Typical spacecraft thruster configurations are often unable to provide full six-degree-of-freedom control and may have unwanted interaction between their attitude control and trajectory control functions, have undesirably high instantaneous electrical power demands, and use more thrusters than desirable. These last two potential problems gain increased significance if a spacecraft is required to have especially small size and mass, and have very low cost.

Two different eight-thruster configurations have been designed that address all these issues. Torque can be generated in either direction around any of three orthogonal axes, a translational force can be generated in either direction parallel to any of three orthogonal axes, and any of these torques and translational forces can be generated using only two thrusters at a time. Certain operations can also be carried out simultaneously using two pairs of thrusters.

In the ideal case, with thrusters that have identical thrust levels, no thrust misalignment, and no plume impingement problems, torque can be provided without accompanying translational force, and with the spacecraft center of mass centered between the thrust lines, translational force can be provided without accompanying torque. Propellant storage design influences how thoroughly this latter, unintended torque is avoided because both propellant consumption and movement of stored propellant due to spacecraft accelerations have the potential to move the spacecraft center of mass.

Fundamentally, it is desirable to make the center of mass of the stored propellant coincide with the center of mass of the dry spacecraft and remain there. That is probably simplest if cold gas propulsion is used and that propellant is stored in a single spherical tank located at the center of mass of the spacecraft. Propellant three-axis center-of-mass stability is also possible with liquid propellant subsystems, but may require different or more complex propellant storage and management than usual.

Even propellant two-axis center-of-mass stability may be sufficient if most translational force generation is needed along the third axis and some level of unwanted torque is allowed to accompany intentional translational force generation perpendicular to that axis. A simple monopropellant hydrazine propulsion subsystem utilizing a diaphragm tank and eight thrusters, for example, can potentially provide six-degree-of-freedom control, have most efficient translational force generation in the direction that it is most needed, and generate that force without significant unwanted torque.

This work was done by David H. Collins of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49578

Topics:
Attitude control Avionics Electric power Flight control systems Hydrazines Liquid propellants Mechanical Components Off-board energy sources On-board energy sources Propellants Spacecraft Steering systems Trajectory control Vehicle acceleration Vehicles
This Brief includes a Technical Support Package (TSP).
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Six-Degree-of-Freedom Control With Only Eight Thrusters

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    This article first appeared in the December, 2015 issue of NASA Tech Briefs Magazine (Vol. 39 No. 12).

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    Overview

    The document titled "Six-Degree-of-Freedom Control With Only Eight Thrusters," produced by NASA's Jet Propulsion Laboratory, discusses innovative thruster configurations that enable spacecraft to achieve comprehensive control over their motion using a minimal number of thrusters. This is particularly relevant for interplanetary and orbital missions where efficient trajectory modification and attitude control are crucial.

    Thrusters generate reactive forces through mass expulsion, allowing spacecraft to adjust their trajectories and orientations. Traditional thruster arrays often face limitations, such as inadequate six-degree-of-freedom control, undesirable coupling between trajectory and attitude control, high instantaneous power demands, and the need for a large number of thrusters. These challenges can be particularly problematic for smaller, cost-sensitive spacecraft.

    The document outlines two specific thruster configurations that can provide the necessary control without requiring more electrical power than what is needed to operate just two thrusters at a time. These configurations allow for torque generation around three orthogonal axes and translational forces along those axes, enabling precise maneuverability. The thruster pairs can be strategically positioned to ensure that the spacecraft's center of mass is optimally aligned, allowing for effective control without unwanted torque or force coupling.

    The first configuration (Option 1) aligns torques and translational forces with the same set of axes (XYZ), but requires thrusters to be placed in at least six different locations. The second configuration (Option 2) allows for thrusters to be located in as few as four areas, although it introduces some complexity as the torque and translational force axes differ in certain instances.

    Overall, the document emphasizes the potential of these special thruster configurations to enhance spacecraft control while minimizing size, cost, and mass. This advancement is significant for future missions, as it addresses the need for efficient and effective control systems in the increasingly complex field of space exploration. The findings presented in this technical support package are part of NASA's broader efforts to develop technologies with wide-ranging applications in aerospace and beyond.