Gradient is developing an aircraft which never has to land. Satellites are expensive to launch, constrained to prescribed orbits, and impossible to maintain and upgrade. High-altitude pseudo-satellites (HAPS) are a new breed of aircraft which collect enough solar energy during the day to remain aloft throughout the night, thereby eliminating the need to land and refuel. This capability provides the same services as our existing satellite networks, but also features station-keeping for persistent ISR, broadband speed data rates for multi-domain operations (MDO), and atmospheric monitoring for weather prediction. Unfortunately, current HAPS attempts cannot attain the goal with existing solar cell and battery technologies. Leading contenders use standard aircraft configurations, but even fully optimized designs offer limited durations under favorable solar conditions (lower latitudes during summer months). Global coverage on the worst winter day necessitates a dramatic improvement in aerodynamic efficiency, to yield an aircraft which operates on substantially less power. Low-power fixed-wing aircraft employ slender wings to minimize induced drag, but this introduces bending and flexibility which requires additional structural material. Helicopter rotors deflect under their own weight which require centrifugal forces pull them taught, but high angular rates and triangular span loading lead to terrible aerodynamic efficiency. What if we borrow the physics of a helicopter (where centrifugal stiffening places elements under tension), to solve the aerodynamic/structural problem within fixed-wing aircraft? Composite materials are phenomenal under tension, but significantly worse under compression. A small amount of radial acceleration alleviates an adverse bending moment, which substantially changes the design problem. Gradient has developed the Tethered Uni-Rotor Network (TURN) system; a novel concept UAS which uses this technique to overcome deficiencies with traditional HAPS aircraft. Rather than a single high aspect ratio flying-wing aircraft (which cannot do the job on its own), consider four such aircraft tethered together in a hub-and-spoke arrangement, where the system operates in a state of rotation and places each of the wings under tension. Compared to fixed-wing aircraft, this approach: reduces structural material by half, doubles the energy storage mass, maximizes aspect ratio which minimizes induced drag, and permits airfoils with 3X improved lift-to-drag ratios. Compared to helicopters, this approach: eliminates downwash field interactions, achieves ideal elliptic span loading, and places substantial mass at the wingtips to slow the angular rate and minimize power. These multiplicative design improvements yield a system which operates on an order of magnitude lower power than fixed-wing aircraft, while still offering hover and vertical takeoff and landing (VTOL) capabilities, which puts a feasible HAPS solution within reach.
