SpaceX and NASA will wait until Sunday morning to fly the CRS-21 Dragon cargo mission to the International Space Station (ISS), after Saturday’s opening launch attempt was called off. “Due to poor weather in the recovery area for today’s attempt,” SpaceX tweeted, “now targeting Sunday, December 6 at 11:17 a.m. EST.” The long-awaited mission is the first flight under the second-phase Commercial Resupply Services (CRS2) contract with NASA and will feature the debut of the cargo-carrying version of Crew Dragon.
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Assuming an on-time launch tomorrow, CRS-21 will autonomously dock at International Docking Adapter (IDA)-3 on the space-facing (or “zenith”) port of the station’s Harmony node at 1:30 p.m. EST Monday. The Dragon is laden with 6,550 pounds (2,970 kg) of equipment, payloads and supplies for the seven-person Expedition 64 crew.
Weather conditions on Saturday—predicted to be only about 50-percent favorable—should improve markedly to 60 percent on Sunday and, in the case of a 48-hour slip to the next available “window” on Tuesday this will increase to 70 percent.
“A low-pressure system is expected to develop across the central Gulf of Mexico overnight and gradually drift eastward Sunday,” noted the 45th Weather Squadron at Patrick Air Force Base in its Saturday morning update. “While conditions are expected to remain dry across Central Florida Sunday morning, the disturbance will generate a mid-level cloud shield across much of the state during the launch window.” Potential showstopper tomorrow include a violation of the Thick Cloud Layer Rule.
Should Sunday’s attempt also come to nought, SpaceX and NASA will align for a backup opportunity on Tuesday, 8 December. “The weather rapidly deteriorates Sunday night into Monday as an upper-level trough dives down from the Upper Midwest into the Deep South, lifting the surface low in the Gulf of Mexico northeastward through Florida,” it was added. Tuesday is presently expected to produce low temperatures and breezy northwesterly winds, with Liftoff Winds expected to pose a threat.
As outlined in AmericaSpace’s CRS-21 preview article, this upcoming mission promises to break a raft of records, including the first-ever autonomous docking by an uncrewed visiting vehicle at the U.S. Operational Segment (USOS).
Previous Dragons, as well as Japan’s H-II Transfer Vehicles (HTVs) and Northrop Grumman Corp. Cygnus cargo ships, were (and are) captured via the station’s 57.7-foot-long (17.6-meter) Canadarm2 robotics and “berthed” at the Earth-facing (“nadir”) Common Berthing Mechanism (CBM) on either the Unity or Harmony nodes. Monitoring the CRS-21 docking from the multi-windowed cupola will be Expedition 64 astronauts Kate Rubins and Victor Glover.
“Rubins and Glover will [monitor] the spacecraft’s telemetry through a communications link between Dragon and the station during the approach and docking phase of the mission, which is about the last one hour and 15 minutes before docking,” NASA told AmericaSpace. “Dragon will be in this phase of flight (inside the Approach Ellipsoid) about an hour less than the operations for the previous version of Dragon.”
Following docking, hatches may be opened within as little as two hours. “An assessment will be made after successful vestibule pressurization and leak checks,” NASA advised us.
Additionally, CRS-21 marks the first occasion that the Crew Access Arm (CCA) on historic Pad 39A at the Kennedy Space Center (KSC) has been utilized to late-load cargo aboard an uncrewed Dragon. And that cargo is substantial, with an expectation that CRS2-class Dragons can lift up to 50 percent more cargo than their CRS1-class predecessors.
The CRS1 contract—signed between NASA and SpaceX back in December 2008—initially called for 12 Dragon flights and a delivery of up to 44,000 pounds (20,000 kg) of cargo to successive ISS crews. The contract was later modified to include eight more missions and by the time the CRS-20 flight launched last March the Dragons had trucked 95,000 pounds (43,000 kg) of cargo uphill and returned 75,000 pounds (34,000 kg) home.
Under the terms of the CRS2 contract, awarded in January 2016, SpaceX will fly at least six more Dragons to the station through 2024. Taking pride of place aboard CRS-21 for tomorrow’s launch is the Bishop commercial airlock, destined for installation onto the starboard port of the Tranquility node.
Preparations of the node for Bishop’s arrival were completed during an Extravehicular Activity (EVA) by Expedition 63 spacewalkers Chris Cassidy and Bob Behnken last summer. The airlock is housed in Dragon’s unpressurized “trunk” and will be extracted by Canadarm2, possibly as soon as late December, in a 5.5-hour operation conducted by the ground-based Robotics Officer (ROBO).
The origins of this first-ever private airlock—which is being developed jointly between Thales Alenia Space, ISS prime contractor Boeing and NanoRacks—date back to the middle of the last decade. In May 2016, a Space Act Agreement (SAA) was signed between NASA and NanoRacks and early the following year Boeing was selected to build Bishop’s Passive CBM, with Thales Alenia Space responsible for the pressure shell, Micrometeoroid Orbital Debris (MMOD) shielding and Multi-Layer Insulation (MLI) for the airlock.
The project passed Critical Design Review (CDR) in March 2018 and early in 2019 Thales Alenia Space announced that it had completed work on Bishop and was preparing it for shipment to NanoRacks’ facility in Houston, Texas.
“The airlock module will provide a broad range of capabilities to our payload customers and expand greatly on the commercial utilization of the station,” noted Brock Howe, NanoRacks’ head of airlock, in a February 2017 press release. Commercial opportunities aboard Bishop are expected to include the kind of cubesat and smallsat deployments currently handled by Japan’s Kibo lab. All told, Bishop reportedly has five times greater capacity for cubesat and smallsat deployments than are currently available on the ISS.
Tomorrow morning’s launch will see the veteran B1058 Falcon 9 first-stage fly its fourth mission in just over six months. The core first saw service to launch Dragon Endeavour crewmen Doug Hurley and Bob Behnken back on 30 May, after which it delivered South Korea’s ANASIS-II military communications satellite to orbit on 20 July and a 60-strong batch of Starlink internet communications satellites on 6 October.
In so doing, it has secured new titles for the shortest launch-to-launch interval of a reusable, orbital-class booster—smashing the 35-year record set by shuttle Atlantis’ STS-61B crew—and the shortest time it has taken a booster to log four flights.