the liquid propellant at the bottom of the tank near the engine propellant inlet prior to ignition of the main engine(s). Engines devoted to this single purpose are typically called ullage motors. Reaction control system thrusters are also often used to settle propellants prior to reignition of a liquid propellant engine in space. Industrial use Ullage is also widely used in industrial or marine settings to describe the empty space in large tanks or holds used to store or carry liquids or bulk solids such as grain. In accordance with IMO regulations, the Code of Federal Regulations, and the ABS Rules for Steel Vessels, certain pressurized tanks on steel ships may not be filled greater than 98% full, although there are exceptions. This is so that the pressure relief valve is always in contact with a gas or vapor. Certain pressure relief valves for chemical use are not designed to relieve the pressure when in contact with liquids. In some cases, the ullage in a ship's hold can be relevant to stability; liquid or dry bulk cargo in a part-filled hold can shift asymmetrically towards one side as the ship heels to one side and the other, reducing the margin of stability when compared with a full hold. Excessive ullage in a tank may contribute to the free surface effect. When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up". See also References Bibliography ABS Rules for Steel Vessels 2007, Part 5C External links Description of Saturn V engines Rocket propulsion Packaging