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Why do base parachutes vent all cells besides 1 and 7? What happened when they tried venting every cell?

The prototype canopies I saw with vents on the outer cells seemed to have some heading issues, but mostly I think it was just that the gains from venting those last two outside cells were very limited.

Firstly, All parachutes provide the vast majority of their lift from the center cells. The purpose of venting is to create an airfoil that pressurizes and begins flying sooner in its deployment. Hence venting the cells that will provide lift.
Secondly, If the end cell is first to inflate it will cause obvious heading issues. An obvious way to promote center cell inflation first is to simply not vent the outer cells.