Human astronauts are such a bother when it comes to space exploration. The space environment is pretty much the opposite of the conditions that humans evolved for, to the point where an unprotected human exposed to space will die horribly in about ninety seconds flat. Even given oxygen to breath, the human organism is quite insistent on a whole host of demands: food, water, comfortable temperature, gravity, absence of deadly radiation; the list goes on and on.
This is why NASA is so fond of robot space probes. Not only do robots not have any of those requirements, but there also is no problem with sending the probes on suicide one-way missions. Human astronauts would tend to complain about that.
But given organic non-cyborg astronauts, your spacecraft design is going to need a habitat module for the crew to live in, and all sorts of supplies to keep them alive. Since Every Gram Counts, it will be important to use every trick in the book to try and miminize the mass cost of all this.
A useful accounting device for consumables is the "man-day" or "person-day".
If your ship has 30 person-days of food and oxygen, it can support:
- 30 persons for 1 day (30 / 30 = 1)
- 15 persons for 2 days (30 / 15 = 2)
- 3 persons for 10 days (30 / 3 = 10)
- one person for 30 days (30 / 1 = 30)
...or any other division of 30.
By the same math, a ship with 30 person-days of supplies facing a 10 day mission could support 3 persons (30 / 10 = 3).
So if the exploration ship Arrow-Back becomes marooned in the trackless wastes of unexplored space and is listed as having 20 person-weeks of life support, it makes it really easy for Mr. Selfish to do the arithmetic and figure that he will survive for twenty weeks instead of one if he murders the other 19 crew members. More democratically, if the rescue ship will arrive in 8 days (1.14 weeks), one can calculate that the supplies will stretch for an extra day with 17 crew members (20 / 1.14 = 17.5, round down to 17). The crew draws straws, and the unlucky two who get the short straws have the opportunity to heroically sacrifice themselves so that the rest of the crew may live.
Naturally the kind way to do the math is when initially planning the mission. Multiply the number of crew members by the duration of the mission in days to get the required number of person-days of consumables (and if you are wise you'll add an additional safety margin). Then you can calculate the mass and volume for each vital life-support consumable.
For instance, a 250 day mission with 5 crew would need 1,250 person-days. If food takes up 2.3 kg and 0.0058 m3 per person-day, you multiply by 1,250 to calculate the spacecraft needs to accommodate 2,875 kg and 7.25 m3 for the food supply.
A useful document with nitty-gritty details about life support Human Integration Design Handbook. This include info on the minimum volume needed for such tasks as exercise and hygiene, range of safe breathing mixes, temperature, humidity, acceleration limits, fire extinguishers, and related matter.
The basic requirements for life support are:
- Breathing Mix: an atmosphere to breath, or the crew will rapidly suffocate. Oxygen must be added as it is consumed and carbon dioxide removed as it is exhaled. Humidity must be maintained at a confortable level. An alarm should be triggered if dangerous contaminants are detected, or the signature of a fire.
- Water: for drinking and hygiene, or the crew will die of thirst (though probably not die of filth).
- Food: for eating, or the last surviving cannibal crew member will starve to death.
- Waste Disposal: or the crew will perish in a sea of sewage.
- Temperature Regulation: or the crew will either freeze or roast to death, probably the latter.
- Radiation Shielding: or deadly radiation will take its toll.
- Artificial Gravity: Need a replacement for gravity or limited duration tours in microgravity or it is death by Old Astronaut Syndrome.
The first three requirements are called "consumables", since they are gradually used up by the crew. Each of those three can be controlled by either an "open" system or a "closed" system.
Open systems are ones where a supply of the consumable in question is lugged along as cargo, enough to last the for the planned duration of the mission. It is renewed by "resupply", by obtaining new supply from a resupply spacecraft, a base, or an orbital supply depot. Things can get ugly if the mission duration becomes unexpectedly prolonged, for instance by a meteor scragging the spacecraft's engine.
Closed systems are ones where the supply of the consumable in question are renewed by some kind of closed ecological life support system. Generally this takes the form of some sort of plants, who use sunlight to turn astronaut sewage and exhaled carbon dioxide into food plants and oxygen.
Note that requirements for consumables can be drastically reduced if some of the crew is placed into suspended animation.
If you want more data on life support than you know what to do with, try reading this NASA document. Otherwise, read on.
For some great notes on spacecraft life support, read Rick Robinson's Rocketpunk Manifesto essay.
As a very rough general rule: one human will need an amount of mass/volume equal to his berthing space for three months of consumables (water, air, food). This was figured with data from submarines, ISS, and Biosphere II. Of course this can be reduced a bit with hydroponics and a closed ecological system. This also makes an attractive option out of freezing one's passengers in cryogenic suspended animation.
Eric Rozier has an on-line calculator that will assist with calculating consumables.
According to NASA, each astronaut consumes approximately 0.835 kilograms (0.560 cubic meters) of oxygen per day. They breath out 0.998 kilograms of carbon dioxide per day.
As a point of reference, a SCUBA tank is pressurized to about 250 bar i.e., 250 times atmospheric pressure. At that pressure, one person day of oxygen takes up about 0.00224 cubic meters.
Stored as liquid oxygen, 0.8 kilograms would take up about 0.0007 cubic meters. This requires extra mass for the cryogenic equipment to keep the oxygen liquid, but the volume savings are impressive.
So as far as pure oxygen goes, you take 0.8 kg for one person-day of oxygen, muliply it by the number of crewbeings on the ship, and then muliply it by the number of days in a standard mission (i.e., desired "endurance time" or time between supply stops) to discover the total oxygen mass requirement. Repeat with the volume figure for the total oxygen volume requirement.You'd be wise to add an additional reserve of about 25% to take account of pressurization of the hull, loss due to various mishaps, and general military paranoia.
However, this is just pure oxygen. This is insanely dangerous to use as the ship's atmosphere, the accident that killed the Apollo 1 crew proved that. In practice one uses a "breathing mix" of oxygen and another gas.
Generally the other gas is nitrogen. The technical term is "nitrox". Terra's normal atmosphere is 78% nitrogen, 21% oxygen, plus various other trace gases. People can suffocate if the level of carbon dioxide rises to 7% to 10%.
The Space Shuttle uses a 79% nitrogen/21% oxygen mix at atmospheric pressure (14.7 psi or 760 mm Hg). The shuttle space suits use 4.3 psi of pure oxygen, which means they have to prebreath pure oxygen while suiting up, or the bends will strike. Setting up the optimal breathable atmosphere is complicated.
The amount of oxygen must be kept under strict limits or oxygen toxicity will harm the crew.
At pressures higher than normal atmospheric pressure nitrogen becomes dangerous to breath. Generally spacecraft do not have to worry about that, unless they are diving into Saturns atmosphere or something similarly extreme. The colloquial term is "rapture of the deep", the technical term is "nitrogen narcosis." Pretty much any other inert gas you replaced nitrogen with will still have a narcosis effect, with the exception of helium. This is the heliox mix used by deep divers. A side effect of heliox is it makes all sounds more high pitched. In particular it makes the human voice sound like Donald Duck. Radios can be outfitted with a "helium de-scrambler" which electronically lowers the pitch of whatever is transmitted.
The Bono Mars Glider uses a heliox atmosphere, but I cannot figure out why.
Low energy requires huge amounts of biomass in plants. Data from Biosphere II indicate roughly seven tons of plant life per person per day, with a need for roughly 4 days for a complete plant aspiration cycle, so call it 25 to 30 tons of plant per crewman. With an average density of 0.5, each ton of greenhouse takes up about 2 cubic meters (m3).
High energy methods take up much less space, but (as the name implies) requires inconveniently large amounts of energy. It also results in lots of messy by-products and waste heat.
Practically, it is easier to flush the CO2 instead of cracking it, and instead bringing along an extra supply of water to crack for oxygen. Water is universally useful with a multitude of handy applications, and takes less energy to crack than CO2.
For future Mars missions, it has been suggested that the life support system should utilize the Sabatier Reaction. This takes in CO2 and hydrogen, and produces water and methane. The water can split by electrolysis into oxygen and hydrogen, with the oxygen used for breathing and the hydrogen used for another batch of CO2. Unfortunately the methane accumulates, and its production eventually uses up all the hydrogen. The reaction does require one atmosphere of pressure, a temperature of about 300°, and a catalyst of nickel or ruthenium on alumina.
For emergency use, it would be wise to pack away a few Oxygen Candles. These are composed of a compound of sodium chlorate and iron. When ignited, they smolder at about 600°C, producing iron oxide (rust), sodium chloride (salt), and approximately 6.5 man-hours of oxygen per kilogram of candle. Molecular Product's Chlorate Candle 33 masses 12.2 kilos, cylindrical can dimensions of 16 cm diameter x 29 height, burns for 50 minutes, and produces 3400 liters of oxygen.
It is not enough to supply oxygen to breath, you also have to remove the carbon dixoide. Bad things happen if the CO2 levels rise too high. NASA says that each astronaut exhales 0.998 kilograms of carbon dioxide per day.
- 0.04 percent - Typical level in Terra's atmmosphere
- At 1 percent - drowsiness
- At 3 percent - impaired hearing, increased heart rate and blood pressure, stupor
- At 5 percent - shortness of breath, headache, dizziness, confusion
- At 8 percent - unconsciousness, muscle tremors, sweating
- Above 8 percent - death
NASA uses Carbon Dioxide Scrubbers. In the Apollo program spacecraft, NASA used lithium hydroxide based scrubbers, which fill up and have to be replaced. Oxygen tanks have enough to last for the duration of the mission, and is gradually used up. Actually it is converted into carbon dioxide and is absorbed into the scrubbers, where it cannot be used any more.
