Ten years on, Northrop Grumman reflects on changes to Solid Rocket Motors from Shuttle to SLS era

by Chris Gebhardt

Ten years ago, on 8 July 2011, two, four-segment Solid Rocket Boosters (SRBs) came to life on LC-39A at the Kennedy Space Center, propelling the orbiter Atlantis and her four person crew upwards toward space on the 135th and final Space Shuttle mission.

It marked the 269th and 270th flight firings of an SRB for the Shuttle program and the 265th and 266th and final recoveries of the iconic rocket motors, which always flew in pairs — not just in terms of a full booster stack but for each individual motor segment as well that was paired with a “propellant twin” upon casting in Utah.

For SLS, two well documented, and visible changes have been made to the boosters, which are technically called Five Segment Reusable Solid Rocket Motor (RSRMV): the addition of a fifth propellant segment and the removal of all recovery and reuse hardware.

However, those changes are more complex than they appear.

“We inherited a lot of our knowledge from Shuttle,” related Matt Mecham, Manager, SLS Project Engineering for Northrop Grumman, in an interview with NASASpaceflight.  “In order to be able to deliver an efficient design that meets requirements, we really relied heavily on digitizing [the Shuttle] data.”

“If you think of the Shuttle program, hundreds, thousands, of different drawings on paper [were needed] to take those concepts through manufacturing and into a vehicle.  What we went through, all the old data, all the old drawings… [we] put those into a digital realm for us in our modeling and simulations room.

That modeling was crucial for understanding — before first flight — how each booster and booster pair would function together during ascent and what they will separately do after detaching from the Core Stage for a destructive dive towards the Atlantic.

As the Solid Rocket Boosters separate, the Core Stage continues to power the Artemis 1 mission toward Earth orbit. (Credit: Mack Crawford for NSF/L2)

“Usually, thrust predictions [from the models] and that kind of thing are pretty spot on with the big motors.  It’s a pretty well-characterized system, even with the subtle differences moving from the four-segment to the five-segment [design].  Propellant’s about the same.  New insulation.  You have to resize the nozzle because of the new thrust characteristics,” noted Matt.

The added fifth segment will produce 20% greater average thrust and 24% greater total impulse over the Shuttle-era design and will marginally increase the overall burn time of the five-segment SRBs to approximately 2 minutes 12 seconds, ten seconds longer than Shuttle.

The new segment will increase the overall thrust each booster is capable of producing, with each five-segment SRB generating a maximum of 3.6 million lbf (16,103 kN) of thrust for a total thrust from just the SRBs of 7.2 million lbf (32,027 kN) of thrust.

Like the Shuttle SRBs, the five-segment SLS boosters will have their propellant grain shaped in such a way to tailor thrust at different parts of flight, allowing the solids to “throttle down” for Max-Q and “throttle back up” thereafter.

Like the new segment caused changes to modeling, the other major difference, not recovering the SRBs for reuse and post-flight inspection, has led to its own set of changes that had to be studied and accounted for.

“There’s a lot more data that gets communicated between the core stage, obviously, and the boosters.  We’ve updated some of those components to accommodate all of that data.  But extra data means extra wiring harnesses, extra lines,” said Matt.

Data regarding SRB flight performance and information from the web of Development Flight Instrument (DFI) attached to the boosters can no longer be stored on the boosters for review after flight.  All of that must now be transmitted to the ground in real-time.

The ground test and operational flight use history of the RSRMV cases for the left-hand booster on the Artemis 1 mission. (Credit: Northrop Grumman)

“Because Artemis 1 is the first flight, it’s somewhat of a test flight.  And we have a large demonstration suite that we call a DFI on the boosters.  All of that information is for us to really learn what these mission profiles do, how they affect the rocket.  I’m talking about loads of environments, anything from temperatures to acceleration, that kind of thing,” said Matt.

“All of that has to come down the telemetry so that we can analyze that.  The flight-specific guidance information is the same way.”

This will help validate some of the modeling’s predicted flight performance vs. what is actually seen — even though the model and real-time flight data are likely to mirror each other quite closely.

Not recovering the boosters also means that Northrop Grumman will not be able to inspect the field joints joining the five motor segments together for any anomalies. 

While issues that would only appear in-flight and not during ground testing are rare, one such event did occur with the RSRM design for the Shuttle program.  In June 1996, STS-78/Columbia revealed that new EPA (Environment Protection Agency) regulations that led to a change in clearing solution was the cause of an in-flight, hot-gas path penetration into the field joints of the SRBs.

