The ornamental design for a rocketpen, as shown and described.
FIG. 1 is a front elevational view of the rocket pen showing my design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a front perspective view thereof; and,
FIG. 8 is a front perspective view thereof with the writing tip shown in an extended position.
The broken lines showing the middle of the bottom in FIG. 4 are included for the purpose of illustrating a portion of the rocket pen that forms no part of the claimed design.