Orbital ATK’s Cygnus resupply ship has berthing at the International Space Station (ISS) on Saturday morning. Cygnus launched to orbit on its second ride on the United Launch Alliance Atlas V and enjoyed a nominal deployment from the rocket – although ULA is investigating why the Centaur upper stage had to modify its burn time to push Cygnus into her desired orbit.
Cygnus OA-6 Arrival:
Immediately following launch, Cygnus – named S.S. Rick Husband – successfully entered its target orbit and deployed its lightweight solar arrays.
Since then, Orbital ATK mission controllers in Dulles, Virginia, spent the early days of Cygnus’ mission uploading and executing the first of a series of rendezvous phasing burns (called DV – Delta Velocity burns) to refine the S.S. Rick Husband’s trajectory toward the Station.
The first DV burn occurred on Wednesday, lasted approximately 10 minutes, and was designed to raise Cygnus from its initial near-circular 230km orbit to the 400km orbit of the ISS.
A similarly long DV burn followed, again to raise Cygnus to its proper orbital altitude.
This was then followed by a planned phasing burn to align Cygnus into the exact orbital corridor of the Station and then by a final set of DV burns on Friday to bring Cygnus to its “Go/No-Go for Joint-Ops” decision point, which it reached roughly 5hrs prior to capture at approximately 01:40 EDT Saturday.
Once Cygnus received the “go” from MCC-H (Mission Control Center – Houston) for Joint Ops, Cygnus slowly approached the Station to the Joint Targeting Reference Point (JTRP), which it arrived at just over 3hrs prior to capture – at approximately 03:30 EDT.
From this point until capture and berthing, every step of the rendezvous required a strong communications link through the JEM (Japanese Experiment Module) PROX system between Cygnus, the ISS, and ground controllers.
This communication structure ensured the ability to manually abort – or at least retreat – Cygnus’ approach to the Station in the event of a problem with the spacecraft or the ISS.
Once at the JTRP, Cygnus stopped relative motion with the ISS and awaited a second Go/No-Go decision from MCC-H.
At this point, Cygnus was in the Joint Operations Phase (JOPS) of approach, as overviewed in documentation acquired by L2.
Approximately 3hrs before capture, and with MCC-H providing a “go” to proceed, Cygnus performed the first of four ADV thruster burns (ADV1) to begin moving closer to Station.
During these proximity ADV burns, Cygnus – until capture – made use of the TriDAR vision system designed by Canadian company Neptec with the support of NASA and the Canadian Space Agency.
TriDAR – tested during several Space Shuttle missions – provides Cygnus controllers with real-time visual guidance for navigation, rendezvous and docking procedures.
After Cygnus’ completed her first two ADV burns, the ISS maneuvered to capture attitude – a 5 minute process that took place just over 2hrs prior to targeted capture time.
Then, with just under 2hrs to go until capture, MCC-H issued another Go/No-Go decision regarding two more ADV burns for Cygnus, which took the spacecraft to its 250m hold point below the ISS.
An hour later, MCC-H gave the “go” for Cygnus to depart the 250m hold point and enter the Keep Out Sphere (KOS) of the ISS.
Cygnus then pulsed its thrusters and enter the KOS.
Up until this point, Orbital ATK controllers at their facility in Dulles had full control over Cygnus.
Once Cygnus entered the KOS, NASA controllers at MCC-H joined the Orbital ATK team for the tricky rendezvous and berthing of Cygnus.
Just under half an hour prior to capture, Cygnus arrived at the 30m Hold Point.
Five minutes later, Cygnus received the “go” to proceed to the capture point, at which time it departed the 30m Hold Point just over 15 minutes prior to capture.
Cygnus then arrived at its capture point 12m from the ISS 8 minutes prior to the first capture attempt by ISS Commander Tim Kopra.
Kopra used the Station’s 17.5m Space Station Remote Manipulator System (SSRMS) robotic arm to grab hold of Cygnus at approximately 06:51 EDT.
After Cygnus was firmly in the SSRMS’s grip, Kopra maneuvered the craft to Node-1 Unity (delivered by Space Shuttle Endeavour during the first ISS construction mission in December 1998) where the craft will be berthed for a planned two month stay at the ISS.
Altas V Launch:
The veteran Atlas V rocket launched on its first attempt at the opening of the available window on Tuesday, lighting up the sky as she gracefully departed SLC-41 at Cape Canaveral.
The Atlas V’s first stage was set to burn for four minutes and 15.5 seconds before its engine shut down.
Mission documentation listed the ascent milestones, although they can alter during flight as Atlas V’s flight computer works to real time parameters. However, the timing of engine shutdown was shown to be six seconds early by those following the launch via the NASA webcast.
