The effect of elastomeric liners on high-pressure liquid propellant combustion oscillations

Liquid propellants are currently being investigated as an alternative to solid propellants in advanced gun designs. The use of liquids may provide advantages in the areas of safety, cost and performance. One area of concern in liquid propellant gun designs is the presence of large pressure oscillations: oscillations of +/- 50 % of the baseline pressure have been observed. These oscillations may result in a degradation of performance and reliability. The source of pressure oscillations and possible mitigation schemes are being studied in the Sandia Liquid Propellant Injector/Combustor. In this research device, the effect of acoustic liners on pressure oscillations in a high-pressure (128 MPa) liquid propellant spray combustor is studied for a hydroxylammonium nitrate-based propellant. In this study we investigated the effects of orientation and coverage fraction of elastomer liners on pressure oscillation reduction. Results for a range of elastomer/steel liner combinations are presented, along with an acoustic model and a correlation the data. The results indicate that: (1) liners can reduce oscillations by approximately three orders of magnitude, (2) the reduction in pressure oscillation is independent of the placement of liners relative to the propellant flow, and (3) the magnitude of the oscillations can be correlated with the acoustic properties of the wall material.