4.3 Pressurized areas
Pressurized areas will include:
- private life areas (lodging);
- public living areas (socializing, meeting, entertainment, shopping places);
- administrative and private offices, laboratories;
- medical facilities;
- production workshops for parts requiring the presence of man;
- maintenance/preparation workshops for orbital shuttles and planetary vehicles (jeeps, pressurized rovers);
- preparation rooms for activities outside the base (EVA), and locks;
- food greenhouses (covered in the next section).
For its grand artificial orbital cities, Gerard O’Neill considered a population density of 200 inhabitants per hectare (1 hectare = 2.47 acres), i.e. 50 sqm floor (538 sqf)/inhabitant. This is the density in central Paris. It seems that this value may be kept for a space colony, although its population would not be comparable to that of a metropolis. Here, it would give: 10 hectares.
Naturally, this individual need for floor space is not all that is to be considered. First of all, the value of 50 m² per inhabitant is to be considered as an average. Just like our city dwellers and perhaps even more, settlers will need open spaces where, during their relaxation periods, they may experience different living environments: patios, gardens, fountains, common public entertainment, shopping or meeting areas. All of this, certainly constrained by the limited size of the living modules and the limited floor space available, but laid out the most cleverly possible in order to provide variety, pleasure, soothing feeling, sense of space, proximity to Nature, terrestrial (plants, aquariums) and Martian (views over the landscape and the sky).
4.4 Nutritional requirements
Feeding the settlers might prove to be one of the most difficult tasks to be carried over in total isolation, not only because of the food tonnage to produce, but also because of the nutritional requirements of the human organism; we should absolutely avoid developing some kind of « space scurvy” resulting from misidentified or poorly covered deficiencies.
With a fairly good consensus among the authors, the need, in terms of mass is assessed to be 1.8 kg / day/person, including 0.6 to 0.7 kg dry mass.
Therefore, for the total population, this represents: 3.6 mT per day (sol) of food in a natural state (not lyophilized). The question might be asked whether a purely vegetarian diet would be sufficient and desirable. The answer would be that it appears possible to offer a wider range, in particular on the basis of fish farming (see §5.4).
4.5 Fluids consumption
For the settlers
On this matter, we also can get estimates made for space crew consumption. We may note:
- potable water: 1.7 l for drinking, 0.8 l for food preparation i.e. a total of 2.5 l/day/person;
- domestic water: 20 l/day/person, although experiments, conducted in particular by the Mars Society in its Arctic simulation base, showed that it was possible to limit consumption down to 4 or 5 liters; however what is possible for a space mission of limited duration is probably not desirable for the comfort of settlers on the long run;
- breathable oxygen: 0.8 kg/day/person.
- In order to get total needs, these daily consumptions must be corrected by the recycling rates:
- for water, we assume an average recycling rate of 90%, which will certainly be achievable at that time , and which will allow limiting the additional daily need down to 4.5 mT/d (for a total flow of 45 mT/d );
- for oxygen, with a recycling rate of 80%, we can go down to 0.3 T/day.
In terms of breathable air (a mixture of 240 hPa O2 and 100 hPa N2), needs come from:
- compensation of leaks of living modules and their interconnections; we’ll see (§5.3) that they could represent a volume of about 300,000 m3, excluding greenhouses, i.e. 100 mT at a pressure of 340 hPa; even a 5% leak rate per year (leaks observed on Biosphere 2 were 3 %/year) would represent an annual oxygen and nitrogen replacement production of a few metric tons only, therefore commensurate with the needs of the building period and, regarding oxygen, completely different of the needs for propulsion (rocket and planetary vehicles);
- losses related to airlocks operations to allow (outdoors) Extra Vehicular Activities (“EVA”): it is likely that, in order to save oxygen and – above all – nitrogen, settlers will avoid depressurizing the airlock directly outside; pumps will be in charge of reducing the pressure level by at least a factor 10 (34 hPa), but probably not down to the external pressure (7 hPa) in order to limit the pump size and the time needed for the operation. Individual EVA (wearing spacesuit) will be relatively rare and the corresponding locks, of small dimensions. Pressurized vehicles will be accessed via ducts, without going through airlock. Ultimately, the largely most significant depressurization / repressurization operations could be those in relation with exit and entry of space vehicles into / out of the maintenance and preparation halls. A rapid assessment allows nevertheless estimating that, even with the dumping of a residue of 10% (34 hPa) in the open air, consumption will not reach the magnitude considered for leakage losses.
The greenhouse atmosphere will mainly be made of nitrogen and oxygen which, on account of photosynthesis, would not have to be replenished by refills or carbon dioxide. Surface, volume and leakage rate of the greenhouse modules being of the same order of magnitude as those of inhabited modules, the consumption of nitrogen will be very limited.
As for crops irrigation, we may assume it will be covered by settlers’ “gray water” (water would follow an overall loop of human use => treatment for irrigation => agricultural use => finishing treatment for human use => re-use).
Indeed, the daily debit for human needs, estimated to be 45,000 liters, corresponds, for a total daily production of food (raw state) of 1.8 kg x 2,000, to a water input of 12.5 l/kg of foodstuffs (which corresponds, for a total cultivated area estimated to be 6 ha (cf. §5.4), to an watering of 0.75 liter/m² culture/day).