You may remember all the excitement during the Apollo 13 disaster, when NASA learned the life-threatening dangers of non-standardization. The crew had to use the Command Modules' scrubber cartridges to replace the ones in the Lunar module. Unfortunately, due to lack of standardization, the CM cartridges would not fit into the LM life support system (CM's were square, LM were cylindrical). They had to rig an adaptor out of duct tape and whatever else was on-board.
In the Space Shuttle, NASA moved to a Regenerative carbon dioxide removal system. Metal-oxide scrubbers remove the CO2 as before. But when they get full, instead of being replaced, they can have the CO2 flushed out by running hot air through it for ten hours. Then they can be reused.
In the TransHab design, they use a fancier system to remove carbon dioxide and replace it with oxygen. Actually it recycles the oxygen, plucking it out of the carbon dioxide molecules and returning it to the atmosphere to be breathed once again.
Note that the system does not affect the nitrogen inert gas, so it stays at the proper level.
In the following specifications, the mass (kg), volume (m3), and electrical power requirements (W) is for equipment sized to handle a six person crew.
First the stale air is pumped through a 4-Bed Molecular Sieve (217.7 kg, 0.6 m3, 733.9 W). It initially removes the water from the air (and sends it to be added to the life support water supply), then it removes the carbon dioxide.
The carbon dioxide and some hydrogen (from a source to be explained shortly) are fed into a Sabatier Reactor (26 kg, 0.01 m3, 227.4 W). They react producing methane and water: CO2 + 4 H2 → CH4 + 2 H2O + energy.
The methane is vented into space. The water is fed into an electrolyser to be split into hydrogen and oxygen. Specifically a Solid Polymer Electrolysis (SPE) Oxygen Generation Subsystem (OGS) (501 kg, 2.36 m3, 2004 W).
The hydrogen is sent back to the Sabatier Reactor to take care of the next batch of carbon dioxide. The oxygen is added to the breathing mix and released into the habitat module's atmosphere.
The TransHab starts out with a tank of high pressure oxygen (20.4 kg, 0.78 m3, 6W, 30 MPa) and a tank of high pressure nitrogen (94.4 kg, 3.6 m3, 6W, 30 MPa). The oxygen tank has three days worth of breathing for six crew, enough to give the Sabatier Reactor time to get started. The nitrogen tank has enough to establish the proper ratio for the breathing mix, and some extra to compensate for any atmosphere leaking into space.
In most space program, they use two breathing mixes for the atmosphere inside the habitat modules and space suits. Low Pressure (pure oxygen at 32.4 kiloPascals [kPa]) or High Pressure (breathing mix at 101.3 kPa). High pressure breathing mix is pretty close to ordinary Terran air at sea level.
- Mix: Name of breathing Mix
- Pressure: Normal atmospheric pressure of breathing mix
- Oxygen Percent: Percentage of the gas that is oxygen
- Anoxia Below Pressure: Death by anoxia if atmospheric pressure drop below this
- Oxygen Toxicity Above Pressure: Death by oxygen toxicity of atmospheric pressure rises above this
- Oxygen Partial Pressure: Pressure of the oxygen component
The important thing to note is that for a low pressure breathing mix, the crew will die of anoxia if the atmospheric pressure falls below 5.3 kPa and the crew will die of oxygen toxicity if the pressure rises above 53.3 kPa.
For a high pressure breathing mix, anoxia lies below 25.2 kPa and oxygen toxicity is above 254.0 kPa.
How do you calculate safe breathing mixes for yourself?
The basic limit is anoxia ocurrs when the Partial Pressure of oxygen drops below 5.3 kPa and oxygen toxicity ocurrs when the partial pressure of oxygen rises above 53.3 kPa.
How do you calculate the partial pressure of oxygen?
pO2 = pMix * O%
pMix = pO2 / O%
- pO2 = partial pressure of oxygen (kPa)
- pMix = pressure of the atmosphere (kPa)
- O% = percentage of breathing mix that is oxygen (0.0 to 1.0)
Low pressure is attractive; since it uses less mass and the atmosphere will escape more slowly through a meteor hole. Unfortunately the required higher oxygen level make living in such an environment as hazardous as chain-smoking inside a napalm factory. NASA found that out the hard way in the Apollo 1 tragedy. Since then NASA always uses high pressure, they use low pressure in space suits only because they cannot avoid it.
This does raise a new problem. There is a chance that the high-oxygen atmosphere will allow a meteor to ignite a fire inside the suit. There isn't a lot of research on this, but NASA seems to think that the main hazard is a fire enlarging the diameter of the breach, not an astronaut-shaped ball of flame.
The increased fire risk is one reason why NASA isn't fond of low-pressure/high oxygen atmospheres in the spacecraft proper. There are other problems as well, the impossibility of air-cooling electronic components and the risk of long-term health problems being two. Setting up the optimal breathable atmosphere is complicated.
A more annoying than serious problem with low pressure atmospheres is the fact that they preclude hot beverages and soups. It is impossible to heat water to a temperature higher than the local boiling point. And the lower the pressure, the lower the boiling point. You may have seen references to this in the directions on certain packaged foods, the "high altitude" directions. The temperature can be increased if one uses a pressure cooker, but safety inspectors might ask if it is worth having a potentially explosive device onboard a spacecraft just so you can have hot coffee.
Decompression sickness (also known as DCS, divers' disease, the bends or caisson disease) is one of the more hideous dangers of living in space.
It occurs when a person has been breathing an atmosphere containing inert gases (generally nitrogen or helium) and they move into an environment with lower pressure. This is commonly when they put on a soft space suit or the room suffers an explosive decompression.
It has all sorts of nasty effects, ranging from joint pain and rashes to paralysis and death. The large joints can suffer deep pain from mild to excruciating. Skin can itch, feel like tiny insects are crawling all over, mottling or marbling, swell, and/or suffer pitting edema. The brain can have sudden mood or behavior changes, confusion, memory loss, hallucinations, seizures, and unconsciousness. The legs can become paralyzed. Headache, fatigue, malaise, loss of balance, vertigo, dizziness, nausea, vomiting, hearing loss, shortness of breath, and urinary or fecal incontinence: the list just goes on and on.
Why does it happen? Well, imagine a can of your favorite carbonated soda beverage. Shake it up, and nothing happens. But when you open it, the soda explodes into foam and sprays everywhere. When you open the container of shaken soda, you lower the pressure on the soda fluid. This allows all the dissolved carbon dioxide in the soda to un-dissolve, creating zillions of carbon dioxide bubbles, forming a foam.
Now imagine that the carbon dioxide is nitrogen, the drink is the poor astronaut's blood in their circulatory system, and the foam is the deadly arterial gas embolisms. That's what causes the bends.
Please note that sometimes the bends can occur if one moves from one habitat to another that has the same pressure, but a different ratio of breathing mix (the technical term is "Isobaric counterdiffusion"). Spacecraft of different nations or models could use different breathing mixes, beware. In fact, rival astromilitaries might deliberately utilize odd-ball breathing mixes, to make life difficult for enemy boarding parties invading their ships.
The bends can be prevented by slow decompression, and by prebreathing. Or by breathing an atmosphere containing no inert gases. Slow decompression works great for deep-sea divers but NASA does not favor it for space flight. An atmosphere with no inert gases (pure oxygen) is an insane fire risk. NASA does not allow a pure oxygen atmosphere in spacecraft and space stations, but will allow it in space suit (in a desperate attempt to lower the suit pressure to the point where the astronaut can move their limbs instead of being trapped into a posture like a star-fish).
So NASA astronauts do a lot of prebreathing. This flushes nitrogen out of the blood stream. NASA uses Terra-normal pressure (14.7 psi) inside the Space Shuttle, but only 0.29 pressure (4.5 psi) with pure oxygen in the space suits. The prebreathing is officially called the In Suit Light Exercise (ISLE) Prebreath Protocol, and unofficially called the "Slow Motion Hokey Pokey".
The astronaut(s) enter the airlock, and the airlock pressure is reduced to 10.2 psi. They breath pure oxygen through masks for 60 minutes (because the air in the airlock contains nitrogen). They then put on their space suits and do an EMU purge (i.e., flush out all the airlock-air that got into the suit while they were putting it on, to get rid of stray nitrogen). The air inside their suits is now also pure oxygen. The airlock pressure is then brought back up to the normal 14.7 psi. They then do 100 minutes of in-suit prebreath. Of those 100 minutes, 50 of them are light-exercise minutes and 50 of them are resting minutes. "Light exercise" is defined as: flex your knees for 4 minutes, rest 1 minute, repeat until 50 minutes has passed. Thus "Slow Motion Hokey Pokey". Now they are ready to open the airlock and step into space.
The innovation was the 50 minutes of exercise. Without it, the entire protocol takes twelve hours instead of one hour and fifty minutes.
If the habitat module's pressure was 12 psi an astronaut could use an 8 psi space suit with no prebreathing required (a pity such suits are currently beyond the state of the art), and for a 4.5 psi suit the prebreathing time would be cut in half.
In case of emergency, when there is no time for prebreathing, NASA helpfully directs the astronauts to gulp aspirin, so they can work in spite of the agonizing pain
Please note that most of the problem is due to the fact that soft space suits have a lower atmospheric pressure than the habitat module. So this can be avoided by using a hard space suit or space pod.
All of the atmospheric controls will be on the life support deck.
