Thruster issue requires clean up operations on Endeavour’s TPS

by Chris Bergin

Engineers are working on a plan to clean up around 16 tiles on Endeavour’s nose, following a ‘leak’ of a ‘syrup type ooze’ from a Vernier thruster.

The small thruster (F5R) on the right side of Endeavour’s FRCS (Forward Reaction Control System) came under evaluation during a desiccant inspection that resulted in a “liquid” leaking on to an engineer’s hand.

Due to the dangers of the hazardous substances that power the thrusters, the engineer was evacuated as a precaution. Checks at a medical center confirmed he had not been exposed to any danger.

Orbiters have six Vernier engines or thrusters which are used to make fine adjustments to the attitude or velocity of the vehicle on orbit.

Each vernier RCS engine has one fuel and one oxidizer solenoid-operated poppet valve. The valves are energized open by an electrical thrust-on command. When the thrust-on command is terminated, the valves are de-energized and closed by spring and pressure loads.

Ahead of launch, the openings to the thrusters are monitored by desiccant covers, which provide a visual indication of moisture contamination for the thrusters.

The inspection was called due to the requirement to changeout the VTPA (Vernier Throat Plug Assembly), which was observed to be lodged into the thruster, with part of its desiccant missing – later found on the MLP (Mobile Launch Platform) zero level. It did not cause any damage to shuttle hardware during its liberation.

Under strict safety restrictions, engineers carried out a visual inspection of the truster and nearby TPS (Thermal Protection System). They found liquid to be present on the filler bar bonded to the internal structure shelf surrounding the 28-00 door cavity, and on a number of tiles.

“A thruster Leak (F5R) exposed 15 tiles to liquid dripping that appeared much like syrup; amber in color,” noted processing information on L2. “The (location) tile is where the drop of fluid initially fell before running down the remainder of the tiles.

A drop of fluid was also observed to drip off the hardware during an inspection by engineers and safety officers, which once again called for workers to be moved away from the immediate area. This again was only a pre-caution, with no workers exposed to any dangerous substances.

Efforts proceeded to the sampling of the contaminant, as a forward plan was created to work on a clean up operation – which will be fully evaluated following vapor checks that noted a small trace of MMH (Monomethylhydrazine) in the “ooze”.

“Documenting the resultant contamination of 16 tiles. Engineering is guiding our efforts to clean up this ‘syrup type ooze’ from the TPS and believe that data (toxic vapor checks) has pinpointed the base constituent (MMH),” added processing information on Tuesday.

“Due to the potential presence of hypergols, stringent safeguards will be required to protect personnel from exposure during this work. The disposition that is in draft will be released as a hazardous operation to carefully remove the contaminants and to perform litmus paper checks to verify removal.

“Once these steps are completed, inspections and engineering evaluations will follow to verify no resultant damage and to note the general appearance of the immediate area. Further dispo to follow to address the findings.

“A carboy was installed in the thruster on Friday to control the leaking thruster; Engineering evaluation continues.”

No timescale has been placed on the clean up operation, though Endeavour has a number of days in hand, prior to her move to Pad 39A to take up the roll as the STS-126 shuttle.

“The team continues work supporting a launch date of 11/14 (NET). The STS-126 payload is scheduled to be transferred to the Pad-A PCR (Payload Changeout Room) on 10/23. STS-126 is scheduled to roll around to Pad-A on 10/25,” added Tuesday processing information.

While Endeavour waits for Atlantis to depart Pad 39A for rollback to the Vehicle Assembly Building (VAB), the completion of fuel loading for Endeavour’s systems on Pad 39B has removed several days of pad flow operations after her “rollaround” to the adjacent pad.

In total, the current schedule shows Endeavour has four contingency days for being ready to launch on November 14.

“S0024, prelaunch propellant servicing, was completed this morning. The team drained and refilled the left and right RCS. All S0024 activities are now complete for flight.”

L2 members: All documentation Рfrom which the above article has quoted snippets Рis available in full in the related L2 sections, updated live.

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