With final launch processing underway on Shuttle Endeavour at Launch Pad-A, NASA managers have outlined several changes to Endeavour’s Main Propulsion System (MPS), Ground Umbilical Carrier Plate (GUCP), and Flight Operations & Integration procedures that will debut on STS-127 next week.
STS-127 MPS Inventory Update:
Taking center stage in the Space Shuttle Program’s (SSP) Flight Readiness Review (FRR) presentation on the Space Shuttle Main Engines (SSMEs) and MPS was the low Over Board Mixture Ratio (OBMR) that has been seen on numerous flights since STS-114 in July 2005.
The lowest of these OBMRs occurred last year on Endeavour’s STS-123 flight. That mission experienced an OBMR of “-2.7 sigma, -0.4 percent” notes the FRR document available for download on L2. This OBMR on STS-123 resulted in all fuel bias being consumed by the SSMEs during Endeavour’s climb to orbit.
While this did not put the vehicle at risk of a Low Level Cutoff – due in large part to the positive Ascent Performance Margin (APM) of the flight – a subsequent review of previous post-Return to Flight missions revealed a decreasing trend in the OBMR.
This decreasing OBMR could result in the loss of performance or payload capacity. This issue, which was discussed at a Technical Interchange Meeting (TIM) in summer 2008, noted that the lower than predicted OBMR was likely due to “facility fuel flowmeter temperature bias at the Subsystem Sequence Controller.”
A temporary mitigation plan was developed in which STS-126 and subsequent flights would launch with an increased Controller Mixture Ratio (CMR) of 6.049. This CMR debuted on STS-126 and was maintained on STS-119 and STS-125. These flights experienced no “out of family” anomalies with the OBMR.
However, a new update has been developed for STS-127 and subsequent flights.
As the SSP FRR presentation notes, “STS-127 will be the first flight of SSME controller constant updates, an updated MPS propellant inventory, and an updated CMR.”
For the SSMEs, an SSME element calculated bias for each engine was developed, thereby allowing each engine to be “corrected” for flight. This correction “reduces variability in active engines to the point where a single inventory can reasonably account for the performance variations” of each engine.
Furthermore, the Flight-derived Dispersion Database (FDD) was updated with all flights through STS-124 and the MPS inventory was updated accordingly.
For STS-127, Endeavour will fly with a CMR of 6.043. This calculation was determined through the new MPS inventory. The new Flight Performance Reserve curve results in a -175 lbm impact to the APM for the flight – an impact that is easily absorbed by Endeavour’s launch capabilities.
Finally, for STS-127, Endeavour will use SSMEs 2045, 2060, and 2054 in positions one, two, and three respectively.
Ground Umbilical Carrier Plate Issues:
Also noted during the SSP FRR process was an installation issue of the Ground Umbilical Carrier Plate (GUCP) on Endeavour’s External Tank (ET-131).
“Interference between GUCA (Ground Umbilical Carrier Assembly) and ET-131 right hand hinge support observed during mate GUCP (Ground Umbilical Carrier Plate) installation in VAB,” notes the backup SSP FRR document.
The installation sequence is used to verify contact with hinge supports and also to verify that no interference exists between the pivot assembly and hinge bracket of the GUCA.
Interference was noted on the Right Hand side of the Right Hand pivot assembly to the Right Hand inside of the bracket.
The main concern revolving around this issue is the fact that interference is not permitted at the hinge location due mainly to the fact that there is a potential to induce un-intended loading on the pyro-bolt assembly – which could affect that separation mechanism at T-0.
To correct this interference, the GUCP was removed and a different unit was installed. However, after this was accomplished, the interference remained.
Both GUCP assemblies were then measured/inspected at KSC; both GUCP assemblies were found to be within engineering specifications and tolerances.
Visual inspections then revealed a slight misalignment between the centerline of the plate and the hinge bracket.
Laser measurements at KSC later confirmed the misalignment and the pivot assembly was modified – and successfully installed – by “locally machining outboard surface (0.1” removed) to create the required gap (0.035” gap provided),” notes the backup SSP FRR document.
A stress analysis revealed an adequate factor of safety for the modified configuration.
Furthermore, the repaired configuration does not impact the GUCP release mechanism or sealing interface between flight and ground equipment.
A root-cause analysis is underway; however, similar conditions have not been observed at MAF (Michoud Assembly Facility) to date – which includes visual inspections of ET-139 and ET-132.
In the future, a spare GUCP will be used at MAF to support fit checks prior to shipment of ETs to Kennedy.
Flight Operations and Integration Office:
In addition to the above changes/modifications, the Flight Operations and Integration Office has identified three items that will see modification for STS-127.
The first change relates to the Cargo Integration Hardware (CIH). STS-127 will be the first flight of parallel latching on Major Cargo Elements.
The capacity of the payload latching switch panel on the aft Flight Deck console of Endeavour is 15 switches corresponding to 15 payload latches in the payload bay. However, STS-127’s payload requires 18 latches – three more than the aft Flight Deck console can support.
To accommodate this requirement, a parallel latching system was implemented on the sill PRLAs (Payload Retention Latch Actuators/Assembly) for the Japanese Experiment Module Exposed Facility and the Japanese Logistics Module Exposed Section.
Thus, all 18 payload latches can be controlled from the 15 switches on Endeavour aft Flight Deck console.
Additionally, all engineering products and integration analyses have been completed (as of May 28) except for one: Verification Loads Analysis (VLA) for the ELM-ES (Japanese Experiment Logistics Module – Exposed Section).
The VLA for this cargo element is being re-performed to include “the unique damping matrix,” notes the FO&I FRR presentation, available for download on L2.
This analysis is being re-performed because five of the VLA cases pertaining to Endeavour’s various abort modes did not include the damping matrix.
No issues are expected as a result of the late performance of this VLA. Currently, on-going analyses indicate that no structural margins will be affected by the implementation of an abort during ascent.
Lastly, the Middeck Engineering Status (MES) reports that the return manifest requirements are still being defined.
Similarly, late stow plans are in place for the “EVA Detent Release Tools (2) required for the Unpressurized Cargo Carrier Attachment System (UCCAS) deploy.” Delivery of these tools is expected on June 8.
Furthermore, no STS-125 FO&I In-flight Anomalies (IFAs) were identified and all open IFAs have been dispositioned by the Space Shuttle Program since STS-119.
In the unlikely event that Endeavour suffers critical damage to her TPS (Thermal Protection System), orbiter Discovery could be readied for launch on STS-128 as early as August 6.
This translates to a 54-day LON (Launch on Need) capability with Discovery – a timespan which greatly overlaps the International Space Station’s capability to support Endeavour’s seven member crew and its own six member crew for up to 74 days.
The CSCS (Contingency Shuttle Crew Support) time on the ISS is limited to 74-days by the CO2 removal system’s capacity on the ISS.
These CSCS calculations are based on a 13 person crew on ISS with no Shuttle crew members returning to Earth on Soyuz and Endeavour remaining docked to the ISS for 28-days.
The calculations also assume that the Waste & Hygiene Compartment, Urine Processing Assembly, and Water Processing Assembly all maintain normal operating levels. Additionally, all 13 crew members would maintain a regular exercise program for the duration of their stay on ISS.
If the LON flight is required, Discovery would fly in her STS-128/17A configuration with her full payload installed for flight.
Also, all STS-128 flight software and products would be flown on Discovery in the event of a LON call-up. This includes the first flight the OI-34 software and the first flight of a new MEDS (Multifunction Electronic Display System) load.
A formal flight Software Readiness Review would be held after call-up of the LON flight.
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.