Reactive software poses stringent and comprehensive requirements: deterministic execution with stringent timing constraints under a tight energy budget. Meeting these requirements is particularly challenging when executing on the increasingly heterogeneous platforms of today. In this paper, we integrate MOCASIN, a design space exploration tool, into LINGUA FRANCA, a programming framework for building deterministic and timed reactive software. We show that this integration enables choosing a desired timing and energy performance at design time. We demonstrate our approach in a satellite attitude control application consisting of periodic real-time tasks and sporadic non-real-time tasks. The latter sporadic tasks are coordinated using quasi-static schedules, computed by MOCASIN, leading to less energy consumption compared to the Linux scheduler under CPU frequency scaling governors such as powersave, schedutil, and ondemand.