The temperature control system seems to be predicated on the assumption that there will always be excess heat within the habitat, so that control devolves to a system for rejecting heat to the external environment. The only method for temperature control within the habitat is via circulation of air. There seems to be no assessment of heat loss from the overall system.
It is noted that the ISRPS units generate some 50% of the heat, but only operate for part of the 24 hour cycle (the day length on Mars, which just happens to be more or less the same as that on Earth). The other big heat generator is the atmosphere management system. If the circulation fans fail then there is no means of getting heat into the habitat and the air quality would fall, unless the air circulation systems are cross connected so that either ECLSS can circulate air through the entire habitat.
In terms of technologies, it is possibly the case that the project is somewhat fixated on what it perceives to be a set of problems in aerospace technologies. Whilst the aerospace industry will clearly have to provide the means of transport and landing, the landed systems are not operating in space, but on a planet, albeit with lower gravity and a very thin atmosphere. The required technologies and knowhow for life support solutions could well lie more in conventional chemical process industries than in aerospace. Mars One and Paragon need to look outside the box labelled ‘aerospace’.
The CO2 removal system in the air treatment system is a case in point. In zero gravity a system that relies only on fixed beds of absorbents and the flow of gas is to be preferred. In the presence of gravity the use of liquid absorbents could well be more appropriate. Small, modularized liquid based CO2 removal systems are used on submarines.
Liquid adsorption might also be appropriate for the production of nitrogen and argon from the Mars air. There may be opportunities to utilize waste heat within the CO2 removal system and there could be significant power savings over the compression system envisaged. Compressing gas simply to let it down through a valve is simple but inherently inefficient.
There are also ways of intensifying processes by the application of centrifugal force. A classic example is the HiGee system and the ‘spinning mop’ developed by the Process Intensification Group in ICI in the mid 1970’s. These types of device can be used to replace gravity separation of liquids and to intensify liquid/vapour contacting processes.
The report states that pressure relief is provided to prevent the habitat pressure exceeding 70.3 kPa, which is also the upper limit defined for pressure control within the habitat. At face value this implies that there is no margin between the permissible upper control pressure and the required relief pressure; this would be unusual in most systems. The rationale states that this pressure limit is to ‘prevent pressure induced structural loads from exceeding design limits’. Figure 4 shows a vent from one of the two habitat modules to (it has to be presumed) the external atmosphere. It is noted that venting from the habitat is ruled out as a means of fire suppression, yet is used as a means of limiting the maximum pressure.
There may well be a trade off between design pressure and weight of construction, but the issue is probably complicated by the differences between the capsule construction and that of the inflatable additional habitat space. It is not stated in the report, but it is likely that the design pressure limit is fixed by the nature of the inflatable section.
Having a relief vent system presents the risk of leakage, inadvertent opening, and failure to reseat, and requires maintenance and testing. It also poses a problem of determining the required relief rate, how many valves are required and where they should be positioned. The only obvious source of excess pressure within the system is from one of the gas storage vessels, due to either a leak or a failure of the pressure control system. It would be far better to design out the need for pressure relief than rely on a pressure vent valve.
The provision of pure oxygen storage within the habitat poses a potential risk should the vessel fail (excess oxygen levels). It might be feasible to position gas pressure vessels external to the habitat, possibly with any internal accumulators limited in volume. An alternative might to house all the vessels in a dedicated space, separate from the rest of the ECLSS system and the living areas, and which can be isolated in an emergency.
Hydrogen leakage from the high pressure electrolyzers poses another risk. High pressure hydrogen fires very easily, and burns with no visible flame. This means that little infra red radiation is emitted so there is little sensation of heat despite the high temperature. It is possible to walk into a hydrogen flame without realizing it is there. Optical sensors require specific tuning to match the UV and IR bands emitted by hydrogen flames.
M1104
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