Shuttle Atlantis has launched on her flagship mission to the Hubble Space Telescope, as STS-125 completed smooth S0007 (Launch Countdown) operations on Monday. Atlantis’ External Tank (ET-130) enjoyed an issue-free tanking, bar the need for the Orbiter Project Office to issue a waiver for a small amount of ice that built up on the LH2 umbilical.
Processing/Countdown (S0007) Review:
The Rotating Service Structure (RSS) was successfully rotated to the park position Sunday evening, which followed the activation of the orbiter’s communications systems and closure of the Payload Bay Doors (PLBDs) – the latter taking longer than planned, due to an interference involving an EVA tether.
An issue was also reported with a backup TACAN (Tactical Air Navigation) system, caused by a misalignment problem. Following realignment, the TACAN was deemed operational and able to support STS-125.
Engineers also noted there was a question about “loctite” on five closeout screws on the star tracker door, noticed when the coupon screws – used to verify if the loctite worked correctly – showed the screws were breaking free at too low a torque. These screws have since been changed out.
The Fuel Cell (F/C) flow-through purge has also completed without issue, followed by their activation around 10pm local time on Sunday.
The STS-125 crew – led by Commander Scott Altman – departed from the Operations and Checkout Building for the pad at around 10:16 am, entering Atlantis around 30 minutes later. Closeouts of Atlantis’ crew hatch – now the crew are onboard – will take place shortly.
“STS-125 S0007 Launch Countdown operations: CTS (Call To Stations) was completed Friday at 1530L. Countdown operations continue to progress per the Barchart and on-time,” noted Monday morning processing information via L2.
“Air to GN2 changeover was completed last night at 2114 EDT. ET tanking began at 0442 EDT and should be complete by 0742 EDT. Launch window opens at 1341 EDT. Preferred time is 1401 EDT.
“Weather forecast 90 percent Go for launch.” However, TAL (TransAtlantic Abort Landing) sites are questionable at this time and the biggest threat to launch at this time. Update: Observed TAL weather has improved and is currently green.
Engineers are also keeping an eye on a formation of ice on the LH2 ET Umbilical – located at the aft of the orbiter. Currently, the estimated mass of the ice is not enough to threaten a scrub, although it will continue to be observed via pad cameras, while the ice inspection team are sent back out to the pad for a closer look. Following discussions, a wavier was issued.
The loading of LOX and LH2 into Atlantis’ External Tank has now completed to the stage of stable replenish. The Engine Cut Off (ECO) sensors were checked out during slow fill and deemed to be operating as required, with good voltages.
As always, engineers will be keeping a close eye on the entire tanking process, though special attention will be paid to the Ground Umbilical Carrier Plate (GUCP) – following the leak during STS-119’s opening launch attempt.
“STS-119 / ET-127 Performance Summary: Pre-launch: 1st loading resulted in scrub/LCC (Launch Commit Criteria) violation due to GH2 leakage at GUCA (>40,000 ppm. Flight seal/QD (Quick Disconnect) replaced,” noted a post mission IFA (In Flight Anomaly) review presentation on L2, written by the MAF (Michoud Assembly Facility).
“Leakage occurred during transition from fast fill to topping. Vent valve opened when 98 percent level sensor indicated wet. Detected by leak detectors (LD 23 & 25) located in ground umbilical shroud. Isolates leak to either ground side quick disconnect (QD) or interface with flight seal.
“Contingency plans (vent valve cycling) unsuccessful in controlling leakage within acceptable limits. Launch scrubbed, flight seal/disconnect replaced. No GH2 leakage observed during subsequent loading. No other anomalous performance observed during loading.”
Following unmating and realignment work, the second launch attempt for Discovery was issue-free. However, an investigation was initiated, in order to ensure STS-125 would not suffer from the same problem.
The mating of the GUCP to Endeavour’s ET-131 – for the STS-400 LON (Launch On Need) requirement on Pad 39B – was also used to aid checks into mating procedures.
The investigation found that the most probable cause identified a momentary breach in flexible flight-seal to bellows probe due to ‘thermal shock’ of GH2/LH2 with vent valve in open position. Tape has also been applied to areas of the GUCP, in order to monitor any change to the alignment since initial mating.
Sunday’s L-1 Engineering Meeting:
A meeting between the crew and engineers overviewed specific flow items, such as the subsystem work on Atlantis during her time in OPF-1 (Orbiter Processing Facility).
“The purpose of this briefing is to provide the Atlantis Flight Crew with a summary of significant vehicle and payload issues that have occurred since the STS-125 TCDT L-1 Day briefing that was presented on 9/23/08,” outlined an expansive L-1 presentation, acquired by L2.
