A liquid propellant rocket engine includes a pump that is disposed along a central axis. The pump includes a purge system, a collection annulus in fluid communication with the purge system, and a drain. The collection annulus has an outer diameter wall, an inner diameter wall, and an end wall. The end wall defines an annular channel that has a channel depth that varies circumferentially. The drain opens to the collection annulus. At the drain, the annular channel has a lowest point at which the channel depth is maximum depth.