How can I add a jetpack ability to my movement script

Hello Everyone! I hope you are well, today I have encountered a small speed-bump which I am hoping you can help me to fix. I have a movement script and I am trying to make it so when the suitEquipped bool is true, When you hold down Jump Key you will fly up until key is released. Can anybody help me figure this out, I have tried to start it off but I don’t know if I am right.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent(typeof(CharacterController))]
public class Fly : MonoBehaviour {

    public float walkSpeed = 6.0f;

    public float runSpeed = 11.0f;

    // If true, diagonal speed (when strafing + moving forward or back) can't exceed normal move speed; otherwise it's about 1.4 times faster
    public bool limitDiagonalSpeed = true;

    // If checked, the run key toggles between running and walking. Otherwise player runs if the key is held down and walks otherwise
    // There must be a button set up in the Input Manager called "Run"
    public bool toggleRun = false;

    public bool suitEquipped;

    public float jumpSpeed = 8.0f;
    public float gravity = 20.0f;

    // Units that player can fall before a falling damage function is run. To disable, type "infinity" in the inspector
    public float fallingDamageThreshold = 10.0f;

    // If the player ends up on a slope which is at least the Slope Limit as set on the character controller, then he will slide down
    public bool slideWhenOverSlopeLimit = false;

    // If checked and the player is on an object tagged "Slide", he will slide down it regardless of the slope limit
    public bool slideOnTaggedObjects = false;

    public float slideSpeed = 12.0f;

    // If checked, then the player can change direction while in the air
    public bool airControl = false;

    // Small amounts of this results in bumping when walking down slopes, but large amounts results in falling too fast
    public float antiBumpFactor = .75f;

    // Player must be grounded for at least this many physics frames before being able to jump again; set to 0 to allow bunny hopping
    public int antiBunnyHopFactor = 1;

    private Vector3 moveDirection = Vector3.zero;
    private bool grounded = false;
    private CharacterController controller;
    private Transform myTransform;
    private float speed;
    private RaycastHit hit;
    private float fallStartLevel;
    private bool falling;
    private float slideLimit;
    private float rayDistance;
    private Vector3 contactPoint;
    private bool playerControl = false;
    private int jumpTimer;

    void Start()
    {
        controller = GetComponent<CharacterController>();
        myTransform = transform;
        speed = walkSpeed;
        rayDistance = controller.height * .5f + controller.radius;
        slideLimit = controller.slopeLimit - .1f;
        jumpTimer = antiBunnyHopFactor;
    }

    void FixedUpdate()
    {
        float inputX = Input.GetAxis("Horizontal");
        float inputY = Input.GetAxis("Vertical");
        // If both horizontal and vertical are used simultaneously, limit speed (if allowed), so the total doesn't exceed normal move speed
        float inputModifyFactor = (inputX != 0.0f && inputY != 0.0f && limitDiagonalSpeed) ? .7071f : 1.0f;

        if (grounded)
        {

            if (suitEquipped == true)
            {
                /////////////////////////////////////////This is where I have tried to start the jetpack sequence
            }
            else
            {
                bool sliding = false;
                // See if surface immediately below should be slid down. We use this normally rather than a ControllerColliderHit point,
                // because that interferes with step climbing amongst other annoyances
                if (Physics.Raycast(myTransform.position, -Vector3.up, out hit, rayDistance))
                {
                    if (Vector3.Angle(hit.normal, Vector3.up) > slideLimit)
                        sliding = true;
                }
                // However, just raycasting straight down from the center can fail when on steep slopes
                // So if the above raycast didn't catch anything, raycast down from the stored ControllerColliderHit point instead
                else
                {
                    Physics.Raycast(contactPoint + Vector3.up, -Vector3.up, out hit);
                    if (Vector3.Angle(hit.normal, Vector3.up) > slideLimit)
                        sliding = true;
                }

                // If we were falling, and we fell a vertical distance greater than the threshold, run a falling damage routine
                if (falling)
                {
                    falling = false;
                    if (myTransform.position.y < fallStartLevel - fallingDamageThreshold)
                        FallingDamageAlert(fallStartLevel - myTransform.position.y);
                }

                // If running isn't on a toggle, then use the appropriate speed depending on whether the run button is down
                if (!toggleRun)
                    speed = Input.GetButton("Run") ? runSpeed : walkSpeed;

                // If sliding (and it's allowed), or if we're on an object tagged "Slide", get a vector pointing down the slope we're on
                if ((sliding && slideWhenOverSlopeLimit) || (slideOnTaggedObjects && hit.collider.tag == "Slide"))
                {
                    Vector3 hitNormal = hit.normal;
                    moveDirection = new Vector3(hitNormal.x, -hitNormal.y, hitNormal.z);
                    Vector3.OrthoNormalize(ref hitNormal, ref moveDirection);
                    moveDirection *= slideSpeed;
                    playerControl = false;
                }
                // Otherwise recalculate moveDirection directly from axes, adding a bit of -y to avoid bumping down inclines
                else
                {
                    moveDirection = new Vector3(inputX * inputModifyFactor, -antiBumpFactor, inputY * inputModifyFactor);
                    moveDirection = myTransform.TransformDirection(moveDirection) * speed;
                    playerControl = true;
                }

                // Jump! But only if the jump button has been released and player has been grounded for a given number of frames
                if (!Input.GetButton("Jump"))
                    jumpTimer++;
                else if (jumpTimer >= antiBunnyHopFactor)
                {
                    moveDirection.y = jumpSpeed;
                    jumpTimer = 0;
                }

            }
        }
        else
        {
            // If we stepped over a cliff or something, set the height at which we started falling
            if (!falling)
            {
                falling = true;
                fallStartLevel = myTransform.position.y;
            }

