Thermal requirements and a need for a very flat mechanical interface led to the development of a copper doubler for the titanium vault on the Juno Spacecraft. The vault is designed to contain the science instruments on the spacecraft, protecting them from damage due to the extreme radiation environment of Jupiter. The titanium used in the vault creates unwanted thermal effects due to the poor thermal conductivity of titanium. To remove heat from the telecommunication equipment mounted to the interior of the vault, a copper thermal doubler was used to spread the thermal loads over the entire area of the radiator (located on the outside of the vault), which decreased the effective thermal resistance through the vault wall. A method of bonding a copper doubler to the titanium preserves the mounting interface flatness to less than 0.005 in. (0.13 mm) while providing a superior thermal path to the radiators, which are fitted with thermal control louvers. The precisely controlled titanium surface, and that of the milled copper doubler with integral spacing features, provides the mechanical interface flatness, structural integrity, and thermal performance required by the telecommunications subsystem.
Utilizing precision milling techniques, a copper doubler was milled down to 0.065 in. (1.7 mm) to within ±0.001 in. (0.03 mm). Then, the panel was undercut by 0.005 in. (0.13 mm) in all areas except where fasteners were placed. Around each fastener a boss was provided for good thermal conduction along with the maintenance of precision spacing of the component mounting surface. Bosses were also added in areas where no fasteners were needed to preserve the spacing between the copper and titanium. Precision milling of the part was chosen after blanchard grinding failed due to warping and galling of the copper. The copper thickness was such that a special vacuum fixture with liquid cooling was required. Weep holes were added to the bosses to allow for the excess bonding material to exit and not be trapped under the panel (which may lead to flatness issues). Finally, the entire panel was bonded to the titanium using a precision 5-mil bond line established by the stand-off structures milled into the doubler.
This technique will result in a precision surface for the attachment of critical hardware that requires such a flat mounting surface, and provides the advantage of a mechanical thermal path at the bolted interfaces (a dry interface) while also proving a thermal interstitial material to fill all other voids (a wet interface) between the thermal doubler and the titanium panel. When properly prepared (including a primer), the RTV affords a bonded joint strength of approximately 500 psi (3.5 MPa) to provide structural integrity for the mounting of other parts.
This work was done by Kelley E. A. Alwood, Phillip A. Yates, Jerry J. Gutierrez, Gerald S. Gaughen, Bradley W. Kinter, Christopher C. Porter, and Terry Bennett of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47296