Author Topic: OA-6 Atlas V anomaly discussion and updates  (Read 101642 times)

Offline oldAtlas_Eguy

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OA-6 Atlas V anomaly discussion and updates
« on: 03/28/2016 01:57 am »
This discussion thread is to provide a consolidated discussion and updates location of the OA-6 anomaly investigation and its implications for future Atlas V flights.

Associated source articles:

https://www.nasaspaceflight.com/2016/03/cygnus-iss-arrival-ula-investigate-atlas-v-launch/

http://www.floridatoday.com/story/tech/science/space/2016/03/24/ula-reviewing-early-shutdown-atlas-v-engine/82205806/
A reddit comment from Tory Bruno about the anomaly:

https://www.reddit.com/r/ula/comments/4bumi1/ula_confirms_engine_issue_on_latest_atlas_launch/d1d4gv4
Quote
Atlas first stage burned just under 6 seconds short. That left a delta-V deficit that Centaur needed to make up. Centaur made up that difference by burning about a minute long, delivering Cygnus into a precise orbit, far better that the accuracy requirement (as is usual). However, this consumed propellant, so Centaur's planned de-orbit burn was shorter. Consequently, Centaur impacted in the ocean a little further downrange than the original plan.

While this highlights the robustness of the system, we don't like anything happening that was not expected and are investigating this thoroughly. That's one of the reasons we have the mission success record we enjoy: Thoroughness. Every bit of data and every feature gets ground to dust by our engineers after every flight. (which is paid for on USAF missions by their ELC contract, BTW...)

In this case, because we consider it an anomaly, a very specific and structured set of investigation protocols is followed with a dedicated team.

We have a pretty good idea about the cause, but won't share until we are sure. That is another important discipline. We avoid jumping to the answer until the investigation is complete. That prevents us from narrowing in on a tempting answer too quickly before all the possibilities have been thoroughly investigated. By approaching investigations in this disciplined manner, you have the highest confidence of arriving at a complete, global, and permanent corrective action.

Over the years, I've found that its poor practice and bad luck to jump to the easy answer before doing the homework.

At present, I am not concerned about impacts to the manifest.

http://spacenews.com/ula-confirms-engine-issue-on-latest-atlas-launch/

http://forum.nasaspaceflight.com/index.php?topic=38233.msg1508007#msg1508007

http://spaceflightnow.com/2016/03/24/atlas-5-rocket-forced-to-improvise-during-tuesdays-climb-to-orbit/

http://spaceflight101.com/cygnus-oa6/potential-performance-hit-suffered-by-atlas-v-a-closer-look-at-the-data/

OA-6 Live event thread reference: http://forum.nasaspaceflight.com/index.php?topic=38233.0 contains the initial discussions about the anomaly.
« Last Edit: 03/28/2016 07:04 pm by Lar »

Offline Kim Keller

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #1 on: 03/28/2016 04:34 pm »
A brief article in Florida Today mentions the AV-064 booster burning LO2 at too high a rate. That implies that something wasn't working right in the mixture ratio control system. Take a look at the on-booster video of first stage flight and you'll see the contrail undergo a sudden change in color which seems to indicate when the mixture changed. This happened about 30 seconds before BECO, so to me that suggests that the RD-180 was not making proper thrust for that period, adding to the performance shortfall. My interpretation of the news report is that the booster burned to LO2 depletion.

http://www.floridatoday.com/story/tech/science/space/2016/03/25/engine-issue-delays-ulas-next-atlas-v-launch/82259686/

Offline oldAtlas_Eguy

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #2 on: 03/28/2016 06:06 pm »
A brief article in Florida Today mentions the AV-064 booster burning LO2 at too high a rate. That implies that something wasn't working right in the mixture ratio control system. Take a look at the on-booster video of first stage flight and you'll see the contrail undergo a sudden change in color which seems to indicate when the mixture changed. This happened about 30 seconds before BECO, so to me that suggests that the RD-180 was not making proper thrust for that period, adding to the performance shortfall. My interpretation of the news report is that the booster burned to LO2 depletion.

http://www.floridatoday.com/story/tech/science/space/2016/03/25/engine-issue-delays-ulas-next-atlas-v-launch/82259686/

I wonder were they got the information that the problem was associated with an mixture ratio error? But just burning to depletion of the LOX would not account for the 77s of increased Centaur burn time it also had a major effect on ISP and with the engine being throttled for max G of 3.5 Gs. a lower ISP means lower thrust and a higher throttle position so that even more prop (and LOX) was being consumed. Also just a 5.5s early shutdown would not account for the DV loss by itself. The loss of ISP would need to be over a longer period than just 30 seconds as well. A 2% ISP loss occuring for 200s combined with burning LOX faster than normal would cause the early shuttdown mainly due to the G level throttling instead of a throttle % level setting.

