The all-powerful Program Requirements Control Board (PRCB) has decided to defer what was expected to be rationale to remove all “spring tabs” from the Rudder Speed Brakes (RSB) on the fleet until STS-119.
One of the tabs liberated from the aft of Discovery during the final part of STS-124’s mission, which had no mission impact. The fleet has since undergone an extensive inspection of their RSBs ahead of STS-125.
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RSB Status Post-124 Event:
During the Flight Control System (FCS) checkout on Flight Day 14 of STS-124, the crew spotted a small object floating away from the aft end of the vehicle – originally though to be ice.
The crew photographed the object and the ground was able to determine that the object was a small component of the Thermal Protection System (TPS) on the trailing edge of the rudder speedbrake known as a spring tab. Post-flight inspection showed that three of these tabs were missing.
This prompted the Orbiter Project to review the history of these spring tabs and to determine if design changes should be made, including the removal of all tabs from the orbiters.
The purpose of the rudder speedbrake thermal barriers and spring tabs are to protect the rudder speedbrake cavity from plume radiation during ascent and to control/maintain the cavity venting requirements during ascent. The tabs are attached to the barriers with three spot welds.
‘Fatigue due to ‘chatter’ during ascent is considered the primary cause for these welds to fail,’ noted outlines to the PRCB.
‘Because the rudder speedbrake is closed until FCS checkout, FCS checkout is the first opportunity for any spring tabs to liberate on orbit. Orbiter is evaluating a redesign to help preclude losses for STS-119 and subs.
Although only the one tab was observed to be floating away from Discovery, post flight inspections found she had lost three of the tabs.
‘Post landing inspection verified RSB condition nominal as predicted,’ noted an expansive 50 page presentation to the PRCB, available to download on L2. ‘Three missing spring tabs identified during quick look inspection.’
Tactile inspections have been performed on each of the three orbiters and discrepant parts – in order to find any problems with welds or the tabs themselves – prior to their repair and replacement.
‘Summary: OV-104 (Atlantis) has 13 discrepant parts – 1 spring tab missing. OV-105 (Endeavour) has 6 discrepant parts – 1 new spring tab replaced this flow on OV-105 was broken at weld. OV-103 (Discovery) has 13 discrepant parts – 3 spring tabs missing,’ added the presentation.
‘Tactile test conducted concurrently with post flight inspections. Discrepant parts are being replaced.’
Following the incident with Discovery, a parallel effort has been taking place on the forward plan, which continues to favor the removal of the tabs from the orbiter – thus removing the liberation threat.
However, computational models failed to give a complete picture of how the RSB would behave during ascent without the tabs. Thus more work is required on ensuring the removal of the tabs can be carried out with no ill effects to the orbiter’s hardware.
That data is set to be placed in front of a PRCB meeting in November to confirm the thermal impacts are within spec – which would allow for the removal of the tabs from STS-119, which Discovery is tasked with in February.
‘There is adequate flight rationale to fly with the current design since the tabs do not present a pad or ascent debris threat since the rudder speedbrake is closed preventing the tabs from releasing (though they could slip),’ summarized information on the forward plan.
‘Furthermore, no issues are expected since flight history has included multiple spring tab releases and there have been no signs of overtemp to components inside the speedbrake.
‘A new model is being generated with an ECD (Expected Completion Date) of November to identify thermal impacts of flying without the tabs in place.’
For the interim there is no problem with flying both STS-125 and STS-126 with the tabs in place, even if there are further liberation events.
The presentation evaluted all stages of flight for potential liberation events, notably during ascent when the orbiter’s RSB ‘burps’ under the pressures of the ride uphill.
‘Ascent: RSB is overdrive closed for ascent phase of the mission. Thermal clips form a cavity that would contain a loose tab – acoustic environment provides high cycle environment that could cause failures,’ explained the presentation.
‘Pressure differential between RSB panels causes slight deflections around 5 seconds and a small gap opening relieving the Delta P known as a ‘Burp’. Resultant gap between thermal barrier and adjacent panel is too small for tab liberation. Thermal assessment accounts for RSB cavity pressure ‘burps’ during ascent.
‘Orbit: Opening of the RSB can liberate loose tabs. Descent/Landing: Tabs found post landing – post heating air buffeting has potential to cause tab failure. More likely while the doors are in the open position.’
As always, flight rationale has been built specifically for Atlantis and Endeavour ahead of their respective flights that will have the tabs still in place. Such rationale is based around the results of liberations during ascent, on orbit and landing – and the recent inspections and repairs that have been carried out.
‘Increased inspection/part replacement mitigates potential for tabs to release. Tabs do not present Pad or Ascent debris threat,’ added the presentation. ‘Venting impacts will not affect flight or release Tab. Thermal impacts will not affect flight. Flight history has shown no signs of any over-temps.
‘Tabs (mainly) have potential for release during on-orbit FCS checkout or Descent/Landing (thus past the point they are used).
Should the November meeting decide against removing the tabs from the fleet, a second option will be taken – one which will redesign the tab to ensure it has less potential of liberating from the vehicle.
‘OPO (Orbiter Project Office) is currently evaluating redesign to help preclude losses (STS-119 and Subs).’
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