Avionics Box Being Cleared For Hubble Flight

Jan 30, 2009
Frank Morring, Jr. morring@aviationweek.com

Engineers at Goddard Space Flight Center will subject a spare avionics box for the Hubble Space Telescope to vibration testing next week as they work to meet a May 12 launch date for the space shuttle Atlantis on the final servicing mission to the orbiting observatory.

Still to come is a 28-day thermal vacuum test of the spare Science Instrument Command and Data Handling (SIC&DH) unit, which should be finished early in March. That gives the Hubble program about a month of wiggle room to meet an April 8 deadline to ship the SIC&DH to Kennedy Space Center.

But the backup flight hardware has been at Goddard for 18 years, and has never been certified for flight. And engineers are using equally aged test equipment to qualify the ground spare without a major disruption in scheduling both for the nine remaining shuttle missions on the flight manifest and the transition to the Orion/Ares I vehicles to follow.

"Part of the problem or the challenge that we have in doing any of the testing on this box is that the test system that was built by the original contractor, which was Fairchild Industries and IBM, was built back in the '80s," says Hubble Program Manager Preston Burch. "The heart of the test system is a VAX-11/780 computer. Well, you can imagine what it's like trying to operate a computer that's that old with software that's that old."

Going into the final stretch, Goddard's Hubble team still has "a couple" of outstanding issues that Burch says probably will wind up originating in the software or in the test configuration. One of them involves a "handshake problem" in the system that allows the SIC&DH to juggle input from all five Hubble instruments at once.

A potentially larger issue involves a startup problem with the unit's A-side science data formatter that disappeared before a clear cause could be identified. The redundant B-side worked perfectly from the start, and when engineers went back to the A-side it gradually began to work normally too.

Burch says a careful teardown inspection -- including x-rays and microscopic examination -- failed to find anything that could have caused the earlier problem. Workmanship in the unit was judged "very good to excellent," and the problem hasn't resurfaced.

By contrast, a power supply control unit problem was traced to short circuits caused by tin whiskers. The well-understood metallic crystals were removed, and technicians installed a barrier material over the parts containing tin to prevent a recurrence.

"Our attitude is that if we get through three months of rigorous environmental testing, and with all of the physical inspection, all the other analysis work and additional testing that we've done, if the problem never resurfaces -- and given the fact that when it has surfaced in the past it has always gone away -- we think that that's acceptable," Burch says of the A-side data formatter.

Despite the unexplained anomaly on the A-side data formatter, replacing the failed unit on the orbiting telescope is the new No. 3 priority for the mission, Burch says, right after replacing the aging gyros and installing the new Wide Field Camera 3 that will increase the "discovery efficiency" of the telescope by a factor of 35 over its present instrument (Aerospace DAILY, Nov. 3, 2008).

The late-breaking problem with the SIC&DH also has forced the Hubble team to work with the Mission Operations Directorate at Mission Control Center-Houston on re-choreographing the five spacewalks planned for the servicing mission. Given its high priority, the SIC&DH changeout has moved into the first extravehicular activity (EVA).

That complicates the timing of subsequent work, which already was tightly packed into the remaining four EVAs. Lead EVA astronaut John Grunsfeld, who will be making his third servicing visit to the Hubble, has found a way to speed repairs to the Advanced Camera for Surveys originally scheduled over two spacewalks.

Still, it remains an open question whether he will be able to complete the job on EVA 3 as currently planned.

Photo: NASA

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