort.

EVTOLs have vastly different battery needs than cars – they have to draw far more power quickly, for longer, to handle the brute force task of a vertical takeoff and landing. Landing particularly poses a challenge, when the battery has been drawn down and voltage is waning but it still must be able to reliably provide lots of power, says Viswanathan, who has worked with a number of electric aviation startups. “It’s like being on Ludicrous Mode but not for 3 seconds, but for 60 to 90 seconds,” he says. “The battery, it’s singeing inside.”

And Joby and other EVTOL developers have yet to prove they can accomplish another crucial task: to reliably gauge the remaining power in a battery when it’s run down, as well as when its capabilities have degraded months into its life, which is key to determining how much farther an electric aircraft can fly. Everyone’s had a laptop suddenly go dark that said it had three more minutes of charge. That’s not acceptable for a vehicle in the sky.

Viswanathan and his graduate student Shashank Sripad, who were given the nickname of battery police by Quartz for work they’ve done reality-checking the range claims of electric vehicles like the Tesla Semi and Audi e-tron, took on the challenge of estimating the plausibility of Joby’s, based on limited specs for the aircraft that the company shared with Forbes.

(Warning: here come a lot of numbers.)  

Bevirt says that Joby has done painstaking work to shave ounces and optimize aerodynamics to produce an aircraft that can fly as far as possible, achieving a maximum lift-to-drag ratio in the “high teens,” comparable to small single-propeller planes that have much cleaner profiles than its six-tiltrotor aircraft. Joby expects its aircraft to have a maximum allowable gross weight of 4,800 pounds, which includes passengers and payload. Viswanathan and Sripad gave Joby the benefit of the doubt and plugged in 300 watt-hours per kilogram for how much energy the battery cells contain (if it’s using a cell that’s currently in production, 270 Wh/kg is more likely, they say). With a top cruise speed of 200 mph, to achieve range of 150 miles plus FAA-required reserves of 45 minutes of flight time, they calculate that Joby’s aircraft would need a battery pack with a total of 200 kilowatt-hours of energy. Adding in the necessary insulation and packaging to prevent thermal runaway in any one cell from igniting a larger fire, they estimate that the assembled battery pack would contain 200 Wh/kg of energy, which they say would be better than anything on the market today. That would mean a battery pack weighing 2,200 pounds. Subtracting that from the max gross weight of 4,800 pounds, as well as 1,000 pounds for the pilot and four passengers, leaves only 1,600 pounds for the airframe, avionics and other onboard systems—a slim 33% of gross weight. That’s about 35% lower than any certified production airplane in history, including ones built under far less stringent rules, notes Zachary Lovering, who developed Airbus’ well-regarded EVTOL demonstrator Vahana.

However, Bevirt says that Joby has done even better than 200 Wh/kg at the pack level, achieving an unheard-of 235 Wh/kg, though he says the figure is deceptive due to integration that makes the battery “hard to decouple from the rest of the aircraft.”

That statement suggests to Viswanathan that Joby has saved weight by integrating the battery’s thermal management system with the cooling mechanisms for other electrical systems on the aircraft. And it may have taken some of the cooling equipment off the aircraft entirely.

In October, Joby filed a patent application for a thermal management system for its batteries that features a ground cooling unit that would be connected to the battery pack while it’s being charged on the landing pad, which the filing says reduces the need for refrigeration equipment onboard the aircraft, saving space and weight, as well as allowing the batteries to be charged more quickly, which generates substantial heat.

Taking Joby at its word, a 235 Wh/kg battery pack would weigh 1,870 pounds, Viswanathan and Sripad estimate, leaving 40% of the aircraft’s gross weight for structures, still extremely low by historical standards.

The bottom line: Either Joby has built an unprecedentedly light and efficient airframe or its range will turn out to be lower.

One way that Joby says it’s achieved its performance goals is by building practically all of the aircraft in-house apart from the battery cells, a strategy that runs counter to the norm in aviation, with its extensive ecosystem of subcontractors and suppliers. “The big bet we’ve made is on vertical integration,” says Bevirt, “and it’s born incredible fruit for us.”

Investors and former coworkers credit Bevirt for making it work. Engineers tend to develop deep, narrow specializations. They say Bevirt has an unusual ability to pull together work across the broad range of technologies that go into an aircraft, from carbon fiber composites to aerodynamics to electronics to software. And he has a knack for shaving off the extra margins and buffers engineers design into individual components that weigh down the whole, says Brian Ballard, who served as president of Joby from 2017 to 2019. “What JoeBen does better than pretty much anyone I’ve ever seen is work with a multifunctional team to get something great that no individual core competency could have created,” says Ballard.

The risk in Joby’s vertical integration strategy: that its built components that work fine for an experimental aircraft but that won’t meet FAA standards. Observers say that risk is compounded by the lack of aerospace experience of Bevirt and the company’s foundational team, none of whom have previously taken an aircraft through the daunting certification process.

Toyota is providing Joby with invaluable engineering help in addition to the roughly $400 million its invested in the company with an eye toward applying automotive mass production techniques to crank out the aircraft in numbers unheard of in aviation. (Bevirt gushes over Toyota engineers’ knack for designing tools and fixtures that have reduced painful assembly processes from an hour to minutes – “It makes you want to do cartwheels,” he says.) But automakers face much laxer safety standards. That raises questions about how far Toyota’s help will go to enable Joby to not only certify its aircraft as safe, but then meet the FAA’s exacting standards to demonstrate that it can be mass produced safely.

Bevirt says that since Joby first flew a prototype in 2017 that validated its design, it’s spent the intervening years building a certifiable production version that it’s now flight testing, with the help of experienced certification experts it’s brought onboard.

“I feel even better today than five or six years ago about the choices we’ve made,” says Bevirt.

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