Keeping momentum during jump?

Hello, I am here to seek how I would go about achieving this goal, I have researched around looking for how this is done and seen rigid body's but I've never used them, so if possible can someone explain how I cna adapt my following code to do this.

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class PlayerController : MonoBehaviour {
7.  
8.     public float defaultmoveSpeed = 8.00f;
9.     public float moveSpeed = 8.00f;
10.     public float jumpSpeed = 15.00f;
11.     public float gravity = 20.0f;
12.     public float sprintSpeed = 16.00f;
13.  
14.  
15.  
16.     private Vector3 moveDirection = Vector3.zero;
17.  
18.     private CharacterController playerController;
19.  
20.  
21.     // Use this for initialization
22.     void Start () {
23.         playerController = GetComponent<CharacterController>();
24.  
25.     }
26.    
27.     // Update is called once per frame
28.     void Update () {
29.         //Movement Section
30.         playerMovement();
31.  
32.  
33.         //methods to which should only be used when player is grounded
34.         if (playerController.isGrounded)
35.         {
36.             jumpMethod();
37.         }
38.        
39.     }
40.  
41.  
42.  
43.  
44.  
45.  
46.     public void jumpMethod()
47.     {
48.         moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
49.         moveDirection = transform.TransformDirection(moveDirection);
50.         moveDirection *= moveSpeed;
51.  
52.         if (Input.GetButtonDown("Jump"))
53.             moveDirection.y = jumpSpeed;
54.     }
55.  
56.  
57.     public void playerMovement()
58.     {
59.         moveDirection.y -= gravity * Time.deltaTime;
60.         playerController.Move(moveDirection * Time.deltaTime);
61.  
62.         if (Input.GetKey("w"))
63.         {
64.             transform.Translate((Vector3.forward) * moveSpeed * Time.deltaTime);
65.         }
66.  
67.         if (Input.GetKey("S"))
68.         {
69.             transform.Translate((Vector3.back) * moveSpeed * Time.deltaTime);
70.         }
71.  
72.         if (Input.GetKey("A"))
73.         {
74.             transform.Translate((Vector3.left) * moveSpeed * Time.deltaTime);
75.         }
76.  
77.         if (Input.GetKey("D"))
78.         {
79.             transform.Translate((Vector3.right) * moveSpeed * Time.deltaTime);
80.         }
81.  
82.         // Sprint key function
83.         if (Input.GetKey("left shift") && Input.GetKey("w"))
84.         {
85.             moveSpeed = sprintSpeed;
86.  
87.         }
88.         else
89.         {
90.  
91.             moveSpeed = defaultmoveSpeed;
92.  
93.         }
94.     }
95. }
96.  
97.  

 

The Goal I want to achieve just so everyone knows is the ability for the user to keep momentum when in air, so if they sprint they'll still be moving that direction when i nthe air.