Crew Weight Allotments and recycling.

Their is a general assumption that everything, or at least the food, needs to be recycled. That of course assumes the material can be recycled. (How exactly do you recycle sock lint back into a sock?) But avoiding that. Do we need to? Or the more basic. What will the crew carry?

The "Space Settlements: A design study" (the book I reference frequently, and STRONGLY recommend) gives detailed mass flows per person, per day for a complete closed ecology. Just to give the bulk totals.

The per person mass allotments.

Tons
Soil (Dry)
22
  721 kg/m^2 .3 m deep
Water in soil 10%
2.2
Biomass
  (animals/plants)
.15
Structures
7.7
Substructures
1.5
Furniture
2.0
Machinery
4
Utilities
2.9
Other
8.0
Total
53.0

This means that about half the per person allotment of mass is for the farm, not the person. It also lists the specific life support per person as

In
O2
0.686 kg
H2O
0.4 kg
Food
2.564
Out
CO2
0.857  kg
H2O
0.857  kg
Food
1.936  kg

Note: that this only lists water consumed, not used for washing or other tasks. David Levine mentioned that Harry Benford's "General Cargo Ship Economics and Design" (UM Report, Aug. 1962) recommends:

170 kg/person for mass of crew + their effects 170 kg/person/day for water (45 gallons per person per day) 10 kg/person/day for provisions And that the U.S. Navy goes with slightly different numbers: 180 kg/person for officers and their effects 104 kg/person for enlisted personnel 151 kg/person/day for water 3.6 kg/person/day for provisions and stores.

I'm assuming the much higher allotments for water include washing and other nonconsumption uses. The Space Settlements book mentioned 200kg of water per person being recycled for all uses including the farms. (Also the mass allotment for personal effects is to low for a mulitiyear journey.)

The first thing that hits me is half the total per-person mass allotment in the closed ecology (24,300 kg) is the farm. (More if you count the mass of the area of the ship that holds the farm.) Yet people only eat about 2.5 - 3.1 kg of food per day. The overall numbers NASA lists (see NASA Web page) are:

Inputs:
Ä 3.1 kg of food (dry weight)
Ä 15-20 kg of water (including drinking water and the water content of food)
Ä 4.4 kg of oxygen

Outputs
Ä 7.3 kg of urine
Ä 8.8 kg of "metabolic water" -- exhaled in breathing and evaporated from the skin
Ä 4.8 kg of carbon dioxide
Ä 0.88 kg of solid waste

From Humans in Space by Harry L. Shipman.

2.5 - 4.6 kg of food per day. Over a 25 year mission (9125 days), that comes to about 22,800 - 41,750 kg of stored food. (Less if we use dehydrated food.) Far from saving weight, for the length of this mission recycling food could add weight. Just the opposite of what people normally assume.

The farms discussed in the "Space Settlements: A design study" lists the farm mass at about 36 tons per person, of which 22 tons is soil. The farm includes everything from farm animals to fish, and all normal grains and vegetables. The idea was to make it capable of providing all normal food needed for a standard North American diet for a population of 10,000. Said diet according to their numbers weighed about 1.67 tons per-person per year. (Which seemed a bit heavy to me.) But to get this farm mass to person ration they had to assumed that with intensive care their farms could produce twice the yield of the best farms on earth.

Anyway, 36 tons per person is about 21 years of food mass at their 1.67 tons per year, or 32 food years at NASA's 3.1 kilo per day. I'm not even going to bother with freeze dried numbers. We won't want to be out that long! Then I realized that the farm design required doubling the internal volume of the habitation centrifuge. Which would add another 20 to 230 tons per person! (the latter if you shield the farm centrifuge from ambient radiation.)

So every way I ran it, the mass for a transportable, self sustaining farm, wound up greater than the stored food mass for the duration of our projected missions. Given that the stored mass would decline as the mission goes on (a good thing for the return flight), stored food would be simpler and more reliable than trying to maintain a running farm during a mission, and the farm would almost double the size of the full g gravitation sections needed in the ship. I decided to dump the idea and assume ultra frozen and dried foods stored in the zero g section of the ship. We could have a couple gardens for fun and fresh Veggies, but I'd assume they were just a couple plants in the corner of peoples apartments. You might do an analysis to see if hydroponics for the veggies would weigh less than storing frozen veggies. I.E. we store the meat, flour, rice, milk, ect.., but grow the fruits and vegetables. But for my purposes I assumed the mass numbers wouldn't show an advantage.

Then I realized that we've never built a closed ecology that worked. (Biosphere II was the only large scale attempt, and it would have suffocated its 'crew' in a couple of months.) On the other have we do have most of a half century of field experience with artificial life support systems. To be precise, nuclear submarines. They synthesize air and water from sea water. We could bring large stockpiles of water on the ship, but its easier to just distill the water and electrolysize the air or pump it through algae tanks. The big advantage of this is not only is ship lighter at the start. It gets lighter as time goes on. You'll presumably eat half the food by the time you get to the target star system. This consumed food can be stored and burned as extra deceleration reaction mass. The rest of the rest burned to decelerate you as you reenter Sol.

A farm of course offers a potential for open ended life support and could add a safety factor if the ship is crippled in the target star system. But an emergency supply of freeze dried food for a couple more decades would not dramatically increase the food weight allotment. On the other hand warehouses are far easier to operate and more reliable than farms, and they don't need full shielding. Just as a guess we probably can assume that most other materials (cloths, furniture, etc..) would be easier to replace from carried stockpiles than to recycle.

Oh, while on the topic of Mass. The drive system people seem to be going through hoops to build a huge, high efficiency (relativistic exhaust) engine to keep the necessary reaction mass amounts down to grams per day. I would suggest that if we aren't going to recycle our -- ah-- food by products. The crew will be providing a few tons of usable mass per day. Dehydrate, incinerate to plasma or ionize, and pump it into the accelerator. With an electromagnetic accelerator (as apposed to a thermal rocket) the type of mass used is unimportant, and for ship design purposes using the same stored mass to feed the crew and drive system is very elegant and efficient.


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