On a related note, forced ventilation in the spacecraft's lifesystem is not optional. In free fall, the warm exhaled carbon dioxide will not rise away from your face. It will just collect in a cloud around your head until you pass out or suffocate. In Arthur C. Clarke's ISLANDS IN THE SKY the apprentices play a practical joke on the main character using this fact and a common match. In the image above the blue dome shaped flame is an actual candle burning in free fall. And in Clarke's "Feathered Friend", he talks about the wisdom of using an animal sentinel to monitor atmospheric quality. Specifically by using the tried and true "canary in a coal mine" technique.
And yes, on Skylab, the area around the the air vent got pretty disgusting quite quickly, as all the floating food particles and assorted dirt from the entire space station got sucked in. In some SF novels the slang name for the air vents is "The Lost and Found Department."
Unpleasant odors in the air is a problem, but there is not much one can do about it. After all, you can't just open up a window to let in some fresh air, not in the vacuum of space. NASA carefully screens all materials, sealants, foods, and everything else to ensure that they do not emit noticeable odor in the pressurized habitat sections of spacecraft and space stations. Such odors can quickly become overpowering in such tight quarters.
Infinitely more serious than annoying odors are harmful atmospheric contaminants. They share the same problem that a spacecraft cannot open the windows to bring in some fresh air. But unlike odors, these can harm or kill.
Basic atmospheric monitors will keep an eye on the breathing mix inside the habitat module for oxygen and carbon dioxide levels. But prudent spacecraft will have monitors for carbon monoxide and other deadly gases, hooked up to strident alarms.
In space no one can hear you scream, but in the habitat module's atmosphere everybody can hear that high-pitched squeaky wheel in the ventilator. And there may be permanent hearing loss from loud noises, say, from rocket engines.
As a point of reference, the normal ambient noise level on the International Space Station is 60 db.
Meteors are probably nothing to worry about. On average a spacecraft will have to wait for a couple of million years to be hit by a meteor larger than a grain of sand. But if you insist, there are a couple of precautions one can take.
For larger ones, use radar. It is surprisingly simple. For complicated reasons that I'm sure you can figure out for yourself, a meteor on a collision course will maintain a constant bearing (it's a geometric matter of similar triangles). So if the radar sees an object whose bearing doesn't change, but whose range is decreasing, it knows that You Have A Problem. (This happens on Earth as well. If you are racing a freight train to cross an intersection, and the image of the front of the train stays on one spot on your windshield, you know that you and the engine will reach the intersection simultaneously. This example was from Heinlein's ROCKET SHIP GALILEO.)
(Ken Burnside used this concept in his starship combat game Attack Vector: Tactical. From the point-of-view of the target, the incoming missile will hit if it stays on one bearing and does not move laterally. So a game aid called a ShellStar is used to detect the presence of lateral motion.)
The solution is simple as well, burn the engine a second or two in any direction (That was from Heinlein's SPACE CADET). One can make an hard-wired link between the radar and the engines, but it might be a good idea to have it sound an alarm first. This will give the crew a second to grab a hand-hold. You did install hand-holds on all the walls, didn't you? And require the crew to strap themselves into their bunks while sleeping.
Having said that, Samuel Birchenough points out that anybody who has played the game Kerbal Space Program know that an object that is not on a fixed bearing can still hit you. If your spacecraft and the other object are in orbit around a planet, the object's bearing will be constantly changing up to the last few kilometers before the collision.
If the habitat module or space suit is punctured, all the air will start rushing out. Unless you and the other occupants want to experience first-hand all the many horrible ways that space kills you, you'd better patch that hole stat!
HULL BREACH EMERGENCY GEAR
An instrument called a Manometer will register a sudden loss of pressure and trigger an alarm. Life support will start high-pressure flood of oxygen, and release some bubbles. The bubbles will rush to the breach, pointing them out to the crew.
The crew will grab an emergency hull patch (thoughtfully affixed near all external hull walls) and seal the breach. The emergency hull patches are metal discs. They look like saucepan covers with a rubber flange around the edge. They will handle a breach up to fifteen centimeters in diameter. Never slap them over the breach, place it on the hull next to the breach and slide it over. Once over the breach, air pressure will hold it in place until you can make more permanent repairs.
A more advanced alternative to bubbles are "plug-ups" or "tag-alongs". These are plastic bubbles full of helium and liquid sealing plastic. The helium is enough to give them neutral buoyancy, so they have no strong tendency to rise or sink. They fly to the breach, pop, and plug it with quick setting goo. Much to the relief of the crew caught in the same room with the breach when the automatic bulkheads slammed shut.
Now you have some breathing space to break out the arc welder and apply a proper patch.
Once a pressurized habitat module or space suit springs a leak in the vacuum of space, all the air starts howling out the hole escaping into the void. Since people generally need air to breath or they die, there is an intense interest in how long it will take the air to go bye-bye.
Assuming Terra-normal pressure and density inside, and zero pressure outside, the effective speed of the air whistling out the breach works out to a smidgen under 400 m/sec. Veteran rocketeers, vacationing on Terra, tend to have a momentary panic if they feel the wind. Their instincts tell them there is a hull breach.
You probably won't use this equation, but to calculate an approximate time it will take for all the air to totally escape:
dm/dt = A * sqrt[ 2 * P0 * rho ]
- dm/dt = the rate (mass per unit time) at which air leaks into vacuum (in rocket engines they call it mDot or ṁ)
- A = area of the hole it's leaking through
- P0 = stagnation pressure in room (far from the hole)
- rho = air density inside the room far from the hole
- sqrt[x] = square root of x
If you want to get fancy and take the atmospheric temperature into account, use Fliegner's Formula (equation from quote below):
dm/dt = 0.04042 * ((A * P0) / sqrt[ T0 ])
- dm/dt = the rate at which air leaks into vacuum (kg/sec)
- P0 = stagnation pressure in room (far from the hole) (Pascals Pa)
- A = area of the hole (m2)
- T0 = stagnation temperature in room (far from the hole) (Kelvin, about 293 K for room temperature 20°C)
TIME TIL DEATH
However, what we (and the hapless people inside the breached compartment) are more interested in is how long it takes the pressure to drop to the deadly level of anoxia, i.e., how long the hapless people have to live before dying of suffocation (equation from quote below).
t = 0.086 * (V / A) * ( ln[ P0/Pƒ ] / sqrt[ T ])
tcabin = 0.00699 * (V / A)
tspacesuit = 0.00910 * (0.06 / A)
- t = time for the pressure to drop to anoxia (seconds)
- tcabin = time for the pressure to drop to anoxia in a habitat module cabin with high pressure (seconds)
- tspacesuit = time for the pressure to drop to anoxia in a spacesuit with low pressure (seconds)
- V = volume of compartment (m3)
- A = Area of the hole (m2)
- P0 = stagnation pressure in room (far from the hole) (Pascals) (cabin normal = 101,300; spacesuit normal 32,400)
- Pƒ = final pressure in room (Pascals) (cabin anoxia = 25,200; spacesuit anoxia = 5,300)
- T = Stagnation temperature in room (far from the hole) (Kelvin) (about 293 K for room temperature ~20°C; note √293 ~ 17.12)
- sqrt[x] = square root of x
- ln[x] = natural logarithm of x
- 0.00699 = 0.086 * ln[ 101,300/25,200 ] / sqrt[ 293 ] (cabin normal to cabin anoxia)
- 0.00910 = 0.086 * ln[ 32,400/5,300 ] / sqrt[ 293 ] (spacesuit normal to spacesuit anoxia)
- 0.04 to 0.06 = approximate volume of air in a space suit between inner suit and astronaut's body (m3)
Remember if the compartment is using high pressure (101.3 kPa) breathing mix anoxia strikes when Pƒ = 25.2 kPa, and with a low pressure (32.4 kPa) breathing mix at Pƒ = 5.3 kPa.
So if a posh passenger cabin of 15 cubic meters with high pressure has a 3 centimeter (one inch) diameter hole (area 7.07×10-4 m2) blown in the bulkhead by a wayward meteor, the inhabitants have an entire 148 seconds (about two and a half minutes) before anoxia strikes.
A .45 calibre ACP bullet has a diameter of .451 inches or 11.5 mm. So if it punches a perfect hole the same diameter as the bullet the hole will have a radius of 0.00575 meters and an area of 0.000104 square meters. This will bring the 15 m3 cabin down to anoxia in about 1008 seconds or 16.8 minutes. The time will drop if the hole is more ragged or if there are multiple holes. Obviously each additional hole cuts the time in half.
Somebody in a space suit doesn't have that kind of time. Dimitri SIyde tells me that according to the NASA EMU LSS/SSA Data Book the “free volume” inside a space suit is from 0.04 to 0.06 cubic meters (the gas volume of the anthropometric clearance between the crewmember and the inside of the suit including the PLSS oxygen ventilating circuit). The space suit uses low pressure. A hole a half-centimeter in diameter has a hole area of 1.96×10-5 square meters. As long as the suit's air tanks can keep up the loss the pressure won't drop. But once the tanks are empty, the pressure will drop to anoxia levels in a mere 27.9 seconds.
Does this mean that crewpeople in a combat spacecraft will do their fighting in space suits? Probably not, for the same reason that crewpeople in combat submarines do not do their fighting while wearing scuba gear. The gear is bulky, confining, and tiring to wear. They will not wear it even though in both cases the vessel is surrounded by stuff you cannot breath. They may, however, wear partial-pressure suits or have emergency space suits handy. Those suits will only protect you for ten minutes or so, but in exchange you won't be hampered like you were wearing three sets of snow-suits simultaneously.
