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The adoption of the hybrid design for large rocket motors has been hindered by the slow regression rate associated with classical hybrid fuels and consequentially, the requirement for complex, multiport fuel grains. High regression rate hybrid fuels (e.g. paraffin) enable simple, single port hybrid propulsion systems for a variety of applications including launch vehicles, solar system exploration and space tourism. The mechanism responsible for the increased regression rate of these fuels is still not fully understood. Therefore, an apparatus has been constructed at Stanford University to visualize the combustion of high regression rate hybrid fuels in order to compare it with the predicted mechanism. It consists of a flow conditioning system and combustion chamber with three windows. The combustion is captured using two high-speed video cameras for both top and side views. The behavior of paraffin-based fuel is compared to that of classical hybrid fuels. Both the experimental design and results of seven tests with five hybrid fuels and gaseous oxygen are presented. Results of two additional tests are included for increased detail. The results are consistent with the droplet entrainment mechanism generally used to explain the high regression rates exhibited by paraffin-based fuels.

Hybrid rockets are promising candidates for in-space motors because of their throttling capability, ability to stop and restart and their tolerance to low temperatures. The advantages of a hybrid system compared to other chemical propulsion systems are quantified through a specific example: an orbit insertion (OI) stage. While the orbit insertion is left somewhat general, it is representative of a Mars orbit insertion (MOI) and will be compared to past MOI engines. For this example, system studies were conducted to design and optimize the hybrid propulsion system. The design is optimized for chamber pressure, O/F ratio, minimal gravity loss, oxidizer and fuel selection. The hybrid OI motor uses a paraffin-based fuel with MON3 (97% N 2 O 4 and 3% NO) as the oxidizer. Areas where further research is required are identified. Nomenclature ACS = attitude control system Isp = specific impulse IRFNA = inhibited red fuming nitric acid MMH = monomethyl hydrazine MOI = Mars orbit insertion MON3 = mixed oxide of nitrogen (97% N 2 O 4 and 3% NO) NTO = nitrogen tetroxide OI = orbit insertion O/F = oxidizer to fuel ratio T m = melting temperature TVC = thrust vector control δ thermal = characteristic thermal thickness ΔV = delta velocity

The discovery of high regression rate liquefying fuels for hybrid motors has generated a renewed interest in visualization experiments. In 2012, the Stanford Combustion Visualization Facility was developed to investigate the combustion of these fuels with gaseous oxygen. A major upgrade of this facility was recently completed. Details of the changes made to the facility are provided. Visual and schlieren images of the combustion of paraffin wax and oxygen at a range of operating pressures are presented, including the first schlieren images of the combustion of paraffin wax and oxygen at supercritical pressures. The nature of the combustion and the boundary layer thickness were seen to change dramatically with increasing chamber pressure, up to approximately the critical pressure of the fuel. Droplet entrainment from the surface of the fuel grain was observed at all pressures. At elevated pressures the combustion of the fuel grain was characterized by very strong surface blowing events comprised of numerous droplets.