The airlock is not needed. Most will tell you that they simply place the lid over the bucket without snapping it closed. I normally just place a piece of plastic sheeting over it to keep the critters out.
The amount of fermentation that happens will produce a huge amount of CO2. This forms a protective "blanket" to guard against the outside world. Additionally, leaving the primary open allows unwanted gasses to more easily escape.
Good friends, go in, and taste some wine with me; And we, like friends, will straightway go together (Julius Caesar, Act 2, Scene 2)