Speed of Sound Characterization of Mixed Oxides of Nitrogen from Freezing Point to 50°C

Widely used since the 1960s, Mixed Oxides of Nitrogen (MON) react hypergolically with monomethylhydrazine (MMH). Despite their prominence in liquid propulsion systems, the properties of MON as a function of temperature have not been well characterized. The MON concentration is determined by the weight percentage of nitric oxide (NO) in the mixture, e.g. MON-3 referring to a 3 wt.% NO mixture, and MON-25 referring to a 25 wt.% NO mixture. There is a strong interest in using high NO concentrations MON, such as MON-25, due to their low freezing points, and as such there is a renewed incentive in carefully documenting the properties of MON relative to temperature. The main objective of this study is to modify a commercial sonic velocimeter to determine the speed of sound of various MON concentrations from NTO to MON-25. Deionized water and isopropyl alcohol, both of which have well documented speed of sounds, were used to validate the measurement method. These calibration tests resulted in sound speeds with a relative uncertainty of 0.7% from the literature at most. This paper details the modifications made to an Anton Paar L-Sonic 5100 sonic velocimeter for use with NTO and MON, and will further present the speed of sound results for NTO and MON-25.17 from -10°C to 50°C in comparison to existing literature data sets. Future work will include testing additional MON grades in order to better document the speed of sound in the oxidizer to improve industry understanding of the oxidizer.