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.

Hatfield's office has just completed a preliminary design review of the two IDAs that NASA plans to send to the station, the first late in 2014 in a Dragon trunk. Essentially a modified APAS at one end and a NASA-developed passive docking interface that meets the international standard at the other, the IDA will be pulled from the Dragon with the station arm, positioned in front of PMA-2 on the front of Harmony, and installed by a pair of spacesuited astronauts.

A second IDA will follow later, for installation on a position to be determined by NASA and its partners. Initially, though, the mechanism on the front of Harmony will be the point of entry for station crews arriving on U.S. commercial vehicles, all of which are being built to meet the basic international standard for spacecraft docking.

Approved by the top human-spaceflight managers at the Canadian, European, Japanese, Russian and U.S. space agencies, the IDSS sets parameters that will allow any spacecraft meeting the standard to dock with any other (AW&ST Oct. 25, 2010, p. 34). Hatfield's office also is developing a NASA Docking System (NDS) that meets the international standard, and will play the most active role in commercial-vehicle dockings at the space station, at least initially.

All four companies working with partial NASA funding under Space Act Agreements (SAAs) to develop commercial crew vehicles—Blue Origin, Boeing, Sierra Nevada and SpaceX—will use the NDS to dock with the station. NASA has also discussed the IDSS with companies that have unfunded SAAs for commercial crew work. The JSC Engineering Directorate is building the first “production representative” of the system, and Boeing will produce the qualification and flight systems. Details of how the systems will be transferred to private companies are still being worked out.

“We have said these visiting spacecraft have to have an IDSS-compatible docking system,” Hatfield says. “So they are basically free to pursue how they want to achieve that.”

The NDS also will be mounted on NASA's Orion Multi-Purpose Crew Vehicle, a holdover from the old Constellation program of human exploration vehicles that is intended for human exploration beyond low Earth orbit. A technical descendent of the Low-Impact Docking System (LIDS) that was under development for Orion under Constellation, the NDS uses an electro-mechanical system of sensors and actuators to detect and damp out loads after initial contact, within parameters set by the international standard (see chart).

“The Russians have in their system springs and dampers that attenuate the forces and moments,” Hatfield says. “We have a closed-loop system that senses forces and moments through a set of load cells, and then dynamically manages a set of actuators to damp that out.”

Detailed work on the international standard is still evolving at the working-group level, Hatfield says, with the “conservative” Russians wanting to see that the U.S. approach works before they adopt it themselves. One issue has to do with the difference in the width of the rings that must meet and align in the soft-capture portion of a docking to form a solid connection in the hard mate that follows. The U.S. system is based on a ring 104 mm (4 in.) across, while the Russians use a 45-mm ring in their APAS. Both systems connect a 1,255-mm “tunnel” that the crew can use to move themselves and their gear between spacecraft.

Also to be determined is a standard for passing utilities—data, power, video and fluids—across the interface. The standards are public documents, although some elements of the NDS design are protected by U.S. International Traffic in Arms Regulations (ITAR).

“If they wanted to build it to print, for example, they would need to have an agreement with us for us to release those details so that we're compliant with export control and ITAR,” says Hatfield. “But that could be a contract, it could be a Space Act Agreement, there are a number of ways to enter into those kinds of agreements that would allow us to give them that data package.”

Theoretically, though, even China could dock at the station if they designed their own active system to meet the IDSS parameters. At the other end of the station, Russia continues to use the probe-and-cone mechanism built into its Soyuz and Progress capsules, which also has been adopted by ESA and the Japan Aerospace Exploration Agency for their unpiloted cargo carriers.

Visiting Vehicle Initial Contact Conditions
Initial Condition Limiting Value
Closing (axial) rate 0.05–0.10 meters per second
Lateral (radial) rate 0.04 meters per second
Pitch/yaw rate 0.15 deg. per second (vector sum of pitch/yaw rate)
Roll rate 0.40 deg. per second
Lateral (radial) misalignment 0.11 meters
Pitch/yaw misalignment 5 deg. (vector sum of pitch/yaw rate)
Source: International Space Station partnership