How to run empty_world.launch with simbody physics engine in Gazebo 9.4.1?

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Hello,

I would like to run a simple robot in an empty world using the simbody physics engine in Gazebo 9.4.1 with ROS Melodic distro. Here is what I am doing right now:

1. In my empty_world.launch file I call this argument for the physics engine:

   arg name="physics" default="simbody"/

2. In my empty.world file I define the parameters of the simbody physics engine following the SDF guidelines.

3. I run the .launch file using

   roslaunch "package name" empty_world.launch

4. Once Gazebo is running, the Physics tab in the Gazebo GUI shows that the engine is ODE but it includes the max_step_size and other parameters that I defined in my empty.world file. So I know that Gazebo is reading my .world file but for some reason it is still saying that it is running the ODE engine.

5. As a sanity check, I can run the following and the Gazebo GUI shows that the engine is simbody.

   gazebo -e simbody

6. The problem is that once Gazebo is running (after sending the command in step 5 above), I cannot get my robot to spawn in the world; the robot launch file is just paused on this line in the Terminal:

   [INFO] [1540949039.735518]: Waiting for service /gazebo/spawn_urdf_model

What am I doing wrong? Are there any resources you recommend for defining a physics engine in Gazebo 9.4.1?

Thank you for your help!