This article is devoted to a review of existing small upper stages, including perspective upper stages under development or testing, actively used in the tasks of launching small spacecraft into target orbit, the relevance of which has recently been increasing against the background of the trend of miniaturization of spacecraft and, consequently, the reduction of payload mass. The paper considers the main classification features of upper stages, including mass, type of propulsion system, design and layout schemes. In particular, the advantages and disadvantages of the above schemes are discussed in terms of layout flexibility, design weight, manufacturability, ease of assembly, and operational reliability. A comparative analysis between them is given for such key parameters as thrust developed by the propulsion system, the mass of the payload to be launched to the target orbit, and the method of propellant supply. The peculiarities of the use of single-component and two-component fuel are also considered, as well as the influence of fuel pair selection on the efficiency and safety of the upper stage. The purpose of this study is to select and justify a scheme for the realization of a small upper stage combining optimal technical characteristics, high reliability, economic efficiency and environmental safety. The key element of scientific novelty is the application of a multi-criteria analysis method to evaluate various designs for small upper stages. This method allows formalizing and balancing conflicting engineering criteria, including mass efficiency, specific impulse, multiple firing capability, technological maturity, etc.

Gimbaled thrust vector control (TVC) systems are widely employed in modern launch vehicles to provide attitude control during powered flight. However, the aerodynamic influence of the circumferential geometric discontinuity introduced by the gimbal joint gap on nozzle internal and external flow remains insufficiently studied in open literature. The purpose of this study is to evaluate aerodynamic losses and flow field modifications of bell-shaped rocket nozzles with the presence of a gimbal joint gap using computational fluid dynamics (CFD) simulation. These calculations were done using the Reynolds-Averaged Navier–Stokes (RANS) model in 2D axisymmetric flow in ANSYS Fluent software. For modelling the flow properties in compressible separated flows, the k-ω SST turbulence model was selected. Two nozzle configurations, with and without the gap of the gimbal joint, were compared. The Sutherland viscosity law and the NASA polynomial thermodynamic data were used for the working fluid and it was modeled as an ideal compressible gas representative of the combustion products of LOX/Kerosene. The exit to throat area expansion ratio for nozzle geometry is 8.97, which means that the nozzle is designed to have a supersonic exit condition. The CFD results showed a good agreement with the theoretical results, showing the validity of the numerical approach. The nozzle studied here operates under overexpanded conditions at sea level, where the nozzle exit static pressure falls below ambient pressure, giving rise to oblique shock waves in the exhaust plume and making the accurate characterisation of flow disturbances particularly important. The presence of the gimbal joint gap was found to introduce aerodynamic losses in Mach number of 6.74%, providing quantitative design-relevant data for gimbaled nozzle systems in launch vehicles.Keywords: gimbal joint gap, bell nozzle, TVC, CFD.

Accurate ballistic trajectory prediction for upper stages on low Earth orbit is challenged by non-stationary atmospheric density driven by solar activity: as density varies, fixed noise covariance matrices in standard Kalman filters become mismatched with actual process statistics, causing degraded estimation accuracy. Non-stationary atmospheric density disrupts fixed-covariance Kalman filters in low Earth orbit, degrading trajectory prediction for upper stages. This paper characterises the quantitative position/velocity accuracy trade-off and performance boundaries of the Sage–Husa Adaptive EKF (AEKF), identifying an empirical sensor-noise approximate crossover region near σ* ≈ 100 m where the AEKF improves velocity accuracy by 38.9% at the cost of a 59% increase in position RMSE.