Discovery Completes Visually Spectacular Docking to Space Station
A healthy orbiter Discovery and her crew have completed a stunning R-bar Pitch Maneuver (RPM) and docking to the International Space Station. Additionally, Discovery’s extremely clean ride to orbit signals the outstanding work of numerous engineers to clear a potential contamination concern within the External Tank following the inadvertent opening of a backup Fill and Drain valve in Discovery during detanking operations following the November 5th launch scrub.
Following a flawless launch on Thursday afternoon and the all-important course correction burns and OBSS (Orbiter Boom Sensor System) inspections of Discovery’s RCC (Reinforced Carbon-Carbon) Wing Leading Edge and Nose Cap panels, as well as her upper flight surfaces and T0 umbilicals yesterday, Discovery is closing on her orbital target: the International Space Station.
With the Terminal Initiation burn (TI) at ~1133 EST this morning, Discovery’s crew began their final approach to the ISS. The TI burn, which makes final course adjustments to the Shuttle orbiter’s approach to the ISS, placed Discovery and her crew on the proper heading to reach a point 600-feet “below” the Space Station along the R-bar.
Once on the R-bar, Mission Control, taking into account the need to avoid potential Longeron shadowing on Discovery, gave Discovery’s crew the go ahead to perform the 180-degree R-bar Pitch Maneuver, or RPM, to allow the flight crew aboard the International Space Station to conduct a photographic survey of Discovery’s underbelly TPS (Thermal Protection System) tiles.
With this maneuver complete, Commander Steve Lindsey performed the TORVO maneuver to take Discovery from the R-bar to the V-bar (Velocity bar) directly in front of the Space Station’s line of travel.
Once properly aligned within the docking cone, Lindsey pulsed Discovery’s thrusters to reduce the orbiter’s speed by two-tenths of a foot per second, effectively slowing Discovery and allowing the ISS to “catch up” with the orbiter.
Then, at a much closer range, Lindsey once again pulsed Discovery’s thrusters to increase the orbiter’s speed by one-tenth of a foot per second, effectively slowing the closing distance between the two spacecraft while still allowing the ISS to “catch up” to Discovery.
Following docking at ~1416 EST, hard seal will be confirmed and air leak checks performed between Discovery’s Orbiter Docking System (ODS) and the Space Station’s Pressurized Mating Adaptor 2 (PMA 2) – although around 40 minutes was lost from the schedule as the alignment and relative motion was dampened out between the two vehicles to allow for hard dock.
Following the successfully completion of leak checks, the hatches will be opened between the two vehicles. The rest of the day was spent removing ELC-4 (Express Logistics Carrier 4) from Discovery’s payload bay and attaching it to the Integrated Truss Structure of the International Space Station.
Although it was not originally clear if they would be able to complete the installtion by the end of the Flight Day, the crew opted to press on with the task, which will result in a wake up time 30 minutes later than scheduled on Flight Day 4.
STS-133 News Articles (over 110 articles): http://www.nasaspaceflight.com/tag/sts-133/
PV13 Incorrect Configuration During ET Drain following Nov. 5th Launch Scrub:
Thursday’s successful launch of Space Shuttle Discovery proved the dedication and hard work of the thousands of engineers around the agency who came together to prepare Discovery for her capstone mission.
While the ET intertank stringers certainly gained a great deal of attention during pre- and post-launch news coverage, a second issue related to the External Tank following the November 5, 2010 launch scrub was also worked by the ET and Main Propulsion System (MPS) teams: the inadvertent opening of the backup PV-13 fill and drain valve inside Discovery.
Following the scrub of Discovery’s Nov. 5, 2010 launch attempt, Launch Control Center (LCC) personnel initiated ET drain operations to remove the nearly 535,000 gallons of LO2 (Liquid Oxygen) and LH2 (Liquid Hydrogen) from ET-137 (External Tank 137) via the primary PV fill and drain valves located inside the aft compartment of orbiter Discovery.
As noted my the anomaly review and recommendation presentation, available for download on L2, “During LH2 drain on November 5th STS-133 launch attempt, PV-13 was incorrectly configured open for approximately 7.5 minutes (tank was about 99% full).”
PV-13, a backup fill and drain valve inside Discovery’s MPS (Main Propulsion System) plumbing, is nominally configured closed during ET drain operations unless the PD3 valve is configured closed – which would only occur during an abort scenario.
Thus, for STS-133’s November 5 ET drain operations, the PD3 valve was in its nominal “open” configuration. PV13 was then accidentally commanded “open,” allowing LH2 to flow from the ET via the PD3 valve, past the open PV13 valve, and into the MPS plumbing inside Orbiter Discovery – causing a concern for potential for the spread of contaminants in the LH2 stream into the SSMEs (Space Shuttle Main Engines).
As part of the anomaly and potential contamination review for this incorrect configuration, NASA engineers identified only one occurrence in Space Shuttle Program (SSP) history where the PV13 valve was opened during ET drain operations.
“In 1989, for STS-30R, waiver (WK1125R1) was processed to conduct entire drain with PV13 open to maintain the ET recirculation line at cryogenic temperatures to prevent possible jacket overpressure or line rupture,” notes the anomaly presentation.
