NASA continues evaluations for ISS Dual Docked Operations

by Chris Gebhardt

Following initially positive opening investigations into the possibility of allowing a Space Shuttle orbiter to be present (i.e. docked) to the International Space Station (ISS) during Russian Progress and/or Soyuz dockings and undockings, Space Shuttle Program and ISS Program officials have decided that more investigation and information is necessary before allowing this scenario to unfold.

DDO Removal from STS-131 and STS-132; STS-131 and 132 Processing Updates:

The decision to delay the formal introduction of Dual Docked Operations (DDO) first came to light via notes from ISS program manager Mike Suffredini. These notes indicated a preference to avoid DDOs for STS-131 – a decision that was later applied to the currently-scheduled May 2010 launch of orbiter Atlantis on the STS-132.

The subject of delaying Discovery’s STS-131 launch to avoid DDO operations was first uttered in response to delays to the veteran orbiter’s rollover to the VAB from OPF-3 due to cold temperatures in Florida – and by extension the VAB.

The final decision to delay Discovery’s launch from March 18 to April 5 was – according to memos obtained by and available on L2 – a result of both the cold temperature delays to Discovery’s rollover and mating operations, which occurred more than a week behind the previous schedule, and the desire to avoid DDO.

“Based on continued cold weather trends and the Program decision to not launch during the dual-docked ops window, we are now targeting the following milestones for STS-131: Orbiter mate 2/22, SSV to the Pad 3/2, Payload to the Pad 3/15, and Launch NET 4/4 (Under Review),” notes a manager’s processing memo dated February 15.

Launch of STS-131 was eventually slipped a further day to April 5 due to what appears to have been a desire to avoid launching on Easter Sunday (April 4).

In all, the realigned schedule gave technicians a week’s worth of contingency days for Discovery’s pad processing flow.

Discovery finally rolled over to the VAB for mating with her External Tank (ET) on February 22 despite initial reports that weather had delayed the rollover to February 23.

Mating operations between Discovery and her ET were extremely smooth thanks to the hard work and continued dedication of the KSC workforce.

Final preparations for Discovery’s March 2 rollout were completed by mid-morning March 1; however, the threat of severe weather at the Kennedy Space Center tomorrow compelled KSC personnel to delay rollout by 24-hours.

STS-131 Specific Articles:

Discovery is now scheduled to rollout to Launch Pad 39A on Wednesday, March 3 at 00:01 EST (12:01a.m.). Call to Stations for rollout is scheduled for 20:00 EST, 8:00p.m.) March 2.

As a result or the rollout delay, the Discovery crew’s TCDT (Terminal Countdown Demonstration Test) has been pushed back by one day to accommodate the new rollout and associated processing dates.

Adding to the busy nature of KSC processing (which is in full swing on all three vehicles), was the arrival of External Tank 136 (ET-136) this afternoon. ET-136 arrived at the Kennedy Space Center’s turn basin via the Pegasus barge just after 1p.m. today.

Offloading of ET-136 – which is destined for use by orbiter Atlantis and the STS-132 mission in May – went very smoothly.

The tank was removed from the barge approximately one hour (1 hour) after the barge arrived on dock at the turn basin and was safely inside the VAB within an hour of first motion out of the barge.

DDO Overview (i.e. Ground Rules):

In all, previous DDO considerations and evaluations determined that DDO (with a docked Shuttle Orbiter at Node-2(Harmony)/PMA-2) was permissible (pending resolution of a few analyses) for dockings and undockings at Service Module aft, DC-1 nadir, FGB (Functional Cargo Block) nadir for undockings only, and MRM-2.

For the case of the FGB nadir port, the DDO docking was ruled out “due to Kurs antenna cone impingement” with the orbiter.

Additional DDO ground rules include mission specific analysis for DDO docking/undockings occurring while a “cargo element is on the Shuttle RMS (Remote Manipulator System),” no DDO activity during an EVA, robotics operations, or on a Shuttle dock/undock day.

Furthermore, the ISS will be under the control of the Russian segment (with Shuttle in a passive mode) during all Russian vehicle dockings. Control of the ISS for undockings will either be controlled by the Russian segment or the Shuttle orbiter’s VRCS (Vernier Reaction Control System) jets, but not both.

Moreover, according to the updated 125 page DDO status master presentation (available for download on L2), DDO is not considered viable during ISS crew direct handover operations.

A direct handover of ISS crewmembers involves a new, three-person subset of astronauts launching on a Russian Soyuz and rendezvousing with the ISS before the outgoing ISS crew subset departs the station. Therefore, ISS occupancy would stand at eight or nine after the new crew’s arrival and before the outgoing crew’s departure.

