Orbital ATK and NASA have conducted the last planned full-scale Space Launch System (SLS) solid rocket booster test firing prior to the first SLS launch in 2018. The five-segment solid rocket motor (RSRMV) for the Qualification Motor-2 (QM-2) test occurred at 9:05 am Mountain Time, 15:05 GMT, following a computer issue resulting in a slight delay.
In what is always a dramatic event, the booster was fired horizontally from the T-97 Large Motor Test Facility at Orbital ATK’s Promontory Propulsion Systems facility in Northern Utah.
In the final hours ahead of the test, Jeremy Redden, Orbital ATK’s Integrated Chief Test Engineer for QM-2, said the test team assembled at 11 pm local (Mountain) time on Monday night to begin final preparations.
The building that houses the motor during assembly and temperature conditioning was rolled back about six hours before ignition.
“We may hold it [rollback] just a little bit longer than that if we think we’re prepared,” he said ahead of the events.
“There’s a lot of operations that have to happen after we roll the building back — we dump a lot of sand down to protect the pad, [we need to get] the final team in place to do all the hydrazine checks, and doing the safing and arming [of the motor].
“There’s this big, long process and in fact we’ve been doing dry-runs of the process for two weeks now.” Redden said that the team will go through two more dry-runs overnight prior to the test.
A computer sequencer issue postponed the original T-0 by one hour.
The QM-2 test was the fifth full-scale test in the SLS booster development program.
(Image taken from the amazing 220mb super slow-mo DM-3 Five Seg Motor Ground Test Video – available in L2 – LINK).
Due to the maturity of many elements of the RSRMV design, flight segments are already being cast with solid propellant, with three segments already going through the casting process.
After the segments go through final assembly at Promontory, it is hoped they will be transported by rail to the Kennedy Space Center late next year.
Similar to the DM-2 test, once the QM-2 motor was fully assembled it was conditioned to fire at the low end of its designed temperature range, with the propellant mean bulk temperature (PMBT) targeting 40-degrees Fahrenheit.
Previous motors were also temperature conditioned to evaluate the design performance at different temperatures in motor’s planned PMBT operating range of 40 to 90 degrees F; for example, the QM-1 motor was tested at the upper end of the temperature range.
As with preparations for the DM-2 test motor, this motor has been “refrigerated” in the T-97 test stand for several weeks in order to chill the PMBT to the test target.
Mr. Redden noted that the PMBT of the motor was around 73 degrees when temperature conditioning started and it took about 50 days of conditioning to reach the 40F target.
“Action time” for the five-segment motor is just over 126 seconds, with the motor generating close to three and a half million pounds of thrust at the beginning of the firing.
The solid propellant cast into the motor segments is shaped to provide peak thrust during the first twenty seconds of firing, then a reduction in thrust corresponding to the SLS vehicle’s maximum aerodynamic pressure region of flight around one minute after liftoff.
The thrust then increases again before rapidly tailing off at the end of the two-minute long burn.
The RSRMV is rated to achieve a maximum thrust of 3.6 million pounds, but that would be the case under more ideal conditions, including a high propellant temperature.
With the QM-2 test motor being subjected to an extreme, long-term chilldown, the peak thrust at this lower boundary condition will be closer to 3.3 million pounds.
The RSRMV is an evolution of the Space Shuttle Reusable Solid Rocket Motor (RSRM) design; originally selected to be the first-stage of the Constellation Program’s Ares I Crew Launch Vehicle, after Constellation was cancelled the design was adopted for SLS in 2011.
The Shuttle RSRM had four segments: the forward, forward-center, aft-center, and aft segments. The RSRMV adds an additional center-center segment in the middle of the motor, stretching its length from over 126 feet to almost 156 feet. The motor cases are a little over 12 feet in diameter.
The RSRMV uses the same steel motor cases flown and reused during the Shuttle Program; the QM-2 test uses cases that were reused across forty different Shuttle flights and seven previous ground tests, including Shuttle launches going back to STS-51F in 1985.
The SLS booster consists of the RSRMV motor, along with the forward assembly that includes the nose cone and forward skirt that houses avionics and the aft assembly that includes the aft skirt that houses the booster thrust vector control (TVC) hardware.
Fully assembled, the SLS booster reaches a length of 177 feet. The SLS booster retains much of the forward and aft assembly hardware unchanged from its Shuttle heritage.
Mr. Redden said that while the motor for this test doesn’t include the booster forward assembly or the systems tunnel that runs along the length of the booster, the aft-skirt will include a mass-simulator for the aft booster separation motors, something that hasn’t been done for a ground test since 1979.
The booster hydraulic thrust vector control system will be tested, as is standard.
“There is a duty cycle that it goes through, so the nozzle will be vectored in different directions; it will have [an] assigned wave [pattern] with increasing frequency that it will exercise during the test,” he added. “It’s the same one we had for QM-1, the same duty cycle.”
The facility was then modified to support both four and five-segment motor tests in 2002 and then refurbished prior to RSRMV motor testing that began in 2009.
The large solid-rocket motors have been tested horizontally throughout the programs.
This is in contrast to assembly of flight motors at the Kennedy Space Center in Florida where the boosters are stacked from bottom (aft) to top (forward), test motors are assembled from forward to aft.
(Images: Via NASA, Orbital ATK and L2 – including SLS renders from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)
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