SpaceX’s CRS-13 Dragon unberthed from the ISS ahead of a return to a Pacific Ocean splashdown on Saturday, concluding the spacecraft’s latest resupply mission. Dragon returned with 4,100 pounds of cargo, highlighting her unique downmass capability for NASA. Splashdown occurred around 10:36 Eastern Time (07:36 local time on the West Coast).
Dragon has enjoyed yet another successful trip into space.
Launched by Falcon 9 (B1035.2) from a returning SLC-40 at Cape Canaveral, Dragon has enjoyed an issue-free orbital trip to the ISS.
Following a smooth rendezvous with the International Space Station, she was berthed to Node-2 Harmony’s nadir port.
The attachment to the orbital outpost marked the delivery of 4,861 lbs (2,205 kg) of cargo to the ISS: 1,080 lbs (490 kg) of Crew Supplies; 1,567 lbs (711 kg) of Science Investigations; 363 lbs (165 kg) of EVA Equipment; 416 lbs (189 kg) of Vehicle Hardware; 11 lbs (5 kg) of Computer Resources; and 1,422 lbs (645 kg) of unpressurized, external cargo in Dragon’s trunk.
Of the science payloads launched to Station, two were external experiments in the trunk of the Dragon, namely the Total and Spectral Solar Irradiance Sensor (TSIS-1) and the Space Debris Sensor.
Both payloads were successfully removed during Dragon’s berthed operations, while the ISS crew removed the multitude of payloads from her pressurized module, before re-filling it with downmass.
Over recent days, the SSRMS moved into operations to prepare for the departure of Dragon.
The ISS crew also checked out the vital communication system that will be employed during the departure of Dragon, such as the Commercial “Off-The-Shelf (COTS) Ultra High Frequency (UHF) Communication Unit (CUCU) and Crew Command Panel (CCP)
Friday night marked the unberthing of the vehicle from Node 2.
Whilst being firmly held in the snares of the Latching End Effector (LEE) on the end of the SSRMS, bolts securely fastening Dragon to the nadir (Earth-facing) Common Berthing Mechanism of Node-2 Harmony were driven to the retract position, freeing Dragon from its berthing port on the Station.
Dragon was then maneuvered on the end of the SSRMS away from the main structure of the ISS to its release point 10 meters from the orbital lab.
For this release, the Dragon will depart via ground controls, with Expedition 54 Flight Engineers Joe Acaba and Scott Tingle monitoring, as opposed to conducting the operation themselves.
The exact time of release is always subject to change from the announced plan due to lighting conditions, communications coverage, and day-of timeline activities. CRS-13’s release was marked as at 3:58 AM Eastern.
Once the LEE snares were released, the SSRMS backed away from Dragon as the craft held its position at the 10m mark.
Once the Station’s arm is cleared to a safe distance, Dragon conducted a series of three small thruster firing departure burns that move the capsule down the R-Bar (Radial Vector) and away from the International Space Station toward Earth (when viewed in relation to ISS orientation and Dragon movements with respect to Earth).
During the initial stage of departure, Dragon was under the control of its own computer programming, with the Station crew and controllers at Mission Control Houston in Texas for NASA having primary control over the spacecraft.
As Dragon pushes down the R-Bar, the largest of the three thruster departure burns imparted enough Delta Velocity (Delta-V) change to Dragon to push it outside of the approach ellipsoid.
The approach ellipsoid is a 4 km by 2 km oval-shaped region around the International Space Station that extends 2 km in front of and 2 kilometers behind the ISS along the velocity vector (V-Bar) and 1 km above and 1 km below the Station along the R-Bar.
Once Dragon cleared the approach ellipsoid 1 km below the ISS, primary control of the vehicle shifted from NASA to SpaceX controllers in Hawthorne, California.
Dragon conducted several hours of free flight activities as controllers at Mission Control SpaceX prepared the vehicle for the end of its mission.
This included the closure of the Guidance Navigation and Control (GNC) bay door on Dragon, creating a perfect thermal protection seal around the entirety of Dragon for entry.
At the appropriate time, Dragon’s Draco thrusters began a 10-minute firing sequence known as the deorbit burn to slow the capsule and place it on to the proper heading for entry into Earth’s atmosphere.
Following the deorbit burn, the umbilicals between Dragon and her external payload trunk were severed ahead of the trunk’s separation from Dragon itself.
Dragon then places its heat shield out in front in preparation for Entry Interface (EI) – the moment Dragon reached the first traces of Earth’s upper atmosphere.
Once EI occurred, Dragon’s Thermal Protection System (TPS) protected it from the searing hot temperatures of reentry formed as the air molecules around Dragon are instantly heated and turned to plasma under the friction created by Dragon’s high velocity.
Dragon’s primary heat shield, called PICA-X, is based on a proprietary variant of NASA’s Phenolic Impregnated Carbon Ablator (PICA) material and is designed to protect Dragon during atmospheric re-entry.
PICA-X is robust enough to protect Dragon not only during ISS return missions but also during high-velocity returns from Lunar and Martian destinations.
Unlike the Dragon capsule, the Dragon trunk destructively burns up in Earth’s atmosphere.
Once safely through the plasma stage of reentry, Dragon’s drogue parachutes deployed, followed by the main chutes designed to ease the vehicle to a splashdown in the Pacific Ocean for recovery.
Recovery was attained by three main recovery vessels which were positioned near Dragon’s return location. The main recovery vehicle had already set sail on Friday.
Fast recovery vessels deployed to begin collecting Dragon’s parachutes as recovery of the capsule itself was conducted by the primary recovery assets.