That seems to be the direction the Artemis team is working toward according to sources interviewed for Ars Technica. To begin with, though I need to detour to kill this fact I got from somewhere and didn't cross check thoroughly before posting it Monday.
Pre-chilling isn't to reach cryogenic temperatures, although the chilling is done by running some of the liquid hydrogen fuel through the fuel tubing of the engine. They chill the engine until the sensors (wherever they are) reach a temperature of about 40 F (5 C).
The source I saw gave the temperature in degrees Rankine - of all things - which said 500 Rankine and I did the conversion. (I mean, seriously, who uses Rankine in their work?) The reality is yes, they do chill them to extremely low temperatures with the -420F liquid hydrogen. My fault for not looking for other sources.
During a news conference on Tuesday evening, NASA's program manager for the SLS rocket, John Honeycutt, said his engineering team believed the engine had actually cooled down from ambient temperature to near the required level but that it was not properly measured by a faulty temperature sensor.
"The way the sensor is behaving does not line up with the physics of the situation," Honeycutt said.
The problem for NASA is that the sensor cannot be easily replaced and would likely necessitate a rollback to the Vehicle Assembly Building at Kennedy Space Center in Florida, a few kilometers from the launch pad. This would delay the launch of the rocket at least into October, and the space agency is starting to get concerned about wear and tear on a rocket that has now been stacked for nearly a full year.
The belief that the engine actually was the right temperature comes from other observations. Other sensors showed that liquid hydrogen was flowing and the pressures were correct. If hydrogen had been blocked from flowing into the engine, or if it had been spilling unrestrained as it would from a cracked open engine, the pressures would have been other than expected.
Honeycutt said his team is working on a "flight rationale" plan that would allow the rocket to launch without getting good data from the temperature sensor on the engine, but based on all the data they have available. It seems to me they're looking for some sort of consensus of the data that they can make themselves feel good about.
The schedule for today was for teams to crawl all over the vehicle looking for issues, including the inspection of an area where there was a small hydrogen leak during Monday's countdown. If the managers are satisfied with the inspections and their flight rationale for dealing with the faulty temperature sensor, the team will start counting down on Thursday. Fueling will begin Saturday morning. Weather looks better than this past Sunday night/Monday morning, but roughly a 25% chance of showers in the morning, and NWS says chances go up as the day goes on, saying the start of the two hour launch window at 2:17 PM EDT looks better than the end.
The things that linger in my mind are things like if the engine isn't chilled properly can it really be expected to meet its specs. Remember how they said they thought the engine cooled down to near the
required temperature? How near and what does that imply? If instead of -420F it's higher but still cryogenic, say -380F (number pulled from "air"), will flight software abort the launch? In the shuttle days, if the engines weren't up to rated pressures and powers within seconds, the software aborted the mission. Blowing up at ignition is bad, but perhaps blowing up in the first couple of minutes of flight is worse. Then I think NASA has never completed fueling this vehicle and not done a static firing of it in this configuration.
Artemis I/SLS on Pad 39B, August 19. Trevor Mahlmann photo.
TIL about the Rankine scale. I don't think I've ever heard of it. Apparently it is to Fahrenheit what Kelvin is to Celsius.
ReplyDeleteAnd it's used in the aerospace industry because of course it is.
I can't wait for the great Kelvin/Rankine catastrophe when we lose another probe.
I think I saw Rankine introduced in one class in my life, but never seen it used for anything. What I think the problem was comes down to a typo. The quote said 500 Rankine, but 50 Rankine works out to be pretty close to what they want: -410F.
DeleteI only knew about Rankine because my HP calculator included and I looked it up.
DeleteLook, let's just light this candle and send it on its way. The delays are costing more than a Starship launch every day.
ReplyDeleteEnough is enough!
I bet that they go for it. This is already a politically-driven program, with the "first woman and first POC" crap, so the pressure to launch for PR will be added to the engineering concerns that you have noted.
ReplyDeleteI sincerely want to see a successful mission. I am afraid that getting one is going to come down to a roll of the dice.
Wait. A critical sensor (measuring engine temps) failed. Okay, where are the other 2? Even SpaceX has more than one sensor per function on their Raptor 2.0s (and there were many many more on 1.0s.)
ReplyDeleteAnd a faulty sensor does not answer the hydrogen leak from, what I heard, was a suspected crack in the tank and the insulation.
It's going to blow up. It's going to scatter garbage all over KSC and damage critical infrastructure (especially over at SpaceX's area.) It's going to be like that scene from the ridiculously stupid movie "Contact" where the rotating hoopy thing goes blooie due to an explosion and pieces parts go everywhere.
Gonna be bad, real bad. Even worse, NASA and Congress will double down on spending even more money on that non-flying dogsqueeze.
I hope I'm wrong. But I don't think I am.
You're probably right. The constant stream of problems with this thing have pushed the team into a corner. The rocket has been stacked too long, it's around the limit of trips to/from the pad on the crawler-transporter, and (among the rumors I've heard) there's concern the SRBs are too old. They built and stacked the segments too long ago. They can't keep rolling it back forth between the VAB and the Pad. Either scrap it or throw it. With luck, if it explodes, it'll provide some new artificial reef structure.
DeleteI honestly didn't even notice there wasn't a word about "but the back up temperature sensor said..." but you're absolutely right. No mission critical parameter should ever have just one sensor.
If you go back in the SLS "mythology" you'll find an example from like 2017 where some NASA dude argues that SLS is better than Falcon Heavy because "SLS is real and Falcon Heavy is a dream."
When Tyson is trotted out to put sincere and furrowed face expressions "to the science" you know it's a PR CYA effort and really its just something foolish.
ReplyDeleteAt Imagineering I worked with a lot of CalTech PhDs and they all laughed at NASA engineering. Basically calling them political pawns in a never ending funding game to hide and not produce in *actual* engineering. I dunno, personally. But Tyson is a real red flag to me. Maybe it will be Bill Nye, next.