Orbital ATK prepares for final casting of SLS EM-1 flight SRBs
Following the highly successful test firing of the Qualification Motor 2 (QM-2) at their proving grounds near Promontory, Utah, Orbital ATK is preparing for the second half of casting operations for the Solid Rocket Booster segments that will power the SLS EM-1 mission through the first two minutes of flight – while at the same time working through a minor issue that is not expected to affect SLS SRB production timelines.
QM-2: success for cold temp firing of SRB:
QM-2 was the second in a two part qualification firing program for the new five-segment SRBs to validate the final configurations, manufacturing processes, propellant design, and operational temperature range for the propulsion element that will provide 86.74% of SLS’s thrust at liftoff.
The first qualification test, QM-1, validated all of these elements in the high-end of the SRB certified operational temperature range; QM-2 tested the low-end temperature range.
While the SRB high-end temperature range never caused a delay or concern for delay during the Shuttle program, the low-end temperature range became a major consideration for flight operations in 1985.
During the 24 January 1985 launch of Shuttle Discovery on STS-51C, liftoff occurred with an ambient air temperature of 53°F – having risen to that from a lower temperature in the preceding overnight hours.
Post-flight recovery and inspection of the STS-51C SRBs revealed charring to the primary O-rings on both SRBs as well as a burn through penetration of the primary O-ring on the center field joint of the right SRB and heavy charring on the secondary O-ring – indicating a near burn through event of the field joint.
Despite this revelation to Morton Thiokol (predecessor to ATK), the reality of minimum operational temperature limits for SRB certified use was not readily understood by everyone one year and four days later.
On 28 January 1986, with a predicted air temperature of just 34°F at the 09:36 EST targeted launch time of STS-51L, Challenger was approved for launch by NASA and Morton Thiokol managers over what, at the time, were unknown to NASA objections from Thiokol engineers due specifically to the hard freeze temperatures reached at the pad in the overnight hours.
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.
A key element revealed at the hearings were that some engineers thought 53°F was cold enough to be problematic while others thought the temperature had to be much lower – at 40°F.
Other testimony complicating understanding of the minimal safe temperatures for the SRBs revolved around whether that 40°F lower limit was air temperature or propellant mean bulk temperature – a significant difference given that the mean bulk propellant temperature for Challenger’s SRBs was calculated to be above both the 40°F and 53°F limits discussed.
Regardless, safety improvements, joint heaters, and a structured minimum allowable air temperature for launch chart was created in the wake of Challenger and adhered to for the duration of the program.
Since 1986, Thiokol – which became ATK and now Orbital ATK – has maintained a dedication to understanding how the SRBs operate in their lower and maximum temperature ranges.
For QM-2, which was fired on 28 June 2016, Jeff Foote, Orbital ATK Vice President of NASA Programs, noted that “QM-2 was a very clean and highly successful test firing. Instrumentation data yield was extremely good, and included some instrumentation that will fly on EM-1.”
At this time, all major disassembly operations of the QM-2 motor have been completed, and Orbital ATK’s teams are continuing to dissect components and make standard measurements in the nozzle and case insulation.
Of particular note, the redesigned nozzle joint 1 performance was “excellent”, and Orbital ATK was able to use QM-2 to gather “valuable dynamics data” on the Aft Booster Separation Motors (BSM) and housing – the first time such data has been gathered on a five segment SRB.
SLS EM-1 booster casting operations:
As with most things in spaceflight, operations usually parallel and overlap with one another.
This was certainly the case with the QM-2 static fire in June and the commencement of casting operations for the flight set SRBs that will propel the EM-1 mission skyward in 2018.
As of this month, Orbital ATK has completed casting of five of the 10 booster segments for the SLS EM-1 mission.
In casting order, Orbital ATK currently has the Left Hand (LH) aft segment, the Right Hand (RH) center/forward segment, LH center/forward, LH forward, and RH aft segments cast.
However, casting operations are currently on hold pending the outcome of an investigation into cracking of a thin adhesive layer within the insulation that was discovered during routine testing of samples that were not taken from the already cast EM-1 segments.
Orbital ATK elaborated on this point to NASASpaceflight.com, stating that “There has been no cracking of any material within the propellant/liner/insulation system in the inspected EM-1 segments and QM segments that have been dissected.
“The question has to do with a thin adhesive layer within the insulation and whether stresses under long-term storage conditions challenge structural margins in very specific regions of some segments.
Mr Foote elaborated, stating that “Over the summer, some insulation samples were tested for the purpose of overall design certification analysis.
“The samples revealed the possibility of some brittle behavior in the adhesive layer under specific test conditions that are outside of normal vehicle operating parameters.
“Until that possibility is fully assessed, a minor change was made to alleviate the concern in certain areas of the segment insulation and that will cause a short schedule pause in segment casting in order to implement.”
At this time, casting is set to resume in early November with the RH forward segment.
This will then be followed by casting of the two center segments, before moving on to cast the RH center/aft, and the LH center/aft segments.
Of note, this minor delay to casting is not expected to have any impact on the overall EM-1 SRB production timeline.
After casting operations are complete, the segments will undergo standard post-casting inspections and verifications before entering final production facility processing in 2017.
Of the five segments already cast, all five of them have “passed through NDT (x-ray) inspection”, notes Mr. Foote.
Under the current production schedule, all 10 segments will be ready for delivery to KSC in September 2017.
As with Shuttle, the SRB segments will make their way across the U.S. via railcar – using the same cars that were used to transport all of the Shuttle SRB segments to Kennedy.
Following the 10 SRB propellant segments across the country will be the two aft skirts – which will ship to KSC in October 2017.
The two forward skirts will then follow in February 2018 ahead of SRB stacking operations in the VAB – which is currently slated to begin in March 2018.
(Images: NASA, Orbital ATK, L2 – via Philip Sloss)