scope:

Description of Driving Event:

The SEPICA instrument, one of 9 instruments flown on the ACE Mission, uses isobutane gas as part of its particle detection scheme. Three proportional counters, one associated with each of SEPICA's 3 detectors (1-high resolution, 2-low resolution), were to be held at a constant pressure by the use of a Commercial-Off-The-Shelf (COTS) bi-metallic micro-machined silicon valve assigned to each of the detectors. Following one month of flawless operations, detector operations indicated pressure being maintained slightly higher than pressure set point. It was thought that this state could be attributed to a loss of valve seat tightness or to a fundamental shift in valve baseline response. Approximately 6 months following launch, pressure in the high resolution detector totally decayed to zero at a rate consistent with that of a normal closed-valve rate. Analysis of the pressure decay and the inability to command the valve seems to indicate that the valve associated with this detector is in a closed state and there exists a lack of ability to flow required current across the system to open the valve. Although unproven, analysis seems to indicate the failure is within the valve. Impact of this failure to mission and instrument science is minimal due to a combination of the availability of high resolution data obtained earlier in the mission, the continuing availability of the other 2 SEPICA low resolution detectors, and the availability of complementary science from other ACE instruments.

Although pre-mission analysis did indicate inconsistent workmanship in the chosen COTS valves, it was determined that these COTS valves were the only acceptable design approach given mission limitations (e.g., power, mass, etc.) and that the risk mitigation approach to select the best valves through instrument team-led filtering process and the performance of extensive ground testing was consistent with the mission/instrument class (i.e., mission was Class C and instrument suite was Class D).

Considerable experience was gained in extensive pre-launch and post-launch ground testing which was performed at on-orbit thermal and vacuum conditions. However due to safety concerns, the majority of life testing on the spare unit at the University of New Hampshire was performed with nitrogen, rather than the on-orbit isobutane gas. The use of nitrogen did not necessarily fully simulate effects in the on-orbit configuration due to the different properties from the on-board gas. Postanomaly evaluation also indicated that, although the spare gas system had undergone some level of vibration test, the valves had been replaced afterward. The flight valves that are being tested in the spare have not been vibrated at all and, therefore, may not be an adequate model of the on-orbit valves.

The valve manufacturer did supply recommended valve operational boundaries (e.g., duty cycle, power, etc). In some of these areas, the on-orbit valves were operated on or near the manufacturer's recommended boundaries and, in the area of duty cycle, the valves were operated outside of the manufacturer's documented recommendations with verbal confirmation from the manufacturer that the out-of-boundary conditions should be acceptable. The selected valve was a commercial product, is no longer produced by the vendor, and vendor-expertise is no longer available.

No on-orbit back-up/redundancy to control the gas flow, possibly in a degraded fashion, in case of failure of the valves was developed or contemplated.