Less than two days since its impressive launch, and only 24 hours later than planned, SpaceX’s remarkable Dragon cargo ship has been successfully captured and berthed to the International Space Station. The snub-nosed craft was seized by the station’s Canadarm2 robotic arm—deftly manipulated by Expedition 34 Commander Kevin Ford—at 5:31 a.m. EST, about half an hour earlier than the time published by NASA yesterday afternoon. The final approach of Dragon ran smoothly and with perfection, and at the time that this article was being prepared the ISS crew had been given a “Go” to pressurize the vestibule between the craft and the Harmony node, ahead of opening the hatches tomorrow.
This mission—the third Dragon to visit the multi-national outpost and the second under SpaceX’s $1.6 billion Commercial Resupply Services (CRS) contract with NASA—had much to prove when it roared into space atop a Falcon 9 booster at 10:10:13 a.m. EST, Friday. The CRS-1 mission, last October, experienced an engine-out anomaly which doomed a small Orbcomm piggyback satellite, but Friday’s CRS-2 launch suffered no such problems … at least, that is, until Dragon separated in orbit. At that point, only one of the ship’s four Draco maneuvering thruster pods was fully operational, a situation which could have potentially ruined the mission, for at least three are required to be in working order for the ISS Program to approve a rendezvous and docking with the space station.
The cause was traced to a blockage in a propellant pressurization line, which threw out-of-sync a number of other pre-programmed tasks, including the deployment of Dragon’s electricity-generating solar arrays. As noted by the website NASASpaceflight.com, Dragon’s initial problems were triggered when the fuel tanks pressurized correctly, but only one oxidiser tank showed its nominal pressure response. The solar arrays were unfurled, nevertheless, and engineers eventually “jack-hammered” the helium isolation system valves by commanding them to cycle on and off several times in succession. At length, this succeeded in clearing the blockage. Dragon’s scheduled arrival at the ISS on 2 March was called off as troubleshooting efforts got underway, but within a matter of hours the other three Draco pods were restored, a critical orbit-adjustment maneuver was satisfactorily performed, and SpaceX CEO Elon Musk tweeted Friday night that Dragon was “back on track.” Yesterday, the two space vehicles drew ever closer, and Expedition 34 crewman Chris Hadfield remarked at one stage that the cargo ship was about 1,770 miles “ahead” of them.
Also on Saturday, NASA announced that the berthing time had been updated to 6:01 a.m. EST. To accomplish the capture and berthing of the cargo ship at the nadir-facing Harmony node, the three U.S. Operating Segment (USOS) crewmen—Ford, Hadfield, and NASA astronaut Tom Marshburn—ate breakfast and set up their laptops and other equipment in the multi-windowed cupola, which would afford them an expansive, panoramic view of the events to follow. Shortly after 2 a.m. Sunday, Ford reported his first visual sighting of Dragon as a star-like blob against the black sky—a star which grew steadily brighter and more pronounced as the time passed.
By 3:40 a.m., the ship had arrived at a position known as the “R-Bar,” or “Earth Radius Vector,” whereby it completed the final stages of its rendezvous with the ISS. Effectively rising to meet its target from “below,” Dragon took advantage of natural gravitational forces to brake its approach and limit the need for thruster firings. Similar R-Bar profiles have been adopted since the Shuttle-Mir era in the mid-1990s. A carefully orchestrated symphony of maneuvers brought the cargo craft to a “Hold Point” about 1.5 miles from the space station, where it passed a “Go/No Go” poll to proceed. Further polls and holds were made at distances of 3,700 feet and 820 feet, with the latter “Hold Point” being reached at 3:55 a.m., as both vehicles flew high above the Black Sea. Ten minutes later, Dragon continued its approach, keeping to the R-Bar path and creeping toward the ISS at a steady, slowpoke pace of less than three inches per second. By 4:20 a.m., it had entered the so-called “Keep Out Sphere”—a collision-avoidance exclusion zone, extending to 650 feet around the ISS—and by the time it reached a distance of 300 feet its rate of closure had slowed to a little under two inches per second.
The minutes passed. At 5 a.m., watched by the ISS crew and an anxious Mission Control Center in Houston, Texas, the cargo ship had reached its next-to-last “Hold Point” at 100 feet, where SpaceX controllers verified good navigational data and the polling of relevant personnel began. Five minutes later, Dragon was cleared to approach to the 30-foot “Capture Point,” within range of the Canadarm2, and proceeded to hold its position. At 5:25 a.m., Capcom Kate Rubins radioed Commander Kevin Ford with a “Go for Capture.” The official time at which the Canadarm2’s Latching End Effector grappled the craft was 5:31 a.m EST., a little over 43 hours after launch. In an intricate operation which lasted several hours and was punctuated by lunch and the need to exercise, Ford and Marshburn maneuvered Dragon toward its berth on the Harmony node, with Hadfield in charge of driving the capture latches into position. Official “berthing” of the cargo ship occurred at 8:56 a.m. EST, and the crew was given a “Go” to pressurize the vestibule an hour later. With this remarkable revival of fortunes, it is confidently expected that the CRS-2 mission will remain docked for three weeks, pending a scheduled departure and return to Earth on 25 March.
Ahead for the crew is the physical unpacking of Dragon, which should begin tomorrow. For Kevin Ford, Oleg Novitsky, and Yevgeni Tarelkin—the first half of the Expedition 34 team, launched last October—today’s berthing makes them the first crew to welcome two Dragons to the station during their residency. They were also aboard at the time of the CRS-1 mission. A little more than half of CRS-2’s load is devoted to ongoing scientific research aboard the space station and includes a pair of GLACIER experiment refrigerators, a spare electronics unit for a MELFI freezer, a carbon dioxide removal assembly, and general crew provisions. Mounted on the unpressurized Trunk are two Heat Rejection Subsystem Grapple Fixtures (HRSGFs), which will be installed on the station’s truss during an EVA by astronauts Chris Cassidy and Luca Parmitano in July.