Numerical and Experimental Investigation of Hybrid Rocket Motors Transient Behavior

As the space business is shifting from pure performances to affordability a renewed interest is growing about hybrid rocket propulsion. Hybrid rocket motors are attractive for their inherent advantages like simplicity, reliability, safety and reduced costs. Moreover hybrid motors are easy to throttle and thus they are ideal candidate when soft-landing or energy management capabilities are required. This thesis is mainly involved with a theoretical/numerical study of hybrid transient behavior. The study of transient behavior is a very important aspect in the development of affordable, efficient, stable hybrid motors, particularly when throttling and controllability is concerned. Moreover transient behavior is important also for motors that work at a fixed operating point, not only in the prediction of ignition and shutdown phases but particularly in the analysis of instabilities. The prediction and reduction of instabilities are one of the main challenge in hybrid propulsion (as in general in all rocket motors). The aim of this doctoral thesis is to investigate and simulate hybrid rocket transient behavior through the development of a numerical code. The numerical code is composed by several independent parts coupled together, each one referring to a different subsystem of the hybrid rocket motor. Due to budget and time constraints it has not been possible to perform a dedicated experimental activity for this thesis. However the numerical results have been compared with experimental data obtained from literature, from CISAS partners (like NAMMO), and from other CISAS experimental activities performed both before and during this doctoral period. Each subsystem of the hybrid propulsion unit and its related codes are described in a different chapter. In the first chapter hybrid boundary layer steady combustion is introduced together with a discussion about the effect of steady hybrid regression physics on the shift of motor operating parameters with time. In the second chapter typical necessary or intentional transient events occurring during the operation of a hybrid rocket (ignition, throttling and shutdown) are classified and described. With chapter 3 begins the description of the several sub-models defining hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the solid grain thermal behavior. The main object of this work is to determine the response of the solid fuel to variations of the heat flux on the surface. A 1D numerical model of transient grain thermal response has been developed with this goal. The model is based on the work performed by Karabeyoglu and solves the temperature profile in the direction normal to the surface. In the first paragraph a model suited for classical polymeric fuels is developed. In the second paragraph the grain model is coupled with the boundary layer response in order to investigate typical hybrid low frequency instabilities. In the third paragraph a version of the original grain model suited for liquefying propellants is developed. In fact recently a new class of fast burning fuels has been discovered at Stanford University. These fuels form a liquid layer on the melting surface during combustion, hence the term 'liquefying fuels'. Entrainment of droplets from the liquid-gas interface creates the desired high regression rate by increasing the rate of fuel mass transfer. Several researchers included people at CISAS have experimental confirmed that paraffin-based fuels burn at surface regression rates 3 to 4 times that of conventional hybrid fuels. Others following studies showed with the use of visualization experiments the presences of waves on the liquid surfaces and droplets entrained by the gas flow, confirming original theoretical predictions. The third paragraph is divided in three parts. In the first part the model developed to predict the regression rate and the thermal profile inside a paraffin fuel is presented. The second part deals with the phenomenology of supercritical entrainment. Finally the third part discusses the problem of the closure of the equations to take into account the space-time variability of the entrainment phenomenon. In chapter 4 the attention is focused on the gas dynamic inside the hybrid combustion chamber. For this purpose two time-varying numerical models are developed. The aim of these unsteady codes is to determine the transient behavior of the main parameters of the hybrid rocket motor. The combustion chamber model represents the core of the hybrid rocket motor simulation. In fact the combustion chamber model gives directly the main parameter of a propulsion system, that is, motor thrust. The sub-models presented in the previous and the next chapters define the input parameters for the combustion chamber model. In fact the grain