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

by Philip Sloss

“Some of the structural reasons that the current Block 1 vehicle throttles are eliminated by the BOLE because those structures that carry the primary load path have all been redesigned. They are designed to this specific load, so there’s no reason to require the engines to throttle any more. And of course that’s a performance gain as well.”

The liquid engines will still have to throttle late in the ascent, well after the boosters separate, to maintain acceleration limits prior to their shutdown.

Borrowing from OmegA technology development, but tailoring for SLS

Some basic aspects of the new BOLE motor design draw on technology development that Northrop Grumman was working on as a part of the OmegA launch vehicle program. Northrop Grumman received an Air Force Research, Development, Test and Evaluation (RDT&E) award in October 2018 to develop the OmegA rocket concept as a part of the U.S. government’s National Security Space Launch (NSSL) program.

Production of the first development units for the multi-stage, solid-propellant vehicle occurred at the company’s Promontory facility in northern Utah, using some of the same infrastructure as the SLS boosters, along with more modern manufacturing technologies. Although the OmegA Common Booster Segment (CBS) concept was not selected in 2020 to continue development, the SLS BOLE motor draws on some of the OmegA technology development and testing.

“The BOLE booster takes advantage of a couple of things,” Tobias said. “One, a lot of qualification work that was done at the hardware and design level.”

“For example, we’re using the same carbon-fiber and resin system for the cases as OmegA did. We’re using the same electric TVC system that CBS and OmegA did, except we’re upgrading it to be single-fault tolerant where the CBS/OmegA system was zero-fault tolerant.”

Credit: Northrop Grumman.

(Photo Caption: The aft skirt for a ground test of the OmegA first stage motor is seen in April 2019 during assembly activities at Northrop Grumman’s Promontory facilities in northern Utah. The BOLE design is taking the electric TVC equipment shown, such as the nozzle gimbaling actuators seen prominently at the two o’clock and four o’clock positions here, and adding the additional fault-tolerance redundancy required in a human-rated launch vehicle.)

“We’re using essentially the same propellant formulation,” Tobias added. “It’s slightly modified for different reference burn rate because the requirements for our booster are different than theirs, [because OmegA was to fly] significantly different kinds of payload missions. But by and large the TVC system is being ported over. So a lot of the major elements of the booster are very, very similar to CBS/OmegA program.”

“The second way we leverage that program is through infrastructure,” Tobias continued. “So the case winding machines, the case machining centers, the ovens that cure the case, the machinery that winds the insulation into the case, the facilities associated with that, these are all infrastructure that were invested in by Northrop Grumman for CBS/OmegA, and they’re directly utilized by the BOLE program.”

As Tobias noted, SLS requires at least single-fault tolerance because it is a human-rated launch vehicle. The electric TVC system adds a third command and control “string” to power the rock and tilt actuators for the BOLE booster nozzle.

“There’s three independent avionics and command and power strings for this system,” Tobias said. “The best way to illustrate this is the [OmegA] CBS actuator has two motor pump assemblies that provide the hydraulic power to drive the actuator. We’re adding a third so we have those three complete, independent strings, both from a command standpoint at the avionics end and at the end-effector end of the architecture at the actuator level.”

The BOLE booster is also being designed as an upgrade to the existing SLS Block 1 vehicle, as opposed to the clean-sheet concept of operations for OmegA. During design, Northrop Grumman and NASA have prioritized integration with SLS launch operations and flight operations.

An example of this is the nose cones for the boosters. Studies were conducted on different shapes than the current vehicle, but the program made the choice to retain the existing style. “We looked at that very early on and traded around some different concepts. 

Ultimately we brought the different concepts to the vehicle, particularly the Chief Engineer for the vehicle,” Reynolds noted. “His desire was to minimize any perturbations to the system for future integration, and so currently the design that we’re holding right now is to maintain a conical nose design.”

Credit: ESA/Arianespace

(Photo Caption: A potential design change to the BOLE SRBs considered but ultimately decided against would have seen the SRB nose cone assemblies altered to more closely resemble those used on the Ariane 5. In the end, NASA and Northrop Grumman decided to keep the conical shaped nose assembly design that currently flies with SLS.)

The BOLE motor is, like OmegA, a segmented design with the same diameter as the Space Shuttle RSRM cases; however, the BOLE motor retains the same five segments and dimensions as the SLS RSRMV, so the composite cases are shorter than the OmegA design.

“The CBS segments were actually longer than the BOLE segments,” Tobias said. “The OmegA rocket was four segments instead of five, and the reason ours are a different length is because we want to minimize the integration impact to the existing SLS Core Stage and launch vehicle to the greatest extent possible. So we’re constraining ourselves to some of the major interfaces so that the rest of the vehicle doesn’t have to redesign around us.”

“For example, the platforms in the VAB. They have some [capability] to move platforms to different heights and different locations. But say [theoretically] we went to six segments, that would be a significant impact to the VAB beyond what they could accommodate.”

“So we’ve tried to go maintain those major interfaces both with the vehicle and with the ground,” Tobias added. “One of the beauties of composite cases is you’re not tied to a certain piece of tooling or a certain forging or that type of stuff. You can change the dimensions however you like. Geometry is much more flexible when you’re dealing with composite cases.”

“Now for the ground we have changed where the booster actually mates to the Mobile Launcher, and we did that because the Block 2 vehicle will be launching from a different Mobile Launcher than the Block 1 vehicle and so we had the design space to go change things without any negative impacts.”

