For most of the time, the model runs at a time slice of 1ms
but as the run time that you select gets longer, the model
increases it - if you are tuning a large parachute for 100s,
you are not likely to be bothered about the ms accuracy of
the initial stages although the launch rod and water phases
run at around 1mstime slice any way.

In this way, you get the accuracy you need, quickly enough
if you are performing a 3D optimisation and lots of people
get to run their simulations at the same time.

If your flight takes 10s to complete, use an appropriate time
limit for the best results. The time slice is chosen by the
program but it is printed out on the results page.

As a guide, you might be interested in how your launch tube
performs so you might want to set the time limit to 250ms.
There, you can see how the acceleration changes within the
thrust phases and between the thrust phases. When you are
interested in sorting out the parachute, your attention will be
turned towards the end of the flight so your concern about
the initial events with the model will be less important to
you although the model will still work the thrust phases out
the same.