Mechanical Design and Vibro-Acoustic Testing of Ultrathin Carbon Foils for a Spacecraft Instrument

IBEX-Hi is an electrostatic analyzer spacecraft instrument designed to measure the energy and flux distribution of energetic neutral atoms (ENAs) emanating from the interaction zone between the Earth’s solar system and the Milky Way galaxy. A key element to this electro-optic instrument is an array of fourteen carbon foils that are used to ionize the ENAs. The foils are comprised of an ultrathin (50–100Å thick) layer of carbon suspended across the surface of an electroformed Nickel wire screen, which in turn is held taught by a metal frame holder. The electroformed orthogonal screen has square wire elements, 12.7 μm thick, with a pitch of 131.1 wires/cm. Each foil holder has an open aperture approximately 5 cm by 2.5 cm. Designing and implementing foil holders with such a large surface area has not been attempted for spaceflight in the past and has proven to be extremely challenging. The delicate carbon foils are subject to fatigue failure from the large acoustic and vibration loads that they will be exposed to during launch of the spacecraft. This paper describes the evolution of the foil holder design from previous space instrument applications to a flight-like IBEX-Hi prototype. Vibro-acoustic qualification tests of the IBEX-Hi prototype instrument and the resulting failure of several foils are summarized. This is followed by a discussion of iterative foil holder design modifications and laser vibrometer modal testing to support future fatigue failure analyses. The results of these activities indicate that there is no strong dependency of the natural frequencies or transmissibilities of the foils on the different foil holder and screen configurations. However, for all foil holder designs, the natural frequencies of the foils were observed to decrease noticeably from exposure to acoustic testing. These test results, when combined with foil holder assembly considerations, suggest that the welded frame and integrated screen designs should be incorporated into the architecture of the IBEX-Hi flight instrument.