In essence, the report simply identifies the main component sections of the life support system and the processing capacities required for each.
As a technical document it seems poorly laid out, with the information poorly presented. There is significant repetition, probably constituting in all around 5 pages of the report, if not more. A summary of the main specifications and a glossary of terms would be useful, and perhaps might be expected. Despite the list of references, only one is referenced explicitly within the report.
The functional layout diagram (Figure 4) is open to criticism. It is not obvious what the boxes around each element of the system represent – this might be expected to represent the envelope of each sub system package, to denote which components were in which system. It is expected that these systems would be modular in construction, with each system being a physically separate assembly. The boxes may be meant to denote this.
The sub-systems will be inside a single capsule, and presumably will therefore share a common space, but one which is connected to but nevertheless separate from, the main living spaces.
There is no distinction between the volume external to the sub-systems and the external Mars environment. Apparent references to external storage are taken to indicate storage external to the given system, but internal to the capsule containing the assemblies. The various vents are simply shown as exhausting external to each sub-system boundary. We are left to presume that venting (unless otherwise stated) means venting to the external Mars environment. Two vents are shown within the boxed area from which they originate, with the label ‘vent to cabin’. Presumably the ECLSS is in a controlled environment in the capsule which is contiguous to the habitat environment.
It can only be surmised from the drawing that the references to internal storage indicate that they are physically internal to the assembly module, and hence an integral part of the assembly, rather than a separate vessel, external to the assembly but still internal to the controlled environment. It would have been far better to show the envelope of the internal capsule space distinct from the external Mars environment.
A route is provided for direct supply of oxygen to the habitat that bypasses the air treatment system and is not mixed with the recycled air. The drawing also shows pure oxygen passing between the two habitat sections, with the implication that there is an oxygen distribution system. Pure oxygen is extremely dangerous. It has to be questioned as to whether this is a sound concept.
No route or mechanism is shown for disposal of dust collected in the inlet air filter. The description only refers to filtration – this may well be inadequate and could impose unnecessary pressure drop. Use of electrostatic precipitation might be considered.
The phraseology used in some places in the report is worth noting. For example, the report says that the processes needed to extract water from the Mars regolith and get nitrogen and argon from the atmosphere are well known. An arguably more accurate assessment would be that there are a variety of well known physical processes that may be suitable for extracting water from Mars regolith and extracting nitrogen and argon from the Mars atmosphere.
In a similar vein, we have the statement that the purity of the extracted water cannot be defined 'with absolute certainty’. This implies that the purity to be expected is already quite well characterized. Would it not be somewhat more accurate to say that data is available on the likely composition of regolith derived water, from, for example the Curiosity program.
Despite the statement on water composition, the report contains no information on the expected composition of the regolith derived water, nor any indication of the possible range of composition that might be expected.
The extraction technique indicated assumes that the water is available as ice and is not chemically bound, allowing the use of only modest heating. There is no information on the temperature required, but it may be in the range 50 -100 C at a guess. Contaminants in the water will depend both on the regolith composition, which is likely to vary depending on the site, and the degree of heating used. The report quotes a working figure of 5% w/w of water ice in the regolith.
The Mars regolith is thought to contain significant quantities of perchlorate, perhaps up to 1% by weight, such that the regolith may well pose a hazard to human health. The presence of an active chlorine compound may be a source for chlorine ingress into the air treatment system. Along with sulphur compounds chlorine may be a poison for the catalysts used in the ECLSS.
Figure 4 shows a single ECLSS unit servicing two habitat volumes. There will be two ECLSS units initially, each associated with a capsule living unit and its attached inflatable (500 m3) living area. We are left to assume that the habitat volumes shown in the diagram are meant to represent a crew capsule and a single inflatable.
The study is based on a single ECLSS having the capacity to service the complete habitat (i.e. all four living units) when operating at a reduced load. This seems to imply that there must be a facility to cross connect at least the air circulation systems to provide air recirculation through the entire system (i.e. all four living areas) if one ECLSS is offline. Nothing along these lines is indicated. It would seem reasonable to provide a method of cross connection for the water systems and possibly the waste systems as well, in addition to the electrical distribution system. If this is not provided, then any failure of an ECLSS component system could force the occupants to temporarily abandon one half of the living system.
There is little or no discussion of what technologies are or may be available for each of the functional areas. It is noticeable that the key parts of the air and water quality systems described appear to be copied from those used on the ISS, and even though simplified schematics of these systems are available from the literature, there are no diagrams showing the systems in more detail. We are treated to a half page description of the CO2 removal system, but no diagram.
The water treatment system uses both activated carbon and ion exchange resins. Activated carbon can be regenerated but the severity of the necessary treatment will depend on what has been absorbed. Ion exchange resins are generally regenerated by treatment with the appropriate chemicals. Since no mention is made of regenerating either material, we are left to assume that regeneration is not envisaged. If so, what life has been assumed?
A similar comment applies to the activated carbon used in the air treatment assembly
An ‘additional purification’ step for the water feed to the electrolysis units is mentioned, but not described. It would appear to be another adsorbent bed. The basic technology for water electrolysis is not described. We are only told that the system will be operated at ‘moderate pressure’, 1000 psia / 69 bara.
Commercially there are two basic types of electrolyzers in use, alkaline systems which use an aqueous solution of KOH, and Polymer Electrolyte Membrane (PEM) units. In a PEM unit, the conduction medium is a solid membrane polymer and precious metals are used in the electrodes due to the acidic environment. These units require very pure water and the product gas pressure is up to around 25-30 bara. In this context 69 bara may not be viewed as particularly ‘moderate’. What experience is there with units operating at 69 bara?
