STS-133: Launch delayed at least 24 hours due to Main Engine Controller issue
Discovery has been given another hurdle to negotiate ahead of her opening launch attempt, after an electrical issue was noted on the redundant Main Engine Controller (MEC) on SSME-3. The original problem had been classed as cleared, following cycling and troubleshooting. However, a Mission Management Team (MMT) meeting later on Tuesday decided to scrub for at least 24 hours to work towards flight rationale.
STS-133 Pad Flow:
STS-133 had remained on track for the retraction of the Rotating Service Structure (RSS) at 7pm on Tuesday evening (local time), following the completion of tasks associated with the loading of the Power Reactant Storage and Distribution (PRSD) system.
“Yesterday’s activities centered on the preparations for and then the loading of the PRSD on-board commodities. Both LO2 and LH2 servicing were completed without issue,” noted Tuesday’s NASA Test Director (NTD) update (L2).
“The pad was re-opened for controlled work at 20:04 (Monday) and the team proceeded with the planned LO2 offload which wrapped up at 00:44 (Tuesday). OMBUU (Orbiter Mid Body Umbilical Unit) demate completed at 01:22 (Tuesday).”
As per S0007 (Launch Countdown) tasks, engineers worked through preparations for the loading of LH2 and LO2 (LOX) into Discovery’s External Tank on Tuesday morning, which included inspections of the LO2 feedline. Tanking was scheduled to begin at 6:27am local time on Wednesday ahead of the delay.
“Flight Crew Systems Stow will pick up. Activation and checkout of the on-board and ground communication system is scheduled for Monday afternoon,” added the NTD report. “Preparations for the Rotating Service Structure (RSS) move will be in work today with the RSS coming back to the park position this evening. First motion is planned for 1900 hrs.”
As expected, an Interim Problem Report (IPR-54) – relating to a missing digit on a timer display – was cleared as no constraint to launch, after it was discussed with STS-133 Commander Steve Lindsey.
“IPR 54: Picked up IPR against Forward Event Timer, 10th digit top left is out. Troubleshooting (T/S) plan is to look at Fault Light documentation. Constraint is S0007.200 15-507. T/S reveals Event Timer operating nominally with one segment out. MR (Material Review) to fly as-is, the test team is in concurrence,” the NTD listed on Tuesday.
The S0007 flow was also two hours down early on Tuesday, due to an outage on the pad’s Gaseous Nitrogen system – requiring the replacement of a couple of valves. Once complete, the flow soon returned back on to the nominal timeline.
“OV-103 / SRB BI-144 / RSRM 112 / ET-137 (Pad-A): S0007 operations continue. We are approximately two hours behind the scheduled timeline as a result of troubleshooting on IPR-0055: GN2 outage is complete. Both Regulators and check valves have been removed and replaced. The GN2 system activation is in work.”
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IPR’s 56 and 57 were also charged to Discovery’s pad flow on the latest report, both of which were closed.
“IPR 56: During sampling the PRSD H2 backup sample melon inlet valve was closed and should have been open. The sample performed on the primary melon was good and the IPR will be closed as human error,” added the NTD report. “IPR 57: The ET pre-press anti ice heater did not come on when activated. Troubleshooting in MLP (Mobile Launch Platform) compartment 33B revealed a loose wire. The wire was tightened and the heater is performing nominally.
Tuesday’s flow also saw engineers and technicians move into SSME (Space Shuttle Main Engine) avionics and pneumatics checkout tasks, although they were put on hold due to the latest problem, IPR-58 – an issue relating to checks on the three redundant Main Engine Controllers (MEC).
“IPR 58 to CCME (SSME Controller Engineer Console): While performing the bus redundancy test on the main engine controllers, CCME reported Engine 1 and 2 performed nominally, however, Engine #3 redundant controller did not activate,” the NTD outlined.
“CCME was expecting approximately 0.5 amp increase on all three phases. Data retrievals showed a very small current spike on Phase A and C, but nothing on Phase B. The controller has a built in ‘safe’ mode that when a phase is missing, it will shut itself down.”
The SSME controllers provides complete and continuous monitoring and control of engine operation. In addition, it performs maintenance and start preparation checks, and collects data for historical and maintenance purposes. The controller is an electronic package that contains five major sections; power supply section, input electronics section, output electronics sections, computer interface section, and digital computer unit.
Pressure, temperature, pump speed, flowrate, and position sensors supply the input signals. Output signals operate spark igniters, solenoid valves, and hydraulic actuators. The controller is dual redundant, which give it normal, fail-operate, and fail-safe operational mode capability. The problem was specific to the redundant controller on ME-3.
“The CCME decided to cycle the Phase B circuit breaker to determine if there was any contamination in the breaker that could cause this problem,” the NTD report continued. “Prior to starting the troubleshooting CCME noticed the redundant controller had been activated without any action by engineering or the SCO (Spacecraft Operator) in the crew module.
“Data retrievals showed the switch scan for the controller cycled on/off on its own, the same signature on Phase A and Phase C were noticed and then shortly after that Phase B came up which ultimately activated the redundant controller.”
An Engineering Review Board (ERB) met early in the morning to discuss the root cause and a troubleshooting plan to verify the function of the Controller.
“Troubleshooting plan: Send a command which will verify the function of the Controller. Power down controller,” noted troubleshooting information (L2). “Cycle CB (Circuit Breaker) five times. Bring up Controller and verify function. Power down Controller. Cycle Switch five times and verify Controller.”
NTD Steven Payne also noted during Tuesday’s media briefing that once switches on the flight deck had cycled the controller, its condition – believed to be related to a transient contamination – had returned to normal.
Mr Payne also noted that only the associated paperwork was outstanding to clear the IPR – which was classed as a constraint to the MEC power up/avionics checkout portion of S0007 (scheduled for the early hours of Wednesday) – from the flow.
The latest entry on KSC logs was at 12:45 on Tuesday, showing no signs the issue would continue to be a problem. “12:45 CCME worked troubleshooting plan which consisted of cycling circuit breakers multiple times. E-3 Controller operation resumed and SSME Final Preps continued. Issue to be further evaluated at ERB.”
However, NASA then announced managers would meet at a 5pm MMT to discuss the data gained throughout the day on the redundant controller, suggesting confidence in the controller’s ability to support the launch might still be in question. The MMT decided to delay the launch.
“The MMT has decided to delay the launch to NET (No Earlier Than) Thursday, November 3,” noted a memo (L2). ” This delay to provide more time to understand IPR 133V-0058 and build flight rationale. Pre-launch activity times for a Thursday launch attempt will follow later.”
Historical records show a MEC IPR associated with STS-90, which was initiated on 3-31-98 and closed on 4-11-98, after a failed electrical circuit breaker was restored on Columbia’s Main Engine 2. An engine controller was also replaced back on STS-43, taking a week to complete at the pad.
UPDATE: Panel L-4 circuit breakers were cycled 5 times overnight and MECs were powered up with no anomalies documented. ERB has met. MMT meeting now taking place.
(Updates will be added to this article. Refer to the updates thread linked above for STS-133 and L2’s live internal updates on the L2 STS-133 Section).
(Images: Lead – Larry Sullivan, MaxQ/NASASpaceflight.com. Photos via L2 documentation, including the SSME ‘bibles’)