Space Exploration Technologies’ (SpaceX) Dragon spacecraft was grappled by the International Space Station’s robotic arm at 9:56 a.m. EDT then berthed to the ISS about an hour later. Dragon’s arrival is one for the history books as the spacecraft is the first commercial spacecraft to deliver cargo to the ISS. SpaceX has met milestone after milestone on the flight with the next major objectives its maneuvers to depart the station then safely return to Earth.
The capture and berthing May 25 followed a series of precise maneuvers May 24 to ensure the SpaceX Dragon and ISS were ready to fly in close formation at 17,500 mph.
Tried and true station rendezvous rules and techniques pioneered by nearly 15 years of shuttle/ISS operations were combined with Dragon’s pioneering new sensors and auto rendezvous capabilities being exercised for the first time.
The SpaceX achievement in orbit is analogous to the 1970s Earthly startup of Fedex–Federal Express–where a tiny company was able to out-compete much larger competitors to become a globally renowned commercial delivery service.
The station’s Canadarm 2 manipulator arm piloted by astronaut Don Pettit captured the Dragon as the nearly 1 million pound outpost and the small commercial spacecraft flew only 32 ft. apart.
Dragon’s rendezvous capabilities were first demonstrated May 24, on Day 3 of the flight, when Dragon performed a series of rendezvous burns that brought it 1.5 mi. below the ISS on the “radius vector” or R-Bar, an imaginary line straight down from the ISS to Earth.
During this “fly-under,” maneuver the Dragon spacecraft established UHF communication with the station using its COTS Ultra-high frequency Communication Unit (CUCU).
Dragon also performed a test of its Relative GPS (RGPS) system, which used the relative positions of the Dragon to the space station to determine the exact location of the ISS within Dragon rendezvous electronics and computers.
The astronauts on the ISS then used the crew command panel (CCP) on board the outpost and the station’s Earth facing windows to monitor the fly-under and to also command Dragon to turn on its strobe light to verify they had a working command link for the critical approach today.
Once the fly-under was complete Dragon fired its 90 lb. thrust Draco engines to begin a loop out in front, then above and behind the station in a racetrack pattern at a distance of between 4-6 mi. This set the Dragon up for a re-rendezvous with the station today.
For its final approach to the ISS before dawn Eastern time this morning, Dragon performed a number of precisely calculated engine burns to once again bring it 1.5 mi. below the station.
A “GO” was then given by the NASA Mission Control Center in Houston to clear Dragon to perform another set of burns to bring it to within 0.87 mi. of the station.
Yet another “GO” decision by Mission Control gave permission for Dragon controllers to then cue software to initiate more thruster firings to very slowly maneuver the Dragon upward to a position 820 ft. below the ISS.
Another key event performed at this point was a test of the Dragon LIDAR laser radar system. This was to confirm that the position and velocity calculations by Dragon’s computers were accurate by ensuring the LIDAR image matched the data obtained with Dragon’s thermal imagers. At one point one of LIDAR systems two lasers had to be re-tuned , delaying the process slightly, said Elon Musk, SpaceX CEO.
Several small precision checkout maneuvers were then initiated before the final approach.
The Dragon flight control team in Hawthorne, Calif., commanded the spacecraft to approach the station from Dragon’s holding position moving upward 98 ft. to 720 ft. under the ISS.
The astronaut crew, using the CCP, then commanded Dragon to retreat by sending a command for the auto firing of thrusters to slowly move it back down the radius vector to the hold point at 850 ft. This test was to ensure that Dragon’s range to the ISS was accurate, and that the flight control team could see that the spacecraft’s acceleration and braking maneuvers were performed as expected.
After holding again briefly at 850 ft. the Hawthorne team again commanded Dragon to once again maneuver back up to the position where the astronaut crew had earlier commanded it to hold before the test of the retreat command.
At this point another “GO” was given by Houston, to allow the Dragon to fly into the “Keep-Out Sphere”, an imaginary sphere drawn 656 ft. around the station to prevent the risk of a collision with the orbiting complex. With that approval, Dragon was to proceed to a position 98 ft. from the station and another automatic hold.
After extensive checks and another approval from NASA in Houston, SpaceX commanded the Dragon to proceed to the 32 ft. position under the ISS, which was the capture point for the station’s Canadarm 2 manipulator arm.
Enroute, however, a second “retreat” command was issued again, this time from the SpaceX control center when controllers wanted to double check the movement of the Dragon up the Radius Vector toward the ISS.
After yet another planned assessment in Houston and Hawthorne, controllers notified the station crew they were “GO” to capture Dragon.
At that point, Expedition 31 crew member Pettit used the 57.7 ft. Canadian station arm to reach out and grapple a fixture on the Dragon spacecraft as the two vehicles flew into an orbital sunset at nearly 260 mi. altitude.
Astronaut Cady Coleman used the same technique on the ISS to capture the Japanese HTV transfer vehicle on an earlier ISS mission. The capture of the Dragon was a challenging maneuver that had only been done with two previous unmanned Japanese transports.
Pettit, with the help of fellow crewmember Andre Kuipers, then guided Dragon to the Earth-facing side of the Harmony Node 2 module where it was attached to the station at about 11 a.m. EDT (see SpaceX image below). The docking was achieved using a common berthing mechanism (CBM) using passive hardware on the Dragon but with active mechanisms on the ISS side. The Node 2 CBM was operated by newly arrived station astronaut Joe Acaba.
The station crew will open the hatch between the Dragon and the station a day after berthing to begin unloading 1,014 lb. of cargo. They will then refill the spacecraft with nearly 1,400 lb. of down cargo including experiment results for return to Earth early next week.
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