Paul Fisher first conceived of the Bullet Space Pen in 1948 and developed his design for the burgeoning space program. His use of pressurized cartridges, thixotropic ink and a tungsten carbide ball tip solved the dilemma of having ink flow always and only when needed. Fisher's first ballpoint pen is arguably the most popular of the 20th century; Fisher Space pens are displayed in both the NASA Museum and the New York Museum of Modern Art.
In the 1950's there were dozens of ballpoint models, and nearly every one took a different refill. So in 1953 Paul Fisher invented the "Universal Refill," which could be used in most pens. But he didn't stop there. After much experimentation he perfected the refill using thixotropic ink; the trick was to have the ink flow when you wanted it to, and not to flow the rest of the time, a problem Fisher solved.