Offline baldusi

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #3 on: 03/28/2016 06:39 pm »
Higher oxidizer mixture would have also meant higher Tc. Given how extreme is that engine, it would have been quite an issue. Particles on the fuel filter, on the other hand, might have reduced the input and the controller might have tried to keep thrust by increasing LOX within limits. It is very difficult to find failures that don't end up in a very rapid fireball with the RD-180.

Offline ugordan

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #4 on: 03/28/2016 06:49 pm »
I wonder were they got the information that the problem was associated with an mixture ratio error?

Let's just say it's not the first time LOX depletion was mentioned.

The loss of ISP would need to be over a longer period than just 30 seconds as well.

My gut feeling is the same, but it's just that - a gut feeling. FWIW, "minor mixture ratio change" was called out at around T+70 seconds.

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #5 on: 03/28/2016 07:08 pm »
FWIW, "minor mixture ratio change" was called out at around T+70 seconds.
That's just a normal callout.

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #6 on: 03/28/2016 07:10 pm »
If it really were consuming LOX faster, then there's a chance that the delta-V shortage was more than you'd assume from just a sudden shutdown 5-6s earlier. The engine thrust and Isp would both be decreased if the mixture ratio control was off, and you'd be staging with a lot of residual Kerosene. Potentially. I agree with Tory though that there's far too little data (especially publicly available data) to say anything with any certainty yet.

~Jon

Offline Lee Jay

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #7 on: 03/28/2016 07:14 pm »
If it really were consuming LOX faster, then there's a chance that the delta-V shortage was more than you'd assume from just a sudden shutdown 5-6s earlier.

Even if they were in closed-loop 3.5g throttling mode?

Offline Space Ghost 1962

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #8 on: 03/28/2016 07:48 pm »
A brief article in Florida Today mentions the AV-064 booster burning LO2 at too high a rate. That implies that something wasn't working right in the mixture ratio control system. Take a look at the on-booster video of first stage flight and you'll see the contrail undergo a sudden change in color which seems to indicate when the mixture changed. This happened about 30 seconds before BECO, so to me that suggests that the RD-180 was not making proper thrust for that period, adding to the performance shortfall. My interpretation of the news report is that the booster burned to LO2 depletion.

http://www.floridatoday.com/story/tech/science/space/2016/03/25/engine-issue-delays-ulas-next-atlas-v-launch/82259686/

Or where something "broke" or eroded. The color is helpful. My interpretation is slightly different.
(1.) I wonder were they got the information that the problem was associated with an mixture ratio error? But just burning to depletion of the LOX would not account for the 77s of increased Centaur burn time it also had a major effect on ISP and with the engine being throttled for max G of 3.5 Gs. a lower ISP means lower thrust and a higher throttle position so that even more prop (and LOX) was being consumed. Also just a 5.5s early shutdown would not account for the DV loss by itself. The loss of ISP would need to be over a longer period than just 30 seconds as well. (2.) A 2% ISP loss occuring for 200s combined with burning LOX faster than normal would cause the early shuttdown mainly due to the G level throttling instead of a throttle % level setting.
1. Me too.
2. As if throttle level impacts anomaly - perhaps look at contrail change more closely?
(1.) Higher oxidizer mixture would have also meant higher Tc. Given how extreme is that engine, it would have been quite an issue. Particles on the fuel filter, on the other hand, might have reduced the input and the controller might have tried to keep thrust by increasing LOX within limits. (2.) It is very difficult to find failures that don't end up in a very rapid fireball with the RD-180.
1.Agree.
2. Its a thoroughbred where it seems that every molecule seems to count.
I wonder were they got the information that the problem was associated with an mixture ratio error?

(1.) Let's just say it's not the first time LOX depletion was mentioned.

The loss of ISP would need to be over a longer period than just 30 seconds as well.

My gut feeling is the same, but it's just that - a gut feeling.
(2.) FWIW, "minor mixture ratio change" was called out at around T+70 seconds.
1. Indeed. This is not an insignificant issue.
2. Thank you, noticed it. Consider acceleration profile against throttling - less structural loading then 5.5G case, some loss in iSP per profile but possibly leveraged by feedback estimation out of box?

Playing the "armchair guessing game" is bounded here by absence of telemetry readouts.

My guess is they have a good idea already, but there's a lot of sweat to prove it by convergence of factors coinciding on a single root cause, and then following it back to prior evidence.

The margins tell you the strengths/weaknesses of this propulsion system. Since it is very mature, one takes this very seriously as you don't expect such surprises late in a program, especially this closely examined.

And flight frequency is "good enough" to cover necessary needs.

