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There are several popular news items about the recent test of Voyager 1's Trajectory Control Thrusters to see if they can be used for attitude control to facilitate communication with Earth. The original attitude control thrusters are slowly degrading. Read more in the NASA News item Voyager 1 Fires Up Thrusters After 37 Years.

In what ways do the two sets of thrusters differ? An answer should include at least their locations and (design) minimum possible impulse per firing for each type. I say design because this may be more available, the actual minimum of the attitude thrusters is changing with time (which is the problem).

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The attitude thrusters and TCMs are mechanically identical, all Aerojet MR-103s. From the Voyager Press Kit:

The 16 thrusters on the mission module each deliver 0.89 N (0.2-lb.) thrust. Four are used to execute trajectory correction maneuvers; the others in two redundant six-thruster branches, to stabilize the spacecraft on its three axes. Only one branch of attitude control thrusters is needed at any time.

The MR-103 is almost a commodity part, used on many spacecraft over 4 decades; the modern version is quite versatile, rated for 0.027 N-sec minimum impulse with a 15ms pulse, and also for 5000 seconds of continuous fire; Astronautix mentions an 8ms minimum duty cycle, presumably for an earlier version of the thruster.

Here's a diagram showing most of the thrusters:

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I've highlighted thruster groups in color and am guessing their purposes, based on their position and orientation, are:

  • The four in blue would be the TCM thrusters (all facing the same direction, not paired).
  • The purple set would provide roll control - two branches each of +roll and -roll
  • The green set would be yaw control (or pitch, depending on how the axes are defined); again two branches for +yaw and -yaw
  • The orange pair would be 2 branches of +pitch (or yaw depending on definitions); there should be a corresponding -pitch pair somewhere, possibly hidden on the underside, forward of this pair.

It's easy to see that, by firing one of the TCM thrusters at a time, you'd get the torque needed for pitch or yaw control, and by sequencing a 90º pitch followed by a yaw, you could effect a roll.

Pitch and yaw control alone (without roll) are probably sufficient to maintain communication with Earth. Since Voyager's antenna bore sight is aligned with the roll axis, roll would not be particularly necessary to maintain communication with Earth. Also, since (according to Table 2-2 on page 10 of the DESCANSO Design and Performance Summary Series Article 4: Voyager Telecommunications) Voyager uses circular polarization rather than linear, arbitrary rotation would not have a significant impact on the polarization of the received signal.

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