Launch Day Frost and Boiling Nitrogen

A Cold Morning at the Launch Field

1On a frosty Saturday in March, the Harlan Middle School rocket club met at a farm field for launch day. At 7:00 a.m., the air was −4 °C, and a thin layer of frost coated the metal launch rails. By 8:30, the sun was shining on the rails. An infrared thermometer showed that the rails were still below 0 °C, and the air was −1 °C. Yet the frost was gone, and no drops of water had dripped onto the ground below the rails.

2While the club waited for the wind to drop, their mentor, Dr. Marguerite Okoye, a retired rocket engineer, gave a demonstration. Large rockets often burn their fuel with liquid oxygen, which boils at −183 °C. Liquid oxygen is dangerous to handle away from a launch site, so she used liquid nitrogen, which boils at −196 °C and behaves in a similar way. Wearing a face shield and insulated gloves, she poured 300 g of liquid nitrogen into a foam cup and 300 g into an open steel vacuum flask. The flask has two walls with almost no air between them. Each container sat on a digital scale that had been set to zero with the empty container on it, and a temperature probe hung in each liquid. The air was now 4 °C, and the students watched from 3 meters away.

3The nitrogen in both containers bubbled the whole time, and a white fog spilled over the rim of the foam cup. Dr. Okoye explained that nitrogen gas cannot be seen; the fog forms when water vapor in the air near the cup loses energy and becomes tiny drops of liquid water. Club members Dawson Tsosie and Yuqing Lin recorded the scale readings every 2 minutes. Both probes read −196 °C at every reading.

4When the demonstration ended, Dr. Okoye said that she never puts a tight lid on a container of liquid nitrogen. She also said that a large rocket's oxygen tank is filled hours before liftoff, and the launch team keeps adding liquid oxygen until shortly before launch.