model of chapter 3 determine the fuel mass flow while the tank and feed lines model of chapter 5 gives the oxidizer mass flow. In the first part of this chapter a global 0D time-varying numerical model of the combustion chamber is developed. The code is then coupled with the grain model described in the previous chapter to account for the transient fuel production. It follows a brief discussion about the main hybrid rocket motor characteristic times and their relative values. In the second part a 1D time-varying numerical model of the combustion chamber is developed. The unsteady 1D code is able to simulate all the features of the 0D code. It should add the acoustic response of the system and the spatial variation of the fluid-dynamic unknowns along the flow direction, increasing the accuracy of the results at the expense of an higher computational effort. Chapter 5 end the description of the several sub-models of the hybrid rocket propulsion system. Together with chapter 3 and 4 it composes the code describing hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the oxidizer path. This includes the sub-systems ahead of the combustion chamber like the pressurization system, the main tank and the feed lines. Moreover it considers also the injector elements and some aspects of droplets vaporization and atomization in the combustion chamber. This work is complementary to the one described in chapter 3, defining the input parameters for the core of the code, that is the chamber gas-dynamic model shown in chapter 4. The main object of this work is to determine how the feed system affects the performance parameters of the hybrid motor with time. For this purpose the prediction of several unknowns like the oxidizer mass flow, tank pressure and the amount of residual gases is obtained through the modeling of the principal subsystem behavior. Moreover the full transient coupling between the feed system and the combustion chamber is also investigated. This chapter is divided in three parts. The topic of the first paragraph regards the main tank and the pressurization system. After a brief description of the main alternatives the discussion goes on with the numerical modeling of the typical solutions adopted for hybrid rockets (i.e. pressure-regulated, blowdown and self-press). First of all a numerical model of a pressure fed tank is developed. The code is able to predict several parameters like masses, densities, temperatures and pressures of the gas in the ullage volume and in the pressurant tank, the pressurant mass flow and the filling level of the tank. The model takes into account several aspects like heat losses, liquid oxidizer evaporation, eventual gas phase combustion of the pressurant gas, the use of by-pass and digital valves. Later a numerical model of a self pressurized tank is developed. The code is able to determine the oxidizer mass, temperature, pressure, density and the vapor/liquid volume/mass fractions during the discharge. The numerical results are compared with experimental hot tests performed at CISAS. The second paragraph takes into account the full transient coupling between the feed system and the combustion chamber. The main challenge is to determine the instantaneous liquid mass flow and the relation between the liquid oxidizer and the gaseous oxidizer that takes part in the hybrid motor combustion processes (i.e. droplets vaporization). In this way it is possible to simulate feed system coupled instabilities. The third paragraph deals with the prediction of the mass flow through the injector elements. In particular the behavior of self-pressurized systems is investigated. In this case the chamber pressure is below the vapor pressure of the liquid inside the tank. Consequently cavitation and flashing occur inside the injector elements. This kind of two-phase flow with vaporization involves several important modeling issues. Different models are compared with cold-flow tests performed at CISAS in order to check the accuracy of their predictions. In chapter 6 some advanced techniques developed to increase the regression rate and combustion efficiency of hybrid rockets are investigated with a particular focus on their influence on the transient behavior of the motor, particularly regarding combustion instabilities. The two methods studied in this thesis are the use of a diaphragm in the midst of the grain and the use of a swirling oxidizer injection. The reason for this choice is related to the fact that both solutions have been tested (among others) at CISAS and look very promising with respect to the overcoming of historical hybrid weaknesses. Even if working in very different ways both methods induce a strong increase of the turbulence level and mixing of the reactants in the combustion chamber, promoting a more complete combustion and an higher heat flux on the grain surface. Beside improving significantly hybrid performances this two techniques can affect the stability behavior of an hybrid motor directly (i.e. modifying the flowfield in the chamber) and indirectly (e.g. reducing the chamber length due to increased regression rate). In the final chapter a summary of the activities carried out and the results achieved is given

