STS-119: LH2 leak during tanking postpones launch to March 15
STS-119′s opening launch attempt has been scrubbed until Sunday, March 15,, following a LH2 leak during tanking. The leak related to a carrier plate that is attached to the intertank area of the ET, with attempts to cycle the associated valve to reduce the leak to acceptable proving to be unsuccessful.
Work at the pad was on the timeline, following an overnight flow that included the highlight of RSS (Rotating Service Structure) retraction.
“S0007 Launch Countdown: SSME Final Preps completed yesterday. Comm. Activation completed at 2033 EDT. RSS retract to the Park position at 0152 EDT,” noted processing information on L2.
“Ascent Switch List was completed at 0429 EDT. Final load preps completed at 0955 EDT. Clear LDA is scheduled for 0910 EDT today. Tanking scheduled to begin at 1155 EDT.”
Tanking has proved to be a smooth process since the ECO (Engine Cut Out) low level sensor anomalies, which resulted in a major modification to the LH2 Feedthrough connector system ahead of STS-122.
Tanking of ET-127 was proceeding to plan, with LH2 fastfill picking up following good results on the ECO sensors.
However, a LH2 leak related to the Ground Umbilical Carrier Plate (GUCP) – which is the interface on the intertank of the ET for the hydrogen vent line - caused a scrub, resulting in a new NET (No Earlier Than) of March 15 to 16, which may see the mission reduced by one or more EVAs, due to the need to undock prior to the launch of the next Russian Soyuz.
With the tank now drained, it will take 24 hours to boil off the remaining LH2 in order to inert tank. Engineers will then again access and look for gross misalignment of QD (Quick Disconnect). Afterwards, engineers will perform a decay check, remove the QD and inspect flight seal.
In around two days time, work will then move towards the installation of a new flight seal, head of decay checks and the reconnection of the vent line.
“In the reinstallation of the flight seal, there is a 30 hour retorque requirement, that pushes us to a Monday launch. The team is looking to examine if there is some wiggle room in the 30 hr torque requirement, then we could potentially get a Sunday launch attempt,” noted a MMT memo acquired by L2.
On Thursday afternoon, engineers managed to access the area and concluded a fix should be in place to allow for a Sunday launch attempt.
Earlier in the count:
The day opened with four IPR (Interim Problem Reports) listed on Wednesday processing information. However, all were classed as non constraints for proceeding through the count towards launch.
“During OAA (Orbiter Access Arm) system pressurization/activation, noted there was a 120 psig delta between the console GN2 pressure transducer and a local gauge,” added the NASA Test Director Status report. “A Heise gauge was installed to determine if the error was with the transducer or the gauge.
“Determined that the local pressure gauge is bad, and the transducer, which is used for launch, is reading correctly. Will leave gauge in place and R/R (Remove and Replace) post-launch. The IPR has been upgraded to a GSE PR (Ground Support Equipment Problem Report) and is no constraint to STS-119.
Two of the four IPRs are status updates on previous issues that have since been resolved, such as the accelerometer units that will provide data for the Constellation Program’s efforts to further understand Thrust Oscillation (TO) effects on the crew. One of the units failed to configure on Tuesday, prior to a battery R&R that resolved the issue.
*Click here for articles on Ares I Thrust Oscillation*
“Battery R&R corrected the problem with the unit and all 9 units are programmed with the launch window data,” updated the NTD report. “This IPR will be upgraded to a PR against the DTO (Detailed Test Objective) unit. No additional work required, in paper clean up.”
The other closed issue relates to the an autocycle valve on the Mobile Launch Platform (MLP). The valve is working as required, but was deemed to be “out of family” due to an electrical issue. Engineers plan to use the valve as required, with a back up contingency in place, should the valve behave unexpectedly.
“Troubleshooting was able to cycle the Hi-Point Bleed valve locally from the pneumatics panel. The valve was subsequently verified to properly work from the Firing Room.
“The valve is in its final launch position and won’t be cycled until post lift-off. The valve is expected to work, however a contingency purge procedure exists in the event it doesn’t cycle after lift-off. No further constraint or work and the IPR will be upgraded to an MLP PR.”
Engineering review to the crew:
The meeting with the crew overviewed specific flow items, such as the subsystem work on Discovery during her time in OPF-1 (Orbiter Processing Facility). The crew were already fully briefed on items such as the Flow Control Valve (FCV) flight rationale work.
Critical subsystems that will be utilized during Discovery’s launch include the Space Shuttle Main Engines (SSMEs), Fuel Cells (FCs) and Auxiliary Power Units (APUs).
“The APUs installed for this mission are APU 1 S/N 310, APU 2 S/N 403, and APU 3 S/N 207,” listed a subsystem overview created for the astronauts. “The APU controllers installed for this mission are Controller 1 S/N 0006, Controller 2 S/N 0004, and Controller 3 S/N 0003.”
