Docking is the paradox of spaceflight. On the one hand, it’s as boring to watch as an apple turning brown; on the other, it’s white-knuckle suspense when you realise that tons of paper-thin metal are one misstep away from destruction. What’s worse, the number of different docking port designs makes compatibility a major concern, so ESA is developing a universal docking mechanism that will allow any spacecraft to lock onto any other.

If you look at the design of the International Space Station (ISS), you’ll see a surprising number of docking ports. The reason for this is because over the past 50 years, the various space agencies have developed their own ways of docking one spacecraft with another, and these mechanisms are rarely compatible with one another. Think of it as the USB port wars of space exploration.

On the ISS, there are basically two kinds of ports. First, there are the berthing ports. These are used for docking unmanned freighters by using the station’s robotic arm to guide them in. The others are the docking ports, which are for spacecraft that can precisely maneuver under their own power in a docking operation.

Space Shuttle docking port (Photo: NASA)

Unfortunately, those docking ports come in more than one flavor. One set uses Russian-designed ports for Russian and European spacecraft. These, in turn, come in a male and female version, which raises compatibility problems that any home workshop enthusiast can relate to.

The other set are the American ones designed for the Space Shuttle. They’re intended for a 100-tonne (110-ton) spaceship and are designed to work when the massive Shuttle presses against it. This is a problem because what works for a Shuttle won’t work so well for a 10-tonne (11-ton) ship that doesn’t have the inertia to activate the locking rings.

ESA’s Common Berthing Mechanism, as it’s called, is designed to pick up where the Americans and Soviets left off in 1975 with the Apollo-Soyuz project. Intended as a "handshake" in space to usher in an era of détente and cool down the Cold War, Apollo-Soyuz involved the rendezvous and docking of an American Apollo Command Service Module (CSM) and a Soyuz spacecraft.

Apollo-Soyuz docking mechanism (Photo: NASA)

This wasn’t as easy as it first seems because the Apollo Command Module used a low-pressure atmosphere of nearly pure oxygen, while the Soyuz uses a mixture closer to ordinary air at normal pressure. Worse, the two spacecraft used completely different docking mechanisms, so engineers came up with an airlock module that had a standard Apollo docking port at one end to fit on the CSM and a new Androgynous Peripheral Attach System (APAS), which was a universal docking port where the docker and the dockee were identical rather than male and female.

This system worked, but the Cold War heated up again and the APAS went on to become the Space Shuttle docking port and the Soviets adapted it for their Mir space station. Otherwise, the “universal” system was just one of several in use.

Rendering of the ESA International Berthing and Docking Mechanism (Image: QinetiQ)

To get things back on track, ESA is developing a new docking system that will be the international standard and will be identical on all spacecraft, so any vehicle can dock or berth with any other craft. According to ESA, this has become especially important because the next generation of spacecraft will all dock rather than berth with the space station.

The clever bit about the Common Berthing Mechanism is that while the rings that hook to one another will be identical, the mechanisms between ring and spacecraft don’t need to be, so they can still be designed to suit a particular type of vehicle. The ESA version, for example, can sense the forces as two vehicles come into contact, which makes it easier to dock with a lighter craft, or with a heavier craft with less of a bump.

According to the space agency, an engineering body is being built in Belgium by Qinetiq Space and will be ready for flight in 2017. The Dream Chaser space plane is currently the prime candidate to carry the mechanism.

Source: ESA

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