STS-134 Agency FRR approves April 29 launch – Storm damage evaluations

by Chris Bergin

Managers at the Agency Flight Readiness Review (FRR) have – as expected – approved April 29 as the launch date for STS-134. The meeting also discussed the health of Endeavour’s External Tank (ET-122), following a bout of severe storm weather at Pad 39A at the end of March – resulting in high wind loads on the stack, lightning events and minor hail damage, all of which has been cleared as no concern for flight.

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Endeavour already has her game face on at Pad 39A, with no Interim Problem Reports (IPRs) of note – at least from her own hardware perspective – for some time now.

The lack of issues has also aided what continues to be a nominal pad flow, which has seen the completion of ordnance installation and the beginning of preparations for the pressurization of the Main Propulsion System (MPS).

“OV-105/SRB BI-145/RSRM 113/ET-122 (Pad-A): LOX storage tank sampling was completed last Friday. ET Camera battery charging was also completed Friday. S0007, Launch countdown preps continue,” noted the NASA Test Director (NTD) report (L2).

“S5009, Final ordnance installation is complete. Range safety system testing was completed. Orbiter/SRB PIC tests and ignition S&A rotation are in work at the time of this writing. It is anticipated that the pad should be re-opened at approximately 0730 EST this morning. S0071 Hyper/MPS pressurization preps call to stations is scheduled for 1600 EST tomorrow. GO2 recharge is scheduled for Wednesday.”

Endeavour’s mission continues to be refined, with one recent memo outlining the current plan for the working mission timeline, one which holds the option of being extended past the 14 day flight into a maximum 16 day primary mission.

“To avoid any confusion, we have removed all what-if timelines, and have left only the official 14+2+2 Flight Plan for you to peruse,” noted one MOD memo (L2). “Once the calm settles in, we will work to re-post the +1 and +2 versions. Until it all changes again.”

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Also mentioned was the latest trajectory data, utilizing the middle ground of 15+1+2 as the baseline. The data relates to Endeavour’s rendezvous with the International Space Station (ISS) and departure – the latter of which will include the STORRM (Sensor Test for Orion Rel-Nav Risk Mitigation) Detailed Test Objective (DTO).

Designed to collect detailed data during rendezvous, proximity ops, and (un)docking, Endeavour’s crew will document, via photographs, all STORRM targets for photogrammetry objectives following Shuttle/ISS hatch opening and prior to Shuttle/ISS hatch closure.

“This trajectory data was generated using the L-25 days ISS vector. It includes a 15+1+2 mission duration and an ISS phasing strategy which results in a STS-134 rendezvous altitude of 187 nm,” added the memo. “The Orbiter ephemeris includes a FD14 midnight undock, followed by a 27 min Vbar separation, a 46 min fly around, a 1.5 fps SEP1 burn (3 fps effective) and STORRM DTO maneuvers starting with SEP2.”

To accomplish the STORRM DTO, just after undocking from the ISS, Endeavour’s Commander and Pilot will perform the standard back out and ISS Flyaround maneuvers through the Sep 1 burn. However, in terms of nominal Shuttle/ISS undockings, Flyarounds, and separations, this is where the similarity for STS-134 will end.

Endeavour’s Sep burns 2 and 3 (including all the mandatory STORRM burns) have been redesigned to facilitate a mini re-rendezvous with the ISS to “accomplish the STORRM re-rendezvous objectives” before completing the flyout of Endeavour to a point “in front” of the ISS in terms of the vehicles’ orbital trajectories and relative positions.

Storm Damage Evaluations:

It was a different type of Storm which gained attention at the latest Program Requirements Control Board (PRCB) meeting – ahead of its subsequent promotion to the Flight Readiness Review – with the recent severe storm weather damage evaluated and found to be of no concern ahead of Endeavour’s final launch.

A large amount of concern was forthcoming via four evaluation presentations (all available on L2), which reviewed the lightning strikes observed near the pad. Thankfully, none were classed as direct strikes, via the array of instrumentation and protection which is in-situ at the pad complex.

“One Lightning Strike Determined to be within 0.45NM, but not within 0.30NM. GSE (Ground Support Equipment) Walkdowns for Lightning. GSE/Facility Lightning Strike Walkdowns – were performed and Complete for 30 March 2011 Strikes,” noted one of the presentations.

“31 March Lightning Strike confirmed (by OTV – TV cameras) to be at Sea, away from LC-39. Did not Trigger 2nd Run of Seq 12, but Walkdowns occurred after both Events. No Damage Found.”

“Lighting assessment: Lightning was detected in the vicinity of KSC LC-39 on March 30 and 31, 2011. Concurs with the April 5 joint ERB conclusions that there are no issues, and retests are not recommended for SSP systems due to that lightning.”

High winds from the storms were also evaluated, this time focusing on the associated hardware with the Ground Umbilical Carrier Plate (GUCP). The concern relates to the wind loads on the vent arm potentially translating on to the GUCP, causing a potential misalignment to form – something which engineers would wish to avoid given the recent leaks from the hardware, including STS-133’s ET-137.