Thought this would be an "easy but proving" mission to launch. Seems to have an added bonus for AF & ULA without cost excepting sweat.

FWIW, "minor mixture ratio change" was called out at around T+70 seconds.
That's just a normal callout.
In all but the context of a significant launch anomaly...
If it really were consuming LOX faster, then there's a chance that the delta-V shortage was more than you'd assume from just a sudden shutdown 5-6s earlier. The engine thrust and Isp would both be decreased if the mixture ratio control was off, and you'd be staging with a lot of residual Kerosene. Potentially. I agree with Tory though that there's far too little data (especially publicly available data) to say anything with any certainty yet.

~Jon
Yes but consider other sources of "propellant" e.g. erosion. All sorts of friction sources robbing iSP not thrust.
If it really were consuming LOX faster, then there's a chance that the delta-V shortage was more than you'd assume from just a sudden shutdown 5-6s earlier.

Even if they were in closed-loop 3.5g throttling mode?
Yes - I'd like to know if throttling was coupled to the shortfall. Mechanism or control feedback.

Offline russianhalo117

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #9 on: 03/28/2016 09:18 pm »
Higher oxidizer mixture would have also meant higher Tc. Given how extreme is that engine, it would have been quite an issue. Particles on the fuel filter, on the other hand, might have reduced the input and the controller might have tried to keep thrust by increasing LOX within limits. It is very difficult to find failures that don't end up in a very rapid fireball with the RD-180.
I wouldn't single out the RD-180. I think you meant the RD-170 Family derivatives and almost all Russian Kerolox engines overall because the pretty much are all based on their predecessors and all relate back to a single engine family during post WWII development.

Offline kevin-rf

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #10 on: 03/28/2016 09:31 pm »
I wouldn't single out the RD-180. I think you meant the RD-170 Family derivatives and almost all Russian Kerolox engines overall because the pretty much are all based on their predecessors and all relate back to a single engine family during post WWII development.
I think you mean the Russian Oxidizer Rich Staged Combustion engines that started with the NK-15... I do not think the earlier engine families where nearly as extreme.
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Offline Newton_V

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #11 on: 03/28/2016 09:49 pm »
Almost all flights are LO2 rich based on tanking.  If it's very cold, you get a larger RP-1 load (weight) since that's based on the ambient temperature, and you get closer to a nominal ration of 2.72

When closed-loop PU, the vehicle will go to the higher MR stop for as long as it needs to.  The PU callout at 70 sec is a typical time for coming off that stop.  In fact, it came off that stop a little earlier, just took him awhile to actually state it.
« Last Edit: 03/28/2016 09:52 pm by Newton_V »

Offline ugordan

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #12 on: 03/28/2016 10:12 pm »
Take a look at the on-booster video of first stage flight and you'll see the contrail undergo a sudden change in color which seems to indicate when the mixture changed.

Thinking of the moment at 4:10 in this video?



You're on to something here (I guess that shouldn't really come as a surprise ;) )
Several amateur videos show the engine plume abruptly brightening at that point in time (30-ish seconds before BECO):

4:40 in this video:


4:07 in this one:


4:09 in this video:


I could believe camera focus change/clouds if it was just one video, but all 3 I've looked yet that caught that period show the same thing.

If the engine suddenly went LOX rich, one would expect less soot in the exhaust to mask the (even otherwise) bright chamber glow.

Offline Lee Jay

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #13 on: 03/28/2016 10:26 pm »
How the heck could it go LOX-rich enough so that 30 seconds of operation in that condition would cause a 6 second shortfall in LOX and NOT have a RUD while operating in that way LOX-rich condition?

Offline ugordan

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #14 on: 03/28/2016 10:48 pm »
Well, people did say it is an impressive engine...

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #16 on: 03/28/2016 11:46 pm »
Would it be enough for a valve to be frozen in position?  Say LXO is entering at full thrust rate and throttling is happening only through fuel rate modulation.  Could it be that simple or do you need more LOX flow than that to deplete that early?
« Last Edit: 03/28/2016 11:47 pm by fthomassy »
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Offline Kim Keller

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #17 on: 03/29/2016 12:17 am »
Thinking of the moment at 4:10 in this video?


Yep, that's it.

Offline jg

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #18 on: 03/29/2016 01:33 am »
LOX leak in the engine nozzle?

Offline WindnWar

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Re: OA-6 Atlas V anomaly discussion and updates
« Reply #19 on: 03/29/2016 06:02 am »
I can't believe the engine didn't fail under those conditions either, talk about cutting it close, 1.3 seconds of margin left in the first stage before you simply fail to achieve orbit.

I wonder how much margin was left in the engine itself by the time it shut down, lox rich has to be pretty brutal on the engine internals.

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