The pros and cons of carrying out a Tanking Test ahead of STS-121 have been revealed in a MSFC document, acquired by NASASpaceflight.com.
The fascinating document, written by NASA/MSFC’s Robert Champion, was presented to last week’s PRCB (Program Requirements Control Board) – and was responsible for ending any possibility of carrying out the cryo-loading of ET-119, which had been pencilled in for June 1.
Live updates, documents, information and educated debate from the global space flight community – join L2. The following information appeared on L2 last weekend.
The original rationale to proceed with a Tanking Test was to verify that the replaced ECO (Engine Cut Off) sensors in ET-119 performed to the required standards, in order to support a smooth countdown a month later.
In full, the test would carry out the following objectives: Cryoshock the LH2 Low Level sensors (ECOS). Evaluate LH2 Low Level system performance under cryogenic conditions. Assess Thermal Protection System (TPS) modification under cryogenic conditions. Evaluate ET Gaseous Hydrogen (GH2) vent line shroud modifications. Demonstrate ET heater performance. Obtain Gaseous Helium (GHe) flowrate instrumentation data to support post-test GH2 prepress performance evaluation. Provide training opportunity.
However, immediate concerns against the Tanking Test negated the requirement, notably, the objectives – except for cryoshock – can be achieved by launch attempt, while the test would expend a thermal and two pressure cycles (one for LOX) on the tank, as well as a cycle on SSME LAI. The test would also raise Fuel Cell supportability concerns.
While a Tanking Test would have benefits in a near-full launch countdown training exercise for the supporting controllers and technicians, added to the confirmation that the ECOs did perform as expected, this was again unbalanced by a list of ‘cons’ against the requirement.
‘Low confidence in repeating due to intermittent failure history,’ noted the document, in rationale against the test. ‘Many items were changed (sensors, tanks, PSB’s, MLP’s, cables, connectors, grounding, etc) between TT1, TT2, Launch attempt 1 and Launch to limit it to one item. ECO Cryoshock theory has not been proven to improve performance. MSFC testing has shown no reduction in sensor resistance due to multiple cryocycles.
‘The Sensors in ET-119 have been through the improved inspection process for selection. ET-120 ECO sensor inspection and test results available next week. Test will have no additional instrumentation to capture failure signature if it should recur. If we have an ECO problem then we treat it as a new issue and troubleshoot accordingly.’
Since the rumours of a Tanking Test came to light, several sources noted their primary concern of cryo-loading the tank due to the potential damage that can occur to the tank’s foam. Loading the tank with supercold propellants leads to ice forming on the skin of the tank, with the potential of super-fine cracks weakening the TPS of the protective foam. That, in turn, raises problems with foam liberation on ascent – which is one of the key goals for NASA during STS-121 to reduce the size and quantity of foam shedding.
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‘Visible Foam cracking is worst on second tanking historically. We have operational controls established to ensure safe flight (ice team inspection),’ the document continued in the ‘con’ rationale against the Tanking Test. Unexpected foam cracks on detank may require rework, delaying launch. Will not know how the foam is performing below surface after cryotanking. We will not have IFR Mod option available after cryotanking.
‘Ice/Frost is partly a function of the natural environment conditions. A June tanking attempt may not reflect environmental conditions on subsequent launch attempts. Except for places where there are already pre-established waivers, the ET design changes and TPS applications have been certified to the cryoconditions expected from a fully loaded ET. We have in excess of 114 loadings to establish the SSP experience base for ice/frost formation in a wide range of environmental conditions.’
While the ‘pros’ for the Tanking Test continued to show weak rationale, giving examples of opportunities for crew training by way of a live exercise, the rationale against didn’t let up in its damning recommendation not to carry out the test.
‘8 weeks from start is a very short time to implement plan,’ added the document on the impact on the schedule for implementation and conduction of the test in the launch schedule flow.
‘Previous tanking test planning was over 1 year. Increased risk of errors in processes and procedures due to short turn around of requirement development and compressed schedule. Compressed time to checkout software mods prior to tanking. Workforce Fatigue and Workload. Reduces pad contingency days from 17 to 12 days with early P/L entry.’