Experimental Investigation of a H2O2 Hybrid Rocket with Different Swirl Injections and Fuels

Hybrid rockets have very interesting characteristics like simplicity, reliability, safety, thrust modulation, environmental friendliness and lower costs, which make them very attractive for several applications like sounding rockets, small launch vehicles, upper stages, hypersonic test-beds and planetary landers. In recent years, advancements have been made to increase hybrid motor performance, and two of the most promising solutions are vortex injection and paraffin-based fuels. Moreover, both technologies can be also used to tailor the fuel regression rate, in the first case varying the swirl intensity, and in the second case with the amount and type of additives. In this way, it is possible not only to design high-performing hybrid motors, but also to adjust their grain and chamber geometries to different mission requirements, particularly regarding thrust and burning time. In this paper, the knowledge about these two technical solutions and their coupling is extended. Three sets of experimental campaigns were performed in the frame of the Italian Space Agency-sponsored PHAEDRA program. The first one investigated a reference paraffin fuel with axial and standard vortex injection. The second campaign tested vortex injection with low values of swirl numbers down to 0.5 with a conventional plastic fuel, namely polyethylene. Finally, the last campaign tested another, lower regressing, paraffin-based fuel with the same low swirl numbers as the second campaign

Numerical and Experimental Investigation of Hybrid Rocket Motors Transient Behavior

As the space business is shifting from pure performances to affordability a renewed interest is growing about hybrid rocket propulsion. Hybrid rocket motors are attractive for their inherent advantages like simplicity, reliability, safety and reduced costs. Moreover hybrid motors are easy to throttle and thus they are ideal candidate when soft-landing or energy management capabilities are required. This thesis is mainly involved with a theoretical/numerical study of hybrid transient behavior. The study of transient behavior is a very important aspect in the development of affordable, efficient, stable hybrid motors, particularly when throttling and controllability is concerned. Moreover transient behavior is important also for motors that work at a fixed operating point, not only in the prediction of ignition and shutdown phases but particularly in the analysis of instabilities. The prediction and reduction of instabilities are one of the main challenge in hybrid propulsion (as in general in all rocket motors). The aim of this doctoral thesis is to investigate and simulate hybrid rocket transient behavior through the development of a numerical code. The numerical code is composed by several independent parts coupled together, each one referring to a different subsystem of the hybrid rocket motor. Due to budget and time constraints it has not been possible to perform a dedicated experimental activity for this thesis. However the numerical results have been compared with experimental data obtained from literature, from CISAS partners (like NAMMO), and from other CISAS experimental activities performed both before and during this doctoral period. Each subsystem of the hybrid propulsion unit and its related codes are described in a different chapter. In the first chapter hybrid boundary layer steady combustion is introduced together with a discussion about the effect of steady hybrid regression physics on the shift of motor operating parameters with time. In the second chapter typical necessary or intentional transient events occurring during the operation of a hybrid rocket (ignition, throttling and shutdown) are classified and described. With chapter 3 begins the description of the several sub-models defining hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the solid grain thermal behavior. The main object of this work is to determine the response of the solid fuel to variations of the heat flux on the surface. A 1D numerical model of transient grain thermal response has been developed with this goal. The model is based on the work performed by Karabeyoglu and solves the temperature profile in the direction normal to the surface. In the first paragraph a model suited for classical polymeric fuels is developed. In the second paragraph the grain model is coupled with the boundary layer response