Fisher Space Pen Refill Cartridge
Unlike ordinary ball pens which rely on gravity to feed ink, the Space Pens replaceable cartridge refill is pressurized. At nearly 50 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 by Fisher.
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 (although we would not reccomend that you take your bullet space pen down to 30 meters!).
Evaporation, wasted ink and back leakage are eliminated. Shelf-life is increased to an estimated 100 years.