Risk assessment debate for STS-128 tank – Rollback is a potential option
Meetings are taking place through Friday to work on risk acceptance for two LO2 Ice Frost Ramps (IFRs) on Discovery’s External Tank (ET-132), following a full review of STS-127′s LO2 IFR liberation event during launch. If managers call for foam loss mitigation on the ramps, the STS-128 stack will rollback to the Vehicle Assembly Building (VAB).
With the S0017 Terminal Countdown Demonstration Test (TCDT) now complete, Discovery remains on a timeline that would result in a launch around August 24 – though no “official” NET (No Earlier Than) date is currently available due to additional work being carried out of the left hand Solid Rocket Booster (SRB).
Currently the pad flow is working on S0024 Prelaunch Propellant Load preparations, GH2 recharge at the GH2 Storage Battery – to be conducted on Monday, and preparations for the closure of Discovery’s Payload Bay Doors (PLBDs).
“S0600 Payload Operations: Payload installation is complete. OBSS/RMS (Orbiter Boom Sensor System/Remote Manipulator System) work continues Friday; IVT’s will be performed tomorrow (Saturday), noted processing information on L2.
“Weekend Work: close PLBD, preps for S0024, HPU (Hydraulic Power Unit) SCAPE (Self-Contained Atmospheric Protection Ensemble – suits used while performing task such as hydrazine loading) to remate QD’s (Quick Disconnects) and preps for HPU hotfire test.”
The HPU hotfire is required due to the changeout of the unit at the pad. This followed issues with the associated Check Valve Filter Assembly (CVFA) – which called for the replacement of the HPU, on-board hydraulic pump, and CVFA itself.
“LH SRB HPU update: HPU R&R is complete,” added Friday processing notes. “Electrical reconnections are planned for tomorrow (Saturday) with SCAPE operations following to reconnect the HPU fuel lines.”
The root cause for the CVFA failure is all too apparent via images taken of the now-dismantled hardware, which show a broken snap ring and a metal fragment within the filter element.
“Disassembly of the check valve filter assembly from STS-128 confirmed that the problem was caused by the check valve in the unit. This has been replaced, and will support launch on August 24,” added notes on the Shuttle Standup/Integration report (on L2).
“The SRB HPU story has caused (the) Orbiter (Project Office to) investigate its APU check valves for commonality. There are no issues.”
August 24 continues to be referenced as the current launch date target on both milestone charts and engineering notes, although managers are evaluating the length of the STS-128 launch window, prior to standing down to allow the Japanese HTV to launch on its debut flight to the International Space Station (ISS).
“Launch opportunities for STS-128/17A show no impact to the HTV1 mission through August 30,” added the Standup. “Launching on August 31 will cause us to lose a +1 day. If the vehicle launches on September 1, we lose the +1 day and one docked day. Flight Day 3 is solid through September 2.”
ET-132 LO2 IFR Debate:
An October launch date would be more realistic, should managers decide to roll the STS-128 stack back to the VAB, for work on the tank’s LO2 ramps.
However, such a call would only be made in the event of ongoing risk assessment evaluations failing to accept the known potential for the LO2 IFRs to shed foam during ascent.
Those risk assessments – which take place ahead of every launch – have not been aided by a liberation event during STS-127′s launch, when ET-131 liberated foam from one of the ramps near the top of the tank – a liberation that exceeded the risk baseline.
“STS-127-I-004 – ET TPS Loss at LO2 Ice Frost Ramp (IFR) 718. Foam loss at IFR 718 exceeds risk baseline,” noted the integrated IFA presentation for STS-127 – available on L2.
“Associated Hazard Reports: Ascent debris impact to SSV (Space Shuttle Vehicle). Cause I – External Debris Impact to the SSV During Ascent. Due to ET Foam (PDL-1034/NCFI-26-007/BX-265). Located at the LO2 Tank Ice/Frost Ramps and BX-265/BX-250/PDL-1034/NCFI-26-007 (type of foam) located on the LO2 Feedline Bellows PDL Drip Lip.”
ET-131 had an eventful ride uphill, liberating several areas of foam – most noticeably from the intertank, but also from the bipod area and the aforementioned LO2 IFRs.
Pull tests on ET-132 were conducted inside the VAB and found the adhesive properties between the foam and the tank’s structure to be within acceptable limits.
However, from an orbiter debris impact standpoint, the IFR liberation was always going to require further evaluation. Checks on ET-132 – including X-rays – were carried out on Discovery’s ET-132, in order to ensure the foam had no underlying problem.
