As work progresses around the country toward the debut of NASA’s SLS rocket in 2018, Orbital ATK is preparing for the final qualification test of the five segment Solid Rocket Booster motor that will power the SLS through first stage flight. That second qualification test, known as QM-2, is targeted for late spring 2016 at the company’s proving ground in Utah.
QM-1: Fixing the propellant void issue
As part of a phased development plan for the massive five-segment Solid Rocket Boosters (SRBs) that will be used in combination with four liquid fueled main engines to power the SLS rocket through first-stage flight, Orbital ATK has progressed through a series of ground tests of the rocket motor.
The most recent test was the Qualification Motor -1 (QM-1) test on 10 March 2015.
That QM-1 test was delayed indefinitely when Orbital ATK discovered small voids between the propellant and the motor casing of the aft segment during standard x-ray inspections to evaluate an insulation lining material change in the SRB motor segments.
The bond between the propellant and insulation, which provides thermal protection for the SRB case, is a critical design feature that undergoes rigorous x-ray inspection to ensure its acceptability for flight.
Given the rigorous test objectives of QM-1 at the upper operating temperature range of SRB certification, Orbital ATK – with NASA’s consent – rejected the original aft segment for QM-1 and proceeded to cast another aft segment.
Curiously, that recast aft segment also contained voids between the propellant and the motor casing.
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.
Evidence quickly began to mount that the issue was related to the use of a new material in the insulation lining between the propellant and the motor casing wall.
Originally, 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.
Non-asbestos (chrysotile-free) rubber insulation was therefore baselined for use on the SLS SRBs.
As development of the SLS 5-segment SRBs continued between 2011 and 2013, then-ATK conducted three DM (Demonstration Motor) tests, all of which occurred with the new insulation and were free of out-of-family propellant void issues.
As the SLS booster design matured from the DM stage to the QM stage, standard development processing and design changes were undertaken as the SRB design moved toward flight maturity.
However, after the DM series of static test firings were complete, NASA requested that ATK conduct additional inspections on the QM series of motors to see if any defects were present as a result of the switch to asbestos-free insulation lining.
When voids were then discovered, an investigation began, culminating with an understanding and fix of the issue.
“We finally figured out that we were getting some off-gassing,”noted Todd May, SLS Program Manager, during an interview with NASASpaceflight.com in December 2014.
“We use Chemlok between the insulation and the casing, and we found that if we lay all but the last couple of layers of insulation, and then a layer of Chemlok, and then put the last layer on, that Chemlok actually (mitigates) out-gassing up through to the barrier to the propellant.
“So we finally were able to pour a new segment with this new process – it’s the cleanest we’ve ever seen.”
In a follow-up interview with NASASpaceflight.com this month, Orbital ATK confirmed that the QM-1 motor test firmly validated the propellant void issue.
According to Orbital ATK, “The QM-1 test results show the fix implemented for the propellant void issue was very successful.
“Perhaps more telling are the results of the x-ray inspections of the QM-2 segments. All five segments completed insulated level x-ray inspection with no detected insulation voids.”
This is of great significance for Orbital ATK and SRB development as this marks the first time that void-free motor segments have been cast.
“During the history of the Space Shuttle program, while segments were all within acceptable limits, a single void-free segment was never produced,” noted Orbital ATK.
“On QM-2, we have produced five void-free segments in a row. And three of these segments have also completed loaded level x-ray with no indications of the propellant void issue.”
QM-2: Validating SRB performance in cold-temperature operating range
With the QM-1 test complete and all post-fire inspections and disassembly efforts complete, Orbital ATK is progressing through their post-fire analysis of the data points collected during the test while at the same time proceeding toward the QM-2 motor test fire.
Currently, Orbital ATK classes preparations for QM-2 as “progressing well.”
All five of the motor segments for QM-2 have completed the casting process and are in various stages of finalization and inspection.
The nozzle and igniter are currently in the assembly and build up stages while the aft skirt and the Thrust Vector Control (TVC) system have completed their hot-fire test and are ready to support the QM-2 test firing.
Moreover, the Test Area at Promontory, Utah, is being prepared to receive the motor segments, which are set to begin shipping to the test stand in November.
When those segments arrive, they will be mated together in the test configuration.
Unlike the QM-1 test which validated SRB performance at the upper end of the operational temperature constraint, QM-2 will test the exact opposite, validating five segment SRB performance at the extreme low-end of the acceptable temperature scale.
The lower temperature range for operational certified use of the SRBs was something that came to prominence in 1986.
However, the reality was that the minimum operational temperature limits for SRB certified use were not readily understood by Morton Thiokol (predecessor to ATK) or NASA in the early years of the Shuttle Program.
During the 24 January 1985 launch of Shuttle Discovery on the STS-51C mission, ambient air temperature at the time of launch was just 53°F.
Post-flight recovery and inspection of the SRBs from STS-51C showed charring to the primary O-rings on both the left and right hand SRBs.
However, it was the burn path penetration through the primary O-ring on the center field joint of the right SRB from STS-51C, coupled with heavy charring on the secondary O-ring (indicating a near burn through event of the field joint), that provided a direct link to a mission one year and four days later.
On 28 January 1986, the Space Shuttle Challenger was approved for launch by NASA and Morton Thiokol managers with a predicted ambient air temperature of just 34°F at the 09:36 EST targeted launch time of STS-51L.
The two hour two minute delay to 11:38 EST resulted in an ambient air temperature of 36°F at the time of Challenger’s launch.
The significantly cold temperatures and Thiokol’s approval for flight precipitated a series of events that led to the destruction of Challenger and the loss of her seven crewmembers.
During the Rogers’ Commission hearings into the loss of Challenger, several Thiokol officials’ testimonies revealed seemingly conflicting understandings on the minimal safe operational temperatures for the SRBs, with some claiming 40°F as the minimum and others revealing concerns that 53°F was cold enough to be problematic.
Other testimony complicating understanding of the minimal safe operational temperatures for the SRBs revolved around whether that 40°F lower limit was air temperature or propellant mean bulk temperature.
Since then, Thiokol became ATK and ATK has now become Orbital ATK. But through those transitions, the dedication to understanding how the SRBs operate in their lower and maximum temperature ranges has remained.
To that end “The QM-2 static test will validate all of the motor ballistic and performance requirements at the bottom end (40°F) of the booster operating requirements,” notes Orbital ATK.
Approximately two months of temperature conditioning in the test stand will be required to chill the motor segments to the necessary temperature for the QM-2 test.
If all processing milestones are met, Orbital ATK plans to conduct the QM-2 test fire – the final test fire before operational use of the five-segment SRBs begin on the SLS EM-1 mission in 2018 – in the late spring 2016.
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