Shuttle Launch Director Mike Leinbach has drawn up a plan to alter the countdown for the upcoming STS-120 launch of Discovery, with an aim to reduce ice build-up on the External Tank (ET).
The plan – which will go to the STS-120 Flight Readiness Review for full approval – involves reducing the T-3 hour hold by 30 minutes, added to ensuring the tanking process does not start before it is scheduled.
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Ice, which is both a debris hazard and a mechanism for liberating foam off the ETs during ascent, builds up on the tank when it’s filled with the super-cold liquid oxygen and liquid hydrogen ahead of launch.
To mitigate ice build-up, the tanks are covered in foam insulation. However, that foam can liberate off the tank during ascent due to the massive aerodynamic stresses the stack undergoes on the ride uphill.
Finding the balance of reducing the amount of foam that can liberate off the tank, versus what is required for thermal protection, has been an ongoing process for NASA, especially after the loss of Columbia.
Another problem relates to small areas of ice that can still build-up in certain areas of the tank. One such area is in the small gaps inside the five LO2 feedline brackets, required to allow the 17 inch diameter pipe to flex as thousands of gallons of propellant are sucked towards the orbiter’s main engines.
Those brackets have previously shed foam, but only came into focus after a liberation event 58 seconds into the launch of Endeavour during STS-118.
Rather unluckily, the piece of foam – along with what was believed to be some of the denser underlying SLA (Super Lightweight Ablative) – bounced off the aft strut of the tank, before rebounding on to Endeavour’s belly, causing a gouge that took several days of evaluations before being cleared as no threat to the orbiter’s re-entry.
No damage was suffered by Endeavour’s structure once the tiles were removed after landing.
That liberation event has been classed as directly related to ice build-up in the area of the bracket, officially listed as ‘mechanically induced cracking of the foam (due to ice bridging) and aeroshear.’
While X-ray tests – called on the next tanks set to fly – determined that cracks already existed in four of ET-120’s (STS-120) brackets, along with one on ET-125 (STS-122), ultimately causing the repair of the brackets, NASA have a plan to reduce ice build-up that can occur on the brackets in the first place.
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The plan, which was hinted at just a few days after STS-118, relates to the earlier-than-scheduled tanking of STS-118’s tank, which is believed to have caused slightly more ice to form on the brackets ahead of launch. This will be mitigated by a two-pronged plan created by Leinbach.
That plan is based around reducing the time between tanking and launch (T-0), whilst keeping to within safety requirements of the countdown.
Finding a way of reducing that timeline evaluated all three of the main built-in holds during final part of the countdown – namely: the three hour hold at T-3 hours. The 10 minute hold at T-20 minutes and the 40 minute hold at T-9 minutes.
Leinbach’s recommendation is to remove 30 minutes from the hold at T-3 hours, which, as a result, would then become a two hour, 30 minute hold.
‘This is a launch probability question, NOT a safety issue as all documented inspections will still be performed,’ Leinbach stressed in his recommendation, before adding his rationale.
‘Eliminate early tanking – no impact to planned timelines. Will hold ET load sequencer initiation until T-6 hrs and counting. Provides contingency to protect ET load start time at T-6 hrs.
‘Reduce hold time at T-3 hrs from 3 hrs to 2.5 hrs – No impact to the FIT (Final Inspection Team) timeline or inspection requirements. Since RTF-2 (STS-121) on-pad inspection timelines have averaged 2 hrs 24min (low 2 hr 6 min – high 2 hr 50 min). For Flight Crew arrival at Pad with FIT inspection in work. Reduced disposition time seen as acceptable by Technical Community.’
‘No change to other holds at T-20 and T-9 min. Not advisable from clock management perspective. Further recommend that the STS-120 LCD is structured with the changes noted and to review post-test for lessons learned. Suggest no further changes without clear evidence that additional time reduction will not significantly impact launch probability.’
The presentation (available on L2) – with a new timeline chart showing the new countdown procedure – does not make it clear if the actions are now baselined into the STS-120 countdown, although it is – as with all changes to a launch – set to go in front of the Flight Readiness Review (FRR) for approval, should it be required.
L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, updated live.
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