错误信息：
[INFO] [1631713794.504229, 358.040000]: Spawn status: SpawnModel: Entity pushed to spawn queue, but spawn service timed out waiting for entity to appear in simulation under the name iris
[ERROR] [1631713794.505728, 358.040000]: Spawn service failed. Exiting.

[vehicle_spawn_isst_15735_2562139104487950788-6] process has died [pid 15835, exit code 1, cmd /opt/ros/melodic/lib/gazebo_ros/spawn_model -sdf -file /home/isst/project/volans/src/simulation/models/iris/iris.sdf -model iris -x 5 -y 0 -z 0 -R 0 -P 0 -Y 0 __name:=vehicle_spawn_isst_15735_2562139104487950788 __log:=/home/isst/.ros/log/cc103de8-162b-11ec-a1d9-0c8268028cc2/vehicle_spawn_isst_15735_2562139104487950788-6.log].
log file: /home/isst/.ros/log/cc103de8-162b-11ec-a1d9-0c8268028cc2/vehicle_spawn_isst_15735_2562139104487950788-6*.log

解决方案是：
将launch文件中.world文件里的sim_time值改为0，即如下内容：

<sim_time>358 36000000</sim_time>

改为：

<sim_time>0</sim_time>