Multifunctional Leaf-Vein Micro-Channel Spar Architecture for Cryogenic Hydrogen Thermal Management and Structural Load-Bearing in Long-Endurance UAVs

Abstract

Long-endurance hydrogen PEM fuel-cell UAVs must pre-heat cryogenic hydrogen from −150°C to 70–85°C at the fuel cell inlet while simultaneously dissipating kilowatts of motor and stack waste heat. Conventional approaches require separate dedicated
subsystems, imposing mass penalties that reduce endurance.

This paper presents a conceptual design for a leaf-vein micro-channel spar architecture in which the primary wing spars of the MARID UAV simultaneously carry structural bending and torsion loads and serve as cryogenic hydrogen thermal conduits. A
hierarchical three-level channel network distributes flow through 160 terminal micro-channels (Di = 1.5 mm) across the wing and tail surfaces, eliminating dedicated pre-heating and waste-heat rejection hardware while routing cold hydrogen through
thermally critical zones to suppress the vehicle's infrared (IR) signature — a key requirement for covert ISR missions.

A one-dimensional finite-segment thermal model demonstrates that ambient spar pre-heating is significant but insufficient at all altitudes. An electric trim heater — 1,220 W at sea level (47% of total electrical demand) to 1,650 W at the tropopause —
closes the energy balance, bringing the outlet temperature to 77.3°C within the required 70–85°C PEM operating window. The 23:1 recirculation pump power is negligible (0.0015 W), and GH2's specific heat (~13,500 J/kg·K) makes it uniquely suited to this
combined thermal-management and propulsion role.