4.1.1 Mechanical/Thermal Design
The environmental aspect of the design of the MMVS is of great importance; since thermal control of the vehicle creates a comfortable space for the crew and a safe operating temperature for the electrical components. The temperature of the vehicle would be determined by the balance of power absorbed with that radiated to deep space.
The thermal protection system consists of various materials applied externally to the outer structural skin of the MMVS to maintain the skin within acceptable temperatures, primarily during the entry phase of the mission. There are four different types of materials designed to insulate the skin of the vehicle against a wide range of temperatures as low as minus 250 degrees F:
?P Reinforced Carbon-Carbon
?P Low- and High- Temperature Reusable Surface Insulation tiles
?P Felt Reusable Surface Insulation blankets
Other thermal materials used are the filler bar and gap fillers, which seal gaps between tiles and between the tiles and the MMVS structure. The seals protect the aluminum and/or graphite epoxy skin of the MMVS by preventing the influx of hot plasma gas. The gap fillers are envelopes of ceramic fiber cloth stuffed with resilient ceramic filler, and sometimes with a metal foil. The filler bar consists of strips of Nomex felt coated with RTV, and is part of the assembly method used for tiles.
There are two different techniques for thermal control of the vehicle: passive and active. The space segment is mainly designed to use passive means of heating or cooling the ship in order to save on power consumption. The passive elements of the thermal architecture depend primarily on the surface properties of the ship. The two major factors on the overall properties are the emmisivity and absorbtivity. The surface finish of the vehicle directly controls these properties. The Passive thermal control system will consist of thermal MLI blankets, low emissivity paints, thermal insulators and interface fillers. The active system shall be applied to the vehicle components which shall be controlled by flight GPC's. A heat exchanger system capable of either heating or removing excess heat from the inside cabin will be operating during the flight.
The active cooling system will consist of multiple heat pump loops couple with rotating external radiators. The heat pump loops can be run clockwise (see diagram below) to heat the interior using the solar flux absorbed by the radiator (radiator positioned perpendicular to the incident flux). The flow can be reversed and the radiator rotated 90 degrees (parallel with the incident flux) to cool the interior.
The loops are similar in design to the current system on the Space Shuttle. This system uses closed loops of freon. The freon loops run throughout the ship maintaining temperature of the various systems. For example, a cooling loop will run through a computer (causing warming) and then warm a hydraulic system before running back through the external radiators. This method allows the precise temperature control of the various systems in use on the ship by varying the temperature of the internal loops. This is done by controlling the orientation of the radiators/absorbers.
Using this system for the temperature control, the ambient conditions can be either cooled or heated simply by orientation of the radiator and direction of the freon flow. Redundancy of this system comes from the fact that there will be multiple independent systems that can be utilized to take over the responsibilities of a faulty loop, and extra pumps and freon onboard to allow for repairs during the mission. The pumps will have a variable pressure output to vary the pressure in the loops and make sure that the freon is being condensed and vaporized at the various points in the loop. The vapour must condense at the cold heat exchanger and vaporize at the warm heat exchanger. There will be an alternate method of pressurization in the system consisting of a bellows pressurized with nitrogen. This will act to maintain the pressure of the system while the loops are reversing and prevent the pumps from getting gaseous freon. These will be automatic processes controlled by the climate control system.
The Limitations for the cooling system are that it is very dependent on electrical energy and the control systems regulating the temperature. Should either of these systems fail, the climate inside the ship could become critically high or low causing a possible mission failure. To prevent this, redundant cooling/heating systems are in use allowing for the loss of a system. The electrical redundancy is discussed later. The system can also be run in a manual mode where the orientation of the external heat exchanger can be controlled by the crew allowing for emergency temperature control.
Redundancies will be built in to ensure the safety of the occupants throughout the flight. Air quality within the capsule must be monitored continuously for oxygen. The pressure will be maintained using pneumatically sealed airlocks and pressure bulkheads where necessary.