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Kerbal Space Program plugin for RCS thruster placement
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Plugin version 0.9.1 Oct 30, 2016
RCSBuildAidToolbar version 0.9.1 Oct 30, 2016
Textures this icon is no longer used. Oct 17, 2014
LICENSE add license file Nov 13, 2015
Makefile fix compilation with KSP 1.2 Sep 21, 2016
RCSBuildAid.version version 0.9.1 Oct 30, 2016
README.asciidoc version 0.9.1 Oct 30, 2016


RCS Build Aid Plugin

Aid tool for balancing RCS thrusters around the center of mass while building a rocket, and a bit more.


  • KSP version 1.2.x



Just copy RCSBuildAid directory into your KSP’s GameData directory.


  • Display thrust and torque forces caused by RCS or engines.

  • Torque by engine gimbals.

  • Delta V readout for MonoPropellant RCS.

  • Dry center of mass marker (DCoM).

  • Center of drag marker (CoD) for parachute placement.

  • Average center of mass marker (ACoM).

  • Ability to resize editor’s overlay markers.

  • Display total amount/mass of resources.


These are the files that comprises RCS Build Aid.

  • Plugins/

    • RCSBuildAid.dll The main plugin.

    • RCSBuildAidToolbar.dll This plugin adds support for blizzy78’s toolbar, and can be deleted if you don’t use it.

    • PluginData/

      • settings.cfg RCS Build Aid config, can be deleted for reset all settings.

  • Textures/

    • iconAppLauncher.png

    • iconToolbar_active.png

    • iconToolbar.png

  • CHANGELOG.asciidoc

  • README.asciidoc


  • RCSBuildAid.version File used by some tools for check for updates, such as KSP-AVC plugin.


This plugin can only be used while in VAB (Vehicular Assembly Building) or in SPH (Space Plane Hangar). RCSBuildAid is be enabled from the application launcher at the bottom of the screen. Once enabled, you should see RCSBuildAid’s window. You can also enable it with a keyboard shortcut, see Settings section.

If you have blizzy78’s toolbar installed, then you can disable the application launcher button or blizzy78’s toolbar from the settings menu.

Balancing RCS

Having balanced RCS means that when you’re translating your vessel won’t rotate and when you are rotating it won’t translate, this is important for easy docking. This depends of the position of your CoM and the placement of your RCS thrusters.

Mode selection

The topmost button in the window is for select the working mode, each having a different purpose. Currently the modes available are:


For balancing translation. The installed RCS will be set to translate your vessel to the current direction, with the green arrow being the actual resulting motion. When balancing translation you want translation motion without any rotation, so you want to place your RCS around your CoM in a way that reduces the torque vector (red arrow) as much as possible.

Is not necessary to completely cancel out the torque force, sometimes might be difficult. If you don’t see the circular arrow then it means that the current torque is acceptable and shouldn’t interfere during normal docking.


For balancing attitude. Only with RCS, reaction wheels and engine’s gimbals not accounted. For balancing attitude you want to reduce the translation vector (green arrow) while keeping the torque vector aligned with the red marker. In most vessels you won’t need to do this since balanced translation implies balanced rotation, but this is not always true.


For balancing engine’s thrust. The engines used in the calculation are always the engines in the bottommost stage, if you want to disable some engines you can move them to a higher stage. This mode also allows changing the direction of gimbals, showing their produced torque.


For balancing parachutes. This mode activates the center of drag (CoD) marker and should help in positioning parachutes so that your vessel will touchdown with the desired attitude and velocity.

Parachutes' drag vectors vary in both magnitude and direction, this is not modelled by RCSBA so its indications are not completely accurate.

Editor overlays


The forces displayed are of 3 types:

Part forces

Forces exerted by parts, they are colored cyan for RCS and yellow for engines.

Translation force or thrust

Colored in green, represents the translation motion of your vessel, essentially your vessel’s thrust. A small green triangle near the tip of the arrow indicates where it should be pointing ideally.

Torque force

Colored in red, represents the resulting torque the thrusters are exerting into your vessel. The effect of this torque is represented by a circular arrow, which is proportional to the expected angular acceleration. When you see a red arrow, it means that in the current configuration and with the given input your vessel will try to rotate, however, depending of your vessel’s mass and of its distribution this rotation might not be noticeable, you can gauge this with the circular arrow. Like in the translation force, a small red triangle would be indicating the ideal direction.



The Center of Mass marker, colored yellow. You should be familiar with this one.


The Dry Center of Mass marker, colored red, indicates the center of mass for your vessel without resources. Which resources to remove when the vessel is "dry" can be configured from the Resources settings.


This is the Average Center of Mass marker, colored orange, shows the middle point between the CoM and DCoM.

