Atlantis completes RPM and docks with the ISS

With her Flight Day 2 (FD-2) Thermal Protection System inspections behind her, Atlantis has docked with the International Space Station (ISS) – following a visually stunning R-bar Pitch Maneuver (RPM). Docking came after engineers cleared a minor ring alignment issue on Atlantis’ Orbiter Docking System (ODS).

STS-129 Status:

The Damage Assessment Team (DAT) are currently evaluating the vast amount of data gained by the Orbiter Boom Sensor System (OBSS), following Flight Day 2’s opening inspections of Atlantis’ TPS, which will be complimented by imagery from the RPM.

“The STS-129 FD2 OBSS Surveys went well. OBSS grapple occurred at 6:12 am and OBSS unberth was at 6:19 am,” confirmed the Mission Evaluation Room (MER) on the completion of the tasks.

“The Stbd T0 umbilical and OMS pod were surveyed at 7:17 am. Stbd RCC was surveyed from 7:33 am to 9:08 am. Nose RCC was surveyed from 9:24 am to 10:00 am. Port RCC was surveyed from 10:22 am to 12:12 am. The Port T0 umbilical and OMS pod were surveyed in between Port RCC scans per procedures.

“OBSS was birthed on the Stbd MPM on the first attempt at 1:34 pm. The analysis team is working in the SES lab making runs to predict the FD2 coverage of the TPS based upon the views crew reconfigured the sensors to during the surveys. The results will be sent out to the DAT and Imagery Analysis team when completed and presented at OPO on FD3.”

Atlantis continues to perform well on orbit, with only the indication anomalies with the ODS ring alignment added to the “funny” list at the MER, per documentation on L2.

“ODS Temporary Loss of Ring Alignment. Occurred twice following Ring Out and similar signatures have been seen on previous flights. ODS is ready to support Docking activities,” noted the MER report, which was expanded on via their latest shift report.

“Ring Alignment Indicator: The loss of ring alignment immediately following the Ring Out command has occurred during previous flights. It is possible during initial ring motor drive operation to experience a temporary loss of alignment which is attributed to an inherent looseness, or slop, in the differential drive train.

“This condition is not known to cause any additional issues to ODS operations during docking and undocking events.

STS-129 Specific Articles:

“The second occurrence of misalignment during ring extension is the first recorded loss of ring alignment one minute into ring extension (at approximately 23.6% linear advance). Review of the initial data plots indicate that a 4 second divergence occurred between Petals 1 and 3 that triggered the sensor ring misalignment.

“Misalignment will occur at ~2 percent delta linear advance between petals in the axial direction and at approximately 2.5 percent delta in plane movement. The mechanism’s centering spring system corrected for the misalignment as designed.

“In discussions with the Russian consultants supporting the mission they felt that many factors may contribute to the loss of alignment. It is possible that friction still exists at certain points in the ball screw mechanism.

A35Atlantis’ ODS system required some troubleshooting work during her Orbiter Processing Facility (OPF) flow. However, the indications are not believed to be related.

“Another factor that they feel is playing a part is the age of the mechanism at this time,” added the MER report. “ODS Engineering and vendor support are in the process of collecting additional data to review possible causes to the ring misalignment. There is a possibility that the loss of alignment may occur again during ring retraction operations.

“Sufficient procedures are in place to address the loss of ring alignment if it were to occur during the docking sequence. Loss of alignment during retract may extend the duration of the docking event during periods when the ring is stopped as the system is allowed to regain alignment but will not prevent docking to ISS.The ODS mechanism is ready to support docking activities.”

Meanwhile, all other rendezvous and docking systems have been checked out, with no issues reported.

“The checkout of the Trajectory Control Sensor (TCS) during Rendezvous Tools Checkout for STS-129 was completed successfully,” added the MER. “TCS is ready to support docking with the ISS. TCS was powered on by the crew, at approximately 5:20 PM, and all data looked nominal. The TCS was powered off at approximately 5:25 PM CDT with the crew indicating completion of Rendezvous Tools Checkout.”

Docking Procedures:

As has become standard procedure for docking day, Atlantis’ crew will perform an NC-4 burn to further phase the vehicle toward the International Space Station (ISS) and increase the vehicle’s apogee and perigee. The major burn — the Terminal Initiation burn – will then place Atlantis on course to arrive 600ft “below” the ISS during the correct lighting conditions for the R-bar Pitch Maneuver (RPM).

Following the RPM (which is scheduled to begin at 10:52 EST), Commander Charlie “Scorch” Hobaugh and Pilot Barry “Butch” Wilmore will maneuver Atlantis “in front” of the ISS along the V-bar (Velocity bar – or the direction of travel of the ISS).

Commander Hobaugh will then fire Atlantis’ thrusters, thereby slowing the Atlantis by a rate of one tenth of a foot per second – allowing the ISS to “catch up” to Atlantis.

If all goes as planned, Atlantis’ Orbiter Docking System and the ISS’s Pressurized Mating Adaptor #2 will connect with one another at ~11:53A.M. EST.

