While NASA prepares for the launch of Shuttle Atlantis on STS-115 at the end of the month, the agency is working on solutions for Discovery’s STS-116 mission, which could suffer a stranglehold on its launch window, due to a problem called YERO.
YERO, or Year End Rollover, could prove to be a problem if Discovery launches late in her window, which would see her on orbit during the change of year to 2007 – which has an adverse effect on flight software.
Information collated from documents available to download on L2.
Previous related article: http://www.nasaspaceflight.com/content/?cid=4640
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A number of documents acquired by this site show NASA is working on both the potential issues that may arise with the orbiter’s reaction to a change of year whilst on orbit – along with a troubleshooting test that will be carried out in October.
‘STS-116 launch window may result in Shuttle operations over the year end,’ noted the troubleshooting outline document. ‘If so, at midnight GMT on Dec. 31 the MTUs (Master Timing Units) will rollover to day 1 and PASS (Primary Avionics System Software) flight software, which increments to day 366, will declare them failed and fault down to internal GPC time.
‘The purpose of this test is to verify that the YERO procedure successfully forces STS-116 PASS FSW to accept MTU time and correctly configures it for continued operations, through landing.’
A post-test briefing will be held on October 31, well ahead of STS-116’s Flight Readiness Review, which could see Discovery’s mission either going ahead as planned – with a solution in place, a restriction of the launch window being reduced to just three days, or the launch window being moved to 2007.
Should a YERO event prove to be unacceptable for flight, the launch window opportunity for Discovery in December will end after December 16, as noted in the YERO outline document: ‘Potential YERO event in flight if launch after December 16.’
NASA has no real experience of how an orbiter would react on orbit with a change of year, given what would have been the only previous YERO flight – STS-32 in 1989 – suffered a launch slip.
However, NASA does have an arsenal of testing resources to fall back on, noted as the four SAIL (Shuttle Avionics Integration Lab) tests, completed at the end of 2005, which was recommended for contingency use only.
Processing with Discovery is going smoothly in the flow, allowing NASA a relaxed timeline in completing evaluations for making a decision on projected impacts – allowing engineers and technicians two months to report back with their findings.
The overview document is clear in its current standing on the impact, with the findings to ‘Determine which launch dates MOD can support – YERO event while docked: Timeline impacts – Post-undock: No ISS resources (comm, att cntl) – YERO not permitted: Launch window impact.
‘If YERO option is permitted – STS-116 Team will: Determine placement of YERO procedure in flight timeline (for portions which can be delayed).’
A full agenda has been tasked on the ‘YERO team’ – allowing for a clear representation of options available for shuttle managers, which will likely report back to various PRCB meetings at the Johnson Space Center.
‘YERO Team will: Determine which YERO steps may be delayed, and for how long. Identify impacts and point-of-contact for affected areas. Develop flight-specific procedure for STS-116 (on-board and ground).
‘Support integrated STS-116 SAIL/MCC test in conjunction with FSW verification team(include other centers, where possible). Revisit MCC rollover test (may piggy-back with ISS BCC test). Work with DT to develop flight-specific YERO sims for STS-116 team (on-orbit and entry teams).
The document continues with a variety of potential impacts, how long they could impact the orbiter, and potential on orbit solutions should YERO manifest itself during the mission during the change of year.
The main concerns are loss of General Purpose Computer function and control of the orbiter for a undefined period of time. Such considerations need to be taken into account when the mission time may be extended into a YERO event, for reasons such as being waived off from the planned landing date due to unacceptable weather at the landing facilities.
The full test in October will give a clearer picture on how NASA should proceed.
The other main issue surrounding the launch window for STS-116 – namely the External Tank shipping date for Atlantis’ LON (Launch On Need – STS-317) requirement is starting to show signs it will arrive at the Kennedy Space Center within the required timelines.