1、Lessons Learned Entry: 2057Lesson Info:a71 Lesson Number: 2057a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 POC Name: Gus Forsberga71 POC Email: gustaf.a.forsbergjpl.nasa.gova71 POC Phone: 818-354-1191Subject: The Vapor Deposited Aluminum (VDA) Coating Process Can Affect P
2、ayload Performance Abstract: Vapor deposited aluminum (VDA) coating, performed according to specification on the OSTM/Jason-2 AMR reflector, caused discontinuities that required rework using a modified VDA process. VDA should not be considered an “off-the-shelf“ process. Evaluate any requirement to
3、perform surface roughening prior to VDA coating, and require an adequate margin in the coating thickness.Description of Driving Event: The objective of the NASA/Caltech Jet Propulsion Laboratory (JPL) Ocean Surface Topography Mission on the Jason-2 satellite (OSTM/Jason-2) is to measure ocean surfac
4、e topography including changes in the global sea level. The Advanced Microwave Radiometer (AMR) instrument aboard the satellite measures radiation from the Earths surface to determine atmospheric water vapor so that radar altimeter instrument time delay errors caused by the water content can be corr
5、ected with great accuracy. A vapor deposited aluminum (VDA) coating was applied during fabrication of the AMR Reflector Structure Assembly (RSA) (Figure 1) to provide the reflector with the desired surface properties. Subsequent JPL tests of VDA witness coupons in June 2006 (Figure 2) revealed a sig
6、nificant deficit in the surface conductivity of these samples that could affect AMR performance (Reference (1). The low conductivity was traced to surface roughening of the reflector surface, performed prior to VDA deposition, that caused discontinuities in the composite and resin surface on a micro
7、n level (Figure 3). Most of the surface roughening resulted from grit blasting, but hand abrasion using green nylon mesh pads also caused degraded surface conductivity. Roughening was performed to reduce specular Provided by IHSNot for ResaleNo reproduction or networking permitted without license fr
8、om IHS-,-,-reflectance (specularity) that could cause damage in the event of anomalous direct pointing at the sun. Figure 1 is a color photo of a high bay cleanroom facility containing the RSA. The RSA appears as a gray dish approximately 5 ft. in diamter with the approximate diameter to thickness r
9、atio of a Frisbee. The dish is mounted on a set of metal girders about 2 ft. high, integral to the dish, mounted on the floor, and canted about 10 to 15 degrees from the vertical.Figure 2 is a color photo of a laboratory setup. Two heavy cables are connected to the front of a large unit of test equi
10、pment with many knobs and a CRT displaying a waveform. The other ends of the two cables are connected to the ends of two waveguides that project at right angles from a copper cylinder about 3 inches in diameter. Three rectangular metal lozenges lie in front of the cylinder.Figure 1. AMR RSA Figure 2
11、. RF resonant cavity measurement of effective 7.17 GHz conductivity for AMR RSA couponsFigure 3 is a black and white SEM image. Several horizontal gray layers are stacked atop one another with gaps between them, similar to a wall constructed of slate tiles. The top layer is not smooth: a portion app
12、ears to be eaten away while another portion of the layer juts up with a pagoda-shaped structure. A box has been added as a visual guide calling attention to this section with the pagoda-shaped structure.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,
13、-Figure 3. SEM image of AMR coupon sample (“basket-weave“ woven composite, no wash coat, aggressive grit blast) showing surface discontinuitiesThe AMR process specification for VDA was approved based on analyses by the project and by independent reviewers and based on a history of use on such JPL fl
14、ight instruments as the Microwave Limb Sounder (MLS) and Cloudsat. Examination of the anomalous, VDA-coated, AMR RSA coupon cross-sections with a scanning electron microscope showed that the samples with the most roughening appeared to have the most discontinuities in the VDA coating and exhibited t
15、he poorest surface conductivity (Reference (2). The root cause of the discontinuities has not been established, but it is believed to be uneven aluminum deposition due to the shadowing effect of surface features created by the roughening process, which is poorly controlled. The anomalous AMR results
16、, as compared to MLS, may be attributable to differences in composite weave characteristics and grit blasting parameters. The OSTM/Jason-2 project relaxed the specularity requirement so that surface roughening of the AMR reflector would not be required. A modified VDA process that did not include su
17、rface roughening was developed and validated using coupon tests. The AMR RSA was reworked, with chemical stripping of the VDA coating and reapplication using the modified process. Low-abrasiveness white nylon mesh pads were used- not to reduce specularity but because coupon tests had shown that thei
18、r use improved adhesion of the VDA. Microwave cavity tests of coupons coated using the new process repeatedly yielded nominal surface conductivity. References: 1. “AMR RSA - VDA Anomaly,“ JPL Problem/Failure Report No. 5388, October 26, 2006.2. Amarit Kitiyakara, “AMR Reflector Structure Assembly (R
19、SA) - VDA Anomaly Summary,“ October 25, 2006.Lesson(s) Learned: VDA coating of spacecraft reflectors is a more sensitive fabrication process than has been previously recognized in VDA process specifications. It is risky to blindly inherit such fabrication processes based on past use if they are not
20、well understood.Recommendation(s): 1. VDA should not be considered an off-the-shelf process: a candidate VDA process should be carefully evaluated to ensure suitability for the application. Do not assume a generic suitability solely based on process use on previous reflectors. 2. Surface roughening
21、using green nylon mesh pads or grit blasting prior to VDA application is a poorly controlled process that may significantly compromise the VDA surface conductivity Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-and should not be used without appropr
22、iate evaluation. AMR coupon tests indicate that surface preparation using a white (i.e., low abrasiveness) nylon mesh pad appears to improve VDA adhesion while not compromising surface conductivity. 3. Ensure an adequate VDA coating thickness margin (Reference (2).Evidence of Recurrence Control Effe
23、ctiveness: JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 6.10.3 (“Engineering Practices: Materials, Processes, and Contamination Control“) in the Jet Propulsion Laboratory standard “Flight Project Practices, Rev. 7“, JPL DocID 58032, September 30, 2
24、008.Documents Related to Lesson: N/AMission Directorate(s): a71 Sciencea71 Exploration SystemsAdditional Key Phrase(s): a71 Systems Engineering and Analysis.Engineering design and project processes and standardsa71 Engineering Design (Phase C/D).Spacecraft and Spacecraft Instrumentsa71 Manufacturing
25、 and Assemblya71 Additional Categories.Flight Equipmenta71 Additional Categories.Hardwarea71 Additional Categories.Parts, Materials, & Processesa71 Additional Categories.Payloadsa71 Additional Categories.SpacecraftAdditional Info: a71 Project: OSTM/Jason-2Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2009-08-27a71 Approval Name: mbella71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
