Method of determining probabilistic operability requirements for a system and its component subsystems

A design method for designing a rocket engine or a space vehicle propulsion system, and its component subsystems, the method comprising:

• a) a determination stage for determining probabilistic operability requirements for said engine or for said system for nominal operating conditions in flight, this determination stage comprising:
• an obtaining step for obtaining a population of operating points of the rocket engine or of the space vehicle propulsion system including at least two subsystems selected from an oxygen turbopump, a hydrogen turbopump, a gas generator, valves, and a propulsion chamber of the rocket engine;
• followed by a construction step for constructing a plurality of predefined limit domains of said space, each representing a different subsystem, each limit domain being constructed as encompassing, around a reference point, a proportion of said population defined in the plane of the subsystem under consideration, corresponding to a projection of the operating points obtained at system level onto the plane of the subsystem under consideration, these limit domains representing observable operating conditions of the rocket engine or of the space vehicle propulsion system when in operation, resulting from various sources of dispersion;
• b) a determination stage for determining probabilistic operability requirements for said engine or said system in order to qualify them, followed, for each subsystem, by a definition step for defining qualifying domains by counting points of the population lying outside a given limit, wherein said qualifying domains introduce relative to the limit domains:
• qualifying directions in which the main modes of failure are critical;
• and
• functional limitations for the subsystems constituted by criteria that must not be exceeded during a stage of development or of production;
• under pain of harming the functional or mechanical integrity of the subsystem in question, or of the rocket engine, or of the space vehicle propulsion system itself, these criteria serving to quantify margins for the subsystems relative to their modes of failure;
• c) an adaptation stage for adapting the domains of said subsystems as a function of the result of the counting for each subsystem and as a function of an overall reliability target defined for the rocket engine or for the space vehicle propulsion system, e.g. the proportion of said defined population for the rocket engine or for the space vehicle propulsion system;
• the associated criteria defined for said subsystem that need to be complied with during a stage of development or a stage of producing said subsystems.