# When this F9 2nd stage engine suddenly moves upon shutdown, is it moving along the rocket's axis?

The GIFs are made from frames of the live broadcast of SpaceX JCSat-16 2nd stage around the time of the second engine cut-off (SECO-2).

It looks like the engine moves outward axially as the thrust ends, although I can't be absolutely sure. This is a funny angle - that nozzle is quite foreshortened in this view, and the lens may be a fisheye and quite close, contributing some distortion.

Watching the video on-line the speed telemetry shows the acceleration drop to zero just before the apparent motion. Is this evidence of some kind of spring-loading or deflection of the engine mount?

The camera could be on a mount which is flexing under acceleration, but because of the sort-of binary step nature of the motion, I don't think that's actually what is happening in this case.

Also the engine is gimbaled, and viewing from this angle with a short focal length lens, it is possible that this is actually rotation about a pivot point further to the right inside the spacecraft rather than axial translation.

I'm curious if this motion can be explained as a relaxation of a hypothetical "shock absorber" as discussed in this question after thrust has stopped.

above: Frames from the broadcast changed to monochrome (R+G+B)/3 and stitched manually into a GIF. There are jumps in time due to the editing of the video on line. The launch time code can be used to estimate time.

above: Two frames from the broadcast changed to monochrome (here just the Blue (B) channel only, in order to improve contrast and ignore thermal radiation) selected 50 frames (50/30 ~ 1.67 seconds) apart.

Full video: shown here use the launch time code T+ 00:27:32 to 00:27:38 for reference.

• Course correction? First the engine tilts, then the shape of the left, bright part of the nozzle (lit by the sun ?) starts to change. Looks like the stage is turning. – asdfex Aug 15 '16 at 8:43
• Supplementary - does anyone know what would be the nature of the debris shown falling off in the first gif? – Mike H Aug 17 '16 at 5:06
• @MikeH it's a good question, and it's asked several times, if I get a chance I'll find one of the better answers for you. There are two classes of "stuff". One class is bits of insulation foil that have come off due to high gee forces and vibrations, the other class is frozen stuff, including water from humidity earlier in the launch sequence. You can check other questions and answers here for "frozen", "ice" and "debris". – uhoh Aug 17 '16 at 5:15
• A bit hot, just above the bell where the debris was first resting for it to have been ice, I guess? So, maybe insulation. – Mike H Aug 18 '16 at 17:51
• @MikeH you might consider asking a separate question also. The engine is using cryogenic propellants so there are really cold things as well as really hot things in this area. Here is oxygen ice possibly being made just as an example. – uhoh Aug 19 '16 at 2:49

This occurs in the very last moments before the engine is shut down. In fact, as you mentioned, the acceleration drops to 0 as this happens. There's a number of possible explanations, including:

• The engine had a slight gimbal that was reverted upon completion of the thrust.
• There isn't any real movement, but rather the difference in the sun angle to re-position the satellite causes it to look like there is an issue.
• Stopping the engine has a slight jerk.

Here are a number of recent SECO cutoffs posted. All shown are of the second SECO. All show similar behavior. Note that the first SECO is rarely shown because it happens about the same time as the 1st stage landing. I went back as far as CRS-8, and all of them are missing the first SECO.

Given all of these, I'm pretty sure the truth is the first case, that the engine was slightly gimballed and returned to it's centered state. This most likely makes the stabilization easier required to properly deploy it's payload.

• Thanks! I'll take a careful look. The Thaicom-8 video link is set to start at t=49m50s which corresponds to launch T+ 00:28:20. At T+ 00:28:24 the video feed switches to another camera viewing from a different direction, and the engine appears to move axially aftwards. But without a baseline of at least a few seconds earlier from the same camera, it's circumstantial. However the EUTELSAT 117 West B and ABS-2A t=44m54s T+ 00:26:54.5 also shows the same apparent axial aftwards motion. On what basis do you conclude these are all gimbaling in the direction that would appear aftwards? – uhoh Aug 15 '16 at 14:13
• I'm not sure I agree with the gimbal theory. For what reason would off-axis thrust be required up until the termination of the second stage burn? I'd imagine it is only used to effect changes in attitude - continually thrusting off-axis would be throwing away delta-V to cosine losses, no? I'm guessing some shock-absorbing properties in the engine mount, possibly to smooth the impulse of startup and shutdown that the payload sees. We see the spring being unsprung so to speak. But this is purely speculative, not sure I can justify posting it as an alternative answer. – Saiboogu Jan 12 '18 at 17:38
• I also think that it is not so likely that there was a long period of time with the engine gimbaled, unless there was an offset between the neutral position and the position where the thrust axis intersects the center of mass, which is in fact a possibility as discussed in the question Do gimbaled engines have to be carefully test-fired to ensure the thrust axis intercepts center of rotation? – uhoh Feb 20 '18 at 2:50