NASA solves YERO problem for shuttle

by Chris Bergin

Shuttle orbiters can now enjoy a happy New Year’s Eve on orbit, following the recommendation to implement a YERO (Year End Rollover) solution that was recently designed by NASA engineers.

The recommendation, which was presented in the form of a 45 page presentation to the all-powerful PRCB (Program Requirements Control Board), involves the implementation of a modification to flight software on the orbiters, and new ground procedures.

**The most comprehensive collection of STS-117 onwards related presentions and mission documentation are available to download on L2 **

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Notes and Presentations, including: The Feb 15 YERO Solution – PRCB. Year-End Roll-Over (YERO) MCC New Year’s Eve Test – Dec 21. PRCB: STS-116 Year End Roll-over (YERO) Options (Nov 2). STS-116 YERO (Year End problem) outline and troubleshoot plan – available to download on L2.

YERO is a problem that was first highlighted when Discovery’s STS-116 launch window was restricted to launching before the mission could overlap the change of year.

In the end, Discovery arrived home before Christmas, moving the focus to any mission that may occur at the end of 2007.

The YERO problem relates to the shuttle orbiter’s computers needing to reset through a change of year, which could cause a glitch on orbit.

While a contingency was already in place – involving the orbiter remaining docked to the International Space Station (ISS) during a YERO event – a final solution was requested by shuttle manager Wayne Hale.

‘The shuttle computers, when they were originally programmed – the basic operating system was built back in the 1970s – it was not envisioned that we would fly across the end of the year, so we didn’t handle it very well,’ noted Hale during the pre-STS-117 flight briefing.

‘We’re making some changes to the way the software works, which will be fairly simple and easy. So if we need to at the end of this year, fly across the end of the year, we will be able to do that.

‘We will have some restrictions on the ground software on the ground at the Kennedy Space Center by way of the launch capability, which will restrict us from launching literally on New Year’s Eve. So there’s probably a day or so that we would not launch at the end of the year.’

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While not speaking of the actual solution, Hale was speaking just a few hours before being shown the presentation that YERO was becoming a closed action, noted as ‘Approve proposed YERO solution and operational concept. Close all parts of action.’

The summary of the solution shows two main elements to ensuring the orbiter won’t be affected by the change of year, involving a recycling of ground systems, and a ‘YERO Reset’ on the orbiter, with modified software to allow a reset to GMT Day 001 during the event.

‘The Integrated solution that best satisfies goals and objectives is to force to entire system to roll over to GMT day 001,’ noted the presentation. ‘MTU (Master Timing Unit) to day 366 in the event of a leap year.’

‘Preflight: KSC will perform PASS GPC IPL (orbiter software) and recycle the Firing Room. In-flight: Flight Software will be modified to enable YERO reset to rollover to GMT Day 001. Orbiter will perform YERO Reset and MCC (Mission Control Center) will perform YERO recycle.’

On the current schedule, Endeavour’s STS-123 is the nearest launch to the end of 2007 – although that flight is widely expected – and unofficially set – to move into 2008. With natural stretch in the manifest, it is likely STS-122 with Discovery will head towards the end of 2007, making this the first potential test of the new YERO solution.

This was touched upon in the document, with STS-122 targeted for the YERO software modification, in readiness for this flight requiring the capability, noting in the presentation: ‘Approve – to implement YERO Reset capability in the Flight Software for STS-122.’

While there is large confidence, noted throughout the presentation, that the plans outlined are the solution to YERO, further testing will be carried out over the year, with the likelihood of a simulation of a YERO event being carried out, following a previous simulation being moved from 2006’s New Year’s Eve into 2007.

Testing is also likely to be carried out at KSC, with a number of elements affecting launch operations, especially in the event of a scrub.

‘In the event of a launch scrub, YERO re-sync procedures cannot be executed prior to ET (External Tank) drain, boiloff and tank inert (scrub plus 24 hours),’ added the presentation. ‘YERO also impacts specific scrub turnaround timelines due to complications with scheduling YERO re-sync around critical/hazardous operations.’

The YERO plan may actually remain unused if all of the remaining shuttle missions launch away the small window that is affected. However, having this plan in place at least adds a number of options, should NASA require them.

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