NASA, European Space Agency (ESA), and contractor personnel conducted an Abort To Orbit (ATO) test case last week for the Orion spacecraft using the Propulsion Qualification Module (PQM) at the White Sands Test Facility (WSTF) in New Mexico. The team at White Sands ran the PQM through a twelve-minute long sequence of engine firings to demonstrate the ability of Orion’s propulsion system to handle a stressing abort case.
In a hypothetical ATO case, Orion would have to pick up the slack from an underperforming Space Launch System (SLS) launch vehicle in the middle of a launch where it was going too fast to immediately return to Earth. At some point during ascent, the safest option would be to separate from the SLS and finish insertion into Earth orbit to provide more time to think through the situation; the August 5 test was meant to help verify that Orion’s propulsion systems are ready to handle that situation if it ever came up in flight.
The PQM is a “battleship” version of the European Service Module (ESM) propulsion subsystem built by ESM prime contractor Airbus Defence and Space. The test article was delivered by Airbus to White Sands in 2017; the ATO test was the thirty-first test of thirty-eight in a two-phase test series to help certify the Service Module propulsion system for its first Orion flight on Artemis 1.
Abort To Orbit test case
Built by Airbus DS for ESA, the ESM has three types of engines: twenty-four reaction control system (RCS) thrusters for attitude control, eight auxiliary (Aux) engines for translational maneuvers, and an Orion Main Engine (OME) for large translational burns. The OME is a refurbished Space Shuttle Orbital Maneuvering System (OMS) engine, which were used in pairs on the Shuttle Orbiters.
The PQM was delivered to the 300 area at White Sands in early 2017 minus its OME, which was installed on-site after arrival. The test article was subsequently lifted into Test Stand 301, which was also used during the Apollo and Shuttle programs. Similar to the human-rated spacecraft from those earlier programs, the ESM and PQM use hypergolic propellant; mixed oxides of nitrogen (MON-3) is the oxidizer and monomethylhydrazine (MMH) is the fuel.
(Photo Caption: Test Stand 301 at White Sands Test Facility in New Mexico a few hours before the ATO test case was run on August 5. The test article is inside the building on the left, surrounded by ground support equipment and facilities. The multi-level stand includes a flame bucket below that directs exhaust down and to the left from this point of view.)
The PQM is a test article for the propulsion subsystem of the ESM; it doesn’t have any equipment that tests other ESM functionality such as electrical power generation, consumable storage, or thermal control. It also is more of a battleship structure than the flight articles, using stronger but heavier materials for its structure and fuel tanks.
The purpose of the article is to test the engines and thrusters with the systems for propellant distribution and pressurization. Another difference from flight modules is that the PQM only has twelve RCS thrusters, omitting one of the twelve-thruster RCS “strings,” and all of the engines are fixed in a down-firing position on the test article; on the Service Module, the OME can be steered and the RCS thrusters are arranged to provide roll, pitch, and yaw control.
An initial series of “blowdown” tests were conducted two years ago prior to the arrival of pressure control assemblies (PCA) that enabled active control of the propellant tank pressures with helium gas. PCAs for both the first ESM, Flight Model-1 (FM-1), and the PQM were delivered in last half of 2018 and a two-phase test series began in October, 2018, with a couple of OME tests to clear FM-1 for shipment to the Kennedy Space Center in Florida.
The tests were divided into steps, with Step 1 using unsaturated propellants and Step 2 using propellant that is saturated with its helium pressurant gas. “We plan a total of thirty-eight runs and we’ve done thirty, we’ve got eight more left,” NASA’s Orion Program Manager, Mark Kirasich, said at White Sands prior to the test, which was the thirty-first overall.
(Photo Caption: PQM fires its translational engines during the long ATO test case on August 5. The exhaust from the firing OMS engine can be seen at the bottom of a diffuser that extends down from its exit nozzle. All eight Aux engines also ran with the OMS as they would in an ATO case.)
The August 5 test was a part of Step 2, where the hypergolic propellant was saturated with helium. “There’s a possibility of [use] after a fair time on the pad, so saturation occurs when the vehicle sits on the pad and the gases are absorbed slowly over time, so it depends on the launch window and how quickly from when you load the vehicle to when you launch whether you would be saturated or not,” Brian Anderson, NASA PQM Program Test Lead at White Sands, said before the test.
“We’re prepped and ready for an ATO, so we have enough propellant onboard to run that ATO profile, and then the COPVs are loaded on one side to where they would be near capacity for what you would call pre-launch, the other side is slightly lower for the conditions we’re at, and then we will consume almost the entire quantity of propellant that’s on PQM today for the Abort To Orbit.”
The test of the ESM propulsion system test article simulated an ATO firing profile. When crewed flights begin with Artemis 2, Orion will have abort capability on launch day from prior to liftoff through orbit insertion. For its first launches on SLS, the launcher’s Solid Rocket Boosters and Core Stage insert both Orion and a fully-loaded upper stage into Earth orbit about eight and a half minutes after liftoff.