Instead, the ship's pressurized inhabitable section will be divided into individual sections by bulkheads, and the connecting airtight hatches will be shut. The air pressure might be lowered a bit.
NASA assumes that each astronaut consumes per day 0.617 kilograms of dry food and 3.909 kilograms of potable water (some mixed in with the food). Astronauts also use 26 kilograms of water per day for personal hygiene.
NASA also assumes that each astronaut excretes 4.254 kg of water per day due to various metabolic processes. For details see below. Some of this water can be reclaimed.
Ken Burnsides and Eric Henry figured that each person has a reserve of 10 liters of water, and requires somewhere between 0.1 and 0.25 liters of water per day to make up for reclamation losses. (Eric used 0.1, Ken used 0.25 mostly due to having worked in a sewage treatment plant)
In the TransHab design, they use a water management subsystem to recover potable water from waste water.
In the following specifications, the mass (kg), volume (m3), and electrical power requirements (W) is for equipment sized to handle a six person crew.
An aluminum potable water storage tank (145.9 kg, 0.54 m3, 5 W) initially contains a three day supply of water for the six crew members. Waste water is sent through a Vapor Phase Catalytic Ammonia Removal (VPCAR) system (1119 kg, 5.5 m3, 6090.7 W). The VPCAR process is a wastewater treatment technology that combines distillation with high-temperature catalytic oxidation of volatile impurities such as ammonia and organic compounds.
The report mentioned that the VPCAR system was selected over a rival system since it had a lower mass, volume, and turnaround time. The VPCAR's drawback was the larger power requirements.
Of course there is the problem of recycling disgust, but that has to be fixed by psychologists, not engineers.
NASA assumes that each astronaut consumes per day 0.617 kilograms of dry food and 3.909 kilograms of potable water.The food is to have an energy content of 11.82 MJ per astronaut per day, about 2,800 food calories. More precisely:
FemaleAstronautDailyCalories = 655 + (9.6 × W) + (1.7 × H) - (4.7 × A)
MaleAstronautDailyCalories = 66 + (13.7 × W) + (5 × H) - (6.8 × A)
- W = weight (kg)
- H = height (cm)
- A = age (years)
The always worth reading Future War Stories has a good article on Military Field Rations and Space Food.
For food, Eric and Ken ran numbers from the USS Wyoming.
TL;DR: 1 person-day of food is 2.3 kg and 0.0058 m3, food storage space is about 0.012 m3. Food supply is 29% frozen, 57% dry, 14% fresh. If you are not interested in how these numbers were derived, skip to the next section.
150 man crew, 90 day cruise (13,500 person-days), 31,500 kg of food (9,000 kg frozen, 18,000 kg dry, 4,500 kg fresh). This is about 2.3 kg of food per man per day.
Frozen meat has a density of about 0.35 kg/liter (which Ken determined experimentally with a kilo of frozen meat in a 2 liter pitcher in his sink). Frozen veggies were less (0.4 kg/liter), so split the difference and use 0.375 kg/liter. 9,000 kg takes up 24,000 liters (24 m3).
Fresh foods have a density of roughly 0.25 kg/liter, due to air packed around the food by the packaging. 4,500 kg takes up 18,000 liters (18 m3).
Dry and canned goods range from densities of 0.25 kg/liter for flour and bread and 1.0 kg/liters for canned goods. Split the difference and use 0.5 kg/liter. 18,000 kilos takes up 36,000 liters (36 m3).
Total volume is 78,000 liters, or 78 cubic meters of food (1000 liters = 1 m3). Assume that we're off on our calculations and round up to 80 m3 as a reserve.
Storage, including refrigeration wastage is usually three times the space, but the Navy has a tradition of doing things in amazingly tight quarters. So we will merely double it, for 160 m3 to store our food.
Add about 1000 liters of water (water for 150 crew for 90 days, plus a reserve) which of course masses 1000 kg.
Add about 3,500 liters of compressed air (0.2 liters per person per day for 90 days, plus a reserve for general pressurization and a 20% safety margin) which masses 1050 kg.
Together air and water add about 5 m3.
There are alternate figures on life support in this document. It specifies the daily requirements of consumables per person as: 0.83 kg Oxygen, 0.62 kg freeze dried food (which would increase to 2.48 kg when the water was added), 3.56 kg water for drinking and food preparation, and 26.0 kg water for hygiene, flushing, laundry, dishes, and related matters. Note that the value for hygiene water is somewhat dependent on technology — if you have sonic showers and the like the requirements may be less.
William Seney notes that the NC State document specify oxygen consumption figures differ considerably from Eric and Ken's estimate. If we assume their value should be 48L per HOUR instead of per DAY (1.38 kg / day) it is much closer.
When the body uses glucose the reaction is:
C6H12O6 + 6 O2 => 6 CO2 + 6 H2O
so a slight excess of water is produced. According to the NC State document this works out to about 0.39L per person per day, which may be enough to replace losses.
For a real Spartan bare-minimum cruise, you can probably use a figure of one m3 per person per day. But this would not be recommended for a cruise of longer than 20 to 30 days. Morale will suffer. And don't even think about feeding your crew food pills.
The bare-minimum of consumables mass looks like 0.98 kg water, 2.3 kg food, and 0.0576 kg air per person per day. About 3.3 kg total, round it up to 4. People actually need 2.72 kg of water, but since food is 75% water, it contains an additional 1.72 kgs.
Our 90 day cruise now has about 165 m3 of bare essentials. Put in niceties like better cooking gear, spare clothing, toilet paper, video games, soda, luxury goods, and you are probably getting close to 240 m3. That will fit in a sphere 8 meters in diameter (about 25 feet).
If the spacecraft has no artificial gravity, you'd better include lots of spices and hot sauce. As the body's internal fluids change their balance, crewmembers will get the equivalent of stuffy noses. This will decrease the sense of taste. Food will taste bland like it does when you have a head cold, and for the same reason.
You'll need more space if you want to include hydroponics for fresh veggies. Roughly 800 liters of hydroponics per person per 'green meal' per week. This also helps CO2 scrubbing and crew morale. About 20 m3 per 25 men, or 120 m3 for our 150 man crew. 3 green meals per week takes about 600 m3.
On NASA shuttle and ISS missions, the astronauts have conventional knives, forks, and spoons; a hot/cold water injector, a warming oven, and scissor to cut open plastic seals.
When the water injector is set to "hot water" the temperature is between 68° and 74° C. The injector can be set to dispense water in one-half ounce increments up to 8 ounces.
Dehydrated food containers have a "septum adapter", i.e., a little airlock to insert the water injector nozzle. Otherwise when you removed the water injector the container would become a water weenie and drench you. For beverages you would then insert into the septum adapter a drinking straw. The straw has a built-in clamp to prevent the drink from spraying all over your face when you take the straw out of your mouth. For foods you wait until it rehydrates, then use the scissors to cut the container open. You make an X-shaped cut, creating four large flaps to help keep the food from escaping (a "spoon-bowl" package).
The warming oven is a forced air convection oven with internal hot plate. Its internal temperature is from 71° to 77° C. It can hold up to 14 food containers at a time: rehydratable packages, thermostabilized pouches or beverage packages.
NASA's space shuttle used fuel cells for power, which create plenty of water usable to rehydrate food. The shuttle meals were mostly dehydrated to save on mass.
The International Space Station on the other hand uses solar panel for power, which do not produce water. While there is some water available from recycling there is not enough for rehydrating food (there is barely enough for powdered drinks). Therefore the ISS uses no dehydrated food, instead is uses frozen and thermostabilized food which already has the water in it.
You can thank Napoleon Bonaparte for the invention of thermostabilized food in foil containers.
Back in the 1800's, it was tough to get food to armies on the move (the age-old problem of Logistics). An army would have to split up and spread out in order to ransack all the villages and farms in the area for food. Napoleon almost lost the war and his life at the Battle of Marengo because of this. While his army was split up, Napoleon's small army segment got ambushed by the entire Austrian army. If the other French groups had not returned in time, Napoleon might not have even been mentioned in the history books.
Determined not to get caught like that again, Napoleon offered a reward of 12,000 francs to the inventor who could preserve food for army rations in large quantities. The prize was won by Nicolas Appert, who basically invented thermostabilizing (your grandmother calls it "home canning" using Mason jars). He had stumbled upon Louis Pasteur's pasteurization process 50 years before Pasteur. The press went wild, waxing poetically on how Appert had established the art of fixing the seasons, so seasonal foods could be enjoyed year round. The French army was pleased as well.
Appert used glass bottles to hold the food. A short time later, Peter Durand figured out how to thermostabilize food inside tin-plated cans. A couple of decades later artists figured out how to make their studios portable by storing their oil paints in tin tubes. Decades later NASA stored thermostabilized foods inside tin tubes for the Mercury mission (but later abandoned them because the mass of the tube was more than the mass of the food it held). Currently NASA supplies thermostabilized food inside foil pouches for the astronauts of the International Space Station.
NASA packages food in single-service disposable containers to avoid the ugly payload mass requirements of a dishwasher. Eating utensils and food trays are cleaned at the hygiene station with premoistened towelettes.
The containers are in one of five standardized dimensions so they will fit the holes in the "dinner table" and the slots in the oven. All five sizes have the same width. They often have build-in velcro pads on the bottom.
The always worth reading Future War Stories has a good article on Military Field Rations and Space Food.
Emergency or Survival food is typically found in emergency re-entry capsules and spacecraft lifeboats (although in reality the latter are a really stupid concept).
