NuSTAR Bus Thermal Design, Test, and On-Orbit Performance

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR) observatory was successfully launched on June 13 th , 2012 from a Pegasus XL launch vehicle into a low earth orbit (LEO). The NuSTAR mission is currently in the operational science phase, imaging the sky in the high energy X-ray spectrum. The spacecraft bus was designed, integrated, and tested by Orbital Sciences Corporation (Orbital). The Instrument was developed by teams from California Institute of Technology (Caltech), Jet Propulsion Laboratory (JPL), University of California – Berkeley (UCB), and other partners. This paper discusses the bus thermal control system (TCS) design, testing, and on-orbit performance. The NuSTAR TCS design was especially challenging due to the 10meter deployable instrument mast and the spacecraft requirement to point at any target in the sky. The full sky requirement leads to the requirement that the spacecraft withstand being exposed to the thermal environments in dramatically different ways. In some attitudes all of the bus radiators are shielded from solar flux, and in others the radiators can be exposed to full sun. A detailed thermal distortion analysis, which included a sensitivity analysis to many variables, was completed on the bus and instrument structure. The NuSTAR spacecraft utilizes four star trackers, each of which has its own unique thermal control challenges due to their different locations on the spacecraft. One of the star trackers required an innovative thermal control system due to its location inside the spacecraft bus cavity, which has a radiative environment temperature higher than the star tracker operational temperature limit. System level thermal vacuum (TVAC) and thermal balance testing presented a unique challenge because NuSTAR was in the instrument stowed configuration. During TVAC testing unique non-intrusive techniques were used to simulate some component dissipations, and special procedures were developed and utilized for difficult to verify heaters. The on-orbit thermal performance of the bus TCS has been excellent. The thermal model was correlated to on-orbit data to allow better prediction of thermal performance over the entire mission. The final analysis of the NuSTAR bus shows it will meet the TCS requirements for the 2-year mission as planned.