in order to investigate typical hybrid low frequency instabilities. In the third paragraph a version of the original grain model suited for liquefying propellants is developed. In fact recently a new class of fast burning fuels has been discovered at Stanford University. These fuels form a liquid layer on the melting surface during combustion, hence the term 'liquefying fuels'. Entrainment of droplets from the liquid-gas interface creates the desired high regression rate by increasing the rate of fuel mass transfer. Several researchers included people at CISAS have experimental confirmed that paraffin-based fuels burn at surface regression rates 3 to 4 times that of conventional hybrid fuels. Others following studies showed with the use of visualization experiments the presences of waves on the liquid surfaces and droplets entrained by the gas flow, confirming original theoretical predictions. The third paragraph is divided in three parts. In the first part the model developed to predict the regression rate and the thermal profile inside a paraffin fuel is presented. The second part deals with the phenomenology of supercritical entrainment. Finally the third part discusses the problem of the closure of the equations to take into account the space-time variability of the entrainment phenomenon. In chapter 4 the attention is focused on the gas dynamic inside the hybrid combustion chamber. For this purpose two time-varying numerical models are developed. The aim of these unsteady codes is to determine the transient behavior of the main parameters of the hybrid rocket motor. The combustion chamber model represents the core of the hybrid rocket motor simulation. In fact the combustion chamber model gives directly the main parameter of a propulsion system, that is, motor thrust. The sub-models presented in the previous and the next chapters define the input parameters for the combustion chamber model. In fact the grain model of chapter 3 determine the fuel mass flow while the tank and feed lines model of chapter 5 gives the oxidizer mass flow. In the first part of this chapter a global 0D time-varying numerical model of the combustion chamber is developed. The code is then coupled with the grain model described in the previous chapter to account for the transient fuel production. It follows a brief discussion about the main hybrid rocket motor characteristic times and their relative values. In the second part a 1D time-varying numerical model of the combustion chamber is developed. The unsteady 1D code is able to simulate all the features of the 0D code. It should add the acoustic response of the system and the spatial variation of the fluid-dynamic unknowns along the flow direction, increasing the accuracy of the results at the expense of an higher computational effort. Chapter 5 end the description of the several sub-models of the hybrid rocket propulsion system. Together with chapter 3 and 4 it composes the code describing hybrid rocket transient behavior. In this chapter the attention is focused on the numerical modeling of the oxidizer path. This includes the sub-systems ahead of the combustion chamber like the pressurization system, the main tank and the feed lines. Moreover it considers also the injector elements and some aspects of droplets vaporization and atomization in the combustion chamber. This work is complementary to the one described in chapter 3, defining the input parameters for the core of the code, that is the chamber gas-dynamic model shown in chapter 4. The main object of this work is to determine how the feed system affects the performance parameters of the hybrid motor with time. For this purpose the prediction of several unknowns like the oxidizer mass flow, tank pressure and the amount of residual gases is obtained through the modeling of the principal subsystem behavior. Moreover the full transient coupling between the feed system and the combustion chamber is also investigated. This chapter is divided in three parts. The topic of the first paragraph regards the main tank and the pressurization system. After a brief description of the main alternatives the discussion goes on with the numerical modeling of the typical solutions adopted for hybrid rockets (i.e. pressure-regulated, blowdown and self-press). First of all a numerical model of a pressure fed tank is developed. The code is able to predict several parameters like masses, densities, temperatures and pressures of the gas in the ullage volume and in the pressurant tank, the pressurant mass flow and the filling level of the tank. The model takes into account several aspects like heat losses, liquid oxidizer evaporation, eventual gas phase combustion of the pressurant gas, the use of by-pass and digital valves. Later a numerical model of a self pressurized tank is developed. The code is able to determine the