Artemis 1 Orion halfway through Plum Brook thermal vacuum test

by Philip Sloss

The NASA Orion spacecraft that will fly the Artemis 1 lunar orbit mission is about halfway through two months of around the clock thermal vacuum testing at Plum Brook Station in Sandusky, Ohio. NASA and prime contractor Lockheed Martin are collecting data on the spacecraft’s behavior and performance while it operates in simulated deep space conditions.

The tests not only verify that the Orion spacecraft and its design meet requirements, but the data collected will help to calibrate analytical models of how the structure reacts to thermal conditions in flight and how the spacecraft’s thermal control system reacts to those conditions and the resulting impact on overall power usage. Functional testing has already been conducted at two cold thermal balance points with current testing being performed at a hot balance point.

The inside of the spacecraft was depressurized to near vacuum conditions early in the test to evaluate system performance in contingency conditions; when the testing returns to a cold thermal balance point, the test team will repressurize the crew cabin to evaluate that capability at extreme temperatures. Following thermal vacuum testing the test chamber at Plum Brook will be reconfigured for electromagnetic inference/compatibility (EMI/EMC) testing that will conclude the campaign in Ohio.

While the Artemis 1 spacecraft goes through its final major development test campaign, work also continues on the second and third Orion missions for Artemis. Assembly of the Artemis 2 spacecraft is progressing on two continents, along with planning and analysis of the trade space for the next Orion to support the Artemis 3 lunar landing mission.

One month into two month thermal vacuum tests

“The test is going fantastic,” NASA Orion Program Manager Mark Kirasich said on January 23. “We’ve had twenty-seven elapsed days and it was initially planned to be a sixty-three day test. It’s all based on observed data and we’ve hit a couple of our milestones quicker, so if the test continues as it’s been going we’ll probably finish a handful of days earlier than Day 63.”

The Orion spacecraft for the Artemis 1 mission is sealed inside the Space Simulation Vacuum Chamber at Plum Brook’s Space Power Facility for the tests after arriving there on November 26 after shipment from Kennedy Space Center (KSC) on NASA’s Super Guppy aircraft. The trip to Plum Brook for this first lunar-capable Orion build was scheduled to test the spacecraft in an in-space configuration before it is returned to the Armstrong Operations and Checkout (O&C) Building at KSC to finish final assembly for launch.

Final assembly of the Crew Module and Service Module was completed in the middle of last year; the spacecraft was then prepared for the Plum Brook tests, which also served in part to get it ready for the Artemis 1 mission. The vehicle is being tested as if it were in space, which require it to be configured for and ready to operate under those conditions.

Credit: NASA/Quentin Schwinn.

(Photo Caption: A convoy of vehicles accompanying the transporter carrying the Artemis 1 Orion spacecraft, lower middle, approaches Plum Brook’s Space Power Facility on November 26. Orion was driven from Lahm Airport in Mansfield, Ohio, to the test facility two days after being flown from where it was assembled at the Kennedy Space Center in Florida.)

For the thermal vacuum tests, the spacecraft (minus its solar arrays) is oriented vertically in the large vacuum chamber on a stand, surrounded by thermal conditioning equipment that simulates those in-space temperatures.  Orion is enclosed in a cryo shroud and heat flux system to chill and heat different parts of the spacecraft and the chamber is pumped down to about three times ten to the minus six (3 × 10-6) Torr or 5.8 × 10-8 pounds per square inch (psi) to simulate vacuum conditions seen in space.

After arrival at Plum Brook just before Thanksgiving, the spacecraft was prepared for testing. It was taken out of its shipping and handling package, placed in support equipment for the test, and moved into the vacuum chamber. Electrical ground support equipment (EGSE) runs up to the spacecraft, including umbilicals to stand in for its missing solar arrays and cabling to carry commands and data to and from the spacecraft and data from test instrumentation deployed all over it back to the control center.

The test team began the around the clock testing just before Christmas. “You ever see those big tankers that go down the interstate with liquid nitrogen? When you’re pumping one of those every four hours it’s a 24/7 operation,” Kirasich explained.

“It is a really big industrial operation. This is Ohio in the middle of the winter; just the support crew to get those tankers in and out as well as the contingency plans we have if there’s a snowstorm, you’ve got to keep those tanker trucks going all of the time to keep the chamber cold.”

With the chamber at vacuum conditions and Orion powered up full-time, the spacecraft was first tested at two cold thermal balance points. “We went to two different cold set points,” Kirasich said. “The first was simulating what we predict the vehicle is going to be at in our most often nominal attitude, where the tail of the spacecraft is pointed at the Sun but all the other surfaces are pointed some degree to deep space.”

Credit: NASA/Bridget Caswell.

(Photo Caption: The Orion spacecraft is rolled into the vacuum chamber at Plum Brook on December 4. The spacecraft is caged inside a heat flux system that is then surrounded by a cryo shroud. The two elements allow the spacecraft to be baked and frozen on all sides while in a vacuum inside the chamber during the two-month long thermal vacuum test.)

