Unlike ordinary ball pens which rely on gravity to feed ink, the Space Pen has a pressurized replaceable cartridge. 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.

All replacement refills use patented Fisher ink technology. Fits the Following Space Pen product categories: Bullet, Astronaut, Telescoping, X-750, Space-Tec, PolicePro, Stowaway, Military, Zero Gravity, Cap-O-Matic, Cartridge, TecTouch, ToughTouch, Backpacker and X-Mark Space Pen products.