Updates SLS Updates

[Urwumpe]

Sounds like they're going to ship the core stage to Kennedy anyway despite the premature cutoff of engines

Yes, they are likely going to finish things at the Cape. Remember, the flight in November will be unmanned, so no need to be extra cautious. Also we all know how sensitive the SSMEs were in the past - they are always looking for a reason to shutdown early. So, it isn't really a full failure yet. It can even be that the issue will be fixed before launch.

 

[Urwumpe]

If the flash + shutdown and the MCF are related I don't know... not sure if they added TVC into the engine controller (they were independent in the Shuttle).


More details about the pre-pressurization issues, the LH2 tank was losing pressure too fast so the He pulses to pressurize the tank were lengthened.

I did not find any information about how the TVC of the SLS core stage is implemented. I did not see any action of it today as well. I only know, it must exist and there has to be an external power source for it, since its unlikely that they installed a hydraulic pump on one of the turbopumps - that would be a major change to the RS-25 and require way more testing than they did (Pumping hydraulic fluid with the needed pressure and volume flow for keeping such engines pointing in the right direction takes a lot of power and would mean tapping off more energy from any turbine = A higher pressure drop).

Also, I had seen some cold gas venting from the intertank area and I am sure, they saw it too, the camera did some closeups to the region. I was surprised they continued with the test despite it, I woud have expected them to investigate, since it was not coming from any of the vents, but seemed to have its origin behind or right next to the oxygen feedline.

 

[GLS]

I did not find any information about how the TVC of the SLS core stage is implemented. I did not see any action of it today as well. I only know, it must exist and there has to be an external power source for it, since its unlikely that they installed a hydraulic pump on one of the turbopumps - that would be a major change to the RS-25 and require way more testing than they did (Pumping hydraulic fluid with the needed pressure and volume flow for keeping such engines pointing in the right direction takes a lot of power and would mean tapping off more energy from any turbine = A higher pressure drop).

I think they placed a turbine in the GH2 line from each engine to the tank, that drives an hydraulic pump for a hyd system per engine. This should be invisible to the engines.
The TVC slews they wanted to do (and apparently started between the MCF and the cutoff) was to verify that the hyd flow was enough to gimbal the engines while they were operating.

Assuming the flash during the gimbal time frame means a TVC issue: it's possible the ATVC box of the Shuttle was merged into the new engine controller... this would explain a TVC issue causing a controller initiated shutdown. Or the ATVC could still be independent of the engine controller, and a TVC issue caused a hyd pressure drop, which could make the controller kill the engine (in the Shuttle it didn't look at hyd pressure).
How does the MCF factor into this I don't know... probably unrelated.

Given how vague the explanation in the press conference was, it is also possible that the flash seen was a test stand flash used to signal the ignition and shutdown commands in the cameras. ?‍

 

[GLS]

I'm not the only one that didn't like the press conference...

https://twitter.com/i/web/status/1350615661849030661

 

[Urwumpe]

I'm not the only one that didn't like the press conference...

https://twitter.com/i/web/status/1350615661849030661

I don't think it is a fair assessment, since "how long the engine was firing" is from an engineering POV a much more complex answer than just "look at those flames".

But yes, it is disappointing. Especially if you consider, that the seven previous green run tests had been without any critical problems.

But again, that the failure happened during a gimballing profile that did not take place at all (those engines did not move at all, not even a degree), suggests that tapping of power from the engines for powering the hydraulics is a plausible explanation of what went wrong. Moving four engines instead of just one requires a much higher volume flow and could mean that the turbine for powering such hydraulics caused a pressure spike upstream into the engine.

Remember, if hydraulics should move, and don't, something else is consuming the power.

Since simulating this is nearly impossible, you need to test it....

 

[N_Molson]

Again the main issue with all this is that they staged a very technical engineering test as a public event ("How to put yourself in trouble" by NASA, Chapter 1). Yes, that Twitter poster misses quite a lot of things. We simply don't know what happened and it probably can't be explained in a few simple words. That's specialist stuff, there are qualified people working on it and that's it. It sure leaves the mere spectator with a unpleasant "and then ?" feeling.

 

[GLS]

I don't think it is a fair assessment, since "how long the engine was firing" is from an engineering POV a much more complex answer than just "look at those flames".