“In general that’s a very cold attitude for most of the vehicle, albeit we keep the aft end warm.”

“So that lasted nine-ish days approximately and thermal balance is not just a cycle, you just don’t go cold,” he explained. “We went cold and then we sat there and we waited for temperatures on the vehicle to stabilize and the test operators they have very strict criteria.”

“There’s about a thousand [temperature] transducers and they have four or five sets of the criteria,” he noted. “For example one is ninety-five percent of the transducers have to be not changing.”

“You have to be within plus or minus [some fraction of] a degree per hour and that’s at every point on the vehicle and the reason you do that is because you use the data to calibrate and ground the thermal models.” Once the temperature of the spacecraft was stable, it was run through system checkouts.

“Throughout this entire test Orion is powered up and in fact we’re going to be running the spacecraft for over one-thousand five-hundred hours throughout this test,” Kirasich noted. “When we hit these balance points we want to run a complete spacecraft functional test, so it’s not just that you keep the spacecraft on and let the thermal control systems operate and then watch them. We then run every system, the comm system, the prop system, every system gets a functional checkout to make sure it operates at these extreme temperatures.”

Kirasich noted that thermal control system performance is providing valuable data. “For example, the coolant pumps are circulating fluid all the time to draw heat away from the avionics boxes and then run them through the radiators and reject them overboard so all of that is working incredibly well,” he said.

“The team is learning because the thermal models ahead of time until you ground them in test data, they’re not perfect. “There’s margin and there’s errors because the models don’t model everything perfectly so we’re finding certain parts of the vehicle react a little bit slower than when the analysis predicted, other areas respond higher.”

“Power balance on the way to the Moon is really important,” he also noted. “[With] the extreme temperatures we are noting heater duty cycles to update our power usage models, so that’s the kind of data you glean from this thing. It’s not only testing ‘did anything break?'”

Credit: NASA/Rad Sinyak.

(Photo Caption: Orion is back in a vertical orientation at Plum Brook on December 1 after being unpacked from its shipping covers and rotated from its horizontal shipping orientation.)

After conditions simulating the tail-to-Sun attitude were tested, the team proceeded to the second cold balance point where they repeated the same testing. “We had heaters on the bottom simulating the Sun, we turned those heaters off so this is simulating like the entire vehicle is at eclipse, so there’s no Sun shining on the vehicle at all, nothing to heat it up,” Kirasich said. “So the temperature on the vehicle went down even further and we did the same thing there, we let the temperature stabilize.”

“Throughout this it was amazing, the vehicle has been performing magnificently,” Kirasich said. “Doug Loverro, the new HEO boss (NASA Human Exploration and Operations Mission Directorate Associate Administrator), was with me on New Year’s Eve [at Plum Brook], he was asking the team some questions.

“Sitting right outside the spacecraft you’ve got these thermal shrouds at minus 250 degrees [Fahrenheit], inside the spacecraft we were controlling exactly where we set the temp to, plus 65 degrees, just beautifully. And he was asking ‘how do you do that?’ and we explained the thermal protection system and the coolant system and the heaters and so it was really good.”

Testing began after a quick false start

The testing began with depressurization of the vacuum chamber on December 26 after a false start. “We did have some excitement at the beginning,” Kirasich said.

“It’s a very big operation. We install the spacecraft and you hook up over a thousand temperature transducers and it’s a thermal vacuum chamber so you have to run all the wiring through very thick walls [that are] hermetically sealed so there’s no leaks and we install heaters and cooling shrouds, that took a number of weeks and then we shut the door and we evacuated the chamber and then what we do is we flood the thermal shroud with this very cold, minus 250 degree liquid nitrogen and things start to cool down.”

Kirasich said the problem showed up on Christmas Eve. “The first day into that, not the spacecraft but some ground support equipment,” he said.

“We have the harness that connects the command and telemetry links between the control room and the spacecraft, we have redundant primary and a secondary lines that run through this very thick wall through the very cold chamber, but it’s a fiber optic line which is a lower temperature limit so we wrapped the entire thing in a thermal blanket with heaters, and guess what we made the thermal blanket zero-fault tolerant.”

“It was a single heater, so as luck would have it about a day into that first cold soak as we were trying to get cold that heater failed,” he noted. “It was Christmas Eve, I was [home] celebrating with my family and the team phoned up and said we need to talk.”

“We could have continued and we would have exceeded the lower operational certification of that fiber optic line and we would have likely failed both the primary and secondary abilities to command and get data from the vehicle. We were a day into it but we decided, nope we better stop and we repress-ed the chamber and we went in and we repaired things.”

“What we did in the ensuing day before we actually got back in the chamber we looked at every single ground support system’s single vulnerability, where a single system could take down a redundant capability that we needed and we improved the system and added some fault tolerance in the areas where we thought we were vulnerable,” he explained. “So that was quite exciting.”

Credit: NASA/Glenn Benson.

