Discovery completes dramatic rollout – Unique SRB repairs to take place at Pad
With around three weeks until the launch of STS-128, Discovery has finally arrived at Pad 39A, following a dramatic 10 hour-plus rollout on Tuesday. Once at the pad, engineers will incorporate around a week’s worth of changeouts associated with the left Solid Rocket Booster (SRB), following issues with a Check Valve Filter Assembly (CVFA) – a system that has never required replaced during a KSC flow until now.
Discovery had spent over a week inside the Vehicle Assembly Building (VAB), undergoing mating with her External Tank (ET-132) and twin SRBs. It’s been an eventful flow ahead of rollout, with a number of engineering challenges noted.
While soft and hard mate procedures went to plan, engineers noted “galled” threads on the LH2 monoball connector – which required repair.
“Orbiter: OV-103 / ET-132 / BI-139/ RSRM 107 (VAB HB-1) S0004 Orbiter/ET mate: Repair to the LH2 monoball connector galled threads was completed Friday along with LH2 Monoball installation, mates, and closeouts,” noted processing information on L2.
One of the final procedures ahead of rollout is the Shuttle Interface Test (S0008) – which checks the connections and interfaces between the orbiter, ET and SRB hardware. This process was completed during Monday processing.
“S0008, Shuttle Interface Test: VAB testing is essentially complete, less SRB hydraulics (IPR-0046). SRB PIC resistance tests, MDM (Multiplexer/Demultiplexer) verifications, and stray voltage tests are complete,” noted Monday morning processing information. “VAB portion of T-0 and Orbiter/ET interface leak checks are also complete.”
One minor issue related to circuit breakers was noted during the tests. However, this was soon cleared, allowing S0008 operations to be completed.
“During preps for S0008 stray voltage checks, noticed that the PIC Rack Circuit breakers were open and should be closed. Unable to determine exactly what opened them. The breakers were closed which solved the problem and Stray Voltage Checks proceeded normally.”
The main problem noted during the weekend – which ultimately caused the one day slip to the scheduled rollout date – related to a Check Valve on the left SRB.
“During B1009 High Pressure Leak Checks, a loud noise was heard shortly after pressure was applied to the LH SRB tilt system. Engineers were notified on console and noticed a rise in the Gas Generator bed temperature, and secured the system,” added processing information.
“After lengthy discussions, the SRB community believes that a check valve failed, which caused the pump and APU (Auxiliary Power Unit) to spin backwards (which caused the noise that was heard), and potentially damaged the flight hardware.
“As a result, the APU, hydraulic pump and check valve filter assembly will likely need to be R&Red. Engineering is estimating at least a week to R&R, retest, and hot fire the APU. Flow management is evaluating the impacts to the integrated pad flow.”
Two HPUs (Hydraulic Power Units) are located in each booster’s aft skirt assembly, each of which include an APU, hydraulic pump, a fuel supply and a hydraulic fluid manifold. The hydraulic power they generate supplies the TVC (Thrust Vector Control) system, allowing for the booster nozzles to steer during their two minutes of ascent.
Additional engineering notes acquired by L2 confirmed that some of the associated hardware will require replacement at the pad. At present, impacts to the late August launch date are still being assessed.
“On Saturday August 1st at approximately 0120 EST during S0008 SRB Hydraulic Initial High Pressure Leak Checks, a loud noise was heard from the Left SRB Aft Skirt when pressure was applied,” outlined the engineering notes.
“After securing hydraulics, data revealed that the Left Tilt SRB GG temperature increased 15 degrees in approx 6 minutes. Also, on station technician and engineering noted the on-board SRB hydraulic pump was too hot to touch.
“An on-board Check Valve Filter Assembly (CVFA) isolates ground hydraulic pressurized flow from the on-board hydraulic pump. If this check valve assembly fails, the hydraulic pump and the SRB APU will spin up backwards. It is suspected that the CVFA failed and spun the on-board hydraulic pump and SRB APU up backwards, which caused the loud noise inside the Left SRB Aft Skirt.
“This failure mode will require R&R of SRB APU, on-board hydraulic pump, and CVFA.”
While engineers are more than capable of troubleshooting at the pad during the scheduled pad flow milestones, this particular hardware has never been replaced whilst the vehicle is in a stacked configuration out at the pad – or inside the VAB for that matter.
“We (Ground Ops) have never replaced a Check Valve Filter Assembly in the Aft Skirt. The paper (procedure) will need to be written and reviewed carefully since these R&Rs are not normally performed. Also, since an SRB APU has not been replaced in a long time and the CVFA has never been replaced by Ground Ops, we have little experience with these R&Rs.
“Therefore, these R&Rs will be worked closely with the TVC team as they have more experience with these types of R&Rs. Retest of these R&Rs includes 20 to 25 OMRS (Operational Maintenance Requirements and Specifications) and an on-pad hotfire of the Left SRB HPU systems (Rock & Tilt).”
Just to add to the challenge, no spare SRB APU is on site, meaning the replacement hardware will need to be removed from a downstream flight’s Aft Skirt “build-up” – its status ahead of stacking – which will subsequently require a new system itself.
“Currently there are no spare SRB APUs or hydraulic pumps. Therefore, the TVC team will be removing an APU and a pump from one of their Aft Skirt build-ups to get us replacement hardware ASAP. USA (United Space Alliance) Ground Ops will be in work writing disposition tomorrow with help/insight from design.”
Otherwise, operations proceeded to plan for STS-128, with the logistics payload arriving at the pad, and repairs to ET-132’s pull test holes completed – the latter allowing for the retraction of the E, D and B platforms that had surrounded the vehicle inside the VAB.
“S0600 Payload Operations: Payload Canister lift/mate with the PCR (Payload Checkout Room) was completed on Friday. The Payload has been transfer to the PGHM (Payload Ground Handling Mechanism),” processing information added.
“TPS Foam Bond Adhesion: 18 additional pull tests were performed on the -Z side of the external tank on Friday. All pull values were above the 30 psi requirement.”
Discovery is hoping to launch no later than the end of the month, so as to allow for the best possible launch window availability for Japan’s debut flight of the HTV cargo vehicle to the International Space Station (ISS) next month.
However, that launch date was nearly put into doubt when a decision to rollback was almost made during today’s trip to 39A, as the crawler-trasporter started to struggle with mud on the crawlerway.
“Turn before 569 is all mud. We are 100yds from CCF road and crawler trucks are screeching bad. Surveyed remainder of crawlerway and it doesn’t look any better. Have 8” gaps in crawlerway,” noted one NASA Test Director.
“Would be OK to return to VAB because we have already squashed the water out. Recommend return to VAB due to high risk of damaging crawler tracks.”
With bad weather closing in on the KSC area for later in the day, having Discovery stuck out in the middle of the crawlerway would have proven to be very undesirable, and further calls to bring the vehicle back into the VAB were being made.
However, engineers decided they could keep the crawler moving and get Discovery to the pad, as much as rollback was being allowed by management has it been required, with Launch Director Mike Leinbach saying “I don’t give a damn about launch date. Let’s protect this vehicle,” at one point over the ground communication links.
Discovery and her STS-128 stack finally arrived for harddown on Pad 39A, over ten hours after departing from the VAB.
Refer to live update pages – linked above – for full rollout coverage and a review of the discussions that took place on a potential rollback.
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.