However, since no more direct handovers are planned for the ISS during the Shuttle’s lifetime, this rule is not expected to effect DDO operations for the remaining two Space Shuttle missions where DDO is still considered viable (STS-134 in July and STS-133 in September).

Additionally, DDO operations have to occur during orbital passes that include “Russian Ground site coverage” to allow the Russians to obtain undisrupted telemetry downlink from their approaching vehicles.

All Soyuz/Progress DDO dockings/undockings must also occur during nominal crew awake periods and all Soyuz/Progress relocation events remain prohibited during Shuttle/ISS mated operations.

Continued Concerns and Evaluations for Particle Impingement:

In all, the continued concern for the orbiter in terms of DDO relates to Soyuz/Progress RCS plume particle impingements and contamination of Orbiter systems, specifically the orbiter’s RCC (Reinforced Carbon-Carbon) Wing Leading Edge panels, windows, payload bay cameras, the SRMS cameras, and the OBSS (Orbiter Boom Sensor System) sensors.

For these environmental analyses, all applicable Russian docking ports were used. As a result, these assessments were able to adequately clear the orbiter’s TPS (Thermal Protection System), the Payload Bay, the Ku antenna, the PLBD radiators, and Trajectory Control Sensor (with post-flight cleaning) from any concerns with regard to plume particle impingement.

Nonetheless, the RCC panels, crew cabin windows, the payload bay cameras, the SRMS cameras, and OBSS sensor packages could not be completely cleared from DDO-related particle contamination.

For the RCC panels, the presentation notes that no “through-damage” to the outer SiC layer is anticipated from Soyuz/Progress RCS particles. Furthermore, all safety margins and RCC damage margins are maintained for DDO particle impingement from any Russian docking port under nominal docking/undocking scenarios (i.e. not including an emergency abort of a docking).

“A significant margin exists between the kinetic energy allowables for RCC damage and that which is predicted from the thrusters of the Soyuz/Progress RCS,” notes the presentation.

Nonetheless, despite these positive margins, the RCC panels were not summarily cleared of DDO concerns by the analysis teams.

“Recommendations/Constraints to DDO: Threat posed to the RCC hardware is still taken seriously and inspections for surface damage will be included as appropriate with the ‘Late Mission’ micrometeoroid and orbital debris (MMOD) inspection that is performed in accordance with operational flight rules.”

The remaining RCC panel concern, however, does not preclude DDO from occurring; nonetheless, the action of not summarily clearing the RCC of all DDO concerns despite the extremely low chance of damage reinforces the strict safety standards NASA adheres to.

The second of the five areas on the Orbiter that could not be completely cleared from DDO particle concerns is the orbiter’s crew cabin windows.

“Analysis/Test: Impact testing and strength testing. Results: Impact testing is completed and impact damage is being measured and characterized,” notes the DDO presentation.

The evaluation criteria for the strength tests are extremely strict. In all, any damage location identified that is deeper than the “allowable depth” will be deemed unacceptable and all damage locations will be evaluated to determine how they interact with each other and affect the strength of the glass.

Strength testing is still on going and will not be completed before March 15. As such, the windows have not been cleared from DDO concerns and will not be cleared until the analyses are complete and the results deemed acceptable.

The third Shuttle-based component that could not be completely cleared from DDO particle contamination concerns were the payload bay cameras.

“Camera lens glass: The exposed front element of the lens of the CTVC (Closed Loop Television Camera) and ITVC cameras (located on the forward and aft bulkheads, the SRMS elbow and wrist locations, and OBSS Sensor Package 1) is a high quality optical glass with a magnesium fluoride anti-reflective coating,” notes the DDO presentation.

These lenses, which are part of the CCTV (Closed Circuit Television) hardware, are the most susceptible to damage and contamination since the lenses can be damaged (pitted) or their optical quality severely reduced by RCS plume particle interference.

Since the damage to these lenses cannot be accurately quantified, protection of the cameras during DDO is necessary to ensure that the cameras remain useful during the mission.

This will be accomplished by pointing the forward and aft bulkhead cameras and SRMS elbow-joint camera away from the Soyuz/Progress’ RCS plume flow.

The SRMS wrist and side-view camera will also be protected from plume flow. If the cameras cannot be protected, video imagery from the cameras for the remainder of the mission would be degraded because of “unwanted glints and/or blurring from residue.”

