Robust Transient Control of Reusable Liquid-Propellant Rocket Engines
Résumé
The current trend towards a more affordable access to space is generally materialising in reusable launchers and engines. From the control perspective, these reusable liquid-propellant rocket engines (LPRE) imply more demanding robustness requirements than expendable ones, mainly because of their multi-restart and thrust-modulation capabilities. Classically, the control system handles LPRE operation at a finite set of predefined points. That approach reduces their throttability domain to a restricted interval in which they are designed to be safe in nominal conditions. Moreover, the operation of their transient phases, which have a great impact on the duration of engine life, is not robust to the possible engine evolution. Hence, the goal of this work is to develop a control loop which is adapted to the whole set of operating phases, transient and steady-state, and which is robust to internal parametric variations. Several blocks have been assembled to constitute the control loop: engine simulation, reference generation and several controllers. First, simulators representative of the gas-generator-cycle (GG) Vulcain 1 and PROMETHEUS engines were built. The purely thermodynamic modelling of the cycle was subsequently adapted to the control framework, obtaining a nonlinear state-space model. The available actuators are continuously controllable valves, binary igniters and binary starters. These actuators are related to discrete events in transient phases. Regarding the start-up operation, the igniter, starter and valves are activated during the first seconds. Up from the end of those activations, the whole system behaves in a fully continuous way. Hence, a different control strategy is proposed for each sub-phase. For the first and discrete sub-phase, a discrete optimisation of events timing is proposed, in which the time differences between events are adapted according to operation criteria and constraints. This trajectory planning, still under implementation, is to be performed off-line. The subsequent continuous sub-phase is feedback controlled to track pre-computed reference trajectories. Apart from the start-up, throttling scenarios also present a dedicated end-state-tracking algorithm. A model-based control method, Model Predictive Control, has been applied in a linearised manner with robustness guarantees to all these scenarios, in which a set of hard state and control constraints must be respected. Tracking of pressure (thrust) and mixture-ratio operating points within the design envelope is achieved in simulation along the continuous sub-phase while respecting constraints. Robustness to variations of the parameters, which are checked to be predominant according to analyses, is also demonstrated.
Origine : Fichiers produits par l'(les) auteur(s)
Loading...