Frank Morring, Jr./Washington and Mark Carreau/Houston

Before the SpaceX Dragon can begin carrying crews to the International Space Station (ISS), it must deliver a new docking mechanism that astronauts will affix permanently in the spot where space shuttles once connected to the orbiting laboratory.

That is good news and bad news or SpaceX. The company can add 750-1,000 lb. of payload to its commercial cargo manifest for the ISS. But any competitor with a docking mechanism that meets the emerging International Docking System Standard (IDSS) will also be able to use it.

The cargo version of Dragon that last month became the first commercial vehicle to reach the ISS includes a “trunk” for unpressurized cargo—a unique capability that will find a market niche that NASA once filled with the space shuttle's payload bay (AW&ST May 21, p. 24). But before it can begin flying astronauts in Dragon's pressurized compartment, SpaceX engineers must change the way their vehicle connects with the space station—from the grapple-and-berth technique used May 25 to a shuttle-style docking.

“In the event that the crew needs to leave for some reason, you don't want to be dependent on a system on the ISS like the arm,” says Skip Hatfield, manager of the development projects office for the ISS program at Johnson Space Center (JSC). “You want to be able to jump in the thing and just depart, in case you're having a bad day, so to speak.”

To reach the station in its demonstration flight, the inaugural cargo Dragon flew in formation 10 meters (33 ft.) below it while NASA astronaut Don Pettit manipulated the 17.6-meter-long Canadarm2 to grapple the unpiloted vehicle from the robotic control station in the ISS cupola. Aided by Andrew Kuipers of the European Space Agency (ESA) and Joe Acaba of NASA, Pettit maneuvered the vehicle to a common berthing mechanism (CBM) on the Harmony node for a hard mate and unloading across the pressurized interface. The process was reversed May 31, when the Dragon left the station to reenter the atmosphere and parachute to a splashdown landing 560 mi. off the coast of Baja California.

The crew version of Dragon also will be designed to link with the station at Harmony, which is nestled between the main European, Japanese and U.S. laboratory modules and attached to them with CBMs that contain the interior hatches. But the Dragon—and other commercial crew vehicles docking with the station—will use a new International Docking Adapter (IDA) that fits onto the Russian-built Androgynous Peripheral Attach System (APAS) at the forward end of Harmony.

Integrated into the Pressurized Mating Adapter (PMA), the Russian system was designed to perform either the passive or active function in a vehicle docking. It mechanically damps out oscillations as the vehicles make contact and then cranks them into a structural connection.