7 Platform one Uncrewed Systems Technology | February/March 2025 Blue Origin and Space X have shown how rocket launchers can return autonomously to Earth, and now researchers in China have published algorithms for powered descent guidance (PDG) for reusable rockets to accomplish high-precision landing, writes Nick Flaherty. This is different from the PDG algorithms for autonomous landings on the Moon or Mars, as the powered descent guidance is required to accommodate nonlinear dynamics and more disturbing flight conditions, including engine-thrust fluctuations and aerodynamic uncertainty. Existing works have considered six-degrees-of-freedom dynamics and the aerodynamic model, but not disturbances in the guidance design. The team of researchers is led by Huifeng Li and Ran Zhang from Beihang University, China. “We formulated a novel problem called endoatmospheric powered descent guidance with disturbance rejection (Endo-PDG-DR) by dividing and conquering disturbances,” said Huifeng Li, professor at the School of Astronautics at Beihang University, whose research interests focus on the field-of-flight vehicle guidance and control. “The disturbances are divided into two parts: modelled and unmodelled disturbances. As a result, two different disturbance rejection strategies are accordingly adopted to deal with the two kinds of disturbances. “The modelled disturbance is proactively exploited by optimising the formulated guidance problem, where the modelled disturbance is augmented as a new state of the dynamics model. “The unmodelled disturbance is reactively attenuated by adjusting the second-order partial derivative of the Hamiltonian of the optimal guidance problem with a parameterised, timevarying, quadratic performance index. “The optimal feedback guidance law unifies two functions: the adaptive optimal steering and disturbance attenuation. Adaptive optimal steering accommodates the disturbance, and the disturbance attenuation compensates for the state perturbation effect induced by the remaining unmodelled disturbance.” All this led to a simple, practical quadraticweighting parameter to compensate for the unmodelled disturbances. Space Powered descent guidance algos for reusable rockets Researchers in the US have developed a flexible, organic, photovoltaic (OPV) solar cell that functions well in space, writes Nick Flaherty. Space missions often decide on gallium arsenide for its high efficiency and resistance to damage from protons, but it is expensive, heavy and inflexible. In contrast, organic solar cells can be flexible and are much lighter. “Silicon semiconductors aren’t stable in space because of proton irradiation coming from the sun,” said Yongxi Li, associate research scientist in electrical and computer engineering at the University of Michigan. “We tested organic photovoltaics with protons because they are considered the most damaging particles in space for electronic materials.” The research shows organic solar cells made with small molecules don’t seem to have trouble with protons, showing no damage after three years of radiation. In contrast, those made with long-chain polymers lost half their efficiency. “We found protons cleave some of the side chains and that leaves an electron trap that degrades solar-cell performance,” said Stephen Forrest, professor of engineering at the University of Michigan. These traps grab onto electrons freed by light hitting the cell, stopping them flowing to the electrodes harvesting the electricity. “You can heal this by thermal annealing or heating the solar cell, but we might find ways to fill the traps with other atoms, eliminating this problem,” said Forrest. The OPVs have the potential to achieve a high specific power of 40 W/g and they can be grown using vacuum thermal evaporation (VPE) rather than a solutionbased process. This VPE allows the cells to be resistant to degradation by 30 keV proton radiation, in contrast to polymerbased OPVs, which suffer a 50% efficiency loss under similar conditions. Radiation-tolerant organic solar cells for space (Image courtesy of University of Michigan) Solar Organic photovoltaic solar cell works in space Two different disturbance rejection strategies are adopted to deal with the two kinds of disturbances (Image courtesy of Beihang University, China)
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