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Space station catches Dragon by the tail

Posted March 4, 2013

SpaceX successfully docked the Dragon capsule with the International Space Station (ISS) Sunday morning at 8:56 a.m. EDT  This is the second of 12 planned cargo resupply missions to the ISS for the Hawthorne, Calif., based company. The procedure involves grabbing the capsule with the station's robotic arm then docking, all performed by astronauts aboard the ISS.

Moving things around in space isn’t easy. Move something the wrong way and it keeps going and going. During the 1965 Gemini 4 mission, astronaut Ed White lost his grasp on a glove during the first space walk. That glove orbited the Earth for about a month before reentering the atmosphere.

One tap the wrong way and a multimillion-dollar satellite can be lost as well. With this in mind, finger-like pincers on the end of a robotic arm won’t work. Unless aligned perfectly, pincers could send an arriving spacecraft like Dragon hurtling away. The design NASA and Canadian Space Agency mechanical engineers came up with is brilliant in its effectiveness and simplicity.

As Dragon approached, astronauts aboard the ISS captured the capsule using the stations robotic arm, CanadaArm2. Their target was the Flight-Releasable Grapple Fixture (FRGF) mounted on the side of Dragon. It looks a bit like a pool cue ball on the end of a stick. The FRGF works with a mechanism on the end of the arm to safely grab the capsule. It is the same system used with original CanadaArm design aboard the five Space Shuttle orbiters.

At the opening of the tubular end effector on the tip of the robotic arm, three cables rotate over each other closing around that ball-tipped rod protruding from the grapple fixture, holding it tightly. The motion is a bit like the iris of a camera closing. The motion of the cables closing grasps the rod, centers it and aligns it in all directions. The arm then locks on making the capsule a part of the station.

Once locked on, astronauts aboard the ISS guide the capsule in for docking with the station’s Harmony module. Dragon will stay docked for about three weeks while 1,268 pounds (575 kilograms) of supplies are offloaded and 2,668 pounds (1,210 kilograms) of samples from completed science research are loaded ahead of a planned splashdown of the capsule off the coast of Baja, Calif., on March 25.

You can get up-close with a Flight-Releasable Grapple Fixture (FRGF) right here in the Triangle. The Museum of Life and Science in Durham features a FRGF on display on the first floor of the space exhibit.

Fixtures like this one are on the side of cargo vehicles like SpaceX’s Dragon capsules, the European Space Agency Automated Transfer Vehicle, as well satellites put in orbit by the shuttle like the Hubble Space Telescope. You can also get an appreciation for the elegant simplicity of the solution by building your own latching end effector with some string and cups.  


Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.

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