Space station benefits from a wide opening

Historically tiny tunnels
Smaller diameters have been the historical norm. For example, back in the late 1960s and early '70s, the tunnel between the Apollo command module and the lunar module was a mere 29 inches wide. Even, now, the Russians are going with the 31-inch width for their next-generation Kliper spacecraft. And because the European Space Agency's brand-new Automated Transfer Vehicle has to link up with Russian hatches, that cargo craft will have to follow the 31-inch standard as well.

People can slide through these smaller tunnels, their shoulders brushing the walls and the hatch rims. Small packets, or golf-bag sized gadgets, can be wrestled in and out. But anything bigger would have to be installed in the space station pre-launch, and stay there forever. This explains why many of the bulkier Russian items that need replacement, such as the Elektron oxygen generators, are long cylinders. That’s the biggest size and shape that can get inside.

On the now-defunct Mir space station, the Russians became overwhelmed with bulky equipment that had broken down or became obsolete, and then couldn’t be discarded. At the end of the station’s life in 2001, the best argument for terminating its flight was that very little of the onboard equipment actually worked at all — but was taking up all the space aboard.

On the U.S. side of the international space station, the size and shape of the hole is matched with the size and shape of the biggest item planned to pass through it — the refrigerator-sized racks. These racks can contain sophisticated scientific equipment, or life-support devices, or even just portable closets.

The modules can be brought on board from the cargo containers, or taken back into the modules for return to Earth, servicing and eventually reinstallation in orbit. They are the heart of the research programs for which the station is being built — and without the right-sized door, they would be stuck on Earth.

It's not a simple matter to build a door that big for space. McCann explained that the pressurization inside the space station puts 20 tons of force on the hatch. Also, the fact that the door is square means the stress isn't distributed uniformly around the edges. For those reasons, the hatch has to be exceptionally strong and stiff.

“It was a challenge to design,” McCann admitted, “but the hatch and the CBM have worked flawlessly on orbit.”

Scientific payoff
Don Thomas, the station's mission scientist, said eight racks' worth of scientific experiments have been transferred from the shuttle to the station so far.

“These facilities have allowed us to conduct a wide range of scientific investigations on how the human being adapts to the microgravity environment ... and have supported a wide range of physical science investigations from crystal growth to the development of advanced materials with unique structures and properties,” he said in his e-mail.

During Discovery's mission, two new research racks are being delivered: a laboratory freezer that can preserve biological samples at a temperature of 112 degrees below zero Fahrenheit (-80 degrees Celsius), and a European incubator that will be used for plant and animal studies.

The space-to-space passageway isn’t finished yet, Thomas said: “On future shuttle missions, six additional scientific racks will be flown including ones that will investigate crystal growth and materials processing in space, combustion processes, and one that will aid in our photography and investigations of the Earth from the ISS.”

Once the shuttle stops flying after 2010, the challenge will be to find new ways to use the plus-size doorway. Perhaps a new cargo carrier can be equipped with compatible mechanisms. Or perhaps a new airlock module can have its own door, big enough for bulky objects.

But the current Common Berthing Mechanism has surplus capability, even now. According to McCann, if needed, the hatch could be opened directly to space, once the air has been released from the Unity module. “If we absolutely had to get something that large into the station, we could,” McCann said.

“You could — not that we ever would,” he added.

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