CRS-9 Dragon completes journey to the Space Station

by Chris Gebhardt

After just over two days of orbital rendezvous activities, SpaceX’s Dragon capsule has completed rendezvous and proximity operations with the International Space Station (ISS) ahead of being grappled by the Station’s robotic arm and berthed to the orbital outpost. Capture occurred 06:56 EDT, with berthing complete at 10:03 EDT.

Dragon’s rendezvous with Station:

Immediately after reaching orbit after a flawless launch Monday night from the Cape Canaveral Air Force Station, FL, Dragon deployed its solar arrays as her control teams in California began preparing for a series of phasing burns with Dragon’s set of four “quads” of thrusters.

2016-07-18-054735The initial phasing burns raised and refined Dragon’s orbit to precisely align the craft into the approach corridor with the Space Station.

For arrival, Dragon was trailing the ISS by just less than 28 km at 02:21 EDT – about 4.5 hrs prior to capture.

At 03:00 EDT, Dragon performed the HA2 burn – an 8sec thruster firing that reduced Dragon’s velocity by 0.37 m/s and further refined the craft’s general approach to the ISS.

This was followed at 03:16 EDT and 03:33 EDT by the HA3-MC1 and HA3-MC2 burns, respectively, which set Dragon up for the CE3 burn.

This CE3 burn was subsequently set to occur at 03:46 EDT, while Dragon was in orbital darkness, and again altered Dragon’s velocity by 0.37 m/s.

After closing to within 6 km of the Station, Dragon then performed the HA4 Approach Initiation burn at 04:16 EDT, at which time the ISS crew began to actively monitor the spacecraft’s arrival.

This six second burn changed Dragon’s velocity by 0.31 m/s, with the HA4-MC1 and -MC2 burns at 04:32 EDT and 04:49 EDT, respectively, aligning Dragon as she closed in on the 350 m point in proximity to the ISS.

Z2dsDuring the approach phase, navigation was aided by Dragon’s relative navigation system, an in-house developed sensor system that was first tested during the CRS-3 mission and has performed flawlessly ever since CRS-4 in 2014.

At 05:05 EDT, Dragon reached the 350 m point and fired her thrusters to hold at this position.

At this time, controllers at SpaceX’s Mission Control Center (MCC-X) in Hawthorne, CA, commanded Dragon to perform a 180 degree Yaw maneuver to place the craft into the proper orientation in relation to ISS for grapple at the end of the approach sequence.

After the yaw maneuver was complete and controllers at MCC-X and MCC Houston (MCC-H) confirmed the health of Dragon’s systems, the spacecraft departed the 350 m hold point at 05:11 EDT before crossing into an orbital sunrise two and a half minutes later.

Crewed DragonDragon then arrived at her next hold point, 250 m below the International Space Station at 05:19 EDT.

Once again, controllers at MCC-X and MCC-H confirmed the health of Dragon’s systems as well as the craft’s orientation in relation to the Station before giving a “go” to press ahead toward capture.

While these series of holds are numerous, they are essential to ensuring a smooth approach and rendezvous for Dragon.

While a Dragon has never misbehaved to date on approach to ISS, controllers on the ground as well as the crew aboard the ISS nonetheless hold the ability throughout the entire approach sequence to manually abort Dragon’s approach through the Commercial Orbital Transportation Services (COTS) Ultra High Frequency (UHF) Communication Unit, or CUCU if an off nominal condition presents during rendezvous.

Dragon at the ISS, via huge unreleased L2 photo collectionFollowing a “go” to proceed, Dragon left the 250 m hold point at 05:31 EDT and crossed the 100 m distance mark from the Station at 05:48 EDT.

Dragon then held her approach at the 30 m point.

After arriving here at 06:05 EDT, teams performed a final assessment of Dragon’s readiness to close to her capture point 10 m below the ISS.

At this point, capture operations were closely aligned with orbital sunrise to ensure that Dragon’s capture by the ISS crew is conducted in orbital daylight.

To this end, Dragon has an “early” window and “prime” window for departure from the 30 m hold point and capture operations at the 10 m hold point.

Dragon through RWS with CUCU support, via L2Dragon departed the 30 m hold point, with a prime window allowing for the earliest departure time from the 30 m hold point at 06:44 EDT, with latest departure for this window at 07:37:31 EDT.

Under the nominal timeline, though, Dragon was to arrive at the 10 m Capture Point (CP) at 06:40 EDT. Due to the smooth arrival, the operations moved up by five minutes.

Once Dragon arrives at the CP, ISS Commander Jeff Williams and Flight Engineer Dr. Kate Rubins – working in the Robotic Work Station in the Cupola lab – extended the Space Station Remote Manipulator System (SSRMS) arm toward Dragon’s grapple fixture.

After receiving a “Go for Capture” call from Houston, Williams and Dr. Rubins used the SSRMS’s camera on the Latching End Effector (as overviewed in a detailed presentation available in L2) to precisely move the SSRMS to grapple posture.

Z5The “Go for Capture” call was given and at this point, Williams and Dr. Rubins took the Space Station’s thrusters to “inhibit” and Dragon was commanded to “free drift.”

Williams and Dr. Rubins then moved the SSRMS over the Dragon’s grapple fixture pin and trigger the capture sequence.

Capture was confirmed at 56 minutes past the hour.

After capture, Dragon was secured by the SSRMS before Williams and Dr. Rubins carefully translated the spacecraft to its pre-install position, 3.5 meters away from Node-2 Harmony’s nadir port.

Once at the pre-install position, Station crewmembers took camcorder and photographic footage of Dragon for post-launch and rendezvous engineering evaluation.

Williams and Dr. Rubins then used the SSRMS to move Dragon to 1.5 m from Node-2, at which point the ISS crew received the final  “go for berthing” call to move Dragon the rest of the way into the Common Berthing Module interface to begin securing the spacecraft to the ISS.

Once Dragon was safely berthed, it marked the second time in just over 37 hours that an uncrewed cargo craft docks/berths to the Station, following the arrival of Progress MS-3 at 00:20 UTC on 19 July.

It also marked the conclusion of yet another busy visiting vehicle period for the ISS with three back-to-back missions over the last 11 days that saw the 9 July docking of the crewed Soyuz MS-1, the 19 July docking of Progress MS-3, and this morning’s arrival of Dragon CRS-9.

The hatch was opened to the spacecraft just several hours after berthing, ahead of schedule.

(Images: NASA, SpaceX, and L2)

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