Almost a year since the launch of its last fully-successful Dragon mission to the International Space Station (ISS), SpaceX has delivered another of its venerable cargo ships toward the orbiting outpost. Liftoff of the CRS-8 mission—flown under the language of the initial $1.6 billion Commercial Resupply Services contract, signed between NASA and SpaceX in December 2008—took place at 4:43 p.m. EDT Friday, 8 April, from Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla. As with SpaceX’s previous ISS-bound missions, the launch window was an “instantaneous” one, imposing additional restrictions and leaving little margin for last-minute technical issues or changeable weather conditions.
About ten minutes after leaving the Cape, Dragon separated from the second stage of its Upgraded Falcon 9 booster and was in the process of deploying its solar arrays and other hardware, ahead of a robotic capture and berthing by Expedition 47 crew member Tim Peake on Sunday morning.
Including today’s spectacular launch, this brings to nine the total number of Dragons lofted towards the ISS in a little less than four years. Under the terms of the initial CRS agreement, both SpaceX and its fellow commercial cargo partner, Orbital ATK—which operates the Cygnus resupply vehicle—were required to conduct a Commercial Orbital Transportation Services (COTS) “Demo” mission to the station, before pressing into their full commitment of dedicated flights. In launching its Demo mission in May 2012, SpaceX became the first commercial entity to accomplish rendezvous and berthing with the space station, when Dragon was captured by Expedition 31 crewmen Don Pettit and Andre Kuipers, using the 57.7-foot-long (17.6-meter) Canadarm2.
With the successful completion of the Demo mission, SpaceX was able to begin its initial round of 12 cargo delivery flights to the space station, conducted through 2016. All told, the original CRS contract required SpaceX to deliver a total of 44,000 pounds (20,000 kg) of equipment and supplies to the ISS. Between October 2012 and April 2015, six Dragons journeyed to the ISS, each berthing at the Earth-facing (or “nadir”) port of the Harmony node. They transported a range of supplies, tools, equipment and research facilities and experiments to several expedition crews, as well as a number of unpressurized payloads inside Dragon’s trunk. The latter have included the Rapid Scatterometer (RapidScat), the Cloud Aerosol Transport System (CATS) and a pair of Heat Rejection Subsystem Grapple Fixtures (HRSGFs), which were later attached to the station’s external radiators during U.S. EVA-22 in July 2013.
In addition to its baseline 12-flight commitment, SpaceX’s roster of Dragons has been augmented by several additional missions under its initial CRS contract and, in January 2016, was one of three recipients of the follow-on CRS2 contract. The latter is expected to see a minimum of six more Dragon flights, to be conducted from 2019 through to the currently timetabled end of ISS operations in December 2024.
A failure-free run of Dragon launches came to a shuddering halt on 28 June 2015, when the CRS-7 mission—laden with the first of two Boeing-built International Docking Adapters (IDAs) for future Commercial Crew applications—was lost at high altitude, just 139 seconds after liftoff. As SpaceX dug in for several months of investigations and corrective actions, the next planned Dragon mission (CRS-8) fell back on the manifest, before eventually winding up in fourth place, after last December’s opening voyage of the Upgraded Falcon 9 and a pair of NASA and commercial payloads in the opening weeks of 2016.
In readiness for Friday’s launch attempt, SpaceX undertook a standard Static Fire Test of the Upgraded Falcon 9’s nine Merlin 1D+ first-stage engines on Tuesday, 5 April. This was followed by a Launch Readiness Review (LRR). On launch day, loading of liquid oxygen and rocket-grade kerosene (known as “RP-1”) commenced about 35 minutes before T-0. The countdown reached its final “Go/No-Go” polling point of all stations at T-13 minutes and the Terminal Count got underway at T-10 minutes.
During this period, the Merlin 1D+ engines were chilled, preparatory to their ignition sequence, and all external power utilities from the Ground Support Equipment (GSE) were disconnected. The “strongback” was retracted from the vehicle at T-5 minutes and the Flight Termination System (FTS) was placed onto internal power and armed. Fueling concluded and the first stage’s propellant tanks attained their proper flight pressures. Finally, at T-60 seconds, the 53 nozzles of the Niagara deluge system were activated, flooding the launch pad surface and flame trench with 30,000 gallons (113,500 liters) of water per minute to suppress acoustic energy at the instant of engine ignition.
All nine Merlins roared to life at T-3 seconds and ramped perfectly up to a combined thrust of 1.5 million pounds (680,000 kg). After computer checks validated their health, the booster departed SLC-40—making it the 21st member of the Falcon 9 family to do so since June 2010—at 4:43 p.m. EDT. Immediately after clearing the tower, it executed a combined pitch, roll and yaw program maneuver to establish itself onto the proper flight azimuth to inject the CRS-8 Dragon into low-Earth orbit.
Passing through the speed of sound at 80 seconds into the ascent, the Upgraded Falcon 9 experienced maximum aerodynamic stress (colloquially known as “Max Q”) on its airframe. Later, two of the first-stage Merlins were throttled back to reduce the rate of acceleration at Main Engine Cutoff (MECO). A little under three minutes after leaving the Cape, the seven remaining Merlins were shut down and the first stage separated from the stack. The turn then came for the restartable second stage, whose single Merlin 1D+ Vacuum engine ignited to continue the climb towards orbit. During its burn, the protective nose fairing—which covers Dragon’s berthing mechanism—was jettisoned and the spacecraft separated from the second stage about ten minutes into the mission. Its two electricity-generating solar arrays were deployed, as was its Guidance and Navigation Control (GNC) Bay Door to expose critical rendezvous sensors. An intricate series of maneuvers to reach the vicinity of the ISS on Sunday morning also got underway.
