A space station is a spacecraft which does not significantly change its orbit but rather acts as a permanent space presence, often to provide services to other vessels in the course of their missions. A small space station can be launched whole by a single rocket, but larger ones can be constructed in stages by docking multiple sections together; such stations can be much larger than what any single rocket could feasibly launch.
Space stations are often used as refueling depots. For example, if a station is placed in Mun orbit, a lander can dock to the station and take some fuel or monopropellant to refill its own tanks for landing and return. A dedicated station's fuel capacity can be far greater than that of any single ship, and holding large amounts of fuel already in orbit can greatly influence the design and operation of subsequent craft.
It is also possible to house Kerbals on a space station. The PPD-10 Hitchhiker Storage Container is especially useful for this purpose, as it can hold four Kerbals and its ladders are arranged to stack on top of each other. The PPD-12 Cupola Module is meant to be incorporated into space stations, both because it is unsuitable for most other mission profiles and because it deliberately resembles an iconic component of the International Space Station. As of 0.24.2, kerbonauts do not require any consumable resources and may be left in space indefinitely.
Space stations can also be used for scientific measuring using the various sensors. By placing a station on an eccentric orbit, measurements at different altitudes can be taken. However, it can be more efficient to perform these tasks with smaller and more maneuverable satellites. As of the version 0.23 update, one can also use the Mobile Processing Lab MPL-LG-2 on a space station to increase efficiency for research and clean the Mystery Goo™ Containment Unit and SC-9001 Science Jr..
Design and construction
A simple space station may be launched as a single unit, and will differ from other craft only in minor ways. The most significant of these is a lack of propulsion. While stations may possess reaction wheels or RCS for basic mobility, they typically lack rocket engines. Other than that, a station may possess electrical systems, one or more command modules, environmental sensors, electric lights, or any other structure.
In order for a station to act as a fuel depot, it must be equipped with clamps for docking. A station will often have several clamps of different sizes on it, to accommodate multiple visiting craft or craft of different designs. Generous space should be allowed around the clamp for visitors to maneuver without colliding with other parts of the station.
An incrementally constructed space station has no practical limit to its size; it may contain as many components as the designer cares to launch. The only cap on extremely large stations is the number of parts they contain, as eventually the user's computer will be unable to simulate the station's physics in real time. Hitting this limit can be staved off by careful choice of the contents of each station module. Module complexity can be decreased by leaving out normally essential systems such as batteries and RCS; when docked to the station they will be able to take advantage of these capabilities in other modules. In order to transport such a module from the launch pad to the station, docking ports or decouplers can be used to mount nonessential systems. They can be removed once the station is assembled, or even beforehand, and the module pushed into place with tugs.
One of the problems with building a space station is the major problem of lag. Any ships with part counts of more than 250 will begin to lag violently on older devices, making any space station of a size that is usable completely impractical because of the lag. This lag problem exists because each part in a ship has its own physics attached to it. Normally this isn't problem, but when there are hundreds of parts it becomes really annoying. And although there are rockets with such high part count, those lose parts after every stage and thus reducing the part count. Usually the payload brought into orbit has then only a fraction of the original part count.
Most space stations include more than one module. A complete space station usually has: A science module, a crew module, a communications module, an electricity module (a module that has batteries and solar arrays, generators, etc. ). A space station that has all of these modules or maybe even more will be a great asset in creating a successful space program in KSP.