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

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.Un motore ibrido a spinta regolabile permette di eseguire dei profili di missione molto particolari come per esempio il soft landing. Le applicazioni correnti della tecnologia propulsiva ibrida sono peculiari di per se, tra loro ci sono razzi sonda, banchi di prova volanti, navicelle per il turismo spaziale e macchine da record di velocità. Molte di loro ad oggi sono dotate di controllo della spinta. Inoltre anche possibili applicazioni future come lanciatori e motori di manovra d'orbita potrebbero beneficiare della throttleabilità. Senza modulazione di spinta il range di applicazioni della tecnologia propulsiva ibrida è molto limitato. Quando la maturità della tecnologia ibrida aumenterà la modulazione della spinta sarà una funzionalità fondamentale qualora non lo fosse già. La modulazione della spinta nei motori ibridi è l'argomento principale della presente attività di ricerca. Questo lavoro è focalizzato nello studio e sviluppo di un motore ibrido multiuso a spinta variabile, in particolare questo avrà una spinta massima di 1 kN e un rapporto di spinta 5:1. Il motore utilizzerà una miscela acquosa di perossido di idrogeno ad alto titolo come ossidante, che sarà spinto verso la camera di combustione con un sistema di pressurizzazione a monte. Due configurazioni di camera di combustione sono presentate: la prima consiste in un grano combustibile di paraffina e una post camera dotata di mixer, la seconda consiste in un’iniezione elicoidale che instaura un vortice all'interno della camera di combustione e un grano in idrocarburi a lunga catena, possibilmente polietilene ad alta densità. Ad ogni modo la campagna di test a spinta modulabile di continuo è stata effettuata con la sola seconda configurazione. In passato al gruppo di propulsione ibrida dell'Università degli Studi di Padova altri ricercatori hanno studiato la throttleabilità sia da un punto di vista teorico che sperimentale, ma questa è la prima volta che dei test dinamici a spinta variabile con un livello continuo di discretizzazione vengono effettuati, e sono effettuati grazie ad una valvola che è stata sviluppata interamente ''in casa'' durante questa tesi. Questo è anche merito della recente disponibilità di fondi presso il gruppo per lo sviluppo di nuovo equipaggiamento. I metodi utilizzati in questo progetto di dottorato sono prevalentemente sperimentali, i risultati ottenuti sono ad ogni modo comparati con quelli provenienti da modelli analitici semplificati. Il sistema di alimentazione ossidante è controllato grazie ad una valvola di controllo di flusso, componente fondamentale in una linea fluidica per motori ibridi a spinta modulabile. Questa valvola è stata sviluppata interamente come parte del progetto di dottorato. I dettagli del design sono presentati nel capitolo 3.La valvola di controllo di flusso è stata sottoposta ad una completa caratterizzazione. I risultati ottenuti con la caratterizzazione sono: la curva caratteristica, l'andamento del coefficiente di scarica, la contropressione massima accettabile, l'andamento delle frequenze di picco della cavitazione con la pressione operativa e alcuni punti della funzione di trasferimento tra portata richiesta e portata ottenuta. Successivamente all'integrazione della valvola col motore una serie di campagne sperimentali cominciò. Il primo passo è stato caratterizzare il comportamento del motore per le due configurazioni proposte per diversi livelli di spinta, mantenuta costante durante i singoli test in modo da poter stabilire la legge di regressione del combustibile e l'efficienza del motore. Successivamente la campagna di test di modulazione di spinta dinamica è stata condotta. I risultati di queste campagne sperimentali sono riportati nel capitolo 4

Numerical Investigation of Nozzle Thermochemical Behaviour in Hybrid Rocket Motors

In this thesis work a physics-based model to predict nozzle thermochemical erosion is presented. This model is based on non-charring materials ablation phenomena and can be used to predict the erosion in the peculiar hybrid rocket motor environment and comparing different propellant composition