2 research outputs found
Tether Dynamics Analysis and Guidance & Control Design for Active Space Debris Removal
No full textRecent years have seen a steep increase in research being performed towards active space debris removal: space debris has proven to be a very real threat to operational spacecraft, and studies indicate that the frequency of collisions will only increase if nothing is done to remove large pieces of debris. In particular, ESA has done studies towards removing Envisat from orbit, after communications were lost and were unable to be reestablished. To this end, a scenario was proposed in which a robotic chaser satellite would use a tether to interface with Envisat, either using a net or a harpoon, and proceed to deorbit the resulting tethered system. This research tackles two main challenges relating to this scenario. First, a suitable model for the tether was developed by discretizing the tether into a number of point masses and massless Kelvin-Voigt elements. The influence of the number of nodes was investigated, showing that increasing the number of nodes used does not significantly increase the fidelity of the solution. Therefore, it was chosen to model the tether with two nodes and three elements. Additionally, multiple combinations of tether length, stiffness, and damping were investigated. Second, a preliminary design for a guidance and control system was developed. This system uses multiple fixed-duration burns as the main deorbit strategy. The guidance system controls the relative state of the chaser with respect to the target during and after these main engine burns: during burns, a hold point is established at the equilibrium length of the tether, and during coasting phases the tether is kept slightly in tension to reduce target and tether motion. At the transition between thrusting and coasting phases, the thrust is gradually throttled down to further decrease these motions. The control system is designed to keep the chaser level with the local horizon at all times. During the design process, the performance of guidance and control systems based on linear quadratic regulators was used as a baseline for the same systems based on sliding-mode control. Furthermore, three different thrust levels for the deorbit burn were examined. It was found that the system based on sliding-mode control offered considerable performance improvements over the LQR-based system: total propellant consumption was reduced by an average of 48\%, while adhering to the same tolerances. Furthermore, it was determined that high thrust levels are desirable for both reducing propellant consumption as well as reducing target rotation. In terms of safety, collision can best be avoided by using lower thrust levels or longer tethers. While this seems to conflict with minimizing required propellant, minimal target rotation and minimal propellant consumption can still be achieved by using long tethers with high stiffness. Finally, it was found that while sensitive to small changes in the initial conditions, the precision of the terminal point is still high enough to allow the target to be deorbited in the South Pacific Ocean. This result was independent of tether model, although higher thrust levels do increase the precision further.Astrodynamics and Space MissionsSpace EngineeringAerospace Engineerin
EFFECT OF FEEDING LEAVES GROWN UNDER DIFFERENT SYSTEM OF MULBERRY CULTIVATION ON SEED CROP REARING OF BOMBYX MORI L.
No full textAn experiment was conducted to analyse and evaluate the leaf quality of mulberry grown under three systems of mulberry cultivation viz. 2"x2", 3\u27x3\u27 spacings and tree plantation for seed crop rearing of B. mori. Rearing and grainage parameters like larval duration, larval weight, survival rate, shell ratio, melting cocoon percentage, moth emergence percentage, laying recovery percentage, fecundity and hatching percentage was studied in both bivoltine (NB4D2) and multivoltine (Pure Mysore) races. For all the parameters 3\u27x3\u27 spacing has given better results during summer and winter seasons compared to other spacing and was significantly higher for low melting cocoon percentage and laying recovery percentage. Whereas tree leaf was found better during rainy season. Hence, it is concluded that 3\u27x3\u27 spacing of mulberry cultivation is better than 2 X 2 spacing during summer and winter seasons whereas tree leaves are recommended during rainy season for seed crop rearing
