Discovery’s Launch Windows and PRSD Requirements Reviewed by MOD

by Chris Gebhardt

As NASA and Space Shuttle Discovery prepare for two important milestones this week in the STS-133/ULF-5 launch campaign (the SSP FRR on Wednesday and the ULF-5 payload to pad delivery on Thursday), flight managers are pressing ahead with their review of the specific PRSD requirements for Discovery as well as her available launch window opportunities in early November.

Pre-launch Considerations, PRSD load, Pad Hold Times, and CO2 Removal:

As it currently stands (pending final approval at the SOMD FRR targeted for Oct. 19), Discovery is set to launch for the 39th and final time in her illustrious career on Monday, November 1, 2010 at 16:40:13 EDT from Launch Pad 39A at the Kennedy Space Center.

Launching from the same launch pad she did at the beginning of her career over 26-years ago, Discovery’s capstone flight is expect to last nearly twice as long as her first mission: nearly 11 days as opposed to only six days on STS-41D. In fact, the entirety of Discovery’s docked mission to the ISS for STS-133/ULF-5 will surpass the amount of time she spent orbiting the Earth during 41D.

In all, the 11+1+2 day flight will see Discovery docked to the ISS for six full days and two partial days (docking day and undocking day). During this docked phase of the ULF-5 flight, two scheduled EVAs (Spacewalks) will be performed by Al Drew and Tim Kopra from the International Space Station’s Quest Airlock and 30 hours of middeck transfers will be completed to and from Discovery and the ISS.

To accomplish this mission, Discovery will carry four full N2 (Nitrogen) tanks – with a minimum of 15 lbs of N2 transferred to the ISS starting on FD-3 (Flight Day 3).

Additionally, Discovery will launch with four H2 and four O2 cryo tank sets to power her Fuel Cells during the flight. A 5th cryo tank set was removed from OV-103 prior to STS-131 as a weight saving measure to increase the amount of up mass possible on the final two flights of the veteran vehicle.

After PRSD (Power Reactant Storage and Distribution) loading is complete on L-2 days, a minimum of 100 lbs of O2 will be offloaded from each of the 4 O2 tanks to assist with center of gravity and Ascent Performance Margin (APM) for the flight.

While a 100 lb O2 offload from each O2 tank is the FRR baseline for the PRSD load, the rather tight APM on STS-133 has led to a discussion of a possible 255 lb max O2 offload from each O2 tank.

An offload of 255 lbs O2 from each O2 tank would result in no O2 transfer to ISS and a “no margin” situation on O2 consumables.

This is not to say that there would literally be no margin of O2, rather that the flight control team would assume there to be “no margin” when setting up Discovery’s deorbit/landing options at the end of the mission.

Additionally, the earlier in the launch window Discovery launched would directly translate to the margin of O2 remaining in her O2 tanks.

Nonetheless, should flight managers decide against the additional O2 offload, Discovery’s Flight Crew could transfer up to 75 lbs of O2 to the ISS during docked operations. Since the ISS O2 tanks can only accept ~75 lbs at this time, Discovery’s crew would not transfer more than that amount of O2 to ISS.

Furthermore, with these cryo loads, the STS-133 launch team will have ~184 hours (7.6 days) of pad hold time available before a 48 hour scrub turnaround would be necessary to top off the limiting commodity – H2 in this case. This pad hold time is based on the completion of H2 PRSD loading by L-45 hours and the completion of O2 loading by L-43 hours.

Using this PRSD load, Discovery’s three Fuel Cells, in addition to providing electrical power for OV-103’s systems, will also provide power to one GLACIER and CGBA and two AEMs from launch through landing, one GLACIER from FD-4 through landing, the PMM (Permanent  Multipurpose Module) Leonardo from launch through FD-6, and Primary Logics and Drivers from prelaunch through docking to “mitigate orbiter PCA issues,” notes the master Mission and Stage Overview presentation from the MOD FRR (Mission Operations Director Flight Readiness Review), available for download on L2.

Finally, for the 11+1+2 day mission, 66 LiOH (Lithium Hydroxide) CO2 scrubbing canisters will be needed. Twenty-nine canisters are already stowed on Discovery’s middeck and four additional canisters will be taken from the pre-positioned stock on the ISS. To accommodate the remaining demand, the ISS’s dual bed CDRA (Carbon Dioxide Removal Assembly) and Vozduhk will provide CO2 removal service for the Orbiter during the docked phase of the mission.

Launch Windows:

With these considerations and procedures in place, Discovery’s launch window is expected to open on November 1, 2010 following the conclusion of the Cocoa Beach Air show at the end of the preceding week.

This will give Discovery’s launch team only seven days to launch the mission before standing down until at least November 24 due to a beta angle exceedance on orbit. This period, known as a Beta Angle Cutout, is used to describe a period of time when the angle of the sun in relation to the orbital plane of the ISS is such that it creates temperatures that violate the safety guidelines of the mated Shuttle/Station complex.

Given this constraint, Discovery’s opening launch attempt on November 1 would occur at ~16:40:13 EDT. The precise second of launch will be determined, as always, during the final hold at T-9 minutes and will reflect real-time tracking and perturbations of the Station’s orbit.

As always, launch will be timed for the millisecond Earth’s rotation carries Launch Pad 39A into the dead-center of the Station’s ground track – called the in-plane launch time. The actual launch window for each launch time listed here begins five (5) minutes prior to and ends five (5) minutes after the listed time for all FD-3 rendezvous opportunities – which exist for every launch day in the early November window.

Therefore, the daily launch times for Discovery’s November launch window, according to the Mission and Stage Overview presentation: Nov. 2 at 16:17:42 EDT, Nov. 3 at 15:51:59 EDT, Nov. 4 at 15:29:28 EDT, Nov. 5 at 15:03:46 EDT, Nov. 6 at 14:41:14 EDT, and Nov. 7 13:15:23 EST. NOTE: The United States transition to Eastern Standard Time at 0200 hours Sunday, Nov. 7, thus making the Nov. 7 attempt 13:15:23 EST.

STS-133 Specific Articles:

In addition to these in-plane times, Discovery will also have FD-4 rendezvous launch capability – based on phasing angles – every other day of the early November launch period stating on the opening day of the window.

FD-4 rendezvous launch opportunities exist on Nov. 1, 3, 5, and 7th. The FD-4 rendezvous launch opportunities translate to an additional 2-3 minutes of launch window past the nominal 10-minute launch window on these days.

Only FD-3 rendezvous launch opportunities exist for Nov. 2, 4, and 6.

As with numerous previous launches on days with FD-4 rendezvous capability, the STS-133 launch team will be allowed to make use of the additional times in order to launch Discovery and will not be constrained to FD-3 launch times only.

Conversely, on Nov. 7 – should that day be necessary and should the launch team, for whatever reason, decide to target a launch time PRIOR to in-plane – some launch times prior to the specified in-plane time may not be available due to phasing constraints between Discovery and the ISS.

(Lead and Pad Photographs: Larry Sullivan, and MaxQ Entertainment. Graphics, L2 presentations)

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