In contrast to conventional ball pens, which feed ink through gravity, the replaceable refill cartridge of the Space Pen® is pressurized. The tungsten carbide ball holds ink at a rate of almost 35 pounds per square inch, allowing the user to write at any angle, even when the paper is tilted. This capability required the improvement of an extraordinary ink.
The shearing action that results in the flow of the new visco-elastic ink, which is comparable to very thick rubber cement, is caused by the rolling ball in its socket. This shearing movement liquifies the thixotropic ink, allowing the pen to write on most substrates, environments and even through water.
