STS-131: SRB flight rationale gained following gyro test failure

by Chris Bergin

With Discovery’s payload now installed into the orbiter’s Payload Bay, all eyes are on Friday’s Agency Flight Readiness Review (FRR). Several items of interest are under review, all of which have flight rationale decisions to allow STS-131 to launch on April 5. One item of interest with the Solid Rocket Boosters (SRBs) already has flight rationale in place, following a test failure on a Rate Gyro Assembly (RGA).

STS-131 Processing Latest:

Discovery remains in a nominal flow towards the April 5 launch date target, as processing milestones continue to be reached with a few days of contingency still in the team’s back pocket. The latest milestone came via the installation of the STS-131 payload.

“OV-103 / ET-135 / SRB BI-142 / RSRM 110 (Pad A): Payload installation into the orbiter was successfully completed. Electrical connections (in work). Payload Integrated Vehicle Testing operations are planned for Friday,” noted the NASA Test Director (NTD) processing latest (L2).

“S1287 Aft compartment closeout for flight continues with completion planned Friday. SCAPE (Self-Contained Atmospheric Protection Ensemble) operations to drain the liquid separator were completed Wednesday. Two waves of LH2 tankers will replenish the LH2 sphere.”

A small Problem Report (PR) was charged to the processing flow, and resolved by the time of the latest processing report, although a second cracked tile has been found during inspections of Discovery’s ceramic inserts – which is undergoing engineering evaluations.

“PR 5547 was taken for a crushed convoluted tubing adjacent to connector in the aft compartment. Repair and retest was completed. An additional tile was discovered cracked on window 6. Engineering evaluation is in work,” added the report, noting the ceramic insert inspections are continuing on other areas of the orbiter, for the purpose of flight rationale.

“Payload Bay Door hinge line ceramic insert engineering evaluation and flight rational development continues.”

An outline on the FRR effort relating to the inserts will be provided on Friday, along with the status of the other items of discussion.

STS-131 Specific Articles:

SRB RGA Item of FRR interest:

Each SRB contains two of the RGAs, with each RGA containing one pitch and one yaw gyro. These provide an output proportional to angular rates about the pitch and yaw axes to the orbiter computers and guidance, navigation and control system during first stage ascent flight in conjunction with the orbiter roll rate gyros until SRB separation.

At SRB separation, a switchover is made from the SRB RGAs to the orbiter RGAs.The SRB RGA rates pass through the orbiter flight aft multiplexers/ demultiplexers to the orbiter GPCs (General Purpose Computers). The RGA rates are then mid-value selected in redundancy management to provide SRB pitch and yaw rates to the user software. The RGAs are designed for 20 missions.

No issues have be reported with the RGAs for several years, until a test failure this month led to an effort to clear the related units for the flight of Discovery’s boosters.

“Observation: SRGA s/n 119 failed Yaw Drift Insensitivity (DI) during pre-installation checkout at the Assembly and Refurbishment Facility (ARF). SRGA s/n 119 failed Pitch Null Offset after passing incoming functional test procedure,” noted a presentation (L2) reviewed by Tuesday’s Space Program Requirements Control Board (PRCB) meeting.

“Concern: Similar failure during (STS-131) ascent could cause erroneous output data from an SRGA.

“Background: SRGA s/n 119 failed Yaw (DI) during 6-position test at ARF on March 1-3, 2010. 1st and 3rd test runs failed, 2nd run passed. s/n 119 failed Pitch Null Offset was observed following incoming functional test procedure. s/n 119 last flew on STS-126, November 14, 2008 (without issue).

“Original design of SRGAs experienced failures of cracked pivots, resulting in a subsequent change of damping fluid (from silicone oil to Krytox oil) and gyroscope materials. SRGA design uses newer ‘Krytox’ gyro, replacing older ‘silicone’ gyro design to reduce vulnerability to shock load induced damage to pivots.

“The PRCB restricted silicone SRGA usage to single flights in June 1987. Krytox design modification development study was authorized in May 1990. Krytox gyros were phased in between STS-70 7/13/95 (1 Krytox gyro in slot 4) and STS-69 9/7/95 (Krytox gyros in all 4 slots).”

Flight history and risk assessment – along with the redundancy in the system – all played a part in aiding the path towards gaining flight rationale for STS-131, while engineers will spend additional team carrying out further checks for downsteam missions in an effort to understand root cause and any potential foward work.

The option to replace the unit(s) at the pad was available, although a there was a slight risk this would push out the launch date by a number of days. This would have been an unlikely option to take, given the confirmed health of Discovery’s SRB RGAs.

“SRGAs with Krytox gyros were certified by test for 24 pyro shocks. No Krytox Gyro pivot/bearing failures since 1995 upgrade. Only one previous Krytox gyro failure of 6-position test, attributed to FOD in bearing. Four SRGAs for redundancy,” noted the risk assessment portion of one of the three main presentations on the problem.

“Criticality 1R/2 for erroneous output; Criticality 1R/3 for loss of output. Hazard: B-00-17, Loss of Vehicle Control. Controlled; Catastrophic, Improbable. SRGAs are tested every flow.

“Schedule Risk: SRGA can be R&R’ed at Pad, requires several shifts. Severity is slight schedule slip (2). Likelihood of failure is Unlikely (2). Mission Safety: Severity is Catastrophic (5). Likelihood is Highly Unlikely (1). Requires multiple failures.”

Given the unit which suffered the problem is not part of the STS-131 stack, along with a good track record for the units that will fly with Discovery’s boosters, flight rationale is already in place ahead of Friday’s Agency FRR.

“Flight rationale for STS-131: All STS-131 SRGAs passed functional test prior to installation into forward skirt (required within 120 days of installation). Verifies flightworthiness between flights. 6-position test is used to test for pivot failures. Shock loads (contribute to pivot failures) do not occur between 6-position test and launch,” the presentation noted.

“STS-131 SRGA ARF test data nominal. SRGA tested during ARF forward skirt Assembly Checkout (ACO), Vehicle Assembly Building (VAB) Shuttle Interface Test (S0008), Terminal Countdown Demonstration Test (S0017) and prelaunch checkout (S0007) at T-3 hours.

“SRGA system is redundant by design. Criticality 1R/2 for erroneous output. Fault Detection Isolation and Recovery (FDIR) deselects first failed SRGA during ascent. Interchangeable Mid-Value Select (IMVS) selection filter rejects failed signals.”

As noted by the presentation, the RGAs will recieve a final health check at T-3 hours in the countdown, although it is highly unlikely they would become a problem given their track record.

L2 members : Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4500 gbs in size

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