Why do hot air balloons have sandbags?

Answer 1

A hot air balloon needs to be slightly negatively bouyant. If a particular design can lift 1000lbs, an ideal flight weight might be 900 lbs so that, in the event of a burner failure, the balloon will slowly descend.

If the payload for a given flight is only 600lbs (fuel, pilot, passengers) then 300 lbs of ballast (sandbags) will be needed to keep the bouyancy in the correct range.

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Sandbags or other ballast is used mostly for helium balloons, not hot air balloons. The rate of descent of a hot air balloon is controlled by the time the burners are burning. If it is going down too fast, you burn longer. You may have two burners but if you lose one, the other one can keep the balloon aloft. they are always much larger than needed.

The design weight is based upon the volume of the balloon and a particular temperture of the air in the balloon which must be below the melting point of the fabric.

Typically you don't need ballast to adjust payload in a hot air balloon. IN a helium balloon, the only way you can go back up after starting a descent (by dumping helium) is the drop some weight. So ballast would be helpful with helium balloons.