SLS showed up, at last
by Jeff Foust
|“It would have been an absolute worst-case nightmare for us,” Free said of the scenario of the SLS caught in the hurricane on a broken crawler. “I don’t think we would have survived it.”|
Ever since the second Artemis 1 launch attempt was scrubbed in early September because of hydrogen leaks during fueling of the core stage, NASA worked to find solutions to the problem (see “Of hydrogen and humility,” The Space Review, September 6, 2022). That ranged from replacing damaged seals in the hydrogen fuel lines to creating what officials called a “kinder, gentler” approach to fueling. In mid-September, NASA went through a tanking test, filling the core stage with liquid hydrogen and liquid oxygen, this time without any leaks.
“All in all, it was a good day. We’re very happy with the results,” said Brad McCain, vice president and general manager of Jacobs Space Operations Group, of that tanking test during a briefing two days later. “We’re very optimistic about our next launch attempt.”
NASA, at the time, was gearing up for a launch attempt in late September or early October, before that launch period closed in early October. However, Hurricane Ian intervened: a day after that briefing, NASA decided to roll the rocket back to the Vehicle Assembly Building (VAB), rather than risk having the vehicle damaged on the pad, where it had been sitting since August.
The rollback to the VAB effectively ruled out a launch in the next launch period in the latter half of October, given the work planned on the vehicle while in the building and the time needed to prepare the vehicle while on the pad. Instead, NASA rolled out the rocket back to the pad November 4 for a launch no earlier than November 14, with November 16 and 19 as backup dates. (NASA also worked to secure another launch date on November 25, Black Friday.
NASA soon got a reminder that the Atlantic hurricane season runs through the end of November. Even before the rollout, meteorologists were tracking a potential tropical storm forming in the Atlantic that could head towards Florida. However, at the time of the rollout, forecasts suggested the system had only a 30% chance of becoming a named tropical storm and no chance of exceeding wind limits on the pad.
“We’ll have impacts from that in terms of the wind, but we’re not looking at any likelihood at this point of see a strong system emerge out of this,” said Mark Burger, launch weather officer with the US Space Force’s Space Launch Delta 45, at a briefing the day before the rollout.
A couple days later, though, what would become Hurricane Nicole became stronger than expected as it churned west towards the Florida coast. NASA decided to leave the rocket at the pad this time even as the risk of high winds, and damage, grew.
Jim Free, NASA associate administrator for exploration systems development, said in a later interview that the storm developed too quickly for the agency to move the rocket back to the VAB before it arrived. “I think the weather folks gave us the best information they could give us. I don’t think anybody saw it coming together to be a hurricane.”
Free said the agency’s concern was that something could happen, like a mechanical issue with the crawler-transporter, that could strand the rocket between the VAB and the pad. “It would have been an absolute worst-case nightmare for us,” he said of that scenario. “I don’t think we would have survived it.”
Nicole came through as a category one hurricane, the weakest on a one-to-five scale, and NASA reported winds that came close to, but did not exceed, certified limits at the pad. Afterwards, NASA reported only minor damage, with the biggest the loss of about three meters of a caulk-like material called RTV, for room temperature vulcanizing, that covers an aerodynamic gap between the Orion crew capsule and its launch abort tower.
|“I feel very confident in the procedures that we’ve worked, but that will be one of those things that will be a big milestone once we get into a steady state fill” of liquid hydrogen, said Parsons. “That’s something we’ll all be watching closely.”|
The loss of that piece of RTV itself was not a major concern: its primary purpose is to smooth the airflow over that gap to avoid heating. “As you fly through the atmosphere, you want a very smooth outer mold line so you don’t trip the boundary layer or cause acoustic noise issues or thermal heating,” said Jim Geffre, Orion vehicle integration manager, in an interview. He said the material, designed to stay in place as the vehicle accelerated to orbit, likely came off in hurricane force winds because of the direction and duration of the winds, not their force.
Engineers, though, were concerned that more material could come off during launch and hit other portions of the vehicle, with the potential of causing damage. That meant doing worse-case analyses to see what would happen if more RTV material came off.
The four core stage engines and tow five-segment solid rocket boosters ignite for liftoff. (credit: NASA/Joel Kowsky)
NASA had already ruled out a Nov. 14 launch because of the time lost to the hurricane, but decided to press ahead with a Nov. 16 launch after completing those analyses. “We agreed that the risk was bounded by current hazards,” said Mike Sarafin, NASA Artemis 1 mission manager, at a pre-launch briefing, even if more material came off during flight. “We’re comfortable flying as-is.”
He said that if RTV material came off, it would most likely hit the launch vehicle stage adapter below the Orion, a conical structure that he said would not be damaged by material impacting it. There was much less of a chance of it hitting further down the core stage or the boosters.
That briefing also revisited the hydrogen leak issues from the past launch attempts. “I feel very confident in the procedures that we’ve worked, but that will be one of those things that will be a big milestone once we get into a steady state fill” of liquid hydrogen, said Jeremy Parsons, deputy program manager for NASA Exploration Ground Systems. “That’s something we’ll all be watching closely.”
As fueling got underway, that confidence seemed well placed. As hydrogen pumped into the core stage, there were no signs of the serious leaks seen in early September. The core stage filled with liquid hydrogen and liquid oxygen, with the upper stage following.