oxidizer mass, temperature, pressure, density and the vapor/liquid volume/mass fractions during the discharge. The numerical results are compared with experimental hot tests performed at CISAS. The second paragraph takes into account the full transient coupling between the feed system and the combustion chamber. The main challenge is to determine the instantaneous liquid mass flow and the relation between the liquid oxidizer and the gaseous oxidizer that takes part in the hybrid motor combustion processes (i.e. droplets vaporization). In this way it is possible to simulate feed system coupled instabilities. The third paragraph deals with the prediction of the mass flow through the injector elements. In particular the behavior of self-pressurized systems is investigated. In this case the chamber pressure is below the vapor pressure of the liquid inside the tank. Consequently cavitation and flashing occur inside the injector elements. This kind of two-phase flow with vaporization involves several important modeling issues. Different models are compared with cold-flow tests performed at CISAS in order to check the accuracy of their predictions. In chapter 6 some advanced techniques developed to increase the regression rate and combustion efficiency of hybrid rockets are investigated with a particular focus on their influence on the transient behavior of the motor, particularly regarding combustion instabilities. The two methods studied in this thesis are the use of a diaphragm in the midst of the grain and the use of a swirling oxidizer injection. The reason for this choice is related to the fact that both solutions have been tested (among others) at CISAS and look very promising with respect to the overcoming of historical hybrid weaknesses. Even if working in very different ways both methods induce a strong increase of the turbulence level and mixing of the reactants in the combustion chamber, promoting a more complete combustion and an higher heat flux on the grain surface. Beside improving significantly hybrid performances this two techniques can affect the stability behavior of an hybrid motor directly (i.e. modifying the flowfield in the chamber) and indirectly (e.g. reducing the chamber length due to increased regression rate). In the final chapter a summary of the activities carried out and the results achieved is given.Man mano che le attività spaziali stanno passando da una fase di ricerca delle prestazioni pure ad una fase di maggior accessibilità, sta crescendo un rinnovato interesse nei confronti della propulsione ibrida. I motori a razzo ibridi sono interessanti per i loro vantaggi intrinseci, come la semplicità, l'affidabilità, la sicurezza e la riduzione dei costi. Inoltre è facile modulare la spinta dei motori ibridi e quindi essi rappresentano un candidato ideale per le applicazioni che richiedono un atterraggio morbido o la gestione dell'energia. Questa tesi riguarda principalmente uno studio teorico/numerico del comportamento transitorio dei motori ibridi. Lo studio del comportamento transitorio è un aspetto molto importante nello sviluppo di motori ibridi stabili, efficienti, in particolare quando sono richiesti throttling e controllabilità. Inoltre il comportamento transitorio è importante anche per motori che operano ad un punto di funzionamento fisso, non solo nella previsione delle fasi di accensione e spegnimento ma soprattutto nell'analisi delle instabilità. La previsione e la riduzione delle instabilità rappresentano una delle principali sfide della propulsione ibrida (come in generale in tutti i propulsori a razzo). Lo scopo di questa tesi di dottorato è quello di indagare e simulare il comportamento transitorio di un propulsore ibrido attraverso lo sviluppo di un codice numerico. Il codice numerico è composto da più parti distinte accoppiate tra loro, ciascuna facente riferimento a un sottosistema differente del motore a razzo ibrido. A causa di vincoli di bilancio e di tempo non è stato possibile effettuare una attività sperimentale dedicata per questa tesi. Tuttavia, i risultati numerici sono stati confrontati con i dati sperimentali ottenuti dalla letteratura, dai partner del CISAS (come Nammo), e da altre attività sperimentali del CISAS effettuate sia prima che durante questo periodo di dottorato. Ogni sottosistema del propulsore ibrido e i suoi relativi codici sono descritti in un capitolo diverso. Nel primo capitolo viene introdotta la fisica stazionaria della combustione ibrida seguita da una discussione sull'effetto che essa ha sulla variazione temporale dei parametri operativi del motore. Nel secondo capitolo vengono classificati e descritti i tipici eventi transitori che avvengono durante il funzionamento di un motore ibrido (accensione, throttling, spegnimento). Nel terzo capitolo inizia la descrizione dei vari modelli che definiscono il comportamento