Graham Warwick After a battery fire destroyed its demonstrator, Lilium has revamped the energy storage system in its eVTOL air taxi.
Graham Warwick
Credit: Lilium
Setbacks come with the territory when developing aircraft, according to Alastair McIntosh, chief technology officer of electric air-taxi developer Lilium.
Former chief engineer on the Rolls-Royce Trent XWB engine for the Airbus A350, McIntosh addressed the February 2020 battery fire during ground maintenance that destroyed Lilium’s demonstrator aircraft.
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“Unfortunately, I’m not the first to break a few engines in my time,” he told a Lilium virtual analysts’ day on June 15. “It’s all about how we manage [the setbacks] and how we respond to the issues.” Lilium’s response was a complete redesign of its energy storage system.
The Munich-based startup is getting ready to fly a second demonstrator that has the new battery system. “We were already moving from cylindrical to pouch cells,” he said. “That was already part of the plan. As part of our flight clearance, we’ve also demonstrated that the modules we’ve designed can contain a thermal runaway and keep the aircraft safe.”
The aircraft now in ground testing is Lilium’s fifth generation of demonstrator, beginning with the Falcon and Dragon subscale models in 2016, the larger two-seat Eagle in 2017 and the five-seat Phoenix in 2019—all flown unmanned.
After the fifth-generation demonstrator flies this year, Lilium’s next aircraft will be the first of at least six certification-conforming prototypes of its planned seven-seat production aircraft, the Jet. The startup is aiming for concurrent European type certification and U.S. validation by the end of 2024.
The Jet has 72 identical battery modules mounted along the sides of the fuselage and fire-walled from the cabin. These power 36 electric ducted fans mounted on the foreplane and wing. Each fan is individually mounted on a flap that tilts to provide vertical and forward thrust as well as all flight control. The fans and flaps are controlled by three dissimilar flight control computers supplied by Honeywell.
The battery selected by Lilium for its seven-seater is a pouch cell with a silicon-rich anode and conventional nickel-manganese-cobalt cathode. The silicon-rich anode increases energy density compared with a more conventional graphite anode. “We believe this combination offers the best compromise of energy and power density at the low state of charge, which determines the effective usable battery capacity,” Lilium said in a May filing with the Securities and Exchange Commission (SEC).
Lithium-silicon batteries are on the cusp of commercial availability, led by companies such as Sila Nanotechnologies. Sila is already working with BMW and Daimler and in January raised $590 million to scale up silicon anode production.
“We spent two years scouting and testing dozens of different cell technologies,” Lilium Chief Program Officer Yves Yemsi said at the Lilium event. “Then last year, we found and secured access to a cell that will meet the unique performance of our vehicle and enable scaled production.”
The cell supplier has not been named, but in its SEC filing, Lilium said: “We have invested in a leading battery technology supplier, securing exclusive rights for the eVTOL market for this chemistry.” Yemsi said Lilium is developing the certification road map and planning the full-scale manufacturing capacity.
The technology selected has demonstrated a cell-level energy density greater than 330 Wh/kg, Lilium said, compared with about 250 Wh/kg for conventional lithium-ion cells. This is sufficient to give the Jet an initial range of 250 km (155 mi.), the company said.
This is less than the 300-km goal originally set for the Jet but is Lilium’s target for entry into service. The company expects energy density and power level at a low state of charge to improve over time, and the Jet is designed to be upgraded as the technology advances.
The cell has demonstrated a power density of 2.8 kW/kg and the ability to be charged to 80% in 15 min. and 100% in 30 min, which is key to high aircraft utilization on Lilium’s regional shuttle networks. In testing, the cell has achieved a life of more than 800 charge-discharge cycles to 80% capacity, says Lilium, which intends to replace batteries 2-3 times a year based on projected flight hours.
The startup’s battery design consists of multiple independent packs, each built up of multiple modules to provide redundancy. The battery casing is designed to contain a multicell thermal runaway within a module, preventing propagation to the remaining modules and packs.
Lilium selected a ducted-fan design to minimize noise, and the Jet is expected to generate a takeoff sound level of 60 dBA at 100 m (330 ft.) compared with 220 m for an open-rotor eVTOL, McIntosh said. Ducted fans require twice the power in hover, but the Jet is expected to spend only 30 sec. in hover. “We see this as a fixed-wing aircraft, not a helicopter,” he said.
Over a 30-min. mission, the Jet will consume only 5% more energy than an open-rotor eVTOL, he said. This assumes a 10-sec. hover for takeoff, 20-sec. transition to wingborne flight, 30-min. cruise at 10% of takeoff power, 20-sec. retransition to thrustborne flight and 20-sec. hover to land. Lilium assumes a 60-sec. hover energy reserve, although reserve requirements have yet to be agreed upon with the regulator.
