1、Lessons Learned Entry: 0594Lesson Info:a71 Lesson Number: 0594a71 Lesson Date: 1998-06-17a71 Submitting Organization: GSFCa71 Submitted by: Ellen L. HerringSubject: ACE SEPICA Micro-machined Silicon Valve On-orbit Anomaly Description of Driving Event: The SEPICA instrument, one of 9 instruments flow
2、n on the ACE Mission, uses isobutane gas as part of its particle detection scheme. Three proportional counters, one associated with each of SEPICAs 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 mi
3、cro-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 fund
4、amental 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 th
5、at 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
6、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 in
7、consistent 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 th
8、e 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 conditi
9、ons. However due to safety concerns, the Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-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 nit
10、rogen did not necessarily fully simulate effects in the on-orbit configuration due to the different properties from the on-board gas. Post-anomaly evaluation also indicated that, although the spare gas system had undergone some level of vibration test, the valves had been replaced afterward. The fli
11、ght 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
12、operated on or near the manufacturers recommended boundaries and, in the area of duty cycle, the valves were operated outside of the manufacturers documented recommendations with verbal confirmation from the manufacturer that the out-of-boundary conditions should be acceptable. The selected valve wa
13、s 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.Lesson(s) Learned: 1. Inconsistencies in Gro
14、und Testing vs. On-Orbit Configuration: Ground testing should mirror on-orbit configuration to the maximum extent possible. Ground hardware should undergo test program as similar as possible to flight units (e.g, thermal, vibration, acoustical, etc.). For those areas that can not be made to be consi
15、stent with on-orbit configuration and/or pre-mission flight unit test program, analysis should be performed to justify and/or extrapolate test results to the on-orbit environment and flight units.2. Use of COTS Products Beyond Recommended Design/Operations: The need to use a small, commercially made
16、 valve was emphasized and recommended by an independent Review Team very early in the program. The choice of the selected valve was driven by what was available in the marketplace, and by spacecraft constraints, cost, and schedule. In order to meet instrument needs, it was necessary to operate the v
17、alve outside of manufacturers recommendations. COTS products should be carefully chosen to be consistent with system defined specifications. Testing or analysis should be performed to identify and understand risks, to the COTS as well as to the mission goals, associated with operating COTS in other
18、than recommended manner.3. Use of Emerging Technology: The selected valve was an immature, emerging-technology product line and was available through only one vendor. The original vendor is no longer producing the valve and has no remaining in-house expertise to support on-orbit anomaly analysis and
19、 resolution. Risk is expected when dealing with emerging technology; however, the customer should attempt to mitigate these risks by fully investigating the stability of the supplier, the future market of the product, and the future availability of engineering analysis support.4. COTS Workmanship: P
20、re-mission analysis indicated inconsistent workmanship on the 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-analyzed valves. Risk mitigation was put in place on the customer side by a filtering, selection process complemented by an extensive grou
21、nd test program. Additional effort should be considered to mitigate risk by partnering with the vendor to improve vendor workmanship prior to customer delivery to provide a higher quality product and/or to include an opportunity for customer inspection before a non-recoverable process (e.g., final s
22、ealing) is performed.5. On-orbit Mitigation: Although the valves were recognized as a high risk element of the instrument development, no analysis was performed on a low-cost, possibly degraded, on-orbit gas control mechanism to fall back on in case of on-orbit valve failure. Given overall mission c
23、onstraints, this was consistent with the approach taken for ACE. However, it is generally recommended that pre-mission analysis should include evaluation of most probable failures and the identification of low cost, degraded on-orbit back-up approaches that could result in continuing science taking
24、in a potentially degraded mode at marginal cost.6. Consequence of Additional Risks of Faster, Better, Cheaper (FBC) Mission Environment: In order to meet the spacecraft limitations, cost, and schedule envelope to fly in the FBC environment, Projects must at times accept higher risk associated with e
25、merging technology, non-space qualified parts, and/or COTS products. Failures, likely to become more frequent in this paradigm, should be expected, accepted, and built upon as a resource to mitigate future mission risks. Required emerging technology such as these low power, low mass micro-valves sho
26、uld be identified to NASA technology programs as a technology to be selected for future initiatives to allow some level of test, verification, and improvement prior to operational flight.Recommendation(s): See Lessons LearnedEvidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesso
27、n: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Hardwarea71 Procurement Small Business & Industrial RelationsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Risk Management/Assessmenta71 Test & VerificationAdditional Info: Approval Info: a71 Approval Date: 1998-06-24a71 Approval Name: Ellen L. Herringa71 Approval Organization: 300a71 Approval Phone Number: 301-286-7393Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
