delaying a simulation till end of a previous one

Hello Matlab community, I'm trying to get a simulation of a water rocket going in simulink. I devided the different stages of the flight into 3 stages:

1) Take off, fuel is water 2) all water out, fuel is compressed air left in the rocket 3) flight with newton friction

Stage 1 and 3 are just going fine but stage 2 is a little tricky. I'm using saint venant / wantzel to calculate the impulse created by the compressed air leaving the rocket. This is devided into 2 stages itself.

1) supercritical 2) subcritical

I tried to realize it using a time triggered switch. But both formulas start at the same time and when the supercritical phase ends the output subcritical is already down to zero. So I need to postbone the 2nd stage till stage 1 is finished.

Since I have to use differential equations I cant use a time triggered sub system.

Hope you guys know a way to solve this =)

Answer 1

[Fangjun Jiang]

I don't think this is a MATLAB or Simulink technique issue. It's your modeling technique issue. How do you decide going from super-critical to sub-critical? There is a criteria, right? Put the logic for that criteria in place in Simulink and use that logic to trig the two different stages.

Answer 2

[Oliver]

I decide to go to subcritical after a certain amout of time. This is easy to calculate. I entered this time in a switch connecting both parts.

Problem is both stages start to run at the same time but subcritical should start later since both parts require different formulas.

Is my logic twisted in this?

[Fangjun Jiang]

That is what I don't understand. You said you can't use a time triggered subsystem but you also said that you go to subcritical after a certain amout of time. You mean you don't know how to use time to trig a subsystem? Use a Clock block, it outputs the simulation time. Compare the time with Constant you'll get true or false or rising edge or falling edge. Then use this signal to trig your subsystems.