#AMRG Presents: "Conceptual Challenges for Crashworthy Battery-Electric Commercial Aircraft" A research summary

What? This study explored the challenge of integrating crashworthy, large batteries into electric commercial aircraft. While safety assessments have been conducted, questions remain about the integration of these batteries under FAR or CS-25 regulations. Current battery technologies and regulatory specifications highlight the need for preventing mechanical deformation in lithium-ion and all-solid-state batteries. More research is needed at both local and global levels to achieve certifiable battery-electric commercial aircraft.

Who? Lennart Lobitz, Alexander Hahn, Daniel Vogt, Tim Luplow, Peter Michalowski, Sebastian Heimbs, and Georg Garnweitne from Technische Universität Braunschweig, Germany.

Where? The full article can be found HERE.

The study explored safety issues in current conceptual aircraft designs, such as crashworthiness of battery-powered aircraft. Existing publications on electric aircraft safety focus mainly on general aviation and failure modes and probabilities of electric components. To stay economically competitive, turn around times of electric aircraft must not be significantly longer than conventional aircraft. This requires high-speed charging that comes at higher risks of battery wear and fire.

The crashworthiness requirements for commercial aircraft are provided in relevant regulations, which typically require sufficient survival space, seat attachment, and inertia forces not exceeding given limits. Conventional aircraft designs follow a systems approach, containing landing gears, high-strength structural cabin floors, crushable subfloors, energy-absorbing seats, and high-mass retention systems. The energy absorption of fuselage structures in large fixed-wing aircraft is crucial for crashworthiness, as they provide sufficient energy absorption due to their high energy absorption and large crash distance. However, the introduction of composite structures has made it more important to provide sufficient energy absorption for crash scenarios for certification. The fuselage diameter plays a significant role in the feasibility of a crashworthy design, especially when the cargo compartment is not allowed to deform.

On the global fuselage level, two typical crash kinematics are identified: unrolling kinematics, which describes frame failure and hinge development, and flattening failure mode, which prevents upward movement of the frame. Kinematic hinges can be inserted or optimized to reduce computational costs. A different approach has been developed, including tensile energy absorbers in the cargo floor areas of the airframe. On the component level, different energy absorbers have been developed to compensate for the brittle behavior of carbon fiber-reinforced polymer (CFRP). The European Program Commercial Aircraft Design For Crash Survivability (CRASURV) investigated the energy absorption capabilities of composite sine wave beams, but experimental tests emphasized a lack of robustness.

Lithium ion batteries (LIB) have been extensively researched, with current LIB reaching over 270 W h kg −1 and 200 W h kg −1 on cell and pack levels, respectively. However, for medium-range aircraft, battery packs with specific energies of more than 800 W h kg −1 are required. Next-generation batteries, like all-solid-state batteries (ASSB), are ideal candidates for use in electric aircraft due to their higher specific energy and intrinsic safety. ASSBs use a solid-state electrolyte, allowing lithium metal as the negative electrode material, enabling higher specific energies of up to 500 W h kg −1 on cell level. However, each class of material has its own challenges that need to be overcome for widespread commercial application.

Battery cell failure can lead to thermal runaway, a devastating failure triggered by uncontrolled rise in cell temperature due to exothermic reactions. This chain reaction produces gaseous reaction products, increasing pressure inside the cell and potentially igniting flammable gas mixtures. The critical temperatures required for reactions can be triggered by various stresses, which can be divided into three categories: thermal, electrical, and mechanical. Thermal stress occurs when the cell heats up to high ambient temperatures, leading to decomposition of the solid electrolyte interface (SEI), resulting in further heating and melting of separators. Electrical stress occurs when the battery is stressed by overcharging, over-discharging, or an external short circuit. Mechanical stress occurs when the battery cell is damaged by external forces, such as plastic deformation from a vehicle crash or penetration of an external object into the cell structure. Vibrations can also cause further stresses, affecting the performance of battery cells. ASSBs offer intrinsic safety and increased specific energy for electric aircraft applications, but their safety is dependent on the specific electrolyte used. The absence of flammable liquid electrolytes does not guarantee an intrinsically safe battery system.

