NASA Evaluates New SLS Booster Materials in Critical Test

As NASA counts down to next fall’s scheduled launch of Artemis-1—the maiden flight of the Space Launch System (SLS) and the first voyage of a human-capable space vehicle to lunar distance since Apollo 17—the agency has conducted a critical evaluation of new solvents for the nozzle materials on the giant rocket’s pair of five-segment Solid Rocket Boosters (SRBs). On 6 August, a 22-second “hot fire” test in the East Test Area at the Marshall Space Flight Center (MSFC) in Huntsville, Ala., allowed engineers to put the nozzle material through the wringer using a subscale test motor.

Video Credit: NASA

Derived from the four-segment SRBs used to launch 135 missions during the 30-year Space Shuttle Program, the five-segment boosters for SLS contain an additional “center” segment, making them larger and far more powerful than their predecessors. Together, they will generate 75 percent of the “punch” to get NASA’s new heavylifter off historic Pad 39B at the Kennedy Space Center (KSC) in Florida and onto the requisite trajectory for its voyages beyond low-Earth orbit.

When fully stacked, each five-segment SRB stands 177 feet (53.9 meters) in height, about 28 feet (8.5 meters) and 16 percent taller than its shuttle-era counterpart. And each booster will produce 3.6 million pounds (1.6 million kg) of propulsive yield at liftoff, an approximately 22-percent thrust hike over its predecessor from yesteryear. When combined with the impulse of four RS-25 engines on the SLS core, the SLS will thus generate a liftoff thrust at T-0 of 8.8 million pounds (3.9 million kg) in its Block I configuration for Artemis-1. To put this into historical context, that figure is about a million pounds (450,000 kg) greater than the peak 7.8 million pounds (3.5 million kg) attained by the Saturn V, which still stands as the largest and most powerful rocket ever brought to operational status.

Though the SLS five-segments boosters are indeed immense, the one tested at MSFC last week appeared quite the opposite. Yet appearances, as with many things in life, are deceptive. The test in MSFC’s East Test Area utilized a “subscale” test motor, measuring 20 feet (6.6 meters) in length and 24 inches (61 cm), burned almost 1,800 pounds (820 kg) of solid propellant and packed a sizeable punch of 23,000 pounds (10,400 kg).

“This 24-inch motor test is to evaluate the material in a solid rocket motor environment,” said Tim Lawrence, manager for motor and booster separation motor systems at MSFC, “and make sure that we don’t get any unexpected changes in how it performs.”

The nozzle construction enables the boosters to provide consistent performance while withstanding the 2,700-degree-Celsius (5,000-degree-Fahrenheit) flame produced as the solid fuel is burned to launch the rocket. Such material changes are checked out in phases on sub-scale to full-scale hardware and this was a significant step in that process. 

Last week’s test snaps at the heels of a previous one at the East Test Area last December, in which the effect of a newer and better-performing solid propellant mixture upon the motor insulation and the nozzle itself was carefully monitored. During that test campaign, three types of insulation were placed onto the motor’s aft dome: a sample of “current” insulation and two specimens of newer insulation, one of which was a variant that reduces the risk of static discharge.

Video Credit: NASA

And in spite of its relative smallness, the subscale test motor was more than sufficient to produce valuable results. “The booster is only 24 inches, but the ability to fire it in a test stand helps us get the data we need to confirm that we want to test it in a larger, full-scale test,” said test conductor Dennis Strickland. The results are expected to help validate the use of new solvents on future SLS boosters beyond Artemis-3—currently envisaged to plant the next man and the first woman on the lunar surface in 2024—and their effects on the nozzle materials during nominal booster operations.

In addition to data about the solvent’s effects on the motor during nominal burn operations, MSFC engineers also gathered information about its performance during the physical assembly of the flight boosters themselves. “The 24-inch motor is large enough that we were able to use the same processes to manufacture the nozzle as are used on the full-scale motor,” said Mr. Lawrence, “and that gives us confidence it will provide a good indication of full-scale performance.”

The December 2019 test of the sub-scale motor served to evaluate the effect of a newer, better-performing solid propellant mixture on the insulation and the nozzle. Photo Credit: NASA

As previously outlined by AmericaSpace, the ten segments of the twin SRBs assigned to Artemis-1 arrived in Florida in mid-June. They are currently located in the Rotation, Processing and Surge Facility (RPSF) at KSC, where they will remain until the rest of the SLS booster—its Launch Vehicle Stage Adapter (LVSA), its Interim Cryogenic Propulsion Stage, its Orion spacecraft and its powerful core stage—are in place for integration and stacking in the cavernous Vehicle Assembly Building (VAB).

But the idea of five-segment SRBs significantly pre-dates the SLS and Artemis era. Even whilst the shuttle program was still underway, plans were laid for a five-segment SRB with a modified nozzle to enhance launch-abort safety, payload-to-orbit capability and reduce costs.  

Video Credit: AmericaSpace

Although the five-segment booster was shelved for the shuttle program after the February 2003 loss of Columbia, it was a central element in NASA’s Constellation Program and underwent a pair of development test-firings in September 2009 and August 2010, ahead of its expected role on the Ares V heavylift rocket. With that program’s cancelation by the Obama Administration, the five-segment boosters gained a new purpose from September 2011 in support of SLS.

Following scaled-down tests early the following year, in October 2012 NASA awarded contracts to “demonstrate innovations” for the five-segment booster,. Elsewhere, the heavylift rocket progressed smoothly through its Preliminary Design Review (PDR) in August 2013 and Critical Design Review (CDR) in October 2015, cementing an architecture which, in addition to the two boosters, would also feature a core stage propelled by a suite of shuttle-heritage RS-25 engines.

In the meantime, Orbital ATK—the result of a 2014 merger between ATK and Orbital Sciences Corp.—conducted the two-minute-long Qualification Motor-1 (QM-1) test-firing in March 2015, which saw the booster punch out 3.6 million pounds (1.6 million kg) of thrust at peak operating temperatures of 32 degrees Celsius (90 degrees Fahrenheit). This was intended to represent the “high end” of its accepted propellant temperature range. And in June 2016, the QM-2 test repeated the exercise at 4.5 degrees Celsius (40 degrees Fahrenheit), the colder end of its propellant temperature range.

According to Northrop Grumman Corp., which completed acquisition of Orbital ATK two years ago, casting of propellant into the aft segment of the left-hand booster—the first component—got underway in April 2016. Since then, in September 2017, the avionics unit to control the ignition and separation of the boosters and communicate with SLS on-board computers completed its final qualification test.

Video Credit: AmericaSpace

Following arrival by rail at the Cape in June, the booster segments for Artemis-1 were transferred to the RPSF, just down the street from the VAB. Its duo of 400,000-pound (180,000 kg) cranes lifted the segments from a horizontal to a vertical position. There they will remain until the SLS core stage—currently just past the midpoint of critical “Green Run” testing at NASA’s Stennis Space Center in Bay St. Louis, Miss.—is ready to accept them. The segments will then be transported into the VAB transfer aisle and lifted by crane into High Bay 3, preparatory for stacking.

Additionally, with SRB segments already at various stages of fabrication for the Artemis-2 and Artemis-3 missions, NASA recently awarded contracts to Northrop Grumman to order long-lead-time items (including propellant ingredients, insulation, cables, nozzle bolts and hardware and carbon cloth) for six further flights from Artemis-4 through Artemis-9.

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