The wavier for STS-30R was processed based primarily on ET cleanliness procedures – procedures which ensure that any potential contamination in the ET LH2 tank remains below 400 microns (S/B 1000). This ET cleanliness limit is based on the SSMEs’ ability to tolerate LH2 fuel stream contamination at quantities less than 400 microns.
Waiver rationale for STS-30R was further based upon the understanding that it would be “Highly improbable that any contamination in the ET could transport into the recirculation system.” This recirculation system includes a recirculation (recirc) line the penetrates the side of the ET.
Additionally, since LH2 has a low density, any contamination would therefore settle at the bottom of the ET LH2 tank or on the ring frame and would not be located in the area of the recirc line exit point; since the “viscosity and dynamic pressure of LH2 is very low in the recirc line exit during drain . contamination would have to be in [the] immediate vicinity of [the] exit to be transported [into] the line.”
Moreover, during ET drain, most of the LH2 flow out of the tank is conducted through the main LH2 feedline from the ET to the Orbiter, not through the recirc line.
Contamination Concerns for STS-133:
The specific contamination concern for STS-133 was based on the fact that during the 7.5 minutes that PV-13 was open, ~2,000 gallons of unfiltered LH2 (with potential contamination) entered the main LH2 transfer line.
When the PV-13 valve was then closed, and the remaining ~300,000 gallons of LH2 drained from the tank through the main LH2 transfer line, any contamination already in the main transfer line could have been deposited in the recirculation line, on valve components, on the Ground Support Equipment (GSE) debris plate, or on the GSE 70 micron filter.
If this was the case, subsequent tanking (fueling) operations could then transport the contamination in the fill line to the Orbiter-side of the 400 micron screen at the bottom on the ET LH2 tank. “At engine start, [the] potential contamination [could then have been] transported to SSMEs through [the] 1000 micron pre-valve screens” with particles up to 1000 microns transported into the engines themselves.
Anomaly and Flight Acceptance Rationale:
From an SSME system perspective, a positive pressure was maintained within the SSME fuel bleed lines during the time that PV-13 was open. This indicates that no flow of LH2 existed into the SSME bleed lines; thus, all potential contamination would have been swept into the LH2 transfer line and GSE filters.
The GSE filters were subsequently removed and replaced (R&Red) to eliminate those potential contaminates from the system.
Therefore, there was a “Very low likelihood that any contamination [would] be either present within the SSMEs or driven to the SSMEs from the MPS system.”
Likewise, a system-wide Risk Acceptance Rationale was performed and no change to the overall system risk was identified. This was concluded due to the “Low likelihood of contamination being entrained in reverse flow through [the] 4″ recirculation line.”
Further Risk Acceptance was based on the fact that “Any contamination trapped in the recirculation line would likely be flushed back into the tank through PD3 on [the] subsequent tanking, then filtered by [the] 400 micron screen.”
Removal and Replacement of the 70 micron ground filter on the MLP and negative Boroscope inspection results of the potentially contaminated MPS LH2 lines also aided Risk Acceptance rationale.
Therefore, a final recommendation was made that “the additional risk that resulted from IPR 133V-0070 condition is acceptable and remains within the current risk classification.”
The incredibly successfully and clean launch of Discovery on Thursday afternoon demonstrated that the team’s belief that the risk for ET contamination was low and that if any contamination existed it would be filtered out by the MPS fuel filters before it could reach the SSMEs was indeed correct.
With Thursday’s successful launch, orbiter Discovery (OV-103) became the first and only vehicle to suffer to a rollback to the VAB during the launch campaigns for both her maiden voyage (STS-41D in 1984) and final voyage (STS-133 in 2010/2011).
For STS-41D, Discovery was rolled back to facilitate destacking and return the OPF (Orbiter Processing Facility) for SSME replacement and inspections following the June 1984 post-SSME start RSLS (Redundant Set Launch Sequence) abort. The STS-133/Discovery stack was rolled back for ET intertank stringer inspections and repairs.
Additionally, Thursday’s successful launch also means that Discovery is (to date) the only Shuttle orbiter to suffer a non-vehicle related day-of-launch liftoff delay on both her maiden and final voyages. On STS-41D (August 30, 1984), launch was delayed six minutes 50 seconds when an aircraft strayed into the launch safety “no fly zone.” STS-133 was delayed two minutes 57 seconds by an Eastern Range computer glitch. (Endeavour is the only other Shuttle orbiter to suffer at non-vehicle related day-of-launch liftoff delay on her maiden voyage.)
With her docking to the ISS today, Discovery will have completed 13 dockings with the ISS, as well as 14 overall dockings to a space station (13 to ISS and 1 to MIR) and 15 rendezvous with a space station – 13 to ISS and 2 to MIR, one docking and one proximity ops approach to 11 meters.
With her 13 dockings to the ISS, Discovery will clinch the record for most dockings to the International Space Station of any vehicle in history – a title she will always retain since no Space Station vehicle currently in development is slated to fly more then 10 times. (Endeavour and Atlantis, assuming STS-134 and STS-135 fly, will each finish their careers with 12 dockings to the ISS.)
(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 Flight Day coverage during the mission. Images used, lead: NASA.gov. Rest via L2 documentation and L2 Historical STS-41D section).