Other researchers have modeled the heat release and temperature rise of a solid-state battery with a lithium lanthanum zirconium oxide (LLZO) separator compared to a conventional LIB in the event of thermal failure due to external heating, internal shorting, and severe failure of the SE. They found that the oxide-based ASSB is safe in the event of external heating, as long as the separator prevents oxygen diffusion from the cathode to the lithium metal. However, in the event of an internal short circuit, the heat release from ASSB and LIB is the same if only the heat release due to the rapid discharge of the battery is considered. ASSBs, LiS, and further next-generation technologies can reach higher specific energies than LIBs, making failure even more severe.

Battery safety regulations in the automotive industry have been developed over years to ensure the safety of battery systems and high-voltage safety within the powertrain. Thermal runaway, vibration, and vehicle impact are critical failure mechanisms of lithium-ion battery packs, emphasizing the importance of sufficient cell spacing and structural support to prevent battery cell movement and detrimental vibration. Battery packs should be placed away from typical crash zones. Post-crash performance limits are similar across different regulations, ensuring the safety and integrity of vehicles and occupants.

In electric general aviation, various options for integrating batteries are being pursued, including wing-integrated batteries, batteries integrated into the side walls of the fuselage, and batteries stored in the cargo compartment below passengers. Propulsion batteries are wing-mounted only in the hybrid-electric SUGAR Volt configuration and in Imothep Reg-Rad, where batteries are located within engine nacelles behind distributed propellers, minimizing cable length. Structural batteries have not been considered in any of the concepts listed above.

The charging strategies of the considered configurations differ mainly between charging the batteries at the gate and exchanging them in-between flights. However, the batteries need to be exchangeable due to maintenance and the aircraft's performance can improve with enhancements in battery technologies. Some certification specifications for large airplanes do not contain any requirements for propulsion batteries, but the regulation allows two different approaches for certification. The first approach aims to prevent thermal runaway propagation from one cell to adjacent cells for worst-case scenarios, but this could lead to impractical designs with substantially reduced specific energies. The second approach ensures continued safe flight and landing (CSFL) in case of thermal runaway in more than one cell, in accordance with fire protection in designated fire zones.

The energy-to-weight ratio of electric aircraft is significantly higher than conventional aircraft due to heavy propulsion batteries increasing battery total capacities. This affects crashworthiness as the energy absorbed in a crash proportionally increases with total mass. Two realistic battery storage options for AEA and HEA conceptual designs are the cargo compartment and engine nacelles. Wing-located batteries offer better accessibility and load alleviation, but may be exposed to vibrations and vibrations. Over-wing mounts may be beneficial from a crashworthiness perspective, but may negatively impact aerodynamics and cabin noise.

The study examines the crash resistance of future battery-electric aircraft by reviewing conventional crash concepts, battery technologies, conceptual aircraft design studies, and regulatory specifications. It reveals that ASSBs, which are prone to thermal runaway, are also prone to mechanical deformation. The cargo compartment offers more space for energy storage than engine nacelles or external pods, but it interferes stronger with global crash concepts. The global crash concept for an aircraft containing batteries in the cargo compartment should favor a flattening failure mode rather than unrolling to avoid penetration of the containers by the frame. The study emphasized the importance of enhancing regulations for commercial aircraft with respect to propulsion batteries and research providing sufficient crashworthiness for battery-electric aircraft. Designing could aim at avoiding the unrolling failure mode and adjusting the cargo cross beam to increase energy absorption capabilities underneath.

Takeaways.

This study does not really convey any information that would likely be surprising to persons involved in AAM and eVTOLs. However, it does provide some insights into the differences between the auto and aviation industries in terms of the ways battery systems are designed and integrated. What is clear is that more research and standards are needed.

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