Trajectory Design & Optimization for Spacecraft with Multiple Engines
Efficient performance of a number of engineering systems is achieved through different phases or modes of operation. In the context of spacecraft trajectory optimization, mission designers are considering a cluster of electric engines in order to configure more capable propulsion systems. However, equipping a spacecraft with multiple electric engines (of the same or different types) compounds the task of optimal trajectory design due to presence of both real-valued inputs (power input to each engine in addition to the direction of thrust vector) and discrete variables (number of active engines). Each engine can be switched on/off independently and "optimal" operating power of each engine depends on the available solar power, which is coupled to the distance from the Sun.
Figure above (left plot) depicts the dependency of the power generated by solar arrays to the distance from the Sun. There are also other factors that affect the net power such as degradation in the efficiency of solar panels due to radiation. Part of the power is used for energizing several sub-systems of a spacecraft. Some portion of the power is used to energize the power processing unit (PPU), which is responsible for modulating the power sent to thrusters. The net power has to be distributed between engines in an "optimal" manner. The right plot depicts the changes in performance of a number of electric thrusters versus the engine input power. Figure below (the left plot) depicts the the trajectory of a spacecraft from earth to comet 67P Churyumov–Gerasimenko (it was the destination of the European Space Agency's Rosetta mission). Here, in our model, the spacecraft is equipped with four BPT-4000 Extra High-Isp engines. The right plot (lower subplot) depicts time history of thrust generated by these four engines, and in the upper sub-plot, the distance from the Sun is shown. This minimum-fuel trajectory consists of three revolutions around the Sun en-route to the comet, whereas the optimal thrust profile has a complex structure. The change in the magnitude of the thrust is an indication that during some portions of flight multiple engines have been used. For instance, during the early phases of flight all four engines are switched on since power input is enough, whereas the thrust value changes as the distance from the Sun increases.
Relevant Publications
-
Ehsan Taheri, John. L. Junkins, Ilya Kolmanovsky, and Anouck Girard, "A Novel Approach for Optimal Trajectory Design with Multiple Operation Modes of Propulsion System, Part 1," Acta Astronautica (under review).
-
Ehsan Taheri, John. L. Junkins, Ilya Kolmanovsky, and Anouck Girard, A Novel Approach for Optimal Trajectory Design with Multiple Operation Modes of Propulsion System, Part 2," Acta Astronautica (under review)