Following the presentation at the PRCB, Shuttle manager Wayne Hale – according to sources – asked who was in favour of the Tanking Test. No one – bar the technicians responsible for the ECO sensors – raised their hands.
Sources also noted that the following element of the document literally changed the minds of some managers who had previously been in favour of the test. The key note that stood out throughout the document was ‘Recommend not conducting the Tanking Test.’ Click image below for larger screenshot.
‘The TPS issues that were identified during the STS-114 IFA were not seen during the tanking tests that preceded the flight,’ noted the document under ‘issues and concerns.’
The ET project noted:
‘The primary issues of subsurface cracks and delaminations could not be visually ascertained and that would not change with performing another test. Only NDE and dissections would provide conclusive results and those inspection techniques are not an option.
‘The RTF foam testing has been based on demonstrating TPS performance for 13 cryocycles, however, the TPS does degrade with each cycle and without clear test objectives that would provide pertinent data, a tanking test would not be a risk reduction exercise.
‘The drain cycle has also been shown to degrade the TPS performance and can potentially cause additional damage. The TPS performance is maintained for this capability, however, as noted in #2, without clear test objectives that would provide pertinent data, a tanking test would not be a risk reduction exercise.
‘The tanking tests performed as part of STS-114 did not clearly identify a consistent ECO sensor system issue each and every time and the testing was insufficient to isolate the issue to any one piece of hardware or system. If a tanking test is performed, it is recommended that ground instrumentation be added that would assist an investigation in the event of an ECO system anomaly during the test.
‘The MLP and GSE checkout and performance is one area that can be verified with a tanking test. Again, the desire is that these systems will be tested and verified at the subsystem level and that a tanking test would not be required to demonstrate these systems.
In the case of STS-121,MLP (1) and Pad (B) will be used. The underlying assumption is that any modifications to these systems would be assessed for possible effects and that the ground valve issue associated with the LH2 tank prepress cycle count experienced on STS-114 has been addressed for STS-121.’
The Reusable Solid Rocket Motor (RSRM) Project noted: ‘The STS-114 tanking test may have contributed to the blistered paint found on RSRM-90B ETA stubs which led to the RSRM 180 day pad stay time waiver (ref. CR S063330) resulting in the 42 day pad stay constraint.’
PH Engineering noted: ‘Planning and performing a test of this magnitude in less than six weeks has the potential for introducing significant risk and increases the chance for mistakes. Every tanking test is different with its own set of requirements. We had over a year to plan for the STS-114 tanking test -plan it into the schedule, time to review and comprehend every requirement, implement software changes and build confidence, and overhaul the test procedure. We did a less than adequate job on Tanking Test 2 last year because of an accelerated schedule and changing requirements.
‘We had numerous errors that were either not caught or resulted in real-time changes/corrections. Due to the workload and the teamâ€™s preparedness, the KSC test team had to stop and call a time out, slipping the test one day. We are on that path again for all the same reasons.’
Electrical Engineering noted: ‘EP recommends the use of ECO system diagnostic instrumentation to allow for failure isolation should an ECO system anomaly occur during this tanking test. The use of this instrumentation could also allow for insight into potential ECO sensor resistance shifts as a result of thermal conditioning that might otherwise not be indicated/observed. In the event of no observed anomalies during the tanking test, this instrumentation would give added confidence in the ECO system for flight of STS-121.’
Orbiter Project noted: ‘Post STS-121/115 Fuel Cell Supportability Concerns. Fuel cells can support STS-121 tanking test if necessary. Norylinsulator plates can begin to leak externally over time due to powerplant operation and start/stop thermal cycles. All three fuel cells removed after STS-114 due to Norylinsulator plate leakage after two tanking tests, a launch scrub, and then flight.
‘Fuel cell powerplants currently installed in OV-103 have very similar number of hours to those installed on OV-103 for STS-114 flow and Norylinsulator plates from the same lot number. Currently have zero spare power plants at KSC with one fuel cell still required for installation in OV-105. Near term fuel cell deliveries from the vendor are scheduled for May, September, and December. Reducing fuel cell operating hours and start/stop thermal cycles helps protect supportability of near term manifest.’
All in all, the huge interdepartmental movement against such a Tanking Test proved to be definitive, leaving the Shuttle Program managers with little option but to delete the requirement from the launch/processing flow.
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