NASA, Northrop Grumman designing new BOLE SRB for SLS Block 2 vehicle

NASA’s Space Launch System (SLS) program and booster element prime contractor Northrop Grumman are developing an upgrade to the current Solid Rocket Boosters (SRBs). The SLS Booster Obsolescence and Life Extension (BOLE) program is in the detailed design phase prior to firing its first ground-test development motor in 2024, followed by a preliminary design review for the boosters that will inaugurate the SLS Block 2 vehicle.

The new solid rocket motor design retains the form and fit of the current motor, but incorporates modern production technology, composite cases, and a new solid propellant formula. NASA SLS and Northrop Grumman are working to integrate the new design with the SLS vehicle and increase performance to Congressionally-mandated levels while minimizing impacts to the design and operations of other flight hardware and launch processing infrastructure.

BOLE upgrade replaces current booster originally designed for Ares I

The BOLE program is a joint effort between NASA and Northrop Grumman to develop a new solid rocket booster design with modern production and manufacturing processes. The new design is intended to replace the current SLS boosters that are based on a five-segment solid rocket motor (RSRMV).

The RSRMV is an evolution of the Space Shuttle Redesigned/Reusable Solid Rocket Motor (RSRM) and was originally designed to be the first-stage of the Constellation Program’s Ares I Crew Launch Vehicle. After Constellation was cancelled, the motor was adapted from its single-stick, first stage application for Ares I to the dual boosters that flank the large Core Stage for SLS.

The current booster uses Shuttle flight hardware and technologies that are becoming obsolete. SLS is an expendable launch vehicle, so in addition to the technology obsolescence, launches will also consume the remaining inventory of Shuttle SRB flight hardware. “When the Shuttle program came to an end, it was decided as the SLS program was starting to ramp up that the project would save enough hardware to give us eight flight sets of the large, structural hardware, the case segments and that sort of thing,” Dave Reynolds, NASA’s Deputy Program Manager for the SLS Booster Element Office, said in a June 25 interview with NASASpaceflight.

Credit: Northrop Grumman.

(Photo Caption: An overview of changes in the new BOLE SRB design when compared to the current design. The new design retains the form and fit of the current boosters, but makes the extensive changes summarized in this Northrop Grumman presentation slide.)

“[We also knew] that at the end of those eight flights we would need a follow-on program that would give us a booster with a minimum of the same capability, but [we] may as well take advantage of the technological advances that have taken place in rocketry over the last 30 years [to] be able to give us a safer, more robust, and more capable booster.”

NASA is developing two major upgrades to the initial operating capability provided by the Block 1 configuration. The Block 1 vehicle combines two RSRMV-based SRBs and a liquid hydrogen, liquid oxygen Core Stage with United Launch Alliance’s Delta IV Heavy second stage as an essentially off-the-shelf, in-space stage called the Interim Cryogenic Propulsion Stage (ICPS). 

The first upgrade that would be phased in is Block 1B, which replaces the ICPS with an in-house Exploration Upper Stage (EUS) that is, like BOLE, tailored to SLS. EUS is currently planned to begin flying on the fourth SLS launch. Adding the BOLE boosters to the Block 1B vehicle is now viewed as the Block 2 configuration. 

An Advanced Boosters competition was part of the initial development roadmap during the first few years of the SLS Program, but was set aside in part due to the budget constraints during that time. The new BOLE motor is being designed to better integrate with SLS Block 1B and at the same time to increase the overall vehicle performance. 

“By focusing on ballistics and changing the propellant to something more modern and higher performing, that necessitates that you have a different nozzle because you have different ballistics, different materials that are coming through. So you need an upgrade on your nozzle material,” Reynolds explained.

“By upgrading your nozzle material and your ballistics you need a different and more robust case and of course they’ve made tremendous strides in composite, carbon-fiber wound filament cases over the past 30, 40 years. And so you are taking advantage of the strengths of the composites. Once you take those three components you also need a new way to attach to the booster structures because it’s a different load path that the booster will set up at that point.”

“If you’re going to redesign that, you may as well optimize it for what the SLS mission is, so you change your attach points and then [when you] put all that together you’re going to have a different TVC [thrust vector control] system and avionics system to be able to drive that,” he added. “So basically the whole booster is an upgrade to a more modern set of technologies, taking advantage of everything.”

In addition to changing the connections from the BOLE boosters to the SLS Core Stage, the new design also moves away from the Shuttle-style connections to the launch platform. “The current booster has a heavy footprint at the point where it attaches to the Mobile Launcher, and that’s because it’s Shuttle heritage,” Reynolds said.

“When Shuttle fired off it’s engines a few seconds before the boosters fired, the whole Shuttle ‘twanged.’ It moved off of center and then it kind of sprung back. Well, that doesn’t happen on SLS.”

“That heavy footprint that bolted the Shuttle down to the pad is no longer necessary,” Reynolds noted. “If you don’t need that heavy footprint attached to the Mobile Launcher any more, then you have a lot more ability to lighten up your aft skirt so that you leave some of that mass on the ground and give yourself some more payload capability for the rocket.”

