The Military Space Pen measures 5.25 inches (133 mm)
and contain a #PR4 Black Ink,
Medium Point Pressurized Fisher Space Pen Refill.
Unlike ordinary ball pens which rely on gravity to feed ink, the Space Pen’s replaceable cartridge is pressurized with nitrogen. 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.
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.
Evaporation, wasted ink and back leakage are eliminated. Shelf-life is increased to an estimated 100 years.