Traditional aerospace design relies on an additive system-of-systems approach, where optimizing one parameter typically introduces penalties in aerodynamic drag or structural mass. This article details a closed-loop, cascaded design methodology for a 40-seat regional transport. By utilizing the cryogenic thermal properties of liquid hydrogen (LH₂) as the primary heat sink, mechanical constraints are bypassed. This enables a structural-propulsion wing core and a tailless, high-aspect-ratio staggered box-wing geometry. The resulting airframe exhibits inherent aerodynamic stability, internal load-path resolution, and zero-moving-parts pneumatic vectoring.
1. Introduction: The Failure of Additive Optimization
The transition to hydrogen aviation using legacy tube-and-wing configurations is fundamentally constrained by the volume-to-weight paradox of LH₂. With a density of approximately 71 kg/m³, accommodating the required fuel volume in a standard fuselage mandates an increase in wetted area, which induces unacceptable form drag. Furthermore, attempting to retrofit conventional turboprop or turbofan engines for hydrogen combustion introduces localized thermal management challenges. Additive optimization—adding components to solve problems created by other components—fails at this thermodynamic limit. A viable LH₂ transport requires an architecture where the fuel, propulsion, and structure operate as an integrated physical loop.
2. The Cryogenic Cascade: Fuel as a Thermal Buffer
Internal combustion engines, specifically horizontally opposed boxer configurations, possess inherent geometric advantages but are traditionally limited by thermal saturation. Conventional cooling mechanisms require large-area external radiators or air-cooled fins, both of which introduce severe profile drag.
This architecture resolves the cooling bottleneck by utilizing the cryogenic state of LH₂ (-253 °C) as an infinite thermal sink. Before injection into the combustion chamber, the LH₂ is routed through integrated heat exchangers within the cylinder block. This sequence executes a phase-change optimization:
The cryogenic fluid absorbs the engine's waste heat, maintaining the block at optimal steady-state operating temperatures without external airflow.
The absorbed heat vaporizes the LH₂, expanding it into a high-pressure gas and pre-heating the fuel, which maximizes combustion efficiency.
By closing the thermal loop internally, the engine block requires zero external cooling geometry, rendering it aerodynamically invisible.
3. Propulsion-Spar Integration
Removing the cooling-drag penalty allows for the physical integration of the propulsion system directly into the primary lifting structure. To fit the engine cores entirely within the natural camber of the upper wing without protruding nacelles, displacement requirements must be scaled down.
A four-engine distributed boxer layout replaces the standard twin-engine configuration. Halving the power requirement per engine reduces the necessary cylinder bore and stroke, yielding ultra-thin, horizontally opposed blocks. These scaled-down engines are embedded flush within the upper wing, effectively acting as reinforced segments of the main spar.
This distributed mass placement across the wingspan provides immediate inertial wing-bending relief. By positioning the dense mechanical mass outward, the engines actively counteract the upward aerodynamic lift vectors at the wing root during flight, allowing the center carry-through structure to be manufactured with less structural mass.
4. Aerodynamic Synthesis: The Staggered Trapezoidal Box-Wing
The distributed propulsion core is structurally supported by a staggered, trapezoidal box-wing geometry. A secondary, high-aspect-ratio lower wing is positioned with a rearward stagger relative to the upper main wing.
The vertical connecting elements are not simple aerodynamic endplates; they act as rigid structural tie-rods. This triangulation prevents the shear-racking and torsional deflection that degrade conventional rectangular box-wings. By coupling the upper and lower spars, the lower wing functions as a tension member under positive-G loads, drastically reducing the required thickness of the main spar.
Aerodynamically, the trapezoidal endplates physically block high-pressure airflow from rolling over the wingtips. This vortex containment significantly reduces the induced drag coefficient. Consequently, the airframe achieves the effective span-efficiency of a much larger conventional wing within a highly compressed physical wingspan, optimizing it for short-field operations.
5. The Tailless Equilibrium: Inherited Safety and Lift Spoiling
The structural and thermodynamic integration culminates in the complete removal of the traditional tail assembly. Eliminating the empennage removes the wetted area responsible for significant parasitic skin friction and the trim drag associated with a downward-lifting horizontal stabilizer.
Longitudinal stability is achieved passively via the rearward-staggered lower wing, which provides continuous pitch-damping. If the angle of attack increases uncommanded, the rearward placement of the lower wing shifts the center of pressure aft, generating a localized lift increase that naturally corrects the pitch angle downward.
During landing, the system utilizes active boundary layer control powered by the engine exhaust. The internal wing plenums route high-velocity gas to trailing-edge Coandă slots. Upon touchdown, fluidic switching valves instantly divert the flow from the upper trailing edge to the lower wing surface slots. This executes an immediate lift-spoil, collapsing the circulation loop and transferring 100% of the vehicle’s mass to the landing gear at a near-zero angle of attack. This flat-landing profile prevents flare-induced ballooning, eliminates tail-strike risk, and maximizes mechanical braking traction for short takeoff and landing (STOL) parameters.
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