Return of the Dragon – CRS-15 ends mission with Pacific splashdown

by Chris Bergin

SpaceX’s CRS-15 Dragon spacecraft has completed the EOM (End Of Mission) phase with a splashdown in the Pacific Ocean on Friday. As per usual – though still unusual for International Space Station (ISS) vehicles – Dragon and her important downmass will be recovered. Release from the Space Station Remote Manipulator System (SSRMS) occurred at 12:38 Eastern, less than six hours ahead of a successful splashdown.

The CRS-15 Dragon was launched on the final Block 4 version of the Falcon 9 rocket just over a month ago. Docking with the ISS a few days later.

The cargo craft carried 1,233 kilograms (2,718 lb) of scientific experiments, 205 kilograms (452 lb) of provisions for the crew, 178 kilograms (392 lb) of hardware for the US orbital segment of the outpost, 12 kilograms (26 lb) of hardware for the Russian part of the station, 21 kilograms (46 lb) of computer equipment and 63 kilograms (139 lb) of hardware to support future spacewalks.

In Dragon’s trunk was NASA’s Ecosystem Spaceborne Thermal Radiometer Experiment (ECOSTRESS), an ecological research payload that was extracted and mounted outside the space station during Dragon’s stay – and is now set to monitor the temperature of plant life on Earth’s surface.

Dragon’s Trunk also transported a replacement Latching End Effector (LEE) for the space station’s CanadArm2 robotic arm.

It is from a current LEE on the SSRMS that will release Dragon to begin the journey home.

On Thursday, ISS Partners and mission managers polled “go” for release thanks to favorable weather conditions forecast in the splashdown zone.

Dragon’s hatch was closed early Friday around 4 a.m. Eastern time. Robotic ground controllers then used the robotic arm to detach Dragon from the Earth-facing port of the Harmony module around 6:30 a.m. Eastern time and will maneuver Dragon into the release position.

With Expedition 56 Flight Engineer Serena Auñón-Chancellor of NASA monitoring Dragon’s systems, the ground controllers released Dragon from the SSRMS at 12:38 p.m. Eastern time.

Once the LEE snares were released, the SSRMS was backed away from Dragon as the craft held its position at the 10 meter mark.

Once the Station’s arm was cleared to a safe distance, Dragon conducted a series of three small thruster firing departure burns that moves 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 pushed 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.

ISS departure zone

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 includes 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 5:23 p.m. Eastern time, SpaceX flight controllers at Hawthorne, California commanded Dragon’s Draco thrusters to fire for 12 minutes and 53 seconds – retrograde – in the deorbit burn that enabled Dragon to slip out of orbit for its descent back to Earth.

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 placed its heat shield out in front in preparation for Entry Interface (EI) – the moment Dragon reached the first traces of Earth’s upper atmosphere.

Dragon re-entry, via SpaceX

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.

Dragon splashdown

Recovery is attained by three main recovery vessels which were positioned near Dragon’s return location. The main recovery vehicle had already set sail earlier this week.

Fast recovery vessels deploy to begin collecting Dragon’s parachutes as recovery of the capsule itself was conducted by the primary recovery assets.

Dragon’s parachute-assisted splashdown occurred around 6:17 p.m. Eastern time, 3:17 p.m. Pacific time, about 410 miles southwest of Long Beach, California. It will take about two days for Dragon to be brought back to the Port of Los Angeles for its cache of cargo and scientific experiments to be unloaded.

The ISS crew spent the majority of the second half of Dragon’s stay loading Dragon’s pressurized capsule with key items to be returned to Earth, including numerous time-sensitive experiments, which will be the first items to be removed.

Dragon will eventually take a road trip to SpaceX’s test center at McGregor in Texas for the complete cargo removal.

CRS-16 is the next Dragon mission to the ISS, currently scheduled for late November. This will be the first time Dragon will have flown on the Block 5 version of the Falcon 9.

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