Technical, Cost Issues Persist For Webb Telescope

By Amy Svitak svitak@aviationweek.com
Source: Aviation Week & Space Technology

ESA is reworking the optical bench, but will not deliver the instrument to NASA in time for ISIM's first cryo-vacuum test, which gets underway in August. The agency says NIRSpec is now reassembled and has passed the first cryo-performance and vibration tests, with a final cryo-test now underway. Shipment to NASA is planned for mid-September, more than a year late but in time to incorporate NIRSPec—still fitted with the faulty microshutter arrays—into the ISIM's second round of cryo-vacuum tests next year.

Meanwhile, NASA is making changes to a spare microshutter array that will be less sensitive to the acoustic environment. The goal is to have ESA swap the array assembly ahead of the ISIM vibration test and third cryo-vacuum test in 2015.

Together with the Mid-Infrared Instrument (MIRI) delivered to NASA in May last year, NIRSpec is one of two instruments Europe is contributing to JWST. Combined with launch atop an Ariane 5 ECA rocket in October 2018, and post-delivery and operational mission support, Europe's JWST contribution comprised €370 million ($500 million) in 2004. “Today, ESA says the figure is around €600 million ($790 million), though that appears to exclude launch costs of around €140 million.

NASA is also awaiting delivery of a second instrument delayed by almost 11 months, the Near-infrared Camera (NIRCam) built by Lockheed Martin Advanced Technology of Palo Alto, Calif. NASA determined the need to electrically ground a mirror on the instrument, a process company spokesman Buddy Nelson says is complete, with delivery expected at the end of July.

In addition to instrument troubles, NASA is grappling with a problem on a key subsystem—a cryo-cooler designed by NASA's Jet Propulsion Laboratory to cool MIRI.

In 2010, NASA realized an essential valve in the cryo-cooler was leaking at rates that exceeded requirements, according to GAO. NASA says the MIRI cryo-cooler is particularly complex because it spans approximately 10 meters—or 33 ft.—through the entire JWST observatory. Following the results of a failure review board, a new valve was designed, but it also did not meet leak-rate requirements. NASA says yet another new valve will not be manufactured in time for use in the first ISIM cryo-vacuum test, and is concurrently developing alternatives.

Problems with a second subsystem, also discovered in 2010, involved the degradation of Teledyne-built infrared flight detectors used by three of JWST's four instruments. As a result, approximately $42 million and 15 months of schedule reserve to replace the detectors were included in the rebaseline of JWST cost and schedule, according to GAO. These additions covered the cost of manufacturing the detectors, fabrication, assembly, and test of new focal plane assemblies, and changing the detectors on three of JWST's four instruments.

Since the manufacturing takes some 30 months, the detectors cannot be delivered until after the second round of ISIM cryo-vacuum tests in 2014. As a result, $2 million of the $42 million in the replan was used to add a third round of cryo-vacuum tests for ISIM.

Another challenge JWST has had to address recently is reducing mass on the spacecraft bus to account for the mass of its electrical wiring harnesses, which turned out to be larger than expected.


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