Northrop Grumman Innovation Systems continues production of solid rocket booster (SRB) segments for the first launches of NASA’s Space Launch System (SLS) rocket at their Promontory Propulsion Systems facility in Northern Utah. Mixing of solid propellant for the SLS boosters and casting it in each of the ten segments for the second flight set is nearing completion as technicians put the final touches on the set for the first launch.



The first set of segments will wait in storage in Utah until next year; in the meantime, NGIS will continue production of the remaining deliverables in the current contract, including the third flight set and a ground test booster. After propellant casting, the segments go through inspections and final assembly before work is complete.

Once NASA is ready to start assembling the first SLS vehicle in Florida, the first flight segments will take a group train ride to the launch site.

EM-1 segments done, stored at Promontory until next year:

Northrop Grumman Innovation Systems (formerly Orbital ATK) produces the five-segment solid rocket motors (RSRMV) that form SLS boosters at their Promontory facility north of Salt Lake City. Cases for a booster are assembled into five segments, lined with insulation, loaded with solid propellant, and outfitted with other booster hardware.

After testing and final assembly work, they are ready to ship to the Kennedy Space Center (KSC) launch site in Florida, where they are joined with a forward assembly that includes the nose cone and forward skirt that houses avionics and an aft assembly that includes the aft skirt that houses the booster thrust vector control (TVC) hardware that helps steer the whole vehicle.

The SLS booster retains much of the reusable case hardware and forward and aft assembly hardware unchanged from its Shuttle heritage.

The RSRMV is an evolution of the Space Shuttle Reusable Solid Rocket Motor (RSRM) design; originally selected to be the first-stage of the Constellation Program’s Ares I Crew Launch Vehicle, after Constellation was cancelled the design was adopted for SLS in 2011. The Shuttle RSRM had four segments: the forward, forward-center, aft-center, and aft segments. The RSRMV adds an additional center-center segment in the middle of the motor.

Production work is essentially complete on the first flight set of ten RSRMV segments for the left and right boosters that flank the Core Stage on the SLS vehicle. “Those are all through propellant loading and final assembly with the exception of I think we’re still doing a little instrumentation work on three of them,” Jeff Foote, Vice President of NASA Programs for Northrop Grumman Innovation Systems, said.

The first flight set will fly on Exploration Mission-1 (EM-1), the first integrated flight of SLS and NASA’s Orion spacecraft.

“All the major work is done, it’s just a little bit of insulation around the flight instrumentation. So those are for all practical purposes complete.” SLS will have a significant amount of development flight instrumentation (DFI) on its first flights measuring pressures, temperatures, and vibrations on the vehicle.

Seven of the segments are already in storage at NGIS’ Promontory facility in Utah north of Salt Lake City. The remaining three segments, the left-hand aft segment, the right-hand aft segment, and the left-hand forward segment, will follow soon.

“For all practical purposes we’re within weeks of all of them being in storage and set to go,” Foote noted.

At KSC, the hardware that combines with the motor segments to comprise a booster is also being readied.

“We have an aft assembly and a forward assembly that are produced at KSC at our booster fab (fabrication) facility down there,” Foote explained. “Those use heritage Space Shuttle hardware, at least the structures.”

“On the forward end we put a nose cap onto a frustum onto a forward skirt and that houses all of the avionics suite,” he continued.

“Those structures are just about ready to mate and the avionics are for the most part done. We’re still doing a few acceptance tests on some boxes and still working on a few cables, but that assembly is happening as we speak.”

“The aft assembly houses the thrust vector control (TVC) system and so there’s really just one main structure there and the TVC system with the two actuators that pin to the nozzle and then a bunch of cables that provide the steering commands and feedback,” Foote noted.

“Those assemblies are getting close to done and we’ll go into what we call acceptance and checkout here before we hand it over to the ground segment (Exploration Ground Systems) to put all three of those elements together to make the deliverable booster.”

The pace of work on the boosters has been adjusted with delays in the EM-1 launch date, as has the date when the motor segments are needed in Florida. Currently, the ten segments are expected to remain in Utah until the middle of next year.

“We’ve got about six months of idle margin in the segments and probably six or seven in the assemblies before their need in support of EM-1 launch date,” Foote said.

“Basically I think the ground segment looks at the launch date and backs up about twelve months, maybe a little less and that’s when they’ll want to receive the hardware,” he added.

Installation of some operational sensors on each booster is being deferred until closer to shipment. Referring to the forward segments, Foote said: “we’ll pull [them] out of storage to put an operational pressure transducer on prior to shipment.”

“We’re going to wait to put it in because it’s operational and it’s got a limited life associated with it,” he explained. “That’s fairly trivial [work] but we don’t want to start clocks on limited life items and then run out of clock down the road and have to do either extensions or remove and replace.”

As during the Shuttle program, the motor segments will be transported by rail from Promontory to KSC. For SLS all of the segments for each flight vehicle are planned to be shipped together. “We’re planning a single transportation of the whole ten segments, a single train,” Foote said. “We could ship them [individually] if the schedule needed it.”

