I took the speed data from SpaceX’s stream at intervals of 30 frames and plotted the acceleration.
In his analysis video, Scott Manley mentions a puff at T+7:06 which corresponds to an increased rate of LOX consumption. He may well be right, but I wonder if there is more going on here.
At T+7:39, there is a much more aggressive puff which corresponds to the acceleration flattening to a constant rate. Previously it was increasing (constant thrust and decreasing vehicle mass) though I can’t say if it was linear or exponential. It would be fascinating to know what was happening throughout the rest of the burn.
I did a quick web search to see if the Falcon 9 second stage does a similar thing during terminal guidance, but found no such indication. I admit I did not do the analysis myself.
So what do you think? Were the last 30 seconds part of the throttle profile or an indication of another more energetic event that doomed the flight?
I’m sure we’ll find out for sure from the investigation report, but I thought it would be fun to speculate a little! :)
As it uses fuel, the acceleration increases because the same thrust is pushing less mass. But at some point, it reaches the max acceleration they want, (which depending on the mission or payload could be 3 Gs to 5 Gs) so they start adjusting the thrust to keep the same acceleration as the mass is reduced. This is typical for most rockets, just with different specific numbers. Especially for something which could eventually be human rated, as humans really don’t like anything above 3 Gs.