Complex sentence: The X- 33 was an unmanned, sub-scale technology demonstrator for the VentureStar under the Space Launch Initiative, a next-generation, commercially operated reusable launch vehicle.
Split into simpler sentences: The X- 33 was an unmanned, sub-scale technology demonstrator for the VentureStar under the Space Launch Initiative. The VentureStar was planned to be a next-generation, commercially operated reusable launch vehicle.