The bar at the bottom of the ship designer is not accurate to the decimal points needed to show mass usage of modules like the Sparrow - which uses 12.6 units. Because of this if you have a Sparrow as one of your weapons and have used up 122.6 units of mass out of 130 it looks like you have used 122 units out of 130 and should be able to add a module that takes only 8 units (such as an Environmental Support Module) but it gets rejected. If armament and/or modules are going to use mass in decimal amounts, the mass used bar must display at the same accuracy.
