Calculate power needed for projectile to reach target

I'm working on a tank game and I have a reticle that is positioned at Input.mousePosition; whenever the player shoots a projectile I want to shoot the projectile at the reticle. Shooting the projectile in the direction of the reticle is easy, I use my fireTransform's forward vector. However I want to calculate how much power should be applied for the projectile to "land" at the reticle's position. But I have no idea how..

Code (CSharp):

1.     [Command]
2.     private void CmdSpawnShell()
3.     {
4.         GameObject instance = Instantiate(shellPrefab, fireTransform.position, fireTransform.rotation) as GameObject;
5.  
6.         instance.GetComponent<Rigidbody>().AddForce(fireTransform.forward * power, ForceMode.Impulse);
7.         NetworkServer.Spawn(instance);
8.     }

 

The arc a ballistic object takes is called its trajectory.

Google up unity calculate trajectory and you'll find a lot of good stuff.

It's just math, nothing to be afraid of.

 

Thank you, @Kurt-Dekker .
I have solved the most crucial problem.. However (I literally SUCK at math and have NO idea what I'm doing).. I would like to remove the time parameter from this function and make the projectile go at a certain speed at all times instead.. Because now, if I aim close to my tank - it takes too long for the projectile to hit the ground. And if I aim far away from my tank the projectile goes waaay too fast. Would you mind helping me solve this?

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.Networking;
3. using System.Collections;
4.  
5. public class TankShooting : NetworkBehaviour {
6.  
7.     [SerializeField] GameObject shellPrefab;
8.     [SerializeField] Transform fireTransform;
9.     [SerializeField] float projTime = .5f;
10.  
11.     IP_Tank_Controller controller;
12.  
13.     private void Start()
14.     {
15.         controller = GetComponent<IP_Tank_Controller>();
16.     }
17.  
18.     private void Update()
19.     {
20.         if (!isLocalPlayer)
21.             return;
22.  
23.         if(Input.GetButtonDown("Fire1"))
24.         {
25.             CmdSpawnShell();
26.         }
27.     }
28.  
29.     [Command]
30.     private void CmdSpawnShell()
31.     {
32.         GameObject instance = Instantiate(shellPrefab, fireTransform.position, fireTransform.rotation) as GameObject;
33.  
34.         instance.GetComponent<Rigidbody>().velocity = CalculateVelocity(controller.reticleTransform.position, fireTransform.position, projTime);
35.         NetworkServer.Spawn(instance);
36.     }
37.  
38.     private Vector3 CalculateVelocity(Vector3 target, Vector3 origin, float time)
39.     {
40.         Vector3 distance = target - origin;
41.         Vector3 distanceXZ = distance;
42.  
43.         distanceXZ.y = 0f;
44.  
45.         float sY = distance.y;
46.         float sXZ = distanceXZ.magnitude;
47.  
48.         float vXZ = sXZ / time;
49.         float vY = sY / time + .5f * Mathf.Abs(Physics.gravity.y) * time;
50.  
51.         Vector3 result = distanceXZ.normalized;
52.         result *= vXZ;
53.         result.y = vY;
54.  
55.         return result;
56.     }
57. }