Description
The objective of the activity is to develop a radiation model taking into account the local radiation environment and solar cell technology to predict, within an absolute accuracy of 2%, the remaining solar cell power factor, taking into account the radiation induced degradation encountered in electric orbit raising (EOR) missions. The model shall be validated on coupons manufactured to engineering model level.
Targeted Improvements:Reach an absolute accuracy of 2% of the solar cells power prediction during Electric Orbit Raising (EOR) compared to state of the art of 7-10%.
Description:Electric orbit raising (EOR) is rapidly becoming a common method for the efficient transfer of spacecraft from the injection to operational orbit. Whereas the traditional direct injection methodpasses quickly through the Earth trapped proton and electron belts, the EOR method provides much greater exposure to the radiation belts, with a corresponding greater degradation of some spacecraft systems. In particular, the solar arrays of the spacecraft are subjected to considerably higher rates of degradation in the inner proton belt, resulting in degradation levels that are comparable tomany years in the operational orbit. Recent experience by spacecraft in this region [e.g. TACSAT-4] shows that there is anunexplained large underestimation of the degradation predicted by the current models and tools. This underestimation could arise from inadequacies in the radiation environment models, the methods and data that convert the radiation environment particle fluxes to degradation, or both. The current process is to use the standard ECSS environmental models with the historical "EQFLUX" or the more recent "SCREAM" solar cell degradation algorithms to determine the array power degradation. These models and algorithms have a proven track record for the long duration geostationary orbits. However, a known deficiency in the standard ECSS models for the radiationbelts is their provision of long-term multi-year average values in a region of the environment where the monthly variability can exceed several orders of magnitude. These models are adequate for the longer time scales, but when applied to a short-term multi-monthEOR trajectory, the dynamic variability of the environment can lead to extreme underestimates of the environment. The current methods for evaluating the solar cell degradation in a high flux proton rich environment could also be inadequately estimating the degradation. This is particularly true for the EQFLUX method that translates all degradation into an equivalent 1 MeV electron flux. Furthermore, the validity of assuming an isotropic environment has to be confirmed.
Therefore, it is proposed to develop new confidence-level-based radiation environment engineering models that will predict the radiation environment as a function of risk of exceedance for a multi-month EOR mission. These models will then be coupled with the radiation degradation parameters of the solar array to produce the expected degradation of the array for the orbit raising phase. The degradation calculation is to include the degradation ofthe solar cells, cover glass, and adhesive, providing a complete assessment of the array upon delivery to the operational orbit. A sensitivity analysis shall be performed to understand the impact of a non-isotropic particle bombardment on the solar cell degradation. An end-to-end error and sensitivity analysis of the process will be performed to establish uncertainties that can form the basisof a margin policy. The study shall be accompanied by irradiation tests on cover glasses and solar cell assemblies or coupons, constructed as cover glassed solar cells with (as a minimum) front side interconnectors applied. Thereby, part of the test specimen shall be irradiated with one proton energy only, while others shall be irradiated subsequently with at least two proton energies (low and high energy) to validate the superposition principle typicallyy applied. The updated models and degradation algorithm will be then be integrated into common space environment and effects tools for ease of use by the engineering community.
Tender Specifics