While the capture joint functioned as intended and stopped the gas before any flight safety or burn through concerns were triggered, the issue was not identified in ground testing prior.  The next mission was delayed while a fix was developed and the program ultimately resumed flights just three months later.

The ground test and operational flight use history of the RSRMV cases for the right-hand booster on the Artemis 1 mission. (Credit: Northrop Grumman)

No other major issues were noted with the RSRMs for the remainder of the Shuttle program.

Speaking to the inability to do such post-flight inspections now, Matt related, “I can tell you from firsthand experience working down at KSC in stacking in the design support role, there are pretty detailed and meticulous inspections that take place of all of the surfaces and the O-rings.”

“They’re cleaned and inspected multiple times to ensure that there are no flaws, there is no particulate, there is nothing, no contaminants, in those joints when they go together.  Then after they are assembled, we do a pretty rigorous leak check that is really sensitive to make sure that all of those joints are sealed at that point to function during flight.”

“All of that again is built upon the Shuttle program.  We continue to use that great knowledge that developed and really designed to really make our product reliable and robust.”

Another element of SLS that relates to the SRBs is the Launch Abort Motor, also built by Northrop Grumman.  This motor on Orion’s Launch Abort System would pull the capsule and crew away from a failing rocket — even while the SRBs are burning.

In this way, should a hot-gas penetration event occur on SLS (and there is no reason to believe one would given the heritage and performance of the system since the redesign after Challenger), the crew onboard would be safe.

In this way, the combination of Shuttle RSRM flight history from 1988 to 2011, ground test firings of the new design RSRMV design coupled with computer modeling, and the Launch Abort System provide enough data that recovery of the boosters for inspection is not a flight safety issue for SLS.

However, these more visible changes and the not-seen considerations they brought to SLS are not the only changes to the SRBs from Shuttle to the new era.

“Overall, on SLS, some things that are new are a new avionic suite.  It’s going to be our navigation control suite that interacts with the overall SLS flight computer.  That’s all been repackaged,” said Matt.

“There’s a new ground umbilical that we’ve developed in our bases with the mobile launch platform.  We have also updated thermal curtains to protect the aft skirt volume — where we have avionics — from the extra heating due to the proximity to the core stage liquid engines.”

An environmental and worker-safety change with the boosters was the elimination of asbestos in the propellant liners placed inside each motor segment.  It is on these liners that the propellant is cast — thus providing some protection to the case during the propellant’s burn.

The elimination of asbestos and testing of the new design showed a propellant void issue between the propellant and the liner.  This led to an investigation, covered extensively by NASASpaceflight, and a resolution that did not involve adding asbestos back into the design.

“That’s all behind us.  I can tell you that we spent quite a bit of effort in resolving that and making sure that the large void issue doesn’t happen.  Part of the way we ensured that is that every time we do a cast, there’s a lot of NDE [non-destructive evaluation] inspections that have been updated and modernized through the years.”

“We do a lot of inspecting of the cast materials on the installation to the propellant to detect any possible voids and ensure that we have a good quality safe product.”

However, it’s not all new for the SLS solid rockets.  The Booster Separation Motors that will push them safely away from the SLS Core Stage are the same and in the same positions as they were for Shuttle.

Credit: NASA

“They’re actually identical to the Space Shuttle design in terms of placement on the booster.  There’s four on the forward assembly and four on the aft skirt, and they’re in the same place,” related Matt.  “That’s one of the aspects of using the legacy hardware, that some of those things we elected to keep in the same place early on just as a, again a stepping stone, a way to move forward.”

In other words, if there wasn’t a design requirement reason to change something, it was largely left as is from Shuttle.

“We are using quite a bit of the older Space Shuttle major structures and other components, and retrofitting others, I guess you’d say, or modifying them to fit the new mission profile.  This is really standing on the shoulders of the Space Shuttle program before us and those great engineers and everybody who worked on it.”

All ten propellant segments for the first two flight SRBs for SLS were delivered to the Kennedy Space Center on 12 June 2020 and commenced stacking operations in the VAB in November.  While some segment casing elements are new, others are being reused from Shuttle.

In all, the oldest segment on Artemis 1 dates to STS-34 and the launch of Atlantis with the Galileo probe for Jupiter in October 1989, while the most-current flight use of an Artemis 1 casing element was on STS-133, and Discovery’s final mission in February 2011.

(Lead image: The aft segments and skirt assemblies of the twin RSRMV boosters for Artemis 1 stacked on the Mobile Launcher in the VAB. Credit: Stephen Marr for NSF/L2.)

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