“This includes problems or issues that have occurred following the decision to rollback from PAD A, remove Atlantis from the integrated stack in the VAB and return to the OPF.
Critical subsystems that will be utilized during Atlantis launch include the Space Shuttle Main Engines (SSMEs), Fuel Cells (FCs) and Auxiliary Power Units (APUs).
The APU is a hydrazine-fuelled, turbine-driven power unit that generates mechanical shaft power to drive a hydraulic pump that produces pressure for the orbiter’s hydraulic system. There are three separate APUs, three hydraulic pumps and three hydraulic systems.
As with every flow, lessons learned from the previous flight of both the program and the orbiter herself are factored into modifications and improvements of the systems. As with STS-125, work on the APUs was outlined.
“Upon return to the OPF, the APU 3 QD (Quick Disconnect) Heater mod was implemented (APU 1&2 circuits modified earlier): This modification replaced the existing fuel fill line heater with a longer heater that provides protection to the previously unheated system 3 GN2 service QD,” outlined the engineering presentation.
“Purpose of mod is to lessen chance of developing on-orbit GN2 QD leakage with resultant fuel tank pressure decay, which has occurred on two recent Shuttle flights (STS-121/OV-103 and STS-123/OV-105). Also, Tempilabels have been affixed to QD flight caps in order to provide post-flight data on heater performance.
“Because of the launch delay, an APU confidence run was performed after return of stack to PAD A. All three APUs performed nominally.”
Also vital to the orbiter power system are the three Fuel Cells, which are located under the payload bay area in the forward portion of the orbiter’s midfuselage. The Fuel Cells operate as independent electrical power sources – each supplying its own isolated, simultaneously operating 28-volt dc bus.
“STS-125: Fuel Cell Total runtime: F/C1, 2036 hours. F/C2, 434 hours (Replaced since last flight F/C operational life: 2600 hours). F/C3, 1130 hours,” listed the presentation.
Atlantis’ three SSMEs, the main engine that can boast an impressive track record, will help power the vehicle’s ride uphill. The reusable power plants flying with Atlantis are engines: 2059 (Main Engine 1), 2044 (Main Engine 2), and 2057 (Main Engine 3).
Debuting on Atlantis’ engines is a software modification, which followed observations of ‘erroneous alarms’ sounding during Return To Landing Site (RTLS) abort simulations.
“DSP Memory Dump Sequence Change: Eliminates erroneous Shuttle Orbiter Cockpit Alarms during RTLS aborts. Discovered during Shuttle Avionics Integration Lab (SAIL) RTLS abort testing,” noted the SSME presentation to the STS-125 SSP (Space Shuttle Program) Flight Readiness Review (FRR) on L2.
“Result of the Shuttle Primary Avionics Software System (PASS) SSME Subsystem Operating Program being active during RTLS and detecting unsolicited SSME Controller ‘Command Accepts’.
“SSME Controller software issues “Command Accepts” for the duration of the post MECO digital signal processor power spectral density (PSD) data memory dump for each SSMEC internal generated command.
“Software Version AD08 changes the DSP Memory Dump to report ‘No Command Received’ for each SSMEC internally generated command and makes the first six words standard in all VDTs. DSP memory dump sequence terminated if vehicle command received. No vehicle command expected during memory dump sequence.”
The nozzles on Atlantis’ SSMEs also required cleaning, after contamination – caused by their extended stay out on the coastal launch pad – was observed by engineers.
“Deposits of green and brown contamination on the tubes of STS-125 nozzles,” added the presentation. “Background: Pre rollout inspection noted green contamination on the tubes of all three nozzles. Green contamination is nickel corrosion products.
“Caused by chlorides in sea salt deposition. Superficial corrosion of tube nickel plating. Isolated brown spots contain iron products and are likely A-286 corrosion products. Affected areas on hotwall and coldwall correspond to onshore wind direction when orbiter is on pad.
“Nozzles cleaned to remove chloride deposits. Post clean leak tests confirmed superficial nature of corrosion. No change from prior leak test results. No pitting through EDNi plating on exposed tube surfaces observed. Reapplication of sodium chromate inhibitor to cold wall aft manifold crevices maintains in-family condition
“Cleaning assumed to have diluted existing protection. STS-125 nozzle cumulative pad time well within family. No abnormal corrosion expected. Large margins to leakage which would result in unacceptable engine performance.”
More will follow during the count.
L2 members: Documentation – from which most of the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.