Why is the ISS shaped so weird?

Chuck Bednar for redOrbit.com – @BednarChuck

The International Space Station (ISS) has been in service for nearly 15 years, been visited by hundreds of astronauts, travelled well over 1.5 billion miles and provided invaluable scientific research. But have you ever stopped to wonder why it looks so… odd?

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Let’s face it, as wonderful as the ISS is, it doesn’t look near as badass as the orbital outposts that are depicted in science fiction (Star Trek’s Deep Space 9 or the Death Star, for example). So why did NASA and its international colleagues choose to give it such an unusual shape?

As it turns out, they didn’t, according to Robert Frost, an instructor and engineer in the US space agency’s Flight Operations Directorate and the author of a Quora article (reprinted by Gizmodo) that dishes the dirt on why the station lacks the sleek designs of its fictional counterparts.

“Where a fictional spacecraft has the luxury of having its design dictated by style, real spacecraft are constrained by budget, tradeoffs, and practicality,” Frost wrote. “Every feature of the ISS can be explained by those words.”

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“We don’t yet have the technology to do construction in space, so we have to assemble a large vehicle in space from launch-able components,” he added. “At the time of the ISS assembly, the two mechanisms for getting a large payload to space were the Space Shuttle Orbiter and the Russian Proton rocket. Those two sentences explain a lot of the ISS appearance.”

Frost went on to explain that the ISS had to be assembled from pieces that were small enough to find in the space shuttle’s payload bay, or in the cargo compartment of a Proton rocket. As such, the maximum length and diameter of these components were limited, forcing crews to rely upon pieces that were primarily cylindrical in shape and able to be linked together.

Parts had to fly themselves

The delivery vehicles also mandated certain other characteristics, he added. While the shuttle could transport unpowered cylinders, remove it from the payload bay and attach is using a robot arm, the Proton rocket had to leave its payload in orbit. The components would then have to fly themselves to the ISS, meaning that all of the Russian modules are active spacecraft, complete with fuel tanks, thrusters, navigation equipment, and a communications array.

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The power required to generate the several laboratories used on the space station also required the use of enormous solar arrays, nearly an entire football field in size, Frost explained. Since the angle to the sun changes while the ISS is in orbit around the Earth, those arrays needed to be able to rotate so that they could always be facing the sun, further complicating the design.

Solar arrays

The energy requirement “dictates that those solar arrays need to have unobstructed paths – unobstructed not just in their rotation, but unobstructed in their line of sight to the sun. So we mount the solar arrays off to the sides and we keep the profile of the rest of the vehicle low,” he said. “Similarly, we need to be able to reject heat to space and thus need to have large radiators. Those radiators need to be able to articulate so that they aren’t in direct sunlight.”

“To hold those solar arrays we need rigid structures that can handle the twisting torques of the solar array rotation and drag,” Frost added. “That’s the big horizontal bar across the vehicle. The trusses are not pressurized modules, but they aren’t wasted space. The trusses are full of equipment like batteries and coolant pumps. The ISS needs to stay powered during night passes, so several very large batteries are needed.”

The NASA engineer added that the station needed to be designed in such a way to give the crew unimpeded access to docking vehicles, the ability to reach berthing ports with its robotic arm and power, data, and consumables connectivity. They also need stable attitude control of the stack, as well as ensuring that the communications and GPS antennae are not blocked by obstructions.

Science fiction is just that – fiction

“When I watch science fiction,” Frost said, “I find myself looking at the smooth, uniform, symmetrical ships and asking myself questions like ‘How do they reject heat? Where are they getting their electrical power? Where are the communications antennae? How does a docking vehicle avoid pluming? … Where the heck did they build that thing? And so on.’”

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“The ISS is all about function over form,” he added. “Every little projection, every little change in color, every change in dimension on the ISS is for a explicit engineering reason.”

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