At the first public reveal of its Midnight eVTOL air taxi last week, Archer Aviation explained some of the details of its proprietary motor and battery pack designs for the all-electric, four-passenger aircraft.
With the goal of getting Midnight to market as soon as possible, Archer has selected several suppliers of already-certified and flight-proven aircraft components to build much of the four-seat air taxi. But when it came to the motors and battery packs, Archer opted to come up with its own designs to ensure that these parts are optimized for this particular aircraft.
“The powertrain itself has to be as lightweight as possible, but still do the job because you want payload,” Michael Schwekutsch, Archer’s senior vice president of engineering, said during a presentation at the Midnight unveiling event in Palo Alto, California. “We have an aircraft that has more than 1,000 pounds of payload … Just imagine [if] we would waste another 200 or 300 pounds on the powertrain and had a payload of 600 or 700 pounds, we wouldn't have a product because we want to fly four passengers in our airplane.”
Archer’s plan is to initially use the Midnight aircraft to transport passengers between congested city centers and airports—especially airports with United Airlines hubs, as United is one of Archer’s major partners, as both an investor and a customer. Earlier this month, Archer and United announced their first air taxi route, which will carry passengers back and forth between Newark Liberty International Airport and the Downtown Manhattan Heliport in New York City’s financial district. Those operations are expected to begin in 2025, with Archer anticipating that the FAA will issue a type certificate for Midnight in late 2024.
Midnight is designed to have a range of up to 100 miles on a single charge, but Archer says the aircraft will primarily fly short trips of about 20 miles. Recharging the aircraft after the average 20-mile mission should only take about 10 minutes, which allows the aircraft to make multiple successive flights in a day with a relatively short turnaround time.
Midnight’s Electric Motor Design
To ensure that Midnight will be capable of carrying four passengers and their luggage, Archer had to design the aircraft to be as lightweight as possible without compromising too much power. This means that Midnight, which weighs about 6,500 pounds, needs to have electric engines that are powerful enough to lift all that weight but light enough to not weigh down the aircraft and reduce its payload capacity. In other words, minimizing the weight of the motors and other components allows Archer to maximize Midnight’s payload.
Archer’s electric motors weigh just 25 kilograms and have a peak power output of 125 kilowatts, with a power-to-weight ratio of 5 kW/kg. These are significantly lighter than existing electric motors for both aircraft and electric vehicles. For example, MagniX’s Magni350 electric propulsion units, which have a peak power output of 350 kW and a power density of 3.2 kW/kg, weigh 131 kg. The Tesla Model 3 weighs 90 kg, has a peak power output of 211 kW, and a power density of 2.3 kW/kg.
“These are not bad designs by any means; they're really just optimized for their applications,” said Alan Tepe, who leads the propulsion inverter design team at Archer. “If we just started out and said, ‘Oh, we're just going to use these other high-performance motors that exist’—if we had taken those motors and put them on the aircraft, the overall propulsion system would weigh about 500 pounds heavier than it does now,” he added. “So really, it just shows the benefit and the need to optimize the motor for a particular application, which we've done.”
The Midnight aircraft has 12 motors to power its 12 propellers, six of which provide lift from the back of the aircraft’s wing, while the other six tilting propellers on the front of the wing can provide both lift and forward propulsion. During cruise flight, Midnight’s fixed wing provides all of the lift, and the propellers are only necessary to take off vertically, hover, and accelerate to transition to wingborne flight.
The front and rear propellers are almost identical, with both types of propellers consisting of 95 percent of the same materials. Using the same materials for both propellers simplifies the manufacturing process, allowing Archer to more quickly and efficiently mass produce the aircraft, Tepe explained.
Archer’s engineers decided on this so-called “12-tilt-six” configuration because it gives the aircraft a high level of redundancy, meaning that it can still safely fly even if it loses multiple propellers.
“One of the big drivers here is you have to make sure that the thrust on all sides of the aircraft is equal in hover because you want to make sure there are no single-point failures,” Tepe said. “If you only have like four propellers and you lose one of the corners, you can't really balance out the load on the other three propellers,” he explained. “So, we have redundancy at the propeller and the engine level, so you can lose propellers on opposite sides and still keep yourself balanced and maintain hover.”
To power the electric motors on Midnight, Archer has come up with a proprietary battery pack design that will utilize cylindrical, type 2170 lithium-ion battery cells supplied by the Taiwanese battery manufacturer Molicel.
These type 2170 battery cells are already widely used in electric vehicles on the market today, “so it comes with favorable economics and a significant supply base … and we're not counting on a technology breakthrough. We're not going with a fancy custom cell that's not going to be available in 2027. It's available today,” said Alex Clarabut, Archer’s director of battery systems.
Midnight contains six 800-volt battery packs, each of which supports two motors at a time. These battery packs are all connected to each other, which allows them to “intelligently shuttle energy back and forth,” Clarabut said. “This is important in case of a battery pack failure, [because] another battery pack can take over, but it also allows you to isolate the batteries from each other,” Clarabut explained. “If there's something that happens on one battery or one engine, other batteries and engines can take over.”
This kind of redundancy is not only important for safety, but it also “enables us to reduce the power requirements at the battery cell level by 20 percent compared to a standard high voltage architecture,” Clarabut said. “If we had to add 20 percent of massive battery cells back into this, it would impact our payload by about 300 pounds. So this architecture is a huge enabler for us.” Clarabut added that Archer’s battery packs with the Molicel cells will be capable of supporting more than 10,000 flights on Midnight, assuming that the average flight is about 20 miles.
Further increasing the safety of the Midnight aircraft, Archer has also come up with a proprietary thermal runaway strategy—a system that prevents fires in the case of a failure in the lithium-ion battery cells. Archer has not disclosed the details of how this thermal runaway strategy works, but Clarabut assured that “if we have a failure of the battery management system, we've got enough redundancy in the system to be able to continue to fly safely.”
Archer has not yet conducted any test flights with its Midnight eVTOL aircraft, but the company has flown its two-seat Maker prototype aircraft extensively over the past year, and Maker's first transition flight is expected to happen in the next few weeks. The company plans to fly Midnight for the first time in the first half of 2023 after it completes the critical design review. A preliminary design review was wrapped up in August.