Propellant management device

A propellant management device (PMD) provides a way to expel propellant in a low-gravity environment. Devices can use surface tension as the primary expulsion device with a combination of baffles, fins, and vanes. The main goal of the PMD is to provide gas-free propellant to the rocket engine.¹

In the absence of gravity, buoyancy forces do not determine liquid and gas position in a vessel. The positions are primarily driven by surface tension.^2: 1 The liquids tend to adhere to the walls and leave a gaseous bubble in the center of the vessel.^3: 2 Propellant management devices (PMDs) are required to provide gas-free operation of the engine.^4: 1

PMDs are typically unique and specially designed for each mission.^2: 1

There are two groups of PMDs, total communication and control-type. A total communication PMD can acquire propellant from anywhere in the tank.^5: 3

There are three types of total communication PMDs: vane, gallery, and pleated-liner.^6: 3

Vanes are used when the spacecraft experiences low acceleration and requires low propellant flow rates. Due to their simple mechanical design, they are low cost and highly reliable.^5: 3 They are typically used in small monopropellant thrusters or to refill another type of PMD: sponges.^5: 5 Vane length (whether it extends partially up the vessel or to the top) is partially determined by the shape of the tank. Cylindrical tanks require full-length vanes since a portion of the propellant could adhere to the forward tank head. Spherical tanks need full-length vanes in a case by case basis. If the acceleration is lateral, partial-length vanes can work.^5: 5

A center post can be added to the tank in addition to the side vanes. This provides a direct path for the propellant to the tank outlet.^4: 4

There are three types of control-type PMDs: sponge, trough, and trap.^6: 4

Sponge PMDs are primarily used to provide the engine with propellant needed for ignition, providing the engine with propellant during a specific maneuver, and propellant control in microgravity environments.^2: 3