Dynamic Challenges of Long Flexible Booms on a Spinning Outer Heliospheric Spacecraft

Historical heliophysics missions have flown variations of an instrument intended to measure the electric fields and plasma waves of the inner heliosphere. A similar instrument is currently being investigated as part of the NASA funded Interstellar Probe (ISP) study. This instrument consists of two pairs of long thin wire antennas (also referenced as booms) deployed radially with small tip masses at the end of each, extending approximately 100 m (or more) tip-to-tip. The booms are deployed on a spinning spacecraft using centripetal acceleration to hold them in place. These booms account for the majority of the spacecraft moments of inertia, even though they represent only a small fraction of the total spacecraft mass. The booms introduce dynamical forces such as coning and nutation which greatly complicate communications through a high gain antenna (HGA) at large distances from the Earth. For a mission to the outer heliosphere, and into interstellar space, careful analyses must be conducted to ensure the HGA can be stably pointed with its boresight close enough to Earth to maintain adequate uplink and downlink data rates for science and engineering data during all major activities, and ensure the health and safety of the spacecraft from launch through end of life. As the booms greatly increase the inertia properties of the spacecraft, they increase the angular momentum of the spacecraft at a given spin rate, requiring more propellant for precession maneuvers and spin rate adjustments. Time must be allocated between maneuvers to allow for nutation to damp, and periodic re-calibrations of the HGA boresight to principal moment of inertia may be required. These further complicate operations, and add risk that must be managed. This paper will detail the various effects, challenges, and risks introduced by long wire antennas to a spinning spacecraft. It will discuss the potential risks and complexities associated with deployment of the booms. It will detail how the static effects such as coning between the HGA boresight and the spin axis, and the transient dynamic events like nutation of the hub and mutual precession of the hub-boom system, can affect downlink rates. It covers some of the operational considerations introduced such as propellant usage and how to precess a system safely with such low frequency modes. This paper includes historical spacecraft dynamics data from the Van Allen Probes, whose Electric Fields and Waves instrument is being considered as a template for an outer heliophysics spacecraft. Finally, it presents some examples of missions that have flown booms similar to the ones being studied, and how they differ from ISP. This paper will detail the various effects, challenges, and risks introduced by long wire antennas to a spinning spacecraft. It will discuss the potential risks and complexities associated with deployment of the booms. It will detail how the static effects such as coning between the HGA boresight and the spin axis, and the transient dynamic events like nutation of the hub and mutual precession of the hub-boom system, can affect downlink rates. It covers some of the operational considerations introduced such as propellant usage and how to precess a system safely with such low frequency modes. This paper includes historical spacecraft dynamics data from the Van Allen Probes, whose Electric Fields and Waves instrument is being considered as a template for an outer heliophysics spacecraft. Finally, it presents some examples of missions that have flown booms similar to the ones being studied, and how they differ from ISP.