Addons or mods are modifications to the official version of the game. They either add new parts, systems and mechanics to the game or replace existing ones. Addons are generally made by the community rather than the developers and can greatly enhance your playing experience. Due to their widespread popularity, Squad offers an official website for exchanging them, the Kerbal SpacePort.
How to install addons
Provided you downloaded an addon from the SpacePort, you should first unpack the .zip archive and then copy the unpacked contents to the appropriate folder in your KSP directory. Here's how.
Open up your KSP directory and go to the "GameData" folder. In another window, find the (unpacked) contents of the .zip archive that you just extracted. As of update 0.20, compatible addons should just show a [mod name here] folder. Copy that folder to the GameData folder, and you're done! All you have to do from there is start the game.
Uninstalling is just as simple as deleting the folder with the name of the addon that you want to remove. However, any ships created with those addons will not load next time you start your game, once uninstalled.
Note: as of the 0.20 update, all of the addon folders are contained in a parent folder (containing the name of the mod downloaded) in the GameData folder. For "legacy" (pre-0.20) addons, the folders were at the root of the .zip file - requiring the user to move the "Parts" and/or "Plugins" folders to the corresponding KSP directory folders. With many mods installed, it may have been hard to tell what parts were mods and what parts weren't when uninstalling. Now, each addon gets its own folder in the KSP directory (instead of sharing one with the game itself), simplifying things.
- Inside of the .zip file will be the folders "Parts", "Plugins", and (occasionally) "Source". There may also be documentation, usually in the form of a "README" text file.
- The "Parts" folder contains the actual ship-building parts you would see in the ship builder interface.
- The "Plugins" folder contains the plugin .dll files that would support the plugin system implemented by the addon.
- The "Source" folder is just the source code, and is only needed by modders wanting to see how the addon was made and maybe attempt to modify it (with permission/attribution, of course).
Plugin-powered addons and parts
For add-ons that utilize plugins to support their core functionality.
|Ferram Aerospace Research||FAR provides a complete aerodynamic overhaul to KSP, including mach effects, stalls, support for fairings, and realistic drag. FAR also allows flaps and air brakes on aircraft, provides valuable aerodynamic data, and gives users much more customization with their control surfaces.||0.9.4||Forum SpacePort||ferram4|
|Kethane Pack||Kethane is a multi-purpose fuel that can be scanned, drilled, and refined into other fuels. It can also be burned in a specialized jet engine for flight on oxygen-free planets like Eve.||0.4.3||Forum, Wiki||Majiir, MMI|
|Romfarer Lazor System||Adds lazors, which can assist in existing areas including vessel control, spacecraft/debris tracking, and map navigation while implementing new features such as a tractor beam, location marking, and teleportation!||v27||Forum SpacePort||Romfarer|
|Carts||Adds rovers and other mobile vehicles to drive around planets on! Can turn on handbrake, cruise control, and lights. Even has a rover with fold-able wheels for a compact fit on a rocket. DISCONTINUED||1.34 (.17)||Forum||Tosh|
|MechJeb||A plugin that adds a ton of in-flight information and automated operations. With it you can autopilot, see true altitude, and countless other features.|| 1.9.8,
|ISA MapSat||A plugin powered addon that allows you to map planets and moons, see anomalies, current position and many facts.||3.3.4||SpacePort||Innsewerants|
|BobCat Ind.||A group of addons dedicated for surface exploration and colonization, now with a dedicated plugin with additional functionality like electricity usage. Current releases include: Mk1 Archer, Mk2 Pretorian, Mk4 RAT, Mk 5 ANT, Charlie's Deli and Shoe Store (at the outlet), and the Falcon Jet.||Various releases||Forum||Bobcat, Crashnburn, CleverWalrus, The Destroyer, BlazingAngel665, Sequence|
|Mission Pack||Mission pack is a group of missions that can help new players learn how to get into an eccentric orbit, getting to the Mun, etc. and for old players to try the game in a new way.||0.85||SpacePort||adammada|
|Damned Robotics||Introduces robotics to KSP. Rotational and linear actuators that are controled withing the game. Enables designing of walkers, robotic arms, docking washers and such. (an updated version by sirkut called "Infernal robotics" brings the mod to 0.22)||DR: 1.3 (KSP 0.19), IR: 0.8b (KSP 0.22)||||plugin: r4mon, models: DYJ|
|KerbTown||A plugin that enables placement of static object (such as buildings) in game. Introduced entirely new type of mods such as Kerbin City Community Project or ICBM launch silo.||0.13||||Razchek|
|Procedural Dynamics - pWings||Procedural wings that allows players to modify the shape from withing SPH/VAB. This project started the procedural mods.||0.5||||DYJ|
Standalone Parts and Part Packs
For add-ons that provide additional parts for spacecraft and spaceplane construction without utilizing plugins.
|Payload Pack||Payload pack gives you the ability to create a wide variation of different payload fairings with the a small amount of parts.||2.0||Forum||Mechanical Mouse Industries|
|Deep Space Mission Pack||This pack aims to provide parts for the upcoming deep space part of the game.||1.8||Forum||kockaspiton|
|Down Under Aerospace & Party Supplies||A collection that includes fuel tanks, engines, boosters, interstage/fairings and a few surprises.||0.5||Forum||Slew|
|ExPI (Extraplanetary Infrastructure||Currently there is a pack of space station parts and a pack of interplanetary probes which is under development.||0.4.1||Forum||Pelf|
|JellyCubes Advanced Weapons R&D||Advanced weaponry from the Advanced Weapons Research and Development division of Cube Corp. For all your intended explosive needs.||Various releases||Forum||JellyGoggles|
|KW Rocketry||Mainly Fuel, Engine and Solid Rocket Parts. 1,2 and 3 meter parts and adapters.||0.5||Forum||Winston/Kickasskyle|
|NovaPunch Remix Pack||Very large parts pack, merger of two of the biggest and best rocket packs for KSP; SIDR by NovaSilisko and Wobbly Rockets by SundayPunch.||1.5||Forum||Tiberion|
|The Truss Pack||Rugged, practical and functional components. Includes many structural parts.||2.0||Forum||Sherbetlemons|
|The Firespitter Pack||Adds propeller plane and helicopter parts.||5.0||Forum||Snjo|
Tools and applications
For external programs that support Kerbal Space Program (e.g. Mod Managers, Orbit Calculators, etc).
|Aerobraking Calculator||An online calculator which computes an aerobraking manoeuver which results in a desired orbit. Works for all planets with an atmosphere.||1.0||Aerobraking Calculator||Alterbaron|
|Faring Factory||A website that allows you to construct your own custom farings using an easy to use web interface. Files can then be downloaded an inserted into the game. (Note that as of 0.18.2, drag has not been implemented, so the farings have no use other than to make your rocket look better)||1.2||Faring Factory||Nathan Nifong|
|Where can I go?||A website calculates the Δv needed to intercept, orbit or land on any celestial body starting from Kerbin or Kerbin's orbit.||?||Where can I go?||?|