Atlantis’ ATVC replacement adds strain to launch date target

by Chris Bergin

While engineers continue to move forward with preparations for the installation of replacement external LH2 Feed-through connector hardware on ET-125, another issue may overlap the tank repair schedule.

The problem relates to the ATVC (Ascent Thrust Vector Control) – specific to the commanding of the Right Solid Rocket Booster (SRB) Rock Actuator. The unit will be replaced over the weekend, although documentation shows Sunday as the deadline for replacing the unit, in order to make the February 2 launch date target for STS-122.

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Each SRB has two hydraulic gimbal servoactuators: one for rock and one for tilt. The servoactuators provide the force and control to gimbal the nozzle for thrust vector control.

The space shuttle ascent thrust vector control (ATVC) portion of the flight control system directs the thrust of the three SSMEs and the two SRB nozzles to control attitude and trajectory during lift-off and ascent.

Commands from the guidance system are transmitted to the ATVC drivers, which transmit signals proportional to the commands to each servoactuator of the main engines and SRBs.

Four independent flight control system channels and four ATVC channels control six main engine and four SRB ATVC drivers, with each driver controlling one hydraulic port on each main and SRB servoactuator.

The issue engineers are tasked with troubleshooting was originally found after the tanking test and subsequent ECO (Engine Cut Off) sensor troubleshooting, when the ‘current monitor signal on ATVC 2, Right Rock Driver, exhibited erratic behavior when commanded to +4 degrees for approximately 20 minutes after power up’ – resulting in an Interim Problem Report (IPR). The issue did not recur after several hours of operation.

‘Ambiguous whether the problem is the current monitor signal, the FA MDM, or the driver output from the ATVC being reflected in the monitor,’ noted a report compiled on Friday. ‘Wiring faults cannot be ruled out.’

Troubleshoot Plan: 

Since the issue was spotted, engineers have struggled to isolate the fault, leading to the problem becoming an undesired Unexplained Anomaly (UA).

‘ATVC was instrumented per previous plan in an attempt to isolate the fault. Fault did not repeat. Further attempts to date have been unsuccessful,’ added the presentation – although it has been rounded down to five specific areas, which includes wiring.

‘With no repetition of the fault, and little time to troubleshoot, the situation is a UA. Traditional UA practice would R&R (Remove and Replace) the ATVC and any other affected components (i.e., wiring) to assure that no parts remain that could have caused the fault.’

However, to support a February 2nd launch date – the official NET (No Earlier Than) target, according to USA, NASA and Lockheed Martin schedules in relation to the completion of ET-125 troubleshooting – the replacement of the ATVC R&R work would need to be completed by Sunday.

‘In order to support a 2/2 launch date, this activity would need to be complete by Sunday 1/6 in order to provide time for hydraulics – on retest and nominal completion of Hydraulics close-out while ECO sensor wiring operations are underway,’ added the presentation.

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This option was one of three presented to shuttle managers on Friday, with R&R holding its own problems, mainly due to time constriants.

‘Time constrained – no opportunities to complete more intensive inspection or testing that might reveal more about the nature of the fault. No way to verify problem is solved unless it can be replicated at the depot – which will be time consuming without further characterization of the problem and relatively easy repetition,’ noted the ‘cons’ on option 1.

‘Risk buy-down is negligible if anomaly does not repeat more frequently or after the unit has been powered up nominally for launch.’

Option 2 – which involved more extensive troubleshooting at the pad – would have led to an additional delay to the target launch date for STS-122, by up to 10 days (from Feb 2). Option 3 was to fly ‘as-is’ – although this was shot down by dissenting opinion.

A split opinion on flying ‘as-is’ – as opposed to the other available options – can be understood via associated documentation, which shows the system has redundancy and does not breach LCC (Launch Commit Criteria) rules.

‘Flight controls are quad redundant and two fault tolerant. Design philosophy oriented towards compensating for random failures in avionics and wiring (not effective for rapid sequential, simultaneous, or common-mode failures).

‘Right Rock signal required for 120 seconds after SRB ignition; ATVC not flight critical after MECO. Response of the system is identical for any random part failure (which can occur at any time), or repetition of a known-but-rare anomaly.
‘No LCC violations are associated with this fault unless the secondary delta pressure exceeds the LCC limit of around 237 psid after the slew test (BTVC-11) or there is a secondary delta pressure exceedance of 2200 psid that causes a channel to be bypassed (BTVC-12) at any time between T-21 and T-10 seconds.

‘Neither of these faults are likely with the observed anomaly – the command is  around 1 degree during the slew test and zero thereafter. There is no indication that the fault causes erratic output without a command input.

‘If problem is thermal-related, ATVC will have been on for at least 6 hours before T-0 allowing time to reach thermal equilibrium.’

A spare ATVC unit is already at the pad, allowing engineers to carrying out the R&R without delay.

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