For the first time in a year, the International Space Station has welcomed the arrival of a Dragon spacecraft from SpaceX following a flawless launch Friday afternoon from the Cape Canaveral Air Force Station. The complex orbital rendezvous culminated Sunday morning at approximately 07:12 EDT (11:12 UTC) with the capture of Dragon ahead of berthing operations.
CRS-8: A flawless launch for Dragon:
Launch of the CRS-8 mission and the voyage of Dragon to the ISS began right on time following a flawless countdown, which saw an expedited propellant loading process in the final 35 minutes prior to liftoff – resulting in full-scale fueling operations proceeding until the T-3 minute and counting mark.
As launch control teams pumped super chilled liquid oxygen into the vehicle, teams at the Dragon flight control center in Hawthorne, California, monitored all pre-launch processes, which resulted in no countdown issues and a right on time liftoff at 16:43:31 EDT.
The latest flight for SpaceX following the 28 June 2015 launch mishap, which resulted in the destruction of the launch vehicle and the CRS-7 Dragon mission to the Space Station, CRS-8 gained notable attention as the rocket propelled itself through first-stage flight.
Following 2 minutes 30 seconds of first-stage operations, all nine Merlin 1D engines at the base of the first stage shutdown, followed immediately by a spring-guided separation of the first stage.
With the first stage gone, the second stage’s single Merlin vacuum 1D engine ignited to begin the second stage phase of flight.
As most eyes turned to the ASDS barge, Of Course I Still Love You, 185 miles northeast of the launch site in the Atlantic Ocean, the second stage continued to perform flawlessly as the first stage guided itself under rocket power to a picture perfect upright landing on the barge.
Shortly thereafter, the single-engine second stage of the Falcon 9 shutdown and, 10mins 30sec after launch, successfully delivered and deployed the Dragon capsule into a 200 x 360 km orbit on a 39-hour rendezvous profile with the Space Station.
CRS-8: Dragon in orbit
Immediately after separating from the second stage, the Dragon spacecraft deployed its solar arrays as its control teams in California began a quick look checkout of the vehicle’s systems prior to committing to the rendezvous profile with the ISS.
With Dragon in good shape, control teams brought the spacecraft’s thruster system online through the pressurization of the system’s propellant tanks and injection of those tanks with gaseous helium.
Dragon then began a series of primary and minor phasing burns, through her set of four “quads” of thrusters, to both raise and refine Dragon’s orbit to precisely align her into the proper approach corridor with the Space Station.
The complex arrival process saw a series of thruster firings from Dragon designed to take her closer to the Station while also holding at distances of 2.5 kilometers, 1,200km, 250 meters, 30m, and 10m from the Station before finally receiving a “go” for grappling.
The timeline was slightly delayed due to extra atmospheric drag during the two days on orbit.
The initial series of orbital maneuvers brought Dragon 2.5 km below ISS, at which point she eold position while her flight controllers and NASA ISS controllers reviewed real-time vehicle performance data.
Then, a Go/No-Go call was made. With a “go” given, Dragon performed the HA2/CE2 pair of thruster burns to bring herself 1.2 km below the ISS.
After holding at this location, teams gave Dragon the “go” to perform the HA3/CE3 pair of burns.
Here, using RGPS and configured with the Station’s own GPS system, Dragon performed the HA4 (Ai) burn to bring herself inside the approach corridor, at which point the ISS crew began actively monitoring the spacecraft’s arrival.
This phase of approach was aided by Dragon’s relative navigation system. This sensor system was developed in house by SpaceX and was initially tested during the CRS-3 mission. It was then promoted to become Dragon’s primary navigation sensor during CRS-4 and has performed flawlessly ever since.
With teams at SpaceX mission control (MCC-X) in California and NASA’s ISS Flight Control Room (FCR) in Houston monitoring all aspects of the rendezvous, Dragon continued her approach to the 250m point, at which she held position again.
