A ground test RS-25 engine has conducted another test firing Thursday afternoon while held in place in a stand at the Stennis Space Center in Mississippi. After Tropical Storm Gordon and most of its associated weather stayed far enough away from the area earlier in the week, the test team was back in the control center preparing for the start of the test.
Development Engine 0525 (E5025) remains installed in the A-1 test stand at Stennis to continue testing to certify affordability changes and operational upgrades for newly-built engines that will start being delivered by RS-25 prime contractor Aerojet Rocketdyne to NASA’s Space Launch System (SLS) Program in the early 2020s.
The test continued evaluating the performance of new hardware and will acceptance test another engine computer to add to the inventory of hardware for the first SLS launches.
Second Retrofit 1b Test
A team of Stennis personnel from NASA, Aerojet Rocketdyne, and facilities contractor Syncom Space Services (S3) conducts RS-25 tests, taking the engine through an event-driven countdown that begins typically begins early on test morning.
The test started when all of the prerequisite steps prior to ignition were complete and the hardware and the people are ready. Ignition typically occurs in the afternoon, Central time. For this test, ignition was just before 2:30 PM local time (Central).
“The primary objective is the Greenrun of ECU FM12,” Philip Benefield, Systems and Requirements Team Lead for the SLS Liquid Engines Office, told NASASpaceflight.com in an email ahead of the test.
“[Other] key development objectives include evaluation of the Hip-bonded Main Combustion Chamber and High Pressure Fuel Pump Insulation.”
The test was planned to run for a duration of 500 seconds, which appeared to be the case during the firing. During the firing, Benefield said that E0525 was to be throttled between 80 and 111 percent of the original Space Shuttle Main Engine (SSME) “rated power level” (RPL) of 375,000 pounds of thrust at sea level, 470,000 pounds thrust at vacuum.
“A majority of the test, [approximately] 340 seconds, will be spent at 111 percent RPL, and [approximately] 80 seconds at 80% RPL,” he added. When handed over from the Space Shuttle Program to SLS, the SSME became known as the RS-25.
As with the last test, Benefield said they were targeting nominal fuel (liquid hydrogen) and LOX (liquid oxygen) start conditions.
This was the second of nine tests planned in the Retrofit 1b series, which is testing the first new “hot-isostatic press” (HIP) bonded main combustion chamber (MCC), a “3-D printed” pogo accumulator assembly, and a new insulation system for the high-pressure fuel turbopump (HPFTP).
The HIP-bonded MCC at the heart of the engine is also a center of attention for this test series. Manufacturing of the MCC at Aerojet Rocketdyne’s facility in Canoga Park, California, now employs the same process the company uses for its RS-68 engines.
“You’ve got the liner and you’ve got the jacket separately, you put that into the furnace and that hot, high-pressure allows you to make that bond between the liner and jacket and that’s what we’re utilizing now on the MCC,” Dan Adamski, Aerojet Rocketdyne’s RS-25 Program Director, explained.
“[It] is exactly the same process that’s used on the RS-68 engine and exactly the same process that we used on the J-2X engine and ultimately what that does, we were able to reduce the cost and the cycle time on the MCC by over 50 percent of what it was for heritage SSME.”
New pogo units are now being built using “selective laser melting” (SLM), an additive manufacturing (also known as “3-D printing”) technique. This test series continues use of the unit first tested in the Retrofit 1a test series that concluded in February.
The new insulation system for the HPFTP also borrows production techniques from the RS-68 program, injecting the insulating material into a mold that fits around the pump. The old system was assembled from several different, individually built pieces and then fitted over the pump in a more labor-intensive process.
The production restart components maintain the form, fit, and function of the heritage SSME designs with the goal of maximizing affordability reductions in production cost and time.
Beginning in December, 2017, the focus of RS-25 ground testing shifted to development and certification of the “production restart” design. Testing from 2015 through most of 2017 helped to certify the already-built “adaptation” engines and a new engine control system to the SLS flight environment.
The first four SLS launches will use engines built during the Shuttle Program and fly them during most of ascent at a throttle setting of 109 percent RPL. Production restart engines will be flown at 111 percent RPL.
In addition to testing the performance of hardware built with the new manufacturing methods, NASA and Aerojet Rocketdyne are acceptance testing new engine controller units (ECU) that will fly on all the SLS launches. The ECU design was certified during the previous adaptation test series and new units produced by Honeywell are being delivered to Stennis.
ECUs for the first two sets of engines and beyond have already been tested and new units will continue to fill out the inventory for the first four launch sets in hot-fire tests in the A-1 stand.
Tropical Storm Gordon stays away
Scheduled for Thursday ahead of the Labor Day holiday weekend in the United States, the formation and path of Tropical Storm Gordon created some uncertainty about whether the test could stay on schedule or would have to move to Friday. Stennis closed to non-essential personnel early Tuesday afternoon and reopened mid-morning on Wednesday.
The center of the storm made landfall along the Mississippi-Alabama border overnight Tuesday into Wednesday and tracked far enough east to avoid direct impacts to either Stennis or NASA’s Michoud Assembly Facility in nearby New Orleans. NASA Stennis News Chief Valerie Buckingham said Stennis had no storm damage or cleanup from the storm.
Late Wednesday, NASA announced the test would be conducted Thursday afternoon.
Early cutoff in last test
The first test of the Retrofit 1b series was conducted on August 14, with NASA Administrator Jim Bridenstine in attendance. That planned 500-second long test was shutdown at 319 seconds while the engine was running at 111 percent.
“The facility fuel transfer valve failed to open,” Benefield explained. “As a result, fuel could not be transferred from the storage barge to the facility fuel run tank during the test. The test was manually terminated by the test conductor when the facility fuel run tank reached too low a level.”
During a test, the engine in the A-1 test stand draws its propellants from “run tanks” built into the stand, but for long running tests it is also equipped with interconnected piping and other infrastructure like pumps that allow the run tanks to be backfilled with additional propellant from barge tankers that are docked adjacent to the stand.
Although it ended ahead of schedule, all of the objectives of the August test were completed prior to the early cutoff.
Following today’s test, Benefield said the next one is currently planned for September 25.