TCM-12: A Crucial Maneuver

TCM-12 is the last maneuver before the swingby. Errors in this maneuver are magnified 300 times into the next maneuver (TCM-13, which is targetted to Jupiter). Needless to say, this is the maneuver to be careful with.

Here's the design:

Maneuver Location: E-6.5d (L+665d), 8/11/1999, 15:30 UTC (a.k.a. GMT) (8:30 AM Wednesday PDT)
Distance to Sun: 0.95 AU
Maneuver Strategy: Correct All Encounter B-Plane Conditions
Engine Type: Main engine
Turn Type: Normal (0.25 ¡/sec)
Total DV magnitude: 12.256 m/s
Burn DV magnitude: 12.243 m/s
Burn Duration: 133.6 sec (2 min 13.6 sec)
Turn Angles: -84.8 degrees (roll), -88.4 degrees (yaw)

We always produce a prediction of the Earth-line delta-V (as you look at the plots of the actual measurements during the maneuver, be sure to refer back to this diagram to see how close it came)

This plot shows us the cost in the next maneuver (TCM-13) for every kilometer that we miss the current aimpoint by.

Observation

In the snap-shot above, you see the effects of the deadband tightening (on the left) and the first roll turn. Remember, this is a plot of the difference between the actual measurements and selected predictions. We happened to include our prediction of the effects of the deadband tightening. Since the data points cluster around zero before and after the event, we can conclude that our model of the effect was correct. On the other hand, if we had predicted the event at exactly the right time, you wouldn't be able to even notice it in this plot (the prediction would've matched reality). Instead, you see a little dip. Now we know what time it really happened!

On the right hand side, you can see the effect of the roll turn. The Cassini spacecraft performs two turns to orient itself in the proper direction for the maneuver. We call the first a roll turn and the second a yaw turn. No predictions for the turns nor the burn are in these plots.

Here you see the effect of the yaw turn. It quite nicely follows the trend in the plot I showed at the top of this page. Furthermore, the yaw turn oriented the spacecraft's antenna in such a way that prohibited further communication. This maneuver, like TCM-9, TCM-10, and TCM-11 was not entirely observable.

I've included this just because it's a nice screen shot of the deadband-tightening, the roll turn, and the yaw turn. (In that order)

Finally, the antenna comes back into view and we can watch the end of the unwind yaw turn! By this time, the burn has been completed. The spacecraft is now reversing its sequence of turns to return to its typical (nominal) orientation. Since these actions are in absolutely the reverse of what you saw earlier, the pattern that plays out is quite symmetric.

It sure is nice to see a plan come together! This was the last planned course correction until August 31 (TCM-13)