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-119, a new modification will debut on the APUs.
“This is the first flight of a new mod which adds a new heater and overtemp thermostat in parallel with the current APU 1 fuel service and test line heater circuits to heat the APU 1 & 2 fuel tank GN2 service QDs (Quick Disconnects).
“This mod installed two new overtemp thermostats, one for each redundant circuit (‘A’ and ‘B’) on APU 1 GN2 service line, and wrapped a new heater, with the two redundant (‘A’ and ‘B’) heating elements built in it, around the APU 1 & 2 fuel tank GN2 pressurant line and QDs.”
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.
Engineers provided an outline on which FCs Discovery will be flying with during her mission, along with their flight history.
“Fuel cell 1, s/n 109, has 354 hours on the stack, 354 hours on the regulator, and 1 flight. The regulator contains the new elastomer material. The cells were made by the drip-fill method. It has PEEK Insulator Plates.
“Fuel cell 2, s/n 107, has 1210 hours on the stack and 1202 hours on the regulator after 3 flights. Fuel cell 3, s/n 116, has 1865 hours on the stack, 5 flights, 353 hours on the regulat after 1 flight.
“Fuel Cell 1, 2, and 3 Diagnostics Tests were performed on 07/25/08; no issues with the results.”
Best known are the three SSMEs, which have an impressive track record during the powered flight stages of Discovery’s ride uphill. The reusable power plants flying with Discovery are engines: 2048 (Main Engine 1), 2051 (Main Engine 2), and 2058 (Main Engine 3).
As per Flight Readiness Review (FRR) documentation, every element of modification or change is listed alongside each engine’s flight history. For STS-119, the FRR presentation for the SSMEs opened with a note relating to the Low Pressure Oxidizer Turbopump (LPOTP), due to a Bearing Ball Process Change initiated by the supplier.
The LPOTP is an axial-flow pump driven by a six-stage turbine powered by liquid oxygen. The flow from the LPOTP is supplied to the High-Pressure Oxidizer Turbopump (HPOTP).
During engine operation, the pressure boost permits the High Pressure Oxidizer Turbine to operate at high speeds without cavitating. The LPOTP operates at approximately 5,150 rpm.
“Reason for Change: Supplier initiated process change,” noted FRR documentation. “Processes Changed: Revised ball blank sintering process and changed factory location. Modified ball grinding and lapping processes and changed factory location.
“One unit LPOTP 2616 – 11 starts, 5500 seconds min. Total hot-fire experience: 13 starts / 7208 seconds.”
The other item of interest relates to the Fuel preburner FG1a elliptical ring, which was observed to be cracking following an acceptance test on another engine (2061). This required documented confidence for the next engines due to fly – with Discovery – to be presented to the FRR.
The FG1a elliptical ring provides access to fuel preburner (FPB) and HPFTP (High Pressure Fuel Turbopump) turbine for post hot fire borescope inspections.
“Cracking of elliptical ring, weld and/or liner is not uncommon,” explained the documentation presented to the FRR. Borescope inspection of ring and weld required post every hot fire.
“Rules limit life of FPB liner and ring to 24,224 seconds with pump out visual inspection of ring at 6,056 second intervals. Erosion, pitting and/or cracking is unacceptable and results in liner and ring replacement.
“Concern is loss of piece greater than critical impact mass (0.142 g). No significant loss of mass experienced in program history. Multiple cracks required to liberate a particle of significant size.”
Also adding confidence was the overall test result for engine 2061, which was deemed to be “in family” for its requirements.
“Engine 2061 Assessment: Detailed post test inspections noted no other hardware anomalies. Engine performance (temps, pressures, flow rates, speeds, etc.) and measured dynamics all as expected and “in family”,” confirmed the FRR presentation.
The reason any cracks warrant an extensive evaluation is due to the threat of FOD (Foreign Object Debris) which would be a serious problem, should a crack liberate FOD into the super fast spinning turbine blades.
However, such is the safety record of the SSMEs during flight and testing, a probability of that threat being realized is a staggering 1 in every 150,000 flights.
“Probability of generating FOD of size to be of concern,” added the FRR presentation. “(However,) requires linkage and intersection of 4 through wall cracks – never experienced in program history.
“Analysis indicates scenario not probable (inconsistent with stress contours). Demonstrated probability = 1/150,000 flights.
“Rationale for Flight: Existing controls remain adequate to preclude significant out of family cracking and loss of significant particle mass. Borescope visual inspection required after every hot fire. Erosion, pitting and/or cracking is unacceptable
“Demonstrated hot fire time on the current flight set indicates that significant fatigue crack growth in one flight is improbable. Crack initiation and growth rate of Engine 2061 is unique.”
Thanks to the FRR process, and the work that is undertaken between every flight, Discovery can look forward to flying with three good SSMEs during her eight and a half minute powered flight into space.
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.