(Photo Caption: Going back to preparations in Florida to go to Plum Brook, the Artemis 1 Orion spacecraft has been rotated to horizontal in the O&C Building at KSC. The spacecraft was lifted into a verticator tool that allowed it to be rotated from its typical vertical assembly and test orientation to the horizontal orientation need to fit in the Super Guppy aircraft.)

Kirasich noted that the team beat their initial estimate for recovery time. “It was a really good Christmas present,” he said.

“When we had that telecon on Christmas Eve the team told me it was going to be a six-day operation because we had to warm everything up before we repressed, then we had to repress, and we had to get the team in. The vehicle is covered, it’s shrouded and we had to take equipment off, put in some scaffolding, and we had this six-day estimate.”

“And so it turned out they got in there and the team was so pumped that we were turned around and we were depressing again in a little bit less than two days so they did a really great job.”

A few days into testing, the Orion crew cabin itself was depressurized. “We did that very early in the test, the interior of the Crew Module has been at a very low pressure, about one psi, and we do this for two reasons,” Kirasich explained. “One we designed Orion to be able to operate if there was a hole in the cabin and we had a leak and the crew had to get home in a vacuum, so we wanted to demonstrate this capability.”

“The second reason is especially on the thermal balance part you’re trying to correlate the thermal models as I mentioned and it turns out when you’re in gravity as opposed to when you’re in space where the effect of gravity is less, [you have] thermal convective effects — you know how heat rises? When you don’t have an atmosphere you can take that uncertainty out and it’s much easier to calibrate the thermal models. We’re trying to take as much margin out of the unknowns of the thermal and power balance models as possible, so that’s why we go to the extremes.”

Kirasich explained that it was decided not to depress the Crew Module pressure vessel all the way to vacuum. “We could have gone all the way to zero [but] it was a risk balancing measure that the engineering team discussed, recommended, and then I accepted,” he said.

“Outside of the spacecraft it’s at a hard vacuum, it’s like ten to the minus 6 Torr, it’s a very deep space vacuum and all the pumps, all the avionics boxes mounted on the outside of the cabin are operating continuously. So we have no reason to believe any of the boxes inside [would have a problem] or any of the things you worry about, the harnesses, any arcing effect in a vacuum [would be] a problem, but it was a risk balancing measure.”

“As a way to get as much data as possible, since the interior still has to survive the rigors of a deep space flight we picked this point as a way to balance risk if you will,” he added. “And we did it quantitatively, we looked at how many thermal objectives we got, we looked at how much of the technical risk we bought down by going to one psi, we looked at how much we reduced the risk of damaging a box for example if you did have a short somewhere. We chose to take it to about one psi and we have been sitting there since early in the test.”

Credit: NASA/Bridget Caswell.

(Photo Caption: The heavily packaged Orion spacecraft is rolled off the Super Guppy aircraft at Mansfield Lahm Regional Airport on November 25 the morning after arrival from KSC. The spacecraft was subsequently lifted onto a flatbed trailer for ground transport to Plum Brook the next day.)

After the second cold thermal balance point, the heat flux system was reactivated to go to a hot thermal extreme. “We did those two cold soaks that I mentioned, one at tail to Sun, one simulating eclipse and then we transition to the other side, the hot side. The way we do this is we turn on heaters, sunlamps if you will, sun simulators, in the equipment right outside the spacecraft so more of the spacecraft is quote ‘seeing the Sun.'”

“It’s the same sort of thing as the first part, we’re doing a thermal balance, so it’s not just waiting until you get to the next temperature. We get to the next temperature and then we sit there and we had the same criteria where we look at all the temperature transducers and make sure ninety-five percent of them have stopped changing.”

At the time of the interview last week, Kirasich noted that the test team was running functional tests of spacecraft systems at that high temperature thermal balance point. Once that testing is complete, they will chill the spacecraft down again in preparation for more tests and repressurizing the Crew Module.

“We’re going to stay depressed all the way until we get to the second cold plateau and at that point we going to repress the cabin and you might say well ‘why do you do it there?'” Kirasich said. “When you repressurize, the lines get very cold because you’ve got that gas expansion it’s just like the way air conditioners work here on Earth and we’re repressurizing at the coldest temperature to provide an extreme test that all the plumbing works, nothing freezes, nothing stops up during this very worst case situation where you have to do a cabin repress under cold environments.”

After the thermal vacuum testing, the chamber will be reconfigured over a couple of weeks to conduct electromagnetic inference/compatibility (EMI/EMC) testing on the spacecraft. Following that, Orion will be removed from the test chamber, rotated back to a horizontal orientation, and repacked for the trip back to KSC.

A caravan of vehicles will first escort the spacecraft on a drive back to Lahm Airport in Mansfield, Ohio, where it will be again loaded on the Super Guppy for a multi-stop flight back to the Shuttle Landing Facility at KSC. The spacecraft will then be rolled back to the Armstrong Operations and Checkout (O&C) Building at KSC where Lockheed Martin’s Assembly, Test, and Launch Operations (ATLO) team will complete final assembly tasks before Orion is turned over to Exploration Ground Systems (EGS) Spacecraft and Offline Operations team to begin launch preparations for Artemis 1.

Related Articles