Building a rocket engine

A rocket engine is crucial for most space-based games and many 2D games as well. In this recipe, we'll cover the minimum that is required to create a thruster that can be used in many different contexts. The following figure shows a thruster with ParticleEmitter:

Building a rocket engine

Getting ready

For this recipe, we need to make sure that we see the debug shapes of physics. To do this, we need to call the bulletAppState.setDebugEnabled(true); statement.

How to do it...

We will begin by setting up some things that are not strictly needed for the rocket engine but will aid the testing. Perform the following steps to build a rocket engine:

  1. First of all we add a floor mesh. For this, we create a new Node class called ground.
  2. To do this, we add RigidBodyControl with PlaneCollisionShape. The plane should face upwards like floors normally do, as follows:
    RigidBodyControl floorControl = new RigidBodyControl(new PlaneCollisionShape(new Plane(new Vector3f(0, 1, 0), 0)), 0);
ground.addControl(floorControl);
floorControl.setPhysicsLocation(new Vector3f(0f, -10, 0f));
  3. We then attach them both to rootNode of the application and physicsSpace of bulletAppState.
  4. Finally, we need to add a key to control the booster. For this, we implement an AnalogListener interface in our application.
  5. Then, add the application to inputManager along with a mapping object called boost that is bound to the Space bar:
    inputManager.addListener(this, "boost");
inputManager.addMapping("boost", new KeyTrigger(KeyInput.KEY_SPACE));
  6. Most of this recipe will be implemented in a class that extends SimpleApplication.
  7. We begin by defining a Node class called spaceShip that will be our spaceship's representation.
  8. We then create a RigidBodyControl instance with BoxCollisionShape and add it to the spaceShip node as follows:
    RigidBodyControl control = new RigidBodyControl(new BoxCollisionShape(new Vector3f(1, 1, 1)), 1);
spaceShip.addControl(control);
  9. Now, we create another Node, which will be our thruster. Give it the name Thruster to be able to identify it more easily later, as follows:
    Node thruster = new Node("Thruster");
  10. We set localTranslation of this so that it will end up at the bottom of the spaceship, as shown in the following line of code:
    thruster.setLocalTranslation(0, -1, 0);
  11. Then, we attach it to the spaceShip node.
  12. Now, we have to attach the spaceShip node to both the rootNode and physicsSpace of bulletAppState.
  13. To control the thruster and make it more reusable, we will create a class called ThrusterControl, extending AbstractControl.
  14. It'll have one field, a Spatial field called thruster, that will store the thruster node.
  15. We will override the setSpatial method and set it by calling getChild("Thruster") on the supplied spatial.
  16. Lastly, we define a new method called fireBooster().
  17. Inside this, we subtract the thruster's location from the spaceship's location and store it in a new Vector3f field called direction as follows:
    Vector3f direction = spatial.getWorldTranslation().subtract(thruster.getWorldTranslation());
  18. Then, we find the RigidBodyControl class in the spatial and call applyImpulse with the direction vector. We use the inverted direction as the relative position that the impulse should originate from. This can be implemented as follows:
    spatial.getControl(RigidBodyControl.class).applyImpulse(direction, direction.negate());
  19. In the application class, we have to make it call the fireBooster method. We do this in the onAnalog method that was added when we implemented the AnalogListener interface:
    if(name.equals("boost") && value > 0){
  spaceShip.getControl(ThrusterControl.class).fireBooster();
}

How it works...

The graphics in this recipe are very minimalistic and mostly rely on the debug mode of BulletAppState to draw them. The physics shapes don't normally have a visual representation since they're not part of the scene graph. Using the debug mode can be very useful during early prototypes.

The RigidBodyControl instance of the spaceship makes sure it's affected by gravity and other forces.

The sole purpose of a thruster is to be able to easily retrieve the position that is relative to the spaceship from where the boosting force needs to be applied. This is why we place it at the bottom of the spaceship. The benefit of using the Control pattern to control a Thruster is that we can apply it to other geometries easily (and even use it in SceneComposer).

The fireBooster method of ThrusterControl takes the position of spaceShip and subtracts the position of the thruster node to get the direction of the force to apply. The relative position of the force is the direct opposite of this direction.

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