FireProjectileFunction.swift

    func fireProjectile(delta:CGFloat) {
    
        if timeSinceLastFire >= 0.15
        {
            let projectile = SKSpriteNode(imageNamed:"laser1")
            projectile.xScale = 0.5
            projectile.yScale = 0.5
            projectile.position = starShip.position
            projectile.position.y += starShip.frame.height / 2
            projectile.zPosition = starShip.zPosition + 1
            
            let speed:CGFloat = 600.0
            let projectileMaxTravel:CGFloat = CGRectGetMaxY(self.frame) + 100

            let distance = projectileMaxTravel - projectile.position.y
            
            let travelTime:NSTimeInterval = NSTimeInterval(distance / speed)

            projectile.runAction(SKAction.sequence([
                SKAction.moveToY(projectileMaxTravel, duration: travelTime),
                SKAction.removeFromParent()]))

            self.addChild(projectile)

            timeSinceLastFire = 0.0
        }
        
        timeSinceLastFire += delta
    }