Rocket Pen utilizes a single stage 1/4A3-3T MINI Model Rocket Engine, and a Pilot G2 pen.
The pen tip is retractable via rotation of the barrel, and the length is under 6 inches.

Note, most rockets don't have a twist in the fins, this one does.  Either hyper instantaneous stabilized flight will be achieved or a bizarre flight pattern will be the result. You are the engineer, manufacturer, and test pilot. Be safe.

Two styles of nose cones are provided, research capsule, and an ogive.
Two styles of Fin/motors are provided, one accepts a 1.5 inch motor, the other accepts a 1.75 inch motor by .50 inch diameter.

A launch pad and parachute swivel are in development.

Skill Level 1: Some experience helpful.
You are the manufacturer.
Play safe.