Under ullage ( English , inter alia, for "free space", "level to the tank ceiling") is meant in the maritime free space or the measured vertical distance between the surface of a liquid in a tank ( level ) and the upper end of the tank.
In general, the aim on ships is to drive tanks only full or empty in order to reduce the stability- reducing effect of free surfaces (sloshing). To give liquids the opportunity to expand when the temperature rises, ship tanks (including tanks on land) are only filled to a certain percentage (e.g. 98%). If the tank was 100% full, there would be the risk that cargo liquid, which heats up during the journey (oil, gasoline, chemicals), expands so much that it escapes into the open via the tank overflows ( overflow ). The remaining free space to be selected depends on the expansion behavior of the liquid (for example, a value of around 1.5% is assumed for palm oil ).
Ullage poses special challenges in space travel. In microgravity or weightlessness , the liquid propellant of rockets is not concentrated at the bottom of the tanks, where the propellant is taken, as it is on the surface of the earth. This makes it difficult to provide a constant supply of fuel to the engines, which rocket engines often require. There are various solutions to the problem, such as ullage motors, which exert thrust shortly before an engine starts and, through acceleration, push the fuel in one direction of the tank. As soon as the engine has started, it takes over this function. Ullage motors can e.g. B. small solid rockets or the Reaction Control System (RCS) of a spaceship.
- J. Bes: Chartering and Shipping Terms: Manual for trampoline and liner shipping . 2nd Edition. Uitgeverij C. De Boer Jr., Hilversum 1968.