Complex sentence: The Mod-0 was also used to test the first steel shell towers, now the dominant tower design, the challenge being to take weight and cost out of the tower while safely passing through a resonance of the soft structure during startup.
Split into simpler sentences: The Mod-0 was also used to test the first steel shell towers, now the dominant tower design. The design challenge was to take weight and cost out of the tower while safely passing through a resonance of the soft structure during startup.