Study and Development of Throttleable Hybrid Rocket Motors

A throttleable rocket motor to perform some particular mission profiles such as soft landing. Current applications of hybrid technology are very peculiar. Among them there are sounding rockets, flying test beds, space tourism and speed world record automobiles. Most of them require throttling. Furthermore throttleability could be an interesting feature to implement on possible future applications such as launchers and in-orbit manoeuvre engines. Without throttleability hybrid rocket applications could be very limited. When the hybrid technology readiness will increase throttleability will be a paramount feature, if it is not already. Hybrid rocket engines throttleability is the topic of the present research activity. This work is focused on the investigation and development of a general purpose throttleable hybrid rocket motor, in particular a 1 kN-class motor with a throttling ratio of 5:1. The engine will use high test peroxide as an oxidizer which will flow to the combustion chamber in a pressure fed fashion. Two combustion chamber configuration are presented: paraffin fuel grain and mixing post combustion chamber or swirled injection (vortex engine) and long chain hydrocarbons fuel grain. However the final dynamic throttling fire test campaign has been carried out with the second motor configuration. In the past at University of Padova hybrid propulsion group other members studied throttleability both from a theoretical and experimental point of view, but this is the first time that dynamic throttling fire tests are carried out with a continuous thrust levels control, performed with an in house developed flow control valve. The recent availability of funds at the group gave us the possibility to develop and characterize new equipment. The tools employed during this doctoral research work are mainly experimental, the obtained results are then compared with the predictions from simplified analytical models. The pressure fed system is controlled by means of a flow control valve, this is a fundamental component in the feeding line of a throttleable hybrid rocket motor. The flow control valve was developed as part of the doctoral activities during this doctoral research period. The detailed design phase is discussed in chapter 3. The flow control valve is composed by valve body and actuation. The variable area cavitating venturi principle was selected for the valve body. Inside the valve a conical pointed pintle alters the throat area of a conventional venturi tube. The variable area cavitating venturi presents different advantages such as independence of the flow from the downstream pressure, uncoupling of tank and combustion chamber environments and precise flow control. The actuation controls the pintle axial position with respect to the venturi throat, an active closed control loop with a feedback on the pintle stroke guarantees the precise positioning. It is possible to implement the system with a control loop closed on the thrust but this strategy was not followed during this thesis. The flow control valve underwent a complete characterization aimed to fully understand its behaviour and limits. The outcomes of this characterization are: characteristic curve, discharge coefficient trend, maximum allowed back pressure, cavitation instabilities peak frequencies, system rise and fall time and some transfer function points. The flow control valve then has been integrated with the test motor, starting a series of fire test campaigns. The first step was to determine the motor behaviour for a series of discrete thrust levels in order to ascertain the motor regression rate power law and combustion chamber efficiency for a constant oxidizer mass flow. This preliminary fire test campaign was carried out for both the proposed motor configurations. Afterwards the dynamic throttling fire test campaign started, four tests were carried out