STS-129: Atlantis in preps for TCDT – SSME nozzle flight rationale

by Chris Bergin

Atlantis is enjoying a relatively smooth pad flow, with only three issues being worked, as engineers prepare the shuttle for the Terminal Countdown Demonstration Test (TCDT) next week. Flight rationale has also been produced for Atlantis’ SSMEs (Space Shuttle Main Engines), following small leaks on both Endeavour and Discovery engine nozzles during the previous two launches.

STS-129 Processing Latest:

The lightning strikes that hit Pad 39A’s infrastructure during the week are not deemed to be an issue, with no concerns noted via S0018 post lightning strike walk downs and data gathering, which were presented to an Engineering Review Board (ERB) on Friday.

Three issues – known as Interim Problem Reports (IPRs) – have been noted on the latest NASA Test Director (NTD) report on STS-129’s flow, all of which are usual for the early stages of pad processing.

“New IPR: LH2 Chill-down Valve in LH2 Storage Area did not open in secondary mode (function normal in primary mode). The constraint will be to the pad clear in launch countdown,” noted the processing update on L2. “New IPR Hardwire safing anti-ice prepress heater did not come on when commanded. Engineering is evaluating and will develop a troubleshooting plan.

“New IPR: During engine Flight Readiness Test (FRT), Engine 3, Fuel Pre-burner igniter channel B failed checkout for 6 samples. The maximum allowed is 3. Subsequent testing of the igniter resulted in 4 of 5 samples passing. This IPR is expected to close with no further work required.”

The main bulk of the nominal processing flow relates to preparations for the TCDT and S0024 prelaunch hypergolic propellant load, following the completion of S0009 operations.

“Shuttle Launch Pad Validation (S0009) is complete (through to the) ET camera functional test,” added the flow report. “Engine Flight Readiness Test (FRT), ball valve seal leak checks, and Aerosurface Frequency Response Test (FRT) all complete.

“Preparations for S0024 prelaunch hypergolic propellant load began Thursday. CTS (Call To Stations) for the main portion of S0024 is currently scheduled for Thursday, October 22nd. HPU (Hydraulic Power Unit) load however will occur on Monday, October 26th to free those assets to support upcoming HPU loading for Ares 1-X on Pad B.

“Preparations for Terminal Countdown Demonstration Test (TCDT) begin Friday. TCDT is scheduled for Tuesday and Wednesday next week. The 2nd part of S0008, Shuttle Interface Test has been re-scheduled for after S0024.”

A4Additional weekend work out at the pad is focusing on the completion on changing out the PV-11 fill and drain valve actuator and GH2 blank off plate installation. The Crawler Transporter that carried STS-129 to Pad 39A last week will also make its way back to the east gate during the middle of Sunday.

SSME engineers will also head to the pad on Tuesday to clean off some chlorides that appear to have been accidentally applied to the nozzles. However, engineers were due to work on the nozzles, per SSME Nozzle mitigation plan – expanded on below.

“There is a document in the system that will allow the team to get out to the pad on Tuesday, October 20, to do nozzle cleaning,” a note on the latest Shuttle Standup/Integration report on L2. “It is believed that chlorides were inadvertently put on the nozzle, due to the type of sponge used to apply the corrosion inhibitor.”

SSME Flight Rationale:

Although the upcoming Space Shuttle Program (SSP) Flight Readiness Review (FRR) will cover the subject in depth, last week’s Program Requirements Control Board (PRCB) meeting included one presentation dedicated to the nozzle leaks suffered during both STS-127 and STS-128’s ascents.

Hundreds of pinhole leaks were observed on both flights, though the total leakage was well within safety limits – and no loss of performance was noted.

Articles for reference:

A3While a new corrosion inhibitor is set to debut on STS-130, flight rationale has been built around additional mitigation procedures for STS-129’s engines – which includes the cycling of reconditioned air into the nozzles during Atlantis’ pad flow.

“Protect nozzle hotwall from environment by installing rollout covers and supplying purge via ECS Air. Purge testing completed in VAB on STS-129 nozzles,” noted the presentation – available to download on L2. “Conditioned (ECS) Air found to be effective. Provide continuous conditioned air routing into the SSME thrust chamber.”

“(Nozzles:) N2035 and N4025 (STS-129) will be cleaned, leak checked, and receive a reapplication of corrosion inhibitor while on Pad. N5011 has never received hotwall corrosion inhibitor application.”

With STS-129 being the last flight ahead of the application of the new inhibitor, managers have created flight rationale for Atlantis’ engines – based on the nominal performance of STS-127 and STS-128’s engines, despite their leaks.

A2“Flight Rationale: Overall leakage rates on N2031 and N4027 remain low with respect to redline limits. Leakage is well within limits safely demonstrated in ground test. Large margin to engine performance effect. STS-127 and STS-128 performance review showed no impact on measurable data,” the presentation noted.

“STS-129 units meet flight leakage criteria. Experience and analysis demonstrate large changes in leak rate due to corrosion are not credible.”

The presentation also covered the change to the OMRSD (Operations Maintenance Requirements Specifications Document), required for the new inhibitor, which will be applied without sponges, thus eliminating the problem with the aforementioned chloride contamination.

“Rationale for changes: changes will implement enhanced corrosion inhibitor on all nozzles,” added the presentation. “RAR-1643 will clean, leak check and re-apply corrosion inhibitor on two of the STS-129 nozzles (E2048/N4025 and E2044/N2035) that likely have chlorides on nozzles. Purge will reduce exposure to corrosive environment and keep nozzles dry.

“Sponges will no longer be used for application of corrosion inhibitor – eliminates source of high chlorides.”

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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