Ice tank, ice fuel, ice reaction mass – Extra reaction mass ( commonly referred to as fuel ) carried externally by a spacecraft. It is simply a block of ice frozen onto a hard point on the outer hull of a ship. Carried like this, no tank is needed and its weight is saved. A bot cuts off blocks of the ice and feeds it to a pipe leading to the engines. Water and oxygen are most often used, both are abundant and available in any system. Nitrogen and ammonia usually have more pressing uses, but are occasionally used. Most other liquids are not abundant enough to be used, except in rare circumstances. Hydrogen and helium don't freeze solid enough to be used in this manner. Any rock or metal can also be used like this ( iron is used more often than nitrogen ), but the energy requirements for cutting up metals and rock make ices the preferred choice. If in close orbit to a star where the ice may be in danger of melting, it will be covered with a 'blanket', a light weight cover to block heat and light. These are very light weight, for example the blanket used for a Goldenrod ice tank weighs only one tonne.