While the new boosters will retain a similar outer mold-line (OML) to the current boosters, the separation system is another area that changes. “The separation dynamics of the Block 2 vehicle are different due to the higher performing boosters, the RS-25s running at two-percent higher power level, and the overall mass and aero (aerodynamics) of the vehicle is different because obviously it’s a larger vehicle than we have,” Tobias explained.

(Photo Caption: A composite of computer-aided design (CAD) graphics showing the design and layout of the aft BOLE attachment to the Core Stage. The graphics also depict the different aft Booster Separation Motor arrangement on the BOLE booster’s aft skirt.)

“So those changes in aerodynamics, those changes in the axial force that the vehicle is experiencing takes you to a different design point for the separation algorithm, which results in changes to the number and location of the BSMs.”

As during Shuttle, and carried over to SLS, groups of smaller solid rockets called Booster Separation Motors (BSM) fire to push the empty SRB cases away from the Core Stage when the bolts connecting them are pyrotechnically severed. In the current separation system, groups of four BSMs are located in the nose cone at the forward end and the aft skirt on the aft end.

The new system of BSMs was designed for the different way the boosters will be attached to the Core Stage and different conditions at SRB separation.

“We were able to optimize with the new attach schema with the vehicle and we were able to optimize the separation algorithm, so we actually have fewer BSMs than the current configuration,” Tobias said. “We just finished that trade study and have now baselined a new architecture. So we actually have three BSMs in the forward end and two in the aft end, and they’re oriented differently. Their net force vectors are pointing in a different direction than the current booster.”

The attachments between the Core Stage and the new boosters are also changing from the old design. “One of the integration changes we’ve made [is to] the aft attach,” Tobias said. “We’ve actually borrowed that design from the Titan program. It provides us increased clearance during separation because it eliminates, in particular, the diagonal strut that is on the Block 1 configurations, and so it gives you a more robust design space for separating the boosters from the Core Stage.”

“Also it’s a simpler design,” Tobias added. “The hardware is not as complex, so we kind of borrowed from our historical design practices in other programs and it’s a fairly minimal change to the Core Stage as well. So we did those things because we’re trying to increase the ease of integration, we’re trying to increase our margins where we can. And if we can make things simpler and more robust things get cheaper in the long run.”

The new SRB design would not be fully needed until the ninth SLS launch, but some elements of the Shuttle SRB inventory have been in storage for over a decade since the final Shuttle flight. As a part of preparing for the obsolescence of some critical elements, the BOLE program is also evaluating possible backup capabilities to support the legacy design.

Between NASA and Northrop Grumman, the SLS Booster element is planning to conduct an experiment on the next full-scale ground test motor for the current RSRMV design, called Flight Support Booster-2. “One secondary objective that we’re targeting, and it looks like we’ll be able to make this happen, is we are actually going to put the OmegA version of the electric TVC system on that booster,” Reynolds said.

“I don’t want to give the wrong impression that that means we’re planning on putting the eTVC system on the first eight flight sets. That’s not the current plan, but like I said, a lot of that hardware that was carried over from Shuttle [and] some of it [has] still not been fully inspected within its storage containers.”

First development motor firing planned for 2024

For the new BOLE motor design, the program is moving towards its first full-scale ground test firing. “Our focus is primarily on what we call Development Motor-1 or DM-1, which will be the first static test motor. And the major objective of that static test motor is to get direct measurement of the actual thrust produced by the booster and to make sure that it is behaving as we designed it and as we intended it to,” Tobias said.

“So motor performance is the major objective out of that test; of course there’s also going to be a lot of component objectives as well but that’s the main focus now. We’ve moved from conceptual design, we’re out of that phase. We’ve made all of our major architecture decisions, and now we’re actually in the detailed design phase for the static test motor.”

“And on the heels of the [first] static test motor will be the preliminary design review,” he added.

Credit: NASA.

(Photo Caption: A NASA presentation slide outlining plans for the two major upgrades from the initial operating capability that will fly on the Artemis 1 mission. The initial Block 1B flight would use the fourth and final set of SLS-adapted Space Shuttle Main Engines; thereafter, the intermediate configuration will begin flying new RS-25 production restart engines. Finally, the BOLE “enhanced performance” booster would enter service on the ninth SLS launch.)

A total of three development motors and two qualification motors would be fired during development prior to the new booster’s first flight. “We have three development motors, our DMs, that are planned and that would be followed by our CDR (Critical Design Review),” Reynolds said.

“Then that would be followed by two qualification motors, followed by our DCR, design certification review, and then it would be a qualified system. That’s a pretty typical development plan for a booster.”

“That’s essentially what we did with the RSRMV, so we’re kind of following that same path,” he added. “And as far as timing goes, the schedule lays us out for being able to accomplish that first DM test in summer of ’24.”

In parallel with detailed design of the BOLE motor and booster system, design analysis of the integrated SLS Block 2 vehicle combining the EUS and BOLE upgrades to the Block 1 configuration is also underway. “DAC-1 (Design Analysis Cycle-1) is the current design cycle,” Tobias said. “Northrop Grumman is currently assessing the loads from this cycle.”

Concepts for the Block 2 vehicle have evolved over the 10 years of the SLS program, but the BOLE upgrade could meet the performance requirement written into law by Congress beginning in 2010. 

“I’ll cautiously answer ‘yes,’ Reynolds said. “There’s always the possibility of more block upgrades on any part of the vehicle, so I say ‘cautiously’ because you never know what customer may come along that needs more capability that drives a compelling need for a block upgrade. But to answer your question succinctly, the EUS plus the BOLE integrated onto the block vehicle that we are currently on gives you that two-step process of the Block 1B and BOLE enables you to go to Block 2 and that is the capability that we’ve been asked to provide from the get-go on SLS.”

Lead image credit: NASA.

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