Plumbing the Space Station
But have you ever stopped to wonder about plumbing ... in space? For example, which way does water flow in a weightless environment? Can toilets flush in free-fall? And if something springs a leak in Earth-orbit, which plumber would you call? There are plenty of choices, but they're all at least 235 miles (378 km) away racing by at 17,000 mph (7.5 km/s).
Designers of the International Space Station (ISS) had to contend with all these questions and many more as they laid out a complex network of tubes, pipes and ducts between the Station's outer skin and its inner walls. Like veins and arteries in the human body, the Station's plumbing circulates vital liquids and gases that keep the crew and the ISS itself in good health.
Above: The U.S. Destiny laboratory module during construction, showing the "standoff" assemblies that contain the pipes, tubing, and ducts that circulate the Station's vital liquids and gases.
"This is kind of an ecologist's dream house," said Dave Williams, system manager for Environmental Control and Life Support Systems (ECLSS) at Johnson Space Center in Houston, Texas. "If you built a house this way you would be reclaiming as much water as possible."
For example, while a house on Earth can simply drain its wastewater to lines leading to a municipal treatment plant, the ISS must carry its own miniature water treatment plant onboard.
This equipment must achieve a higher level of cleanliness than its earthly counterparts for several reasons. Unlike most municipal systems, the ISS system recycles the urine of both the crew and the laboratory animals and returns it to the drinking water supply -- and the health of the crew is of particular concern in space. Like plumbers, there are few doctors nearby! Microbes are a danger even to the Station itself, as exemplified by the problems on Mir with fungal growth. Keeping microbe levels in the water supply to an absolute minimum is an important part of ensuring the longevity of the Station.
Operating "in a bottle" also complicates the plumbing of the Station because the crew can't simply open a window to get some fresh air. Tubes carry pressurized oxygen and nitrogen from the Shuttle to storage tanks on the ISS. Ducts move cabin air from all parts of the Station to the carbon dioxide scrubbers and back, ensuring that the dangerous gas doesn't build up in any forgotten corner.
Below: This diagram shows the flow of recyclable ("regenerative") resources in the Space Station's Environmental Control and Life Support System (ECLSS), which is being developed jointly by NASA's Marshall Space Flight Center in Huntsville, Alabama, and the Johnson Space Center in Houston, Texas.
To be certain cabin air is safe, a mass spectrometer routinely analyzes the gas content of the air. Another network of tubes draws air samples from many different spots around the Station and feeds this air to the spectrometer, which looks at levels of oxygen, carbon dioxoide, and other gases.
"So if we know, for instance, there's some crew activity in a particular location that day, we can tell the computer to sample more frequently there," Williams said.
The oxygen tanks -- in addition to providing a backup supply of oxygen to replenish cabin air -- attach to yet another set of tubes that supply low-pressure oxygen to the modules. Receptacles in the modules allow the crew to tap into these lines with their emergency breathing apparatuses, extending the 15-minute supply built into the breathing apparatuses so that the crew can take their time handling the emergency.
And this collective network of tubing and hardware, which is far more elaborate than that of the typical house, must be compact, lightweight, corrosion-resistant, leak-resistant, microbe-resistant, and highly dependable. To meet this tall order, the pipes of the Space Station are variously made from titanium, stainless steel, or Teflon wrapped in metal mesh. In comparison, household plumbing is typically made of inexpensive PVC and copper.
Along with the unique demands of a "ship in a bottle," the plumbing on the ISS must operate without the assistance of gravity.
Above: What happens to water flowing through pipes in orbital free fall? These photos show water and air flowing through clear pipes. The left photo was captured in Earth's gravity -- the air rises and forms a layer on top of the water because gases are less dense than liquids. The right photo, captured in free fall (equivalent to zero-G), shows that density differences are irrelevant. The air can form a core flowing through the center of the pipe, surrounded by water. [more information]
When building a house on Earth, it's enough to just lay the pipe and then let gravity or the pressure of the city water supply create the flow. In the mutual free fall of Earth orbit, liquids and gases would stagnate on their own.
"You have to look at the lack of gravity carefully," Williams said. "Because normally fluids would just sit there, unless you had the head pressure to force them. In a house, you can count on gravity when you flush a toilet to take that water and put it out in the sewer."
To keep the fluids flowing, the ISS plumbing system includes dozens of pumps and fans that create the pressure needed to coax the liquids and gases into moving.
The mutual free fall environment also places special demands on the design of bathroom and faucet fixtures. Mass-produced fixtures like those found found in a typical home won't work on the ISS.
Left: This "hygiene center" on the ISS looks a bit different than a typical bathroom sink! Crew members need specialized hardware to perform basic bathroom necessities. [on image to enlarge]
The lavatory on the ISS looks markedly different than a bathroom here on the ground. A conventional toilet would not function at all without gravity. The ISS uses specialized equipment to meet these bodily needs.
"We have to have active components to help remove the feces and urine away from the astronaut," Williams said. The two machines that separately handle these two body functions both use air flow created by suction to facilitate waste removal.
With a little practice, no doubt, it seems just like home. And that's the goal of the most far-out plumbing in the solar system -- to work so well that the crew takes it for granted. After all, building a new home in space is a full time job and nobody up there wants to waste time calling the plumber.
This is the third in a five-part series of articles about the construction of the ISS. The first examined the Station's architecture and structural design; the second described the Station's unique thermal control systems. Future installments will explore the power and ergonomics of the Station.
International Space Station -- NASA's Web page for the International Space Station
Water on the Space Station -- Science@NASA article: Rationing and recycling will be an essential part of life on the International Space Station. In this article, Science@NASA explores where the crew will get their water and how they will (re)use it.
Breathing Easy on the Space Station -- Science@NASA article: Life support systems on the ISS provide oxygen, absorb carbon dioxide, and manage vaporous emissions from the astronauts themselves. It's all part of breathing easy in our new home in space.
Advanced Life Support Web Page -- from the Johnson Space Center
Environmental Control and Life Support Systems -- describes the life support systems being developed at Marshall Space Flight Center
This Quicktime movie shows a freely falling fluid in action. Credit: microgravity researcher Dr. John Pojman
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