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

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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.

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