Simulating multiple rbcar's in Gazebo

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I try to run multiple rbcar's on one ROS Master and tried to start a car as a group but the Controller does not start.

I created a launch file for multiple rbcar's which looks like this:

<?xml version="1.0"?>
<launch>

<!-- roslaunch arguments -->
<arg name="debug" default="false"/>
<arg name="gui" default="true"/>

<!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched -->
<include file="$(find gazebo_ros)/launch/empty_world.launch">
<arg name="world_name" value="$(find rbcar_gazebo)/worlds/ba_city.world"/>
<arg name="debug" value="$(arg debug)" />
<arg name="gui" value="$(arg gui)" />
<arg name="paused" value="false"/>
<arg name="use_sim_time" value="true"/>
<arg name="headless" value="false"/>
</include>

<group ns="car1">
<include file="$(find rbcar_gazebo)/launch/rbcar.launch"/>
</group>

</launch>

and includes rbcar.launch:

<?xml version="1.0"?>
<launch>

<!-- rbcar_control launch file -->
<include file="$(find rbcar_control)/launch/rbcar_control.launch" />
<include file="$(find rbcar_robot_control)/launch/rbcar_robot_control.launch"/>

<!-- Load the URDF into the ROS Parameter Server -->
<param name="robot_description"
       command="$(find xacro)/xacro.py '$(find rbcar_description)/robots/rbcar.urdf.xacro'" />

<!-- Call a python script to the run a service call to gazebo_ros to spawn a URDF robot --> 
<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
    args="-urdf -model rbcar -param robot_description -z 1"/>

</launch>

Do somebody know a solution for my Problem ?