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NASA clears SpaceX for Monday cargo launch
Posted: April 13, 2014

NASA managers Sunday cleared SpaceX to press ahead with launch of a Falcon 9 rocket and Dragon cargo ship Monday on a two-day flight to the International Space Station, concluding the lab can be safely operated until a faulty computer can be replaced during a contingency spacewalk next week.

File photo of the International Space Station. Photo credit: NASA
With forecasters predicting an 80 percent chance of acceptable weather, the SpaceX Falcon 9 version 1.1 rocket is scheduled for blastoff from launch complex 40 at the Cape Canaveral Air Force Station at 4:58 p.m. EDT (GMT-4).

If all goes well, the company's uncrewed Dragon cargo capsule will reach the space station early Wednesday, pulling up to within about 30 feet and standing by until the lab's robot arm can lock onto a grapple fixture around 7:11 a.m.

From there, the Dragon capsule, loaded with 4,600 pounds of equipment and supplies -- including a new spacesuit, spare parts and science equipment -- will be maneuvered into place for berthing at the Earth-facing port of the station's forward Harmony module.

This will be the third commercial space station resupply flight carried out by SpaceX under a $1.6 billion contract with NASA calling for at least 12 cargo missions to deliver some 44,000 pounds of equipment and supplies.

Launch originally was planned for last month, but the flight was delayed, first by concern about contamination in an unpressurized section of the cargo ship and later by a short circuit that damaged a critical Air Force tracking radar.

The contamination issue was dismissed earlier and with the radar system back in operation, SpaceX was in the final stages of launch preparations when a multiplexer-demultiplexer, or MDM, failed aboard the station Friday.

Mounted in the central S0 section of the station's solar power and cooling truss, the MDM is one of more than a dozen compact computer units that control a wide variety of functions. In this case, MDM EXT-2 served as a backup computer used to position the station's solar arrays, control a robot arm transporter and operate part of the station's cooling system.

NASA flight rules require full redundancy before a visiting vehicle can be cleared for launch, and flight controllers spent the weekend assessing potential work arounds to determine whether to proceed with the SpaceX launch or whether to order a delay until the station crew could carry out a contingency spacewalk to replace MDM EXT-2.

The concern was what might happen if the primary MDM failed for some reason before the backup could be replaced.

As it turns out, the current angle between the sun and the plane of the station's orbit, known as the beta angle, happens to be favorable for solar power generation. Space station Program Manager Mike Suffredini said the solar arrays will be locked in a favorable position before the Dragon's arrival to ensure sufficient power even if the primary MDM failed.

In addition, the robot arm's mobile transporter, used to carry the arm to various work sites on the station's solar power truss, will be moved from work site 4, where it would block the MDM replacement, to work site 2, where it can be safely parked for the duration.

"Because of the beta we're in, we're able to essentially get back the redundancy we need because we can position the solar arrays such that we're OK," Suffredini said. "The thermal control system will take care of itself (even with a subsequent MDM failure), it has all its protections. Even though we can't command it, it can still operate itself.

"The mobile transporter we'd lose, but ... once we move it off work site 4 so we can get to the MDMs, it's not a huge deal. And then the last big system is the (solar array positioning system). We can operate the (solar arrays) in a pretty much fixed position."

"So once you sort though those issues, you realize you have full redundancy on the thermal control system, we're going to move the MT today we we'll have it out of the way so we can do the EVA, and we have the solar array position that protects us for many days, then we're in a good position."

After the Dragon cargo ship is berthed to the station, the crew will turn its attention to a planned two-and-a-half-hour spacewalk, no earlier than April 22, to replace the faulty MDM.

Spare MDMs are available inside the space station and two yet-to-be-identified crew members will carry the replacement to the S0 truss, pull out the faulty unit and install the spare. Replacing an MDM is one of a dozen procedures all NASA crew members are trained to carry out in the event of critical failures.

NASA will follow strict safety guidelines to prevent any recurrence of a potentially catastrophic water backup that flooded European Space Agency astronaut Luca Parmitano's helmet during a spacewalk last year.

Investigators believe the backup occurred when silica contamination built up over multiple spacewalks and eventually clogged an internal filter in a backpack pump assembly used to separate air and water.

The suits that will be used for the upcoming spacewalk -- serial numbers 3005 and 3011, the suit Parmitano wore -- are equipped with new components, clean filters and fresh water. Suit 3011 was used without incident for two spacewalks last December.

"We believe we're in a very good posture for the EVA," Suffredini said.