STS-135: Atlantis moving through S0007 tasks – Weather a concern for launch
Atlantis and her engineers are now working through the complex set of tasks relating to S0007 – better known as the launch countdown – with no major issues currently under evaluation. However, Friday’s countdown does have at least one challenge to overcome, related to the stormy weather which is expected to roll in over the Kennedy Space Center (KSC) late in the count.
Providing Atlantis doesn’t suffer a major issue, resulting a lengthy turnaround, the start of S0007 operations marks the last time the iconic clock near the turn basin at KSC started ticking back from T-43 hours for a Space Shuttle launch.
Known as S0007 Operations, Call-to-Stations (CTS) occured at at 1230 EDT – with the clocks ticking 30 minutes later – ahead of Friday’s launch window, which ranges from 1121:46 – 1131:46 EDT, for a Flight Day 3 rendezvous; with an option to extend to 1135:05 EDT for a Flight Day 4 rendezvous.
Launch opportunities exist from July 8 to July 10, prior to the current plan to standdown for the Delta IV GPS launch, as the range undergoes reconfiguration for the Cape Canaveral launch, resulting in a constraint which stretches from July 11 to July 15.
The next earliest launch opportunity is 16 July, per the current plan, which can be – if required – negotiated with the the United Launch Alliance (ULA) to potentially juggle the range priority back to Atlantis.
With Atlantis’ Main Propulsion System (MPS) and Hyper pressurization tasks completed at the end of last week, S1287 operations were finalized, relating to orbiter aft closeouts. This resulted in the installation of the aft flight door and post closeout aft completion.
The APU (Auxiliary Power Unit) post-door aft confidence test has also been completed, with nominal results, as the orbiter was buttoned up for her big day. Also listed was the completion of the ODS (Orbiter Docking System) confidence tests, along with final preps on the MPS ahead of launch day.
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Only two issues required a level of engineering discussion, namely final clearance for a waiver to be issued on the Interim Problem Report (IPR-55) relating to a slight breach of limitations during pressurization tasks on the Left Orbiter Manuevering System (LOMS), in addition to work on a problematic ground TACAN unit.
“IPR 135V-0055, helium regulator locked up above desired limit, update: An ERB (Engineering Review Board) will be held today (Monday) to discuss waiver rationale. This will be followed by a noon board,” noted the NASA Test Director (NTD) report (L2), with the waiver expected to be confirmed later in the day.
“On Friday, a problem with the ground TACAN station Transfer Control Unit (TCU) was detected. Weekend troubleshooting identified the power supply to the primary TCU as the root cause. Repair and retest is scheduled Monday upon completion of STA (Shuttle Trainer Aircraft) operations.”
The ground-based TACAN and VHF omnirange TACAN stations constitute a global navigation system for military and civilian aircraft operating at L-band frequencies (1 gigahertz). The orbiter is equipped with three TACAN sets that operate redundantly. Each TACAN has two antennas: one on the orbiter’s lower forward fuselage and one on the orbiter’s upper forward fuselage.
The onboard TACAN sets are used for external navigation and for the orbiter during the entry phase and return-to-launch-site abort. Normally, several ground stations will be used after leaving L-band communications blackout and during the terminal area energy management phases. TACAN’s maximum range is 400 nautical miles (460 statute miles).
With the crew arriving at the Shuttle Landing Facility (SLF) in their T-38 jets on Monday, the latest on the weather for Friday’s launch attempt remains pessimistic, as much as the ever-changable Florida weather may improve matters as the week progresses.
“Synoptic Discussion: The Bermuda high pressure ridge is slightly north of the area, and weak, east winds will cause showers and thunderstorms to develop along the sea breeze inland today,” noted the latest weather briefing by the 35th Weather Squadron (L2).
“On Wednesday, a tropical wave in the vicinity of the Turks and Caicos Islands is moving west-northwest and will impact Florida on Thursday, increasing moisture and bringing scattered showers and isolated thunderstorms by Thursday afternoon and evening.
“By Friday, the wave will move off to the northwest, but the residual moisture will provide the ingredients necessary for showers and thunderstorms to develop as the sea breeze forms Friday late morning. Our primary concerns for launch are showers and thunderstorms within 20 nautical miles of the Shuttle Landing Facility and cumulus clouds near the flight path.”
