The launch campaign for NASA’s Artemis 2 mission, humanity’s first crewed return to the Moon in over 50 years, is now well underway. The Artemis program’s first crew, Commander Reid Wiseman, Pilot Victor Glover, and Mission Specialists Christina Koch and Jeremy Hansen, are currently in training for their historic flight, and recently visited launch pad 39B at Kennedy Space Center to practice procedures for launch day. Space Launch System and Orion hardware for the mission are each in various stages of development as well. Most recently, on Monday, September 25th, segments of the solid rocket boosters for Artemis 2 arrived by rail at Kennedy Space Center in Florida, having departed Northrop Grumman’s facility in Utah a few days prior. These critical components form the backbone of the SLS stack, and will be the first parts of the vehicle to be assembled – but what makes them unique? And how have they evolved from their familiar space shuttle heritage?
The solid rocket boosters are one of the most visibly shuttle-derived components of the Space Launch System stack. Originally known as the Reusable Solid Rocket Motor, or RSRM, the boosters provided the bulk of thrust to the shuttle stack at liftoff, burning for just over two minutes before separation. True to their name, the RSRMs then parachuted to a soft splashdown in the Atlantic, where they were recovered and their components refurbished for reuse.
The RSRMs were a common target for improvement and uprating during the shuttle program, and a modified five-segment version dubbed RSRMV (“V” denoting “five” in Roman numerals) was considered – first for the shuttle itself, and later for use in NASA’s now-defunct Constellation program, targeting the Moon and Mars. Now, the RSRMV design has gained flight experience as part of the Artemis program, reprising their role to lift the massive Space Launch System off the pad and propel humanity to the stars once again.
At a glance, the RSRMV design is remarkably similar to its predecessor. In fact, alongside the RS-25 main engines on the core stage and Orion’s AJ10 engine, the RSRMVs represent Artemis hardware which has previously flown on space shuttle missions. The steel casings for each motor segment are pulled directly from existing shuttle stock, with a few structural stiffeners removed where they are no longer necessary.
Other components retain more or less their original shuttle-era designs. The forward assemblies, comprising the nose cap, frustum, and forward skirt at the top of each booster, are among these, albeit without the parachutes and recovery hardware used for shuttle missions. The aft skirts, housing the thrust vector control (TVC) systems used to steer the rocket nozzle during ascent, are also effectively the same as their shuttle incarnation.
However, the five-segment RSRMVs are much more than merely a stretched version of the original shuttle boosters. Orbital ATK (now NGIS) took the opportunity to implement a number of improvements, in addition to changes necessary to adapt the design from the shuttle stack to support SLS.
Much of the new design work for the RSRMV revolves around the insulation used throughout the booster. Whereas the original RSRM design used asbestos silica-filled nitrile butadiene rubber (AS-NBR), the toxicity of asbestos has driven its replacement with polybenzimidazole (PBI). The resulting PBI-NBR insulation had previously been introduced into the igniters, where additional structural reinforcement has since been added, and the inhibitors, which are insulation spacers located between each propellant segment to help control burn rate.
Most recently, PBI-NBR has been extended to the propellant liner insulation (PLI), which sits between the solid propellant and the steel casing throughout the entire booster. This was not without difficulty; the new material produced significant off-gassing during the curing process, which was found to create voids where the propellant and liner had separated. Changes in the manufacturing procedure, aimed at preventing and removing gas buildup, were able to fully eliminate this issue.
The rocket nozzle at the end of each RSRMV booster has been made larger than that of the RSRM in every respect, being longer and with a wider throat and exit area to support a higher thrust and mass flow. As a result, the expansion ratio has been reduced slightly, from 7.72 to 7.2. The nozzle area has also seen a switch from AS-NBR to PBI-NBR, and improvements to the flexible gimbal joint have been made to address abnormal erosion seen during early tests.
The solid propellant itself has also been modified to suit the needs of the SLS vehicle. Notably, the presence of an iron oxide catalyst in the propellant mixture has been subtly altered in order to reduce the overall burn rate. The solid propellant in both the RSRM and RSRMV designs is cast with a hollow core to allow combustion to flow through the booster. At the fore end of the booster, additional star-shaped fins of propellant extend into the core region to provide additional surface area for combustion, and thus an initial boost in thrust during flight, which gradually erodes away. The RSRMV design has increased the number of fins from 11 to 12, and they have been slightly lengthened, strengthening this “boost” effect for the SLS flight profile.
One major improvement to the original RSRM during the shuttle program focused on the boosters’ field joints. Field joints are where the individual segments are connected on-site in Florida during final assembly and stacking for launch (“in the field”), and were the subject of investigation following their role in the Challenger disaster. The rubber O-ring seals in one of these joints had failed due to low outside temperatures, prompting the inclusion of electric heaters for the remainder of the shuttle program. The RSRMV design used for SLS, however, has incorporated a new O-ring material which is more tolerant to low temperatures. As a result, the now-obsolete joint heaters have been removed.
A handful of other changes can be found scattered about the RSRMV design. Because the boosters attach to the SLS core stage in a different way than on the shuttle’s External Tank, the positions of an attach cylinder (with mounting hardware) and a stiffener cylinder at the base of each booster have been swapped. Each booster features a brand new avionics suite as well, modernizing their control systems. The flight termination system (FTS) charge has been lengthened to more completely destroy a wayward booster, but its activation time has been delayed to allow Orion’s launch abort system to safely clear the SLS stack in the event of a failure.
Though widespread in their scope, each of these many changes helps to make the RSRMV a safer and more capable component of the Space Launch System, as well as of the Artemis program as a whole. Having finally arrived in Florida after several days of transit across the country, the twin five-segment boosters will now loiter there over the winter as the other elements of the stack aggregate at the space center. Soon enough, early next year, the boosters will come together in their flight configuration atop the mobile launcher, ready to propel humans beyond the Earth once again.