To compensate for the gradual depletion of propellants during an engine burn, and the consequent expansion of the gaseous space in the tank, a regulated pressure system feeds in a steady supply of gas to keep the tank at the same pressure as it was at the start of the burn. This system typically pumps in compressed helium or nitrogen gas, which are inert and incombustible.

It is usually preferable to regulate the pressurant gas with a non-venting dome loaded regulator. Dome loaded regulators are able to be opened or closed remotely by pressurizing or releasing a small volume of pressurized gas in a separate port in the regulator.

Blowdown is a condition where the tank’s initial volume of compressed gas, or ullage, is used to push the propellant out the drain, and expanding in the process. By expanding to occupy the space left behind by the liquid propellant, the ullage experiences a drop in pressure that also reducing the flow rate of the liquid propellant, as well as the thrust produced by the rocket motor. This also reduces nozzle efficiency over time, as the flow becomes over-expanded. The nominal loss of thrust and performance is not as bad as it may seem, as high thrust is most critical at liftoff, and the over-expansion is slightly mitigated by the drop in atmospheric pressure during flight. Replacing the mass of the regulator hardware with extra tankage is sometimes a positive tradeoff, and the reduction in complexity makes it far more robust and usually cheaper than an equivalent regulated pressure system. The changing feed pressures makes it less than ideal for regenerative rocket motors.

A less common, but also simple and capable to significant hardware reductions. One tank is situated inside an outer tank, with an open top that is sealed by a moving piston. Only the outer tank needs to be pressurized, so the inner tank can use thin walls. This design is not suitable for cryogenic rockets, and is usually used with storable hypergols, with extensive experience, training, and safety equipment of course.

With both propellants draining at the bottom, and one propellant volume providing the pressurant for both liquids, there is no need to run feed lines from very far. Some designs even incorporate a monolithic endcap/main valve assembly, with pyro valves built into the tank end.

Popularized in recent years by Half Cat Rocketry, the stacked piston uses a moving piston to push a propellant upwards using the pressure from the ullage gas of the propellant below it. By rerouting the fuel to the bottom, it can be fed into the motor. Due to the stacked nature of the tanks, it’s easier to make a rocket much narrower.