            // If air control is allowed, check movement but don't touch the y component
            if (airControl && playerControl)
            {
                moveDirection.x = inputX * speed * inputModifyFactor;
                moveDirection.z = inputY * speed * inputModifyFactor;
                moveDirection = myTransform.TransformDirection(moveDirection);
            }
        }

        // Apply gravity
        moveDirection.y -= gravity * Time.deltaTime;

        // Move the controller, and set grounded true or false depending on whether we're standing on something
        grounded = (controller.Move(moveDirection * Time.deltaTime) & CollisionFlags.Below) != 0;
    }

    void Update()
    {
        // If the run button is set to toggle, then switch between walk/run speed. (We use Update for this...
        // FixedUpdate is a poor place to use GetButtonDown, since it doesn't necessarily run every frame and can miss the event)
        if (toggleRun && grounded && Input.GetButtonDown("Run"))
            speed = (speed == walkSpeed ? runSpeed : walkSpeed);
    }

    // Store point that we're in contact with for use in FixedUpdate if needed
    void OnControllerColliderHit(ControllerColliderHit hit)
    {
        contactPoint = hit.point;
    }

    // If falling damage occured, this is the place to do something about it. You can make the player
    // have hitpoints and remove some of them based on the distance fallen, add sound effects, etc.
    void FallingDamageAlert(float fallDistance)
    {
        print("Ouch! Fell " + fallDistance + " units!");
    }
}

Simply add a value to moveDirection.y when both suitEquipped is true, and Jump Key is held.

I obviously cannot test your code without the rest of your project, but this should work (add it before the last line in FixedUpdate():

    // Apply jetpack if equipped and jump button pressed
    if (suitEquipped == true && Input.GetButton("Jump"))
    {
        moveDirection.y += jumpSpeed;
    }

Here is your whole FixedUpdate() method:

void FixedUpdate()
{
    float inputX = Input.GetAxis("Horizontal");
    float inputY = Input.GetAxis("Vertical");
    // If both horizontal and vertical are used simultaneously, limit speed (if allowed), so the total doesn't exceed normal move speed
    float inputModifyFactor = (inputX != 0.0f && inputY != 0.0f && limitDiagonalSpeed) ? .7071f : 1.0f;

    if (grounded)
    {
            bool sliding = false;
            // See if surface immediately below should be slid down. We use this normally rather than a ControllerColliderHit point,
            // because that interferes with step climbing amongst other annoyances
            if (Physics.Raycast(myTransform.position, -Vector3.up, out hit, rayDistance))
            {
                if (Vector3.Angle(hit.normal, Vector3.up) > slideLimit)
                    sliding = true;
            }
            // However, just raycasting straight down from the center can fail when on steep slopes
            // So if the above raycast didn't catch anything, raycast down from the stored ControllerColliderHit point instead
            else
            {
                Physics.Raycast(contactPoint + Vector3.up, -Vector3.up, out hit);
                if (Vector3.Angle(hit.normal, Vector3.up) > slideLimit)
                    sliding = true;
            }

            // If we were falling, and we fell a vertical distance greater than the threshold, run a falling damage routine
            if (falling)
            {
                falling = false;
                if (myTransform.position.y < fallStartLevel - fallingDamageThreshold)
                    FallingDamageAlert(fallStartLevel - myTransform.position.y);
            }

            // If running isn't on a toggle, then use the appropriate speed depending on whether the run button is down
            if (!toggleRun)
                speed = Input.GetButton("Run") ? runSpeed : walkSpeed;

            // If sliding (and it's allowed), or if we're on an object tagged "Slide", get a vector pointing down the slope we're on
            if ((sliding && slideWhenOverSlopeLimit) || (slideOnTaggedObjects && hit.collider.tag == "Slide"))
            {
                Vector3 hitNormal = hit.normal;
                moveDirection = new Vector3(hitNormal.x, -hitNormal.y, hitNormal.z);
                Vector3.OrthoNormalize(ref hitNormal, ref moveDirection);
                moveDirection *= slideSpeed;
                playerControl = false;
            }
            // Otherwise recalculate moveDirection directly from axes, adding a bit of -y to avoid bumping down inclines
            else
            {
                moveDirection = new Vector3(inputX * inputModifyFactor, -antiBumpFactor, inputY * inputModifyFactor);
                moveDirection = myTransform.TransformDirection(moveDirection) * speed;
                playerControl = true;
            }

            // Jump! But only if the jump button has been released and player has been grounded for a given number of frames
            if (!Input.GetButton("Jump"))
                jumpTimer++;
            else if (jumpTimer >= antiBunnyHopFactor)
            {
                moveDirection.y = jumpSpeed;
                jumpTimer = 0;
            }
    }
    else
    {
        // If we stepped over a cliff or something, set the height at which we started falling
        if (!falling)
        {
            falling = true;
            fallStartLevel = myTransform.position.y;
        }

        // If air control is allowed, check movement but don't touch the y component
        if (airControl && playerControl)
        {
            moveDirection.x = inputX * speed * inputModifyFactor;
            moveDirection.z = inputY * speed * inputModifyFactor;
            moveDirection = myTransform.TransformDirection(moveDirection);
        }
    }

    // Apply gravity
    moveDirection.y -= gravity * Time.deltaTime;

    // Apply jetpack if equipped and jump button pressed
    if (suitEquipped == true && Input.GetButton("Jump"))
    {
        moveDirection.y += jumpSpeed;
    }

    // Move the controller, and set grounded true or false depending on whether we're standing on something
    grounded = (controller.Move(moveDirection * Time.deltaTime) & CollisionFlags.Below) != 0;
}