For this mission, was fortunate to have Philip Sloss at the Johnson Space Center for the final four Ascent Simulations for STS-129. Afterward, Philip was fortunate enough to discuss ISS rendezvous operations with Bill Tracy.

Tracy, who was lead Flight Dynamics Officer (FDO) for STS-46 and several subsequent flights, was lead FDO for Atlantis’ flagship mission to the Hubble Space Telescope this year and will be lead rendezvous FDO for STS-130 next year.

During the interview, Tracy described the duties of the rendezvous FDO.

A2As rendezvous FDO “I actually support the ascent shift – not as ascent FDO, but by working the backroom during the launch. We put the rendezvous together between MECO (Main Engine Cutoff) and OMS-2 (Orbital Maneuvering System burn #2). And we only get about twenty minutes to so.”

Basically, the rendezvous FDO has to provide the ascent FDO with the target post-OMS-2 altitude and an assessment of the planar error.

“Now if we have an underspeed or ascent performance issue, my job is to recover from that. Basically, you have to ask yourself: ‘how much extra propellant does it require to do the rendezvous?’ and ‘Is the rendezvous even possible any more?'”

In that 20-minute period after MECO (Main Engine Cutoff), the rendezvous FDO has to determine the rendezvous plan and inform the ascent FDO how to execute that plan.

“A lot of prep work goes into that – we don’t start from scratch. We can’t start from scratch and come together with a rendezvous that quickly. What you do is you come up with your nominal plan and then you try to foresee as many different failure scenarios as possible and try to devise plans that will save you from each of those failures. It’s challenging, but it’s a lot of fun.”

Later in the interview, Tracy explained how the rendezvous FDO executes a nominal rendezvous with the ISS.

“The one thing you have to remember that the rendezvous is an exercise in energy management.”

Depending the mission, the orbiter could have a very large phase angle (closing on the ISS at a rate of 800 – 1000 miles every orbit) or a very small phase angle (closing on the ISS at a rate of 200 miles per orbit or less).

When the rendezvous FDO arrives on console the day of rendezvous he has “to look at whether or not he has to do one burn or two burns before we get into our star-tracker pass.”

“The goal is to put us at our final phasing burn of the day, called NC-4, at about 40 nautical miles behind the station. We do our NC-4 burn at about orbital noon, give or take a little bit. We then go from noon to sunset… at which point our star tracker pass with the ISS begins.

“Up until that point we’ve done nothing but use ground tracking. It’s not until we actually get to that star tracker pass that we get the onboard, optical sensors looking at our target in relation to the orbiter. That tells us how good our ground tracking was.

“We will then do the first onboard targeted burn, based on that newly-acquired relative motion data that was acquired from the star tracker. We call this burn ‘NCC.’ Hopefully it’s very small – less than one foot per second in all three axes: X, Y, and Z.”

In fact, the sole purpose of this burn is to place the orbiter on a course the will place it in the precise spot necessary for the TI burn.

The TI burn, which occurs eight nautical miles “behind” and 1,200 feet “below” the ISS, is designed to both slow the orbiter’s approach to the station and place the orbiter 600 feet “below” the ISS at a particular MET (Mission Elapsed Time).

Here, in a reversal from all previous mission burns, the crew actually handles the calculation of the TI burn, with the ground teams verifying the accuracy of the TI burn calculations and determining if any changes are required.

“Between NCC and TI what I’m doing is ground targeting of the TI burn solution, just like the crew is doing,” states Tracy. “But I’m doing something that the crew cannot; I’m going to be calculating the TI delay solution — the delay of the rendezvous for an orbit (or more).”

Tracy went on to explain this delay in more detail. “The crew cannot target TI delay. So I’m relatively busy between NCC and TI because I’m doing not just the mirroring of the crew’s onboard solution for TI, but I’m also computing a TI delay solution.

“With that, we can literally stop ourselves eight miles behind the station and hold position there for however long we want. Then, when we’re ready to come in, we’ll do another TI solution.”

These solutions (both the nominal TI and the TI delay) are then transmitted to the crew via a maneuver Preliminary Advisory Data (PAD) – the way in which the FDO communicates the burn solutions to the crew (i.e. time of ignition, duration of the burn, which OMS engines will be used, and any other information necessary for the execution of the burn.)

Later, Tracy went discussed the role of the rendezvous FDO following the TI burn.

“Once we get the go for TI, the activity in the control center starts to tail off a little bit. We still have four more burns – called mid-course corrections (MC) – that are hopefully pretty small.

“Once MC-4 is done, the activity level of the crew picks up a lot. They’re looking out the windows; they’re seeing the station; they come up on the R-bar and they do the R-bar Pitch Maneuver. This is pretty much them following the procedures and doing what they were trained to do with as little input from the ground as possible.”

After executing the RPM, the crew will perform the TORVA (Twice Orbital Rate V-Bar Approach) and align themselves on the V-bar with the ISS. The crew will then fire the orbiter’s thrusters, slowing her velocity just enough to allow the Station to “catch up” with them.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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