Unlike ordinary ball pens which rely on gravity to feed ink, the replaceable refill cartridge of the Space Pen® is pressurized. At nearly 35 pounds per square inch, ink is continuously fed to the tungsten carbide ball, allowing the user to write at any angle, even upside down. This advancement required the development of a special ink.

The new visco-elastic ink, with a consistency similar to that of very thick rubber cement, flows as a result of the shearing action of the rolling ball in its socket. This shearing action liquifies the solid gel thixotropic ink, allowing the pen to write smoothly and dependably on most surfaces and even under water.