NGIS has a single transporter to move the large, RSRMV-sized segments, so they will be moved one by one from the factory area to the train. “It’s about one a day, so it’ll take us a couple of weeks to move over to our Corrine rail facility and to load them onto the rail cars and then make up the train and have it ready to go,” he noted.

Casting EM-2 segments complete early next year:

The production of the set of motor segments for the second SLS launch has overlapped with activities on the first. Cases move through different sites at the factory going from empty, bare metal cylinders to loaded, flight-ready segments.

Currently forecast to be for the Exploration Mission-2 (EM-2) launch, casting of propellant into those case segments began last year and is now close to completion. “We’ve put propellant in seven of the ten for EM-2,” Foote said.

“Our next up is the end of October and then early December and we’ll finish up with EM-2 propellant loading the end of January. Six of those seven are through our non-destructive inspection process, the seventh is on its way through there, so once they reach completion of NDT (non-destructive testing) they go into final assembly, they have systems tunnels bonded to them, external insulation, and then the instrumentation and close out.”

After propellant casting, the cases go through lengthy inspection and final assembly work on site; most of the EM-2 segments are already in the final assembly area for that work. “I believe five of the seven are in final assembly, one is in NDT and one is just getting to final assembly and getting started,” he noted.

SLS is still in development prior to its first flight and the amount and composition of development instrumentation and equipment is still being decided. For the boosters, Foote said the plan is put the same DFI package on the hardware for all three flights in the current production contract. “It’s basically the same package as EM-1, the first three all have the same DFI package,” he said.

In addition to the development flight instrumentation (DFI) on its first flights, SLS will have prominent visual aids. The photogrammetry markings are painted down the length of one part of the boosters for cameras mounted elsewhere on the vehicle.

The photogrammetry markings were understood at one point to be only intended for the EM-1 first flight vehicle; however, NGIS indicated the current plan is also to carry the checkerboard paint job on the second and third booster sets, with a few modifications.

FSB-1, EM-3 segments:

NGIS is under contract to produce boosters for the first three SLS flights, along with a ground test motor called a “Flight Support Booster” (FSB) that will be fired in a test stand at Promontory. “We’ll cast FSB starting in the first quarter and it’ll finish up mid-year and then we’ll start EM-3 right on the heels of that,” Foote said.

The FSB-1 test will be used in part to help qualify aspects of the booster certification that must evolve to meet changing requirements, such as updated environmental regulations, along with other design changes.

“It re-qualifies some materials that are becoming obsolete or that are obsolete in terms of propellant ingredients, nozzle processing changes again tied to things in processing that are either obsolete due to environmental regulations or changes there and then we’ve got a change to our pyrotechnic safe and arm device,” Foote explained.

“That’s not essential to the test in a full-scale sense but it’s desirable to test like we’re going to fly.”

The ground test is currently scheduled for some time in 2020. “It’ll be somewhere between the first quarter and the third quarter of 2020,” Foote said.

“There’s a few things that are still in work to know if it’s end of the first quarter or somewhere in the second or third quarter and that’s based on some other work that feeds into that test and we’ve still got a few prerequisite items that we’re working the development schedules on.”

Throat plug debris concern:

One other area of work at Promontory is testing options for changes to the booster throat plugs. A debris concern was identified for the plugs after they intentionally fragment as they exit the throat of the booster nozzles in the moments after the boosters ignite.

The debris concern is that some of the fragments could rebound off of the sound suppression water that is being deluged onto and around the launch platform and impact the four RS-25 engine nozzles at the bottom of the Core Stage. The throat plug itself was redesigned from Shuttle to SLS to address the significant change in the positioning of the engines and boosters.

The Exploration Ground Systems (EGS) program at KSC is looking at the water deluge system on the Mobile Launcher and at the launch pad and SLS and NGIS are looking at the throat plugs. “There’s a multi-path solution in work,” Foote explained.

“Part of that is debris control through the deluge system and really exploring is [debris] absorbed by the water and to what extent and/or does it deflect off of the water. And if it deflects where could that debris go and what are the concerns there.”

“A second path being worked is by us internally to look at alternative materials and design construction of the plug and there’s a couple of different paths there and that’s in work in support of the vehicle solution,” he added.

The primary purpose of the plugs is to protect the inside of the booster, which is predominantly the precisely machined solid propellant, from the outside environment. This includes not just the ambient outside environment at the launch site, but also the relatively long run-time of the RS-25 engines on the launch pad before the boosters start.

Although very similar or identical to Shuttle, the boosters and engines were separated by greater distances on Shuttle. On SLS, there are now four engines instead of three and they are now “inline” and very close to the booster nozzles.

The throat plugs and what they are protecting are closer to the pressure spike at ignition of each RS-25 and both the high temperatures and acoustics during the six seconds all four engines run while the vehicle is held down at the pad before liftoff.

Changes to the throat plugs to protect against the higher environmental factors on SLS could change the level of risk of possible debris, and so different options are being looked at to balance the different concerns.

“We’re conducting tests here at the material level and kind of the ‘stitch-together’ level of the plug,” Foote explained.

“No plans at this point for full-scale firing. Obviously we have FSB sitting out there, so we’ll see how that fits together once the down-select of the solution for the vehicle is made and tie that objective together, but we’re not to that point yet.”