A year after discovering small voids between the solid propellant and the outer casing of the aft segment of a test motor for the massive 5-segment SRBs for NASA’s upcoming Space Launch System (SLS) rocket, ATK is making firm progress in their investigation and the mitigating factors toward correcting the issue ahead of the Qualification Motor -1 (QM-1) firing.
Solid rocket motor designs utilize a specialized rubber insulation between the external case and the propellant.
The bond between the propellant and insulation, which provides thermal protection for the case, is a critical design feature that undergoes rigorous x-ray inspection to assure its acceptability for flight. These inspections are conducted on all solid rocket motors to ensure they are free of defects.
In the late fall of 2012, as ATK was preparing for the first Qualification Motor (QM-1) test of the new 5-segment variant of the Space Shuttle’s Solid Rocket Boosters (SRBs), small voids between the propellant and the motor casing of the aft segment were discovered during standard x-ray inspections to evaluate an insulation lining material change in the SRB motor segments.
The solid rocket motor segments have a layer of insulation with a liner coating to form a contiguous bond with propellant (known as the bond line). Voids at the propellant bond line are readily detectable with x-ray inspections, and when found during the SLS inspection led to a change in the Propellant/Liner/Insulation (PLI) manufacturing process.
ATK, with NASA’s approval, rejected the first QM-1 aft segment and proceeded to recast another to ensure that the voids would not interfere with the test objectives of QM-1. However, the second test article had similar voids, leading ATK and NASA to investigate the new rubber insulation material.
“We had adequate schedule margin when the voids were discovered to do a thorough evaluation of the voids. We felt this was important to ensure we could determine a root cause and take corrective action,” said Harry Reed, deputy program manager, ATK 5-Segment Booster program – to NASASpaceFlight.com
With two aft segments now displaying the same issue, ATK expanded their investigation to determine what was causing the propellant to de-bond from the casing wall and form voids.
Quickly, evidence began to mount that the issues were related to use of a new material in the insulation lining between the propellant and the motor casing wall.
Originally, the Shuttle SRB motor segment insulation lining was made of chrysotile-filled rubber: chrysotile being the most-common mineral form of asbestos.
While asbestos minerals contain useful sound absorption, strength, and resistance to fire, heat, electrical, and chemical damage properties, the silicate mineral holds severe health risks when inhaled over a prolonged period of time.
Over the last decade, the removal of asbestos products in ATK deliverables – specifically in the insulation linings of the SRBs – had been a stated priority of the company.
So high was this priority that non-asbestos (chrysotile-free) rubber insulation was on a path to be qualified for use on the Shuttle SRBs – even though that particular kind of insulation never flew before the end of the Shuttle Program in 2011.
Nevertheless, the qualification efforts for flight on the Shuttle SRBs meant that the new insulation material had been thoroughly tested through numerous static test firings of SRBs at ATK’s proving ground near Promontory, Utah.
With qualification ground testing complete, chrysotile-free insulation was then baselined for use on the Ares I rocket. After that rocket’s cancellation, the baseline use shifted to the SRBs for the SLS rocket.
Thus, the switch to chrysotile-free insulation was made, and the old asbestos-containing liners were rendered obsolete upon the retirement of the Shuttle fleet.
As development of the SLS 5-segment SRBs continued between 2011 and 2013, three DM (Demonstration Motor) tests were carried out – all of which occurred with the new insulation and without out-of-family void issues.
As the SLS booster design matured from the DM stage to the QM, standard development processing and design changes were undertaken as the SRB design moved toward flight status.
However, after the DM series of static test firings was complete, NASA initiated a request that ATK conduct additional inspections on the QM series of motors segments to see if any defects were present as a result of the switch to asbestos-free insulation lining.
Following the x-ray discovery of the voids within the propellant of the two aft QM-1 motor segments, ATK placed segment processing on hold until an investigation could determine primary/root cause(s) and corrective actions.
With confirmation of a PLI unbond and propellant void, ATK began a “deep dive assessment in support of Knowledge Point Share” meetings with NASA to determine the root cause of the PLI unbond and void issue.
At an initial meeting with engineers from the Marshall Space Flight Center, the investigation team prioritized the dissection of the QM-1B segment, the one that showed signs of an unbond, as the first site for a PLI sample extraction and detailed examination.
Moreover, ATK and NASA agreed to cast an aft segment – with added scrutiny – following the DM-series manufacturing processes, not the QM-series manufacturing processes, to assess whether the PLI void issue was due to an unidentified effect of manufacturing changes. These activities were a part of the root cause investigation.
This DM series segment received the name Process Simulation Article-1 (PSA-1).
By November 2013, ATK had finished PSA-1 aft dome’s cure, trim, and finalization and had begun insulation lay-up of the barrel section.
Additionally, ATK had begun constructing an Integrated Logic Flow (ILF) to outline all of the testing (sub-scale and full-scale) and key decision points necessary for the down-selecting of changes to be incorporated on the Process Simulation Article-2 (PSA-2, full scale aft segment, loaded with live propellant) – which would follow a new-and-improved QM motor production process fine-tuned over the course of the root cause investigation.
By the end of 2013, PSA-1’s insulation cure was complete, and Ultrasonic Testing (UT) inspections revealed the presence of a void in the insulation by the aft dome factory joint.
Follow-up x-ray inspections showed the void to be three small voids in close proximity.
This discovery provided valuable corollary data to inspection results of Shuttle and Ares I DM series motor segments where voids had been detected in the same area of aft segment motors (again, all within family for DM series motors).