The spacecraft’s Launch Abort System (LAS) is jettisoned around three and a half minutes after liftoff which is still early in the ascent. “It probably depends on where you had an issue if you have to abort to orbit, so the scenario would be if SLS is having an issue at some stage and you’ve already jettisoned the launch abort tower, maybe the safest place is for the astronauts to get to orbit as opposed to some kind of ballistic re-entry, so this would bring them to an orbit,” Jim Withrow explained before the test.
He is the PQM Project Manager for the European Integration Office (EIO) at NASA Glenn Research Center. “The mission would be over as far as their final destination. What this would do is get them to a safe orbit, allow them to assess the situation, and come home.”
(Photo Caption: A Mode 4 abort graphic from early in the overhaul of the Orion program following its cancellation and compromise rebirth early in the decade. In the case of an SLS underperformance, the Interim Cryogenic Propulsion Stage (ICPS) could be also used to help close an ATO case, similar to the Contingency Orbit Insertion abort capability that Apollo had on Saturn V launches.)
On SLS, Orion has three abort modes: Mode 1 uses the LAS while it is still attached, Mode 2 is the ballistic, “untargeted abort splashdown,” and Mode 4 is an ATO. Orion was originally developed to launch on the Constellation Crew Launch Vehicle, which also had a “targeted abort landing” Mode 3; on SLS, the Mode 2 and Mode 4 capabilities overlap and Mode 3 is not required.
Longest OMS engine firing in history
Media was invited to observe the test from the blockhouse about one-hundred meters away from the test stand. Everyone in the area shelters inside the blockhouse before the test due to the toxic propellants involved and the high-pressure helium tanks.
WSTF is well away from the closest large population in Las Cruces, but the local winds are monitored for personnel working outside the immediate area and the staff at one of the Tracking and Data Relay Satellite System (TDRSS) ground station facilities were asked to shelter in place during the test because they were downwind.
The helium tanks are composite-overwrapped pressure vessels (COPV) which are pressurized to approximately 5000 pounds per square inch (psi) prior to starting the test. One of the final steps prior to starting the test profile was for a couple of test team members to leave the shelter in the blockhouse and secure the ground-side helium systems for each commodity, fuel and oxidizer, after the tanks were pressurized for the test one at a time.
The test profile was initialized in the mid-afternoon and after a five-second lead-in, the over twelve-minute long firing sequence started with start of the RCS thrusters by themselves and then the Aux engines. “We have about a thirty second period of time when we’re going to bring the RCS engines online, we’re going to let them heat up and they go through kind of a gas purge process,” Withrow said before the test.
“We’ll shut them off, we’ll bring on the Aux engines and they’ll run for about twenty seconds or so and then we’ll shut them off for about a half second,” he explained. “During that half second period everything will be off and then the OMS engine, all eight Aux engines, and two RCS roll engines are all going to come on at the same time, that’s about fifty seconds into the test.”
The August 5 test was the longest duration single firing of an OMS engine in its history. “The OMS engine will run for a little over eleven minutes and thirty seconds, the Aux engines will run for that same period of time, and the RCS engines will run intermittently,” Withrow said.
“Once that is complete there will be a little bit of RCS firing just after the Aux engines and the OMS engine shut off and the test will be complete at about somewhere shy of about thirteen minutes and then there will be about a ten minute period where we’ll be nitrogen purging the OMS engine. So the profile will be over within about twelve minutes but the purging happens again for about ten minutes.”
The test ran to full duration, with the system performing within operational parameters. Afterwards, the team is doing detailed analysis of system performance and incrementally working towards certifying the propulsion system and its elements to fly for the first time on Artemis 1.
(Photo Caption: Support equipment at Test Stand 301 for the hypergolic propellant used by the PQM. MMH is the fuel, monomethylhydrazine, and N2O4 is the oxidizer, nitrogen tetroxide, also called mixed oxides of nitrogen (MON-3).)
The ATO test is one of the stress cases in the ground qualification series, not only testing the engines and propellant distribution but pressurization. “One of the primary objectives is the pressurization control system, it has to do its job correctly for these engines to fire correctly,” Withrow said.
Head or ullage pressure at the top of the propellant tanks is required to keep the engines running, and the tanks can’t be completely filled to leave a little room to apply the necessary pressure.
“We don’t have pumps that pump the propellant into the engines, what we do is we have helium in the tank that is used to provide the force to drive the propellants out to the engines themselves,” Withrow said. “So you have to have a little bit of gas in the propellant tanks to give you that pressure and so we fill them up, we have these tanks filled up as you would for a flight mission just that there’s a little small amount of ullage.”
“As soon as the engines start firing, the pressurization control system will be feeding helium in but if you had zero ullage, so in other words, if the tanks were a hundred percent full then you would not be able to actually provide any pressure, there would be nothing to provide pressure.”
Withrow added that another reason not to completely fill the tanks is thermal expansion. “If you did that with the propellant tanks here and you filled them all the way up, we would potentially have expansion, too,” he said. “We don’t want these propellants to leak.”
During the test, the PCAs continuously fed helium into the propellant tanks as they emptied to maintain the necessary ullage pressure to continue to operate the OMS and Aux engines for the length of the test.