Using steam (water vapour) in the tube has great advantages
  • Cooling
  • Low energy vacuum
  • Increases speed of sound
Cooling is very efficient by boiling water, and ejecting the steam into the tube
 
Vacuum pumping energy is greatly reduced, because the steam only needs to be pumped to a low-pressure condenser
 
The Natural Steam Vacuum is a very low energy solution for 3kPa pressure
 
Steam has a higher speed of sound, reducing the Mach no. of the pod, reducing aerodynamic drag and shock waves