How Hot is the Rocket Engine?
We've all used thermometers to measure temperature but sometimes we need to measure temperature in places where you can't put a thermometer — for example, inside a rocket engine. One of the best methods for those really hard-to-get-to places is based on laser spectroscopy. Light from a laser pointed at the region of interest is absorbed by molecules and some of it is re-emitted back towards the source where it can be measured. The re-emitted light is shifted in frequency (changed in wavelength) by amounts that depend on the molecule and also on the temperature and pressure of the surrounding gas because of collisions among the molecules. The single, sharp frequency from the laser is thus smeared out into a whole spectrum of frequencies whose shape can be used to determine the temperature provided that the molecular transitions and line shapes are known. Because it is the main component of the air, it is convenient to use nitrogen and its molecular properties are known from room temperture to temperatures of about 1200 degrees Celsius from experimental measurements. Although this is a large range, it is far short of what is needed for some applications.
Sheldon Green (NASA/GISS) and Winifred Huo (NASA/Ames) have shown that it is possible to predict the molecular line shape parameters from basic laws of physics, by solving equations of quantum theory. Although these equations are difficult, by using supercomputers at NASA's Numerical Aerodynamic Simulation facility, they were able to predict accurate values for the known temperature range, room temperature to 1200 degrees Celsius. These calculations are now being extended both to lower temperatures, which will be useful for measurements in wind tunnels, and to higher temperatures which are required for measurements in engines, flames, and explosions.
Green, S., and W. M. Huo 1996. Quantum calculations for line shapes in Raman spectra of molecular nitrogen. J. Chem. Phys. 104, 7590-7598.