How do I conserve momentum

Hi I am trying to make a basic flight sim in and am currently working on the first person camera aspect of it. This feels more or less like a real flight sim as long as I hold down the forward button but as soon as I let go I stop. How do I make it so I conserve momentum?

  public class CameraController : MonoBehaviour
     {
         public Rigidbody rb;
         private float speed = 25.0f;
         private float rotationSpeed = 120.0f;
         private float pitchSpeed = 60.0f;
         //private bool brake = false;
         private void Start() {
             rb.GetComponent<Rigidbody>();     
         }
         void Update () {
     
             float translation = Input.GetAxis("Vertical") * speed; //sets the movement params
             float rotation = Input.GetAxis("Horizontal") * rotationSpeed;
             float pitch = Input.GetAxis("Pitch") * pitchSpeed;
     
             translation *= Time.deltaTime;
             rotation *= Time.deltaTime;//normalizing per framerate
             pitch *= Time.deltaTime;
     
             //Vector3 movement = new Vector3(0, pitch, translation);
             //rb.velocity = movement;
             transform.Translate(0, 0, translation);
             transform.Rotate(pitch, 0, rotation);
         }
     
     }