Fits all of our Fisher Space Pens (except the Q4 Multi-Function Pen) Unlike ordinary ball pens which rely on gravity to feed ink the Space Pen's replaceable cartridge 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. 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. The cartridges of conventional ball point pens are open to permit ink to be fed to the point. The secret behind the Fisher Space Pen lies in the unique design characteristics of the ink and the high precision manufacturing tolerances of the ball point and socket. The ink is fed to the ball point by gas pressure permitting the pen to write in any position. An additional benefit of the closed design is that it keeps the pen from drying out giving the Fisher Space Pen an estimated 100 year shelf life.