An Engineering Review Board (ERB) meeting has taken place on Monday to discuss options relating to the leak on Discovery’s Right Reaction Control System (RRCS). Five troubleshooting procedures have failed to correct the issue with a helium isolation valve – with additional troubleshooting set to take place. Should those efforts also fail, flight rationale would be required to avoid a rollback to receive a donated system from Endeavour.
STS-131 Processing and Troubleshooting Latest:
While the RRCS issue is being discussed, engineers will continue with the pad flow for STS-131, pushing forward with the realigned schedule that will see Discovery’s payload arriving at 39A on Friday (delayed again). The Agency level Flight Readiness Review (FRR) remains on track for March 26.
“OV-103 / ET-135 / SRB BI-142 / RSRM 110 (Pad A): S0024 Hypergolic Propellant Servicing was completed less APS (Aft Propulsion System) QD (Quick Disconnect) de-mates (IPR-0033),” noted the NASA Test Director (NTD) on Monday processing information (L2).
IPR (Interim Problem Report) 33 is the latest issue to be charged against Discovery’s OMS/RCS system during hypergolic loading – although all were resolved, bar the decrease (leak) in the helium tank pressure on the RRCS, in unison with the RRCS fuel propellant tank when the tank was vented.
“New IPR 0033 to OMS/RCS: While performing the Aft RCS fuel tank vents, the RRCS Fuel He tank pressure dropped, indicating a leak through the He Isolation Valves. All attempts to resolve this issue were unsuccessful. The test team decision was to continue with hypergolic fuel load with this IPR condition.”
Via an overview presentation provided to the ERB on Monday (and acquired by L2), the real time findings on Friday evening were expanded on – along with the listing of the five troubleshooting attempts that have taken place so far to resolve the leak, all of which were unsuccessful.
“IPR 131V-0033: RRCS Fuel GHe Tank pressure decreased during fuel propellant tank venting (indicating excessive Helium Isolation Valve leakage). Troubleshooting was performed to attempt to establish a measurable leak rate, seat valve(s), and again try to establish a measureable leak rate. Currently the flow through the closed valves has been limited to the controlled vent of the propellant tank (~ 5 million scch),” noted the presentation.
“Troubleshooting Performed to date: 1) Pressurize helium tank and monitor for a pressure increase upstream of the GHe iso – No Joy: Helium tank and propellant tank pressurized at the same rate, pressure subsequently vented. 2) Cycle LV301, GHe Iso ‘B’ valve OPEN then CLOSE. Pressurize helium tank and monitor for a pressure increase upstream of the GHe isolation valves – No Joy: Helium tank and propellant tank pressurized at the same rate.
“3) Cycle LV303, GHe Iso ‘A’ valve OPEN then CLOSE. Pressurize helium tank and monitor for a pressure increase upstream of the GHe isolation valves – No Joy: Helium tank and propellant tank pressurized at the same rate. 4) Pressurize helium tank (and prop tank) to above reg lock up. Vent propellant tank in an attempt to seat the GHe isolation valves – No Joy: Helium tank and propellant tank vent at the same rate.
“5) Pressurize helium tank (and prop tank) to above reg lock up. While venting through propellant tank cycle GHe isolation valve(s) in an attempt to seat the valves: a) Cycle LV303, GHe Iso ‘A’ valve OPEN then CLOSE – performed under flow a total of times, b) Cycle LV301, GHe Iso ‘B’ valve OPEN then CLOSE – performed under flow a total of 3 times – No Joy: Helium tank and propellant tank vent at the same rate.”
Discovery’s Right OMS Pod – and associated hardware such as the RRCS – is known as RP03 (RP=Right Pod). Historical documentation shows this hardware has performed without a major problem since debuting on STS-96.
STS-131 Specific Articles: http://www.nasaspaceflight.com/tag/sts-131/
“RP03 Past Performance: (He Isos) Checked out in OPF at start of flow. Leakage was well in-spec (51 scch). Historically have been good since at least STS-96,” the presentation noted, before showing the historical performance of the associated valves.
“Series regulators: Primary stages. Response testing performed, results in spec. Leak testing was not performed. Since STS-116, one of primary regs creeps after press to flight although did not repeat on STS-128. Secondary stage 252 & 329 scch in the OPF this flow. RV last tested 7/2008, results in spec.”
Three clear options are available to managers. Firstly, engineers will attempt to troubleshoot the issue at the Pad, with the results passing through to the potential creation of a waiver in the Launch Commit Criteria (LCC) – likely to be discussed for approval at the Agency FRR – to allow Discovery to fly “as-is” via flight rationale. Should those options become unacceptable, a repair would be undertaken.
A repair cannot be carried out at the pad, and would result in rollback to the Vehicle Assembly Building (VAB), demating of Discovery from the External Tank/SRB stack, rollback to her Orbiter Processing Facility (OPF), demate Discovery’s right OMS Pod for repairs in the Hypergolic Maintenance Facility (HMF), whilst Endeavour’s Right OMS is donated to Discovery. An expected three month delay would result in the launch schedule.
“Options: Fly as is – activate system for flight and monitor reg health. Troubleshoot at the pad. Repair – cannot be performed at the pad. Swap pod with OV-105 (Endeavour) RP04 (‘quickest’ option),” the presentation added. “HMF Repair, Three weeks required after pod removal, (donated to Endeavour).”
Additional troubleshooting at the pad is likely to be the first option to be evaluated, with three clear options already outlined – although each path holds no guarantee of success.
“Troubleshooting Ideas: Could cycle valves from cockpit. Each valve cycled nine times via LPS (Launch Processing System) already. No different performance expected, same circuitry used by LPS or cockpit switch,” noted options presented to the ERB.
“Run regulator flow tests. Portable reg panel needs to be reactivated. Panel ready to support – TBD (To Be Determined, around one week). 700 psia inlet pressures (1500 in OPF). Panel designed to test reg response only. Any other test capability, e.g. reg & iso leak rates TBD.
“Try ‘baking’ valve by extended powering of valve Soften Teflon of main seat to improve sealing. May not help if the pilot is leaking. Connection demate in doghouse door required along with subsequent SCAN retest.”
Flight rationale is by no means out of the question, but will be subject to the condition and performance of the related redundancy in the system.
With the current indications pointing to at least one of the two parallel helium isolation valves as leaking, two helium system regulators – a primary and a secondary – will be tested to verify they are functioning correctly.
These two regulators are downstream of the isolation valves, and are used to maintain helium pressure to the fuel tank. If there are problems found with one or both regulators – during testing this week – flight rationale will be very hard to approve.
“Each He iso valve is one of three series valves: He Iso, Primary Reg Stage, Secondary Reg Stage. If all three leak a Burst Disk/Pressure Relief valve protects downstream components from over pressurization,” explained the ERB presentation.
“Depending on leak rate, could use thruster firings to help control tank pressures. Thruster firings limited while docked to ISS (International Space Station).”
Once test equipment has been stationed at the pad, the helium system will be brought to flight pressure and engineers will monitor the panels to ascertain whether the regulators function properly. Even if both regulators are deemed to be in a good condition, a flight rationale debate will still be required.
More information and articles will follow, as information is collated into L2.
L2 members : Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4500 gbs in size