To: Checkerboard
Dear Check,
The air rushing out of the neck of the balloon is not reacting against the surrounding air. It is the pressure differential between the front of the balloon and the rear of the balloon. What you said about the balloon in outer space is true. The pressure differential would be greater due to a lack of surrounding atmosphere and the absence of any drag.
When fluid is contained under pressure the internal pressure acts at right angles to the containing wall. That means that when the container is sealed the pressure on the containing walls is equal. When you open up one end of the container the fluid under pressure flows out of the opening and the differeential pressure between the front and rear causes the container to move in the direction of the highest pressure.
Whether you are talking about a balloon, a rocket engine or a jet engine the principle is the same.
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The Cat