Then, a few hours before the scheduled 1:04 am EST liftoff time, came the dreaded news: there was a hydrogen leak after all, in ground equipment used to replenish the core stage. Engineers suspected there were nuts loose in a valve in that line, allowing hydrogen to leak above acceptable levels. Mission managers decided to send a three-person “red team” (or “red crew”) to the pad to fix the problem.
While red teams performed pad repairs during shuttle missions, they were not watched as closely as this trio, monitored by cameras as they drove up to the pad and went inside the structure to tighten the nuts on the valve.
Mike Bolger, Exploration Ground Systems program manager, later described the repairs as an iterative process: tightening the nuts and then cycling the valve to see if it remained loose. By the third time, “they were really solid and we felt good that the packing nuts were tight and we had a good valve.”
The red team left the pad and controllers started flowing hydrogen through the valve. “We resolved that problem and no longer had a leak in the base,” he said.
With the leak resolved, and no other major issues, launch director Charlie Blackwell-Thompson gave the approval to go into the terminal countdown from a hold at T-10 minutes, with liftoff scheduled now for 1:47 am. As the final minutes ticked off, the crowd looked towards Launch Complex 39B, partially preparing for liftoff but also steeling themselves up for another hold. Past countdown rehearsals had never gotten all the way to the planned cutoff at about T-10 seconds, so there was every reason to believe that some other issue would come in those final seconds to halt the countdown.
Those issues never arose. Instead, the SLS ignited its four RS-25 core stage engines, followed by its two five-segment solid rocket boosters, and the rocket rose off the pad, at long last. After all the problems with its development, years of delays, and months of scrubs, the rocket’s ascent to orbit went very nearly to plan. “All indications were that the system performed spot-on,” Sarafin said at a November 18 briefing, noting that a full review of the SLS performance was still ongoing.
That spot-on performance was illustrated by the core stage putting Orion and its Interim Cryogenic Propulsion System (ICPS) upper stage into an orbit with a few kilometers as planned. The translunar injection burn by the ICPS put Orion on its required trajectory to the Moon, where it performed a powered flyby maneuver November 21 to send it towards distant retrograde orbit.
|“We’re Mission Control,” Nelson said Artemis 1 mission contingency planning. “We always have backup plans on backup plans, right?”|
All attention now is on Orion. The successful launch and lunar insertion ensures that NASA has a “priority-one” mission, Sarafin said after launch. The top priority of the uncrewed Artemis 1 mission is to demonstrate the reentry of the crew capsule at lunar return velocities. The second priority is to test other Orion systems, followed by recovery of the capsule after splashdown and miscellaneous other flight objectives. He said at the November 18 briefing that, for the second and fourth priorities, NASA had completed 20% of the objectives with another 40% in progess.
That work will be done in a mission lasting about 25.5 days, with splashdown around midday December 11 off the California coast. That is considered a “short-class” mission by NASA, versus “long-class” missions lasting up to 42 days. Those longer missions would have spent more time in the distant retrograde orbit.
The Orion spacecraft flies by the Moon during a November 21 maneuver to send the spacecraft to a distant retorgrade orbit. (credit: NASA)
The mission timeline still enables all flight objectives to be met, but with less margin for error. “We did proofs of concept early on to demonstrate that you can fit all the objectives the program has decided they want us to accomplish into the shortest class mission,” said Emily Nelson, NASA chief flight director, in a pre-launch interview. “But, it’s much more of a Tetris puzzle to make sure all of those things fit in.”
Each potential launch opportunity required a different timeline, she said, but controllers resisted the urge to use the delay from late summer to further adjust mission timelines. They did, though, craft many contingency plans in case something went wrong at any phase of the mission. “We’re Mission Control. We always have backup plans on backup plans, right?”
There will be, at any given time, a little more than a dozen people in Mission Control during Artemis 1, with about the same number in back rooms supporting those controllers. That includes a capcom, even though there are no astronauts in the capsule to communicate with. “Capcom will be there listening and following along,” she said, particularly during key operations, “basically as a training opportunity for next time.”
Geffre, meanwhile, will be among those closely monitoring the health of the spacecraft over the course of the mission. He said there are more than 1,000 development flight instrumentation channels on the spacecraft collecting data on aspects of Orion’s performance during the mission, some of which will be transmitted back but others that will be offloaded after landing.
“Ss soon as we get all that data back, we're going to turn it over to the engineers and they're going to find out all those areas where we can push the envelope a little bit more and other areas where we have to sharpen the pencil,” he said. “We’re really looking forward to that activity.”
The launch, though, offered a wave of relief for NASA in general and the Space Launch System team in particular, getting the rocket off the ground after years of delays. The red team turned into heroes of the hour with their repairs to fix the hydrogen leak, earning them NASA’s equivalent of 15 minutes of fame: a photo op with NASA administrator Bill Nelson and an appearance on NASA TV shortly after liftoff.
“The rocket’s alive. It’s creaking, it’s making venting noises, it’s pretty scary,” said Trent Annis, one of the red team members, on NASA TV. “My heart was pumping.”
“It was a low moment when we first saw the leak,” Bolger said, bringing up at a post-launch briefing hydrogen leaks in past launch attempts, “but really a high moment when we recognized we’d solved the problem.”
Or, as Annis put in, “Yeah, we showed up today.”
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