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Hybrid rocket engines have several advantages over liquid or solid engines but have traditionally seen limited use due to a low thrust-to-weight ratio. The Cascading Multi-Stage Impinging jet (CAMUI) hybrid rocket engine addresses this by using a series of impinging jets and fountain-like flow paths to improve mixing and increase heat transfer to the solid fuel surface. This thesis investigates the scaling of the CAMUI regression model over an extensive engine size range for the first time, comparing the model to actual firing data from over 30 hybrid rocket firing tests, ranging from 100 mm-diameter-2.5 kN-class engines to 400 mm-diameter-40 kN commercial class engines. With the development of a time-stepped regression simulator, identification and quantification of potential regression simulation uncertainties were performed. Burn time, specifically the effect of long start-up transients, was identified as the most critical. Further investigation into the impact of the transient burn time resulted in the proposed method of using burn time equivalent. This method considerably reduced the errors arising from the long start-up transients. Though done for the CAMUI engine, it was shown to be valid for other hybrid rocket engines. Lastly, with the proposed regression model corrections implemented, the simulator was used to design the successfully fired 40 kN-class engine. The developed model showed good scalability when evaluating the overall regression simulation model in the order of magnitude scaling range of 100 mm-diameter-2.5 kN-class engines to 400 mm-diameter-40 kN. This validates the scalability of the CAMUI concepts as well as its regression modeling to large-scale engines, opening the door for commercial implementation of CAMUI for suborbital and orbital-class rockets.

An understanding of the scalability of hybrid rocket regression models is critical for the enlargement and commercialization of small-scale engines developed within universities and similar research institutions. This paper investigates the fuel regression rates of recent 40 kN thrust-class motor experiments, which were designed based on fuel regression rate correlations of 2.5 kN thrust-class motors from previous research. The results show that fuel regression rates of the 40 kN experiments were within 26% of predictions made using correlations based on 2.5 kN experiments.

Burn time errors caused by various start-up transient effects have a significant influence on the regression modelling of hybrid rockets. Their influence is especially pronounced in the simulation model of the Cascaded Multi Impinging Jet (CAMUI) hybrid rocket engine. This paper analyses these transient burn time errors and their effect on the regression simulations for short burn time engines. To address these errors, the equivalent burn time is introduced and is defined as the time the engine would burn if it were burning at its steady-state level throughout the burn time to achieve the measured total impulse. The accuracy of the regression simulation with and without the use of equivalent burn time is then finally compared. Equivalent burn time is shown to address the burn time issue successfully for port regression and, therefore, also for other types of cylindrical port hybrid rocket engines. For the CAMUI-specific impinging jet fore-end and back-end surfaces, though, the results are inconclusive.

A recently developed reconstruction technique is used to investigate graphite nozzle erosion in two scales of hybrid rocket motors, 30-N-thrust class and 2000-N-thrust class, using oxygen as the oxidizer and high-density polyethylene as the fuel. Thermocouple measurements taken from within the nozzles are used to estimate nozzle throat wall temperature. Forty-four static firing tests were conducted under varying experimental conditions to confirm the validity of the reconstruction technique results, to investigate the conditions at the onset of erosion, and to formulate an empirical predictive model of nozzle erosion rate. Results show that a single formula that treats the combustion gas as a single oxidizing agent for which heterogenous rate constants are functions of equivalence ratio can satisfactorily replicate the erosion rate of graphite by a combustion gas containing multiple oxidizing species. Furthermore, the chemical-kinetic-limited conditions of the onset of nozzle erosion are specified by a novel empirical correlation, which shows that erosion begins at lower temperature and pressure in oxidizer-rich combustion gas than in fuel-rich combustion gas.