I am guessing you are referring to a 2 stage or a cluster, correct?
If so, you basically need to select ignition of 2nd stage at some event, i.e., burnout of booster, ejection charge. You will find a pull down menu in yor sustainer motor config. Again, I am assuming this is 2 stage. Really need more info about what you are trying to do.
Hi sorry about the clarity and category issues, it's my first post. What I mean is launch the model rocket from an rc plane like virgin orbital. This is theoretical so please do not worry about legal issues.
OpenRocket is not equipped to deal with something like that, if for no other reason than the plane-launched rockets launch horizontally, and then angle upwards. OR can't simulate a horizontal launch, and even if it could it couldn't them simulate the behavior of changing trajectory. And if you *don't* change directory, you'd be simulating an air-to-surface missile.
You can set the initial altitude to the launch altitude, and by the use of plugins you could in theory simulate a turn to vertical (but that would be a substantial undertaking, I sure don't know how to do it.)