As such, officials are currently forecasting a 60 percent chance of a weather violation at launch time, a probability which improves for later launch attempts.
“The same concerns exist for the following two days, but due to the launch time moving earlier and a slightly dryer atmosphere on Sunday, the threat of sea breeze development during the launch window decreases each day; therefore, the probability of KSC weather prohibiting launch decreases as well.”
Ideally, STS-135 will want to launch at the first attempt after a nominal countdown, which would result in a Flight Day 3 rendezvous and docking, something which would also aid the potential for managers to opt to extend the mission by one extra day.
STS-135, also designated ISS Utilization and Logistics Flight-7 (ULF-7), is launching with a full set of mission objectives, but the situation is complicated by the combination of the widely-discussed smaller crew size with the shorter mission duration, since the available Shuttle orbiter for this mission is the one that can’t receive power from the ISS.
A complicated set of circumstances meant that while Atlantis remained available to fly STS-135 rather than being retired a few years ago, the orbiter didn’t receive the modifications necessary to use the Station-to-Shuttle Power Transfer System (SSPTS) for an extended stay at the Station.
One of the other consequences is that this will be the only time that Atlantis carries an MPLM to the Station and back, even though she’s been manifested to do so on at least two previous occasions.
The timeline remains very tight during the docked part of the mission and there remains a desire to return as much equipment and trash from Station back to the ground as possible; close to launch, with most details of the mission now finalized, mission planners took another look at Atlantis’ consumables situation to see how far away they are from being able to squeeze another day in, docked to ISS.
During the preflight briefings, STS-135 Lead Flight Director Kwatsi Alibaruho explained that while they’re still a little short, there are a few ways to make up the deficit – if Atlantis launches on-time:
“We are flying full cryogenic tanks; our cryogenic oxygen and hydrogen tanks are used to generate electrical power, which is the limiting consumable that drives how long we can stay on orbit,” he explained. “Based on our analysis and based on our power requirements for this mission, we are approximately 80 kilowatt-hours away from having sufficient margin to extend from twelve days to thirteen days.
“We assessed some options for making up that deficit; one option we’ve discussed with the ISS program and that we’ve agreed to, is that if we do launch on time, we will not power the heaters that are used to keep the MPLM warm in the payload bay. We won’t power them between flight day one and flight day four, where we install the MPLM, and we won’t power them after we unberth the MPLM and bring it home.”
Confirmed at the Flight Readiness Review (FRR), the mitigation of not powering the MPLM heaters via the orbiter’s consumables will buy additional margin to aid a positive decision on the mission extension evaluations.
“We’ve done analysis which suggests that the temperatures can be maintained within acceptable limits without those heaters – those heaters really provided additional capability and redundancy – so by not powering those heaters, we’re going to save most of the power that we need to save in order to make up that extra day,” added Mr Alibaruho.
“The remaining deficit – which is to the tune of right around 25 kilowatt-hours – we think we can make that up with what we call a cryo ‘overload.’
“When we’re loading the cryogenics, the oxygen and hydrogen into Atlantis just prior to launch, (and) typically when we top it off and fill it up there’s just that little bit extra that we’re able to get in the tanks above and beyond the spec’d volume of the tanks. We call that our sort-of ‘cryo overload’ and historically we’ve seen a consistent trend of how much additional…cryogenic oxygen and hydrogen we can get.
“If we get what we’ve seen typically, our analysis says that will make up the remaining deficit, so (with) the combination of cryo overload and not powering the MPLM heaters, we think that’s going to get us there – if we launch on-time.”
Should this procedure fail to buy the expected margins, other elements of mitigation are available to the talented engineering teams on the ground.
“If we don’t get the cryo overload that we expect, we might be able to make up that deficit with some very modest additional powerdowns, again nothing that’s critical that we would need during our travel to and from the Space Station, (for example) turning down the lights when we don’t need them, that sort of thing,” the Flight Director noted.
“The sort of power-saving measures you might use in your own home. So we think we are actually very close to getting it if we get off on-time.”
(Images: Via Brian Papke (Lead and VAB) and Larry Sullivan (T-38 and crew) MaxQ Entertainment/NASASpaceflight.com and L2 content. Further articles on STS-135′s status in work, driven by L2′s fast expanding STS-135 Special Section which is already into the FRR content and live flow coverage, plus more.
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