Sixty-five hours after departing Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla., in a blaze of fire and light, SpaceX’s sixth dedicated Dragon cargo mission (CRS-6) was successfully berthed at the Earth-facing (or “nadir”) port of the Harmony node on the International Space Station (ISS) at 9:29 a.m. EDT Friday, 17 April. As described in AmericaSpace’s CRS-6 launch report, this is the sixth of 12 flights under the terms of SpaceX’s $1.6 billion Commercial Resupply Services (CRS) contract with NASA and it ferried about 4,390 pounds (1,990 kg) of payloads, provisions, tools, and scientific experiments to the incumbent Expedition 43 crew. Dragon will remain an integral part of the ISS for about five weeks, before it returns to Earth in late-May.
Capture of the CRS-6 spacecraft with the 57.7-foot-long (17.6-meter) Canadarm2 robotic arm was accomplished by Expedition 43 Flight Engineer Samantha Cristoforetti—a European Space Agency (ESA) astronaut, who became Italy’s first woman in space when she launched last November aboard Soyuz TMA-15M—and Commander Terry Virts of NASA, both of whom were based in the station’s multi-windowed cupola, at 6:55 a.m. EDT Friday. At the point of capture, the ISS and Dragon were flying about 250 miles (400 km) above the Northern Pacific Ocean. Slightly over 2.5 hours later, following a carefully orchestrated symphony of robotic activity in orbit, the cargo ship was berthed perfectly at the Harmony nadir interface. Current plans call for the hatches into Dragon to be opened early Saturday morning.
On 7 April, the Expedition 43 crew—which, in addition to Virts and Cristoforetti, also includes veteran Russian cosmonauts Anton Shkaplerov and Gennadi Padalka and One-Year crewmen Scott Kelly and Mikhail Kornienko—participated in extensive robotic preparations for Dragon’s arrival. Friday morning began early and the astronauts began setting up their laptops and other equipment in the cupola, which would afford them a panoramic view of the final stages of the rendezvous as it unfolded.
Overnight Thursday, Dragon conducted a series of burns of its Draco thrusters to draw closer to the ISS, along the so-called “R-Bar” (or “Earth Radius Vector”), an imaginary line running from the center of Earth to the space station. By thus approaching its quarry from “below,” the cargo ship took advantage of natural gravitational forces to brake its final approach and limit the need for additional burns. Similar R-Bar rendezvous profiles have been adopted since the shuttle-Mir era in the 1990s. These maneuvers brought Dragon to a “Hold Point” about 1.5 miles (2.4 km) from the ISS, whereupon it was required to pass a “Go/No-Go” poll of flight controllers before advancing closer.
Further polls and hold points were made at distances of 3,700 feet (1,130 meters) and 820 feet (250 meters), the latter of which was reached at 5:27 a.m. EDT Friday. After a “Go/No-Go” poll and systems checkout, Dragon resumed its approach 13 minutes later, at 5:40 a.m. Moving at a steady, slowpoke pace of less than 3 inches (7.6 cm) per second, it entered the Keep-Out Sphere (KOS)—a collision-avoidance exclusion zone, extending 650 feet (200 meters) around the ISS—and slowed its rate of approach to just under 2 inches (5 cm) per second. By 6:14 a.m., the snub-nosed cargo ship reached its next-to-last Hold Point at 90 feet (30 meters) and was cleared to commence its final approach by 6:27 a.m. Its next target was 30 feet (10 meters), the “Capture Point,” which would position it within range of Canadarm2’s Latching End Effector (LEE). The mechanical arm is part of Canada’s contribution to the ISS and builds upon the heritage of the original “Canadarm,” the shuttle’s Remote Manipulator System (RMS), which supported dozens of missions and a wide range of construction, retrieval, deployment, and repair tasks from November 1981 until July 2011.
Today’s grapple of CRS-6 marked the 14th overall capture of a cargo ship by Canadarm2, which was installed aboard the ISS by the STS-100 shuttle crew, way back in April 2001. Since September 2009, the “Big Arm” has supported the capture and berthing of four H-II Transfer Vehicles (HTVs) on behalf of the Japan Aerospace Exploration Agency (JAXA), seven Dragons—including the Commercial Orbital Transportation Services (COTS) Demo in May 2012 and the six dedicated CRS missions—and three Cygnus resupply ships for Orbital Sciences Corp.
Inward motion toward the ISS resumed, as Dragon pulsed its Draco thrusters to continue the final approach, reaching the Capture Point by 6:44 a.m. The cargo ship was commanded into Free Drift, deactivating its thrusters, ahead of capture. The official time at which Canadarm2’s LEE seized Dragon was at 6:55 a.m., as the new arrival and the space station flew about 250 miles (400 km) above the Northern Pacific Ocean. Messages of congratulations flooded in from around the world, with Britain’s Tim Peake—who will fly to the ISS aboard Soyuz TMA-19M in November—tweeting “Great job @AstroSamantha and @AstroTerry capturing #SpaceX Dragon this morning!” Added veteran ISS resident Catherine “Cady” Coleman: “Woo Hoo! @AstroSamantha did a marvelous job capturing #Dragon. Quiet, Precise, steady: That’s our Sam! She & @AstroTerry made a great team!” From aboard the station itself, Scott Kelly tweeted his congratulations, whilst Cristoforetti herself later tweeted “A #Dragon came knocking at our door. Thought it’d be nice to grab it & see what’s in it.”
Over the following 2.5 hours, flight controllers and the Expedition 43 crew worked to maneuver Dragon to its eventual berthing point at the Earth-facing (or “nadir”) port of the Harmony node. The physical berthing of the cargo ship occurred in two parts, with the astronauts overseeing “First Stage Capture,” in which hooks from Harmony’s nadir Common Berthing Mechanism (CBM) extended and grabbed Dragon to pull their respective CBMs into contact. This was followed by “Second-Stage Capture,” in which a series of 16 bolts—four gangs of four bolts apiece—were driven to rigidize the two spacecraft in a tight, mechanized embrace. This was confirmed by NASA as having been successfully completed at 9:29 a.m. EDT, about 154 minutes after the initial grapple.
The Expedition 43 crew were then given a “Go” to pressurize the vestibule leading from the Harmony nadir hatch into Dragon, which is bringing about 4,390 pounds (1,990 kg) of payloads, provisions, tools, and scientific experiments to the ISS. Among its cargo is freshly brewed coffee for the ISSpresso machine, prompting Cristoforetti to tweet—and mockingly paraphrase Capt. Kathryn Janeway from “The Cloud,” the sixth episode of Star Trek: Voyager—“There’s coffee in that nebula … ehm, I mean … in that #Dragon.”
Current plans call for Dragon to depart in about five weeks’ time for its return to Earth. Dragon is presently the only visiting vehicle capable of returning large quantities of payloads safely back to Earth; all other cargo craft—Russia’s Progress, Japan’s HTV, Europe’s Automated Transfer Vehicle (ATV), and Orbital Sciences’ Cygnus—are not designed to be recovered and are intentionally destroyed during re-entry. Dragon, on the other hand, returns to a parachute-assisted splashdown in the Pacific Ocean, off the coast of Baja California.
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