Rocket propulsion performance is characterized by parameters as thrust, specific impulse, and propellant mass ratio. The propellant mass ratio is affected by propellant residuals, which is often overlooked in hybrid rocket design. Such residual is the mass of propellant which cannot be used for propulsion purposes and is left in the vehicle after performing its mission objective. In case of liquid and solid rocket design it is state-of-the art that propellant residuals are small so that they do not affect the usage of both propulsion types for high performance missions with high ideal velocity changes. In authors' view the residual topic is not recognized by its importance and impact on performance in published hybrid rocket studies, which motivates this work. When hybrid rockets are concerned, due to their specific working characteristics the propellant residual size may have implications on their usability for missions with high ideal velocity requirements. For this reason, residual minimization might be another important design challenge for this rocket propulsion type, beside enhancement of fuel regression rate and combustion efficiency. An overview of the different origins and types of hybrid residuals is presented and reviewed for its relevance. Three different types of residuals are identified: the first part is related to the oxidizer that remains in the tank and in the feed lines. The second part can be mainly attributed to fuel grain design and spatial regression behaviour. The third part of residuals results from uncertainties in important design parameters of the hybrid rocket propulsion system, which determine thrust, pressure and O/F-mixture (and therefore this part of residuals). These design parameters, which are used to model oxidizer injection and fuel regression behaviour as well as nozzle mass flow are identified and introduced in the governing equations of hybrid motor operation to investigate the effect of uncertainties on the rocket velocity change bandwidth, which can be delivered by the specific hybrid rocket propulsion unit. This investigation is done by numerical simulations and in addition by existing analytical solutions.
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