The ornamental design for a helmetretentionsystem, as shown and described.
FIG. 1 is a front perspective view of a helmet retention system in accordance with our new design;
FIG. 2 is a top plan view of the helmet retention system shown in FIG. 1;
FIG. 3 is a bottom plan view of the helmet retention system shown in FIG. 1;
FIG. 4 is a left-side elevational view of the helmet retention system shown in FIG. 1; and,
FIG. 5 is a right-side elevational view of the helmet retention system shown in FIG. 1.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.