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The Terrestrial Planet Finder Interferometer (TPF-I) is a space-based NASA mission for the direct detection of Earth-like planets orbiting nearby stars. At the mid-infrared wavelength range of interest, a sun-like star is ~107 times brighter than an earth-like planet, with an angular offset of ~50 mas. A set of formation-flying collector telescopes direct the incoming light to a common location where the beams are combined and detected. The relative locations of the collecting apertures, the way that the beams are routed to the combiner, and the relative amplitudes and phases with which they are combined constitute the architecture of the system. This paper evaluates six of the most promising solutions: the Linear Dual Chopped Bracewell (DCB), X-Array, Diamond DCB, Z-Array, Linear-3 and Triangle architectures.
Each architecture is constrained to fit inside the shroud of a Delta IV Heavy launch vehicle using a parametric model for mass and volume. Both single and dual launch options are considered. The maximum separation between spacecraft is limited by stray light considerations. Given these constraints, the performance of each architecture is assessed by modeling the number of stars that can be surveyed and characterized spectroscopically during the mission lifetime, and by modeling the imaging properties of the configuration and the robustness to failures. The cost and risk for each architecture depends on a number of factors, including the number of launches, and mass margin. Quantitative metrics are used where possible.
A matrix of the architectures and ~30 weighted discriminators was formed. Each architecture was assigned a score for each discriminator. Then the scores were multiplied by the weights and summed to give a total score for each architecture. The X-Array and Linear DCB were judged to be the strongest candidates. The simplicity of the three-collector architectures was not rated to be sufficient to compensate for their reduced performance and increased risk. The decision process is subjective, but transparent and easily adapted to accommodate new architectures and differing priorities.
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