There is one system (it's big bucks too) that has a single 1000w HPS bulb. The plants turn around the bulb like on a Ferris wheel. When they reach the bottom they get a drink. It turns very slowly and yes the plants at the top are upside down. I hear it does work. They grow like 60-100 plants with a single bulb. There is a version that has 2 bulbs end to end as well.

Wish I could find the link but someone will post it.