Laboratory-scale hybrid rocket motor uncertainty analysis

Laboratory-scale hybrid rocket motors often provide the means to evaluate the burning rates of new fuel formulations. While the issue of scaling to larger sizes can be significant, the uncertainty level of the laboratory-scale results also needs evaluation. This work quantitatively evaluates the uncertainty of key experimental results for a particular hybrid rocket motor. The scope includes calculation of the uncertainties in the fuel regression rate, oxidizer flux, motor characteristic velocity, and the oxidizer-to-fuel mass ratio for a laboratory-scale motor. The motor burned gaseous oxygen and hydroxyl-terminated polybutadiene at conditions characteristic of a preburner. The accepted uncertainty methodology of Coleman and Steele established the approach for the study. The results show that the typical uncertainty values are +/- 8.7% in the determination of the fuel regression rate, +/- 10.8% in the oxidizer flux, +/- 10.5% for motor characteristic velocity, and +/- 9.2% for the oxidizer to-fuel ratio. Measurement uncertainties in the diameters of the initial fuel grain, initial port, and nozzle throat contributed significantly to these conclusions. Conceptual biases in determining the burn duration, the average chamber pressure, and the average burning rate have a significant influence. This evaluation revealed the important measurement limitations and potential improvements for the particular test conditions and data reduction equations used. However, the methodology presented is in generally applicable to hybrid rochet testing.