As pre-planned, six seconds after engine shutdown, the spent stage separated, with the Centaur’s RL10C-1 engine entering its prestart phase. Ten seconds after stage separation, the Centaur engine ignited to begin a single burn that was pre-planned for a duration of thirteen minutes and 38 seconds.
However, this burn lasted over a minute longer than noted in the pre-flight mission documentation.
Notably, Centaur did its job despite this issue and delivered Cygnus into the correct 230km orbit and into its correct RAAN (Right Ascension of the Ascending Node), which ultimately classed the mission as a success.
However, due to the booster’s shortfall, Centaur had to all-but deplete its propellent to push Cygnus into the nominal separation parameters.
It is understood that the extra push via Centaur resulted in Cygnus being 300 km further downrange than expected at spacecraft separation. This parameter held no notable impact on the spacecraft’s mission, with any “in-track error” easily compensated for by adjusting the timing and duration of phasing burns to get to the rendezvous point.
Should the compensation in the phasing burns be confirmed, very little – if any – impact on Cygnus propellant usage would be suffered, along with no impact on the timing to get to the ISS. “No concerns just some math and planning,” as one source portrayed it.
While Centaur delivered Cygnus into her correct orbit, the second stage still had one final job to complete: to deorbit itself into an area pre-noted in the NOTAM (Notice To Airmen) zone over the ocean near Australia.
The stage’s RL10C-1 started up as expected for the disposal burn, but due to the depletion of its propellent from having to burn longer than planned, it shut down shortly after the burn began.
As a result, the stage entered a contingency sequence for just such an occurrence but was unable to restart the engine due to “running on fumes”. It is understood, however, that Centaur still re-entered over water, south of New Zealand instead of south of (central to western) Australia.
The review of the issue – currently believed to be related to an imbalance in the fuel mix ratio usage during the RD-180 burn with the Atlas V booster – is already taking place as part of the nominal post-launch review at ULA.
The results will then be discussed with partners, such as the USAF, and has already fed into Atlas V’s next mission in May, which involves the launch of the MUOS-5 communications satellite for the US Navy.
This mission has now been delayed slightly to May 12 to allow ULA time to “further review the data anomaly experienced during the OA-6 mission. The delay will allow additional time to review the data and to confirm readiness for the MUOS-5 mission.”
Since her debut in 2002, the Atlas V has enjoyed a near-perfect success record, with just one partial failure which did not result in loss of mission. That anomaly occurred during the June 2007 launch of NROL-30, a pair of ocean reconnaissance SIGINT satellites for the National Reconnaissance Office.
A faulty valve resulted in the Centaur leaking hydrogen and cutting off early at the end of its second burn. This left the satellites in a lower-than-planned orbit; however, they were still able to maneuver into their final orbit.
Despite Atlas V’s impressive track record, ULA is preparing to bring a new rocket online by the end of the decade.
Due in part to geopolitical and US political considerations regarding Russian-built engines for the Atlas V fleet and also to increasing competition from SpaceX, ULA announced in September 2014 that it had entered into a partnership with Blue Origin to develop a new series of liquid oxygen and methane engines for a new first stage booster.
This successor – initially referred to as a next generation launch system – was announced in April 2015 with the name Vulcan.
Development of this system is proceeding to plan with ULA announcing it had completed the Preliminary Design Review (PDR) for the “Vulcan Centaur launch vehicle” this week.
Passing this milestone will allow ULA to refine and test key elements of the design while executing a current manifest of 14 launches in 2016 while remaining on track to debut the Vulcan in 2019 on a test flight.
“The completion of the Vulcan Centaur rocket’s PDR is the first of several major and very exciting milestones in the launch vehicle’s development,” noted Tory Bruno, ULA president and chief executive officer. “We have a strong path to get to a 2019 flight test of this new, highly-capable American launch vehicle.”
The Vulcan is designed to launch with dual Blue Origin BE-4 engines. However, Aerojet Rocketdyne is also developing the AR1 engine which could power the Vulcan Centaur.
ULA notes that their strategic partnership for American main engines – along with Orbital ATK for the solid rocket boosters and RUAG Space for domestically-produced composite structures – enables collaborative development of Vulcan to maximize the value of this new launch capability.
“Vulcan Centaur will revolutionize spaceflight and provide affordable, reliable access to space with an American main engine,” added Mark Peller, ULA’s program manager for major development.
(Images: via NASA, ULA, Orbital ATK and Jacques van Oene/Spacepatches.nl – also L2 Artist Nathan Koga – plus L2’s Cygnus Section – Containing presentations, videos, a vast set of unreleased hi-res images, interactive high-level updates and more, with additional images via Orbital ATK and NASA)