Dawn Status Report - April 2006
Fri Apr 7, 2006 at 20:24 UTC
Keyur Patel has been named the new Project Manager at JPL. Keyur's recent experience was as Deputy Project Manager on Deep Impact through its successful encounter with Tempel 1. Ann Grandfield has been promoted to Project Manager (from Deputy) at Orbital Sciences Corporation.
There is good news regarding the ion propulsion system components, namely the xenon tank and the power processing units, which raised concerns of NASA managers and contributed to their decision to direct the project to stand down. The Dawn flight xenon tank has been an item of concern since a qualification tank and a flight spare tank ruptured at lower-than-expected pressures during testing. The tank is a composite overwrapped pressure vessel (COPV) with a titanium liner. While the actual flight tank had been tested and performed at pressures well above the expected flight and ground pressures, there was lingering concern that the cause of the qual and spare tank failures needed to be understood to fully qualify the flight tank. An Independent Review Team (IRT) team was formed to review the issues.
The project decided to reduce the xenon load from 450 to 425 kg to add an extra margin of safety (with no impact on planned science operations). The IRT team concluded that the tank was flightworthy in Sept 2005, and was succeeded by an Independent Technical Authority (ITA) review. After extensive materials testing, the cause of the qual/spare tank failures was determined to be due to manufacturing anomalies that had no bearing on the integrity of the flight tank. The ITA issued their draft report on March 17th, concluding that the Dawn flight xenon tank was flightworthy.
Image Credit: NASA/JPL
Dawn flight COPV tank. The tank, which was integrated last fall, has a safety factor >2 for a xenon load of 425 kg.
Several test failures of the Power Processing Units (PPUs) of the ion propulsion system also were a cause of concern that contributed to the mission stand down. The PPUs had experienced parts failures and unplanned shutdowns during testing. A PPU tiger team was formed to assess the PPU design and provide independent assessment of the cause of the testing anomalies. The team concluded that the design was sound, devised methods to test for defects in parts similar to the ones that failed, and explained the shutdowns as indicating the possible need to adjust an internal parameter value. The tiger team recommended an extended "run-in" test of the two identical units, in vacuum for 500-1000 hours, to validate parts quality and operational stability. The run-in testing procedures have been written and tested and the actual testing is due to start immediately.
The Dawn PPU design was inherited from the Deep Space 1 (DS1) mission. The PPU flown on DS1 operated flawlessly for 16265 hours, while the two Dawn PPUs together are expected to operate for about 48000 hours. A minor change was made to the DS1 PPU design to improve the response to a serious grid short.
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