Troubled STS-135 mission timeline under review

by Chris Gebhardt

With just over two months to go before the farewell flight of NASA’s long-running Space Shuttle Program, mission managers for the STS-135/ULF7 flight of middle-child orbiter Atlantis are expressing rather significant concerns over the mission’s stretched-thin timeline. In fact, as it currently stands, the already maxed-out terminal flight of the Shuttle program is simply too jam-packed to accomplish all of the important and long sought-after requirements of the mission.

The Troubled Timeline and Potential Get-wells:

With too much mission content and not enough flight time, STS-135/ULF7 flight planners are facing a very tough decision: either cut down the mission content to fit within the 12+0+2 day flight, or potentially violate Shuttle flight rules and execute a descending node reentry at the end of the mission and preserve all currently-planned flight content.

These decisions are being faced after all previous attempts to right the timeline fell short. According to the STS-135 troubled timeline presentation, available for download on L2, “Russia has initially agreed to provide 25 hours of ISS crew time to assist with STS-135/ULF 7 mission objectives. This is not enough to meet the current transfer requirements.”

Even with this 25 hours of Russian ISS crew time, the STS-135 middeck transfer timeline currently sits at 28hrs 40mins – 1hr 20mins shy of the required 30hrs of crew time for cargo transfer to/from Atlantis’s middeck.

More pressingly, however, is the timeline for generic MPLM Raffaello (Multi-Purpose Logistics Module Raffaello) transfer ops, which currently stand at only 123.5 hours of the necessary 140 hours.

As such, further “Mission content will need to be reduced to meet transfer mission objectives.”

Among the various items on the potential chopping block are a mated reboost maneuver of the ISS, the deployment of ZSR, oxygen and nitrogen transfer to the ISS, the docked water dump from Atlantis, certain middeck payloads from the mission manifest, the mission’s only planned EVA/spacewalk (an EVA that would use the Station crew, not Atlantis’s crew), and even more Public Affairs events than have already been deleted.

Get-well breakdown and considerations:

As part of the extensive evaluation into how much time each of the potentially delete-able mission events could buy back in terms of crew transfer time, a detailed assessment was conducted and outlined in the STS-135 timeline presentation.

In all, four groupings of potential mission content deletions were presented.

Under grouping #1, deletion of the reboost would buy back +3.5hrs of MPLM transfer time, of the ZSR deployment would buy back 1hr 5mins of MPLM transfer time, of the oxygen transfer would buy back 1hr of MPLM transfer and 1hr 5mins of middeck transfer time, of the nitrogen transfer would buy back 1hr 15mins of MPLM and 50mins of middeck transfer time, and of the water dump potentially 1hr 20mins of MPLM transfer time.

In total, deleting this group of activities would take MLPM transfer time to 131hrs 40mins – still 8hrs 20mins shy of the 140hrs of required time.

However, this group of deletions would take total middeck crew transfer time to 30hrs 35mins, thereby meeting that transfer time requirement.

Therefore, if group #1 was chosen, additional content would have to be deleted.

To this end, deletion group #2 would result in a further 7hrs of bought back transfer time. To accomplish this, the following middeck payloads would have to be deleted from the mission entirely: MYCO (1hr 30mins of time), micro-4 (1hr 5mins of time), NLP vaccine (35mins), 3 AEM status checks (45mins), GLACIER (1hr), and integrated immune (no time, just listed for completeness).

Additionally, the CP7 ETVCG removal and replacement get ahead during the mission’s sole EVA/spacewalk would also be deleted under group #2, buying back 2hrs of transfer time.

This round of deletions, coupled with those of group #1 would result in total MPLM crew transfer time of 138hrs 35mins, still 2hrs 25mins short of the 140hrs requirement.

Option three would see retention of the above mission activities in exchange for the deletion of the only EVA of the mission.

According to the mission timeline evaluation presentation, this would buy back 26hrs 30mins of mission transfer time. “Obviously major impacts to overall mission objectives, but fixes transfer mission requirements.”

This would also reduce the amount of time the Russian crew would need to assist with transfer mission objectives. However, as further noted by the presentation, “If launch slips are realized then this could prove useful as the time between STS-135/ULF 7 and Russian EVA #29 is reduced.”

However, should the EVA be deemed necessary for execution during STS-135’s docked timeline, flight designers are also evaluating the deletion of several if not all Public Affairs events during the final Space Shuttle mission.

In all, deleting the PAO event on Flight Day (FD) 6 would buy back 2hrs 30mins, the two PAO events on FD7 would buy back 3hrs of time, the two events on FD8 would buy back 2hrs 45mins, the 2 events on FD9 would buy back a whopping 20hrs of time, and the Joint Crew News Conference and in-flight Crew Photo would buy back 11hr 5mins of time. 

STS-135 Specific Articles: 

However, this is likely to be the first option decided AGAINST since PAO events have already been reduced from the original mission timeline request and “Reducing PAO events for the final flight of the Shuttle Program is probably not realistic.”

Descending Node Reentry?

As was highlighted with STS-120 in Oct/Nov 2007 and with STS-131 in April 2010, landing the Shuttle orbiter on a descending node entry (one that takes the vehicle over the heartland of the United States during atmospheric reentry) instead of the ascending node entries that are usually flown (a profile that takes the vehicle over the Pacific ocean, Central America, and the Gulf of Mexico during atmospheric reentry) would add (or buy) just over 28hrs of time into the STS-135/ULF7 mission timeline.

This would fix the mission transfer requirements and significantly reduce the amount of Russian crew time required for STS-135 mission transfer requirements. Further, the Shuttle crew’s sleep timeline would shift 2hrs later over the course of the mission instead of 5.5hrs earlier.

However, descending nodes entries are prohibited by current Shuttle Flight Rules in the summer months due to the threat of noctilucent clouds – rare high altitude clouds comprised of mini ice crystals.

Also, a note in the mission timeline evaluation presentation regarding the possible “non-starter” nature of this flight rule and the questioning of what the threat of flying through a noctilucent cloud on the final vehicle and program flight would be could point to a possible relaxing of this flight rule for STS-135 – though official confirmation of this is still a ways away.

Continued/Current threats to STS-135 as of mid-April:

Because Atlantis lacks the Station-to-Shuttle Power Transfer System (SSPTS), coupled with the desire to leave the International Space Station in the best possible condition upon Space Shuttle retirement, Atlantis is launching into the flight with the maximum mission duration possible for her: a 12+0+2 day flight (12 nominally planned flight days, ZERO mission extension days, and 2 landing contingency days).

As such, there is no potential flight day to add to the mission – hence the current timeline predicament.

Furthermore, if all the necessary FD2 Thermal Protection System inspection data cannot be collected on FD2, time from the docked mission will have to be taken to collect the data and would result in “significant impacts.”

Likewise, a Focused Inspection requirement – though slim – would take further time away from transfer operations.

Moreover, ISS payload requirements for STS-135 have not yet been finalized and will be highly dependent on when STS-135 actually launches.

Finally, the last and potentially greatest threat listed by the timeline presentation is the proposed Soyuz fly-about of the Shuttle/Station complex for the purpose of gaining historical photos.

Given the already extremely tight mission timeline, it is unlikely that enough time could be found in the STS-135 docked mission timeline to accomplish the Soyuz fly-about.

(Numerous articles will follow. L2 members refer to STS-135 coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used: Larry Sullivan, MaxQ Entertainment/, L2 Presentations and

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