STS-133: Workhorse Discovery exemplifying routine orbiter excellence

by Chris Bergin

With Discovery enjoying her final docked mission – with glowing praise from her crew during nearly every media event – the workhorse of the fleet is providing no headaches for her engineering teams on the ground during her swansong flight. No issues of note are being worked on the orbiter in one of the “cleanest” missions in shuttle history, otherwise known as the routine excellence from the fleet during recent years.

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When Discovery began suffering from launch delays, due to problems with her External Tank (ET-137), some people joked the orbiter had conspired with her problematic orange friend from New Orleans, in order to resist her impending retirement date. Ironically, Discovery’s superb performance on orbit has resulted in just that, with two additional docked days added by the Mission Management Team (MMT).

The allowance for the additional docked days on the ISS is based mainly on the performance of the Fuel Cells, as they demand sustenance from the orbiter’s PRSD (Power Reactant Storage and Distributation) tank loads, eased by the help from the Station-to-Shuttle Power Transfer System (SSPTS).

Some achievement, given Discovery had been slightly handicapped by the removal of one of her PRSD tanks ahead of the mission during her processing flow, in order to make allowances for the heavy payload she was tasked with delivering to the International Space Station (ISS).

“The MMT approved an additional day on-orbit at the meeting. The first landing opportunity at KSC is now targeted for Wednesday, March 9 at approximately 11:59 EST,” noted the MMT (L2). “An orbit adjustment is anticipated (in the calculations) and could affect the landing time. The new landing time will be posted once it is available.”

The change to the mission timeline will also allow Discovery to achieve yet another historic milestone, one of being in space for a full one year period over her career of 39 flights.

“The Orbiter Discovery’s performance continues to be outstanding,” opened one of the numerous Mission Evaluation Room (MER) reports (L2) on Discovery’s docked mission, which recently included a reboost of the station via the orbiter’s Reaction Control System (RCS) thrusters.

“The crew started the reboost at 062/14:03:37 GMT (6/16:10:12 MET) and terminated the burn at 062/14:32:26 GMT (6/16:39:01MET). The reboost resulted in a total velocity change of 3.3 feet per second (when including the attitude maneuvers) and an altitude increase of 0.92 nautical miles,” added MER notes.

“The right RCS fuel pressure reached a secondary lockup pressure of 263 psia eight hours before the reboost. Following reboost, fuel pressure dropped down to 248 psia. Leak rate will be recalculated after right pod stabilization.”

Although classed as an extremely minor issue, with zero mission impact, the Right RCS Fuel Helium Primary Regulator Creep is the only MER item which can be classed as related to Discovery’s primary hardware. It will continue to be monitored, as it was during the nominal reboost.

“RRCS Fuel primary reg creep continues to be monitored. Eight hours prior to reboost activity, leakage appeared to stop while pressure was at 263 (secondary Lock up pressure). Following Reboost, ullage pressure was at 248 psia. At this time the pod environment is still stabilizing therefore leak rate will not be calculated.”

At the latest count on MER documentation, 25 of 38 primary thrusters have been fired. All required RCS thrusters will be fired – as planned – during the routine checkouts on EOM-1 (End Of Mission -1 day), as the crew prepare to take Discovery back through the atmosphere one final time.

Before that historic return takes place, the crew still have a large amount of work to do on Station, as the goal of leaving the giant laboratory in the best possible configuration continues to be achieved.

With the two extra docked days, the strain on post STS-133 stage operations is being greatly reduced, with the advantage of six extra crewmembers aiding the outfitting of the newly delivered PMM (Permanent Multipurpose Module) Leonardo.

Although there is a large amount of public, media – and event Presidential – interest in seeing the crew rush into the unpacking of the newest member of the ISS team – Robonaut 2 (R2) – the primary goal of outfitting includes the transfer of launch hardware used to protect the PMM’s cargo from the stresses of Discovery’s ride uphill.

The materials include foam packaging, which is being transferred to the HTV-2, ahead of its planned departure from the ISS for a pre-programmed entry into a disposal corridor. Supplies from the HTV-2 also have to be transferred via carefully choreographed procedures, in order to maintain the correct center of gravity in the modules.

These operations continued on Flight Day 10, as work elsewhere on the Station took place, relating to maintenance on the Oxygen Generation Assembly (OGA), which has been waiting for the arrival of Discovery to deliever a remediation kit, whilst providing a ride home to Earth for the OGA’s hydrogen dome in Discovery’s middeck.

Work also took place on the Lab Carbon Dioxide Removal Assembly (CDRA), while the Russians also evaluated problems with their own CO2 scrubber – the Vozdukh – which once again failed late on Flight Day 9.

Once Discovery departs the orbital outpost, she’ll be leaving the Station with a parting donation of spare oxygen. Although it is hard to calculate exactly how many pounds of O2 the Station will be left with – due to changes in tank pressures relating to solar beta angles the ISS flies through – the Station will be left in a very healthy situation.

“On the math from the Environmental Control and Life Support System (ECLSS) officer, we’ll leave with the O2 tanks full on ISS when Discovery departs,” noted ISS Lead Flight Director Royce Renfrew at the latest Mission Status Briefing on Flight Day 9.

The crew are also tasked with adding to the ISS’ water supplies, with the usual shuttle mission task of filling Contingency Water Containers (CWCs). According to MER status, two of the CWC bags were filled with water and iodine on FD 8 as an example of the operation. CWC-I number 7 was filled with 41.4 lbm water and CWC-I number 8 was filled with 42.7 lbm water.

Saturday was the last full day the STS-133 crew will spend on the ISS, as they prepare for their farewells and hatch closure between the two vehicles on Sunday (Flight Day 11). Discovery is scheduled to undock from the ISS at 6:03am Central on Flight Day 12.

(Numerous articles will follow. L2 members refer to STS-133 live coverage sections for internal coverage, presentations, images and and updates from engineers and managers – which is now into full and extensive live Flight Day coverage during the mission. Images used: Lead via Graphics via L2).

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