They also may or may not be stored in the ship proper to help deal with a temporary interruption of the food supply (such as a catastrophic malfunction in the CELSS). If all the algae got incinerated by a solar proton storm, the crew will need something to eat while a new crop of algae is grown to harvest. You see this in a couple of episodes of Star Trek: Enterprise, where emergency rations are used when the food replicator is non-functional.
Real-world emergency rations typically are nutrient bars containing about 2,400 calories (enough food for an entire day). Emergency rations are optimized to be portable (low mass + low volume), require no preparation, and stable for prolonged periods (years).
Flavor is not a design consideration, the starving will eat anything. Even if Nutraloaf has been ruled "cruel and unusual punishment". And unlike Humanitarian daily rations, being acceptable to a variety of religious and ethnic groups is also not a design consideration. It is practically impossible to make a food ration which is acceptable to all groups.
In science fiction, emergency rations on re-entry capsules are to help you survive being marooned on an uninhabited planet long enough to figure out what part of the local flora and fauna are edible. In reality, the chance local flora and fauna even existing are remote, edible or otherwise. Especially if you are limited to our Solar System.
The most famous science fiction field ration is the Federation Space Forces Emergency Ration, Extraterrestrial, Type Three (aka 'Extee 3' or 'estefee') from Little Fuzzy by H. Beam Piper.
Military field rations are portable easily prepared food issued to soldiers deployed in the field. In the US Army they are called MREs for Meal Ready to Eat. Around World War I these were called "iron rations".
Field Rations are optimized to be portable (low mass + low volume), easy to prepare, and reasonably tasty. You have to sit down to eat them, though. MREs used by the US Army have a mass of 510 to 740 grams, depending on its menu.
The US Army Aviation uses Aircrew Build-to-Order Meal Modules (ABOMM). These are MREs designed so that a pilot can eat them while simultaneously piloting an airplane: without the use of utensils, in a confined space, and needing only one hand. MREs typically require two hands to eat, while it is not recommended for the pilot to take both hands off the control yoke.
First Strike rations are for people on the go. This can range from a First-in scout traveling from their hypothetical starship to explore and evaluate a hypothetical habitable planet to an asteroid miner living for several days in their space suit and subsisting on whatever they can squeeze through their helmet's chow-lock.
First Strike rations are optimized to be ultra-portable (excessively low mass + low volume), need little or no preparation and are easy to eat while walking or during an EVA (eaten out of hand without the use of utensils). A FS ration have about half the mass of an MRE. Strikers gotta move fast, they can't be weighted down by bricks of MREs. Yes, first strike rations have no mass in free fall, but all the inertia will still be there. Which means Every gram counts. Especially if you are an asteroid miner with a Spartan propellant budget.
|Waste per astronaut per day|
|Dry Feces||0.032 kg|
|Fecal Water||0.091 kg|
|Dry Urine||0.059 kg|
|Urine Water||1.886 kg|
|Dry Perspiration||0.018 kg|
An attempt should be made to reclaim the water. 4.254 kg of water per astronaut per day is too much to just throw away.
Male astronauts will use approximately 28 grams per day of toilet paper, minimum. Female will use 64 grams per day, because unlike their male counterparts, they wipe after urination. There will be about 5.1 grams per day of waste clinging to the toilet paper.
More toilet paper is required than is necessary under one gravity since in microgravity the fecal material has no particular inclination to separate from the body.
Women experience menstruation for 4 to 6 days occurring every 26 to 34 days. The total amount of menses is about 113.4 grams, of which about 28 grams is solids and the rest is water. This is highly variable. Approximately 104 grams of menstrual pads or tampons are consumed each period, again highly variable.
Female crew members on the International Space Station use medication to prevent menstruation for up to six months, but this may not be acceptable for longer missions.
Naoto Kimura mentioned that "Oh-gee Whiz" would be a good brandname for space toilet.
Astronaut hygiene is complicated.
NASA allocates 26 kilograms of water per astronaut per day for personal hygiene. This will be rationed, so use it wisely. It is no fun to go around all day with a head full of shampoo.
Bath and showers are very difficult in free fall. The crew will probably be reduced to sponge-baths or maybe a shower while zipped up in a bag. In Robert Silverberg's 1968 novel World's Fair 1992 he mentions "sonic showers" which use sound waves to remove dirt with no water required. In Babylon 5, only the command and VIPs have water showers, the rest have to make do with "vibe showers". And in Andre Norton's space novels, the bathing room is called the "fresher" which presumably is short for "refresher". As TV Tropes says: Our Showers Are Different.
People who have gone camping are familiar with how surprisingly difficult it is to keep clean in the absence of running water. As do city-folk living in houses near a water main break who have to make do without tap water for a few days. You tend to take for granted the luxury of accessing unlimited amounts of water out of the faucet. In the space environment, water is strictly limited, and what water there is performs poorly as a cleansing agent in free fall.
Crew will probably be required to take showers "navy style", since wet navy ships also have limited water (non-salt water at least). You turn on the shower water to get your body wet. You then turn off the water to conserve it while you lather up with soap. Then you turn the water back on to rinse off.
Keeping the habitat module clean is also a challenge. Water is limited, water does not clean things very well in free fall, and the limited atmosphere prevents one from using any alternate cleanser that it toxic or has a disagreeable odor.
And as mentioned elsewhere, any free floating garbage tends to accumulate on the air-intake vents. The vents on the Skylab space station quickly became quite disgusting with random bits of rotting food and dust particles.
Once space travel matures to the point where spacecraft are regularly traveling between locations with habitable environments, the ships will tend to become infested with vermin. Rats and cockroaches invaded seagoing vessels about five minutes after ships were invented. And after thousands of years they are still a problem. Spacecraft are highly unlikely to be free of the problem, unless ships are single-use items and only travel to airless worlds.
And we all know how indestructible roaches and rats are. Atomic radiation just slows them up a little. Only slightly more vulnerable are bed bugs.
Possible solutions include fanatical cleaning of the habitat module, periodic temporary removal of the hab module's breathable atmosphere to create vacuum, Ship's cat or other animals, and tiny hunter-killer robots.
If vacuum doesn't finish them off you have a real problem. You might have to go to the next level and flood the place with poison gas/insecticide. The trouble with that is purging the module of the poison afterwards so it doesn't kill the crew. And how tense things will get if an emergency crops up right in the middle of the gassing.
But if radiation mutates the vermin to the point where they actually become intelligent, well you are truly up sewage pulsar with no gravity generator.
About this time Lieutenant Sulu's pet rat Mickey escapes. Later a random crew member chuckles that he just saw Mickey in the corridor, and the funny rat looked like it was dancing. Both McCoy and Sulu turn the color of library paste. McCoy bolts back to Sick Bay while Sulu explains that dancing was a symptom of a rat who had Bubonic Plague. Egads.
The rat leads the crew on a merry chase over the next few weeks, while McCoy worriedly tells Captain Kirk that Bubonic Plague is so ancient that the medical records do not list the cure. Finally they give up on half measures, put the entire crew in space suits, and flood the Enterprise with poison gas. Lieutenant Uhuru writes "The Ballad of Mickey the Space Rat" and performs it for the crew.
The valiant crew then successfuly completes the mission. Space cafard starts setting in again. And suddenly Mickey reappears, dancing away.
The crew freaks out, grimly determined that they are not going to spend their shore leave quarantined on board. A level by level search finally corners Mickey, who is shot by about thirteen phaser-bolts simultaneously. His body is vaporized.
Privately, Dr. McCoy comes clean to Captain Kirk. Mickey did not have the plague, this was all a ruse to fight space cafard. McCoy taught Mickey how to dance, and held him in an oxygen tent while the ship was being gassed. And yes, the medical records do contain the cure for Bubonic Plague, you fell for it Captain.
Due to events recounted in the prior Solar Queen novel, the free traders were given compensation in the form of another (late) free trader's trade rights to the planet Sargol. The cat-people there have a few odd types of perfume for trade. Not very profitable, certainly nothing to attract the attention of the megacorporations.
However, the dear departed free trader had discovered on his last trip something called Koros gemstones. A small pouch of these beauties had almost caused a riot among the bidding gem merchants at an inner planet trading mart. Incredibly valuable. And more than enough to bring the megacorporations sniffing around.
The traders of the Solar Queen finally manage to obtain some Koros stones, after they discover the cat-people of Sargol love catnip even more that the ship's cat. A representative of the Inter-Solar megacorporation appears, becoming very annoyed to find the Solar Queen. Inter-Solar was hoping to poach some stones before the Queen showed up. The crew of the Queen is suspicious, the I-S man seems to be up to something nefarious.
A few days on the trip back home to Terra, the crew starts falling sick. Headaches, then falling into a coma. They fear it is some sort of plague, but then the medic notices small puncture wounds. The ship's cat does not catch anything, but the captain has a weird pet called a Hoobat (a nightmare combination of crab, parrot and toad). It manages to lure out of hiding some Sargolian chameleon insects with coma-inducing stingers. The Hoobat can make a hypnotic noise (by rubbing its claws together) which attracts the insects. It seems that the last load of perfume wood from Sargol had been seeded with the bugs by the I-S man.
When the Solar Queen leaves hyperspace and approaches Terra, they find that I-S has spread the word far and wide that the free trader is a plague ship, and should be quarantined. At least long enough for I-S to clean Sargol out of all its Koros stones. Hilarity ensues but our heroes manage to triumph over the evil megacoporations.
Be that as it may, the real peril may not be from macroscopic pests at all. Microorganisms could be much more serious. Even if they were not being carried by plague rats.
If you are lucky the mutant germs are just a new kind of plague.