In terms of needs for the hydroponic cultivation of tomatoes in heated greenhouse10 and within a closed loop, literature indicates11an input possibly reduced to 15 l/kg. Knowing that tomatoes have a rather average irrigation yield (1,000 l/kg of dry matter in the field, versus 500 for potatoes), we will take as input required, a mean value between these two crops, i.e. a need a little over 11 l/kg, in fair relation with the capacity of 12.5 l/kg considered (a loss of 10% is still allowed).
For propulsion (Spatial and planetary)
From the traffic assumptions used in §4.1 and the way to manage it, by means of orbital shuttles (see §5.1.1), between Mars orbit and the surface of the planet, we may estimate the need for corresponding propellant, by making a rough sizing assessment of these vessels. With, for the cargo version, a payload of 50 mT descending to the surface and of 10 mT ascending from it, we get the following need per cycle: 50 mT of propellant (H2/O2) ascending, 17.5 mT of propellant (CH4/O2) descending12. On the basis of 25 flights per year (see §5.1.1), and taking a flat margin rate of 10% for losses in fueling operations (33% for H2), we get a need per (terrestrial) year13 for space flights of (rounded figures):
- 220 mT of liquid hydrogen
- 100 mT of liquid methane
- 1,400 mT of liquid oxygen
- 2,100 mT of liquid methane
- 8,400 mT of liquid oxygen
- carbon dioxide: it is the easiest resource to tap since it makes 93% of the atmosphere;
- hydrogen: for the production of plastics (e.g. polyethylene).
10 Functioning with an optimal humidity rate of 70 %, which lowers the transpiring of plants and therefore their need for water.
11 Document of « Centre technique interprofessionnel des Fruits et Légumes » (CtiFL), September 2005
12 The choice of methane for descent, rather than hydrogen, would be allowed by the low ΔV needed for deorbiting, final braking et final approach (~800 m/s); it would ease the managing of propellants during the orbital phase of the mission.
13 The synodical revolution being 26 months, the need for one year is 12/26th of the need for one revolution.
Planetary transport vehicles (mostly ten pressurized rovers and a few dozen jeeps, see 5.1.2) and, more than anything else, mining heavy engines used in remote sites (drilling machines, scrapers, mechanical shovels, trucks) will consume electric energy supplied by fuel cells (or combustion engine generators) operating with the CH4/O2 couple (methanol may be preferred). It is more difficult to assess the consumption that this will represent. Based on the assessments of §5.1.2 with regards to the fleet of vehicles, we would need:
|Type||Unit Power (HP)||Active Nb/Site||Nb Sites||Installed Power (HP)||Employed14 Rate||Power Consumption (HP)|
|Heavy eq’pm’t Vehicles||500||1||4||2000||25%||500|
|Total Power (HP)||1100|
Assuming that the engines (internal combustion or electric, fed by methane/oxygen power cells) function with an overall yield of 25% and taking as CH4 combustion heat output 48 MJ/kg of methane, we may deduce a need by sol of 5.84 mT of liquid methane and 23.38 mT of liquid oxygen.
Or, for each terrestrial year, about:
We see that the global motorization needs, resulting mainly from the heavy equipment needed for construction building and mining, largely dominate the needs for astronautic traffic. Naturally, these tonnages are very dependent upon the level of exploitation of resources and upon the above estimates of average power consumed by the corresponding engines. But, in any event, it is a key item in terms of capacity to be provided.
Note: methane being obtained from carbon dioxide and hydrogen through the Sabatier process (see §5.5.4), in a ratio of 1 kg of hydrogen for 4 kg of methane, the total of 2,200 mT requires, from the beginning, an additional production of hydrogen of 550 mT/year.
Besides propellant production, the two fluids mentioned hereafter will be mainly used for the production of plastics and steelmaking:
Since we assume that productions will be limited to the needs required for the settlement in a construction phase (no export), the quantities of hydrogen to produce, difficult to assess, remain low compared to what the synthesis of methane for motorization will require.
14 Average functioning power over one day, relatively to one unit power (maximum).
4.6 Recap of main needs
It is obvious that in the absence of detailed research on the functioning mode of the colony and the design of its equipment, the above need assessments remain very hazy and, for many of them, highly dependent on questionable assumptions. However, they allow visualizing an order of magnitude of means that would have to be implemented to allow the functioning of a colony of 2,000 residents as considered here.
These requirements are as follows:
|Interplanetary passenger traffic||270||pax / synodical revolution (26 months)|
|Interplanetary cargo traffic||600||mT / synodical revolution|
|Electric Power, installed||160||MW (artificial lighting for greenhouses)|
|Inhabited areas surface||10||ha|
|Food||3.6||mT / sol (ready to be eaten)|
|Water (inhabitants and greenhouses)||4.5||mT / sol (after 90 % recycling)|
|Water (for H2/O2 production): from||19.5||mT / sol (6,950 T/year)|
|to||31.6||mT / sol (1150 T/year)*|
|Oxygen (mostly liquid)||8||mT / sol (10 000 T/year)|
|Hydrogen||2.2||mT / sol (770 T/year)|
|Methane||6.2||mT / sol (220 T/year)|
* according to process chosen: oxygen can be obtained by electrolysis and/or thermic breaking up of atmospheric carbon dioxide (cf. §5.3).