Helium balloon simulation in Gazebo

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Hello,

First, I'm using ROS Indigo with Gazebo 2. To simulate a helium balloon attached to a ground robot in Gazebo, I add a prismatic joint and a balloon link to a pre-existing robot model, and I disable the gravity for the balloon link. However, when I spawn it into Gazebo, it automatically re-enables the gravity for my balloon link... so the balloon crashes on the robot.

Simplified version of my URDF file :

<?xml version="1.0"?>
<robot name="my_robot">

  <!-- Cube base link -->
  <link name="base_link">
    <inertial>
      <mass value="1"/>
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
    </inertial>
    <visual>
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
      <material name="white"/>
    </visual>
    <collision>
      <origin xyz="0 0 0.5" rpy="0 0 0"/>
      <geometry>
        <box size="1 1 1"/>
      </geometry>
    </collision>
  </link>

  <gazebo reference="base_link"> 
    <material>Gazebo/White</material> 
  </gazebo>

  <!-- Joint between cube and balloon -->
  <joint name="balloon_joint" type="prismatic">
    <origin xyz="0 0 3.0" rpy="0 -1.57 0" /> 
    <parent link="base_link" /> 
    <child link="balloon_link"/>
    <limit lower="-3" upper="10" effort="10" velocity="5"/>
    <gravity>0</gravity>
  </joint>

  <gazebo reference="balloon_joint"/>

  <!-- Balloon link --> 
  <link name="balloon_link">
    <inertial>
      <mass value="1"/> 
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/> 
    </inertial> 
    <visual> 
      <origin xyz="1 0 0" rpy="0 0 0" /> 
      <geometry> 
        <sphere radius="1" /> 
      </geometry> 
      <material name="blue" /> 
    </visual>
    <gravity>0</gravity>
    <collision> 
       <origin xyz="1 0 0" rpy="0 0 0" /> 
      <geometry> 
        <sphere radius="1" /> 
      </geometry>
    </collision> 
  </link>

 <gazebo reference="balloon_link"> 
    <material>Gazebo/Blue</material> 
    <gravity>0</gravity> 
  </gazebo>

</robot>

with the launch file:

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

  <arg name="world_name" default="worlds/empty.world"/>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(arg world_name)"/>
    <arg name="paused" value="false"/>
    <arg name="use_sim_time" value="true"/>
    <arg name="gui" value="true"/>
    <arg name="headless" value="false"/>
    <arg name="debug" value="false"/>
  </include>

  <param name="robot_description" textfile="$(find playground)/urdf/description.urdf"/>

  <node name="spawn_model" pkg="gazebo_ros" type="spawn_model" output="screen"
    args="$-unpause -urdf -param robot_description -model base_link"/>

</launch>

When I roslaunch the launch file, the balloon has crashed on the cube; it's clear that gravity has been re-enabled for the balloon, since if I make a rosservice call to apply an upward force to the balloon, it will fall again on the cube.

I figured out that, when I convert this URDF into a SDF file using gzsdf, Gazebo automatically overrides my gravity tags. I tried to remove the "<gravity>1</gravity>" tags of the SDF and directly spawn the SDF file into Gazebo; however, robot-state-publisher didn't work anymore (it only accepts URDF files), so it's not a solution for me.

How can I prevent Gazebo from automatically re-enabling gravity for my balloon link?

Thanks.