JL's Jetpack Solution

local CAS = game:GetService("ContextActionService")

local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()

local thruster = Instance.new("BodyVelocity")
thruster.Velocity = Vector3.new(0, 20, 0)
thruster.MaxForce = Vector3.new()
thruster.Parent = character.HumanoidRootPart

local jetpack_on = false

local function jetpack()
    jetpack_on = not jetpack_on
    print("Jetpack:", jetpack_on)
    if jetpack_on then
        thruster.MaxForce = Vector3.new(0, 5000, 0)
        return
    end
    thruster.MaxForce = Vector3.new()
end

CAS:BindAction("Jetpack", jetpack, false, Enum.KeyCode.LeftControl)