Following yesterday’s eventful launch, the second dedicated Dragon cargo ship has settled into orbit and is en-route to a docking with the International Space Station, after what SpaceX CEO Elon Musk described as a “frightening” first few hours. According to Expedition 34 crewman Chris Hadfield, who tweeted from the ISS earlier today, the Dragon’s early problems appear to now be “fixed” and the astronauts were in the process of “re-planning the rendezvous and grapple schedule” for the robotic visitor. Current thinking is that the spacecraft will arrive in the vicinity of the station tomorrow (Sunday) for capture by the 57-foot-long Canadarm2 robotic arm and berthing on the Earth-facing Harmony node.
The present CRS-2 mission—the third Dragon to visit the station and SpaceX’s second under its $1.6 billion Commercial Resupply Services contract with NASA—was launched in seemingly perfect fashion from Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla., at 10:10:13 a.m. EST, 1 March. Unlike last October’s CRS-1 launch anomaly, the nine Merlin-1C engines on the first stage of the Falcon 9 rocket appear to have performed well, as did the single Merlin-1C Vacuum engine on the second stage. However, shortly after the separation of the Dragon, the situation went distinctly awry.
Three of the four Draco thruster pods did not activate properly, due to a blockage in a propellant pressurization line. This threw out of sync several pre-programmed tasks, including the deployment of the solar arrays, and the scheduled 2 March ISS docking was scrubbed. Although two thrusters will suffice to get to the station, at least three are needed to satisfy NASA’s ISS Program that the Dragon is fully-controllable. There were other woes. If the thrusters could not be brought on-line, the cargo craft’s 120-mile perigee would be too low to enable it to remain in orbit for more than a couple of days.
At length, SpaceX engineers worked through the troubles, bringing all four Draco pods on-line, performing the critical orbit-raising maneuver, and re-establishing the snub-nosed spacecraft on course for the station. “Orbit raising burn successful,” Elon Musk tweeted yesterday, the excitement undeniable in his words. “Dragon back on track.”
By the time Chris Hadfield sent his own tweet today, the cargo ship—whose 1,490 pounds of payload is housed both in its pressurized cabin and, for the first time, in its large unpressurized “Trunk” section—was about 1,770 miles “ahead” of the ISS. From his perspective, Elon Musk described the event as “a glitch of some kind … not serious,” but nevertheless illustrative of the reality that launching spacecraft remains a business laden with risk. “There’s no leakage,” he acquiesced of the Draco propellant line trouble. “There’s no debris or fluid or gas leakage that we’re aware of. All systems seem to be intact and functioning quite well.”
If Dragon receives an all-clear to approach the ISS tomorrow, it will do so by adopting an “R-Bar” (or “Earth Radius Vector”) rendezvous profile. This follows an imaginary line, extending from the center of the Earth to the space station, meaning that the cargo craft will effectively rise to meet it from “below.” In doing so, Dragon will take advantage of natural gravitational forces to brake its final approach and limit the need for thruster firings.
As it draws closer to its target, the spacecraft will establish an ultra-high-frequency link using its UHF communications unit (CUCU), which enables the space station’s incumbent Expedition 34 crew—led by Commander Kevin Ford—to monitor the final approach. A carefully orchestrated symphony of maneuvers will bring the Dragon to a position about 1.5 miles from the ISS, where a “Go/No-Go” decision to proceed will be made. Further Go/No-Go decisions will be made at distances of 3,700 feet, then 820 feet, then 100 feet, and finally 30 feet, before Dragon is captured by U.S. Operating Segment (USOS) crewmen Ford and Tom Marshburn, using the Canadarm2. Both men will be based in the station’s multi-windowed cupola, which will afford them expansive panoramic views of the rendezvous and docking at the nadir-facing Harmony node.
During these incremental steps toward its quarry, Dragon will employ its close-range guidance instruments, including lidar and thermal imaging equipment, to confirm the accuracy of its position and velocity. After initial berthing at Harmony, the vestibule between the two spacecraft will be pressurized Sunday, and Ford and his crew will open the hatch to begin a three-week process of unloading Dragon’s cargo. A little more than half of the load is devoted to ongoing scientific research 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.
The remainder of the CRS-2 mission remains to be seen. The pre-launch press kit outlined a four-week voyage, with the pressurized capsule re-entering and splashing down, a few hundred miles off the coast of Southern California, 25 March. Such plans may require revision, based upon recent events. All in all, SpaceX’s CRS contract with NASA calls for 12 dedicated missions—of which this is the second—and requires the delivery of some 44,000 pounds of payload to the ISS.