The new TVC system in the BOLE design is an electric, battery-powered system, which will replace the current Shuttle heritage system that is powered by toxic hydrazine fuel. “Similarly to the motor case structures, we saved several of the Shuttle hydrazine-driven TVC systems for the first eight flight sets, but after that we were going to have to go back to the assembly line, many of which have been closed down for 20 years or more,” Reynolds noted.

“[We could] fire them back up, or we could move to a more state-of-the-art, modern design. And since the OmegA program had already done the pathfinding for us on the eTVC (electric TVC) system, we were able to easily take advantage of that. That has the additional benefits of giving us the possibility of eliminating a hazardous material from the booster: the hydrazine.”

Credit: NASA.

(Photo Caption: Graphic comparing the different versions of SLS in development. The SLS Block 1 Crew configuration will fly three Orion lunar missions before being superseded by the Block 1B Crew vehicle. Both Block 1B and Block 2 still have Cargo configurations in design/development, but work on a Block 1 Cargo vehicle was discontinued after the Europa Clipper spacecraft was finally taken off the SLS manifest due to compatibility and availability issues.)

“Nobody likes working with hydrazine if they can avoid it, and so by switching over to an electric, battery-driven design you get to eliminate some of those challenges with the con-ops [concept of operations] that we’ve had to deal with all throughout the Shuttle days and these early days of SLS,” he added.

The new design also uses a different propellant mix from the Shuttle solid motors, which allows more of the higher-impulse propellant to be stored. “The booster itself is heavier, but it’s primarily heavier because it has more propellant in it,” Mark Tobias, Northrop Grumman’s Deputy Chief Engineer for the SLS Booster Element, said.


“We were able to pack more propellant [in, because] the propellant that we’re using is a higher density than the current propellant. Composite cases are obviously much lighter than steel cases for a constant set of design requirements, so we were able to go increase the internal chamber pressure of the motor to provide more thrust for the same weight.”

“We allowed ourselves to go [up] to about a two, three hundred psi (pounds per square inch) increase in [chamber] pressure with no mass penalty because we went to composite cases,” Tobias explained. “So the boosters actually weigh more but it’s because they’re carrying more propellant than the current ones.”

The thrust trace for the BOLE motor is also tailored to the SLS vehicle compared to the Constellation/Ares I-designed RSRMV. Solid rocket motors may not have the fine throttling control of some liquid engines, but their thrust is designed to change for different phases of flight.

The thrust trace is a plot of thrust versus time showing how the motor performance is designed to vary during the action time from ignition to burnout. Tailoring the the extra impulse with the BOLE motor to the SLS vehicle and ascent trajectory generated a lot of discussion.

Credit: Northrop Grumman.

(Photo Caption: A NASA slide comparing the thrust trace of the SLS Block 1B and Block 2 and exergy performance analysis of the two vehicle configurations. The BOLE motor thrust trace is tailored to the SLS flight profile, both improving payload performance and reducing operational constraints.)

“When we were designing the thrust trace, max dynamic pressure and actually the time at which max dynamic pressure occurs was a topic of intense discussion,” Tobias said. “There was a tremendous amount of design iteration around the various different Mach number and dynamic pressure regimes, and ultimately the Booster [element] working with the [SLS Program] vehicle design team settled on a design constraint where we actually constrain the dynamic pressure as a function of Mach number.”

“Essentially, what that constraint is, is it tells the booster you can only put so much impulse in the transonic portion of vehicle flight. So we essentially detuned the booster in that transonic portion of flight ,and then we tuned it back up in the later portions of flight in order to get performance back,” he added. “So dynamic pressure was a primary driver in the thrust trace design. It took us quite a bit of effort, several months of design iteration to get that right.”

For SLS Block 1 launches, the four Aerojet Rocketdyne RS-25 engines in the Core Stage could have a busier throttle profile during the first stage of flight than they did as a trio during Space Shuttle launches. During the first stage of Shuttle launches while the SRBs were firing, the three Space Shuttle Main Engines (their original name) would throttle down for several seconds in the transonic, maximum dynamic pressure (Max-Q) portion of ascent.

On early SLS launches, the engines may be asked to nominally throttle down around Max-Q in addition to a mandatory throttle-down around the time of SRB separation to reduce loading on the forward Core Stage-Booster attach points as the solid motor thrust tails off.

In addition to the heritage RSRMV thrust trace, the heritage Shuttle SRB case structures were designed for the Shuttle-scale forces. The heritage structures for the forward and aft assemblies of the current SLS boosters were reinforced, but throttling the RS-25 engines is still necessary in places during first-stage ascent to keep the loads within limits.

The new BOLE motor structures and thrust trace are designed to reduce the liquid-engine throttling demands. “One of the goals of the BOLE program was to get rid of some of those operational constraints. So with the BOLE booster, the RS-25s don’t have to throttle at all during first stage portion of ascent,” Tobias noted.

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