Following DDO, the ground processing flow would have to examine each lens to determine if “lens cleaning or replacement” is necessary to maintain optical standards.

Forward work for this issue has been identified as the creation of an SRMS protect position that would allow for the necessary protection of all SRMS cameras – a procedure complicated by the need to use the SRMS to protect the OBSS’ various sensor packages during DDO ops.

For the OBSS, the primary DDO particle concern relates to the two sensor packages that are critical in determining the health of the orbiter’s TPS while on-orbit.

Assuming that the OBSS remains in its primary berthing location in the orbiter’s payload bay, “Sensor Package 1 – LDRI (Laser Range Dynamic Imaging) particle fluence for all port locations does receive particle impacts to some degree,” notes the DDO presentation.

This scenario would result in damage to the “thermal silver tape” located on the outside of the sensor.

“In the worst case scenario during flight, the LDRI would operate at higher temperatures; however, it is unlikely that LDRI temperature would exceed the margin for adequate data quality.”

The presentation further notes that, even if LDRI were to experience higher operating temperatures, the likelihood of failure of the LDRI sensor is extremely unlikely and actually classified as “no risk of failure.”

Nonetheless, a cooling-off period would be instated for the LDRI to maintain safe operating temperatures if the silver thermal tape were to be damaged during DDO.

Additionally, “In the OBSS stowed position, the ITVC/LDRI lenses are pointing in the Orbiter aft/ISS nadir direction,” notes the presentation. “This is the surface of the greatest concern in order to protect the optics. The ITVC/LDRI optics are not to be in the line-of-sight of the source of particle fluence.”

Likewise for DDO dockings to DC-1 nadir and FGB nadir, the analysis team has strongly recommended that the OBSS be moved to a DDO protect position underneath the belly of the orbiter or that Russian vehicle DDO be prohibited from these docking ports.

Moreover, analysis for Sensor Package #2 on the OBSS (the LCH and IDC sensors) shows that these sensor lens surfaces are likewise located in the “orbiter zenith/ISS aft surface which in all vehicle operations is the most impacted area.”

For this sensor package, DDOs at DC-1 nadir and SM aft are “drastically worse” than DDOs at FGB nadir and MRM2.

“Based on the DTO-829 (Detailed Test Object 829) test results of a fused silica lens, which LCH has, similar results are expected,” notes the DDO status presentation. “The LCS performance may be impacted for the duration of the mission by contamination of the window or permanently impacted by pitting of the window.”

Conversely, “Assuming the LCS experiences a similar 2 percent pitting effect as the SRMS camera during the test, the effect of pitting on performance of the LCS should be negligible, resulting in a ~2 percent drop in intensity due to scattering effects.”

In all, the analysis teams have recommended that the OBSS be moved to a DDO protect position to ensure the protection of all OBSS sensor systems.

If this cannot be accomplished – for whatever reason – the silver tape of the LDRI sensor will have to be repaired post-flight and a series of cool-down periods for the LDRI sensor might have to be performed during TPS late-inspection operations.

For this OBSS protect position, the original plan by the MOD (Mission Operations Directorate) had the SRMS grappling the OBSS and maneuvering it “underneath” the orbiter.

In this fashion, all OBSS sensors and all but one SRMS camera would be protected from any Soyuz/Progress RCS thruster plume particles.

The one camera on the SRMS that would not be protected would be the side-view camera – which the flight crew would need to monitor clearances.

However, if the SRMS and OBSS were to be placed in a configuration that would protect all OBSS sensors and all SRMS cameras, an additional 10-15 minutes would have to be added to the timeline, additional crew training would be required, and additional clearance monitoring from ISS-based assets would be required.

As always, continued analyses and investigations will be performed and all orbiter systems formally cleared before DDO is cleared for use.

However, as of this writing, DDO will be avoided for STS-131 and STS-132.

While this now holds little impact to Discovery’s STS-131 mission, the avoidance of DDO for STS-132 reduces the mission’s launch window.

Right now, STS-132 (and the final flight of Atlantis) remains on target for a launch on May 14, 2010 at roughly 14:28 EDT to deliver MRM1 to the ISS.

According to an STS-132 Flight Manager memo, “The (Program) did decide to avoid Dual Docked Operations (DDO) for STS-132/ULF4; this includes both the Soyuz 21S undocking and Soyuz 23S docking.

“With that said, the STS-132/ULF4 launch opportunities, avoiding DDO, are May 14-20 and May 30-June 5. These dates assume the current planned undocking and docking dates for 21S and 23S.”

L2 members : Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4500 gbs in size

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