“LO2 Ice Frost Ramp Loss: Additional NDE (Non Destructive Evaluation) options pursued for ET-132 at KSC included X-ray and THz,” added the ET-127 IFA presentational to the PRCB, noting good news on the health of ET-132’s ramps. “All observations were within performance expectations.”
“NDE performed on ET-132 LO2 Ice Frost Ramp. Process assessment of ET-132 Sta. 718 IFR Terahertz scan identified no issues. No anomalies noted (voids, cracks, etc.). Scans limited (whole ramp not scanned),” added the investigation presentation.
As to why the IFR debate is still ongoing – despite the positive findings – relates to the continuous process of working on risk assessments for the Flight Readiness Reviews (FRR).
Thursday’s Program Requirements Control Board (PRCB) meeting conducted a second review into STS-127′s IFAs (In Flight Anomalies), again dominated by the foam liberation events during ascent (presentations available on L2).
This followed last week’s PRCB meeting, which took a first look at the mission’s IFAs, prior to yesterday’s expanded overview, which provided the first real managerial discussion on the IFA’s findings, specifically on the foam losses.
“Will be the first discussion regarding IFAs on ET-131 used for STS-127/2JA. This will focus on the intertank and the LO2 IFR and the Bipod area,” Shuttle manager John Shannon noted ahead of the PRCB meeting via the Standup.
“When we were initially discussing rollout, the focus was on what types of data to collect. Resolution of open work and the entire picture of the story will be told. (Analysis is) being done at this time, along with assessment of the terahertz and plug-pull data.
“The discussion will revolve around the question of whether or not we have done enough work to characterize the situation and risk appropriately. We will not launch until all questions have been fully answered.”
Following the PRCB meeting, engineering notes – provided to KSC engineers on Friday morning and acquired by L2 – suggested that plans are being drawn up to return the STS-128 stack to the VAB for work on two IFRs, in the event of a negative risk assessment.
“The teams will meet again today (Friday) at noon to discuss the potential LO2 Tank Ice/Frost Ramp rework on ET-132 (STS-128),” outlined the engineering notes.
“A meeting was held Thursday afternoon (PRCB) to discuss access in the Integration Cell (in the VAB) for potential IFR rework for ramps at (stations) XT-593 to XT-828, pending a final Program decision for STS-128 rollback.
“Concerns still exist regarding foam debris documented on STS-127 from XT-718 ramp (the main liberation event, though two other ramps also shed a small amount of foam). Program is weighing options to mitigate or accept risk at LO2 IFR locations.
Should managers decide that rollback is the best option, a large amount of scaffolding would be built around the top of ET-132, allowing for reworks to be conducted on the two ramps in question.
“Team of USA (United Space Alliance), LM/LSS (Lockheed Martin), and NASA laid out a preliminary plan for Integration Cell access that involves scaffolding from B-Roof to rework ramps 828 to 760, then reconfig access with scaffolding from E-Roof to rework ramps 718 to 634,” added the notes.
“Ramp at 593 would likely be accessed from C-Main (or retracted version). Engineering and Ops will be performing assessment walkdowns on Friday to firm plans for potential rework.”
While no information on exactly what type of rework would be undertaken is available at this time, it would likely mirror the procedures to modify the LH2 IFRs – located further down the tank. This involved the trimming of foam from the ramps – thus reducing the potential mass of any liberation.
Also, plans are already in work to install a camera facing out of the pilot window to film how the LO2 IFRs behave during ascent.
“Recent photos have been taken of the view seen of the ET from the pilot’s seat through window 4,” added another memo outlining the plan.
“The photographs were taken in order to see the best location (best view of the tank as well as not interfering with the Pilot’s view) that a minicam could be mounted on the flight deck to monitor foam loss on the upper part of the tank during ascent.
“Further research and discussion will need to be completed before making a decision on the location. Note: Image currently blurred by window cover on the outside of window.”
In summary, managers are faced with two decisions. Firstly, a decision could be made to accept the risk assessments as within safety parameters. Such a decision would be made during either/or both the SSP FRR or the Agency FRR – the latter of which approves the flight to proceed and confirms the launch date.
Secondly, if managers decide the risk is unacceptable for flight, plans will be presented to reduce the risk by engineering work on the IFRs. However, due to access and environmental conditions, rollback to the VAB would become the only option available.
A second article will follow ahead of the SSP FRR to further outline the IFR issue.
L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.