A review of Dragon systems and guidance followed, resulting in a “go” condition for Dragon to continue her approach to the ISS.
Dragon then closed to a 100m distance from the ISS and held position again.
Following another series of alignment and system checks, Dragon received a “go” to proceed to the 30m hold point.
While these series of holds are numerous, they are nonetheless essential to ensuring a smooth approach and rendezvous profile for Dragon.
While the vehicle itself, ground controllers, and the crew aboard the ISS have the ability to abort Dragon’s approach to the Space Station via the Commercial Orbital Transportation Services (COTS) Ultra High Frequency (UHF) Communication Unit, or CUCU, it is never desirable to have to abort the rendezvous of an incoming visiting vehicle.
Nonetheless, the ability to abort Dragon’s rendezvous at any point in the timeline exists to protect the International Space Station from a potentially hazardous impact should the spacecraft not behave as anticipated.
To this end, the CUCU provides a bi-directional, half-duplex communications link between Dragon and the ISS using ISS UHF Space to Space Station Radio antennas, which provides a communications path between MCC-X and Dragon during proximity operations and a command security link between the ISS and the spacecraft.
After holding at the 30m point, Dragon then proceeded to her capture point, 10m below the Space Station.
Once Dragon arrived and held at the 10m capture point, ISS Commander Tim Kopra and British astronaut Tim Peake – working in the Robotic Work Station (RWS) in the Cupola lab – began extending the Space Station Remote Manipulator System (SSRMS) arm toward Dragon’s grapple fixture.
After receiving a “Go for Capture” call from Houston, Kopra and Peake 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.
With the Station’s thrusters inhibited and Dragon in free drift, Kopra and Peake moved the SSRMS over the grapple fixture pin on Dragon to trigger the capture sequence ahead of pre-berthing.
Capture was marked at 07:23 EDT (11:23 UTC).
After capture, Dragon was then secured by the SSRMS before Kopra and Peake will carefully translated the spacecraft to its pre-install position, 3.5 meters away from the Station’s Node-2 Harmony nadir port.
Once at the pre-install position, Station crewmembers took camcorder and photographic footage of Dragon through for engineering evaluation of Dragon.
The SSRMS was then moved Dragon to 1.5m, at which point the ISS crew awaited the “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 to the ISS, a unique milestone was reached: the presence of both U.S. commercial resupply vehicles at the Space Station at the same time – with Orbital ATK’s Cygnus craft currently berthed at Node-1 Unity.
It also marks the conclusion of a busy visiting vehicle period for the ISS with four back-to-back missions over the last three weeks that saw the 18/19 March launch and docking of Soyuz, the 25 March arrival of Cygnus, the 2 April arrival of Progress, and this morning’s arrival of Dragon.
CRS-8: One month at Station
In all, the CRS-8 Dragon will spend one month and one day berthed to the International Space Station.
Also during this time, the ISS crew will unpack and attach the first module element to arrive at the Station since Discovery’s STS-133 mission left the MPLM Leonardo at the Station in 2011.
Weighing in at 1,413 kilograms, Bigelow’s expandable BEAM module will be removed from the trunk of the Dragon spacecraft and translated to its attachment location on the ISS via the SSRMS.
Once attached to the ISS, a complex series of checkouts will take place prior to the module being expanded to its full configuration.
The module itself, an experiment designed to prove the usefulness of expandable module technology on the ISS following two previous and still on-going solo expandable module flights, will enjoy an anticipated two year run attached to the ISS.
However, Dragon will depart long before that two-year experiment ends.
Under the current mission timeline, CRS-8 Dragon will be unberth from the International Space Station on 11 May 2016 after it is loaded with thousands of kilograms of return experiments and equipment.
Following unberthing, Dragon will perform a controlled reentry into Earth’s atmosphere with an anticipated splashdown in the Pacific Ocean off the southwestern coast of California.
(Images: SpaceX, NASA and L2’s hundreds of unreleased hi-res Dragon ISS mission photos.)