If you ain't, you might be dealing with the functional equivalent of the Andromeda Strain or Mutant 59: The Plastic Eaters (i.e., a plague capable of destroying all life or all technology on a planet). The medical service may order your spacecraft intercepted with a nuclear warhead, which may or may not help matters.
If the dire dice of destiny land on you with both sixes down, your vermin infestation might turn out to be a super-vermin infestation. The classic example is the xenomorph from the Alien movie. But in novels, it usually takes the form of rats or something have have mutated to the extent that they actually become intelligent.
The Moties have sub-species. One of them is called the "watchmakers", they are semi-intelligent miniature Moties and are used by Motie engineers as sort of mobile tools. They are very clever and breed like rabbits. I think you can see where this is going.
The MacArthur accepts a breeding pair, who promptly escape and disappear into the duct work. The anti-rat ferrets are worthless and the watchmakers are too good at hiding for humans to find. So the crew vents the entire atmosphere to vacuum and figure the problem is solved. The crew does not realize that watchmakers can make their own pressure-tight hab modules.
It isn't until a few weeks later that Captain Blaine realizes they are still infested with the beasties, who are busy reengineering the entire ship. The crew tries to exterminate the watchmakers with a frontal assault with laser handguns, but are defeated by a combination of watchmaker counter-attack and unexpectedly redesigned ship systems that do not operate quite the way they used to.
The crew is evacuated to the Lenin but are only allowed to board after being strip-searched. Especially after they have to fight off a few animated space suits full of watchmakers. The Lenin destroys the MacArthur, which takes an inordinate amount of time since the watchmakers have drastically improved the MacArthur defensive force fields.
The Motie aliens do not have a problem with watchmakers because [A] their watchmakers are not feral so they can order them around to a limited extent and [B] Moties don't care if their ships are redesigned while in flight.
NASA directs that the temperature inside a habitat module should be from 291.5 K to 299.8 K with the nominal temperature 295.2 K. That's 18.4°C ⇒ 22°C ⇒ 26.7°C in metric (and 65°F ⇒ 72°F ⇒ 80°F for those poor benighted folk still using Imperial)
NASA assumes that each astronaut emits 11.82 MJ of heat per day.
This is the job of the Spacecraft Thermal Control Systems.
Temperature inside the habitat module is prevented from getting too cold by hull thermal insulation (to prevent the internal heat from escaping), and by adding heat from sources such as electrical resistance heaters.
Temperature inside the habitat module is prevented from getting too hot by hull thermal insulation (to prevent heat from the sun from entering), and by removing heat using heat radiators. In the TransHab design, it needs approximately 96 kilograms of heat radiators and internal thermal equipment per person.
Radiation shielding has its own separate page.
You want to limit the acute dose of radiation to under 0.1 Grays, and the astronaut career chronic dose to under 4.0 Sieverts.
What this boils down to is supplying the habitat module with a small radiation shielded room called a storm cellar or biowell. It will need radiation shielding to the tune of 500 grams per square centimeter of surface. Since this very expensive in terms of mass, storm cellars will be as small as the designers think they can get away with. With the restriction that all the crew has to physically fit inside, and they might have to shelter there for several days.
If the spacecraft has a fission, fusion, or antimatter power plant or propulsion system, it will require an anti-radiation shadow shield to protect the crew.
If the spacecraft is a combat spacecraft who will have to face radiation from nuclear warheads and particle beam weapons, it is going to require lots of very massy armor.
Supplying artificial gravity has its own separate page.
It is unknown what the minimum amount of artifical gravity is required for health. The only data we have are for 1.0 g on Terra, 437 days in 0.0 g in the ISS, and a few hours at 0.16 g for the Apollo lunar vists. NASA limits astronauts on the International Space Station to 180 day visits.
So, for instance, if the minimum required gravity for health is above 0.4 g, Martian colonists living on the ground will still need regular visits to the centrifuge.
Pretty much all the reports I could find on the subject conclude with something like "I have no idea, more research is needed."
Effects of prolonged microgravity include:
- permanent degraded vision (so far only observed in male astronauts, may be due to enzyme polymorphisms that increases astronaut vulnerability to bodily fluid shift in free fall)
- accelerated aging
- bone damage (1% to 1.5% per month, like osteoporosis)
- kidney stones
- muscle damage
- immune system changes
- cardiovascular changes
- red blood cell loss
- fluid redistribution
- fluid loss
- electrolyte imbalances
- vertigo and spatial disorientation
- space adaptation syndrome aka "drop sickness"
- loss of exercise capacity
- degraded smell and taste
- weight loss
- changes in posture and stature
- changes in coordination
A centrifuge would provide gravity to prevent these dire medical effects, but they are a major pain to attach to a spacecraft.
A possible compromise is the personal centrifuge. This is a centrifuge a few meters long, just big enough for one man to strap in, spin up about 30 RPM, and do some exercises. Yes, this will probably give them severe motion sickness, but it will only be for the duration of the exercise period. This will only help some but not all of the damage done to the body by microgravity.
It is tempting to just forget about spin gravity, and just have everybody float around while the ship is not under thrust. One can be an optimist and assume future medical advances will discovere treatment for all the hideous effects. Marshall Savage suggests electro stimulation therapy of the muscles (Ken Burnside says rocket crewmen will have to wear their "jerk-jammies" when they sleep). One would hope that a medical cure will be found for the nausea induced by free fall, or "drop sickness" (they say that the first six months are the worse).
But the only way to guarantee 100% freedom from all of the nasty medical effects is with full 1 g artificial gravity.
Remember the fundamental rule of rocket design: Every Gram Counts.
The spacecraft will have to lug along inconveniently large masses of air, food, and water ("consumables") so that the astronauts can live. And if the ship runs out while in a remote location, the crew will be reduced to a castaways in a lifeboat situation drawing straws to see who dies. With the added constraint that castaways in a lifeboat at least have unlimited access to breathable air. The fact that consumables run out at all will limit the duration of any given mission.
Which explains NASA's burning interest in Closed Ecological Life Support Systems (CELSS). In theory the only input such a system needs is energy, either sunlight or some power source to run grow lights. The advantages are:
- The astronauts will have air, food, and water forever (or until the equipment breaks down or the energy input stops)
- After a certain mission duration, it will be cheaper (in terms of mass) to use a CELSS instead of transporting consumables. With a primitive CELSS this happens at about 145 days, increasing the efficiency will bring the break-even duration point lower. The mass of the CELSS is constant regardless of mission duration, the mass of consumables goes up with mission duration.
The main functions of a CELSS are:
- Turn astronaut's exhaled carbon dioxide into oxygen
- Turn astronaut poop and table scraps into food
- Turn astronaut pee and washing wastewater into drinkable water
The current lines of research focus on doing this the same way Terra's ecosystem does: by using plants. In order to make the CELSS hyper-efficient they have to use hyper-efficient plants. Which explains the focus on algae.
Currently the state of the art is nowhere near achieving a 100% efficient CELSS. But an efficiency of over 75% or so would be a huge help. Sometimes a sub-100% system is called a Controlled Ecological Life Support Systems. The system might also be partial, such as a system which can 100% recycle carbon dioxide into oxygen, but cannot recycle food at all.
In NASA jargon, a closed environment life support system based on algae is called a "yoghurt box", one based on hydroponic leafy plants is called a "salad machine", and one based on a fish farm is called a "sushi maker".
Problems with creating and maintaining a balanced CELSS include carefully controlling the amount of plants consuming carbon dioxide (so they don't gobble more CO2 than the astronauts can produce, resulting in plant death from asphyxiation), and small-closed-loop-ecology buffering problems. The latter means that the smaller your CELSS system is, the more rapid and violent the results from tiny changes. With a closed-loop ecology the size of Terra, tiny changes can take months to years to show any effects, and those will be mild. With a small spacecraft CELSS, tiny changes can cause immediate and drastic effects.
Since maintaining the balance of a CELSS is so tricky, it will be a major undertaking to keep things stable if the number of crew members changes. If the number goes down (by a group on a landing mission the the Martian surface, or by crew casualties) the amount of plants will have to be cut back. Adding new crew is more of a problem. Leafy plants take time to grow to useful size when increasing square meters of cultivation. Algae is less of a problem since single celled plants multiply at a speed that puts rabbits to shame. It will probably be only a few hours to grow the biomass of algae enough to accommodate the new crew.
Another concern is the shipboard supply of phosphorus and nitrogen, since these are biological bottlenecks.
Of course there is the problem of recycling disgust, but that has to be fixed by psychologists, not engineers.
Terra's ecosystem fundamentally works by plants feeding animals while the animals feed the plants. Pretty much all spacecraft CELSS system try to utilize plants to feed the astronauts. Replacing plants with something else seems like a silly attempt to re-invent the wheel. Why do all that work when Mother Nature has already done it for you, for free?
The plants ingest water, carbon dioxide, sunlight, and some trace elements. Using Mother Nature's photosynthesis process, the plants output carbohydrates and oxygen. The astronauts consume plant carbohydrates and oxygen. Using human digestion, the human body is nourished, while producing water, carbon dioxide, and some trace elements. If you can balance the system, it will keep spinning as long as there is sunlight available.
Photosynthesis essentially splits water into hydrogen and oxygen, spits out the oxygen, and fuses hydrogen and carbon dioxide to form carbohydrates.
According to Chris Wolfe, NASA's best estimate is that the amount of leafy plants needed to handle one astronaut's carbon dioxide exhalation will produce only half of that astronaut's food. This is a problem. If the CELSS is producing all of the crew's food, it will have twice as many plants as needed, which will rapidly deplete the atmosphere of carbon dioxide, which will suffocate all the plants. It will also overproduce oxygen, but that can be extracted from the air and put into tanks for latter breathing, for rocket oxidizer, or exported and sold.