As the investigation continued into March 2014, ATK’s Evaluation Article-1 (EA-1) – designed to allow for comparison testing of insulation lay-up techniques used inthe current QM series motor production process (used in Joint 1 of EA-1) and newly-identified insulation lay-up techniques (used in Joint 2) – had completed x-ray evaluations.
These x-rays revealed that Joint 2 (using the newly identified lay-up technique known as wallpaper butt-joints) was clear of any voids.
However, Joint 1 was shown to possess two small voids in a location where the area of the seal ply insulation overlaps the joint.
With these results in hand, the investigation teams made the recommendation to use the new wallpaper butt-joint lay-up process on EA-1’s 3rd Joint: a Joint upon which the engineering teams had forgone insulation lay-up until evaluations on the two lay-up options could be completed and evaluated.
EA-1 proved a crucial step in the investigation process as its test results were used to determine the new insulation lay-up process for PSA-2 – the aft segment test article thatincorporates all of the corrective actions the investigation team was developing and then would be loaded with live propellant to determine whether these actions had eliminated the occurrence of PLI voids.
By April, this lay-up process was approved for use on PSA-2, and ATK was given permission from NASA to proceed with PSA-2 through the insulation lay-up process using the new wallpaper butt-joint process from EA-1 that resulted in no insulation voids.
Meanwhile, during the investigation process, ATK developed a new x-ray technique to inspect joint regions of the segments that were not accessible with prior x-ray techniques.
Based on the new technique, ATK proceeded with the disassembly (i.e. detachment) of the remaining four motor segments of QM-1 to facilitate x-ray inspections of them with the new x-ray angle (developed from work performed on the QM-1B aft motor segment) to determine with added certainty that no PLI unbonds and/or voids were present in those segments.
Of the four QM-1 segments re-inspected, three passed with no concerns while the Center/Aft segment was found to have one area of concern.
The area was found to have an x-ray indication “similar” in appearance to the insulation void, PLI unbond, and propellant void areas seen on QM-1A and QM-1B aft motor segments.
The location of the indication on the Center/Aft segment was in an area classed as “very benign,” meaning the area is exposed only for combustion late in the burn cycle of the motor segment – in the last 5 seconds of the burn.
(At the time of this writing, a formal decision that the Center/Aft segment is acceptable for use has been reached, contingent on the completion of some open actions, including a sub-scale test.)
By the end of May 2014, PSA-1 (produced with the DM process) had completed production, and x-rays and evaluations had revealed the presence of 64 insulation voids and one PLI defect.
(Insulation voids are those found deep within the rubber, not in the bond line, and are historically more common and acceptable; the voids of concern are those at the bond line. With the enhancements to the EA-1 process, voids within the insulation have not recurred.)
For comparison, SLS Developmental Motor-3 (DM-3), ground-tested on 8 September 2011, had a total of 163 non-propellant insulation voids within the rubber across the 5-segments. These voids are well understood and not considered problematic when properly characterized.
(Image taken from the amazing 220mb super slow-mo DM-3 Five Seg Motor Ground Test Video – available in L2 – LINK).
On DM-3 they were acceptable, and the motor had a successful test firing. PSA-1 has 64 voids and one PLI defect in a single segment.
While those numbers initially seem high, the total of 64 voids is within the historical Development Motor family, and PSA-1-fabricated as part of a void investigation—received significantly more thorough inspections then previous DM series motor segments.
As part of the DM series, motor segments are not subjected to the kinds of inspections QM series motors are because the entire test series is designed to narrow down processing and safety needs for the final flight-production process. Thus, QM series motors receive more stringent inspections than DM series motors, and flight-set motors receive continuous thorough inspections through the life of the program they serve.
In this case, the DM series motors passed all of ATK’s and NASA’s inspections and test firings. It wasn’t until ATK was proceeding toward QM series motor segments that NASA requested more thorough inspections of the QM series motors to determine whether the switch to non-asbestos containing insulation liners was having a previously unseen effect.
“The beauty of the solid rocket motor inspection system is that defects will be found and solutions reached to ensure the motors delivered will perform with the highest reliability,” said Reed. “This is a requirement to ensure SLS is a safe and reliable system for human exploration of deep space.”
With this newly acquired PSA-1 information in hand, ATK engineers re-reviewed the developed fault tree of the investigation and determined that the evidence and corrective actions to date were “consistent with the fault tree elements and the data obtained from the QM-1B dissection.”
Additional evaluations on dry cycle testing – part of the investigation – revealed a potential need to reevaluate the humidity environmental requirement during the processing/production timelines.
The exact effect, or how great an effect, humidity might contribute to the issue during the production process is still unknown, and ATK is continuing to investigate.
Presently, QM-1B and the QM-1A segment are being monitored and routinely re-inspected to determine the growth rate – if any – of the PLI defects. This continuous inspection for growth is due to the need to understand whether the voids can propagate or whether they are static after they form.
This understanding will potentially influence forward procedures and policies regarding void and PLI unbond detections.
Meanwhile, PSA-2 is proceeding toward propellant load operations in mid-October, followed by x-ray in early November to determine that the void issue has been successfully addressed.
While no definitive date has been set for the QM-1 static test firing, a target of early 2015 is likely.
(Images: Via ATK, NASA and L2 content from L2′s SLS specific L2 section, which includes the SRS “Bibles” – numerous non-restricted presentations all in one place.)
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