Complex sentence: But in addition to testing the rockets structurally and the new engines, SA-2 was designed for Project Highwater, an experiment to release a large quantity of water into the upper atmosphere to investigate its effects.
Split into simpler sentences: But in addition to testing the rockets structurally and the new engines, SA-2 was designed for Project Highwater. This was an experiment to release a large quantity of water into the upper atmosphere to investigate its effects.