Miniaturized spacecraft, such as the well-known CubeSats, have become more and more popular in the last decade, allowing for dramatic reductions of the mission costs. However, it is not fully clear at the present moment which access to space option is the most convenient and affordable one for this class of satellites, with micro-launchers still penalized by their intrinsically bad structural efficiency and propulsion performance. It seems to be preferable to use onboard propulsion for the orbital transfer of these small spacecraft after being inserted in Low Earth Orbit as a piggyback payload of a larger launcher. However, in turn, this increased autonomy due to the presence of a propulsion system raises significant issues in terms of safety to the other surrounding spacecraft in orbit. Strict measures are expected to be applied in the future regulations, in scenarios such as a thrust-direction failure in a small spacecraft, with consequent damage to other spacecraft creating debris. This problem becomes even more critical in Low Earth Orbit due to the presence of the International Space Station. This paper presents a general overview of the current access to space options for small satellites, and the existing propulsion technology for their orbital transfer. A preliminary analysis is performed on the design requirements related to potential scenarios and recovery actions related to propulsion system failures for small satellites operating in Low Earth Orbit. @en

The article reviews the main forms of flow-induced instabilities detected in the liquid propellant turbopumps systems of modern rocket engines, with special reference to rotating stall, rotating cavitation, cavitation surge and higher order surge modes, illustrating their characteristics, origin and damage potential.@en

After a brief review of the fundamental aspects of cavitation relevant to the operation of high-performance inducers and turbopumps, the article summarizes their application to the analysis of pumping systems, illustrates the scaling of cavitation phenomena from model tests to full-scale operation, describes the occurrence of flow-induced instabilities in turbomachinery, and introduces the concepts of static and dynamic instability of pumping systems and their generalization to cavitating turbopump systems.@en

The ideal flow model for the preliminary design and performance prediction of radial turbopumps presented in the companion paper of the present volume (d’Agostino et al. 2017) is here interfaced with the calculation of the boundary layers inside the blade channels and other major forms of flow losses, with the aim of developing an effective tool for rapid parametric optimization of the machine performance and geometry under appropriate design constraints, such as assigned values of the specific speed, flow coefficient, and blade solidity. A mixed-flow turbopump, with a six-bladed impeller, a vaneless diffuser, a single-spiral volute and nondimensional performance characteristics similar to those typically used in liquid propellant rocket engine feed systems, has been designed, parametrically optimized and manufactured in accordance with the indications of the present model. The pumping and suction performance of the machine has been determined in a series of tests in the cavitating pump rotordynamic test facility (CPRTF). Under fully wetted flow conditions the measured pumping characteristics of the machine (hydraulic head and efficiency as functions of the flow coefficient) proved to be in excellent agreement with the model predictions, thus successfully confirming the validity of the proposed model as an effective tool for rapid and efficient design of high-performance centrifugal turbopumps.

The present work has been supported by the European Space Agency under Contract No. 40001025856/10/NL/SFe. The authors would like to express their gratitude to Dr. Giorgio Saccoccia of ESA-ESTEC for his constant and friendly encouragement.
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A reduced-order model for the preliminary design and performance prediction of radial turbopumps is illustrated. The model expresses the 3D, incompressible, inviscid, irrotational flow through helical blades with slow axial variations of their pitch and backsweep angles by superposing a 2D axial vorticity correction to a fully guided forced vortex flow with axisymmetric stagnation velocity in the meridional plane. Application of the relevant governing equations allows for the closed-form definition of the impeller geometry and flowfield in terms of a reduced number of controlling parameters. Mass and momentum conservations are used for coupling the flow leaving the impeller with the 2D reduced-order models of the flow in the diffuser and/or the volute, as well as for the evaluation of the mixing losses in the transfer between successive components of the machine. This information completes the geometric definition of the turbopump and determines its ideal noncavitating performance in accordance with the resulting flowfield. As a consequence of the neglect of viscous effects, the slip factor predicted by the present model exceeds those obtained from theoretical/semi-empirical formulas reported in literature for centrifugal pumps, but correctly captures their trend.

The present work has been supported by the European Space Agency under Contract No. 40001025856/10/NL/SFe. The authors would like to express their gratitude to Dr. Giorgio Saccoccia of ESA-ESTEC for his constant and friendly encouragement.
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