Under the guidelines established by the European Space Agency (ESA), a specific effort was devoted to define the preliminary design concepts for a Crew Transport Vehicle (CTV) compatible with the Ariane 5 launcher. The mission objectives of this vehicle include the possibility of transporting 4 people (and a limited amount of pressurized payload) to the International Space Station Alpha (ISSA), and returning them to Earth safely.
Different options were identified at system level, however a modular vehicle was commonly adopted:
a Crew Module (CM) designed to withstand the typical phases of the atmospheric re-entry and provide an adequate environment for the crew during all the mission
a Resource Module (RM) envisaged to provide the propulsion provisions for orbital transfer and deorbiting; in addition it carries all the necessary resources to support the mission from lift-off until separation from the CM.
The Thermal Control for such a vehicle has to cope with a wide range of environmental conditions and operational modes. In particular, the ascent and re-entry phases imply the presence of significant aerothermal fluxes on the external surfaces, whereas the typical space environment of low Earth orbit may induce severe cold boundary conditions for the design. The Thermal Architecture of the CTV relies on the combined action of three different subsystems.
The Thermal Protection subsystem is mainly based on external ablative insulation, which limits the temperature excursion of the primary structure during the re-entry phases (below 120 km altitude) until touch-down and post-landing. The Air Conditioning Section (part of the Environmental Control and Life Support Subsystem) is designed to provide comfortable thermal conditions for the crew inside the pressurized compartment, collecting heat loads due to external environment, to spacionics or to crew metabolism.
These loads are transferred from the air loop to the water coolant loops of the Active Thermal Control Section (part of the Thermal Control Subsystem) which also collect the waste heat received from the electronic units mounted on dedicated cold plates. In orbital conditions the resulting thermal power is then transported by a low freezing temperature coolant fluid to a set of space radiators located on the Resource Module and finally rejected to space. Other types of heat sink devices (based on expendable fluids) can be specifically operated during the various mission phases.
The Passive Thermal Control Section is a major influence in the global design architecture:
- thermal insulations in the Crew Module have to be sized to prevent excessive temperatures on the crew cabin walls or on the equipment of the unpressurized compartment during reentry in the various regions of the vehicle (mainly in the Resource Module) the low temperature insulations are to be carefully distributed and trimmed to minimize the need for heaters during the orbital phases (the Crew Transport Vehicle may stay docked at the station in dormant mode for a duration of 6 months).
The major design features of the CTV thermal control are highlighted, taking into account the spacionic architecture (mainly the Data Management and the Electrical Power Supply), the mission timeline, and the constraints related to the failure tolerance criteria adopted at System level.