We can infer that Paragon may be assuming the use of PEM units.
The description of the thermal control system gives no hint of what the circulation fluid is. The suggestion of the possible use of buried pipes to dissipate the heat seems to be an after thought – the system would have to be constructed before the ECLSS could be started up.
In the section on in-situ water production is the statement that the evaporation process would ideally be driven by waste heat; it is suggested that the heat from the gas compression process could be used. In part quite possibly, with the compressor heat used as and when available, and if it is of sufficient temperature to heat the regolith to the temperature required. The nature of the two systems and their likely operating pattern means that such heat integration requires more thought than shown in the report. The regolith system as described is a batch process, the compressor would be running only as demanded to generate make-up inert gas. At the very least an electric heating element is required to provide heat to the evaporator if and when the temperature of the TCS from the compressor is insufficient.
Mars air contains low levels of water vapour, at perhaps 0.005 Pa to 0.4 Pa, so possibly as high as 400 ppm in the air intake. Low levels, but has it been considered when proposing the use of cryogenic freezing to separate out CO2?
Paragon seem a bit confused as to whether the in-situ processing module is the ISRPS ( In-Situ Resource Processing System) or the ISRU ( In-Situ Resource Unit?), or even the ISRUS. (Providing a list of acronyms would probably have helped.)
The wet waste processing system is described only in the vaguest of terms, despite its critical role in determining the habitat water balance. According to Table 5, more than half the potable water usage will end up in this recovery system, with some 97% of it being recovered.
The system is described as being ‘passive’, but reference is made in the brief description to heat input. No source of heat is identified and none is shown in Figure 4. Whatever the source and means of getting heat into the mixture it will have to done efficiently and with some means of control. If it does not work efficiently then they may well find they need a bigger tank.
The description refers to the ‘residual wet waste (sludge)’ being stored. There is no description or indication in Figure 4 of how the sludge is to be removed from the ‘wet waste storage’ (where the water is recovered) to the ‘wet waste residual solids storage’.
Table 10 gives a power usage of 1.5 kW for the wet waste processing, which is 10% of the averaged power usage. What is this power used for? Agitation? Sludge handling?
While we are looking at water recycling, it would be useful to have some explanation for the assumptions in Table 5 on the % of recycling for each potable water usage. What is the basis for assuming the wet waste processing system can recover 97% of the water input to it? Why is the ‘recovery’ figure for the water used in the electrolysis section to provide make up oxygen for metabolic requirements set at 75%? If this is supposed to represent the water generation due to oxidation of foodstuffs (in the crew’s bodies) by the equivalent amount of oxygen then a figure of around 50% would seem more appropriate.
The mixture of urine, faecal matter, food waste and gray water will probably have to be sterilized either by heat or by chemicals, otherwise gases such as methane, ammonia, CO2 and H2S could and probably will form, loading the returning air flow to the ARS with levels of these gases. There is no vent shown on the waste tank. Concentrating up the liquid wastes in the tank risks precipitating salts. It is not difficult to imagine that the membrane would get choked with solids. As noted previously the description refers to a wet sludge tank, with the implication that partially dewatered solids are separated or removed from the waste tank.
The report states that this concept has been put forward to avoid introducing the complications arising from direct urine and faecal matter processing. The report has given the appearance of reducing this possibly complex area to nothing more than a tank and a membrane.
As is practiced on the ISS, the astronauts may well have to adopt a diet , high in protein and low in fibre, to minimize production of faecal matter along with rigid rationing and control of food consumption.
The choice to mix these wastes is surely a mistaken policy. Separation of urine and gray water from faecal matter and other solids opens up the way for efficient water recovery. The residual concentrated liquids can then be stored with the solid wastes. Even if the same concept of a tank and a simple humidity exchange membrane is retained, it must be easier and more robust to apply this to liquid rather than mixed waste.
It could of course be argued that details such as possible heat integration, consideration of other technologies, and fleshing out of systems such as the waste water recovery are all part of the next step of development. But it is hard to dispel the feeling that the system concept has been more or less thrown together, based simply on existing technical solutions from the ISS or vague concepts such as the wet waste processing system.
Does this report constitute ‘an initial conceptual design study’? How long is a piece of string? In this case it seems rather short. What the report has delivered seems to fall short of the original description, both in terms the scope and the level of detail provided. Perhaps there are other documents provided to Mars One from Paragon, but we can only judge on what we see.
What Might Have Been Expected?
- a summary of the pre- and post- crew arrival system requirements
- an initial specification for all the input and interconnecting streams, including size specification for regolith
- a glossary of terms
- basic block flowsheets for each subsystem
- a brief review of possible technologies for each system
- breakdown in ISRU mass/volume/energy usage between air compression and regolith processing (why these systems are lumped together is a bit puzzling – they are two distinct and non-related processes)
- basis for assumptions implicit or explicitly stated in the study (e.g. efficiency for air compressor and air unit fans, specific energy consumption for water electrolysis, percentage of electrical power dissipated as heat, etc)
- an indicative typical system mass and energy balance
That assumptions may be nominal figures should not preclude disclosing them - the report does for instance display the assumed power efficiency for cryogenic cooling of the Mars air.
Bedrock - Valles Marineris [credit : NASA]
M1102
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