The Frontier of Space

The best-known wind tunnels are vast, bellowing monsters that soak up the local power supply and drive the neighbors nuts. Last week Dr. Richard G. Folsom of the University of California described a quieter and trickier tunnel. Built with Navy and Air Force funds, it is a stainless steel tube only 5 ft. long and 18 in. in diameter. Its purpose: to simulate aerodynamic conditions near the earth's outer frontier--the atmosphere 50 miles up.

At this altitude and above, the air is so thin that it does not act as a normal gas. Its molecules are in motion, but instead of colliding with one another every 10,000th of an inch, as they do at sea level, they travel many feet between collisions. When a solid body passes through such a rarefied atmosphere, it behaves as if it were moving in space containing a few ping-pong balls in rapid, random motion.

To simulate these peculiar conditions, California scientists use a peculiar apparatus: a "molecular beam" developed by Physicist Franklin C. Hurlbut. First, all possible air is pumped from the stainless steel tube (which takes a week of pumping). At one end of the tube is a small "source chamber" containing nitrogen gas. When this is heated by a furnace, the nitrogen molecules pick up kinetic energy and zigzag through the chamber at great speed. Those that happen to be shooting in the right direction pass through a hole one-fiftieth of an inch in diameter that leads to the evacuated tube.

The nitrogen molecules enter the tube as a "beam" that can be deflected and controlled almost like a beam of light. The hotter the source chamber, the faster the molecules move. When the temperature in the source chamber is 1,000DEG C., the molecules in the beam speed at 1,800 m.p.h. Models of aerodynamic surfaces placed in this beam behave just as if they were moving at 1,800 m.p.h. through the ping-pong-ball atmosphere on the frontier of space.

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