SpaceX’s Dragon spacecraft was captured by the International Space Station (ISS) and successfully berthed, following her Easter Sunday arrival. After only a slight hiccup with her quad thrusters, the spacecraft has enjoyed a smooth passage towards the orbital outpost, ahead of a complex set of requirements that culminated in her capture and berthing.
Dragon began her journey into space on Friday, atop of the Falcon 9 v1.1 launch vehicle, the first time the spacecraft had been paired with the upgraded rocket.
A quick look review of the ascent performance noted no issues with the launch vehicle, as has been the case with all four launches of the beefed up F9.
With a camera capturing the separation of the Dragon from the Upper Stage – a beautiful sight, showing off Dragon’s trunk, and external payloads – the spacecraft soon spread her wings via solar array deployment.
Soon after, Dragon’s propulsion system – a set of four “quads” of thrusters on the Dragon, vital for attitude control and required burns en route to its destination – were put through their “priming phase”, as Dragon prepared to fire up the thruster systems by pressurizing the fuel tanks via the injection of gaseous helium.
This involved the propellant system opening valves between the Helium tanks and the Fuel and Oxidizer tanks to pressurize the system. An isolation valve failed, requiring a secondary valve to come into play, resolving the requirements of the priming and the first burn to send Dragon en route to the Station.
The arrival into the ISS’ back yard involved a whole series of thruster firings, each taking her closer to the station; holding at distances of 2,500, 1,200, 250, 30 and 10 meters, before finally being grappled by the Canadarm2 Remote Manipulator System, and attached to the nadir port of the Harmony module.
The series of finite maneuvers began as Dragon caught up to the Station via the Height Adjustment (HA) and Co-Elliptical (CE) burn points, bringing Dragon 2.5 km below ISS. A Go/No-Go was performed for the HA3/CE2 burn pair bringing Dragon to 1.2 km below ISS.
The HA3/CE3 burn pair, using RGPS and configured with the ISS’ own GPS system, were then conducted, followed by the HA4 (Ai) burn, taking Dragon inside the corridor where the crew began to monitor the spacecraft’s approach.
With both SpaceX mission control in California, and NASA’s ISS Flight Control Room (FCR) in Houston monitoring, Dragon arrived and held at 250 meters distance from the Station, where checks of Dragon’s LIDAR system were conducted, a key element of hardware that has a heritage of testing via the Space Shuttle Discovery during her STS-133 mission.
With all parties are satisfied with Dragon’s performance – and ability to abort if required – Dragon was given a “Go” to approach to 30 meters distance from the Station where she automatically pause.
At all points, the ability to abort could be made by controllers on the ground, the Dragon herself and the ISS crew – via the Commercial Orbital Transportation Services (COTS) Ultra High Frequency (UHF) Communication Unit, or CUCU, which rode in the middeck stowage locker on Atlantis during STS-129 late in 2009, before being handed over to ISS crew members ahead of the Dragon flights.
The CUCU provides a bi-directional, half-duplex communications link between Dragon and ISS using existing ISS UHF Space to Space Station Radio (SSSR) antennas, which provides a communication path between MCC-X (SpaceX) and Dragon during proximity operations and a command security between ISS and Dragon.
Dragon did suffer from some communication dropouts during arrival. However, the dropouts and subsequent system reboots only took around 10 seconds to remedy. Such an issue is only a problem if the dropouts had been 50 seconds in length, in which event Dragon would have aborted. The issue cleared closer to capture.
Proceeding from 30m to the Capture Point at 10 meters out, Dragon automatically held position again, allowing the ISS’ robotic assets – already translated to the pre-capture position – to make the move towards the Dragon via controls in the Cupola RWS.
Upon receiving the “Go for Capture” call from Houston, the ISS crew armed the SSRMS capture command and began tracking the vehicle through the camera on the Latching End Effector (LEE) of the SSRMS, as overviewed in a detailed presentation available in L2 – Link).
With the ISS’ thrusters inhibited and Dragon confirmed to be in free drift, the arm’s LEE translated over the Grapple Fixture (GF) pin on Dragon to trigger the capture sequence ahead of pre-berthing.
Capture was on time at 11:14 GMT.
The Dragon, secured by the SSRMS, was then carefully translated to the pre-install set-up position, 3.5 meters away from the Station’s module, allowing the crew to take camcorder and camera footage of the vehicle through the Node 2 windows.
This footage will be downlinked to the ground for engineers to evaluate the condition of the Dragon spacecraft.
The SSRMS then translated Dragon to the second pre-install position, at a distance of 1.5 meters out. Desats were inhibited prior to the translation of the Dragon into Common Berthing Module (CBM) interface to begin the securing of the spacecraft to the ISS.
A “Go” at this point was marked by all four Ready To Latch (RTL) indicators providing confirmation on the RWS panel.
As has been seen with previous Dragon arrivals – and indeed new additions to the Station itself – the spacecraft was put through first stage capture tasks, allowing the SSRMS to go limp, ahead of second stage capture, officially marking Dragon’s berthing with the ISS at 14:06 GMT.
With all of the ISS berthing milestones part of the pre-planned schedule, the ISS crew will decide when to open the hatch to the Dragon, which can vary depending on the allowances in the crew’s timeline. Hatch took place early on Monday.
The CRS-3/SpX-3 is carrying a complement of 1,580kg/3,476lb of payload, an increase from the previous limit of 800kg, afforded by the increased upmass capabilities of the Falcon 9 v1.1.
A multitude of cargo includes a GLACIER and two MERLIN freezers for transporting ISS experiment samples.
The external payload in Dragon’s trunk includes the Optical Payload for Lasercomm Science (OPALS) – which will demonstrate high-bandwidth space to ground laser communications, and the High Definition Earth Viewing (HDEV) package consisting of four commercial HD video cameras.
The CRS-3 mission also involves the delivery of a replacement Extravehicular Mobility Unit (EMU), allowing for the return of a faulty suit on the same vehicle when it returns to Earth. This spacesuit relay is enabled by a specially built rack inside the Dragon.
(Images: via L2’s SpaceX Special Section, which includes over 1,000 unreleased hi res images from Dragon’s three flights to the ISS. Other images via NASA and SpaceX)
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