The increasing interest towards reaching Low Earth Orbits for both manned and unmanned missions makes disposable rocket launchers an unsustainable choice both economically and environmentally. Many attempts were made to reduce space flight costs and debris, mainly recovering and refurbishing parts of launch systems to be employed for several flights. Nevertheless, the turning point that could cut down the costs connected to space access would be the realisation of a fully reusable Single-Stage-To-Orbit vehicle. In this sense, development of a suitable propulsion system able to provide an effective alternative to conventional rockets is the main issue. Among all technologies, the most attractive choices appear to be Rocket-Based Combined Cycles (RBCC) and precooled hybrid air-breathing-rocket engines, particularly Synergetic Air-Breathing Rocket Engine (SABRE). These architectures combine the high specific impulse of air-breathing devices for the first phase of the ascent and the ability to work in vacuum conditions of rockets. RBCC engines unite different working modes, namely ejector, ramjet, scramjet and pure rocket, to generate thrust from steady conditions on ground to orbit insertion. SABRE is the propulsion unit designed for the SKYLON spaceplane. It exploits a helium closed cycle to cool down incoming air to cryogenic temperatures, compresses the airflow above 140 bar and pumps it into a combustion chamber. Liquid hydrogen is employed to cool down helium and to sustain combustion in the engine. For rocket working mode, air intake is closed, and stored liquid oxygen work as oxidizer in the combustion chamber. Aim of this thesis is to demonstrate the advantages connected with combined cycle engines. A preliminary overview of different propulsion architectures proposed for SSTO spaceplanes will validate the selection of SABRE and RBCC as the most suitable systems. Numerical models will be developed to compute and compare performance parameters of the selected candidates in terms of thrust, specific impulse and specific fuel consumption. Due to the lack of data available, performance computation is based on simplified models: RBCC engine is developed on the basis of propulsion system designed for Hyperion spaceplane, whereas SABRE is modeled according to data found in literature. To provide a significant comparison, a hypothetical constant dynamic pressure trajectory is selected to compute performance parameters according to flight altitude and Mach number for air-breathing working modes. Eventually, mass ratios are computed with Tsiolkovsky equation for each engine, considering a 5000 kg payload to be inserted into a 300 km high orbit. Results of numerical simulations are compared with a hypothetical rocket launcher, based on properties of the space shuttle main engine, to demonstrate the unsatisfactory behavior of a non-airbreathing SSTO vehicle. A spaceplane configuration will be adopted for combined cycle engines, whereas for the rocket baseline a conventional vertical take off slender body will be considered. Simulation results show that SABRE and RBCC engines would lead to a reduction of propellant necessity, decreasing the total take-off masses of 58.34% and 84.31% respectively of the total take-off mass. Despite its advantages in terms of mass savings, RBCC engine requires long range and residence times in atmosphere due to high velocity reached before rocket mode transition. Such requirement could increase heating of the spaceplane surface. Furthermore, ramjet and scramjet phases present a significantly lower thrust with respect to other working modes, leading to small acceleration and increased time-to-orbit. In this sense, thanks to its high thrust and high specific impulse in air-breathing phase of the ascent, SABRE demonstrates to be the most attractive choice in perspective of a SSTO launcher development.
Il recente incremento di voli verso orbite basse, legato a missioni di trasporto umano e cargo, rende insostenibile l’utilizzo di lanciatori sacrificabili sia dal punto di vista economico che ambientale. Diversi tentativi sono stati effettuati per ridurre il costo e i detriti generati dai lanci spaziali, indirizzati prevalentemente al recupero e riutilizzo di parti dei lanciatori stessi. Tuttavia, una svolta decisiva verso il taglio dei costi e dell’inquinamento connessi a missioni spaziali deriverebbe esclusivamente dalla realizzazione di un veicolo SSTO completamente riutilizzabile. A tal proposito, la sfida principale è la realizzazione di un sistema propulsivo adeguato in grado di fornire una valida alternativa agli endoreattori convenzionali. Tra le tecnologie proposte le più promettenti risultano essere gli RBCC ( Rocket-Based Combined Cycles ) e i motori ibridi preraffreddati che uniscono modalità air-breathing e a razzo, in particolare SABRE ( Synergetic Air-Breathing Rocket Engine ). Queste architetture combinano gli alti impulsi specifici della propulsione atmosferica con l’abilità dei razzi di operare nel vuoto. I motori RBCC operano come eiettori, ramjet, scramjet e infine come endoreattori, al fine di generare spinta da condizioni statiche a terra fino all’inserimento in orbita. Il motore SABRE è l’unità propulsiva sviluppata per lo spazioplano SKYLON. Esso sfrutta un ciclo chiuso ad elio per raffreddare l’aria entrante a temperature prossime alla liquefazione. L’aria viene poi compressa oltre i 140 bar e iniettata in camera di combustione, dove reagisce con l’idrogeno. In modalità razzo, la presa d’aria viene chiusa e l’idrogeno viene bruciato con ossigeno liquido conservato nei serbatoi. Obiettivo di questa tesi è dimostrare i vantaggi apportati dall’utilizzo dei cicli combinati. Una rassegna preliminare di differenti tecnologie propulsive proposte negli anni per spazioplani SSTO sarà seguita da una discussione che validi la scelta dei motori RBCC e SABRE come migliori candidati. Successivamente, due modelli numerici verranno sviluppati per calcolare e