Scalability for Rocket Nozzle Flows Based on Subscale and Full-Scale Testing

When flow separation and side-load behavior is investigated for different nozzle concepts or when design considerations for advanced nozzle concepts are experimentally screened, the initial experimental effort is generally limited to subscale experiments with high-quality diagnostics. However, one fundamental question always arises: How should these test results be scaled to a potential full-scale design or application? Different approaches exist, including the use of characteristic numbers for normalization and analytical and numerical models. It is shown that analytical models, being based on physical origins and dominated by inviscid flow phenomena, may result in high accuracy for full-scale prediction. In contrast, it is shown that for flow regions dominated by viscous effects significant differences between cold- and hot-gas experiments are observed, which makes a direct scaling from cold-gas subscale to hot-gas full-scale difficult. For scaling, an intermediate step in the scaling rationale with hot-firing tests is foreseen. Demonstrated by results from hot-firing tests with Vulcain subscale nozzles, it is shown that a geometrical scaling is possible for similar hot-gas flow properties. These test results, obtained with a 40-kN thrust chamber at representative combustion chamber operation conditions, are presented and discussed in detail.