Update from ULA - I'll write it up: Centennial, Colo., (April 29, 2016) -- ULA successfully delivered the OA-6 Cygnus spacecraft to its precise orbit as planned on March 22. During the launch, the system experienced a premature first stage shutdown. Atlas is a robust system. The Centaur upper stage compensated for the first stage anomaly, delivering Cygnus to a precise orbit, well within the required accuracy. The ULA engineering team has reviewed the data and has determined an anomaly with the RD-180 Mixture Ratio Control Valve (MRCV) assembly caused a reduction in fuel flow during the boost phase of the flight. In addition to analysis and testing, all RD-180 engines are being inspected. Last Friday, in preparation for the MUOS-5 launch, the Atlas V completed the Launch Vehicle on Stand (LVOS) operation, erecting the Atlas V into the Vertical Integration Facility at Cape Canaveral Air Force Station. LVOS will allow configuration of the vehicle to support RD-180 engine inspections and confirm all engine components are ready for launch. The Atlas V MUOS-5 launch is targeted for early summer; a new launch date has not been secured on the Eastern Range. The impact to the remainder of the Atlas V manifest is in review with new launch dates being coordinated with our customers. All missions manifested for 2016 are expected to be successfully executed by the end of the year, including OSIRIS-REx, which will remain in early September to support its critical science window.
I'm pretty sure ITAR greatly simplifies the communications for solving this sort of anomalies.
Centaur used, and maybe still uses, capacitance probes to detect propellant in its propellant utilization system. Such probes provide a continuous reading, rather than a point result. Early Atlas (the original Atlas) PU systems used an array of point sensors. I'm not sure what Atlas 5 CCB uses.http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680027035.pdfA mixture ratio valve is visible in the attached RD-180 schematic. Atlas "owns" the sensors, but all of the actual PU controls are part of the RD-180. - Ed Kyle
Quote from: edkyle99 on 04/11/2016 04:25 pmCentaur used, and maybe still uses, capacitance probes to detect propellant in its propellant utilization system. Such probes provide a continuous reading, rather than a point result. Early Atlas (the original Atlas) PU systems used an array of point sensors. I'm not sure what Atlas 5 CCB uses.http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680027035.pdfA mixture ratio valve is visible in the attached RD-180 schematic. Atlas "owns" the sensors, but all of the actual PU controls are part of the RD-180. - Ed KyleEd (or others) I stared at that diagram for a while (awesome diagram by the way, nice find) and I can't find the MRCV, is it the thing marked MR Control EHA
It becomes another ? as to whether this inspection would even be able to detect the problem that had occurred. For any assurance as to the capability of an inspection to catch the problem the confidence level of the root cause of the anomaly must be very high >80%.
Quote from: Lar on 04/29/2016 03:05 pmQuote from: edkyle99 on 04/11/2016 04:25 pmCentaur used, and maybe still uses, capacitance probes to detect propellant in its propellant utilization system. Such probes provide a continuous reading, rather than a point result. Early Atlas (the original Atlas) PU systems used an array of point sensors. I'm not sure what Atlas 5 CCB uses.http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680027035.pdfA mixture ratio valve is visible in the attached RD-180 schematic. Atlas "owns" the sensors, but all of the actual PU controls are part of the RD-180. - Ed KyleEd (or others) I stared at that diagram for a while (awesome diagram by the way, nice find) and I can't find the MRCV, is it the thing marked MR Control EHA Yes, mixture ratio electro-hydraulic actuator (EHA) valve. See Section A. Engine Heritage and Description:http://www.ulalaunch.com/uploads/docs/Published_Papers/Supporting_Technologies/Incorporation_of_RD-180_Failure_Response_Features_in_the_Atlas_V_Booster_Emergency_Detection_System_2011.pdfQuoteIt becomes another ? as to whether this inspection would even be able to detect the problem that had occurred. For any assurance as to the capability of an inspection to catch the problem the confidence level of the root cause of the anomaly must be very high >80%.According to the paper above, the mixture ratio control electro-hydraulic valve is part of a closed hydraulic system that is exercised during hot fire acceptance test and expected to be relatively immune from contamination after the test firing since the hydraulic system is closed. So the valve would presumably have been tested at the component level, then tested again during the hot fire, presumably successfully both times. The paper says that the hot fire runs through mixture ratio extremes, so the valve would have been run through a range of operational settings.So it sounds like a rigorous QA process in which the suspect valve would have been well-exercised before flight.
Maybe it's just me, but this seems like good news rather than bad. They moved from "we aren't sure" to "we know what it was" and they announced they are going to contain all of the 2016 launches. When's the last time SpaceX made a committment like that?
I know its a press release, but this isn't much of a description of a root cause. I hope we get more info.
Where's the bad news?
Exactly. I don't understand how this can be viewed as particularly good news. The narrowing-down of the fault to the engine also comes at a bad time politically as it might give more ammo to certain politicians who want to dump the RD-180 as soon as yesterday. "See, the engine is unreliable anyway!"