Developing a Foil Mask for the Janus XCAT Instrument Entrance Aperture, 15-R8198
Inclusive Dates: 11/15/10 – 03/15/11
Background — A spaceborne X-ray coded aperture (XCAT) instrument is being proposed as a complement to the Joint Astrophysical Nascent Universe Satellite (JANUS) instrument suite. The mission of XCAT is to detect gamma ray bursts (GRBs) and provide the necessary pointing information to the spacecraft to train the observatory's infrared telescope in the same direction for more detailed observations. An integral component of the XCAT instrument is the entrance aperture mask. Typically, these masks are constructed of very thin foils with significant flatness requirements. The foils are of a porous design that uses a random grid pattern to achieve the required transmissibility. SwRI is proposing the JANUS mission to the NASA Explorer Program Announcement of Opportunity (AO), announced November 1, 2010.
This project proposes to test the ability of the foil to withstand launch acoustic loads. Several candidate foil mask designs and configuration permutations were tested. With the completion of the acoustic testing, resources allowed additional thermal and random vibration testing to further characterize the foils in the expected launch and space environments.
Test masks were mounted into frames as would be expected in a flight design. Two frame styles were considered for this study. One design included interior pane support ribs, while the second frame style omitted the interior pane support ribs, which would be preferable from a science viewpoint, and was thought to be the worst-case mask support condition.
A test enclosure was design and fabricated to provide a cavity similar in volume to the expected flight design (see Figure 1). While the volume was flight-like, the wall thickness of the enclosure was significantly thicker than the expected flight design. This was deemed necessary because of the lack of actual flight design details and a desire to minimize the variables affecting the mask response. The assembly included a back panel and mounting provisions for attaching of the test article to the acoustic chamber as well as the vibration facility.
Another fixture was developed to use in random vibration testing of the test articles.
Finite element modal analyses were performed to determine the response of the enclosure to the environmental testing and to ensure coupling would not occur between the enclosure and the mask.
Accomplishments — Several candidate foil mask designs and configuration permutations were tested to determine their ability to withstand launch and orbit environments. Multiple mask variants were tested with positive results in all cases. Even post-structural damage caused by a handling mishap proved to not lead to acoustic failure. The proposed JANUS coded aperture mask design appears to be quite tolerant of Taurus level acoustic loading. This is likely because of the perforated nature of the mask.
Thermal testing was conducted on a sample mask. While not suffering a catastrophic failure, the masks did exhibit unacceptable behavior in the thermal testing. Permanent deformation occurred in the foil section of the mask. It should be noted that the thermal test was very much an extreme temperature excursion test that was not necessarily indicative of the actual expected performance on orbit. Further thermal environment definition will likely also indicate a much smaller temperature range will reduce the thermally induced strain on the mask.
The mask was exposed to standard launch vehicle random vibration as well. The result of this test was structural failure prior to reaching the two minute planned test time. Additional work on better characterizing the vibration and thermal environments, along with actual flight foil assembly design, will be required to demonstrate survivability for these types of loads. Some limited characterization of the masks was conducted using a laser velocimeter while exposing the mask to low-level vibration. This data could be used in further development efforts of a flight mask support design.
As to the original intent of this project, the screens have been shown to be capable of surviving exposure to the expected acoustic environment. Additional work on better characterizing the vibration and thermal environments, along with actual flight foil assembly design, will be required to demonstrate survivability for these types of loads.