transitorio dei motori ibridi. In questo capitolo l'attenzione è focalizzata nella modellazione numerica del comportamento termico del grano solido. L'obiettivo principale è quello di determinare la risposta del combustibile solido alle variazioni di flusso termico sulla superficie. A tal fine è stato sviluppato un modello numerico monodimensionale della risposta termica transitoria del grano. Il modello è basato sul lavoro di Karabeyoglu e risolve il profilo termico nella direzione normale alla superficie. Nel primo paragrafo viene sviluppato il modello base per combustibili polimerici. Nel secondo paragrafo il modello è accoppiato con la risposta dello strato limite allo scopo di simulare le tipiche instabilità a bassa frequenza dell'ibrido. Nel terzo paragrafo il modello base viene esteso per simulare combustibili che formano uno strato fuso sulla superficie del grano. Difatti recentemente è stata scoperta da ricercatori dell'università di Stanford una nuova classe di combustibili ad elevata velocità di regressione. Questi combustibili formano uno strato di liquido sulla superficie fusa durante la combustione. Grazie all'entrainment di goccioline di combustibile la velocità di regressione è aumentata considerevolmente a causa del nuovo meccanismo di trasporto di massa. Diversi ricercatori hanno confermato una velocità di regressione pari a 3-4 volte quella dei combustibili ibridi tradizionali. Studi successivi hanno mostrato tramite esperimenti visivi la presenza di onde sulla superficie liquida e di goccioline trasportate dalla corrente, confermando le previsioni iniziali. Il terzo paragrafo è diviso in tre parti. Nella prima parte è presentato il modello sviluppato per predire il profilo di temperatura e il regression rate in un combustibile a base di paraffina. Nella seconda parte viene discussa la fenomenologia dell'entrainment supercritico. Nella terza parte viene discusso il problema della chiusura delle equazioni per tener conto della variabilità spaziale e temporale del fenomeno dell'entrainment. Nel quarto capitolo l'attenzione è concentrata sulla gasdinamica della camera di combustione. A tal fine sono stati sviluppati due modelli numerici transitori. L'obiettivo di questi codici è di determinare il comportamento transitorio dei principali parametri del motore ibrido. Il modello della camera di combustione rappresenta il cuore della simulazione del motore ibrido. Difatti questo modello fornisce direttamente il parametro principale di un sistema propulsivo, cioè la spinta. I modelli dei capitoli precedente e successivo definiscono i parametri di ingresso per il modello della camera di combustione. Infatti il modello del grano del capitolo 3 determina la portata di combustibile mentre il modello del serbatoio e delle linee di alimentazione del capitolo 5 fornisce la portata di ossidante. Nella prima parte di questo capitolo viene sviluppato un modello non-stazionario globale della camera di combustione. Il codice viene poi accoppiato con il modello del grano descritto nel capitolo precedente per tener conto della produzione transitoria di combustibile. Segue una breve discussione sui tempi caratteristici di un motore ibrido e la loro relativa grandezza. Nella seconda parte viene sviluppato un codice monodimensionale non-stazionario della camera di combustione. Il codice transiente monodimensionale è in grado di simulare tutti gli aspetti già trattati dal codice zero-dimensionale. Esso aggiunge la risposta acustica del sistema e la variazione spaziale delle grandezze fluidodinamiche nella direzione del flusso, incrementando l'accuratezza a scapito di un maggiore costo computazionale. Il quinto capitolo termina la descrizione dei vari modelli del sistema propulsivo ibrido. Insieme ai capitoli 3 e 4 compone il codice che descrive il comportamento transitorio dei motore ibridi. In questo capitolo l'attenzione è concentrata sula modellazione numerica del percorso dell'ossidante. Ciò include tutto ciò che si trova a monte della camera di combustione, come il sistema di pressurizzazione, il serbatoio principale, le linee di adduzione. Inoltre considera anche la piastra di iniezione e alcuni aspetti dell'atomizzazione ed evaporazione del liquido nella camera di combustione. Questa parte è complementare con quella descritta nel capitolo 3 e definisce i parametri d'ingresso per il cuore del codice, cioè la gasdinamica della camera di combustione del capitolo 4. Il principale obiettivo di questo lavoro è determinare come il sistema di alimentazione influenza i principali parametri prestazionali del motore nel tempo. Per questo motivo varie incognite come la portata di ossidante, la pressione nel serbatoio e la quantità di gas residuo vengono determinate attraverso la modellazione del comportamento dei vari sottosistemi. Inoltre viene indagato anche l'accoppiamento non-stazionario