 thumb|right|300px|iss propulsion module (nasa) the iss propulsion module was proposed as a backup to functions performed by the zvezda service module and progress spacecraft. critical iss functionality such as guidance, navigation, control and propulsion are provided only by russian (zvezda and progress) and the european (atv). a propulsion module would have been needed for iss altitude maintenance and reboost, debris avoidance maneuvers, attitude control and propellant supply in the event the zvezda service module was not available (launch failure, etc.) to the international space station. if the zvezda had not been available, the interim control module would have been used at first. it only had a lifetime of three years; then the propulsion module would have been necessary. the iss requires an average 7,000&nbsp;kg of propellant each year for altitude maintenance, debris avoidance and attitude control. based on current usage, it will need 105,000&nbsp;kg through 2014. a propulsion module would have provided reserve propellant for one year of iss orbit life in case of supply interruption. a propulsion module would have been attached to the unity node of the iss. multiple supply vehicles are required to satisfy the iss's 7,000&nbsp;kg annual average propellant need. the current plan for six progress m1 spacecraft per year meets that need. the propulsion module would hold 9808&nbsp;kg fuel. progress m holds 1100&nbsp;kg; progress m1 holds 1950&nbsp;kg. esa atv holds 4,000&nbsp;kg. the cancelled u.s. interim control module holds 5000&nbsp;kg of fuel. a shuttle orbiter iss generic reboost had 232&nbsp;kg of fuel available. an orbiter max reboost mission had 1626&nbsp;kg of reboost fuel available. zarya fgb [russian: Ð¤ÐÐ - Ð¤ÑÐ½ÐºÑÐ¸Ð¾Ð½Ð°Ð»ÑÐ½ÑÐ¹ ÐÑÑÐ·Ð¾Ð²Ð¾Ð¹ ÐÐ»Ð¾Ðº, english: functional (or operational) cargo block (or module)] holds 5,500&nbsp;kg and the zvezda service module holds 860&nbsp;kg, however, it is generally preferred to keep the main thrusters on zarya and zvezda in reserve, as they have finite lifespans. the propulsion module, besides being part of the backup plan for if the service module was not available, was intended to be an american-owned propulsion system for the station and planned as a late addition. however, the original design was over budget and late. an alternative design, the "node x" design, which was built around an improperly fabricated hul
Components_of_the_International_Space_Station