What am I doing wrong with this launch script?

I am attempting to make a script that will launch a rocket using the new explosions and DestroyJointRadiusPercent feature that will fire when I click a button (part button)

print("Launching.")

Main = game.Workspace.Rocket.Head
Remotebomb = script.Parent.Parent
LandMine = script.Parent
Booster = game.Workspace.Rocket.Booster

function onClicked()
    explosion = Instance.new("Explosion")
    explosion.BlastRadius = 50
    explosion.BlastPressure = 1000
    explosion.DestroyJointRadiusPercent = 0
    explosion.Position = Booster.Position --change the "Bomb" to any model name to make that model explode
    explosion.Parent = game.Workspace
    wait(1)
    explosion = Instance.new("Explosion")
    explosion.BlastRadius = 50
    explosion.BlastPressure = 1000
    explosion.DestroyJointRadiusPercent = 0
    explosion.Position = Booster.Position
if
    Main.velocity.y > 10000
    then
    wait(2)
    explosion = Instance.new("Explosion")
    explosion.BlastRadius = 50
    explosion.BlastPressure = 750
    explosion.DestroyJointRadiusPercent = 0
    explosion.Position = Booster.Position
else
    explosion = Instance.new("Explosion")
    explosion.BlastRadius = 50
    explosion.BlastPressure = 5000
    explosion.DestroyJointRadiusPercent = 0
    explosion.Position = Booster.Position
loop(5)
    if Main.velocity.y > 20000
        then
        wait(5)
        explosion = Instance.new("Explosion")
        explosion.BlastRadius = 50
        explosion.BlastPressure = 2500
        explosion.DestroyJointRadiusPercent = 0
        explosion.Position = Booster.Position
    end
end
end
script.Parent.ClickDetector.MouseClick:connect(onClicked)

This is what I get out of the Output:

00:34:47.751 - velocity is not a valid member of Part 00:34:47.752 - Script 'Workspace.Model.Explosion trigger.Script', Line 22 00:34:47.753 - Stack End