kOS allows you to write scripts that communicate with scripts running on other processors within the same vessel (inter-processor communication) or on other vessels (inter-vessel communication).


While you are able to send messages to vessels that are unloaded, the receiving vessel must be loaded in order to use and reply to the message. This is because kOS is unable to run on an unloaded vessel. The loaded status of a vessel depends on proximity to the active vessel (usually a sphere a couple of kilometers in radius) as well as current situation (landed, orbit, suborbit, etc). For more information about how vessels are loaded, check the loaddistance documentation page. In order to have the receiving vessel reply when unloaded, it will need to be set to the KUNIVERSE:ACTIVEVESSEL or the load distance needs to be adjusted.


The basic unit of data that is sent between two entities (CPUs or vessels) is called a Message. Messages can contain any primitive (scalars, strings, booleans) data types as well as any serializable types. This allows for a lot of flexibility. It is up to you as a script author to decide what those messages will need to contain in order to achieve a specific task.

kOS will automatically add 3 values to every message that is sent: Message:SENTAT, Message:RECEIVEDAT and Message:SENDER. The original data that the sender has sent can be accessed using Message:CONTENT.

Message queues

Whenever a message is received by a CPU or a vessel it is added to its MessageQueue. Message queues store messages in order they were received in and allow the recipient to read them in the same order.

It is important to understand that every CPU has its own message queue, but also every vessel as a whole has its own queue. A vessel that has 2 processors has 3 message queues in total: one for each of the CPUs and one for the vessel. Why does the vessel has its own, separate queue? Well, if it hadn’t then you as a sender would have to know the names of processors on board the target vessel in order to send the message to a specific CPU. That would complicate the whole process - you would have to store those names for every vessel you plan on contacting in the future. When every vessel has a separate queue the sender doesn’t have to worry about how that message will be handled by the recipient. It also allows you to easily differentiate between messages coming from other CPUs on board the same vessel and messages coming from other vessels. The downside is of course that in certain complex cases it might be necessary for a CPU to operate on two message queues - its own and vessel’s.

There is one major difference in how CPU and vessel queues are handled. Messages on vessel queues are persisted when changing scenes in KSP while messages on CPU queues are not. This difference stems from the fact that when you’re switching from one vessel to another that is further than 2.5km away KSP actually saves the game, constructs a new scene and loads the game from that previously created save file. If messages hadn’t been added to the save file they would be lost and any kind of long distance inter-vessel communication would be impossible. Obviously in the case of inter-processor communication all processors are part of the same vessel. No scene changes are required and hence no need to persist messages.


Connection structure represents your ability to communicate with a processor or a vessel. Whenever you want to send a message you need to obtain a connection first.

Inter-vessel communication

First we’ll have a look at the scenario where we want to send messages between two processors on different vessels. As the first step we must obtain the Vessel structure associated with the target vessel. We can do that using:

SET V TO VESSEL("vesselname").

Sending messages

Once we have a Vessel structure associated with the vessel we want to send the message to we can easily obtain a Connection to that vessel using Vessel:CONNECTION. Next we’re going to send a message using Connection:SENDMESSAGE. This is an example of how the whole thing could look:

SET MESSAGE TO "HELLO". // can be any serializable value or a primitive
PRINT "Delay is " + C:DELAY + " seconds".
  PRINT "Message sent!".

Receiving messages

We now switch to the second vessel (in the example above it was named “probe.”). It should have a message in its message queue. To access the queue from the current processor we use the SHIP:MESSAGES suffix:


Inter-processor communication

This will be very similar to how inter-vessel communication was done. As the first step we must obtain the kOSProcessor structure associated with the target CPU.

Accessing processors

The easiest way of accessing the processor’s kOSProcessor structure (as long as your CPUs have their name tags set) is to use the following function:

  • volumeOrNameTag – (Volume | String) can be either an instance of Volume or a string

Depending on the type of the parameter value will either return the processor associated with the given Volume or the processor with the given name tag.

