Variations and Control of Thrust and Mixture Ratio in Hybrid Rocket Motors

Abstract

Hybrid rocket motors have several attracting characteristics such as simplicity, low cost, safety, reliability, environmental friendliness. In particular, hybrid rockets can provide complex and flexible thrust profiles not possible with solid rockets in a simpler way than liquid rockets, controlling only a single fluid. Unfortunately, the drawback of this feature is that the mixture ratio cannot be directly controlled but depends on the specific regression rate law. Therefore, in the general case the mixture ratio changes with time and with throttling. Thrust could also change with time for a fixed oxidizer flow. Moreover, propellant residuals are generated by the mixture ratio shift if the throttling profile is not known in advance. The penalties incurred could be more or less significant depending on the mission profile and requirements. In this paper, some proposed ways to mitigate or eliminate these issues are recalled, quantitatively analysed and compared with the standard case. In particular, the addition of energetic additives to influence the regression rate law, the injection of oxidizer in the post-chamber and the altering-intensity swirling-oxidizer-flow injection are discussed. The first option exploits the pressure dependency of the fuel regression to mitigate the shift during throttling. The other two techniques can control both the mixture ratio and thrust, at least in a certain range, at the expense of an increase of the architecture complexity. Moreover, some other options like pulse width modulation or multi-chamber configuration are also presented. Finally, a review of the techniques to achieve high throttling ratios keeping motor stability and efficiency is also discussed.

Bridging the Technology Gap: Strategies for Hybrid Rocket Engines

“…Typical technical hurdles (similar to those around throttling with liquid engines) involve the choice of throttling method. These include flow control valves, variation of the injection area, and secondary gas injection [71,82]. Another difficulty is the nonlinear flow dynamics of the ball valves [83].…”

Localized characteristic velocity (c*) for rocket combustion analysis based on gas temperature and composition via laser absorption spectroscopy

“…Further details on the implementation of this technique are discussed in [12,15]. With K(r) known for each transition, spatially-resolved temperature is obtained through the ratio of two integrated spectral absorption coefficients, as shown in equation (7):…”

“…The primary metric for assessing rocket combustion efficiency is the characteristic velocity (c * ), which reflects the thermal energy density of the working fluid. Traditionally, combustion chamber properties are assumed to be homogeneous or uniform and c * is determined via chamber pressure and total mass flow rate measurements, as these properties vary little in the spatial domain of a steady-state combustor [6][7][8]. Inasmuch, the traditional c * metric reflects a global indicator of numerous combustion processes, which often do vary spatially, and does little to inform the specific loss mechanisms that result in decreased combustion efficiency.…”

Challenges of Ablatively Cooled Hybrid Rockets for Satellites or Upper Stages

“…This occurs under a very limited amount of the total impulse; the high upper stage sensitivity to specific impulse losses induces a non-negligible effect on the payload mass. Some more complex hybrid architectures are able to compensate the mixture ratio shift during throttling [42][43][44][45][46][47][48]. A simpler intermediate option that can be used for a pre-determined mission profile is the use of a concentric double fuel grain.…”