The solution is to recover the carbon trapped in the non-edible parts of the plants harvested for food.
The easiest way is to burn those parts and feed the generated carbon dioxide to hydroponics. The ash will contain other nutrients. Another solution is to feed the leftovers to a supercritical water oxidation unit and let it generate the carbon dioxide.
For CELSS uses, plants are generally grown by using some species of soil-less cultivation. Using actual soil to grow plants, with all of its active cultures and other messy ingredients, is far to unreliable to use in a life-or-death system.
Plants require light in order to perform photosynthesis, but using direct sunlight is a problem. First off you'll have to filter out ultraviolet and other frequencies harmful to plants. A transparent window allowing direct sunlight to illuminate the plants will also allow deadly solar storm radiation to fry them to a crisp. Granted plants tolerate up to 1 Sievert per year, but lets be reasonable here. There ain't no such thing as transparent radiation shielding (as yet).
In theory a shielded shutter will protect the plants from solar storms, but it is yet another possible point of failure. This is unacceptable if you are relying upon your plants to allow the crew to keep on breathing. If the storm detector fails, if the shutter actuators fail, if the crew forget to close manual shutters, all of these mean death by asphyxiation.
A fiber optic pipe fed with filtered sunlight and containing numerous bends to defeat radiation might work. But then if the spacecraft moves further away from Sol than Terra's orbit, the intensity of sunlight drops off rather alarmingly due to the inverse square law. That moron Freeman Lowell took far too long to figure this out in the movie Silent Running.
When you figure in that plants can only use certain wavelengths of sunlight, you might as well give up and use artificial grow-lights. LEDs are best, due to their relatively low waste heat. Feed the LEDs with electricity from the power generator of your choice.
The wavelengths used by photosynthesis are called photosynthetically-active radiation (PAR). They lie in a band from 400 to 700 nanometers, more or less the visible-light spectrum. Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. This is why plant leaves appear yellowish-green to our eyesight, the chlorophyll doesn't eat those wavelengths so they spit them out. PAR is usually measured in bizarre units of "µmol photons m−2s−1" because photosynthesis is a quantum mechanical process (i.e., a bizarre process).
According to Chris Wolfe, most plants flourish under 26 mol PAR per square meter per day (26,000,000 µmol PAR/m2/d). Assuming that 12 hours (43,200 seconds) of the day are daylight and 12 hours are nightime, this means the plants are enjoying about 601 µmol PAR per square meter per second (µmol m−2s−1, the negative exponents are a cute way of saying "per" or "divided by").
LEDs can produce about 1.7 µmol PAR per watt-second or 6,120 µmol PAR per watt-hour. Supplying 601 µmol m−2s−1 will require about 354 watts (601/1.7 = 354) per square meter.
In other words, your hydroponics LED lights are going to need about 354 watts of electricity per square meter of hydroponic plants, for 12 hours out of every day. Certain plants require different amounts of mols PAR per square meter, and different numbers of illumated hours per day, but this is a good back-of-the-envelope value. Lettuce and spinach want about 250 µmol m−2s−1 (not 601), while tomatoes and cucumbers can get by on only 100 µmol m−2s−1
If I am reading the reports properly, algaculture requires a wee bit more. Chlorella algae wants 450 µmol PAR/m2, and Spirulina's optimal value is about 120 µmol PAR/m2.
So doing the math, Chlorella's 450 µmol PAR/m2 will need about 265 watts/m2 of LED electricity, while Spirulina needs 71 watts/m2. Since algae is typically cultivated in tubes or tanks, I am unsure how to translate the square meters of illumination into volumes of algae culture. The thickness will not be much, because algae is so good at harvesting photons that is is practically opaque.
As previously mentioned, plants will probably be grown by using soil-less methods.
- Hydroponics: growing plants without live bacterial-infested soil ("salad machine")
- Solution Culture: no substrate, just nutrient water
- Static solution culture: plants in jars of still nutrient water
- Continuous-flow solution culture: flows of nutrient water passing over plant roots
- Aeroponics: plant roots are in the air, misted with a fog of nutrient water
- Algaculture: microalgae cells cultivated in either static or continuous flow solutions cultures ("yoghurt box")
- Medium Culture: using a sterile solid substrate bathed in nutrient water
- Aquaponics: solution or medium cultures in association with tanks of fish and other sea food ("sushi maker")
Growing leafy plants for food is not as efficient as growing algae or other single-cell plant. But it is quite a bit easier. The basic idea is to grow food plants not in soil, but instead in nutrient filled water or in an inert material bathed in nutrient filled water.
Medium cultures can use all sorts of substrates: rock wool, baked clay pellets, glass waste growstones, coco peat, parboiled rice husks, Perlite, Vermiculite, pumice, sand, gravel, wood fibre, sheep wool, brick shards, and polystyrene packing peanuts have all been used.
Algae is cultivated in photobioreactors. These try to hold the algae cultures in thin layers because the little greenies are so good at absorbing the light that any algae that is too deep will get no light at all. The concentration of algae is typically something like 5×108 algae cells per mililiter of water.
In 1965, the Russian CELSS experiment BIOS-3 determined that 8 m2 of exposed Chlorella could remove carbon dioxide and replace oxygen within the sealed environment for a single human (I am assuming this is in a very shallow tray). The official figure for Chlorella oxygen production is 25 to 400 femtoMol O2/cell/hour. If am I doing the math correctly, at a concentration of 5×108 cells/ml, this translates into about 0.0032 kg of O2 produced per hour per liter, and 0.768 kg of O2 produced per day per liter. Since astronauts require 0.835 kg of O2 per day, this implies they would need 1.09 liters of chlorella culture.
In THE MILLENNIAL PROJECT, Marshall Savage sings the praises of Spirulina algae. However, you'd best take the following with a grain of salt. There is often a long distance between the ideal and the real.
Anyway, Spirulina is apparently almost the perfect food, nutritional wise. A pity it tastes like green slime (though Savage maintains that genetic engineering can change the flavor). Spirulina is highly digestible since it contains no cellulose. It is 65% protein by weight and contains all eight essential amino acids in quantities equivalent to meat and milk. It also has almost all the vitamins, with the glaring exception of vitamin C (I guess rocketmen will become "limeys" again). It is also a little sparse on carbohydrates. Savage calculates that it will be possible to achieve production rates of 100 grams (dry weight) of algae per liter of water per day. It breaks down 6 liters of algae water per person, supplying both food and oxygen, while consuming sunlight (or grow-lights), CO2 and sewage. 6 liters of algae water will produce 600 grams of "food" (540 grams is 2500 calories, an average daily food requirement), 600 liters of oxygen, and consume 720 liters of CO2 and an unspecified amount of nutrient salts extracted from sewage. Since food is generally 75% water, 600 grams of dry food will convert into about 2.4 kg of moist food, which compares favorably with the 2.3 kg on the USS Wyoming.
600 liters of oxygen is about 0.8574 kg of oxygen, which is above the NASA requirement of 0.835 kg of oxygen per astronaut per day.
NASA commissioned a study back in 1988 to determine how difficult it would be to cultivate Spirulina as part of a closed ecological life support system.
Dr. John Schilling mentions a possible pitfall:
There are other things you have to be mindful of when cultivating Spirulina. From the Swedish Medical Center:
SF writers with an evil turn of mind will see some interesting plot possibilites in these facts. The ship's food supply could become contaminated by an incompetent repair of the algae system utilizing lead pipes, an algae culture supplier with poor quality control, or deliberate sabotage.
The advantage of algae is that it can theoretically form a closed ecological cycle. This means that 6 liters of algae water, one human, some equipment, and sunlight can keep the human supplied with food and oxygen forever. Theoretically, of course. 0.006 m3 per person compared to 90 m3 per person is a strong argument for lots of green slime dinners for enlisted Solar Guard rocketmen. Of course the Biosphere II fiasco shows how far we are from actually achieving a closed ecological cycle. Don't forget the 0.25 liters of water per person per day to make up for reclamation losses.
William Seney points out that as a luxury, some of the algae can be diverted to feed fish such as carp, catfish or tilapia for an occasional treat.
And you'd better keep the algae tanks far from the atomic drive. The last thing you want is for the little green darlings to mutate into something you can't eat. Or worse: something that is really inefficient at producing oxygen.
Christopher Huff begs to differ:
There were some figures in a report on a cruder life-support set up written in 1953. This used Chlorella algae, which isn't quite as good as Spirulina since it has an indigestible cellulose cell wall. The figures assume a Chlorella culture density of 55 grams per liter of water and a daily yield of 2.5 grams per liter. Savage's 100 grams per liter sounds a little optimistic, and 2.5 sounds a little pessimistic. The truth is probably somewhere in between.
At a yield of 2.5 g/l, to provide one rocketeer with 500 grams of food (instead of Savage's 600 grams) will require 200 liters of algae culture.
Urine is passed through an absorption tube to remove excess salt (which would kill the algae) but retaining urea and other nitrogen compounds the algae needs. Faeces are irradiated with ultraviolet to kill all bacteria and added to the urine. This is fed to the main algae tank along with pressurized carbon dioxide (previously removed from the air with calcium oxide). A pump sends a flow of algae culture to the growth trays under filtered sunlight. The culture then passes through a centrifugal separator on its way back to the main tank. The separator performs two functions:  removing excess gas to maintain a pressure equilibrium with the carbon dioxide injection and  periodically harvesting algae for food. Harvest will occur once a day, extracting 500 grams of algae from nine liters of culture per person. The pump will be controlled such that the algae on the average will experience two minutes of sunlight then three minutes in the darkness of the main tank before it starts the cycle anew.
