Understand now. I didn't realize I could change separation time in OR

So if I plot separation as you say, what I am seeing at the expected separation time is the booster appears to go from -2 Gs to about -3.5 Gs (opposite of what you said earlier??) and the sustainers is rather constant. The booster is slowing down more than the sustainer, correct? If so I should have had separation? Am I reading this right?

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It looks to me like the change in acceleration is from -2 G's just before separation to about -2.1 G's just after. This is comparing the acceleration of the stack (before separation) to the sustainer only (after separation). The acceleration shown is just another way to say whether the rocket is slowing down or speeding up. In your simulation, the deceleration is a bit more (at -2.1) for the sustainer than for the stack. So, the simulation is saying that the sustainer, after separation, will slow down a bit faster than the stack prior to separation. At first blush, that would say that the parts won't separate.

The problem with this approach is that you want to know what the sustainer drag is while it is still with the booster rather than after it separates (which is what you get with the simulation). The difference in the two cases is the base drag of the sustainer, which is not present in the stack configuration, but is present after separation. I don't really know if it's particularly rigorous, but the way I account for this is to determine the percentage of the total drag on the sustainer that is base drag and then adjust the sustainer deceleration to account for it. Rocksim gives the drag coefficients associated with the nose/body, the fins, and the base drag. I don't know if open rocket does this or not. But let's say the base drag is 10% of the total for the sustainer at the separation velocity of interest. If you have -2.1 G's total for the sustainer, then -1 G is due to gravity and -1.1 G's is due to drag. If base drag is removed, then the acceleration due to drag drops to 0.9 x -1.1 = -0.99 g's, or a total deceleration of -1.99 G's. So, you have -2G's for the stack and -1.99 G's for the sustainer when it's part of the stack. Clear as mud? With that result, you might conclude that there is essentially no force between the parts, and they might separate or they might not. Sort of consistent with your flight results.

By the way, if anyone out there thinks I'm wrong with the above approach, please speak up. I need to do this calculation from time to time, and if I'm off base, I'd like to know.

Jim