Helium Tanks

Helium tanks are pressurized to 3000 psi. They are used to actuate the valves and to 
pressurize the propellant tanks. The amount of He needed to pressurize tanks is determined 
in part by the amount of pressure loss in the propellant feed system. With less pressure 
loss the vehicle requires less Helium. The Helium itself isn't heavy, but 1lb of He 
requires 10lb of hardware. Saving He saves weight.