With the successful delivery of CRS-8 to low-Earth orbit, SpaceX’s second mission of the day got underway. Shortly after the separation of the Upgraded Falcon 9’s first stage, efforts to return the hardware to a soft landing on the deck of the Autonomous Spaceport Drone Ship (ASDS) in the Atlantic Ocean kicked into high gear. As outlined previously by AmericaSpace, a number of attempts have been made to land Falcon 9 first stages on the ASDS, with varying degrees of success.
A series of “controlled oceanic touchdowns” in April, July and September 2014 were followed by four attempts to physically land on the ASDS. The first, in January 2015, saw the first stage reach the deck, but impact at a 45-degree angle and explode, whilst the second, in April 2015, landed with excessive lateral velocity and toppled over upon impact. Then, in January 2016, the final Falcon 9 v1.1 came close to a bull’s-eye landing on the ASDS, but was thwarted by the failure of one of its landing legs to properly latch into position. More recently, last month’s first-stage hardware from the SES-9 launch reached the deck—against many odds, having endured a high-energy re-entry and a truncated series of descent burns—but “impacted hard” on the ASDS and was lost.
With today’s launch, however, the Falcon-9 first stage executed its offshore landing flawlessly, marking another big milestone accomplishment for the company as they continue evolving the booster into a rapidly reusable (and cheaper) launch system.
VIDEO: Falcon-9 Drone Ship Landing April 8, 2016 (credit: SpaceX)
“Like landing on a postage stamp”, said Elon Musk this afternoon, briefing the press at Kennedy Space Center after the launch.
As with previous Dragons, CRS-8 will approach the space station along the “R-Bar” (or “Earth Radius Vector”), which provides an imaginary line from Earth’s center toward the ISS, effectively approaching its quarry from “below”. In so doing, Dragon will take advantage of natural gravitational forces to brake its final approach and reduce the need to perform excessive numbers of thruster burns. By Sunday morning, it will reach a “Hold Point” about 1.5 miles (2.4 km) from the station, whereupon it must pass a “Go/No-Go” poll of flight controllers in order to draw nearer.
Further polls and holds will be made at distances of 3,700 feet (1,130 meters) and 820 feet (250 meters), after which Dragon will creep toward its target at less than 3 inches (7.6 cm) per second. Critically, at 650 feet (200 meters), it will enter the “Keep-Out Sphere” (KOS), which provides a collision avoidance exclusion zone, and its rate of closure will be slowed yet further to just under 2 inches (5 cm) per second. After clearance has been granted for the robotic visitor to advance to the 30-foot (10-meter) “Capture Point,” the final stage of the rendezvous will get underway, bringing Dragon within range of Canadarm2. Current plans call for Expedition 47 astronauts Tim Peake and Jeff Williams, based in the multi-windowed cupola, to grapple Dragon at about 7 a.m. EDT Sunday. They will be assisted by Commander Tim Kopra, who led last month’s Orbital ATK OA-6 Cygnus grapple.
The Robotics Officer (ROBO) in the Mission Control Center (MCC) at the Johnson Space Center (JSC) in Houston, Texas, will then command the physical berthing of the cargo ship to the nadir CBM of the Harmony node by 10 a.m. EDT. Berthing will occur in two stages, with the Expedition 47 crew overseeing “First Stage Capture”, in which hooks from the node’s nadir CBM will extend to snare the cargo ship and pull their respective CBMs into a tight mechanized embrace. “Second Stage Capture” will then rigidize the two connected vehicles, by driving 16 bolts, effectively establishing Dragon as part of the ISS for the next month. Shortly afterwards, the Expedition 47 crew will be given a “Go” to pressurize the vestibule leading from the Harmony nadir hatch into the cargo ship.
“The cargo will allow investigators to use microgravity conditions to test the viability of expandable space habitats, assess the impact of antibodies on muscle wasting, use protein crystal growth to aid the design of new disease-fighting drugs and investigate how microbes could affect the health of the crew and their equipment over a long duration mission,” said NASA Deputy Administrator Dava Newman.
With the arrival of CRS-8, this will mark the first occasion that as many as six visiting vehicles—whether piloted or unpiloted—have been simultaneously in residence at the ISS. In addition to the Soyuz TMA-19M and Soyuz TMA-20M spacecraft, which delivered both halves of the incumbent Expedition 47 crew to the station in December 2015 and March 2016, there are also a pair of Russian Progress resupply freighters and Orbital ATK’s OA-6 Cygnus cargo ship. With the arrival of Dragon, this will leave four vehicles docked at the station’s Russian Orbital Segment (ROS) and two berthed at the nadir ports of the Unity and Harmony nodes on the U.S. Orbital Segment (USOS). It will also mark the first occasion that both of NASA’s Commercial Resupply Services providers, SpaceX and Orbital ATK, are represented on-station at the same time.
And 2016 is shaping up to mark the first year that one of NASA’s commercial providers will have staged as many as three missions in a single calendar year. That achievement was narrowly missed in 2015, when the back-to-back successes of the CRS-5 and CRS-6 Dragons was tempered by the failure of CRS-7. Today’s launch of CRS-8 is expected to herald three Dragon missions in 2016, which will see CRS-9 in late June deliver an International Docking Adapter (IDA) and CRS-10 in August drop off a multitude of payloads, including the Department of Defense’s Space Test Program (STP)-H5, with the Lightning Imaging Sensor (LIS), and the Stratospheric Aerosol and Gas Experiment (SAGE)-III to enhance the station’s capability as a research platform. In parallel, Orbital ATK’s first Cygnus cargo flight of 2016 is currently underway, with the OA-5 mission slated for June and OA-7 for December.
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