“Winds on 3-30-2011 at Pad A were recorded to 78.7-knot from the Southwest (2200-2600). The following concerns due to the high winds will addressed:. Concern 1: Did the high winds move the vehicle outside of the tested excursions? Concern 2: Are the loads induced into the Umbilical including 7-Inch QD/GUCP by the high winds a concern?”

Thankfully, the evaluations resulted in positive results, as the analysis showed the 78.7-knot gust was within tested limits, noting the Haunch Pivot Arm angle was within tested limits and that the axial load into 7-inch QD, GUCP and Pyrotechnic bolt were all within the required parameters.

“Analysis shows the 78.7-Knot gust recorded is well within the tested system limits. Pivot Arm and Vent Line angles are less than tested excursions. Pivot Arm angle controls axial load in Vent Line. Calculations show a 5.25 safety factor at the 7-In/GUCP Interface,” added the GUCP specific presentation.

“The peak gust was 30 degrees perpendicular to vent line centerline (perpendicular wind used in calculation). Inter-tank access arm helps protect the vent line from winds (not used in calculations). 2 g’s dynamic factor very conservative.

“Pending completion of nominal planned work, the GH2 vent line including 7-in QD/GUCP are ready to support STS-134 launch countdown.”

The final evaluation related to the hail damage to the top of ET-122, which only resulted in minor Thermal Protection System (TPS) damage, well within acceptable limits.

“Assessment: External Tank assessment of 03/30-03/31 severe weather event and affect on ET-122,” noted the Lockheed Martin presentation on the tank. “Actions Taken: Performed visual and hands on inspection of ET hardware. Impacts primarily on the -Y / -Z quadrant. No indications / impacts observed on composite nose cone.

“Visual indications / minor damage observed on LO2 tank and Intertank acreage TPS and components. Majority of TPS impacts < 0.10” deep. Characterized as superficial or witness marks (no discernible depth). Performed sampling of ‘worst-case’ depths using scale (defects >0.10” deep). Performed loads evaluation of 78.7 kt peak wind. Peak wind condition exceeds design certification environment (74.5 kt).”

Although the flight rationale was always expected – given the lack of any noticeable damage – the depth of the evaluation is a testament to the amount of data and flight history the Shuttle Program utilizes to ensure they are confident the flight hardware is in an acceptable condition.

“Rationale for Acceptance Summary: Composite hardware is damage tolerant. No visual indications. Testing at 20 ft-lbs impact energy result barely visible damage. Max impact energy for ET-122 hail event is 0.62 ft-lbs (0.47” hail at 107 mph winds),” the presentation added.

“Bounded by ET-124 Max impact energy of 5.92 ft-lbs. Residual hail defect locations do not provide a leak path across the GO2 Vent Seal Land surface. Similar to condition accepted after launch scrub (after vent hood retraction). No continuous leak path over face of dock seal land. Aero recession performance assessed to be nominal.

“ET-122 damage is bounded by ET-124 hail damage testing. Surrounding TPS analyzed at higher recession rates around the cavity. Residual crushed foam ablates at nominal rates as demonstrated by test (crush damage 5, 10, 15 and 20 percent for 1” thick TPS). Crushed foam liberates as normal ablation products. Thermal assessment. No impact to ascent or entry requirements.”

ET-124’s data is highlighted by the evaluations, due to the damage it suffered from a hail storm ahead of the STS-117 launch in 2007, resulting in the rollback of the stack for repairs in the Vehicle Assembly Building (VAB). However, the presentation also used data from ET-129 as part of the rationale. Even post-Return To Flight data was used to back up the readiness of the tank to fly.

“Condition enveloped by similar condition analyzed for STS-126 / ET-129. No impact to icing and launch probability requirements. Configuration of ET-122 hail damage sites would be encompassed by the icing tests done for ET-124 hail damage to meet an 85 percent Launch Probability requirement. No impact to propellant quality requirements. Insignificant quantity of impact sites to affect propellant quality.

“TPS Debris Assessment: Undetected crushed foam due to collateral damage was a major emphasis during RTF II. Large amount of mechanical property and performance testing previously performed to understand performance of both detected and undetected crushed foam. Results of tests showed no impact to TPS performance for crush levels (up to) 20 percent (visually detectable).”

With the ERB debating the status of the vehicle ahead of the PRCB, along with the PRCB then providing the documentation to clear the tank to fly, the Agency FRR did not have to spend much time in concurring with the evaluations, and passing ET-122 to fly as-is.

Another article will follow on the specifics of ET-122’s FRR documentation, including the previous tank (ET-137) performance during STS-133’s launch – and the history of the 10 year old tank which is set to fly with Endeavour.

(Numerous articles will follow. L2 members refer to STS-134 coverage sections for internal coverage, presentations, images and and updates from engineers and managers. Images used: All via L2 content and L2 presentations).

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