RSE35
To further minimize the costs of pressure fed systems the source of the pressure must be defined and accounted for. Propellants can be pressurized with an external tank of high pressure gas like helium and a regulator system or a self pressurizing liquid like nitrous oxide or propane can be used with the vapor pressure pushing the propellants into the combustion chamber. (see RSE # 29) With the pressure and flow drop as the tank pressure falls, the thrust will decay to a lower level at the end of burning. But as we saw in RSE # 34, this has only a small effect on ISP, and the total impulse of the vehicle is only slightly impaired. Due the longer burn duration, gravity losses increase. However, without the added weight of the gas pressure tanks, the mass ratio of the vehicle improves which more than compensates. Great cost savings are realized by eliminating the expensive pressurant tanks and the high gas flow regulators.
The huge Sea Dragon launch vehicle proposal used a combination of gas pressurization and a unique staged vapor pressurization system. A vapor pressurized liquid forced a low boiling point liquid into the main tanks where the latent heat of the main propellant vaporized it to gas. The system combined simplicity, low weight, and low cost.