ullage n : the amount that a container (as a wine bottle or tank) lacks of being full
- In a wine bottle, the empty space between the cork and the top of the wine.
- In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel
- In an industrial setting, the empty space in a tank, such as for fuel
Ullage refers to the unfilled space in a container of liquid.
EtymologyThe word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was created, to fill a barrel up to the bung hole. In turn, a noun ouillage was created, which was the immediate source of our word, first recorded in Norman English about 1300, at first in the sense of the amount of liquid needed to fill a barrel up to the bung hole.
Wine and spiritsBy an obvious extension, ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. And it is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes.
In liquid rockets, ullage is the space within a fuel tank above the liquid propellant. This term derives from the term 'ullage' in winemaking, where it refers to the space above the liquid in a container such as a barrel or wine bottle.
Liquid, cryogenic rockets keep their propellant in tanks. These tanks are never completely filled in order to allow for the expansion of the cold liquid propellant. On the ground, the space between the top of the propellant load and the top of the tank is known as "ullage space".
In zero-gravity conditions the gas may float around and threaten to be sucked into the engines, which is typically very undesirable. Small rocket engines are sometimes used which settles the propellant prior to the main engine ignition. These are called ullage motors.