But yes, it is disappointing. Especially if you consider, that the seven previous green run tests had been without any critical problems.

I'm not expecting a root cause to be found in a short timeframe, but doing a press conference 3 hours after the event, and they didn't even look at a console/log/whatever for the ignition and shutdown times before it? Plus, if they know a controller send a FID, they know what it was, why not tell us? Instead we get PR keywords... ?
"T seconds after ignition a redline for sensor X/check Y/component Z was violated and the test terminated. We will determine the cause of that failure in the following days." Doesn't look so hard. ?‍

But again, that the failure happened during a gimballing profile that did not take place at all (those engines did not move at all, not even a degree), suggests that tapping of power from the engines for powering the hydraulics is a plausible explanation of what went wrong. Moving four engines instead of just one requires a much higher volume flow and could mean that the turbine for powering such hydraulics caused a pressure spike upstream into the engine.

Remember, if hydraulics should move, and don't, something else is consuming the power.

Since simulating this is nearly impossible, you need to test it....

AFAIK, there is an hydraulic system for each engine (engine valves + TVC), driven by a CAPU powered by tapping the GH2 that goes from that engine to the tank. 4 engines, 4 CAPUs, 4 hyd systems, all independent.
How do you know the gimballing didn't happen? There was a clock in the corner of the image and it stopped at (I think) 67.7 seconds and the gimballing was to start at 60s... I don't think a failure right at the first gimbal command would take 7 seconds to stop the show. I think they gimballed, probably not at the desired rate, eventually violating a position error limit and thus stopping the test.

 

[GLS]

Again the main issue with all this is that they staged a very technical engineering test as a public event ("How to put yourself in trouble" by NASA, Chapter 1). Yes, that Twitter poster misses quite a lot of things. We simply don't know what happened and it probably can't be explained in a few simple words. That's specialist stuff, there are qualified people working on it and that's it. It sure leaves the mere spectator with a unpleasant "and then ?" feeling.

The level of preparedness for an off-nominal result was very patent in the script the PAO woman read at the end of the webcast: "very successful test". ?

 

[malcontent]

A test is for working out bugs, right? so finding a bug means a successful test!

Twitter comment saying this test was a congressional requirement? Is that true? Which part, the test itself, or making a performative display of something they would do anyway?

 

[DaveS]

I'm not expecting a root cause to be found in a short timeframe, but doing a press conference 3 hours after the event, and they didn't even look at a console/log/whatever for the ignition and shutdown times before it? Plus, if they know a controller send a FID, they know what it was, why not tell us? Instead we get PR keywords... ?
"T seconds after ignition a redline for sensor X/check Y/component Z was violated and the test terminated. We will determine the cause of that failure in the following days." Doesn't look so hard.

It isn't. For the STS-55 RSLS Abort, the NASA Test Conductor (NTD) asked the Shuttle Project Engineer (SPE) folks about what caused the RSLS Abort just 15 minutes after the abort had happened amidst the procedures of safing the vehicle. Here's a video which should start at that point. If it doesn't, the exchange happens 24 minutes and 26 seconds into the video.

Transcript:
-NTD: SPE, NTD
-SPE: SPE, go ahead
-NTD: What caused the abort?
-I'll let CSME(LCC Firing Room console position in charge of the SSMEs) brief us but the only indication we got was Engine 3 limit exceeded but I need further from SME but I understand Oxidizer Pre-Burner pressure low
-NTD: CSME
-CSME: This is CSME. We got an indication that a we had a down limit parameter exceeded on Oxidizer Pre-Burner purge pressure and I think maybe data showed it spiked at 111 PSI, the allowable is 50. And it checked from start to start+2.28 seconds. And it looks like we had a failure of a check-valve.
NTD: And I copy.

 

[GLS]

down limit parameter exceeded on Oxidizer Pre-Burner purge pressure

This is the FID.
I just checked and here it is: FID 013-414, OPB purge pressure redline exceeded
For sure when the number pops up in the screen, the corresponding text is also shown, otherwise the controllers would have to memorize hundreds of these things.

