Evaluation techniques for optical analysis of hybrid rocket propulsion

Abstract

fuel grains for high thrust applications. The multiport design increases the residual mass of unburnt fuel and thereby decreases the delivered specific impulse. Instabilities are also more likely with this type of design. Liquefying hybrid rocket fuels were only found recently. Cryogenic solid n-pentane showed regression rates 5-10 times higher than polymeric hybrid fuels (Carrick and Larson, 1995, Larson et al., 1996, DeRose et al., 1997). Paraffin-based fuels, tested at Stanford University, show a 3-5 times higher regression rate at similar mass fluxes compared to polymers (Karabeyoglu et al., 2002, Karabeyoglu, 1998). This is achieved by a different combustion mechanism. Paraffin fuels form a liquid layer on the fuel surface during the combustion (Karabeyoglu et al., 2002). It is expected that the low viscosity and surface tension of the liquid fuel enable an additional mass transfer by entrainment of liquid droplets. The gas flow over the surface induces liquid layer instabilities, which produce the droplet entrainment (Karabeyoglu and Cantwell, 2002), see Figure 1. Optical results of this entrainment process of low viscosity liquefying fuels are shown in Figure 2 and in (Kobald and Schlechtriem, 2013). Scale-up tests were done and confirmed that the theory is also applicable for engines at larger scale (Karabeyoglu et al., 2004). Recent tests with different paraffin-based fuels and GOX showed an exponential relation between the liquid layer viscosity and the overall regression rate (Kobald et al., 2014). An example of a liquefying paraffin-based fuel during combustion is shown in Figure 2. This image has been recorded with the optical combustion chamber setup at the DLR Lampoldshausen, which is described in this article. In the image droplet entrainment is clearly seen as well as the wave-like movement and liftoff of the flame along fuel surface. Previous research with this optical chamber focused on the visualization of droplet entrainment and the application of a Black and White Schlieren setup to visualize density gradients and flow patterns during the combustion (Kobald and Schlechtriem, 2013, Kobald et al., 2013a)