tra il sistema di iniezione e la camera di combustione. Questo capitolo è diviso in tre parti. Il primo paragrafo riguarda il sistema di pressurizzazione. Dopo una breve descrizione delle principali alternative la discussione continua con la modellazione numerica delle principali soluzioni adottate nei motori ibridi (pressure-regulated, blowdown e autopressurizzato). Prima di tutto viene sviluppato un modello numerico di un sistema pressure-fed. Il codice è in grado di predire svariati parametri tra cui le masse, temperature, densità e pressioni del gas nel serbatoio principale e in quello del pressurizzante, la portata di pressurizzante e il livello di riempimento del serbatoio. Il modello considera vari aspetti tra cui gli scambi termici, l'evaporazione del liquido, la combustione finale in fase gassosa, l'uso di by-pass e valvole digitali. Successivamente viene sviluppato un modello numerico di un sistema autopressurizzato. Il codice è in grado di predire la temperatura, densità, pressione dell'ossidante assieme al titolo della miscela. I risultati numerici vengono comparati con i test sperimentali condotti dal CISAS. Il secondo paragrafo considera l'accoppiamento non-stazionario tra il sistema di iniezione e la camera di combustione. La principale difficoltà deriva dalla determinazione della portata istantanea di liquido e della relazione che lega la portata di liquido a quella di gas che partecipa alla combustione. In questo modo è possibile simulare le instabilità dovute a tale accoppiamento. Il terzo paragrafo riguarda la determinazione della portata di massa attraverso la piastra di iniezione. In particolare viene investigato il comportamento di sistemi autopressurizzanti. In questo caso la pressione in camera di combustione è al di sotto della pressione di vapore dell'ossidante nel serbatoio. Per questo motivo nell'iniettore si sviluppano importanti fenomeni di cavitazione. Questo tipo di flussi bifasici coinvolgono diversi aspetti di modellazione. Tre modelli differenti sono comparati con test sperimentali effettuati dal CISAS con l'obiettivo di determinare l'accuratezza delle previsioni numeriche. Nel sesto capitolo vengono analizzate alcune tecniche avanzate per aumentare la velocità di regressione e l'efficienza dei motori ibridi con una particolare attenzione al loro effetto sul comportamento transitorio del motore, soprattutto sulle instabilità. I due metodi studiati in questa tesi sono l'uso di un diaframma in mezzo al grano e l'utilizzo di un'iniezione swirl. La ragione di questa scelta è legata al fatto che entrambe le tecniche sono state testate (tra gli altri) dal CISAS e risultano essere molto promettenti a riguardo del superamento degli storici punti deboli dei motori ibridi. Anche se funzionanti con principi molto diversi entrambi i metodi inducono un elevato incremento della turbolenza e del miscelamento di reagenti nella camera di combustione, promuovendo il completamento della combustione e un più elevato flusso termico a parete. Oltre ad incrementare notevolmente le prestazioni dei motori ibridi queste due tecniche possono influenzare anche il comportamento transitorio di un motore sia direttamente (modificando il campo fluido all'interno della camera di combustione), sia indirettamente (ad esempio riducendo la lunghezza della camera per via di una maggiore velocità di regressione). Nell'ultimo capitolo vengono riassunte le attività svolte ed i risultati ottenuti

Challenges of Ablatively Cooled Hybrid Rockets for Satellites or Upper Stages

Ablative-cooled hybrid rockets could potentially combine a similar versatility of a liquid propulsion system with a much simplified architecture. These characteristics make this kind of propulsion attractive, among others, for applications such as satellites and upper stages. In this paper, the use of hybrid rockets for those situations is reviewed. It is shown that, for a competitive implementation, several challenges need to be addressed, which are not the general ones often discussed in the hybrid literature. In particular, the optimal thrust to burning time ratio, which is often relatively low in liquid engines, has a deep impact on the grain geometry, that, in turn, must comply some constrains. The regression rate sometime needs to be tailored in order to avoid unreasonable grain shapes, with the consequence that the dimensional trends start to follow some sort of counter-intuitive behavior. The length to diameter ratio of the hybrid combustion chamber imposes some packaging issues in order to compact the whole propulsion system. Finally, the heat soak-back during long off phases between multiple burns could compromise the integrity of the case and of the solid fuel. Therefore, if the advantages of hybrid propulsion are to be exploited, the aspects mentioned in this paper shall be carefully considered and properly faced