Press release: https://www.nasa.gov/press-release/...-rocket-core-stage-for-artemis-i-moon-mission
The team successfully completed the countdown and ignited the engines, but the engines shut down a little more than one minute into the hot fire. Teams are assessing the data to determine what caused the early shutdown, and will determine a path forward.

Again, all broad strokes and no details at all. It looks more and more like they are covering the a** of a certain aerospace giant at all costs.

Anyway, at least they posted some stills:

 

[Urwumpe]

How do you know the gimballing didn't happen?

Because the engines did not move. Not even slightly. There was also no slow motion of the engines AFTER shutdown, which would happen, if the engines had been slightly displaced from vertical.

If they would have moved the engine by merely 1°, you would have noticed it visually easily, those are large rocket engines and the gimbal is far away from the camera plane.

 

[GLS]

Because the engines did not move. Not even slightly. There was also no slow motion of the engines AFTER shutdown, which would happen, if the engines had been slightly displaced from vertical.

If they would have moved the engine by merely 1°, you would have noticed it visually easily, those are large rocket engines and the gimbal is far away from the camera plane.

The first video where the engines are visible after 60s (gimbal start time) is when they are already shutting down. Maybe they didn't gimbal... I have no way to tell.

If there is an hyd failure, then the engines stay where they are. If hyd pressure drops below what the actuator needs, the actuator lock valve closes and "locks" the hyd fluid in the piston, so it doesn't move (it will eventually drift, but not visually). This assumes the design didn't change from what was used in the Shuttle.

Yes, even a small nudge is very visible (actually, I saw the small motion during initial command to 0º during the final minutes of the countdown).

 

[N_Molson]

Teams are assessing the data to determine what caused the early shutdown, and will determine a path forward.

Seems already more reasonable than the "Cool now let's ship everything to KSC" that that PAO served us yesterday...

 

[GLS]

Just watched the test again and here is more wood to add to the fire.
The clock in the top right corner of the facility cameras starts counting at engine ignition command and stops at 67.7 seconds. This puts the cutoff +/- at T+60s (judging from the H2 burnoff igniters starting at T-12s and also engines going to 109% at T0 right before the camera view changes).
A TVC issue is still on the table, but given how close the cutoff is to the engines throttling down to 95% (right before the TVC test), an engine issue cannot be discarded. The first thing that jumps to mind is an hydraulic lockup in an engine. This would not be a problem during steady state operation, but for throttling it needs hyd pressure to move the valves, so at that point the controller would go into "hydraulic lockup mode". This doesn't shutdown the engine by itself (it happened on STS-3), but as this a test, and in the interest of safety, it is possible that it would mean the end of it. This fits with what was said yesterday, that "an engine sent a signal to the core to shutdown".
Interesting how it all comes back to hydraulics...

After shutdown the test controllers are heard saying that all 4 engines are in "post-shutdown standby" (the regular post firing controller mode) so they obviously have telemetry from the engine controllers, thus discarding a full blackout of power in them or a data path issue. A pneumatic shutdown (which is how the engines are killed without hyd pressure) also ends up in post-shutdown standby, so no conclusions on this can be taken.

Post firing, it is visible that the white covers at the top of the nozzles are ripped, especially in the area between the engines.

 

[DaveS]

Post firing, it is visible that the white covers at the top of the nozzles are ripped, especially in the area between the engines.

I just went back a took a look at the post-cut off views and all the thermal insulation still look good. Even the aluminum tape that covers the conical base of the Engine Section, looks intact.

 

[GLS]

I just went back a took a look at the post-cut off views and all the thermal insulation still look good. Even the aluminum tape that covers the conical base of the Engine Section, looks intact.

Another camera view shows that other engines have a similar cut to the one on the left.

 

[DaveS]

Another camera view shows that other engines have a similar cut to the one on the left.

That's the kind of damage they expect to have to fix even after a successful hot fire. They always budgeted a month for the refurbishment work before getting the stage of the test stand and loaded aboard the barge for the trip to KSC.

 

[Urwumpe]

That's the kind of damage they expect to have to fix even after a successful hot fire. They always budgeted a month for the refurbishment work before getting the stage of the test stand and loaded aboard the barge for the trip to KSC.

Yes - also that does not mean, that the damage is one that can occur in-flight or has any effects on the performance.

 

[kuddel]

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