STS-133 launch slip impacts HTV-2 and ATV-2 missions

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Following the further delay of the STS-133/ULF-5 mission into late February, International Space Station (ISS) Program Managers are readjusting the mission plans for Europe’s Automated Transfer Vehicle-2 (ATV-2) “Johannes Kepler” and Japan’s H-II Transfer Vehicle-2 (HTV-2) “Kounotori”. Both vehicles will be present at the ISS during the STS-133 mission, which should provide for some stunning views during Discovery’s flyaround post-undocking.

ISS Visiting Vehicle Status for ATV-2/HTV-2:

ATV-2 is dictating the choice between Discovery’s February 24th and 27th launch dates, with the deciding factor being the length of time between ATV-2’s launch and docking to the ISS. ATV-2 is currently planned to launch on February 15th, and dock to the ISS at the Service Module (SM) Aft port on February 26th.

That means an eleven day free flight for ATV-2 – eight days of minimum free-flight time which covers any phase angle in the ISS’s orbit, and three extra days to allow for three consecutive launch attempts for ATV-2 without having to adjust the docking date.

If Discovery were to launch on the 24th February, then under current plans, Discovery and ATV-2 would be docking to the ISS on the same date – 26th February. This is not doable due to the amount of work that would be placed on the ISS crew.

As such, the ISS Program (ISSP) is currently in discussions with the European Space Agency (ESA) with a view to eliminating the three extra days from ATV-2’s free flight, which would allow ATV-2 to dock on 23rd February, instead of the currently planned 26th. Should this agreement be made, this would allow Discovery to launch on the 24th – one day after ATV-2’s docking.

The major issue with this plan is that if ATV-2’s launch is scrubbed by one day, then Discovery’s launch will also need to be delayed by one day, due to the fact that the three extra days have been removed from ATV-2’s free flight, and so no time exists in the free flight for launch slippage.

If no agreement can be made, Discovery will launch on February 27th – one day after ATV-2’s docking.

Discovery’s launch slip has also affected the robotics operations for HTV-2, currently planned to launch on 20th January, for a rendezvous and berthing to the ISS on 27th January. The launch window, however, extends from the 20th to the 28th January, with launch attempts to be made every three days.

HTV-2 will be a challenging mission from a robotics perspective, with multiple operations planned that include use of the Space Station Remote Manipulator System (SSRMS) “Candarm2″, the Special Purpose Dextrous Manipulator (SPDM) “Dextre”, the Mobile Base System (MBS), the Mobile Transporter (MT), and the Japanese Experiment Module Remote Manipulator System (JEM RMS).

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Shuttle Discovery is dictating the robotics operations due to the fact that she is carrying the ExPrESS [Expedite the Processing of Experiments to Space Station] Logistics Carrier-4 (ELC-4) in her Payload Bay (PLB), which is the install site for both of HTV-2’s external Orbital Replacement Unit (ORU) payloads – the Flex Hose Rotary Coupler (FHRC) and Cargo Transportation Container-4 (CTC-4).

Due to the fact that Discovery’s launch date has been pushed into the late February window, there will now be insufficient time to complete all ORU transfers to the ISS prior to HTV-2’s departure date on 28th March. If STS-133’s launch date is pushed beyond the late February window, then both ORUs will have no install location on ISS prior to HTV-2’s departure.

A full overview of HTV-2 robotics operations was detailed in an expansive set of HTV-2 Mission Operations Directorate (MOD) Flight Readiness Review (FRR) documents, available to download on L2.

On HTV-2 Flight Day-7 (FD-7), the HTV will be captured and berthed to the Node 2 Nadir Common Berthing Mechanism (CBM) by the SSRMS, which will be based on the Node 2 Power and Data Grapple Fixture (PDGF).

A few hours after the berthing, the SSRMS will grapple the Exposed Pallet (EP) in HTV-2’s Unpressurised Logistics Carrier (ULC), in order to provide power to the EP’s two ORU payloads.

A few days after the berthing, the SSRMS will extract the EP from HTV-2’s ULC and hand it off to the JEM RMS. The JEM RMS will then attach the EP to the Japanese Exposed Facility (JEF) via the JEF’s Exposed Facility Unit (EFU), and the EP’s Payload Interface Unit (PIU).

When connected together, the EFU and PIU become an Equipment Exchange Unit (EEU).

A few days later, the SSRMS will retrieve the SPDM from MBS PDGF number 2. The SPDM, on the end of the SSRMS, will then retrieve the FHRC and CTC-4 from the EP on the JEF.

Two plans are being considered that would either place both ORUs on the SPDM’s Enhanced ORU Temporary Platform (EOTP), which was delivered during STS-132 last May, or one ORU on the EOTP and another on one of the SPDM’s “arms”, which may better distribute the heater power draw of the two ORUs.

The SPDM will then be placed on the MBS. Having ORUs on the SPDM during STS-133 may carry some issues, however, as one of STS-133’s EVA tasks is to install a CLPA lens cover over the SPDM’s video camera.

In the days following the ORU transfers to the SPDM, the JEM RMS will grapple and remove the EP from the JEF, and hand it off to the SSRMS. The SSRMS will then re-insert the empty EP into the ULC.

This plan ensures that both ORUs are transferred to ISS and the EP re-inserted into the ULC before HTV-2 has to depart the station.

The next task for the SSRMS, still based on the Node 2 PDGF, will be to grapple HTV-2 and relocate it from the Node 2 Nadir CBM to the Node 2 Zenith CBM. The relocation is a Launch Commit Criteria (LCC) for STS-133, as HTV-2 would preclude payloads from being removed from Discovery’s PLB if it were berthed to Node 2 Nadir.

This means that Discovery cannot launch until the relocation has been performed, due to the fact that, should a problem arise during HTV-2’s unberthing from the ISS, Discovery would be unable to perform her mission objectives of installing the Permanent Multipurpose Module (PMM) and ELC-4 onto the ISS.

The Mobile Servicing System (MSS), which includes the Mobile Transporter, MBS, SSRMS, and SPDM, will then be reconfigured to support the STS-133 mission. All of the aforementioned operations are time-critical, as they must be completed before Discovery launches. If any problems or issues arise during these robotics operations, very little time will exist to perform troubleshooting.

Following the successful installation of ELC-4 onto the S3 Nadir Inboard Payload Attach System (PAS), and Discovery’s departure from the ISS, the SSRMS will position itself on the MBS, and the MT (with MBS attached) will then translate from Worksite 5 (WS-5) to WS-2. The SSRMS will then grapple the SPDM, and the SPDM will install the FHRC and CTC-4 onto ELC-4.

On the third and final day of the ORU transfer operations, the SSRMS will place the SPDM back onto the MBS, the MT will translate from WS-2 to WS-5, and the SSRMS will then be relocated to the Node 2 PDGF.

It is unclear at this time as to whether HTV-2 will be relocated back to Node 2 Nadir prior to unberthing. Whichever option is chosen, the final task for the SSRMS will be to unberth HTV-2 from Node 2 Nadir or Zenith, and release it to free-fly once again.

(Images used: Lead – NASA.gov. Within the article  – via L2 presentations).

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