The Orion European Service Module (ESM) Propulsion Qualification Module (PQM) is in the opening phases of testing – including hot firings – at the White Sands Test Facility in New Mexico. Bolted to a test stand, the PQM is being taken through a series of test campaigns by a team of personnel from prime contractor Airbus Defence and Space, the European Space Agency (ESA), and NASA.
After initial propellant loading and characterization of subsystem behavior, hot-fire testing of the thrusters and engines to help qualify the propulsion subsystem of the ESM has just started.
“The number one purpose of this test activity is to qualify the propulsion subsystem,” Steve Barsi said in an interview with NASASpaceflight.com. Barsi is the ESM Propulsion Subsystem Manager in NASA’s Orion Program.
“This is one of the only subsystem verification activities that we have on the books. Normally you do testing at engine level or subassembly level, but the purpose of this test is really to bring everything together and try to test it in an integrated manner.
“[The ESM] engines have been tested at the thruster or engine level, but this is the first time where we’re bringing them all together and understanding interactions between the engines and what effects the [propellant] feed system has on the operation.”
The ESM has three types of engines: twenty-four reaction control system (RCS) thrusters for attitude control, eight auxiliary engines for translational maneuvers, and an Orbital Maneuvering System engine (OMS-E) for large translational burns. The PQM only has twelve RCS thrusters, omitting one of the twelve-thruster RCS “strings.”
Although the RCS thrusters and the auxiliary engines are undergoing individual hot-fire testing above and beyond these tests, the PQM testing also provides an opportunity to hot-fire the main OMS-E engine.
Originally the main engine for the Space Shuttle Orbital Maneuvering System, it was re-purposed for the ESM. Although the engine used in these tests will only be used for ground testing, this will be the first time OMS engines have been fired since the Shuttle era.
“A second pretty big objective [of the PQM tests] is to go delta-qualify the OMS engine,” Barsi explained. “Over the past probably two years or so, we’ve been readying the OMS engine for flight. That involved using a Shuttle engine, taking it apart, rebuilding it and it underwent environmental qualification testing at the Johnson Space Center.
“The specific objective related to the OMS engine is after it was subjected to a vibration environment that exceeded its previous use on Shuttle, we want to confirm that the engine can still operate normally under the environment that it’s expected to see in flight.”
Airbus Defence and Space is the prime contractor for the ESM; in addition to the propulsion subsystem being tested with the PQM, the flight model ESM also provides electrical power, thermal control, and consumable storage for the overall Orion spacecraft.
Final integration of the PQM was done by OHB Sweden in Stockholm, which was completed in January. The module was then shipped to the United States and arrived at White Sands in February for initial installation in Test Stand 301 there.
In addition to the engines, the PQM has propellant storage and distribution and pressurization assemblies, along with avionics for their command and control. Mr. Barsi said the initial hot-fire tests will look at the system behavior without pressure control.
“The test campaign is phased over two parts. The first part is related to blowdown testing — what I mean by that is there’s no active pressure regulation of the propellant tanks.
“We’re trying to get some early data and these are shorter duration burns — it’s not actively controlling the pressure in the propellant tanks. That total test campaign is approximately three weeks.
“The next phase of testing is active pressure regulation and so we’re still waiting on some hardware to perform that testing and that should arrive later this year. Once that hardware arrives and it’s integrated we’re looking at a few months of testing.”
The test team consists of approximately thirty people, including personnel from Airbus (both on-site at White Sands and in Bremen, Germany), ESA (in White Sands and Europe), and NASA (from White Sands and Glenn Research Center). The on-site personnel will be working from a blockhouse near the test stand.
“It’s within walking distance of the test cell, but it’s far enough away to keep those safety limits that have been defined,” Barsi noted. The ESM uses hypergolic propellants which are highly toxic; mixed oxides of nitrogen (MON-3) is the oxidizer and monomethylhydrazine (MMH) is the fuel.
The overall test campaign is divided up into different phases and will hot-fire different combinations of the engines.
“There’s about six overall separate campaigns,” Barsi added. “The different campaigns are intended to test different things, so some of the specific campaigns evaluate system performance with saturated versus unsaturated propellant. That’s referring to the amount of helium dissolved in the fuel and the oxidizer.
“Some of the sequences have to do with running through specific burn sequences, so we’re looking at firing the auxiliary engines and the OMS engine simultaneously. And then we’re looking at firing the aux engines and the RCS engines simultaneously and looking to see if there are any pressure transients in the system.”
Each of the test campaigns also has several sequences within it.
“There are multiple sequences and each sequence has multiple firings in it. We use inputs from the Guidance Navigation and Control (GNC) team to help define what those sequences are. Some of the sequences are pretty short — so the first test we do is just doing short firings of the engines just to do a general health check on the system.
“This will be the first time we’re firing engines with the accompanying ESM propulsion feed system. So some sequences just have single burns in them [and] some have multiple engine burns — it’s all intended to qualify the subsystem.”
“The most stressing case from a subsystem burn profile is an abort profile — that’s the profile where all the engines are firing at the same time,” he noted, adding that in an abort the engines could run around twice as long as a more typical burn.
The PQM will be operated in ambient conditions for these tests as compared to normal operation in Earth orbit or cislunar space, with some provisions for that environment.
“[We have] a diffuser with a blowoff plate, so…some of the [tests] we’re going to be running with a simulated vacuum start for the OMS engine,” Barsi noted.
In addition to the single-string RCS on the PQM versus flight models, the PQM structure and propellant tanks are also different, but Barsi explained that the qualification article is being used to focus on characterizing the behavior of propellant distribution, pressurization of the system, and engine firings.
“Structurally it’s a much beefier system, the ground article. The tanks are much beefier, too; but in terms of what we’re looking to get out the test, all the line routing geometry are the same. That’s a pretty important parameter to understand what the pressure drop is from the tanks to the engines and understand what the overall hydraulic performance of the system is.
“All the engines are flight-representative, most of the valves are flight-representative. The big differences are structure and the propellant tanks.”
The avionics for the propulsion subsystem are also contained with the PQM.
“The way the subsystem is structured, the active regulation unit and avionics for that…[are] part of the subsystem,” Barsi continued. “So that is flight representative; there’s obviously not a flight computer for the PQM, there’s a ground computer that supposed to send commands to activate specific burn sequences.
Both the flight models and the PQM use helium to pressurize the propellant tanks, but Barsi noted that the OMS engine retains it’s own pressurization pack.
“The OMS carries its own little tank and that’s nitrogen and that’s used to pneumatically actuate some of the valves and then purge the engine of fuel at the end of every burn,” he noted.
Barsi said that the test campaigns are planned to be completed early next year. After that, the PQM will be decontaminated and eventually will be removed from the test stand.
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