CRS-17 Dragon returns home from ISS mission

by Chris Bergin

SpaceX’s CRS-17 Dragon spacecraft concluded the EOM (End Of Mission) phase of her mission following unberthing from the International Space Station (ISS) on Monday. Dragon’s release from the Space Station Remote Manipulator System (SSRMS) occurred at 16:00 UTC, with splashdown in the Pacific Ocean around 21:10 UTC on Monday.

The CRS-17 Dragon was launched on Falcon 9 B1056.1 last month.  The booster also returned without issue, allowing it to be prepared for a future mission.

CRS-17 mission delivered 338 kilograms (745 pounds) of supplies and provisions for the space station’s crew, 357 kilograms (787 lb) of hardware for the US and International segments of the station, 75 kilograms (165 lb) of computer equipment, 10 kilograms (22 lb) of hardware to support EVAs and 11 kilograms (24 lb) of hardware that is being flown on behalf of the Russian Federal Space Agency, Roskosmos.

Dragon’s cargo includes components for the space station’s new water storage system, filters for a new waste management system, wire harnesses and depressurization indicators that will be used to support future commercial crew flights and the POLARS system that will facilitate transportation of scientific experiments at low temperatures.

Dragon’s unpressurized Trunk contains hardware that will be mounted on the outside of the space station. This includes the Orbiting Carbon Observatory 3 (OCO-3) payload, which will use three spectrometers to measure the density and distribution of carbon dioxide in Earth’s atmosphere.

The Trunk also contained Space Test Program ‚Äď Houston 6 (STP-H6), a package of eight technology development and demonstration payloads for the¬†US Air Force‚Äôs Space Test Program (STP). STP-H6 includes experiments that will test instruments for ionospheric and plasma research, demonstrating computing resources for in-orbit image processing, validating the use of modulated X-rays for communications and testing systems for future satellites.

After both payloads were removed for installation on the ISS, the robotic assets then

Ahead of the return trip, Dragon was packed with downmass and the hatch closed. Robotic ground controllers then used the robotic arm to detach Dragon from the Earth-facing port of the Harmony module to maneuver Dragon into the release position.

Dragon was unberthed by the Space Station Remote Manipulator System (SSRMS) at 12:35 UTC before being released from the robotic arm at 16:00 UTC.

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 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.

SpaceX flight controllers at Hawthorne, California commanded Dragon’s Draco thrusters to fire for 12 minutes and 53 seconds Рretrograde Рin 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) ‚Äď the moment Dragon reaches the first traces of Earth‚Äôs 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 in the last few days.

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

Dragon’s parachute-assisted splashdown usually occurs 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-18 is the next Dragon mission to the ISS, currently scheduled for July.

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