Create a New Scene with Node3D as the root.
In the Viewport look for these icons
click on the 3 vertical dots.
Click on the buttons to add a Sun (DirectionalLight3D) and Sky (WorldEnvironment) to the Scene. Then add a Camera3D so that we can look at the scene.
Now we can add a floor for the rocket to stand on.
Add a New CSGBox3D Node as a child of Level.
CSG stands for Constructive Solid Geometry and allows you to prototype a level by combining simple shapes.
Set the size of the floor to 30 x 8 x 5
Position the floor at -4.0 on the y axis to lower it so the surface is at ground level.
To give the floor a colour we must add a New StandardMaterial3D
Then click on the Sphere which appears to open the Material Settings.
Search for the Albedo property and use the Color Picker to choose a colour.
I have chosen brown.
Select the Floor node and open the Groups dock
Add a New Group and call it Floor
Make sure that the Floor Node is in the Floor group
This will allow us to detect when the player has hit the floor.
Add the Player scene to the Level scene (instantiate)
Test the game and you should be able to control your rocket.