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/d1d4gv4QuoteAtlas 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.
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.
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?
The loss of ISP would need to be over a longer period than just 30 seconds as well.
FWIW, "minor mixture ratio change" was called out at around T+70 seconds.
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.
(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.) 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.
Quote from: oldAtlas_Eguy on 03/28/2016 06:06 pmI 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.Quote from: oldAtlas_Eguy on 03/28/2016 06:06 pmThe 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.
Quote from: ugordan on 03/28/2016 06:49 pmFWIW, "minor mixture ratio change" was called out at around T+70 seconds.That's just a normal callout.
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
Quote from: jongoff on 03/28/2016 07:10 pmIf 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?
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.
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?