A fresh batch of urine and faeces is added immediately after algae harvest, to give the algae twenty four hours to consume it. So by next harvest there is no human excretions contaminating the food (you hope).
Now for the answer you've been waiting for. Dr. Bowman estimates that the equipment will mass approximately 50 kg, plus 200 kg per man for algae culture. Since the equipment is such a small fraction of the total, mass savings depend upon getting the algae yield higher than 2.5 g/l. Such as Savage's 100 g/l Spirulina with 6 kg per man of algae culture.
Dr. Bowman points out that when one compares an algae system with merely stocking crates of food, the break-even point occurs at a mission of 145 days (about five months). Below this time it takes less mass to bring crates of food, as the mission duration rises above 145 days the algae tanks get more and more attractive.
Other SF novels have suggested vats of yeast or tissue cultures of meat ( in vitro meat ) to supplement food supplies. H. Beam Piper's Terro-Human series had spaceships equipped with "carniculture" tanks. A. Bertram Chandler's Rim World stories featured spaceships with all sorts of food vats. Tissue-culture for meat, hydroponics for vegetables, algae and yeast for single-celled food.
But unless they can re-cycle wastes from the crew, it seems more efficient to just carry more boxed food.
Currently scientist can only grow tissue cultures as a single sheet of cells, making them thicker will require figuring out how to make them grow blood vessels to nourish all the cells ("vascularization"). But some technicians figure that they can grow lots of meat cell sheets, then laminate the sheet layers together to approximate a slab of meat.
There are researchers exploring several different strategies to make full-blown vascularization. But it ain't easy. Strategies include material functionalization, scaffold design, microfabrication, bioreactor development, endothelial cell seeding, modular assembly, and in vivo systems. See link for details.
The joke name for this process is "In Meatro"
If you are trying a closed cycle with tissue cultures, you will have to deal with the problem of the Food Chain. Typically each higher level of the pyramid has one-tenth the biomass of the one below, for reasons you can read about in the link. What this means is that you will have to feed ten meals worth of algae to the meat tissue culture in order to produce one meal worth of meat. Even on Terra, this is the reason why meat is more expensive than vegetables.
Obviously the food chain effect also applies to diverting some of the algae to fatten up some fish as a special meal.
At least the tissue culture helps increase the meat ratio. For instance, an entire cow is about 40% edible meat. The rest is bones, hooves, hide, and other inedible parts. Tissue cultures would theoretically turn that up to 100% edible meat. Granted the inedible parts can be recycled via supercritical Water Oxidation, but the inefficiency of wasting all that algae food energy on growing inedible bones kills this idea dead. Not that it would have been practical to bring a cow along on your spaceship in the first place.
As a side note, the idea of lab grown in vitro meat has caused some controversy among the vegetarian community. Any person who is vegetarian on the basis of avoiding animal cruelty, should have no objection to eating in vitro meat. But some vegetarians still maintain that one should not eat meat because of Reasons.
Of course things can become a real moral quagmire, as Sir Arthur C. Clarke points out in his disturbing short story The Food of the Gods.
There is an alternative between eating algae and the daunting task of growing vascularized meat tissue cultures, but you ain't gonna like it.
There are quite a few edible insects that will happily eat algae. Since they are live, they make their own vascularization. They are very efficient at converting algae into insect meat. And a much higher percentage of insect body mass is edible meat.
Yes, most people from western cultures find the thought of eating bugs to be incredibly disgusting. However the astronauts are already drinking recycled urine so it just takes some training. Processing will help, a compressed-protein bar composed of finely ground insects will be easier to eat than a plate full of microwaved bugs with too many legs.
In the following table, the "Algae for 1 kg of animal" is how much algae an entire animal will need to eat in order to increase its weight by one kilogram. "Edible meat" is the percentage of the animal's mass that is edible. "Algae for 1 kg of meat" is how much algae the entire animal will need to eat in order to increase its edible meat mass by one kilogram (i.e., reciprocal of edible meat percent times algae for 1 kg of animal).
You can probably use the "Algae for 1 kg of animal" figure as a ballpark figure for a tissue culture.
1 kg of animal
|Edible meat||Algae for|
1 kg of meat
|Cow||10 kg||40%||25.0 kg|
|Pig||5 kg||55%||9.1 kg|
|Chicken||2.5 kg||55%||4.6 kg|
|Cricket||1.7 kg||80%||2.1 kg|
If you figure a beef tissue culture requires 10 kg of algae for each new kilogram of beef, the freaking live crickets are still more efficient.
At harvest time, insects are killed by freeze-drying, sun-drying or boiling (in space, exposing them to vacuum probably counts as freeze-drying). They can be processed and consumed in three ways: as whole insects; in ground or paste form; and as an extract of protein, fat or chitin for fortifying food and feed products. Insects are also fried live and consumed, but a deep-fat fryer in microgravity is insanely dangerous. Some species need to have their legs and wings removed before eating.
In practice, extracting insect protein is probably not worth the effort. Needs lots of exotic chemicals and equipment, and reduces the percentage of edible mass. It is easier just to grind them into powder or paste and make bug-burgers.
For more details than you really want to know, read the report.
Science fiction authors could use this as an interesting bit of historical detail. Old-timer spacers can tell tales about back in olden days when they had to eat bugs. You young whipper-snapper spacers have it easy nowadays, what with your vascularized filet mignon tissue cultures.
A shmoo is a fictional cartoon creature created by Al Capp, they first appeared in his classic comic strip Li'l Abner in 1948. Shmoos were prolific, required no food (only air), are delicious and nutritious, have no bones or other waste, and are eager to be eaten. (Ironically, they are the greatest menance to humanity ever known. Not because they are bad, but because they are good.)
Oddly enough, shmoos share many common traits with one-celled yeast. Yeast even looks a little like a shmoo. When a yeast cell senses the mating pheromone, it initiate polarized growth towards the mating partner, creating the characteristic outline of a shmoo. The process is called "shmooing", which shows that biologists have a sense of humor. As to the matter of the deliciousness of yeast, see the exerpt from Lucky Starr and the Oceans of Venus below.
In the real world, left-over brewers' yeast is used to create such foods as Marmite and Vegemite. Even in 1902 people realized that it was a criminal waste to just throw away the huge quantities of perfectly edible yeast protein that was a by-product of making beer. Marmite and Vegemite are still being sold today. Actually in Australia, Vegemite is more or less a food staple.
In science fiction, one occasionally encounters the term "dole yeast". In future societies that have some form of social welfare system for unemployed people, the food given is generally a portion of unpalatable raw yeast, since that is usually the cheapest food available. Single-cell protein is very inexpensive, especially if you grow it on minimally processed sewage.
"Aquaponics" is a way of raising both plants and meat in one tank. You use an over-sized deep hydroponic tank to grow the plants. Below the plants you raise fish. The fish are fed food pellets. The hydroponic nutrient media is supplemented by the waste the fish excrete. The plants consume the nutrients, purifying the water and keeping the fish healthy ("rhizofiltration"). The system is more stable than a standard hydroponic rig, since the larger tank will buffer and moderate any changes. The larger volume of water also means you can get away with a more dilute solution of nutrients.
Not just standard fish can be cultivated, the system can also be used for shellfish such as lobsters, shrimp, clams and oysters.
In NASA jargon an aquaculture system is called a "sushi maker".
If you really want to get back to basics, you can try to synthesize food in a laboratory, with no plants or animals involved. It is probably harder and less efficient than growning food, but might be the only thing left if there is an utter disaster in the CELSS. The crew will call this the horror of Food Pills, and they will be right.
NASA looked into this between the 1960s and 1970s.
Wastes have to be fed to the algae, or whatever. But it would be nice to turn the astronaut poop into sterile chemicals first instead of infecting the algae tanks with E. coli bacteria. And the problem of reducing to useable form plant stalks, fish bones, chicken feathers, and other tough scraps. Not to mention all the plastic bag bits.
Enter the Supercritical water oxidation (SCWO) unit.
By placing water at temperatures and pressures above the thermodynamic critical point, it turns into a fluid that combines the worst properties of a blast furnance and sulphuric acid. You feed anything into one of these hellfire-in-a-box thingies and nothing is going to come out the other end except water, oxidized chemicals, and mineral ash. This happens at about 374.1°C and 22.12 Mpa.
The only estimates I've managed to find (Parametric Model of a Lunar Base for Mass and Cost Estimates by Peter Eckart) for a SCWO unit are:
- Mass: 150 kg per person being supported
- Expendibles required: 10 kg per person per year
- Volume: 0.5 m3 per person
- Power required: 0.36 kilowatts per person
- Heat load: 0.09 thermal kilowatts per person
- Liquid waste input: 27.18 kg per person per day
- Solid waste input: 0.15 kg per person per day
Waste products from the astronaut's septic tanks and tablescraps are run through the SCWO. The appropriate output chemicals are fed to the Spirulina, which multiplies in meters of transparent tubes run under filtered sunlight. Filtered because raw sunlight in outer space is quite deadly to algae, and it isn't too healthy for humans either.
There is more information on SWO units here. The first reference describes a facility with a volume of just over 20 cubic metres that can process 7.5L per minute, more than enough for a crew of 300. (30L/person/day - 20 hours a day). Thanks to William Seney for these link.
General Atomics has some developed some SWO units for waste disposal.