Ullage or headspace is the unfilled space in a container, particularly with a liquid. Etymology The word ullage comes from the Latin word oculus, used by the Romans to refer to a barrel cork hole. This word was, in turn, taken in medieval French as oeil, from which a verb ouiller was created, meaning to fill a barrel to full capacity. Around 1300, the word ouillage was created by the Normans to refer to the amount of liquid needed to fill a barrel to capacity. Alcoholic beverages In winemaking, ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. 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. By further extension, in brewing and beer retail, ullage is the residue of beer left in a barrel that has been emptied. When calculating tax returns and the like, licensed premises owners, landlords or managers can factor in the duty on ullage, or unavoidable barrel wastage. Ullage therefore has come to be used as a general term, in the licensed trade, for waste beer whether at the barrel or at the bar tap or pump. However, what customers leave in their glasses does not count as ullage, because it has been sold. Rocketry Liquid propellant rockets and spacecraft store their propellants in tanks. Cryogenic tanks are never completely filled, to prevent severe pressure drop in the tank after engine start. On the ground, or in the continued gravitational field of Earth during rocket-propelled ascent, the space between the top of the propellant load and the top of the tank is known as "ullage space". Ullage pressure is a critical measurable during powered rocket flight, because it affects tank structural integrity and engine Net Positive Suction Pressure (NPSP). In the weightless condition in space without engine thrust, empty space occurs in partially-filled tanks, and the ullage space becomes distributed across much of the tank in a heterogenous mixture of masses of liquid amidst many gaseous regions. Under these conditions, liquid floats away from the engine intake, which is undesirable for stable engine operation. The application of a small force, from a small rocket engine for example, are sometimes used to provide sufficient acceleration to reaggregate (settle)