AddRelativeTorque not rotating

Questions & Answers
1

I’m trying to make a fireball curve towards a player but it appears to not be working.

The fireball is off the ground, ignoring gravity and does not have constraints.
It has a mass of 1, drag and angular drag are 0.

HomingStrength is set to 1000.

UpdateHoming_Dumb() is called in FixedUpdate().

    void UpdateHoming_Dumb()
    {
        Rigidbody rigidBody = GetComponent<Rigidbody>();
        if (Vector3.Dot(rigidBody.velocity, homingTarget.position - transform.position) >= 0)//if target is in front of spell
        {            
            if (Vector3.Dot(Vector3.Cross(rigidBody.velocity.normalized, Vector3.up), homingTarget.position - transform.position) >= 0)//if target is to the left of spell
            {
                Debug.Log("LEFT" + rigidBody.velocity);
                rigidBody.AddRelativeTorque(Vector3.up * -homingStrength);
            }
            else//if target is to the right of spell
            {
                Debug.Log("RIGHT" + rigidBody.velocity);
                rigidBody.AddRelativeTorque(Vector3.up * homingStrength);
            }
        }
        else//if target is behind spell (over shot)
        {
            //Debug.Log("BACK");
        }
    }
2

I just tested your code and it works just fine.

“Just fine” meaning: The object turns.
Needless to say, you use the velocity of your object to determine the movement direction. So you should have a velocity at the start. I took the liberty of rewriting your code a little to work:

using UnityEngine;
using System.Collections;
[RequireComponent(typeof(Rigidbody))]
public class HomingTest : MonoBehaviour {
	public Transform homingTarget;
	public float homingStrength = 1000.0f;
	Rigidbody rigidBody;
	public float epsilon = 0.01f;
	// Use this for initialization
	void Start () {
		rigidBody = GetComponent<Rigidbody>();
		rigidBody.AddForce(transform.forward,ForceMode.Impulse);
	}
	
	// Update is called once per frame
	void FixedUpdate () {
		UpdateHoming_Dumb();
		rigidBody.AddForce(transform.forward,ForceMode.Acceleration);
	}
	
	void UpdateHoming_Dumb()
	{
			float homingDir = Vector3.Dot(-transform.right, homingTarget.position - transform.position);        
			if (homingDir >= epsilon)//if target is to the left of spell
			{
				Debug.Log("LEFT");
				rigidBody.AddRelativeTorque(Vector3.up * -homingStrength);
			}
			else if(homingDir <= -epsilon)//if target is to the right of spell
			{
				Debug.Log("RIGHT");
				rigidBody.AddRelativeTorque(Vector3.up * homingStrength);
			}
			else
			{
				Debug.Log("STRAIGHT");
				rigidBody.AddTorque(rigidBody.angularVelocity*-1,ForceMode.VelocityChange);
			}
		
	}
}