A list of all processors can be obtained using the List command:


Finally, processors can be accessed directly, like other parts and modules:


Sending and receiving messages

Then we can use kOSProcessor:CONNECTION to get the connection to that processor. This is how sender’s code could look like:

SET MESSAGE TO "undock". // can be any serializable value or a primitive
  PRINT "Message sent!".

The receiving CPU will use CORE:MESSAGES to access its message queue:

WAIT UNTIL NOT CORE:MESSAGES:EMPTY. // make sure we've received something
  PRINT "Undocking!!!".
} ELSE {
  PRINT "Unexpected message: " + RECEIVED:CONTENT.

Connectivity Managers


New in version v1.0.2: The concept of selectable connectivity managers was added after KSP introduced a stock communications system (CommNet). kOS was updated to support both CommNet and RemoteTech. Other mods may be supported or provide their own support in the future.

kOS can implement communications over a variaty of connectivity configurations. We refer to these options as “Connectivity Managers.” You can slect the active manager from the kOS section of KSP’s Difficulty Settings. If the currently selected manager no longer is available, or if a new manager becomes available, you will be prompted with a dialog box to select the manager you want to use.

Connectivity Managers

By default kOS supports the following list of managers. If a manager is not currently available (because the required mod isn’t installed, or the system is disabled in the settings) it will not be shown in lists of available managers.

This manager will permit all connectivity at all times. Connections between vessels, home, and control always show as being connected. This is the equivalent of setting CONFIG:RT to False under the former system. It is possible kOS will be unable to use all features if this manager is selected while communication limitations are enforced by another mod or a setting.

This manager will use KSP’s stock CommNet implementation to monitor connections. It will only be available if CommNet is enabled in the KSP difficulty settings.


Active Vessel: CommNet has limitations on updating connections for vessels which are not the active vessel. The best way to ensure that a connection is updated is to include one of the kinds of antenna that can act as a relay on one or both of the vessels.


Debris Vessel: To save on computer load, CommNet cuts all “debris” vessels out of the system when loading the scene and does not track them. Therefore any vessel that was of type “debris” when the scene was first loaded will be unable to have a connection, regardless of what antennas it may have on it, and regardless of whether you change its vessel type to something other than “debris” during the scene. The vessel type has to be something other than “debris” at the moment the scene is first loaded in order for it to be able to have CommNet connections. If you have a “debris” vessel that has no communication connections, you can change the vessel type from “debris” to something else, and then go to the tracking station and come back, then that can cause the vessel to have a connection again.

This manager will use the RemoteTech mod to monitor connections. It will only be available if RemoteTech is installed. You can access more detailed information and methods using the RemoteTech Addon


Take care when configuring your game for connectivity. Enabling multiple systems at the same time may result in unexpected behaviors both for the game connectivity itself, and for kOS’s connectivity manager. kOS only supports selecting one connectivity manager at a time and you should ensure that only the corresponding in game connectivity system is enabled.

You can check communication status between vessels by checking the Vessel:CONNECTION. To monitor the status for home and control connections the following bound variables are available. This allows you to monitor the basic network status using a single unified system, regardless of which connectivity manager is selected.


Returns a Connection representing the CPU Vessel’s communication line to a network “home” node. This home node may be the KSC ground station, or other ground stations added by the CommNet settings or RemoteTech. Functionally, this connection may be used to determine if the archive volume is accessible.


Attempting to send a message to the “home” connection will result in an error message. While this connection uses the same structure as when sending inter-vessel and inter-processor messages, message support is not included.


Returns a Connection representing the CPU Vessel’s communication line to a control source. This may be the same as the HOMECONNECTION, or it may represent a local crewed command pod, or it may represent a connection to a control station. When using the CommNetConnectivityManager this should show as connected whenever a vessel has partial manned control, or full control. Functionally this may be used to determine if terminal input is available, and what the potential signal delay may be for this input.


Attempting to send a message to the “control” connection will result in an error message. While this connection uses the same structure as when sending inter-vessel and inter-processor messages, message support is not included.