Tank battle back on – this time with STS-120

by Chris Bergin

Engineers at the Michoud Assembly Facility (MAF) are no strangers to the battle of protecting the shuttle launch schedule by shipping their External Tanks on time – and this battle has now moved to ET-120.

The tank is set to fly on STS-120, Discovery’s October mission to install the Node 2 ‘Harmony’ module on the ISS. Ongoing Return To Flight (RTF) modifications on the tank are now expected to take an additional four weeks.

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ET-120 has had an eventful life already, having been one of four ETs that were sent back to MAF in New Orleans from the Kennedy Space Center for modifications, following the loss of Columbia.

ET-120 was originally set to fly on Discovery’s RTF mission STS-114, until it was replaced by ET-121 following ECO (Engine Cut Off) sensor problems during tanking. 

The tank was also at the center of the NASA decision to remove the PAL ramps from all tanks ahead of STS-121, after visible cracks were observed on ET-120’s foam surface. The PAL ramps came into focus following a liberation event during STS-114, where a chunk of the ramp came loose during ascent – although questions remain on the actual cause of the foam release.

With ET-120 being recycled for flight, work is still being completed on the final stages of its RTF modifications, most notably on the Ice Frost Ramps (IFRs) validation procedures.

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The tank was set to leave MAF for the five day transport by barge on June 4, but that date has now slipped twice this month.

However, the slip isn’t damaging the processing flow of Discovery at present, with a 30 day buffer – aided by the recent changes to the launch manifest which also changed the flight from Atlantis to Discovery – before there’s an impact to the required date for ET mate with the Solid Rocket Boosters.

‘Completion is about 24 days down to schedule (was June 4),’ noted PRCB (Program Requirements Control Board) information earlier in the month on L2. ‘Challenges with non-recurring verification and validation is driving the late completion, but KSC says they have about 30 days they can absorb with no impact to ET Mate.’

24 days became four weeks this week, as per information in the latest Shuttle Standup/Integration report (L2), although Lockheed Martin sources note they are confident that any further delays will not breach the 30 day ET mate buffer.

‘ET-120 progressing at MAF. Critical path still runs through station 1593 ice/frost ramp V&V (verification and validation),’ added the Standup report. ‘Showing themselves as four weeks down on schedule. Working through non-recurring scope. Making progress putting pressurization lines down and on remainder of RTF mods.’

STS-120, commanded by US Air Force Colonel Pam Melroy, will deliver launch package 10A, which consists of the US Node 2 (with four DC-to-DC Converter Unit (DDCU) racks and three Zero-g Stowage Racks (ZSR) installed), a Power and Data Grapple Fixture (PDGF) for the station’s robot arm, and a Shuttle Power Distribution Unit (SPDU).

Node 2 – which will also provide a docking port for future shuttle missions – will be the first pressurized habitable module delivered to the station since the Quest Airlock was installed in June, 2001. Also on this flight, the P6 solar arrays will be moved from the Z1 truss on top of the Unity module to its final position at the port end of the truss.

The mission will have new elements to deal with, which were handled by the PRCB (Program Requirements Control Board) re-baselining in June 2006.

‘STS-120 was originally baselined in the FDRD on January 23, 2003 with a February 19, 2004 launch date. Following the Columbia accident, the mission was removed from the FDRD. Recently the ISSP has decided to accelerate the ISS International Partner (IP) Module missions by one flight. This includes the STS-120/ISS-10A (Node 2) mission,’ noted the presentation.

‘The STS-120 mission requirements have been updated to include not only the Node 2 installation but also the definite transfer of the OBSS Integrated Boom Assembly and the P6 relocation task (previously planned for the STS-119 (ISS-15A) mission). The STS-119 mission has been deferred until after the delivery of the IP Modules.

Mission objectives include: Unberthing Node 2 from PLB and install on ISS Node 1 Port ACBM. A Crew Rotation, involving Expedition 15 Flight Engineer Clayton Anderson, who will return on STS-120, which also carries his replacement, Daniel Tani, to the station. Tani will return on shuttle mission STS-122.

Other mission elements for the six docked days on the ISS include the Transfer Water and Critical Items. Transfer OBSS IBA to ISS. Relocation of P6 from Z1 to P5 and re-activate P6, which requires back-to-back EVAs. Protect P6 52 Hr Deact-to Reactivation thermal constraint. Get Ahead Tasks – and ISS Reboost and Flyaround Imagery – Prop Margin Dependent.

Following the re-baseline, the mission may be extended past its initial 11 day mission, with the post-Columbia mission elements coming into play. This includes Late Inspection, which is required after undocking to check for any damage on orbit from space debris.

‘Late inspection cannot be accommodated with current mission content; mission duration increase to 13+1+2 would be required to support pre-undocked late inspection but not possible on non-SSPTS OV-104 vehicle; moving the OBSS IBA transfer task to STS-118/ISS-13A.1 would allow pre/post-undock late inspection,’ added the presentation.

At present, the OBSS (Orbiter Boom Sensor System) role in future missions includes stowing the arm on the ISS due to clearance issues with some future payloads. Recently, NASA cancelled the option of building a mini-boom to mitigate this issue.

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