Why does the F9 second stage roll back and forth during deployment of each RADARSAT satellite?

Question

GIF

Watching the SpaceX video RADARSAT Constellation Mission after T+ 52:00 we can see at least the first two satellites deploy.

In each case, the second stage rolls one way during release of one of the satellites then stops and rolls back the other way.

Why are the back-and-forth rolls necessary, and isn't it a bad idea to release a heavy, off-axis mounted payload in the axial direction during a roll?

If the deployment was radial like in some ISRO cubesats I can understand, but these are axial deployments but from off-axis locations. I don't understand what's going on with all these rolls, especially as the satellite disengages from the 2nd stage.

Answer 1

There are several different reasons why mission planners chose to give release these satellites while the 2nd stage was rotating. Most of these are to reduce the possibility of the worst case scenarios.

When satellites are initially released by their launch vehicle they start up in a safe mode with minimal systems running, their very first tasks are to make contact with a ground station get 3 axis stabilised (control their orientation) and start generating electricity using their solar panels. The telemetry antennas are usually low frequency and omnidirectional, so if the satellite is slowly rotating you can be certain it will be facing the earth at least some of the time even if the satellite fails to get control of its orientation. This reduces the probability of the craft getting stuck with the antenna facing away from the earth an unable to communicate with its controllers.

Similarly a slow roll means that the solar panels are guaranteed to be in sunlight some of the time meaning that a certain low level of power generation is assured. Also in case any active thermal control systems take longer than planned to start working a slow roll evenly heats and cools the satellite reducing the change of extreme heating or cooling on any one component.

There is an additional reason in the case of the RADARSAT launch. Since before release the rotation means that the stage and satellites would be rotating about their combined centre of mass. So when they are released a small linear acceleration is applied to the satellite (and an opposing one to the 2nd stage). This will help to give each of the three satellites slightly different trajectories reducing the possibility of them coming into contact with each other shortly after release. This is probably the reason why they changed the rotation between the release of each satellite.

Answer 2

The axial vs radial deployment doesn't really matter. If they are released at different orientations or at a different rate of rotation: the satellites will have slightly different velocities and will drift apart. This is important as no kind of maneuvering or even reorientation is particularly safe with satellites in the near touching conditions they are packed into the fairings in.

The elaborate series of rolls could be for a number of reasons. My guess is that its the result of the off-centre rotating release generating an impulse and a torque on the space craft (as hinted at). It is likely to be easier to control the conditions of the release by resetting the roll to a known state and starting again than trying to compensate for the dynamics of releasing successive masses.

However there are a myriad of potential reasons that have nothing to do with deployment dynamics. Perhaps the spacecraft needs to maintain a certain range of roll angles to keep an antennae pointed towards earth...