ExploderBg

using UnityEngine;
using System.Collections;

public class ExploderBg : MonoBehaviour {
    public GameObject ExplosionPrefab;
    public int HitCount = 1;
    private int HitTracker = 1;
    public float ImpactThreshold = 15.0f;
    public float LifeSpan = 10.0f;

    void OnCollisionEnter (Collision collision) {
        if (collision.relativeVelocity.magnitude > ImpactThreshold){
            HitTracker += 1;
        }
    }
    void FixedUpdate(){
        if (HitTracker > HitCount) {
            var bullet = (GameObject)Instantiate (
                ExplosionPrefab,
                transform.position,
                transform.rotation);
            Destroy (gameObject);
            Destroy (bullet, LifeSpan);
        }
    }
}