STS-129: Main Engine Ignition Acoustic debate leads FRR

by Chris Bergin

Managers at the STS-129 Agency Flight Readiness Review (FRR) have been discussing an issue relating to an attach point between the aft Reaction Control System and OMS Pod, which may be around 20 flights beyond their predicted fatigue lifespan. Due to potential structural integrity problems via localized acoustics during Main Engine Ignition a forward plan has been created, one which accommodates the November 16 launch date.

STS-129 Processing Latest:

Engineers are back at Pad 39A, following Wednesday’s test launch of Ares I-X. The Rotating Service Structure (RSS) was rotated to its park position just after midday, in readiness to accept the STS-129 payload canister.

“Following the Ares 1-X launch yesterday, Pad A was re-opened and OV-104 (Atlantis) powered up to continue processing activities,” noted Thursday processing information on L2. “The RSS is scheduled to be moved to the park position today to accept the payload later today.

“Orbiter aft closeouts for flight continue. Preparations for the EMU (EVA Mobility Unit) functional are in work with CTS (Call To Stations) scheduled for tomorrow. Preparations for S0017 Terminal Countdown Demonstration Test (TCDT) picked up yesterday. CTS for S0017 is planned for Monday, November 2nd.

The only troubleshooting being worked at present relates to one of Atlantis’ Space Shuttle Main Engine (SSME) controllers, while the replaced onboard Waste Collection System (WCS) is undergoing testing.

“IPR (Interim Problem Report) SSME controller Actuator Calibration (ACAL) coefficient miscompare update: Hydraulic operations are being performed to support the troubleshooting efforts that are in work,” the report added. WCS functional testing started yesterday and is scheduled to be completed on Saturday.”

Launch Date:

As previously reported, November 16 is the current NET (No Earlier Than) launch date – providing the Atlas V/Intelsat mission launches from next door’s Cape Canaveral on November 14. A 24 hour scrub turnaround for Atlas would result in STS-129 slipping to November 17.

STS-129 Specific Articles:

How long STS-129 will have the Eastern Range for is being based on negotiations with the Delta IV launch with the WGS satellite.

A312Earlier in the week, Mission Management Team (MMT) head Mike Moses intimated that STS-129 is down to a one to two day launch window, via the Delta IV team agreeing to a two day slip to their launch date.

“The Eastern Range has just approved our STS-129 launch date request for 11/16 and 17. The Delta IV WGS launch has agreed to slip two days to the right to accommodate 129,” noted a memo from Mr Moses (L2). “The 11/16 attempt is also contingent upon the Atlas V Intelsat launch occurring on their first attempt (11/14). If they need their backup day, 129 will move to 11/17.”

However, as of Thursday’s 8th Floor News memo from MOD, the possibility of a window as long as the 16th to 19th was mentioned.

“STS-129 – Atlantis is at the Pad in preparation for her November launch. Range conflicts with the Delta IV launch still exist, however, negotiations are in work which may provide launch opportunities through November 19th.”

FRR Decision: Proceed with a NET of November 16.

Agency FRR:

The final Flight Readiness Review for STS-129 is hosted at the Kennedy Space Center (KSC), fine tuning and approving documentation that had been presented at the two-day Space Shuttle Program (SSP) FRR last week in Houston.

Although it was only classed as an outstanding issue in SSP FRR documentation – all 26 presentations available on L2 – a Special PRCB (Program Requirements Control Board) meeting was called earlier this week to discuss a potential problem with part of the orbiter’s structure – and any negative effect the SSME’s may have as they roar to life.

“The first topic focused on a Main Engine Ignition Acoustic issue, which the community is viewing as a constraint to launch of STS-129,” added notes from the meeting, acquired by L2. “For those that weren’t tied into the STS-129 SSP FRR last week, this acoustic loads issue was talked there and the community has been trying to get their arms around this problem ever since.

a26“The issue concerns recent acoustic environment analysis that suggests that the attach pins that mate the aft RCS stinger structure to the OMS pod may be ~20 flights beyond their predicted fatigue lifespan. The pinch point occurs over a 0.5 second span, roughly three seconds prior to liftoff, where the main engine acoustic environment is inducing a significant pressure load, not only on the stinger attach points, but on the entire aft structure of the vehicle.

Although structural fatigue may sound dramatic, the attach point in question is only one of four on each stinger. Engineering data – via recent inspections – is already to hand on the same location on veteran orbiter Discovery, which shows the hardware is in good shape.

“Each stinger has 4 attach points and 1 of them, the upper inboard, is the crux of the problem (it sees loads in 3 axes while the others only see loads in 1 or 2 axes),” the notes continue. “A boroscope inspection of the upper inboard stinger attach point on Discovery was performed in the OPF (Orbiter Processing Facility) and no gross structural issues were observed.”

The evaluations are now centering around providing instrumentation in the area in question, which would allow for engineering data on how much stress the acoustics of the SSMEs place on the stinger/attach points during the short window of the engines igniting.

“The problem is that critical damage that could lead to catastrophic conditions would start with a crack (much like the Flow Control Valve issue that was recently worked prior to STS-119) and it requires OMDP (Orbiter Maintenance Depot Period)-level disassembly to get to these attach points for a dye penetrant/NDE (Non Destructive Evaluation) inspection of these attach points.

A4“It is an understatement to say there is a lot of uncertainty in the acoustic environment. There are limited microphone instruments in the aft part of the vehicle, none of which are in close proximity to the stinger, and there is a lot of complex, structural shadowing and MLP (Mobile Launch Platform) interactions that makes it very difficult to extrapolate an acoustic reading at a dissimilar location into an imparted load on a structural attach point inside of the stinger.”

*Click here to see a SSME Ignition in superslow 8mm footage*

The forward plan from a paperwork standpoint revolves around creating a risk assessment for the hardware, which is commonplace for the program. For the interim, several actions will be carried out with the Orbiter team, in order to improve their data and understanding of the potential issue.

“Even if the engineering team is able to sharpen their pencils and they come up with acceptable fatigue life margin for the stinger attach points, there will still be some risk-acceptance required by the Shuttle Program because the answer will be primarily derived from an analytical model and not NDE inspection data,” added the notes.

“(SSP Manager) John Shannon handed out the following actions to the team. No decisions were made concerning this MEI acoustic issue:

“1) Orbiter will investigate instrumenting the upper inboard Atlantis (STS-129) stinger attach point with gauges to quantify the actual loads imparted to it. This action will help provide flight rationale for future flights, but not for STS-129.

“2) SE&I will investigate instrumenting the MLP to attempt to better understand the environment. This too will provide flight rationale for future flights. 3) KSC (Kennedy Space Center) ground ops will boroscope the upper inboard attach point on Atlantis and Endeavour.

“4) The Boeing loads team will continue to construct a dynamic-loads model from scratch, and will validate it using strain gauge and accelerometer flight data from STS-1 through STS-5. This model could show there is analytical, structural margin in the attach points.

The Special PRCB also noted that there are indications a few other areas appear to be showing fatigue issues, but still have plenty of lifespan for upcoming flights.

“There are some TPS (Thermal Protection System) tile locations and a backface panel door that are showing fatigue lifespan issues as well, but the engineering team is very close to proving that the TPS and panel door still have plenty of structural margin.

“The second topic was very straightforward – the PRCB approved the redesign of an attachment structure for the potty, which will fly for the first time on STS-130. The attachment structure for the STS-129 potty has cracks on it, but NDE of the structure proves it has sufficient fatigue margin to fly safely on STS-129.”

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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