How do I set a constant speed on a 2d joystick movement script?,Constant speed on 2d joystick top-down movement

Hi, I'm struggling with how to make it so that my spaceship has a constant speed, and joystick movement doesn't change the speed, only what direction the spaceship is flying. Also, I want to have a my spaceship rotate slower, around a 'circle' like not a literal one. But i want to make it so that you cant make an instant u-turn. If you want a reference, i'm trying to make my movement exactly like the game 'Missiles'. Here's my movement script.

public class Airplane : MonoBehaviour { public Rigidbody2D rb; public VariableJoystick joystick; public float speed;

 Vector2 moveDirection;
 
 // Update is called once per frame
 void Update()
 {
     MyInput();
 }

 void FixedUpdate()
 {
     Movement();
 }

 void MyInput()
 {
     float x = joystick.Horizontal;
     float y = joystick.Vertical;

     moveDirection = new Vector2(x, y);
     
 } 

 void Movement()
 {
     rb.velocity = moveDirection * speed;

     if (rb.velocity != Vector2.zero)
     {
         transform.up = rb.velocity;
     }
 }

}