Spectroscopy Reveals Fluorine’s Role in Rocket Propellant
Michael D. Wheeler
CHAMPAIGN, Ill. -- A high-speed CCD camera and spectrometer played an important part in analyzing how fluorine gas enhances the combustion of boron in propellants -- an advance that could have an impact on the development of more energetic propellants for solid-fuel rockets.
Getting the highest amount of energy per pound of propellant is a key factor in rocket performance. While liquid fuels provide more energy per pound than solid propellants, they are expensive and difficult to store because they must be cryogenically cooled. Solid propellants are less costly and are relatively safe and reliable, but they are far less energetic.
As an alternative, researchers are exploring the use of highly energetic metals such as boron, which has a high heating value per unit mass and volume, as an additive. The drawback is that a thin layer of boron oxide forms on the particles' surface, hindering combustion and delaying ignition.
At the University of Illinois, Martin Spalding, a doctoral candidate, and professors Herman Krier and Rodney Burton investigated if highly reactive fluorine would strip the boron of oxidation and improve the ignition time. To test their theory, they conducted experiments inside a 12-m-long shock tube. The group measured the spectra of the burning metal in argon and oxygen at high pressure and temperature, monitoring the effects of different concentrations of fluorine gas.
The group used an image-converting streak camera from Cordin Co. of Salt Lake City to capture time-resolved spectra gathered with an HR-320 spectrometer from Instruments SA (now Jobin Yvon/Horiba). The data revealed that the fluorine accelerated both the rate of ignition and combustion.
"Since a single test takes about two hours to perform, it is important to maximize the amount of data which can be recorded," Spalding said. He added that the researchers could have used a high-speed film camera, but that it would have provided only about four frames of data per test. The streak camera records 20 or more spectra in a single test as well as the output. He explained that this setup made it possible to trigger the streak camera system from the shock tube, whereas film cameras would have required the "firing" of the shock tube to be synchronized with camera operation.
The group said its next step is to find a means to safely incorporate fluorine into a solid propellant.
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