Well I posted about 2 months ago about designing a propulsion and ballast system completely run on compressed gas. Here is the update.
CO2 did not work well as people said. But Nitrogen is cheaper then CO2, is not liquid and does not cool the same way. We are running a 90 PSI grinder at 15PSI attached to a 19:1 drill gear box. Over the winter I will be playing with ballast system.