All forces are referenced to one of these markers, you select which one from the GUI. Checking how forces change depending of where your CoM is will help you to understand how to balance best your vessel, eg, for a vessel that docks with almost no fuel it would be best to balance RCS around the DCoM and ignore the CoM marker. If you want your vessel to be balanced in most situations you need to build your vessel in a way that keeps both center of masses as close as possible, or if you can’t achieve this, balancing around the ACoM marker would be the best compromise.


The Center of Drag, only enabled while in parachute mode. Shows the resultant drag force.

User interface

RCS Build Aid’s window have several sections showing some information and options.

Select mode

Shows some information regarding the current mode in use, usually the thrust and torque magnitudes, and buttons for change the current direction and center of mass (CoM, DCoM or ACoM).


Torque magnitude.


Thrust magnitude. In engine mode, there’s a toggle for select between thrust at vacuum or at sea level of the selected planet.


Active center of mass of the vessel, click for change.

Direction (Translation mode)

Current movement direction, click for change or use the shortcuts.

Rotation (Attitude and Engine modes)

Similar to direction, but for attitude changes. In Engine mode there will be an "R" button for reset gimbals position.

ΔV (Translation mode only)

Available delta velocity from RCS at current direction.

The delta V readout has some issues, see Known issues section.
Burn time (Translation mode only)

Time RCS will last until running out of fuel at current direction.

Body (Engine mode only)

Selected body for TWR calculations, click for change.

TWR (Engine mode only)

Thrust to weigth ratio at sea level of the selected celestial body.

Vt (Parachute mode only)

Terminal velocity of the vessel for the selected planet and altitude.

Vessel mass

Shows some information regarding the mass of the vessel.

Wet Mass

Total mass of the fully fueled vessel.

Dry Mass/Fuel Mass

Mass of the vessel without fuel / Mass of the fuel, click in label to toggle.


Shows a list of resources currently in your vessel, displaying the total mass or the total amount of each (click in the "Mass" label for change the reading).

You can select which resources should be used for calculating the DCoM position and the vessel’s dry mass, disabled resources will be removed and enabled ones will remain. For example, if you want to know your vessel’s mass and DCoM position when only the MonoPropellant is completely consumed, only disable the MonoPropellant.


Options for show, hide or change the size of the CoM markers.


You can open the settings menu from the small s button in the title bar of RCSBuildAid window.

Use application launcher

Disable/Enable RCSBuildAid’s button in the application launcher.

Use blizzy’s toolbar

Disable/Enable RCSBuildAid’s button in blizzy’s toolbar.

Show in Action Groups

Don’t hide RCSBuildAid when switching to the action groups screen in the editor.

Marker autoscaling

The CoM markers will change size depending of the camera distance (if you get the camera closer to your vessel the markers will shrink).


Enable RCSBuildAid with a keyboard shortcut, disabled by default. You might assign any key but no key combinations. Using ESC disables it.

Keyboard shortcuts

You can change direction from the GUI, or alternatively with the same keys for translating your vessel.

For translation mode:


Set direction towards fore (forward).


Set direction towards aft (backward).


Set direction towards starboard (right).


Set direction towards port (left).


Set direction towards ventral (down).


Set direction towards dorsal (up).

For Attitude and Engine mode:


Set rotation to roll left.


Set rotation to roll right.


Set rotation to yaw right.


Set rotation to yaw left.


Set rotation to pitch down.


Set rotation to pitch up.

Using the keys when no mode is enabled will automatically enable translation or the last used mode. Setting the same direction twice disables current mode.

Incompatible plugins

This plugin will work only with parts using stock modules, such as ModuleRCS or ModuleEngine. Modded parts that use other modules will not be detected by RCS Build Aid.

Known Issues

  • Delta V readout for RCS isn’t very smart and only works for the stock RCS that use monopropellant, there will be no dV readout if the RCS uses other fuel or a different flow mode (like Vernor RCS).

  • Parachute mode only works with stock aerodynamics and parachutes, will not work with mods such as FerramAerospaceResearch and RealChute.



Before compiling with make, you need to create an enviroment variable pointing KSP instalation:

export KSPDIR=/home/user/KSP


This project picks its references for a local directory for avoid commiting the absolute path of library files into the repository. So if you want this project to find its references without changing any project files you have create a local directory named Libraries and copy (or symlink) the directory Managed from a KSP install and Toolbar.dll from blizzy78’s toolbar mod (if you wish to compile RCSBuildAidToolbar.dll).

  Managed -> ${KSP_DIR}/KSP_Data/Managed
  Toolbar.dll -> ${KSP_DIR}/GameData/000_Toolbar/Toolbar.dll

Reporting Bugs

You can report bugs or issues directly to GitHub:


This plugin is distributed under the terms of the LGPLv3.

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program.  If not, see <>.
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