confrontare spinta, impulsi specifici e consumi specifici delle tecnologie selezionate. A causa della scarsa disponibilità di informazioni, tali modelli si baseranno su diverse semplificazioni. Il motore RBCC verrà sviluppato partendo dalla configurazione proposta per lo spazioplano Hyperion, mentre il modello relativo a SABRE sarà costruito sulla base di dati raccolti in letteratura. Per portare a termine un confronto significativo, una traiettoria ipotetica a pressione dinamica costante sarà selezionata per le fasi di funzionamento air-breathing. Infine, l’equazione di Tsiolkovsky verrà utilizzata per calcolare le masse di strutture e propellente di spazioplani costruiti sulla base dei motori selezionati. A tal fine verrà considerata una missione finalizzata all’immissione in orbita di un carico pagante di 5000 kg a 300 km di quota. I risultati saranno confrontati con quelli ottenuti da un riferimento costituito da un lanciatore verticale SSTO propulso da endoreattori modellati sulla base dei motori SSME di Shuttle. I risultati conclusivi dimostreranno che i motori SABRE e RBCC apportano un vantaggio notevole rispetto agli endoreattori convenzionali, con masse al decollo ridotte rispettivamente del 58.34 % e 84.31 % rispetto al razzo di riferimento. Sebbene RBCC possa sembrare il miglior candidato in termini di riduzione di massa, il confronto tra traiettorie air-breathing rivelerà la sua necessità di prolungati tempi di permanenza in atmosfera per raggiungere la velocità di attivazione della modalità endoreattore, condizione che potrebbe aumentare le sollecitazioni termiche sulla struttura. Inoltre, le modalità ramjet e scramjet mostreranno spinte esigue, riducendo le accelerazioni e incrementando il tempo necessario al raggiungimento dell’orbita. Alla luce dei risultati, l’alto impulso specifico e la spinta elevata di SABRE lo rendono il candidato ideale in ottica di una rapida ascesa verso il completamento di una missione SSTO.
Propulsion system analysis of spaceplanes for SSTO
Salvi, Stefano
2019/2020
Abstract
The increasing interest towards reaching Low Earth Orbits for both manned and unmanned missions makes disposable rocket launchers an unsustainable choice both economically and environmentally. Many attempts were made to reduce space flight costs and debris, mainly recovering and refurbishing parts of launch systems to be employed for several flights. Nevertheless, the turning point that could cut down the costs connected to space access would be the realisation of a fully reusable Single-Stage-To-Orbit vehicle. In this sense, development of a suitable propulsion system able to provide an effective alternative to conventional rockets is the main issue. Among all technologies, the most attractive choices appear to be Rocket-Based Combined Cycles (RBCC) and precooled hybrid air-breathing-rocket engines, particularly Synergetic Air-Breathing Rocket Engine (SABRE). These architectures combine the high specific impulse of air-breathing devices for the first phase of the ascent and the ability to work in vacuum conditions of rockets. RBCC engines unite different working modes, namely ejector, ramjet, scramjet and pure rocket, to generate thrust from steady conditions on ground to orbit insertion. SABRE is the propulsion unit designed for the SKYLON spaceplane. It exploits a helium closed cycle to cool down incoming air to cryogenic temperatures, compresses the airflow above 140 bar and pumps it into a combustion chamber. Liquid hydrogen is employed to cool down helium and to sustain combustion in the engine. For rocket working mode, air intake is closed, and stored liquid oxygen work as oxidizer in the combustion chamber. Aim of this thesis is to demonstrate the advantages connected with combined cycle engines. A preliminary overview of different propulsion architectures proposed for SSTO spaceplanes will validate the selection of SABRE and RBCC as the most suitable systems. Numerical models will be developed to compute and compare performance parameters of the selected candidates in terms of thrust, specific impulse and specific fuel consumption. Due to the lack of data available, performance computation is based on simplified models: RBCC engine is developed on the basis of propulsion system designed for Hyperion spaceplane, whereas SABRE is modeled according to data found in literature. To provide a significant comparison, a hypothetical constant dynamic pressure trajectory is selected to compute performance parameters according to flight altitude and Mach number for air-breathing working modes. Eventually, mass ratios are computed with Tsiolkovsky equation for each engine, considering a 5000 kg payload to be inserted into a 300 km high orbit. Results of numerical simulations are compared with a hypothetical rocket launcher, based on properties of the space shuttle main engine, to demonstrate the unsatisfactory behavior of a non-airbreathing SSTO vehicle. A spaceplane configuration will be adopted for combined cycle engines, whereas for the rocket baseline a conventional vertical take off slender body will be considered. Simulation results show that SABRE and RBCC engines would lead to a reduction of propellant necessity, decreasing the total take-off masses of 58.34% and 84.31% respectively of the total take-off mass. Despite its advantages in terms of mass savings, RBCC engine requires long range and residence times in atmosphere due to high velocity reached before rocket mode transition. Such requirement could increase heating of the spaceplane surface. Furthermore, ramjet and scramjet phases present a significantly lower thrust with respect to other working modes, leading to small acceleration and increased time-to-orbit. In this sense, thanks to its high thrust and high specific impulse in air-breathing phase of the ascent, SABRE demonstrates to be the most attractive choice in perspective of a SSTO launcher development.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/173326