Orbital ATK engineers have concluded a thorough examination of the Qualification Motor -1 (QM-1) following its Static Fire test in March. The motor performed as expected, providing a green light to proceed to the QM-2 Static Fire next spring – the final test that will confirm the five segment boosters are ready to fly with the Space Launch System (SLS). Casting of the QM-2 motor segments will take place over the summer.
The QM-1 test was the first of two “qualification” test firings of the five segment booster that will provide the majority of the thrust for SLS during first stage flight.
The move to the five segment version began with the Constellation Program (CxP) as a single in-line first stage for Ares I, before transitioning back to the traditional use of two boosters on either side of the core stage for SLS.
Testing of the larger motor involved three Demonstration Motor (DM) Static Fire tests, with the results feeding into QM-1.
Preparations for the QM-1 test began with the first casting operation of the Forward Segment back in July 16, 2012 at Orbital ATK’s Promontory facility.
However, in early 2013, the QM-1 test was slipped, due to issues found during the inspection of the aft segment, which was found to have about a two foot-wide area where propellant had debonded from the inside of the segment wall.
Months of evaluations, recasting and further examinations resulted the confirmation of a Propellant/Liner/Insulation (PLI) unbond and propellant void, requiring engineers to begin “deep dive assessment” meetings – in cooperation with NASA – to determine the path forward.
“We ran thousands of tests from the smallest material property test to five full-scale engineering and process simulation articles,” noted Alex Priskos, manager of the SLS Boosters Office at NASA’s Marshall Space Flight Center. “There were a lot of nights and weekends involved. It was an intensive investigation.”
The root cause was traced to the new asbestos-free insulation, selected to comply with environmental regulations.
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 Orbital 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.
However, as discovered during the investigation, after the insulation was cured in the motor case segments, it emitted gas bubbles when the segments were filled with warm propellant. The release of gas bubbles created unbonds at the interface between the propellant and insulation, as well as voids in the propellant.
Numerous mitigation processes were looked into, concentrating on the area between adhesive used between the metal case and the adjacent insulation. This included the installation of an additional thin sheet of rubber insulation on top of the adhesive layer to add structural strength to the barrier.
This was known as the “wallpaper butt-joint lay-up process”, which – along with a number of processing changes – solved the issue as a segment was cast with no voids discovered. In fact, it was the most defect-free segment ever produced, showing not only a solution to the void issue, but also an improvement to the quality of the segments.
“It’s been a tough road, but we’ve come through this with new processes, new repair techniques, and new inspection techniques that help ensure this is the safest booster ever built,” Mr. Priskos added.
“The team did an amazing job. We will continue to gain additional data about this insulator because we want to make sure that we can reproduce this success time and time again. We face that challenge with every booster we make.”
With 102 design objectives, supported by more than 530 instrumentation channels, the motor was put through its paces during a “high temperature (90 degrees F mean bulk temperature)” test.
The quick-look review showed the motor performed as designed. However, the full test results required the motor to be disassembled and inspected in detail.
While there’s huge public interest in the return and reuse of SpaceX’s Falcon 9 v1.1 first stages, mainly due to their propulsive landing technology and reusability, SRBs also boasted a large amount of reusability – returning back home under parachute to be recovered from the Atlantic Ocean after providing their two minutes of thrust during the Shuttle era.
Numerous elements of the QM-1 booster followed that tradition, with hardware – such as casings – from previously flown Shuttle SRB hardware used during the test.
A key benefit of returning the hardware was the allowance to fully investigate the performance of the motor via post-flight inspections.
These extensive investigation reports would show the condition of the hardware, down to the finite detail, such as noting even the slightest blob of grease that were deemed to be out of place, even after the two minutes of fury during ascent, followed by a splashdown into the ocean.
The QM-1 motor has now undergone a similar process, after being cleaned down following the test – aided by the robotic CO2 fire extinguisher that was seen in use during the end of the test, protecting the internal workings of the motor to allow for a detailed inspection.
The inspections of the motor hardware shows the nozzle and insulation performed as expected, ballistics performance parameters met requirements, and the thrust vector control and avionics system provided the required command and control of the motor nozzle position – exactly as planned.
“These test results, along with the many other milestones being achieved across the program, show SLS is on track to preserve our nation’s leadership in space exploration,” added Charlie Precourt, Vice President and General Manager of Orbital ATK’s Propulsion Systems Division.
The preparations for next spring’s QM-2 test, the final test before the production of the Exploration Mission -1 (EM-1) booster set, are already taking place.
According to L2 updates, casting of the aft segment will take place this month, with the remaining segments cast through the rest of the summer. The forward segment is scheduled for casting on June 1; center/forward segment on July 20; center/center segment on August 10 and the center/aft segment on August 31.
Thanks to the hard work involved with QM-1’s production, investigation and success, the QM-2 processing flow it likely to enjoy a much smoother path.
“Ground tests are very important – we strongly believe in testing before flight to ensure lessons-learned occur on the ground and not during a mission,” added Mr. Precourt.
“With each test we have learned things that enable us to modify the configuration to best meet the needs for the upcoming first flight.”
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