SLS continuing engine upgrades, tech development to support launcher evolution

by Philip Sloss

Currently, NASA has used selective laser melting or 3-D printing machines to construct channel wall engine nozzle prototypes up to approximately 65% full scale. “Right now, that’s limited by the size of the build box on the machines, it has to do with how well you can control the laser,” Heflin explained.

“That technology will continue to grow and that build box for that volume that they can build will get larger and larger over time. And that continues to open the door for larger pieces. And we’ve been working that; got a ton of really good subscale and component work that we’ve been able to do to try to push that for potential future affordability.”

Upper stage design leveraging commercial RL10 production and tech development

The Liquid Engines Office also manages SLS’s use of Aerojet Rocketdyne’s RL10 engine in its upper stages. One RL10 engine powers the ICPS upper stage of the Block 1 vehicle. The stage is a close derivative of ULA’s Delta Cryogenic Second Stage; the ICPS is also produced and distributed by ULA and uses the same RL10B-2 configuration as the Delta stage.

Beyond the first three or four launches, SLS is developing the larger Exploration Upper Stage (EUS) in-house with prime contractor Boeing. EUS is a four RL10 engine design that will be the upper stage for the higher-performance Block 1B vehicle.

NASA contracted with Aerojet Rocketdyne in late 2016 for the production of ten RL10 engines to be used beginning with the second SLS launch and the program has received over half the set from the West Palm Beach production facility. “Six have been delivered, two of those are being utilized on ICPS flights,” Heflin noted.

“We have four additional that are still in the factory, those aren’t scheduled to be delivered until next year, but they are on schedule.” At the time of the 10-engine order, the space agency intended to retire the Block 1 vehicle after a single flight; since then, at least two more Block 1 launches are planned as a part of the Artemis program that was born and branded last year.

Credit: NASA.

(Photo Caption: An expanded view of the SLS Block 1B vehicle in its Cargo configuration.  Block 1B replaces the commercial ICPS upper stage with the in-house developed Exploration Upper Stage, which is beginning its Critical Design Review. The SLS Liquid Engines Office manages the RS-25 Core Stage engines and the RL10 EUS engines.)

“First of all, RL10 is a commercial engine, and I am incredibly proud that we have delivered six engines without changes to those engines,” Heflin said. “We set out to buy them as they were built and not have to make changes to meet our vehicle or NASA requirements and we have been successful in doing that.”

“They are coming right off the production line, and they are the same engine that Aerojet Rocketdyne builds for all their customers.  Their nozzles — they have different sized cones and they bolt on the cones to fit the customer. But the core of the engine is exactly the same as they’re built for other customers, which was a huge accomplishment for us.”

In addition to the certification effort to integrate the commercial engine into the vehicle, NASA also verified that the engine met human-rating requirements for flying crews to the Moon on SLS. The RL10 engine is also ready to fly crews to the International Space Station on Boeing’s Starliner Commercial Crew spacecraft as a part of the Atlas V launch vehicle’s Centaur upper stage.

“We did have to go through the process of showing that that engine meets or exceeds our requirements and that it will perform as expected on this vehicle, just like we had to do for RS-25,” Heflin said. “That activity has been completed for the engine; there’s still a little bit that has to be done as the Exploration Upper Stage design matures, so we’ll have to revisit some of that.”

“We went through and analyzed what was known at the time, what we had agreed to as a program. And their models and their data get refined as their design matures; so we’ll have to go back and look at some of that, but the certification for Exploration Upper Stage is progressing really well. We did the same thing for all the human rating requirements, we had to make sure that they met all of our NASA requirements, and that work has been completed.”

The SLS Program is beginning the critical design review (CDR) for Exploration Upper Stage and the Block 1B vehicle; the materials for the multi-level review are being submitted and the final overall review is expected to be completed in December 2020.

Credit: Aerojet Rocketdyne.

(Photo Caption: Four of the RL10 engines produced by Aerojet Rocketdyne for the SLS Program at the engine maker’s West Palm Beach facility. The engines are being stored by Aerojet Rocketdyne at their Stennis engine facility for SLS. Not shown here are the different nozzle extensions that will be used for the different SLS upper stages. The Block 1 ICPS uses a multi-cone, in-flight deployed extension; the EUS for Block 1B will have fixed extensions installed before pre-launch vehicle integration.)

Qualification of the RL10 for use with SLS was done through testing and analysis. “We had to put the engine through some vibration testing to show that it would meet the vibratory loads that we were predicting. And all that work was completed last year,” Heflin said. “It [wasn’t] a full-up engine hot-fire test series; it was more laboratory testing.”

“There was some hot-fire testing to show performance at some of our conditions, but all of that work is behind us now.” The six RL10 engines delivered by Aerojet Rocketdyne to NASA are being stored by the engine maker at their Stennis final assembly facility, where RS-25 and RS-68 engines are integrated.

ULA will integrate ICPS engines at their Decatur, Alabama, rocket production factory. The engines for EUS will stay at Stennis until Boeing is ready to begin installing them on EUS flight articles as they are assembled at the Michoud Assembly Facility in New Orleans.

The main difference in the flight configuration for the RL10 on ICPS versus EUS is the nozzle extensions. The RL10B-2 engine used on the Delta Cryogenic Second Stage and ICPS includes a three-part nozzle extension; the bottom two large cones of the extension are stowed around the engine for launch and then deployed in flight following stage separation and before engine ignition. The RL10C-3 engine, that will be used on EUS, will use fixed nozzle extension cones that don’t require in-flight deployment.

NASA and SLS are also hoping to leverage manufacturing improvements from technology development that Aerojet Rocketdyne is leading for the RL10 engine. “Most of what we are focused on internal to my office and Marshall engineering is RS-25 focus, but Aerojet Rocketdyne is doing a lot of work in the additive world for RL10 to make the engine more affordable for all of their customers, not just for us,” Heflin said.

“So they’re looking at it because it’s a commercial product, and they’re investing their own money in a lot of the additive work. We’ll benefit from it certainly with future acquisitions of what will be lower-cost engines. We know what’s going on, and they’re open and sharing with us. But we’re not driving it.”

The engine maker has test-fired a 3-D printed injector and combustion chamber in a prototype of the next-generation version of the upper stage engine, called the RL10C-X, at their West Palm Beach facility.


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